strain gauge based accelerometer

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1. Introduction

Among the range of mechanical sensors (position, speed, acceleration, shock), there are

two types of the sensors, one passive, the other active, whose measure forces and deformations.

Passive sensors are mostly used in mechanics. These sensors can be resistive, capacitive or

inductive. Inductive sensors are often used for displacement measurements. On the other hand,

resistive sensors are often used for deformation measurements and are sometimes called,

somewhat incorrectly, constraint gauges.

2. Strain Gauge

A strain gauge is a device used to measure the strain of an object. Invented by Edward E.

Simmons and Arthur C. Ruge in 1938, the most common type of strain gauge consists of an

insulating flexible backing which supports a metallic foil pattern. The gauge is attached to the

object by a suitable adhesive, such as cyanoacrylate. As the object is deformed, the foil is

deformed, causing its electrical resistance to change. This resistance change, usually measured

using a Wheatstone bridge, is related to the strain by the quantity known as the gauge factor.

Strain gauges are used in many instruments that produce mechanical strain because of the

affect being measured.In their own right, they are used to measure the strain in a structure being

stretched or compressed.The strain gauge element is a very thin wire that is formed into the

shape shown. This produces a long wire all in one direction but on a small surface area. The

element is often formed by etching a thin foil on a plastic backing. The completed element is

then glued to the surface of the material or component that will be strained. The axis of the strain

gauge is aligned with the direction of the strain. When the component is stretched or compressed,

the length of the resistance wire is changed. This produces a corresponding change in the

electrical resistance.
http://en.wikipedia.org/wiki/Strain_(materials_science)http://en.wikipedia.org/wiki/Edward_E._Simmonshttp://en.wikipedia.org/wiki/Edward_E._Simmonshttp://en.wikipedia.org/wiki/Arthur_C._Rugehttp://en.wikipedia.org/wiki/Electrical_insulationhttp://en.wikipedia.org/wiki/Cyanoacrylatehttp://en.wikipedia.org/wiki/Electrical_resistancehttp://en.wikipedia.org/wiki/Wheatstone_bridgehttp://en.wikipedia.org/wiki/Edward_E._Simmonshttp://en.wikipedia.org/wiki/Edward_E._Simmonshttp://en.wikipedia.org/wiki/Arthur_C._Rugehttp://en.wikipedia.org/wiki/Electrical_insulationhttp://en.wikipedia.org/wiki/Cyanoacrylatehttp://en.wikipedia.org/wiki/Electrical_resistancehttp://en.wikipedia.org/wiki/Wheatstone_bridgehttp://en.wikipedia.org/wiki/Strain_(materials_science)


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3. Accelerometer

Accelerometers are widely used to measure tilt, inertial forces, shock, and vibration. They

find wide usage in automotive, medical, industrial control, and other applications. The use of

accelerometers to measure acceleration is a relatively new technique in biomechanics. Their use

in biomechanics is not widespread, possibly due to the problems associated with these

measurements. There are various methods in biomechanics that are used to measure either linear

and/or acceleration. Some involve the capture of images at known intervals of time (e.g.,

cinematography, videography, and stroboscopic photography) form which the second time

derivative of position is determined. These optical methods have inherent problems associated

with errors in position data which result in erratic acceleration parameter. Accelerometers are the

most recent developments in the electronic measurement of acceleration. There are different

types of accelerometers. The type is based on the measurement technique employed within the

accelerometer. The types of accelerometer are :

a) Piezoelectric:

Piezoelectric devices are sensitive to changes in the acceleration applied to the element

during compression. The charge output of the piezo element varies with the applied

acceleration. There are several different internal designs. Piezo accelerometers cannot

measure to DC (0 Hz), but can measure to very high frequencies (in excess of 50 kHz).

b) Strain gauge (piezoresistive):

A mass under load is instrumented using a full-bridge strain gauge. The output of the

bridge varies with the applied acceleration. Strain gauge accelerometers can measure to

DC, but do not have as high a frequency response as piezo devices (typically under 1


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kHz). The transducer may also be fabricated from micro-machined components, often in

silicon, rather than tradition mechanical components. These devices usually have a wider

frequency response than mechanical transducers and are more rugged.

c) Capacitive:

An accelerometer can be built from a capacitive element. In these a mass attached to

plates vibrates and the change in capacitance (gap) between these plates is proportional to

the applied acceleration. These transducers also measure to DC and are generally more

rugged than mechanical strain gauge accelerometers.

d) Servo:

An older design, not often used these days except in specialized applications. A servo

accelerometer contains a mass whose position is controlled by a servo feedback

mechanism. The feedback signal is proportional to acceleration. Servo accelerometers

can be quite fragile, but can measure very low levels of acceleration. They also respond

to DC.

1. Strain Gauge Based Accelerometer

Strain gauge accelerometers, often called "piezoresistive" accelerometers, use strain

gauges acting as arms of a Wheatstone bridge to convert mechanical strain to a DC output

voltage. The gauges are either mounted to the spring, or between the seismic mass and the

stationary frame. In the picture, strain gauge windings, which contribute to the spring action, are


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stressed (two in tension, two in compression), and a DC output voltage is generated by the four

arms of the bridge that is proportional to the applied acceleration.

These accelerometers can be made more sensitive with the use of semiconductor gauges

and stiffer springs, yielding a higher frequency response and output signal amplitude. And unlike

other types of accelerometers, strain gauge accelerometers respond to steady-state accelerations.

