hidrolik vs elektrik

7/29/2019 Hidrolik vs Elektrik

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O + P lhydraulik und Pneumatik 46 (2002) Nr. 4 1

1. Introduction

Hydraulic systems are in widespread use throughout all fields of modern engineering

applications. The pump geometry causes volume flow pulsations, which lead to suchdisturbing effects as increased noise development or vibrations, for example, either

directly or after being converted into structure-borne and/or air-born noise. In addition

to this, the pulsations put a strain on the pipeline systems and the systems

connected up downstream, particularly in the vicinity of the resonant frequency.

The experiments performed in the anechoic room at the Institute of Fluid Power

Transmission and Control at RWTH Aachen (IFAS) included an investigation into the

noise development of hydraulic pumps, for example, in order to optimise the

Automated self-regulating system for a low reflection line

termination (RALA)

Dipl.-Ing. S. Schellinger

Dipl.-Ing. E. Goenechea

Institute for Fluid Power Drives and Controls (IFAS), RWTH Aachen

Hydraulic components have been the subject of air-borne, fluid-borne and

structure-borne noise measurements conducted in the anechoic room at the

IFAS for many years now. In an effort to avoid the influences exerted on the

studied components by the hydraulic circuit in the anechoic room, a so-called

RALA, which is a low reflection line termination (RALA is an abbreviation of the

German term ReflexionsarmerLeitungsabschluss), is used to terminate the

high-pressure side of the circuit. This article describes an algorithm, which is

capable of setting the optimum operating point for a RALA of this nature. In

future, this will enable measurements to be made with working points which

alter in real time.


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vibration, radiation and/or pulsation behaviour. Only the sound phenomenon

attributable to the pump should be measured and evaluated to enable a comparison

of the results measured for all pumps. Standing waves, which may be produced byreflections at the end of the line, would make the results measured for the tested unit

dependent on the system connected to the pump.

One possible means of suppressing these reflections has already been developed at

the IFAS. The RALA comprises an adjustable shutter, with volume connected up

downstream.

Analogous to a so-called terminating resistor, which is used to prevent reflections at

the end of the cable in an electrical system, the RALA is fitted between the pumpoutput and the load in a hydraulic system.

This means that a reflection-free state can be achieved as a function of pressure,

temperature and volume flow for every operating point by altering the shutter. The

pulsation at the RALA end of the tested unit is therefore no longer determined by the

geometry of the hydraulic system.

Up to now, the RALA developed at the IFAS has been trimmed manually, by

comparing the shapes of the two pressure signals and then turning an adjusting

screw to reduce or increase the shutter aperture. Using this method, the ideal

operating point was characterised by the fact that, apart from phase and amplitude,

the two pressure signals were identical.

The development of the self-regulating system has led to a situation in which the

pressure signals can now be analysed by computer and adjustment of the shutter

aperture is controlled by a motorised, automated system on the basis of this data, so

that the RALA adopts the optimum reflection-free operating point for every working

point of the tested unit without any interaction.


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Figure 2: The hydraulic circuit with low reflection line termination RALA (5) and

motorised adjusting unit (9 & 10)

2. Use of the RALA

The RALA is part of a hydraulic circuit, as shown in Figure 2. The circuit essentially

comprises a hydraulic pump (1) with drive motor (2), the high-pressure line (3) with

two dynamic pressure measuring points (4a and 4b) and the RALA (5). A pressure

control valve (6) is fitted downstream of the RALA in the hydraulic circuit to simulate a

load. A static pressure sensor (7) and a temperature sensor (8) enable monitoring of

the operating pressure and temperature. The high-pressure line, which runs between

the pump and the RALA must be as smooth and free of interference as possible. The

two dynamic pressure sensors (4a and 4b) are positioned one meter apart and

register the pressure pulsation. Made up of piezoelectric elements, the pressure

sensors supply charge fluctuations, which are converted into voltage signals by


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measuring amplifiers. These voltage signals used to be compared visually until the

self-regulating system was developed. A hydraulic actuator was used to adjust the

shutter of the RALA manually for optimum trimming of the line for the respectiveworking point. The automated self-regulating system now performs this function and

controls the shutter aperture in such a way as to ensure that the reflection-free state

is adopted at all times.

