Strain Gauge

Laboratory Report

By Akash Sherchan

Student I.D. #: 1124433

University of Warwick

Summary

The strain gauge laboratory session had a lot of aims and one of the main aims was to provide us with experience in using circuits based on operational amplifiers and examine the characteristics of these circuits when they amplify DC signals. Another aim was to also investigate the use and characteristics of resistive strain gauges. The overall objective was to understand the how these widely used transducers are used and how they behave. This will be done through custom pre-built electronic circuits which will then be used to amplify signals from a resistive strain gauge when weighed down with washers. Table of Contents

Introduction

Method

Results

Graphical Representation

Analysis and Discussion of Results

Conclusion

Bibliography

Appendix

Introduction:

The aim of the laboratory was to gain experience in the use of Operational-Amplifiers and the study of their characteristics. This report will cover the method used to study Op-Amps and analyse the results thus explaining and showing the behaviour and Op-Amps. This laboratory had 3 sections looking at a non-inverting amplifier, inverting amplifier and a strain gauge bridge (differential op-amp) respectively. Where:

• V+ : non-inverting input

• V−: inverting input

• Vout: output

• VS+: positive power supply

• VS−: negative power supply

Where:

• V+ : non-inverting input

• V−: inverting input

• Vout: output

• VS+: positive power supply

• VS−: negative power supply

Operational-Amplifiers are key components in many electronic circuits and were first used in analogue computers to carry out mathematical functions such as addition, multiplication, and integration. Ideally the op-amp only amplifies the difference in voltage between the two inputs.

Vout (Output) = [V+ (+ Input) – V- (- input)] X Gain

There are 3 main op-amps available and these 3 will also be used in this laboratory. They are: * Non-inverting

* Inverting

* Differential

Circuits also rely on op-amps to be almost ideal. To be an ideal op-amp they must have the following characteristics: * Infinite Bandwidth (ability to amplify AC signals)

* Infinite Gain

* Infinite input impedance

* Zero Noise (no effect of electrical and environmental noise)

Method:

This laboratory was split into 3 separate parts, looking at a non-inverting amplifier, inverting amplifier and a strain gauge bridge (differential op-amp) respectively. When setting up the operational amplifiers we had to use resistors between the values of 1kΩ to 1MΩ. This is because the OP177 is not an ideal op-amp and has internal resistances. Using the values within this range the equations we use are still accurate. The apparatus used were circuits based on op-amps, specifically the OP177 op-amp. The pin connection of this device and other apparatus used are shown below :

Figure 1: Basic op-amp connections

Figure 1: Basic op-amp connections

Sockets for resistors

Sockets for resistors

Figure 2: Actual test board used in the laboratory

Figure 2: Actual test board used in the laboratory

Figure 3: Laboratory test set-up with equipment

Figure 3: Laboratory test set-up with equipment

1) Non-inverting Amplifier

1.1 The first step was to set up a non-inverting amplifier using figure 4. However, we had to set up the circuit so we would have a gain of about 15, using ‘preferred value’ resistors. So with a certain amount of resistors with their own specific values we had to calculate and pick two resistors to use for this circuit. We did this with the equation to calculate the gain of a non-inverting amplifier:

Gain = V0/Vi = R1 + R2 / R2

Figure 4

Figure 4

I used the resistors of values 33 kΩ and 470 kΩ to make a real gain of 15.24 for this circuit. 1.2 We then physically set up the circuits as shown in figure 3 and placed the...