Sensor amplification #
Sensors produce real small voltages, so we need to amplify them.
Strain gauges #
A strain gauge is a small wire that changes resistance when it is stretched. The strain gauges we’ll use in ME 30 are made of constantan, an alloy of copper and nickel. The constantan is attached in a serpentine shape to a polyimide film. The 14 sections in the serpentine shape means that the wire behaves like 14 resistors in series, each of which increases in resistance when it’s stretched. When the strain gauge is not stretched, it has a resistance of 120 ohms. When stretched, it increases to 121 or 122 ohms.
If we put the strain gauge in series with a normal 120 ohm resistor to make a voltage divider supplied with 3.3 V, the voltage between the two resistors will be around 1.65 V. The voltage will go up and down in proportion to strain, but the voltage change is so small that it will be hard to detect.
Amplification #
To make the signal bigger, we’ll use an amplifier. You can buy a strain gauge amplifier like the HX711, but we’re going to build one using a chip called an operational amplifier, or op-amp. We’re also going to deploy a technique that will make the sensor more accurate.
The technique we’ll use is called a Wheatstone bridge (invented in 1833 by Samuel H. Christie; Wheatstone just blathered on about it). The idea is to add a second pair of resistors in parallel with the original voltage divider and then measure the voltage difference between the two central nodes. It’s like our voltage is a bridge across the middle of the circuit.
The Wheatstone bridge has two major advantages.
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Resistors change resistance when they heat up, which means that your sensor signal will slowly drift with the temperature in the room, and it gets worse because the current running through the resistors heats them up even more. When comparing the two sides of the bridge, they are both at roughly the same temperature, so the temperature drift is the same on both sides, which means it cancels out in the difference.
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It is easier to measure small voltages accurately than large voltages. Every op-amp has the unfortunate property that it responds slightly nonlinearly to input voltage. If we just try to amplify a 1.65 V voltage relative to ground …….. acch, not sure how to explain this well yet. This is called the common-mode rejection ratio, or CMRR. The LM324 opamps that we will be using have a CMRR of around 10,000 (80 dB).