3. The reflection-free terminating resistor

Theoretically speaking, the RALA constitutes an infinitely long line in the hydraulic

system. The pulsation wave continues to runs along it until it is transformed by

dissipation. The reflection factor is zero. There is an electrical analogy to this

situation.

Figure 3: Analogies between a hydraulic pipeline and an electrical cable


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A terminating resistor is connected to the end of an electrical cable to prevent

induced waves being reflected. All waves are absorbed by the terminating resistor as

long as it matches the line impedance.There are also electrical analogies for other components in a hydraulic system. This

is illustrated in Figure 3 with reference to a hydraulic pipeline and an electric cable.

Figure 4: Equivalent electrical circuit diagram of a hydraulic line and an electricalcable

Figure 4 shows the equivalent circuit diagram of a line, whereby its characteristic

wave impedance is given by:

CjG

LjRZline +

+=

)( where f 2= (1)

The dashed values are per unit length, i.e. they are given in [unit per metre].

Figure 5: Equivalent electrical circuit diagram of the hydraulic circuit


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Figure 5 shows the equivalent electrical circuit diagram of the hydraulic circuit. The

impedance of the pump and the line impedance are combined into Z1 upstream of thefirst dynamic pressure, and the impedance of the pressure control valve (DBV (6) in

Figure 2) is combined with the line impedance from the pressure control valve to the

oil tank, that constitutes the hydraulic zero potential (p = 0 bar), into Z2. The pressure

pulsations at the two measuring points correspond to the alternating voltage U1 and

U2 in the equivalent electrical circuit diagram. The terminating resistance is given by:

2min 1 ZsCRZ volumeshutteratingter += (2)

The reflection factor is determined by:

lineatingter

lineatingter

ZZ

ZZsr

+

=

min

min)( (3)

In conjunction with (2), this gives the frequency-dependent reflection factor at the

RALA:

)(

)(

)(

)()(

20

10

KKs

KKs

ZR

ZRsr

lineshutter

lineshutter

++

++

+

= (4)

where

2

0

1

ZCK

volume = (5)

)(

11

lineshuttervolume ZRCK = (6)

)(

12

lineshuttervolume ZRCK

+= (7)

As far as the trimmed RALA is concerned, the shutter resistance is the same as the

line impedance

( lineshutter ZR = ) and the reflection factor is therefore r(s) = 0.


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Figure 6: Pole/zero diagram of a low pass

The following applies if the shutter of the RALA has been closed too far:

lineshutter ZR > :20

100)( KKs

KKsHsr++++= where K1>K2. If the reflection factor is interpreted

as being a transfer function, the value of the zero (numerator from formula 4) is greater than

the value of the pole (denominator from formula 4) ( PSNS > ). The pole and zero are

entered in the diagram in Figure 6. This corresponds to the pole/zero diagram for a low pass.

If the shutter of the RALA has been opened too far, given by:

lineshutter ZR < :20

100)(

KKs

KKsHsr

++

++= where K1


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4. The algorithm

The algorithm is based on the assumption that an ideally trimmed RALA prevents

reflection of the pressure pulsations. The reflection factor of a poorly trimmed RALA,

on the other hand, has either high pass or low pass characteristics. In this case,

excitation takes the form of the dynamic pressure fluctuation produced by the volume

flow pulsation of the connected pump.

The wave timetables for the three operating states show the way in which the

respective operating state can be detected from the dynamic pressure signal using

Fast Fourier Transformation (FFT). To illustrate this, the Fourier coefficients shown in

the diagram of the wave timetables have been restricted to two orders of different

frequencies.

Figure 8: Wave timetable of the trimmed RALA


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The amplitudes of the pressure pulsations in the hydraulic system are attenuated by

around 6 dB for all operating frequencies in the experimental set-up. As far as the

forward wave is concerned, the ratio of the two harmonics at the first pressure sensorremains the same after running through the one-meter long pipe to the second

pressure sensor (Figure 8). The reflective properties of the RALA are what alter the

ratio of the harmonics at the two pressure sensors, enabling the algorithm to detect

the current operating state and to correct it in order to achieve a reflection-free state.

