Fault Detection of Solenoid Valve Using Current Signature

Analysis

By

Atia Adrees

M.Sc in Control Systems Engineering

August 2009

Surpervisor : Mrs Linda Gray.

Executive Summary

Solenoid valves are frequently used in industry to control flow of fluids and gases by running

and stopping current through them. Solenoids are used to open and close bleed valves in

heavy duty gas turbine engines during acceleration to rated speed and de-acceleration from

rated speed to protect axial compressor from surging and stalling conditions. Therefore,

condition monitoring of solenoids plays important role in gas turbine engines. The purpose of

this project is to use Motor Current Signature Analysis for condition monitoring and fault

detection of solenoid valves. The approach used is to detect changes in the driving motor

current signal characteristics and extract information about the solenoid valve’s health. The

project is based on the experiment using a tailored test rig. A suitable computer model of

solenoid valve is also developed.

The experimental investigation is a series of experiments conducted on the solenoid test rig.

First CS of solenoid with normal and healthy spring was obtained to establish baseline

current signature. Then load was increased slowly to the critical value (when plunger doesn’t

move), current data was collected and examined in time domain. Second fault was injected by

replacing the normal, stiff spring with loose spring. To see the effect of damaged return

spring, one of the turns of the spring was scrapped gradually until it broke and current data

was collected for various degree of damage.

Time domain Current signature analysis is used to study the effect of varying load, obtained

results show that CSA has capabilities not only to detect changes in load but it has potential

to make an assessment about the severity of the problem. Similarly examination of current

signature for loose and damaged spring reveals that CSA has abilities to identify loose or

broken spring. By comparing CS of solenoid with loose and damaged against baseline CS, it

is very easy and simple to make an assessment about the severity of the fault.

A mathematical model of solenoid valve is also developed in simulink. Some parameters are

measured directly and others are found heuristically. Both experiment and model show that

CS of solenoid valve has particular features in time domain, variation in load, loose or

damaged spring effect these features .By comparing against the baseline CS these faults can

be identified and quantified.

Acknowledgements

I would like to express my gratitude to Andy Mills for his continuous help, guidance and

valuable advice throughout this project.

I would also like to thank Mrs Linda Gray for her support and kind attention. Thanks to all

members of 307 for their ever available help and support.

CONTENTS

Executive Summary

Acknowledgements

List of Contents

List of Figures

List of tables

CHAPTER 1 Condition Monitoring

Introduction 1

1.1) Maintenance Strategies and Condition Monitoring 1

1.2) Common Failure Modes in Motors 2

1.21) Stator Electrical Failures 2

1.22) Rotor Electrical Failures 2

1.23) Mechanical Failure Modes 3

1.3) Advantages of Condition Monitoring 3

1.4) Thermal Monitoring 3

1.5) Visual Inspection 4

1.6) Oil Analysis/Wear Debris Monitoring 4

1.7) Vibration Monitoring 4

1.8) Acoustic Monitoring 5

Summary

1.9) Aims and Objectives of Project 6

CHAPTER 2 Current Signature Analysis

Introduction 8

2.1) Background to Current Signature Analysis 8

2.2) What is Current Signature Analysis 8

2.21) Why Use Current Signature Analysis 9

2.22) Potential Advantages of Current Signature Analysis 9

2.3) Applications of Current Signature Analysis 10

2.31) Current Signature Analysis for Induction Motors 10

2.4) Current Signature Analysis for D.C Motors 11

2.5) Current Signature Analysis for Gear Box 12

2.6) Electrical Signature Analysis for Motor Operated Valves 13

2.64) Electrical Signature Analysis for Solenoid Valves 14

2.65) Electrical Signature Analysis for Alternators and Generators 15

Summary 15

CHAPTER 3 Current Sensing Technologies 17

Introduction 17

3.1) Current Shunt 17

3.2) Hall Device 18

3.3) Rogowski Coil 20

3.4) Optical Current Measurement Technique 20

3.41) Free Path Technique 20

3.42) Closed Path Technique 21

3.5) Transformers 21

3.6) Current sensing Power MOSFETs 21

Summary

CHAPTER 4 Hall sensor Test Rig 24

Introduction 24

4.1) Test Rig Construction 24

4.2) ACS712 (Hall Sensor) 24

4.3) Instrumentation Amplifier 25

4.4) Description of the Test Rig 25

4.7) Non Intrusive Current Measurement with Hall Sensor 31

4.5) Current Measurement with Shunt 32

Summary 34

CHAPTER 5 Solenoid Test Rig and Fault Simulation 35

Introduction 35

5.1) Solenoids 35

5.11) An Overview of Solenoid valves and Principle of Operation 36

5.12) Common Faults in Solenoid Valves 36

5.2) Solenoid Rig Description 37

5.3) Solenoid Valve Modelling 39

5.31) Electromagnetic Subsystem 39

5.32) Mechanical Subsystem 40

5.4) Solenoid Model 41

5.41) Solenoid Forces Model 42

5.42) Solenoid Inductance Model 42

5.43) Model of Solenoid Mechanical subsystem 43

5.44) Model of solenoid Applied Current 43

Summary 44

CHAPTER 6 Solenoid Fault Detection using CSA 45

Introduction 45

6.1) Current Signature for Healthy Solenoid Valve 45

6.2) Current Signature with Increased Load 47

6.3) Current Signature of Solenoid with spring of less Stiffness 49

6.4) Current Signature with Damaged Spring 49

Summary 55

CHAPTER 7 Conclusions and Future Work

7.1) Review of the Objectives and Achievements

7.2) Conclusions

7.3) Future Work

References

Appendix A Datasheet of INA126

Appendix B Datasheet of ACS714

List of Figures

Hall Sensor Test Rig

Figure 4.1 : Hall Sensor test rig

Figure 4.2 : Hall Sensor Out Put Signal

Figure 4.3 : Bandwidth of Hall Sensor

Figure 4.4 : Bode Plot of Hall Sensor Out Put Signal at various frequencies

Figure 4.5 : Power Density Estimate of Hall sensor output signal

Figure 4.6 : Current Shunt Test Rig

Figure 4.7: Simulink Model

Figure 4.8: Voltage across Shunt

Figure 4.9: Power Density Estimate of Signal across Shunt

Solenoid Test Rig and Fault Simulation

Figure 5.1 : Magnetic field in Solenoids

Figure 5.2 : Solenoid

Figure 5.3 : Solenoid Plunger

Figure 5.4 : Solenoid Test Rig

Figure 5.5 : Experimental Setup

Figure 5.6 : Solenoid Valve Model

Solenoid Fault Detection using Current Signature Analysis

Figure 6.1 :

List of Tables

Table 3.1: Comparison of Current sensing technologies

Table5.1: Results of Series of Experiments to Evaluate System Degradation

Table 5.2: Results of series of experiments to evaluate spring damage

Condition Monitoring

CHAPTER 1

Introduction

Electrical and electromechanical devices pervade all areas of modern life at both domestic

and industrial level. Although reliability of electrical and eletromechanical devices is high but

these devices do fail and their unscheduled maintenance in some occasions leads to heavy

financial losses and in some cases their failure can lead to loss of lives. Therefore it is very

important to monitor the condition of those devices which play vital role in our lives. The

continuous monitoring of equipment can give early signs of any malfunctioning or incipient

fault and maintenance can be scheduled.

This chapter presents three main approaches to machinery maintenance and advantages offer

by condition monitoring. Since advantages offered by condition monitoring are widely

accepted which has led to different condition monitoring techniques. Advantages and

disadvantages of each condition monitoring method are explained later in this chapter.

1.1) Maintenance Strategies and Condition Monitoring

Peter[1]

states that there are three main approaches to maintenance of machinery.

(1) Breakdown maintenance

(2) Scheduled maintenance

(3) Preventive maintenance

Plan (1) demands no more than a ’run it until it breaks’ and then maintenance is carried out.

This approach is slowly being abandoned by industry [2]

.

Method (2) relates to interrupting machinery at regular intervals for the purpose of

maintenance and this policy is still being used in many areas even though it is expensive if

can’t allow breakdown due to financial or safety implications. [1]

Strategy (3) specifies the maintenance tasks to be carried out based upon machinery

condition, in order to prevent equipment breakdown, by carrying out repairs, servicing, or

component replacement[2]

.This approach is based upon fault detection, severity assessment

and fault diagnosis. This process is collectively called condition monitoring[1]

. The objective

of machinery condition monitoring is to obtain the earliest possible warning of incipient fault

or malfunctioning and to assist in the diagnosis of the subsequent cause.

Most large motors use 3-phase power supply which consists of three AC supplies of same

frequency and amplitude, but 120º out of phase with each other. Most motors are considered

an inductive load; therefore voltage in each phase leads the current by 90º[3]

. Being simple,

rugged, low-priced, easy to maintain, highly reliable and acceptable efficient, induction

motors are most widely used machines in industry[2]

.Induction motors (asynchronous) have

been used as prime movers for various equipment such as gearboxes, pumps, fans,

compressors since their introduction[3]

.

On the other hand D.C motors are able to provide speed that can easily and effectively

adjusted over a wide range of operating conditions as suggested by Wildi[4]

.

Third major electromechanical device is synchronous motors, the synchronous motors has the

advantage of being operated under a wide range of power factors, and are much better suited

for bulk power generation[5]

.

Therefore condition monitoring of three major electromechanical devices plays important

role in industry.

1.2) Common Electrical Failure Modes in Motors

1.21) Stator Electrical Failures

The most common electrical failure mode is that of insulation failure in the stator winding,

leading to short circuit current paths, overheating and consequential burn-out. The insulation

system is one of the weakest components in all types of electrical machine[6]

.

1.22) Stator End Winding Faults

Foreign bodies inside a machine, such as steel washers, nuts or small portions of insulations,

get thrown around by the rotor.Damage is caused by these objects, usually in the stator end

winding region, where insulation is damaged[1]

.

1.23) Rotor Electrical Failures

Cracking or breakage of the conductors in the rotor is also a common electrical failure

mode[7]

.This could occur either in the slots bars or in the end-rings. In particular, the

machines that undergo frequent starts under high load develop cracked and eventually broken

bars or end-rings sections[8]

.

1.24) Mechanical Failure Modes

Common mechanical problems include bearing wear, rotor imbalance and air gap distortion

(eccentricity). Bo[2]

reports that most common mechanical faults are associated with bearings.