Figure 1: Piezoresistive accelerometer

4.1 Application

1) Car Alarms

2) Tilt or Inclination

3) Patient Monitors

4) Inertial Forces

5) Laptop Computer Disc Drive Protection

6) Airbag Crash Sensors


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7) Car Navigation systems

8) Elevator Controls

9) Shock or Vibration

10) Machine Monitoring

4.2 General Principle

A seismic mass is placed on an elastic return blade equipped with two or four

piezoresistive gauges in a Wheatstone Bridge. The blade flexion is translated into gauged

deformation. These gauges enable conversion of the acceleration into an electric quantity, since

the received signal is proportional to the acceleration of the moving object.

Figure 2: Principle of piezoresistive strain gauge accelerometer

The resistivity variation depends on material, resistivity, doping level, type of doping

agent and the crystallographic direction in which the material is machined, and the resistivity

itself is given by the concentration of the doping agent. The gauge factor of silicon varies as

follows:

[+100 to + 175] for type P;

[100 to 140] for type N.


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The important parameters are:

gauge factor K;

temperature coefficient of resistance;

temperature coefficient of gauge factor.

Gauge factor K is initially determined by doping level but also depends on temperature.

Figure 3 shows that the gauge factor and temperature coefficients are inversely proportional to

the level of doping.

Figure 3: Effects of the doping level and temperature on silicon type P

4.3 Assembly of the gauges

Flat Gauge


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These gauges have 2 relatively broad mounting plates, joined together by a narrow central

element (Figure 4). In this configuration, strains are concentrated in a miniature element, the

surface of which is polished, free from any potential unwanted strain.

Figure 4: Diagram of a flat gauge

The strain induced by fixing will be kept to a small fraction of the useful strain at the

throttling level. These strain gauges are made of a single silicon crystal with a high degree of

purity. The silicon doped with phosphorus gives a negative gauge factor, while doping with

boron gives a positive gauge factor.

Carved gauges

We can use the notched gauge principle by development of chemical etching. Figure 5

shows a carved monolithic element: the notches releasing the gauges and the seismic masses are

the un-etched parts of silicon.


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Figure 5: Monolithic sensitive element for accelerometer 7270

The linearity and the sensitivity are optimized because of:

the monolithic structure;

the extremely small size to ensure a very high force/weight ratio;

the freedom of the gauges.

The resonance at several MHz and the linear range of more than 100,000g exceeds the

performance of former sensors. These sensors are particularly stable because there are no

adhesive joints between the mass, the gauges and the substrate.


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Figure 6: Expose view of Piezoresistive accelerometer

4.4 Features and limits of these accelerometers

4.4.1 Sensitivity, frequency response

The sensitivity is defined by equation:

S = (m/a)= (V m / a )= S1*S2

Vm = output voltage of the Wheatstone Bridge

= deformation

a = acceleration

S2 is the electric sensitivity of the Wheatstone Bridge formed by the 4 gauges.

S2 = Vm /

S1 characterizes the response of the mechanical part of the accelerometer:


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S1 = /a

The sensitivity varies from 1 to 25 mV/g according to thegauge.

4.4.2 Bandwith

The accelerometers for measurement at continuous and low frequencies have critical

damping = 1. The useful bandwidth extends from 0 to of the resonance frequency.

Piezoresistive accelerometers have a factor of merit defined by equation:

= S.f02

f0 = natural frequency

S = sensitivity

= factor of merit

For a given construction and technology, we cannot have, at the same time, an

accelerometer having high sensitivity and wide bandwidth. Moreover, the higher the damping of

the accelerometer is, the bigger the subsequent phase shift becomes. The typical frequency

response range is from 0 to 3,000 Hz.

4.4.3 Influence of temperature

This can take 3 different forms:

Influence on zero.

Influence on sensitivity.

Due to thermal variations of the Youngs modulus and the gauge coefficient, the

sensitivity decreases as the temperature increases. The order of magnitude is 1 to 2.10-4 of

nominal sensitivity/C. The resulting error can reach a small percentage of the value at 20C but

this error is stable. It can thus be partially corrected by calculation.


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Variation of the damping coefficient: The piezoresistive accelerometers are damped using

silicon oil. When temperature increases, the kinematic viscosity decreases and then the damping

coefficient also decreases. The achievable working temperature is 50 to 150C. Above 150C

doping is no longer effective.

4.5 Technological limitations: connecting cable

Connecting cables bring a deterioration of the transmitted signal at the entry to the signal

conditioner, which is related to its length and its frequency. In the case of a significant length of

cable, the accelerometer with constant current instead of constant voltage can be supplied in

order to eliminate the influence of the resistance of the cable. The accelerometer requires a very

stable source of power.

4.5Technological limitations: shocks and vibrations

Accelerometers are sensitive to shocks and vibrations. For inertial navigation it is

necessary to equip them with a mechanical filter eliminating high frequencies and to place them

in an enclosure protected from shocks. The main advantages and disadvantages of piezoresistive

accelerometers are summarized below.

Advantages Disadvantages

high sensitivity no significant linearity

low cost high sensitivity to temperature

quite high bandwidth generally average performance

simple data processing the lower the sensitivity, the higher the

bandwidthpossible miniaturization

possibility of obtaining a very high

natural frequency (> 30 KHz)


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INSTRUMENTATION AND AVIONICS

STRAIN GAUGE BASED ACCELEROMETER


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Mohd Nazri B Ismail

0436303

strain gauge based accelerometer

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