An untrimmed RALA reflects pressure pulsations as a function of the frequency,

whereby the reflection factor has low pass characteristics, i.e. low frequencies are

reflected to a greater extent than high frequencies if the shutter has been closed toofar (Figure 9).

Figure 9: Wave timetable of the hydraulic system with low pass termination


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If the shutter has been opened too far, the reflection factor adopts high pass

characteristics, i.e. high frequencies are reflected to a greater extent than low

frequencies (Figure 10).

Figure 10: Wave timetable of the hydraulic system with high pass termination

The algorithm is based on the Fourier Transformation (FT), which maps the signals

from the time domain onto the frequency domain. In mathematical terms, this

transformation is described by

+

= dtetfF tj )()( (8)

and is founded on the theory that each signal can be broken down into a series of

sinusoidal oscillations of different frequencies and amplitudes. The American

scientists Cooley und Tukey have devised a number of algorithms which reduce the

computing time required to form the Fourier integral considerably. These have

become well known as the Fast Fourier Transformation (FFT) and are frequently


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used as they reduce the computing time to a minimum while maintaining sufficient

accuracy.

Each of the two dynamic pressure sensors detects the sum of the forward and returnpulsation waves at the two measuring points 4a and 4b (refer to Figure 2). The

control algorithm then forms the FFT for both pressure signals. These pressure

signals may contain harmonics up to the tenth order, for instance. The control

algorithm uses two of these harmonics for the calculation and produces ratios for

both measuring points. The RALA has assumed a reflection-free state if the two

ratios are the same. If the difference between the two ratios does not equal zero, the

shutter must be opened or closed according to the sign preceding the difference.5. The hardware

The algorithm described above was initially tested on a mobile RALA with electrical

adjustment. The electrical components of the automated RALA can be divided into

three areas: signal recording, signal processing and control signal output.

The pulsation signals are recorded by two dynamic pressure sensors. The

construction of the sensors is such that they supply a charge signal which is

proportional to the pulsation signal. This charge signal is converted into a voltagesignal in the measuring amplifiers and is routed to the line input of a sound card in

the computer (Figure 11).


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Figure 11: Block diagram of the electrical system for the automated RALA

The computer processes the signals by applying the above-mentioned algorithm to

them in order to subsequently transmit a control signal to the motor controller via theRS232 interface. A power pack feeds a 24 V / 10 A power supply to this motor

controller, which then controls the servo motor for the shutter. With 50 to 60

revolutions per minute, this servo motor produces 10 Nm of torque. The RALA has

been designed for operating pressures up to 250 bar. The 5 Nm maximum torque

required to adjust the RALA was determined at its adjusting spindle at this operating

pressure.

When the end position of the spindle is reached - determined by the geometry of the

RALA and interpreted by a ten-turn limit switch potentiometer - a limit switch analyser

(Figure 12) generates a TTL signal, which is sent to the motor controller (Figure 11)

and inhibits the appropriate direction of rotation for the motor. This means, for

example, that when the shutter is closed, the end of the spindle cannot be run into it

any further, thereby preventing the spindle becoming jammed in the shutter aperture.


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Figure 12: Circuit diagram of the limit switch analyser

In Figure 12, component Ref01 supplies a exactly 10 V as the reference voltage.

This 10 V supply is fed to three potentiometers: the setting potentiometer for the

upper limiting value (end mark 1), the setting potentiometer for the lower limiting

value (end mark 2) and the potentiometer on the outside of the RALA, which reads

out the current position. The potentiometers act as voltage dividers here, which

means that they supply a voltage between zero and ten volts according to their

respective positions. Component TL074 contains four operational amplifiers (OP),

whereby two of them are used. The voltage from the external potentiometer is

supplied to the negative input of one operational amplifier and the positive input of

the other. The voltages supplied by the other two potentiometers are fed into the

other operational amplifier inputs. The operational amplifier compares the two applied

voltages in the following way: if the voltage at the positive input (+) is greater than at

the negative input (-), voltage is applied to the output. The output voltage is used to

give a visual indication with an LED, to generate a TTL signal and to switch a relay. If

one of the LEDs lights up, this means that the RALA has reached an end position.