However, bearing faults are often initiated by the effects of other faults conditions.

Eccentricity of the rotor give rise to vibration due to imbalanced magnetic pull and it

becomes worse when the resultant asymmetric heating leads to thermal bending of the rotor

shaft. The problem of eccentricity is particularly critical when the air gap distance is small 7]

.

1.3) The Advantages of Condition Monitoring

The idea of the scheduled shutdown leads to the notion of monitoring. As condition

monitoring means the continuous evaluation of the health of machinery, plant and equipment

throughout its serviceable life and should be designed to attempt to recognise the

development of faults at an early stage.

Therefore it is generally accepted that condition monitoring offers many advantages

including[1,2,7]

Avoiding unexpected catastrophic breakdowns with expensive or dangerous

consequences.

Reducing the number of machine overhauls to a minimum and thereby reducing

maintenance costs.

Eliminating unnecessary intervention and subsequent risk of introducing faults on

previously healthy machines.

Reducing the intervention time and thereby minimising production loss (as the fault to be

repaired is known in advance and overhauls can be scheduled when most convenient).

The purpose of condition is to obtain data that would aid in evaluating machine’s health[2]

.

Measurements of various parameters are used for condition monitoring. Bradley states that

the optimum method for condition monitoring highly depends on the kind of processes, or

machines being monitored and the maintenance services targets[7]

. Monitoring objectives and

costs should take into account selecting condition monitoring techniques. The most widely

used condition monitoring methods are given below.

1.4) Thermal Monitoring

Thermal monitoring is well recognised condition monitoring tool for assisting in reduction of

maintenance costs. This technique makes use of the infrared energy that all surfaces emit

above a temperature of absolute zero. This technique involves monitoring of temperature and

thermal patterns of a machine during operation[9]

. A potential problem is indicated by thermal

signature different from the normal thermal pattern.

Thermal monitoring improves the ability to predict equipment failure. One disadvantage of

this technique is that it requires constant atmospheric temperature and density of airborne

particles of the environment in which the instrument is working because changes in

atmospheric temperature and density of airborne particles can affect reliability and accuracy

of the measurements taken[10]

.

One of the strengths of thermography is that it could be used on a wide variety of equipment

including pumps, motors, bearings, pulleys, fans, drives, conveyors [6]

.Traditional

thermocouples, fibre optic sensor and infrared thermograghy can be used for thermal

monitoring, depending upon the application [11]

.

1.41) Visual Inspection

Visual inspection is one of the most basic forms of condition monitoring. Techniques used

range from simply using magnifying glasses or low power microscope to light assisted

devices such as stroboscopes, hand –held vibration pick-ups and infrared cameras and

endoscopes [1]

.Visual inspection has been found particularly useful for detection of cracks,

corrosion, leakage and sub-surface defects [6]

.

1.42) Oil Analysis/Wear Debris Monitoring

This method forecasts health of machine by studying the wear particles in lubricant[2]

.The

continuous trending of wear rate monitors the performance of the machine and its

components, and provides early warning and diagnosis. It is claimed Oil condition

monitoring can sense danger earlier than vibration techniques [6]

.

1.43) Vibration Monitoring

Vibration analysis is widely used condition monitoring technique [12]

.All rotating machines

vibrate and produce noise. These characteristics contain valuable information about

machine’s health. This technique monitors the response of the machine or equipment to the

different forces which are applied to it. The response is analysed to extract information about

the health of the machine. Vibration analysis is generally employed for trouble-shooting and

fault diagnosis of rotating machinery and equipment [13]

.

Accelerometers with a wide frequency range and temperature capability collect vibration

signals. These signals are analyzed using different signal processing techniques such as peak,

root mean square (RMS) values, crest factor, frequency content, cepstrum, wavelet and

higher order spectral analyses [1]

.

Vibration monitoring is not suitable for all-purpose condition monitoring; some type of

machine faults may not produce significant changes in vibration signature when compared

with the baseline vibration signal [14]

. The overall vibration signal from machine is the sum of

many components and structures to which it may be coupled. However, mechanical defects

generate vibration signals at different frequencies, which can often be related to specific

machine fault conditions [15]

.

Vibration monitoring is the most common condition monitoring technique applied to

industrial machinery. However, despite the fact that vibration monitoring offers the widest

coverage of all failure prevention techniques, and it has been widely used for online

monitoring for rotating machines such as motors, pumps, gearboxes, difficulties can still arise

from the location of the accelerometer, since the detected signals could be a combination of

different sources[14]

. As result the signal could be a function of nonlinear, non-stationary

sources, and may be influenced by transmission path [2]

.

1.44) Acoustic Monitoring

Operating machines produce vibration and generate noise, and this technique analyses the

generated noise.Generally microphones are used to pick up acoustic signals for analysis and

the relevant monitoring information about the health of the machine is extracted [15]

.

Due to non-intrusive data collection through inexpensive, easy to mount and sensitive

microphones acoustic monitoring is an attractive option for online condition monitoring,

including gearboxes, bearings, tools and engines[2,16]

.Laboratory results have shown that

acoustics monitoring could be particularly useful for the condition monitoring of gearboxes[6]

.

Acoustic monitoring has its own advantages when the problem of noise is under

investigation. It provides vital information about noise sources and generation mechanism.

However, one disadvantage is the contamination of the acoustics signals by the background

noise and acoustic environment definitely plays an important part in condition monitoring of

machines [6]

.

Summary

It is very clear that condition monitoring of electrical and electromechanical equipment offers

many indisputable advantages which has given rise to different condition monitoring

techniques. Each technique has its advantages and disadvantages depending upon the

application. Most of these techniques are often both expensive and labour intensive. The high

cost of implementing and operating these techniques frequently limits their use to critical

machines.

Therefore primary objective of this project is to present an economical, nonintrusive and

reliable monitoring technique capable of screening a large number of parts in machines.

1.5) Aims and Objectives of Project

The aim of this project is to investigate faults in motor driven equipment by monitoring

supply current and voltage (Electrical Signature Analysis). The intension is not to suggest

that ESA be considered as a replacement monitoring tool to conventional techniques. It is

simply to highlight its advantages and explore the capabilities of ESA for condition

monitoring and fault detection.

The aim is to confirm ESA as a new means to detect mechanical faults in electrical,

electromechanical and motor operated equipment.

1.51) Objective 1

Collect information regarding different current sensing technologies. Highlight their main

advantages and disadvantages.

1.52) Objective 2

Evaluate most promising technologies by building test rig for each. This will provide an

opportunity to explore limitations and advantages of each technology for condition

monitoring purpose.

1.53) Objective 3

Use non-invasive and invasive ESA methods and compare them. Compare sensitivity of

current measurement with wires of different types of shielding.

1.54) Objective 4

To collect and analyse current signature for healthy solenoid to establish baseline signature,

for use in fault detection and to investigate the of the current signature in case of faulty

solenoid.

1.55) Objective 5

To introduce specific faults in solenoid and both measure and predict its effect on the supply

current.

New Objective

During project it is decided that a solenoid model will be built which will help to understand

electromechanical dynamics and effect of various parameters on the current signature of

solenoid.

Current Signature Analysis

CHAPTER 2 Introduction

This chapter describes the need of current signature analysis as condition monitoring tool, its

potential advantages being non intrusive but diagnostic and prognostic information

comparable to conventional condition monitoring techniques. Brief account of the successful

applications of CSA is also given in this chapter.

2.1) Background to Current Signature Analysis

For many years organizations and individuals have been using a variety of condition

monitoring techniques to monitor the health of rotating machines such as motors, generators,

pumps, reciprocating compressors, etc to reduce the production costs, improve the reliability

of the operation and provide improved service to customers as discussed by Han and

Y.H.Song [11]

.

Electrical current signature analysis presents a potential breakthrough in its ability to detect

and quantify mechanical defects, degradations in electrical and electromechanical equipment

and unwanted changes in process conditions without interfering with the operation of the

monitored equipment [17]

.

2.2) What is Motor Current Signature Analysis?

Motor current signature analysis is the technique of measuring motor current and analyzing it

to detect various faults in motor and its driven system. Current signature analysis has become

the most popular condition monitoring tool because of its ability to detect the most common

machine faults very easily and its capability of identifying faults in electric machine driven

equipment.

While electrical signature analysis is the technique of measuring motor’s current & voltage

signals and analyzing them to detect various faults [18]

. The major difference between motor

current signature analysis and electrical signature analysis is that current signature provides

information from the point of test towards the load and voltage informs the user what is from the

point of test towards the supply. This provides the opportunity to quickly establish where a

particular signature exists.

2.21) Why Use Current Signature Analysis?

Most commonly used condition monitoring techniques are vibration monitoring, acoustic

monitoring, thermal and oil analysis; these techniques are often both expensive and labour

intensive [6]

. The high cost of implementing and operating these techniques frequently limits

their use to critical machines.

The main objective of this project is to present an economical, and reliable monitoring

technique capable of screening a large number of parts in machines. However, the system

must be capable to identify and locate a piece of malfunctioning machinery with the

parsimonious use of transducers.

2.22) Potential Advantages of Current Signature Analysis

Current signature analysis is a particularly powerful application where it is necessary to

monitor the equipment and machines from a remote point (due to accessibility or hazardous

environments), and/or where non-intrusive techniques are needed. Current signature analysis

could enhance the reliability of equipment by providing early indication of malfunctioning

and faults [17]

.

Advantages of applying the current signature analysis compared to other condition

monitoring techniques have been summarized below [17, 19]

.

Electrical current signature analysis is versatile and powerful condition monitoring tool

because it provides diagnostic and prognostic information comparable to conventional

condition monitoring techniques but only requires access to electrical lines carrying input

or output power rather than to the equipment itself means truly non-intrusive, technology.

It is possible to perform onboard and remote analysis even continuous monitoring if

desired. This technique has already been tested on and successfully applied to a wide

variety of systems, including military, industrial, and consumer equipment.

Rapid measurements can be performed as frequently as desired by relatively unskilled

personnel. Current spectrum is not affected by the current transducer or probe location.

Reduction of noise levels on the measured signal compared to acoustic and vibration

analysis.

It can be used in places where conventional methods are inapplicable.

