Engineering

Engineering



Engineering

Introduction

In this lab, you will discover about damage gages and the Wheatstone connection circuit. You will glimpse how they can be

used for damage and force measurement. You will change a living program to assess the dynamic characteristics of a second-order system.

 

Teaching Objectives

* Gain functional know-how with opposition strain-measurement techniques.

* Learn about the Wheatstone connection and how it is utilised in strain measurement.

* Use a beam instrumented with damage gages as a force measurement device.

* Use damage gages to assess the natural frequency and damping in a beam.

* Design a force transducer for assessing push from a model rocket motor.

 

Preparatory Reading:

Procedure

Part 1: Strain Gages and the Wheatstone Bridge

The steel foil damage gages utilised in this lab

are resistors with a nominal (unstrained) opposition of 120 ohms. As they are put in stress, their opposition increases; as they are compressed, their opposition decreases. The Wheatstone connection presents a way to alter these alterations in opposition to alterations in voltage, which are so straightforward to work with. These voltages can be trained, conveyed, or retained digitally.

Figure 1:  Wheatstone Bridge Circuit Figure 1 (Graphic1.png)

 

Figure 1 displays a Wheatstone bridge configuration.

* Four resistors are attached in an end-to-end fashion.

* The input or excitation voltage is attached to the bridge between peak and base nodes of the circuit.

* The yield is the distinction between the voltages at the left

node and the voltage at the right node.

* An excitation voltage is needed to alter the change in

resistance (in the legs of the bridge) to a change in voltage at the yield of the bridge.

Figure 2:  equation (1) Figure 2 (Graphic2.png)

When construction a Wheatstone connection with damage gages, all four resistors have the identical nominal value. Bridges can be constructed in the next configurations:

 

* Quarter Bridge-One damage gage and three fix resistors

* Half Bridge- Two damage gages and two repaired resistors

* Full Bridge- Four damage gages

 

Figure 3:  Quarter Bridge Configuration Figure 3 (Graphic3.png)

Quarter Bridges

Figure 3 shows a quarter connection configurations. The quarter connection has one hardworking leg, i.e., one leg with an altering resistance. From formula (1) overhead we can draw from a sign for the yield voltage as a function of the opposition change ?R:

Figure 4:  equation (2) Figure 4 (Graphic4.png)

Half and Full Bridges

Figure 5 and Figure 6 display half-bridge and full-bridge configurations respectively.

 

* Half bridge: two hardworking legs, one in stress and one in

compression. These legs are adjacent legs in the bridge.

* Full bridge: four hardworking legs, two in stress and two in

compression. The gages in stress are on converse legs of the bridge. Using formula 1 and Figure 1 as a guide, derive signs for the yield voltage of the half-bridge and full-bridge circuits.

A half connection could be made with two gages in stress on converse legs. When would this be useful? What would be the major difficulty with managing this?

Part 2: Calibration of the Strain-gagged Cantilever Beam

Your TA will supply an aluminium beam instrumented with damage ...