TAMPERE UNIVERSITY OF TECHNOLOGY

EDE-33106 Machinery Monitoring

Exercise 4: Strain Gauge Measurement

Group 6

Mithun Chowdhury

Student ID: 240874

Email: mithun.chowdhury@student.tut.fi

The LabVIEW block diagram and front panel are given below:

Figure: LabVIEW Block Diagram

Figure: LabVIEW Front Panel (1)

Figure: LabVIEW Front Panel (2)

1. Explain the operation of Wheatstone bridge in the case of 1/4 – bridge.

Answer:

A Wheatstone bridge is a network of four resistive legs. One or more of these legs can be active sensing elements.

Figure: Basic Wheatstone bridge Circuit Diagram

Wheatstone bridge consists of:

• Four resistors. • Supply voltage (typically 2 ... 10V), • Output voltage = proportional to the strain • one or all of the resistors are strain gauges • other resistors are ”normal resistors” • all resistors must have same resistance value that the bridge is in balance

The ¼-bridge measures either axial or bending strain. It has the following characteristics:

• A single active strain-gauge element is mounted in the principle direction of axial or bending strain.

• A passive quarter-bridge completion resistor (dummy resistor) is required in addition to half-bridge completion.

• Temperature variation in specimen decreases the accuracy of the measurements.

Output of Wheatstone bridge with ¼-bridge:

Bridge output (Vo) are corresponding strain are:

Corresponding stress is:

2. Write down the measurement installation, type of strain gauge and type of glue.

Measurement Installation:

To do the strain gauge measurement, on a steel beam the given strain gauge was placed. Before that the place was cleaned with acetone and using a grinding paper. Then the bridge was

connected to the amplifier and balanced the bridge to value of 0. After that the cross-section dimensions of the beam was calculated (a=1.2cm and b=0.8cm). Then placed a mass in the end of the beam and measure the position (56cm). Finally the data acquisition card was connected.

Type of strain gauge:

The strain gauge used was bonded metallic strain gage.

This metallic strain gage consists of a very fine wire or, more commonly, metallic foil arranged in a grid pattern. The grid pattern maximizes the amount of metallic wire or foil subject to strain in the parallel direction. The cross-sectional area of the grid is minimized to reduce the effect of shear strain and Poisson Strain. The grid is bonded to a thin backing, called the carrier, which is attached directly to the test specimen.

Type of Glue:

Cyano Acrylate based cement is used as glue for fixing.

3. How you scale the output of 1mV/V in to relative strain when the gage factor has the value of 2.

Gauge factor is defined as the ratio of fractional change in electrical resistance to the fractional change in length (strain). When the gage factor has the value of 2, almost always strain gauges are used in a bridge configuration with a voltage or current excitation source.

In figure VEX, that is applied across the bridge. The output voltage of the bridge, VO, will be equal to

4. Short explanation:

The goal of Strain gauge measurement experiment was to get familiar with data acquisition card and to know the principle of strain gauge measurement. After measurement installation done, I made measurement by taking the mass off and placing in the end of the beam, then computed the stress in the beam and compared with the measured value. An excitation to the beam was given after take-off the mass, and measured the free vibration time signal, so called logarithmic decrement. Finally nominal angular frequency was calculated by given formula. In this measurement ¼ bridge installation was used.