It has ability to perform functionality of vibration monitoring suggested by R. R. Schoen,

T. G. Habetler, and F. Kamran et al [20]

.

2.3) Applications of Electrical Current Signature Analysis.

Electrical current signature Analysis has been successfully used for induction motors, D.C

motors and motor operated valves [18, 19, 20]

.

Most large motors use 3-phase power supply which consists of three AC supplies of same

frequency and amplitude, but 120º out of phase with each other. Most motors are considered

an inductive load; therefore voltage in each phase leads the current by 90º [4]

. Being simple,

rugged, low-priced, easy to maintain, highly reliable and acceptable efficient, induction

motors are most widely used machines in industry[8]

.Induction motors (asynchronous) have

been used as prime movers for various equipment such as gearboxes, pumps, fans,

compressors since their introduction[6]

.

On the other hand D.C motors are able to provide speed that can easily and effectively

adjusted over a wide range of operating conditions [1]

.

Third major electromechanical device is synchronous motors, the synchronous motors has the

advantage of being operated under a wide range of power factors and are much better suited

for bulk power generation[5]

.Therefore condition monitoring of three major electromechanical

devices plays important role in industry.

Different techniques have been used to assess the condition and performance of motors.

Electrical current has been used successfully to detect faults in induction motors and has

become an established tool [6, 17, 18, 19, 20

]

2.31) Induction Motor Broken Rotor Bar Detection by Electrical Signature Analysis

Broken rotor bar faults cause’s sidebands spaced at twice slip frequency around the

fundamental electrical supply frequency in current spectrum [2, 18]

.

Howard [18]

states in his article that broken rotor bar signature was enough unique to be

identified easily and magnitude of the peaks of sideband frequencies could be used to determine

the rotor condition.

Bo [2] explains in detail the effect of normal, half broken and full broken rotor bar on the peak of

the sideband frequencies. As the severity of fault increases peak of sideband frequencies

increases.

2.32) Angular Misalignment Fault in Induction Motor

Identification of angular misalignment faults by electrical signature analysis requires to

perform another signal conditioning of motor current signal [19]

.This procedure is called RMS

modulation, which is carried out to eliminate the line frequency. In demodulated motor

current signature, running motor peak speed is prominent. However, the presence of

misalignment will result in high peaks at running motor speed and at its harmonics [17]

.

2.33) Foundations Looseness in Induction Motor

Uneven or loose foundations bolt will lead to foundation looseness. This fault can be easily

identified by looking at the RMS demodulated spectrum. Foundation looseness results in two

high peaks at half of running motor speed [19]

.

2.34) Bearing faults in Induction Motor

All bearings have set of unique defect frequencies which are specified by the manufacturer

and the presence of these defect frequencies faults allows the identification of bearing. When

current signature is examined for bearing faults, presence of high peaks at these bearing

defect frequencies indicate bearing faults. The severity of faults is assessed by the amplitude

of these peaks [20]

.

2.35) Stator Mechanical Faults in Induction Motor.

Stator mechanical faults typically comprise wedges, laminations and core damage [2]

.Current

signature is able to identify other static eccentricity, dynamic eccentricity in induction motors

[12].

2.4) D.C Motors

Current signature analysis has become an established field for A.C induction motor’s online

condition monitoring. While it is a developing tool for D.C motors fault diagnosis and

prognosis[21]

.Current signature analysis both in time and frequency domains has been used

successfully to detect shorted armature windings, shorted field windings ,rotor faults, off

magnetic neutral plane brush positions[21,22]

.

2.41) Turn to Turn Short

Many turn to turn or commutator bar to bar faults arise from carbon dust build up. Carbon

dust from the brushes build up on the commutator creating a short circuit between

commutator bars [21]

, turn to turn also results from insulation failure.

In case of no fault, the time domain current signature analysis shows no modulation of carrier

frequency but faulty condition waveforms have modulation of carrier frequency [22]

.Current

signature frequency spectrum shows, with fault condition amplitude of the harmonics is

significantly increased as compared to no fault condition [19]

.

2.42) Coil Group Short

The time domain current spectrum shows increase in modulation of carrier frequency as

compared to turn to turn short. Current signature frequency spectrum shows an increase in the

harmonics as compared to turn to turn short [17]

.

2.43) Coil to Coil Short

The time domain current signature with coil to coil short results an increase in modulation of

carrier frequency. Current spectrum in frequency domain indicates an increase in the

amplitude of harmonics as compared to coil group short.

2.44) Brush Position

Detecting the brushes when they are off the magnetic neutral axis is difficult task, especially

if the motor is inaccessible during operation. Voltage analysis in time domain provides

sufficient information about the location of brushes. If brushes are at the centre, there is no

noise on the voltage waveforms but when brushes are off, voltage waveforms in time domain

have a lot of hash [22]

.

2.45) Rotor Faults

Rotor faults in brushless D.C machines arise from eccentricities, damaged rotor magnets,

misalignments and asymmetries [23]

.Satish investigated the effect of different rotor faults on

current spectrum of brushless D.C drive with only torque control loop and concluded the fault

harmonics are visible in current spectrum. However change in the current is relatively small

with only one loop. But addition of speed loop results in increased amplitude of harmonics

[22].

As diagnostics of induction motor has proved that rotor conditions affect certain

characteristics frequency components in the machine stator current [17, 18]

.Similarly rotor

conditions in dc motor affect certain characteristics frequency component in the machine

stator current [23]

.

2.5) Current Signature Analysis for Gearbox

Mohammad

[6] discovered that 3-phase motors can be used as transducer to diagnose

mechanical faults beyond the immediate vicinity. He used current signature analysis to reveal

faults in helical gearbox.

Gears have been used as a mean of power transmission for long time. They are used usually

to transfer a change of rotational speed. Gearboxes consist of set, sets of gears mounted on

shafts and supported by bearings. The entire system is enclosed within housing, with

lubrication. A power source such as an electric motor drives the gearbox input shaft, normally

at a relatively high speed. The gears inside the gearbox transmit a reduced speed to the output

shaft. When the output shaft speed is reduced, generally output torque is increased [24]

.

Gears in general operate in groups of two, or more, with teeth of one engaging the teeth of

other, this enables them to transfer power without slippage. When teeth of two gears are

meshed, turning one gear will cause the other to rotate; such arrangement allows the speed

and direction of rotation to be changed. The gear with fewer teeth are called pinion. The

speed of the rotation when the gear drives the pinion and reduced when pinion drives the gear [3]

.

2.51) Types of Gear Failure

Often, increased vibration and noise levels from gears are commonly associated with

equipment failure. Gear failures can be classified into two types.

Fracture of gear tooth, where a lot of section of tooth breaks away, usually occurs at the

root of the tooth-due to tooth root bending fatigue.

Damage or destruction of the working surfaces of the gear tooth-surface wear and fatigue.

Mohammad [3]

used current signatures to investigate broken tooth. The presence of

symmetrical series of sidebands spaced around the running speed reflect shaft speed

variation, and is if the motor was acting as modulator with the line frequency as the carrier

and the shaft speed as the modulating signal[25]

.

2.6) Electrical Signature Analysis for Motor Operated Valves

Initially electrical signature analysis techniques were developed to monitor the performance

and condition of motor operated valves in nuclear power plant safety related systems

remotely [17]

.

Since then electrical signature analysis has been effectively used for performance and

condition monitoring of large number of consumer and industrial equipment such as air

compressors, water pumps, large chillers, fans of various types and automobile alternators

and helicopter generator, home appliances and tools[17]

.

Monitoring of motor Operated Valves by electrical signature analysis has led to the

development of several signal conditioning and signature analysis methods which use electric

motor as transducer [19]

.These methods provide means of detecting small time dependent load

and speed changes generated anywhere within motor operated valves system and converting

them into useful information which can be used to detect degradation and incipient failure [25]

.

Haynes et al has developed a signature analysis method and apparatus for monitoring the

condition and performance of electric motor driven mechanical devices. This method analysis

an electric signal proportional to measured motor current using frequency domain signal

analysis techniques to provide a current noise signature for which various operating

characteristics of the motor operated devices could be observed [25]

.

It has been discovered that current noise signature contains the sum of all the mechanical load

changes which refer back to the electric motor drive. These characteristics are separated on

frequency and amplitude basis such that the source of various changes in load could be

identified such as periodic gear mesh loading, friction events at frequencies corresponding to

their origin and other load varying characteristics of the device. Therefore current noise

signatures recorded at different periods during the operating life of the device may be

compared to determine aging and wear or abnormal operating characteristics without

interrupting the normal operation of the device [21]

.

H.D Hynes describes how electrical signature can be used to monitor the condition of motor

operated valves with the help of motor operated gate valve operation. A typical motor

operated gate valve has a motor operator capable of providing necessary torque that, when

applied to the stem threads of a rising stem valve, produces enough stem thrust to open and

close valve. Forces opposing stem travel include stem packing friction, stem ejection force

due to internal fluid pressure and gate to guide friction induced by high fluid flow [25]

.

In typical motor operator, worm gear drives the drive sleeve through lug to lug contact. This

helps the motor to start with relatively unloaded condition and can reach full speed before the

worm gear and sleeve engage. This transient running load that occur at this engagement is

called ” hammerblow” and can be easily identified in a simple time domain waveform

obtained by rms-to-dc conversation of measures current signal[25]

.

H.D.Hynes also reports during the beginning of an opening stroke for an 18-in gate valve, in

addition to the hmmerblow, an increase in running current is also observed in current

waveforms. This increase is result of friction between the valve stem and the stem packing

rings when the stem begins to move [25]

.Amplitudes and the times of occurrences of these

features are very useful parameters and may be trended over time for condition monitoring

[20].The differential between hammerblow and initial stem movement provides information of

clearance between the stem nut and stem thread surfaces.

Similarly the time between initial stem movement and gate unseating represents the gate and

stem coupling surfaces. A precise measurement of these times can give an early indication of

wear [20]

.

2.7) Solenoid Valves

Solenoid valves are electromechanical devices, often used to control the flow of liquids and

gases. These valves are controlled by electric current through solenoid (which is a wire coil).

Solenoid valves are the most widely used control components in fluidics. Their operations

involve shut off, release, dose, distribute or mix fluids. Solenoids present fast and safe

switching, high reliability, long service life, low control power and design. Therefore their

condition monitoring plays important role in most applications.

