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Fylde Electronic Laboratories Ltd: Dynamic Strain Gauge Measurements using the FE-537-SGA.

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Introduction

The Wheatstone bridge circuit has been the mainstay of strain gauge measurement for many years and is often configured for constant current operation. The following note describes an alternative method for deploying constant current gauge energisation which offers advantages for dynamic strain measurements.

The technique requires the application of an extremely precise and stable constant current via only 2 wires to a remote gauge. The developed dynamic signal is recovered from the same conductors by the use of an A.C. coupled amplifier having high input impedance and high common mode rejection. This method of energisation provides an excellent signal to noise ratio which combines with the simplicity of a 2 wire connection to develop a precise dynamic measurement over a large dynamic range from below 1 Hz to as high as 100 kHz. No bridge completion is required and there is no need to balance as required by Wheatstone bridge type circuits.

Schematic of a Dynamic Strain Gauge Amplifier System

Schematic Diagram

How the Constant Current system works

The balanced differential Constant Current supply is preset at 5, 10 or 20 mA. The precision and stability of this current source is very high since any noise will be amplified.

The input impedance of the amplifier and the output impedance of the current sources is arranged to be sufficiently high to avoid loading the gauge and thus introducing non- linearities into the measurement.
Because the current is constant, the connections to the gauge may be long and may exhibit resistance without detriment to the calibration; i.e., the voltage developed across the gauge is dependent only on the current supplied and the resistance of the gauge.

As the gauge value changes dynamically, so a small A.C. voltage is set up which the amplifier receives without attenuation and amplifies as required. The A.C coupled amplifier is able to ignore the standing D.C. offset presented to it's inputs. The use of a balanced differential supply and high common mode rejection means that the gauge is able to move electrically by several volts positive or negative without creating interference to the signal developed at the amplifier output.

Calibration

Standard shunt calibration methods cannot be employed with A.C. coupled amplifiers, unless the shunt value can be switched in and out of circuit sufficiently rapidly to create a dynamic calibration which is in band for the amplifier frequency range.

For 1/4 bridge (single resistor element) gauges, the FE-537-SGA provides an alternative calibration method which dynamically adds a precise low resistance which directly simulates the resistance variation of the gauge.

For the Fortunately, calibration is straight forward with amplifiers and systems of this type, because:-

 

1. The output of the gauge depends only on the strain signal, the gauge factor of the strain gauge and the voltage developed across the gauge.

 

Voltage across the gauge

VG = I x RG


Signal developed by the gauge

s = VG x Strain x Gauge Factor

2. The gain of the amplifier is precise and may be calibrated. Reliability and stability are high.

 

3. The current is extremely precise.

 

For confidence though, the voltage across the gauge (& cable) may be measured locally and will be equal to :-

VG + Vl = I x (RG + rl)

Where

RG = gauge resistance
rl = line resistance
I  = Constant Current (14.28mA)
Vl = Voltage across line resistance
VG = Voltage across gauge

Providing that the measured returned voltage is as expected given the approximate line resistance employed, then the user can have confidence that calibration will be correct. The measurement of the returned voltage before and after strain measurements gives confidence that both gauge and cables are in good order.


The FE-537-GA Dynamic Strain Gauge Amplifier Data Sheet

The FE-537-SGA Dynamic Strain Gauge Amplifier is a standard module in the FYLDE range of "blue panel" instrumentation modules. One or more channels may be housed in a range of cases to suit the requirement.

The amplifier consists of the following main sections:-

1. Mains or DC Power Supply
2. Constant Current Strain Gauge Supply
3. AC Coupled Low Noise Amplifier
4. Low Pass Filter
5. High Pass Filter
6. Output Stage
7. Calibration Circuit
8. Overload Circuit

Thus the module contains all of the required elements to make accurate dynamic strain measurements and may be employed on full bridge as well as fractional bridge applications.

A shortform and detailed data sheets are available on request for blue panel modules.


The FE-1823 Dynamic Strain Gauge Amplifier Data Sheet

The FE-1823 Dynamic Strain Gauge Amplifier is a standard module in the FYLDE 1800 series range of instrumentation modules. It provides two channels of energisation and amplification and is suitable for use in demanding environments such as flight test.


 

 

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