The motor controller uses the two TTL signals to inhibit the motor's respective

direction of rotation. When one of the end marks is reached, the relay switches a


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pulsation signal to earth. This measure was initially intended to improve the reliability

of the system as a whole, with the information about reaching one of the end marks

being received by both algorithm and computer. However, the algorithm has provedto be so reliable in service that this information is not needed. The measuring

amplifier can now supply the pulsation signal to the computer directly, without having

to pass through the analyser.

An adapter cable is required to supply the pulsation signals to the line input on the sound card.

This cable has two BNC connectors of the standard type used in metrology at one end and a

3.5 mm stereo jack plug of the standard type used in PC and hi-fi technology at the other end.The pin assignments are in accordance with the DIN stereo standard for the left and right-

hand channels when the experimental set-up is arranged in the following way from left to

right: pump, first pressure sensor, second pressure sensor, RALA. This adapter also offers a

means of making the pressure signals audible for the human ear through a stereo system as the

pulsation signals are within the 50 Hz to 5000 Hz range.

As a rule, the levels used in measuring systems are between 10 V and + 10 V,

whereas the levels used in hi-fi technology are generally between 1 V and + 1 V.

To reduce the level of the pressure signals down to sound card level, the gain of the

measuring amplifier must be reduced to such an extent that the maximum amplitude

does not exceed 1 volt.

The pressure signals may be input and the control signal for the motor output via the

sound card. In this case, the motor should be controlled by means of a pulse-width-

modulated signal (PWM). The computer is currently reading the two pressure signals

in via the sound card and outputs the control signal for the motor via the serial RS232

interface.

6. Determining the quality of the reflection-free state

The forms of the two pressure signals are compared in the time domain in order to

determine the quality of the RALA setting. With the distance between the two

pressure sensors amounting to approximately one metre, a wave peak (marked in

green in Figure 13) reaches the second sensor approx. 0.77 ms (where


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smcl /1300= ) later than the first. Dissipation weakens the amplitude by around

factor 2)( =G . This corresponds to an attenuation of dBGA 6)(log20)( == .

Figure 13: The time signals for the pulsations

Time lag (phase) and amplitude attenuation both depend on numerous factors (e.g.

temperature and medium). An algorithm which compares the forms of the two signals


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in the time domain and, as a result, reflects the quality of reflection freedom,

therefore presupposes dynamic compensation of these differences.

The result is output to a virtual oscilloscope. Figure 13(b) shows the oscilloscope for

a perfectly trimmed RALA. Figure 13(c), on the other hand, shows an RALA setting

that needs to be re-adjusted. The curve forms are not the same in spite of the

standardisation. The green curve representing the difference between the two

pulsation signals, which is a straight line in the ideal case of the perfectly trimmed

RALA (refer to Figure 13(b)), degenerates into a waveform in Figure 13(c).

The area under the difference curve is evaluated in order to assess the setting of theRALA. The value of the difference curve and the integral of this value must be

calculated for this. The RALA is perfectly trimmed if the value of the integral is zero.

The RALA requires re-adjustment if the value is greater than zero.

7. Conclusion

The algorithm discussed in this article offers a faster, more accurate means of

evaluating a hydraulic pump than has been possible up to now. This is because

previous methods defined the parameter fields to be measured, which werecharacterised by discrete rpm, pressure and volume flow values, as well as

temperature values where applicable. The RALA was adjusted manually for each

working point. The RALA had to be trimmed before conducting the measurements.

The algorithm finds the optimum reflection-free working point quickly and reliably. As

a result of this, the developed algorithm can be used to run through extensive ranges

of pressure, rpm and volume flow values while the RALA adjusts automatically in real

time. It is now possible to conduct measurements during pump acceleration, whichcover the entire rpm range of the pump and this data can then undergo an order

analysis, for example. The acquisition of cascade diagrams in real time is also

conceivable, indicating FFT spectra of the pressure pulsation at the high-pressure

connection of a pump as a function of the pressure. Measuring techniques like this

and the ability to measure up to eight sound, pressure and/or acceleration signals at

the same time facilitate the localisation of resonance effects within a pump

considerably.

hidrolik vs elektrik

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