Since mechanical and electrical perturbations occurring in motor and its driven system are

indicated by the changing electrical spectrum [19]

.Therefore, by monitoring the current

through the solenoid, performance and condition monitoring of the solenoids valves can be

performed as suggested by E.D.Blakeman and R.C.Kryter [26]

.

NASA is developing a “smart” current signature sensor which will enable nonintrusive

continuous condition monitoring of solenoid valves without interrupting the normal operation [27]

. It is expected that this smart sensor will be capable of indicating solenoid-valve failures

to perform preventive repairs. This sensor will noninvasively measure and analyze steady-

state and transient components of the magnetic field, indirectly electric current in solenoid

valve during normal operation [27]

.

This sensor uses the fact that unique current signatures of the solenoid, especially when

solenoid energies and de-energies are changed by electrical and mechanical deterioration of

the solenoid and valve parts. Solenoid current signatures comprise characteristic peaks and

valleys that repeat periodically during every operating cycle. Each feature has well defined

magnitudes and shape. M.Taghizadeh et al suggests that result of electrical and/or mechanical

deterioration, the time and magnitude of both peaks and valleys change; these changes

provide indications of potential trouble [28]

.

2.8) Electrical Signature Analysis for Alternators and Generators

Electrical Signature Analysis provides the means to collect diagnostic related information

from electric motor driven devices through a remote non-intrusive current and voltage

measurement on a motor power cable [17, 19]

. The resulting signal after appropriate signal

processing is capable to indicate mechanical and electrical perturbations occurring in the

motor and its driven system. This technique has been successfully applied to many different

types of AC and DC motors with various driven machines. In electromechanical systems,

variations in load and speed produce correlated changes in current and voltage. Therefore

voltage output signals of electro-magnetic devices such as generators and alternators are used

to monitor their condition and performance. The main objective of this study is to investigate

diagnostic techniques from established non-intrusive ESA methods and apply them for sound

reduction in automobile alternators, fault detection of rotor and gearbox in helicopter and

aircraft generators, and diagnosis of gear problems and electrical winding health for airliner

auxiliary power unit generators[21]

.

ORNL has demonstrated the similarity between mechanical vibration, motor current, and

generator voltage signatures on commercially available air compressor. The compressor is

belt driven by motor and the belt driven by motor is standard automotive alternator. Tension

in the belt can be adjusted to engage or disengage while alternator and motor still running.

[29]

.

The alternator output signal was filtered to remove dc voltage component and then low pass

filtered to attenuate the pole pass frequency and high frequency components. The frequency

spectrum of conditioned signal includes all important electrical and mechanical events

frequencies, proving alternators a very source of condition monitoring of equipment [29]

.

Summary

Literature survey confirms that current signature analysis has a potential to be versatile and

non invasive condition monitoring tool. It is an established condition monitoring tool for

induction motors which are widely used in industry. But it is a developing fault detection

method for DC motors, motor operated valves, alternators and generators. Since mechanical

and electrical perturbations occurring in motor and its driven system are indicated by the

changing electrical signature, therefore it is possible to monitor the devices by examining the

supplied current which reveals all necessary information about system’s electrical and

mechanical health.

Current Sensing Technologies

CHAPTER 3

Introduction

Since purpose of this project is to use current signature analysis for condition monitoring,

therefore this chapter presents merits and advantages of each available current sensing

technology. Six up to date current sensing technologies including current shunt, Hall Device,

Rogowski Coil, Optical Current measurement technique, Transformer and power MOSFETS

are discussed in detail.

3.1) Current Shunt

The simplest method of measuring current would be to determine the voltage drop across a

low value resistor (shunt) through which the current passes. Current shunts have been used

successfully at both high currents (500 amp) and high frequencies (100MHz) but usually not

for both simultaneously.

Keyworth V.E [30]

reports that for higher continuous current ratings the thermal requirements

become incompatible with the high bandwidth requirements i.e. the measurements limits of

the shunt are determined by heat dissipation, and the effect of the reactance on frequency

response.

For high current the resistance must be kept low, to minimise the thermal power generated in

order to maintain the reasonable shunt temperature necessary for its stability. The measured

voltage drop should also be independent of the magnetic field forces.

At high frequencies the inductance of the shunt may become significant, therefore shunt of

appropriate resistance must be used to insure that product of frequency and inductance does

not become comparable with the resistance value. Neal J.A.G [32]

describes the conditions,

being incompatible, prevents the use of this method where both high currents and high

frequencies may be encountered.

All shunts have de-rating factor for continuous use, 66% being the most common. Using

continuously for more than two minutes de-rating factor must be applied. There are also

thermal limits on use of shunts, at 80C0 thermal drift starts, at 120

0 drift becomes a significant

problem at 140C0 the manganin alloy is permanently damaged due to annealing resulting in the

resistance[33]

.

If the current being measured is at high voltage, and then this high voltage will also be

present in the connecting leads to and in the measuring device which is highly undesirable.

Current transformers and Hall current sensor can provide this high voltage isolation. But

current shunts are more accurate and cheaper than Hall devices below 80C0.

3.2) Hall Device

Hall effect was discovered by Edwin Herbert Hall in 1879.He proposed that a magnetic field

applied to the current carrying conductor would generate “a state of stress in the conductor,

the electricity pressing towards one side of the wire” [34]

.Although Hall was working on this

effect some years before the identification of electrons but his intuitive approach of “pressed”

electricity was appropriate. It is now well known phenomenon that when a magnetic field is

applied at right angles to the direction of flow of current, an electric field transverse to the

direction of flow of current and magnetic field sets up. This electromotive force is a direct

function of the applied external magnetic field, from which fact emerges a large number of

potential application.

The Hall coefficient is the ratio of the induced electric field to the product of the current

density and the applied magnetic field. It is the property of the material of the conductor and

its value depends on the type, number and properties of the charge carriers that constitute the

current. All conducting material exhibit Hall effect due to the nature of current in a

conductor. Semiconductor materials are able to produce much larger Hall EMF than metals

and all modern Hall devices are made of semiconductors.

Current is formed, due to the movement of small charge carriers, typically electrons, holes or

both. All moving charges experience a force in presence of a magnetic field which is not

parallel to their motion. In absence of such magnetic field, charge carriers follow straight path

but when a perpendicular magnetic field is applied, their path is curved accumulating them on

one side of the material. This separation of charges leads to an electric field that opposes the

migration of more charge carriers establishing a steady electric potential for as long as the

charge is flowing.

Hall voltage VH is given by

eq(4.1)edn

BIVH

Where I = current

B = magnetic flux density

d= depth of the plate

n= charge carrier density

e= electron charge

Hall coefficient RH is given by

eq(4.2)ne

1R H

Therefore the Hall effect is very effective tool to measure carrier density or the magnetic

field. Hall plate is characterised by its sensitivity which is given by following equation.

eq(4.3)d

RS H

The sensitivity “S” of the Hall Plate is directly proportional to Hall coefficient and inversely

proportional to the thickness “d” of the Hall plate.

The choice of a suitable material for Hall device depends upon the application, its electrical

characteristics, operating temperatures and other parameters such as charge mobility and

energy gap as suggested by Putley [35]

. A low charge mobility means for a given thickness of

the plate resistance will be high, leading to an enhanced power dissipation. This is turn leads

to rise in temperature and to the onset of intrinsic conduction. Intrinsic conduction is

responsible for increase in number of charge carrier ”n” and a reduction of the Hall

coefficient ”RH”. However, a material with a high energy gap will enable to continue to

function at higher temperatures.

Hall effect devices when properly packaged are impervious to dust, water, dirt and water

making them better for position sensing than optical and electromechanical sensing. When

current (electrons) flows through a conductor, a magnetic field is produced, therefore a

contactless current sensor can be made using Hall device. This contactless measurement does

not need additional resistance and gives isolation for high voltage and high current

measurement which enhances the safety of the measuring instrument.

Neal J.A.G [32]

states complications arise when the Hall device is used for either very high

current or very high frequencies. At very high current the core may saturate, which together

with the high field strength and considerable temperature dependence of “RH”, will cause

deviation in the Hall voltage. If signal amplifier are used in the immediate vicinity of the Hall

probe, these may also be affected by the high magnetic fields associated with the high

current.

Rizvi S.N.A [36]

describes the main features of the technique as follows

Good accuracy and resolution

Widely used as clip on format

Limited bandwidth, which is generally much less than 100 kHz.

3.3) Rogowski Coil

Rogowski coil current transducer has been used for measuring and detecting electric current

measurement for decades. A Rogowski coil for the current measurement is a wire wound coil

wrapped on the toroid arranged to complete a loop around the path of the current to be

measured. The output voltage of the Rogowski coil is proportional to the rate of change of

current as given by following equation

eq(4.4).Vdt

di

L

AμVt

Where A is the area of each turn, L is the of the coil, μ is the permeability of the material. For

given coil the sensitivity can be adjusted over an enormous range selecting the appropriate

values of the integrator components. The same flexible coil can be used to measure the

current ranging from few milli amperes to several mega amperes.

Rizvi S.N.A [35]

gives the main features of the Rogowski coil

No D.C response.

Do not suffer from the saturation effects and consequently can have a large dynamic

range.

Often used to measure high transient currents.

Wide bandwidth up to MHz

Clip-over and flexible types available.

3.4) Optical Current Measurement Technique.

The working principle behind the optical current measurement technique is the underlying

fact that when a linearly polarized light passes through a medium under the influence of the

magnetic field, the direction of plane of polarization of light will in general be rotated. This

phenomenon is known as Faraday effect.

3.41) Free Path Technique

The free path technique make use of a light beam directed upwards from the ground to pass

freely through the atmosphere to interrogate a passive transducer element (glass) attached to a

high voltage line. The beam, with the required information impressed on it and directed down

to be analyzed by the ground based receiver. Roger A.J [37]

states the advantages of this

simple technique.

The light passes freely through the atmosphere, from earth potential to high voltage point,

and so no expensive insulation is necessary.

Low cost.

The sensitivity depends simply on length of the glass element.

The bandwidth is as large as that of the photo detector and its amplifier.

Roger A.J [37]

gives one important drawback of this simple technique.

It is very sensitive to the vibrations in the conductor bar, as the vibrations in the

conductor varies the amount of light which is returned to the photo detector and thus give

rise to noise.

