Introduction to Strain Gages
Introduction
Voltage Divider Circuit
Bridge Circuit
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        A strain gage is a resistor in which the resistance changes with strain.  A strain gage is a thin piece of conducting material that may look like the drawing below.  (Although, there are also semiconductor strain gages - not covered here.)  It is literally glued on to the device where you want to measure strain.  If you want more information on strain gages, it is a good idea to check with manufacturers.  Here are some good links.
        Now, assume that you have a strain gage glued on a device and the device is under stress.  When the device-under-test is put under stress it may elongate or shrink, and the strain gage is sensitive to that small change in geometry.

The small elongation in the strain gage produces a small change in the resistance of the strain gage.  Small as it is, it is what we need to use to get a voltage indicative of the strain in the bar.  To convert that small change in resistance into a usable signal is not impossible, but it takes a little doing.  Often, the strain gage is used is a bridge circuit like this one.

        What are you trying to do in this lesson?


Sensors In Voltage Divider Circuits

        The kind of sensor that we will examine is a resistive sensor, and to make things specific we will look at using a strain gage to make mechanical measurements of strain.  Here's that sensor (Rs) in a voltage divider with another resistor, Ra.

Let's examine what happens in this circuit.  Some of the things that happen in this circuit include the following.

        Now, if we have some sense of the resistance change, then we can think about how we will sense that change.  Because the change is very very small we will have to worry about how we are going to use that very small change in resistance  We have at least a couple of options for how we can make that measurement.

        Let's compute the output voltage for the voltage divider.

V1 = Vin Rs/( Ra + Rs)

        Now, let's compute some typical values.         Now, we can check what happens when the resistance changes by a typical small amount. We can compute the output for both cases.         That's a pretty small change in the output voltage. If the supply voltage is 5v, that change becomes:         Now we can define what the problem really is.
Using A Bridge Circuit

        A bridge circuit can help with our problem.  Here's a bridge circuit.

We will choose Ra and Rb to have the same value.  That will produce 2.5 volts at the middle of the left branch.

        There are some implications of this result with the bridge circuit. There are some other alternatives also.         An important consideration when using bridge circuits is choice of values for those resistors that have values you can choose.  In the bridge we just considered, only the strain gage resistance was fixed.  That leads to a question. Let's imagine that you have a strain gage.  Let's also assume that you have measured the thermistor, and you know the following. The question is "How to build the bridge?".  We'll work on an answer to that question starting next.  We will assume that the supply voltage is five (5) volts.         Let's look at the implications of one choice.  We'll look at Circuit 1. What we have demonstrated is that we get a very small voltage change with this choice of resistors for the bridge.  There is always the possibility that a different choice of resistors would produce better results.  Let's check that out.  Let's look at Circuit 2.  Here is what we noted above for Circuit 2.         Now, with no strain the bridge is balance, and with a 5 volt supply, we would have 10/11 of five volts or 0.4545454 volts.  When the load is applied and the sensor resistance changes, the voltage from the sensor side of the bridge is going to be:
Problem

        Compute the output voltage from the loaded bridge for Circuit 3.



        What can we conclude from this? What might we think about now?
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