Strain Gage Measurement

Introduction

When a force is exerted on an object, the length of the object will change. The ratio of the change in length to the original length is called strain. A strain gage is a small section of very fine wire that changes electrical resistance when its dimensions are changed. In use, a strain gage is attached to the surface of the object to be measured in such a way that when the mechanism is deformed, the conducting material is also deformed.

Consider two strain gages that are glued to the top and bottom of a metal cantilever beam. When a force is applied to the beam, it bends down in proportion to the force (Hooke’s law). As the bar bends downward, the top of the bar stretches as it is placed in tension. The top strain gage is also stretched, and its resistance increases slightly because the length of gage wire increases while the width of the wire decreases. For the strain gage on the bottom of the bar, all of these effects are reversed – the bottom of the beam is placed in compression and the gage decreases in resistance. These changes in resistance are small, but they can be measured accurately with the proper circuit. A well-known circuit designed to convert small changes in resistance to changes in voltage is a Wheatstone bridge circuit.

Design Objectives

• Measure strain on a cantilever beam as a load is applied to the end of the beam
• Build a Wheatstone bridge circuit with two strain gages
• Measure the output voltage of a circuit and convert it to strain
• Use a Vernier Instrumentation Amplifier to amplify voltage readings