3.42) Closed Path Technique

The enclosed path technique provides a protected propagation path for light beam. Two

possible approaches in sensor construction under this technique are

The winding of the optical fibre around the current carrying conductor.

The encircling of the current carrying conductor with the glass element.

However, the most dominant approach has been winding of an optical fibre around the

current carrying conductor. However, vibrations and other fibre related problems do limit the

use of fibre as a current sensing element.

Kanoi M .Tl [39]

constructed a current sensor based on the bulk glass approach. It was found

that the optical system has good linearity and errors were well within 0.4% in the current

range of 0.2kA to 2kA rms, a variation of ±0.3% in the temperature range of -20º C to 90

ºC.

3.5) Transformers

A Current transformer is a traditional approach and possibly the most widely used current

transducer. Current transducers are inherently identical to the two winding voltage

transformer. A simplest form of transformer consists of two sets of electrically insulated coils

of wire arranged in a manner such that change in current in one (primary) coil will produce

change in voltage in (secondary) windings.

The main features of current transformers can be described as

Passive, requires no power supply.

Low cost.

Available in clip format.

Good accuracy and long term stability.

Dynamic range limited by the core saturation effects.

No dc response

3.6) Current Sensing Power MOSFETs

Current sensing power MOSFETs are effective tool to measure load current that eradicate the

need for a current shunt resistor as claimed in their data sheet.

Current sense FET technology depends upon the close matching of transistors cells within the

power MOSFETs. Power MOSFETs are made up of many thousands of transistor cells in

parallel. Elements within the device are identical and drain current is shared equally among

them. The on-state resistance of the MOSFETs depends on the number of cells in parallel in

given chip area, large the number of cell is parallel on the cell; lower the on state resistance

[38].

The source connection of several cells is isolated from the majority of cells and brought out

onto a separate sense pin. Now power MOSFET can be taken as two parallel transistors with

common gate and drain but separate sources.

The sense cell passes only fraction of the load current proportion to the ratio of their area.

This ratio is typically 500:1 and known as sense ratio denoted by n.

eq(4.5)n

RIV senseD

sense

The sense current mirrors the drain signal closely. It is claimed in their datasheet that they are

economic alternative to current shunts. But the information given on the datasheets is

ambiguous.

Summary

It is very clear from literature survey that each current sensing technology has its own

advantages and disadvantages and choice of technology depends upon the application. For

current measurement which involves AC and DC signal, Hall effect device, current shunt and

optical fibre can be used.

Measurement with current shunts is simplest method, presents if not infinite but very large

bandwidth. But measurements which involve high currents and high bandwidths thermal

conditions become incompatible. The use of current shunts also add unwanted extra

resistance in the circuit resulting into power losses .In case of current being measured at high

voltage, high voltage will be present to and in the measuring device which is not safe

.measurements are always intrusive.

Hall effect current sensors presents solution to all the problems with shunt, negligible

resistance leading to negligible power losses in circuit, in case of currents at high potential,

high voltages are not present in the measuring instruments leads. Intrusive and nonintrusive

measurements can be made. The disadvantage is the limited bandwidth.

Optical fibre can be used non intrusive measurement of AC and DC signal. Their bandwidth

depends upon the phtodetector but it is complicated as compared to Hall effect sensors and

current shunts.

Current measurement which includes only AC signal, transformers, and Rogowski coil can be

used. The main advantages offered by current transformer are the passive, non intrusive,

economic current measurements but dynamic range is limited by core saturation. While

Rogowski coil presents, non intrusive, very large bandwidth and large dynamic range. The

main features are summarized below in the table.

Current

Shunt

Hall

Device

Transformer Rogowski

coil

Optical current

Measurement

D.C response yes yes no no yes

Band width limited Less than

100 kHz

In MHz in MHz Depends upon

photo detector

Contactless

measurement

no yes yes yes yes

Hall Sensor Test Rig

CHAPTER 4

Introduction

As described in chapter3 that Hall Current sensors are widely used for current measurement

because of their performance, availability, cost and their ability to measure both alternating

and direct current. Their capability of invasive and non invasive current measurements made

them very strong candidate for current measurement.

Current shunts (measuring current across low value resistor) are the simplest way and have

been used to measure high currents and high frequencies. This approach is simple and

economical. Therefore test rigs for Hall sensor and current shunt are constructed. Current

measured with shunt and Hall sensor is compared. Bandwidth of Hall sensor is verified and

effect of high frequencies across shunt is explored.

4.1) Hall Sensor Test Rig Construction

Allegro’s ACS712 fully integrated, Hall effect based linear current sensor is used for test rig

because of extremely stable output offset voltage and total output error 1.5%.Since zero

current output voltage is Vcc× 0.5 and output voltage is proportional to 185mV/A. Therefore

offset needed to be cancelled; otherwise a small desired signal would reside on large offset

voltage. Working with small currents amplification of desired signal is also required.

As only small currents are under considerations therefore amplification is required. For offset

cancellation and amplification INA126 precision instrumentation amplifier is selected

because of simplicity and performance. Instrumentation gain can be adjusted only with one

resistance between pins one and eight.

Function generator is used to supply current. The signal generator can provide maximum

current of 0.3amps. Frequencies can be varied from 1Hz to 1MHz.Therefore effect of higher

frequencies can be seen on shunt current measurement and Hall sensor. Also magnitude of

measured current with current shunt and Hall sensor is compared.

4.2) ACS712

ACS712 is fully integrated Hall effect based linear current sensor and low resistance current

sensor. It is widely used in industry for AC and DC current sensing. It consists of linear Hall

sensor circuit with a copper conduction path near the surface of the die. Applied current flows

through this copper conduction path and generates a magnetic field which is sensed by the

integrated Hall IC and converted into voltage.

The output of the device has a positive slope when increasing current flows from pins 1 and

2, to pins 3 and 4, which is the path used for current sensing. The device can tolerate current

up to five times over current conditions because of the thickness of the copper conductor.

ACS712 comes with a filter pin for noise management. The capacitor at this pin adjusts the

bandwidth of the output signal.

4.3) Instrumentation Amplifier

An instrumentation amplifier is a differential amplifier with buffer stage, which eliminates

the need of input impedance matching and thus making it particularly suitable for

measurements. Additional characteristics include very low DC offset, very high open loop

gain, very high common mode rejection ratio, low drift and low noise. Instrumentation

amplifiers are used for great accuracy and stability.

The INA126 precision instrumentation amplifier is used in test rig for accurate and low noise

differential signal acquisition. The output is referred to the output reference (Ref) terminal

which is generally grounded.

The output of the amplifier = (Vin+ - Vin

-) G eq (4.1)

Where G is gain of amplifier, Vin+ is signal applied at input pin 3 and Vin

- is signal applied at

pin 4.Gain of the amplifier is set by connecting external resistor, RG between pins 1 and 8.

eq(4.2)R

KΩ805G

The 80KΩ term in above equation comes from the internal metal film resistors which are

laser trimmed to accurate absolute values. The accuracy and temperature coefficient of these

resistors are included in the gain accuracy and drift measurement.

4.4) Description of the Test Rig

.Terminals1 and 2 of Hall sensor are fused internally, similarly terminals 3 and 4. Current to

be sensed flows through terminals 1, 2 to terminals 3, 4.Since output of ACS712 is 185mV

per ampere and offset voltage is 2.5V for 5V Vcc. Therefore output of Hall sensor is very

small AC signal residing on 2.5 DC signal for signal generator with maximum current of 0.3

amp.

Hall Sensor Device resistance to adjust gain Instrumentation Amplifier

Variable resistor capacitor to adjust bandwidth

Fig (4.1)

Therefore Precision amplifier INA126 is used to reduce DC offset and amplify AC signal. A

voltage divider made with variable resistor provides 2.5V at pin 2 of INA126; output of Hall

sensor is connected to pin3 of INA26. Gain is set to a factor of 19 by connecting a resistance

of 6.5KΩ between pins 1 and 8 of INA126.The capacitor at pin6 of ACS712 set the

bandwidth of Hall sensor. The internal filter resistance of ACS712 is 1.7KΩ. By using

following equation

eq(4.3)HzRC2

1f c

As R is known bandwidth is set to 1 KHz with C= 100× 10-9

Farads

Experimental set up to measure current through Hall Sensor is shown below.Pc1

Fig (4.2)

As applied current to be sensed passes through the copper conduction path located near the

surface of the die, it generates a magnetic field which is sensed by the integrated circuit Hall

effect device. The output is given by voltage proportional to the applied current.

In above case applied current is sinusoidal which results into proportional sinusoidal voltage

at the output pin. As the output voltage is most of the time less than peak value, therefore

peak value does not represent the true effect .Instead root mean square is the effective value

for varying signal which is equivalent to the DC value of same effect.

Vrms of the sinusoidal signal= 0.7×Vpeak eq (4.4)

Vpeak =0.6 V

Vrms=0.42 V

As output of Hall sensor is amplified by instrumentation amplifier by factor of 19.

Vrms=0.42/19=0.022V

185mV results from = 1amp

0.021 V =0.113amp

0 2 4 6 8 10 12 140.8

1

1.2

1.4

1.6

1.8

2

2.2

X: 0.7593

Y: 2.109

X: 0.2741

Y: 0.9082

output from Hall Sensor

time

voltage

Bandwidth of Hall Sensor

The fig (4.3) below shows the effect increasing frequencies on the output of Hall sensor.

Fig (4.3)

Above fig shows amplitude of the signal falls significantly at 1 KHz. To confirm that

amplitude of the signal is dropped by 3dB at 1 KHz which is adjusted bandwidth of the

sensor, Bode plot is plotted shown in fig (4.4)

Fig (4.4)

0 2 4 6 8 10 120.5

1

1.5

2

1Hz 10 Hz 100 Hz

1KHz

10 KHz

100

101

102

103

104

105

-14

-12

-10

-8

-6

-4

-2

0

2

4

X: 628

Y: 3.694

radians/sec

20lo

g10a

mpl

itude

bandwidth of Hall Sensor

X: 6280

Y: 0.7485

Amplitude falls by 3dB at 6280 rad/sec.

ω =2πf eq (4.5)

ω = 680 rad/sec

f = I KHz

This is the same bandwidth as adjusted by capacitor C at filter pin6 of Hall sensor.

Signal to Noise Ratio

Signal to noise ratio of the Hall sensor output signal is calculated by using Welch method.

Fig (4.5)

SNR= eq(4.6)noise

signal

P

P

= 510

50 dB

Hall sensor’s Output in Absence of Applied Current

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5-80

-70

-60

-50

-40

-30

-20

-10

0

10

Frequency (kHz): 0.09999847

Pow er/frequency (dB/Hz): -9.211306

Frequency (kHz): 0.107872

Pow er/frequency (dB/Hz): -54.30496

Frequency (kHz): 0.1054993

Pow er/frequency (dB/Hz): -64.04458

Frequency (kHz): 0.1054993

Pow er/frequency (dB/Hz): -64.04458

Frequency (kHz)

Pow

er/

frequency (

dB

/Hz)

Power Spectral Density Estimate via Welch

Fig (4.6)

Hall sensor show some kind of effect in the vicinity of power supplies, due to lack of time it

was investigated properly. The top fig in above graphs is output of Hall sensor when there is

no applied current and it is very close to power supply.

It is very clear from above graphs that the noise decreases significantly as Hall sensor is

moved away from the batteries.

4.5) Non intrusive Measurements with Hall Sensor

Hall sensors are available for non intrusive current measurements. Like Honeywell’s

CSLA1CD 9729 is an open loop contactless current sensor. It consists of Hall effect sensor

mounted in an air gap of magnetic core. The current carrying wire is passed through the

aperture of the sensor, magnetic field (proportional to current) produced by the applied

6 8 10 120.4

0.5

0.6

0.7

0.8

X: 10.86

Y: 0.5957

voltage

24cm X: 12.59

Y: 0.7324

0 2 4 6 8 10 12 14 160.55

0.6

0.65

0.7

0.75

X: 5.766

Y: 0.5957

40 cm

X: 7.091

Y: 0.7178

50 52 54 56 58 60 62 64 66 680.55

0.6

0.65

0.7

0.7564 cm

time

current is sensed by the Hall effect sensor which produces voltage proportional to current.

Honeywell presents large variety of Hall effect sensor for non intrusive current

measurements.

Fig (4.7)

http://www.directindustry.com/prod/honeywell-sensing-and-control/open-loop-current-

sensor-12365-306124.html

4.6) Current Measurement with Shunt

A power resistor is connected across function generator to measure current supplied by

function generator as shown in fig (4.7).

Function Generator

Computer

Fig (4.8)

A simulink model shown below is used to observe voltage across shunt.

Advantech scope PCI-1711[auto]

Analog input

1M 100KH 10KH 1KH 100HZ 10HZ 1HZ

1.00 HZ

load

shunt

Data acquisition card

Fig (4.9)

The voltage across 15Ω power resistor is shown is shown below

Fig (4.10)

Since measured signal is AC signal, therefore RMS value of current is calculated.

Vrms = 0.7×Vpeak eq (4.7)

Vpeak=1.1 V

Vrms=0.77 V

R=7Ω

0 5 10 15 20 25-1.5

-1

-0.5

0

0.5

1

1.5

time

voltage

voltage across shunt

Analog output 1

Irms= eq(4.8)R

Vrms

Irms= 0.1amp

Signal to noise Ratio

Signal to noise ratio of the voltage signal across shunt resistor is calculated by using Welch

method.

Fig (4.11)

SNR = eq(4.9)7.47

52

P

P

noise

signal

Bandwidth of Shunt

It was expected that at small currents shunts have unlimited bandwidth but it is found that

used current shunt (100W) has definite band width. It behaves differently at 1MHz,

amplitude of the signal does not drop but it increases significantly. But another current shunt

(10W) has indefinite bandwidth.

-0.04 -0.02 0 0.02 0.04 0.06 0.08-120

-100

-80

-60

-40

-20

0

20

Frequency (kHz)

Pow

er/

frequency (

dB

/Hz)

Power Spectral Density Estimate via Welch

Frequency (kHz): 0.0100708

Pow er/frequency (dB/Hz): 2.007075

Summary

Hall sensor and current shunt are used to measure the current supplied by function generator.

It is expected that Hall sensor has limited bandwidth as described in their datasheets but

current shunt has unlimited bandwidth. Experiments results proved bandwidth of Hall sensor

is limited as expected ,while shunts has much larger but limited bandwidth. The signal to

noise ratio of signal across the shunt is higher than the signal to noise ratio of Hall sensor

output signal. But measurement with current shunt adds extra resistance in the circuit causing

power loss which is mostly undesirable. At the same time current shunts are heavy and big

elements for higher currents, their output drifts at higher currents. They always make

intrusive current measurements.

Hall sensor’s output can be made smoother by adding extra filter circuit at the output. They

only add negligible extra resistance providing very small power loss in the circuit. Hall

sensor circuits for current measurements are small and light in weight. Non intrusive

measurements can be made with Hall sensors.

Solenoid Test Rig and Fault Simulation

CHAPTER 5

Introduction

To monitor the effect of common solenoid failure modes on the current signature, a test rig is

built. A solenoid is mounted on the plateform in such way that the load on the plunger can be

varied .Current through the solenoid can be measured with Hall sensor rig as described in

chapter4. The return spring of the solenoid can be replaced with less stiff spring and spring

can be damaged gradually by scrapping one its turns to see the effect of loose and damaged

spring.

5.1) Solenoid

A solenoid is three dimensional coil. In Physics the term solenoid refers to loop of wire, often

wrapped around a metallic core. When current is passed through the coil, magnetic field is

produced inside the solenoid. Solenoids are very important because they can produce

controlled and uniform magnetic field.

In solenoids a large magnetic field is created along its axis in z direction as shown in figure

(5.1).The magnetic field components in the other directions are cancelled by the magnetic

field components of neighbouring turns and because of this cancellation effect the magnetic

field outside the solenoid is also very weak. Outside the solenoid the magnetic field is nearly

zero, for a solenoid which is long as compared to diameter of its coil. Inside the solenoid the

fields due to individual turns add together giving rise to strong field at its centre.

http://plasma.kulgun.net/sol_page/ fig (5.1)

Since magnetic field always exits in closed loops, therefore an argument can be made that as

magnetic lines goes up in the solenoid, they must go down outside to complete loops. As the

volume outside the solenoid is much greater than the volume inside the solenoid, therefore

density of magnetic field is greatly reduced outside the solenoid.

Magnetic field B due to solenoid without core is given by the following equation.

InμB 0 eq (5.1)

Where n = number of turns per meter

I = current passing through solenoid

μ0 = permeability of air.

The inclusion of metal core increases strength of magnetic field and equation of B becomes

InμμB 0r eq (5.2)

Where μr is the relative permeability of the material of the core.

5.1) An Overview of Solenoid Valves and Principle of Operation

A solenoid valve is an electromechanical device, often used to control flow of liquids and

gases by running and stopping the current through solenoid. Solenoids valve is the most

frequently used control element in fluidics. Solenoids present fast switching, high reliability,

low control power and compact design.

A solenoid valve consists of two main parts the solenoid and the valve. Solenoid converts

electrical energy into mechanical energy when current passes through it, this mechanical

energy is used to open or close the valve. A return spring can be used to keep the valve open

or close while the valve is not activated.

Solenoids are used in gas turbine engine to open and close bleed valves. Bleed valves are

employed by heavy duty gas turbine engines to safeguard the axial compressor against

surging and stalling conditions during acceleration to rated speed and de acceleration from

rated speed[39]

.Therefore, it is very important to monitor their health since they play focal part

to protect axial compressor.

5.12) Common Faults in Solenoid Valves

Common failure modes in solenoid valves can be summarized as follows

1) Delayed plunger motion due to increase in load (friction).

When solenoids are energised, plunger pulls into its coil which results in closing or opening

the valve operated by solenoids. Since often solenoids valves are employed to control flow of

fluids and gases, sometimes these fluids and gases are sticky which leads to an increase of

load (friction) on the plunger causing degradation of system performance.

2) Incomplete plunger movement

In this case plunger moves but does not complete its stroke which results into not fully

opened or closed valve in its activated state.

3) Broken or loose spring due to wear and tear

Third common fault is broken or loose return spring which can result in not fully opened or

closed valve in its non activated state.

5.2) Solenoid Rig Description

The solenoid in fig (5.1) DC operated solenoid with return spring when current passes

through solenoid, it produces strong magnetic field which flows through and around the

solenoid frame. Since the solenoid frame is made up of steel, it strengthens the magnetic

field.

Fig (5.2)

The plunger as shown in fig (5.2) has cylindrical shape with round cross section. It has

positive cone one end and pin at the other end. When plunger is placed in the solenoid frame,

magnetic field is reinforced because positive cone of the plunger fits into the negative cone

frame bottom and this offer bigger area for flux transfer.

Fig (5.3)

Solenoid is mounted on a platform with brackets as shown in fig (5.3)

screw(L) for changing load fig (5.4)

The plunger pin is passed through the shaft which acts as normal load.The shaft passes

through steel block, by tighting the screw on the top of seel block as shown in fig(5.3) the

load can be increased on the plunger.

When potential difference is applied across the sploenoid, current flows through solenoid

which sensed is sensed by Hall sensor shown in fig(4.)

Since the built rig is capable of varying load on the shaft,therefore first two faults can be

smiulated by tighting the screw(L) on the shaft.Third fault can be smiulated by replacing the

stiff spring with loose spring, and (damaging)scrapping its one of turn gradually till it breaks.

5.3) Solenoid Valve Modelling

Solenoid valve is complicated system whose characteristics are ruled by correlated

electromagnetic and mechanical subsystems. To model solenoid valve, different subsystems

of a solenoid are examined and equations of the systems are derived in form of nonlinear

state equations. State variables selected for this purpose are

i. Plunger position x(t)

ii. Spool velocity x.(t)

iii. Solenoid current

It is tried to make simple but accurate model, with a reasonable number of parameters, so it

can be used as effective simulation tool.

5.31) Electromagnetic Subsystem

Electromagnetic subsystem comprises an electrical and magnetic circuit in the solenoid. The

solenoid model needs to represent the transformation of the input voltage to an electro-

magnetic force on the plunger of the valve. The electrical system consists of actual coil which

is represented by varying inductance L in series with a resistance R of the coil.

The magnetic system includes a fixed core surrounded by the coil turns and plunger which

moves due to magnetic force. The electromagnetic force for this system is as follows

Ni = Hc lc + HgIg = HcIcq eq (5.3)

Where N= number of coil turns

i = current

Hc = magnetic field in the core

Hg = magnetic field in the air gap

lc=length of the magnetic field in core

lg=length of the magnetic field in air gap

assumption is made that air gap 0, therefore flux fringing can be ignored

leq = lc +μc/μ0 lg = lc + 2μr(xt - x) eq(5.4)

Where lg =2(xt –x0) eq (5.5)

μ0, μc and μr = permeability of air, permeability of material of the core and relative

permeability of core respectively.

As Hc =φ/Ae μc eq (5.6)

L i =Nφ eq (5.7)

Where φ =magnetic flux

Ae = effective cross sectional area of flux path

The variable inductance L (t) is given as a function of spool position

x(t))(x2μl

μAN

l

μANL(t)

trc

ce

2

eq

ce

2

eq (5.8)

the state equation of this electromagnetic is stated as

i(t)L(t)dt

di(t)RV(t) eq (5.09)

V (t) = applied voltage

Differentiating eq (5.8)

ce

2

.2

r

2

trc

.

ce

2

r

μAN

x(t)(t)L2μ

x(t)](x2μ[l

x(t)μAN2μL(t)

dt

d

eq (5.10)

Substituting dL (t)/dt from eq (5.10) in eq (5.09) results following equation

ce

2

.

r

μAN

x(t)i(t)L(t)2uRi(t)][V(t)

L(t)

1i(t)

dt

d eq (5.11)

The magnetic force of solenoid is given by equation below

5.32) Mechanical Subsystem

The mechanical subsystem includes mass, spring and damper under the effect of magnetic

field and force of friction. According to Newton’s 2nd

law of motion, this system can be

represented by the equation below

eq(5.12)μ2A

φ2F

oe

2

m

(t)Fk(x(t)x(t)dt

dbx(t)

dt

dm m2

2

s eq (5.12)

5.4) Solenoid Model

Fig (5.5)

Current Model

Forces Models

Inductance model

Solenoid Magnetic Force

xd2 xd1

x1

x

springForce1

50

k3

i

1

s

hard stops for

solenoid

forceBalance

[xd1]

damping

0.5

b

Subtract1

Subtract

Scope1

20

R

Product4

Product3

Product2

Product1

Product

u2

Math

Function2

u2

Math

Function1

u2

Math

Function

L

4

K4

0.5

K2

1000

K1

1

s

Integrator2

1

s

Integrator1

1

s

Integrator

[0]

IC

[xd1]

Goto8

[x]

Goto7

[i]

Goto6

[Fm]

Goto5

[Ld1]

Goto4

[V]

Goto3

[L]

Goto1

[id1]

Goto

[i]

From8

[L]

From7

[Ld1]

From6

[xd1]

From5

[id1]

From4

[L]

From3

[V]

From2

[x]

From11

[Fm]

From10

[L]

From1

[xd1]

From

0

Ff

10

Constant

0.1

1/m

L

Ld1

i

id1

5.41) Solenoid forces Model

This part of simulink model represents magnetic force of solenoid as given by equation by eq

(5.12).

By finding the exact parameter of modelled solenoid very similar wave shape can be obtained

by this model. Due to lack of time, unavailability of suitable equipment, it was not possible to

find exact values of used solenoid. But fig shows below that model provides similar signature

as expected by h solenoid magnetic force.

Fig (5.6)

5.42) Solenoid Inductance Model

It models the change in inductance as solenoid is energized as shown in fig below.

Fig (5.7)

5.43) Model of Solenoid Mechanical Subsystem

This part of the models shows how the mechanical and magnetic forces interact, which affect

the speed of plunger, larger magnetic force results in higher acceleration. Presence of friction

and spring slows the plunger speed.

Fig (5.8)

This figure shows as the power is turned on, plunger starts its rapid movement.

5.44) Model of Solenoid Applied Current

It shows as discussed in chapter 2 that it comprises of peaks and valleys which can be seen in

the fig below. Any degradation in mechanical or electrical will change their amplitude and

values.

Fig (5.9)

As the waveforms of current, acceleration and inductance shows that this model can be used

to simulate faults with appropriate values of modelled solenoid.

Summary

Solenoid rig is made successfully which is capable of changing load. Therefore most

common faults can be simulated in this rig and current signature will be captured with Hall

sensor rig. Solenoid simulink model can be used to analyse the system behaviour by using

correct parameter of the modelled system and faults can be simulated in the model and their

effect on current signature spectrum and other parameters can be observed.

Solenoid Valve Fault Detection using Current Signature Analysis

CHAPTER 6

Introduction

It is explained in chapter 5 that a test rig using solenoid valve is constructed which is capable

of changing load on the valve. The current through the solenoid is measured with the Hall

sensor rig. First baseline current signature is obtained with normal operating load, and then

load is increased gradually to see effect on the current signature.

The spring of solenoid is replaced with spring of lower stiffness, CS is obtained and

compared with baseline CS, one turn of the spring is damaged and CS is examined to identify

the damaged spring.

6.1) Current Signature of Solenoid Valve with Normal Load

Experimental set up is shown in the fig below.

l

Ddddddd

Fig(6.1)

Current through the solenoid is measured with Hall sensor and CS is captured with simulink

model. Current Signature with normal load is shown below in fig(6.2)

P.C

Data acquisition card

Solenoid

Hall Current Sensor

Power supply

5

vv

V

Power supply

Fig(6.2)

In above current signature 0.6V is offset by Hall sensor with no input current, at 2 seconds

solenoid valve is turned on, current flows through the solenoid and sensed by Hall sensor,

therefore voltage at the output pin of Hall sensor increases. There are three main features

when solenoid is turned on.

1) Time to reach first peak (tP)

2) Time to reach valley (tV)

3) Time to reach peak2 (tP2)

Third peak occurs when solenoid is de-energized, it is not expected that there would be any

significant changes in this feature corresponding to changes in load or spring condition.

First peak feature (1) occurs at the beginning of the plunger movement during the initial

phase of the current inrush transient. The valley occurs when plunger has completed its

movement and second peak corresponds to the maximum value of the AC component of the

signal as the current increases to its steady state value during the latter part of the transient[1]

.

It is expected that increase in load will result in an increase in time to reach the first peak as

plunger need more force to accommodate extra load. Therefore plunger will take longer time

to energize. A broken, weak or “loose” (less stiff) return spring will lead to smaller tV since

valve will close more quickly as suggested by E.D.Blakeman and R.C.Kryter.

6.2) Current Signature with Increased Load

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 50.5

1

1.5

2

2.5

3

X: 2.08

Y: 0.6201

X: 2.3

Y: 2.344

X: 2.396

Y: 2.383

time

voltage

current signature with normal load

X: 2.333

Y: 1.665

The fig below shows the effect of increased load on current signature.

Fig(6.3)

Time to reach valley with increased load= 0.4498 sec

Time to reach valley with normal load= 0.252 sec

It is easily seen in above CS of the solenoid that time to reach first peak and time to reach

valley is increased as expected. Due to increase in load, the plunger requires more force to

close the valve, therefore it energies in magnetic field for longer period to gain enough

energy to close the valve. There is slight change in the depth of valley but it is not significant

to identify any change.

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 50.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

2.2

2.4

X: 1.183

Y: 1.65

X: 1.285

Y: 2.397X: 1.153

Y: 2.373

current signature with increased load

time

voltage

X: 0.7087

Y: 0.6006

Fig(6.4)

In the above figure it is obvious that time to reach first peak and valley has increased

significantly with further increase in load .There is a little change in depth of the valley.

Again change in time to reach first peak and valley is radical. CS is able to identify the

change in load easily which is further strengthened by increase in depth of the valley. Beyond

this point any further increase in load stops plunger to move and valve does not close which

is very clear from the CS shown in fig( 6.5).

Fig(6.5)

6.3) Current Signature of Solenoid with Spring of Less Stiffness

0 1 2 3 4 5 60.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

2.2

2.4 X: 2.38

Y: 2.188

X: 0.7549

Y: 0.6055

time

volta

ge

critical friction

X: 2.407

Y: 1.47

0 2 4 6 8 10 12 140.5

1

1.5

2

2.5

3

time

volta

ge

plunger unable to move

Return spring of solenoid is replaced with spring of less stiffness and it is expected that time

to reach valley will decrease since the valve will close more quickly.

Fig(6.6)

The time to reach first valley is decreased as expected.

With stiff spring (normal load) time to reach valley = 0.252 sec

With less stiff spring (normal load) time to reach valley=.208 sec

It can be observed that depth of the valley is also increased but it is the characteristic of

material of the replaced spring.

6.4) Current Signature with Damaged Spring

One of the turns of the spring has been scrapped down; CS of the solenoid is captured with

damaged spring.

Fig(6.7)

The time to reach valley has reduced as expected, since valve takes less time to close.Now

time to reach valley = 0.13sec which shows that the change in time to reach valley of CS of

the solenoid with damaged spring is appreciable to identify this kind of faults. The fig below

shows when one of the turn is at the verge of breaking

2.5 3 3.5 4 4.5 50.5

0.6

0.7

0.8

0.9

1

1.1

1.2

1.3

1.4

1.5

X: 3.121

Y: 0.6006

X: 3.349

Y: 0.6006

X: 3.313

Y: 1.25

X: 3.568

Y: 1.313

spring in normal condition

time

volta

ge

0 0.5 1 1.5 2 2.5 3 3.5 4 4.50.5

0.6

0.7

0.8

0.9

1

1.1

1.2

1.3

1.4

X: 1.187

Y: 0.5957

X: 1.322

Y: 0.6152

X: 1.478

Y: 1.318

time

volta

ge

spring with damaged coil

X: 1.293

Y: 1.182

Fig(6.8)

Now the time to reach valley has significantly decreased and it is very easy to identify any

abnormality in the spring by CS by comparing it with baseline signature.

Fig(6.9)

As expected with broken spring time to reach above discussed features is has become very

small. Above results prove that CS in time domain is able to identify the increase in load,

loose and damaged spring. Therefore a series of experiments were performed to relate

degradation in system to corresponding change in the above discussed features.

With normal load and step change in voltage, CS is taken for one complete cycle of solenoid

valve(when solenoid energies and de energies).. Then with normal load, voltage is increased

0 1 2 3 4 5 6 70.5

1

1.5

2

X: 3.266

Y: 0.6006

X: 3.359

Y: 1.06

X: 3.382

Y: 0.5957

X: 3.56

Y: 1.372

spring nearly broken

time

volta

ge

12.5 13 13.5 14 14.50.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

2.2

2.4

X: 13.36

Y: 0.6201

X: 13.4

Y: 1.06

X: 13.41

Y: 0.6934

time

volta

ge

broken spring

X: 13.53

Y: 2.075

slowly from zero to the point when plunger starts its movement. The voltage is noted down.

This procedure is repeated by gradually increasing the load.

From ohm law

eq(5.1)R

IV

Current I for the voltage when plunger moves is calculated. From Chapter 5 the magnetic

force of attraction of the solenoid is given by the equation below

eq(5.2)μAN

i(t)L(t)F

oe

2

22

m

eq(5.3)i(t)αF 2

m

eq(5.4)L(t)αF 2

m

Since L(t) acts as constant when voltage is changed slowly and magnetic force of attraction

becomes proportional to square of the current.

Fm = Fl + Fn + Fs eq(5.5)

Where Fm = force exerted by magnetic field

Fl = force exerted by increase in load

Fn = force by Newton’s 2nd law

Fs = force due to spring

At normal operating conditions

Fm = Fn + Fs eq(5.6)

As the load increases, magnetic force required to overcome this extra force also increases.

Therefore by calculating change in current due to change in load, an estimate of force exerted

by load can be made.

A series of experiments are done with varying load, change in current is calculated and

features are extracted for step change in voltage as described above.

Voltage Force α

(V/R)2

% degradation Time to reach

peak1

Time to reach

valley

Time to reach

peak2

3.1 0.197 0 0.1328 0.1398 0.4238

3.22 0.21 5 0.168 0.174 0.4800

3.29 0.221 9.5 0.17 0.18 0.4820

3.62 0.2644 26 0.176 0.186 0.515

3.83 0.295 38 0.238 0.25 0.523

4.21 0.36 64 0.249 0.261 0.58

4.53 0.41 84 0.325 0.35 0.618

Table(5.1)

These plots are used to extract features with step voltage.

Fig(6.10)

A series of experiments are performed with normal load, loose spring and gradually

damaging spring by scrapping one of its turns in the same manner as explained above, with

step voltage features are extracted from CS and by slowly increasing the voltage, minimum

voltage to pull the plunger in is measured.

Table of results with loose ,gradually damaged and broken spring is given below.

Voltage

V

Force α

(V/R )2 amp

% damage Time to

reach peak1

Time to

reach valley

Time to

reach peak2

1.34 0.037 loose 0.07 0.078 0.352

1.25 0.031 19 0.066 0.071 0.34

.87 0.015 71 0.6 0.065 0.32

0.7 0.01 88 0.053 0.058 0.279

0.6 0.0064 100 0.05 0.053 0.272

Table(5.2)

0 0.5 1 1.5 2 2.5 3 3.5-5

0

5

voltage

0% degradation

0 1 2 3 4 5-5

0

5

voltage

5% degradation

13 13.5 14 14.5 15 15.5-5

0

5

voltage

9.5% degradation

0 0.5 1 1.5 2 2.5 3 3.5 4-5

0

5

voltage

26% degradation

0 0.5 1 1.5 2 2.5 3 3.5 4-5

0

5

voltage

38% degradation

0 1 2 3 4 5-5

0

5

voltage

64% degradation

0 1 2 3 4 5-5

0

5

time

voltage

84 degradation

0 1 2 3 4 5 6 7 8-10

0

10

time

voltage

100% degradation

These plots are used to extract features for loose and damaged spring for step voltage change.

Fig(6.11)

These features in table one and table two are plotted in three dimensions as shown in

fig(6.12), it is very clear and easy to identify the trend as load increases and in second case

of damaged spring. As load increases features on the 3D plot move up and replacement of

stiff spring by loose spring moves the features down ward direction.

Fig(6.12)

0 2 4 6-5

0

5loose spring

0 1 2 3 4-5

0

519% damage

5 6 7 8-5

0

571% damage

0 1 2 3 4 5 6-5

0

5

time

volta

ge

88% damage

0 1 2 3 4 5-5

0

5

time

volta

ge

100% damage

0

0.1

0.2

0.3

0.4

0

0.1

0.2

0.3

0.4

0.2

0.3

0.4

0.5

0.6

0.7

time to reach peak1time to reach valley

time

to r

each

pea

k2

loose spring

19% damage71% damage

normal system

5% degradation

9.5% degradation

26%

38% degradation

64% degradation

84% degradation

88% degradationbroken spring

In this figure(6.13) degradation/damage of the system is plotted against the first two features.

Fig(6.13)

Summary

The current signature of the solenoid valve under normal operating conditions comprises of

peaks and valleys. In time amplitude current signature, first peak represents the time when

plunger begins its movement. The valley occurs when plunger has completed its movement

and second peak represents the maximum value of final phase of transient current which now

increases to its maximum value. It was expected that change in load will lead to an increase

in time to reach first peak, time to reach valley and time to reach peak2. Its means plunger of

solenoid valve with increased load will start its rapid movement later as compared to the

plunger of solenoid valve with normal load. Above results proves that increased in load

changes the time to reach first peak and time to reach valley and time to reach peak2 as

expected.

Current Signature Analysis is also able to detect any changes (reduction in stiffness or

damaged spring) in the return spring. Above graph results show that current signature has

capabilities to detect any abnormalities with the return spring and comparing current

signature with base line CS an assessment about the severity of the fault can be made.

Fig(6.10) and fig(6.11) shows that CSA is an effective condition monitoring tool for solenoid

valves. It has abilities to detect faults in solenoid valves and also an assessment about the

severity of damage can also be made by comparing the faulty state CSA against baseline

CSA.

0

0.1

0.2

0.3

0.4

0

0.1

0.2

0.3

0.40

20

40

60

80

100

time to reach peak1time to reach valley

degr

adat

ion/

dam

age

normal system

5% degradation

26% degradation

38% degradation

64% degradation

84% degradation

loose spring

19% damage

71% damage

88% damage

broken

Conclusions and Future Work

CHAPTER 7

7.1) Review of Objectives and Achievements

Objective 1

Collect information regarding different current sensing technologies. Highlight their main

advantages and disadvantages.

Achievement

A comprehensive literature survey was done to gather information about different current

sensing technologies. Merits and demerits of each technology were explored which are

presented in chapter 3.

Objective 2

Evaluate most promising technologies by building test rig for each. This will provide an

opportunity to explore limitations and advantages of each technology for condition

monitoring purpose.

Achievement

Current shunt and Hall effect based current sensor were selected for current measurement.

Test rig for each technology was built and series of experiment including bandwidth

confirmation, signal to noise ratio, behaviour at higher currents were done to investigate their

behaviour as described in chapter 4.

Objective 3

Use non-invasive and invasive ESA methods and compare them. To compare sensitivity of

current measurement with wires of different types of shielding.

Achievement

It was found during literature survey and been confirmed by the experiment that Hall sensors

used for intrusive measurement are unable to perform non intrusive measurements. But

electromagnetic effect on the Hall sensor output was discovered in the vicinity of power

supplies.

Objective 4

To collect and analyse current signature for healthy solenoid to establish baseline spectrum,

for use in fault detection and to investigate the spectral content of the current signature in

case of faulty solenoid.

Achievement

A solenoid rig was constructed and using Hall sensor rig baseline current signature was

obtained .It was found CS of solenoid valves consists of peaks and valleys, these valleys and

peaks occur when solenoid energises and de-energises.

Objective 5

To introduce specific faults in solenoid and both measure and predict its effect on the supply

current.

Achievement

Load on the solenoid plunger was gradually increased until plunger was unable to move,

current signature for each load was captured, analysed and features were extracted to see the

effect of increasing load on the CS of solenoid valve. Solenoid’s healthy return spring was

replaced by loose spring, CS was captured and features were compared against the baseline

CS. Return spring was damaged gradually until it broke by scrapping one of its turn and CS

was captured for degree of damage.

Conclusions

Literature survey and evaluation of different current sensing leads to the table (3.1) in chapter

3. Each technology has its own merits and demerits and choice of current sensor depends

upon the application.

The current signature of solenoids consists of peaks and valleys. These valleys and peaks

occur when solenoid energises and de –energises. Three important features can be extracted

from solenoid CS, time to reach first peak, time to reach valley and time to reach second

peak. First peak arises when plunger starts its rapid movement, valley occur when it has

completed its movement and second peak occur when energy stored by varying inductance is

released back.

Series of conducted experiments proved that as load increases time to reach first peak, time to

reach valley increases. Results plotted in three dimension shows that degradation of the

system CSA can be used to identify the change in load (which is the most common failure

mode of solenoid valves). CSA has abilities to detect loose return spring or damaged return

spring faults. An assessment about the severity of this fault can also be made by comparing it

against baseline CS

Mathematical solenoid model give current, acceleration and inductance waveforms as

expected. Substituting of accurate values of the system being modelled, can give similar

results and faults can be simulated by this model.

7.2) Recommended Future Work

This project has shown that CSA is very effective condition monitoring tool, since it has

potential to diagnosis faults and by proper feature extraction an assessment about the severity

of the problem is also possible. There are many areas where future work can be extended.

Condition monitoring of Hydraulic valves( Torque Motor servo valves)

Condition monitoring of PWM valves

Condition monitoring of generators

More accurate values for Solenoid model parameters.

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Research Studies.

2) Liang, B. (2001). Condition Monitoring and Fault Diagnosis of Three Phase Induction

Motors. (PhD Thesis, UMIST, 2001).

3) Thompson,W.T.(1994). On Line Current Monitoring to Detect Electrical and

Mechanical Faults in Three-Phase Induction Drives. Proceeding of IEEE conference on Life

Management of power plants, pp66-73.

1) Wildi,T.(1997). Electric Machines, Drives and Power Systems(3rd

Edition ).Englewood

Cliff,N.J : Prentice Hall.

2) Selmon, G.R. and Straughen,(1980). A. Electric Machines. Reading, Mass. ; London :

Addison-Wesley.

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