ENGN/PHYS 207 Fall 2018 Assignment #5 Final Report Due Date: 5pm Wed Oct 31, 2018

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1 ENGN/PHYS 207 Fall 2018 Assignment #5 Final Report Due Date: 5pm Wed Oct 31, 2018 Circuits You ll Build 1. Instrumentation Amplifier Circuit with reference offset voltage and user selected gain. 2. Strain measurement system: Instrumentation amplifier interfaced to Wheatstone Bridge and Arduino Lab Skills You ll Learn 1. Implementing Instrumentation Amplifiers 2. Arduino if-else logic to respond to measurements 1 Vibration Sensor In this portion of the lab, we ll just build and explore with a Wheatstone Bridge strain gauge + instrumention amp + buffer + Arduino interface. There is nothing to write-up, but you do need to provide a proof of concept demonstration to the instructor. This portion of the lab is to get your feet wet and give you exposure to an Instrumentation Amplifier, and more practice with the Arduino (which is just such a wonderful and powerful tool at your finger tips). Now you will use actual strain gauge elements (see Fig. 3(a)) configured as a Wheatstone Bridge to measure deflections in a beam (e.g., see Figure 3(b) and 3(c)). Where would you do something similar in real life? There are many examples! For instance, you might test the structural integrity of a bridge with a strain gauge in a WB bridge configuration; you might examine the response of an airplane wing to an impact; you might test how earthquake isolated a building is; and so on. One very exciting new biomedical application is cuffless blood pressure monitoring; the strain in the arteries varies with time as blood pumps through in pulsatile fashion. (I did promise that circuits is actually useful...hopefully this helps convince you). The output of strain gauge bridge is quite small (approximately 2 20 mv), just barely large enough to see on your oscilloscope. We will amplify this signal to make it easier to see and measure. To do so, we ll make use of a very, very useful piece of electronics called an instrumentation amplifier. We ll use the model known as the INA Amplifiers are kind of like cars. By analogy, general category is automobile (amplifier); a specific model is a Honda Civic (INA 126). 1

2 Figure 1: Wheatstone bridge output amplified by INA126 instrumentation amplifier. The reference pin of INA 126 is driven by an op-amp voltage follower with voltage divider input. The output of the instrumentation amp is to be connected to an Arduino analog input pin. Notice that all power connections for the WB, TL082, and INA126 are +5V and GND, provided by the Arduino power supply pins. Later in the term, we ll discuss how its inner workings. For now, you just need to know that it amplifies the difference of two inputs by a gain factor G, and adds the reference voltage: V out = G(V + V ) + V REF. (1) where V + V is the WB output signal (i.e. the straing gauge s electrical signal), and the voltage gain G = k/R G should be set to about 50 in this case with appropriate value of resistor R G. The voltage V REF is the output bias that allows us to center the signal around 2.5V instead of ground, allowing us to see both positive and negative swings in the WB output. For instance, if your bridge has an output V W B = ±10 mv (possible to read with a scope, but just barely), the amplifier will multiply this by a factor of 50 and output V amp = ±500 mv = ± 0.5 V (much easier to cleanly read on a scope). Additionally, the output is recentered about V REF = 2.5 V. This is important to see both positive and negative swings in the bridge output. Else, we d lose the bottom half, since the output can t go below 0V. Detailed instructions how to wire your circuit are as follows. Obtain a strain gauge device from the instructor. It has 4 wires: 2 to power the WB and 2 2

3 Figure 2: Pinout of INA126. Note that pin 1 is in the upper left corner, with the chip properly oriented. for the output signal. Bridge excitation: Connect the Red wire to Arduino +5V power supply pin. Connect the Black wire to GND. Route WB output signal to instrumentation amplifier: Connect INA126 pin 2 to the White wire/conductor of the bridge circuit. Similarly, connect INA126 pin 3 to the Green conductor of the bridge. Route power connections to your amplifier: Connect INA126 pin 4 to GND and pin 7 to +5 V. Setting the gain of the amplifier: Connect an appropriate value gain-setting resistor R G between pins 1 and 8 of the INA126. Set the gain of the INA126 to be G 50. Provide a reference for the amplifier that biases the output to about 2.5 V. Build a voltage divider out of R 1 and R 2, where the voltage across R 2 is about 2.5 V (half way between GND and +5 V). Connect the voltage divider to the TL082 connected as a voltage follower aka a buffer. The buffer serves to isolate the voltage divider from the INA 126. Connect the output of the buffer to INA126 pin 5. Measure the amplified bridge output with an Arduino analog read. (Alternatively, you can use an oscilloscope to debug as necessary.) Connect the output of the INA126 (pin 6) to Arduino analog input A0. Open the Arduino IDE, then upload a sketch for making analog measurements. See Lab 2 for a refresher as needed for the analogread() command. The online help at arduino.cc is great too. Lastly, include an if-else statement that will make the built-in LED illuminate when the voltage exceeds a low or high voltage limit. This corresponds to too bending beyond a threshold in either direction. Note that you must first configure the LED pin as an output, see the Blink.ino example sketch setup() section for how to do this in one line of code. For if-else statements, check out a good bit tutorial from the proverbial horse s mouth: https: // Note you will replace the 120 threshold value used in this example with some sensible voltage based on your WB + instrumentation amp output. Also, note that you need a way for the LED to turn back off, so you might write a block of Arduino code like this in the loop() section: int LEDpin = 13; //built in LED is connected to pin 13 float Vhigh = 4; //define high side threshold 3

4 float Vlow = 1; // define low side threshold if (Vout > Vhigh Vout < Vlow){ // if the output is too high or to low digitalwrite(ledpin, HIGH); // set digital line to high = +5 V, lights up LED } else { digitalwrite( LEDpin, LOW); // otherwise, turn it off, no warning } Use a C-clamp to fasten your the beam, with attached strain gauge sensors to the bridge. Orient your beam so that the strain gauges are near the table, the eyelet for loading your bridge at the distal end. Set the beam to oscillate. Can you clearly see the signal changing on the Arduino serial plotter output that corresponds to the vibrations of the beam? If so, success you ve just a vibration sensor! Show the instructor and you are finished :) 4

5 (a) (b) (c) Figure 3: (a) Strain gauge element. The resistance changes when thin metal film comb shape deforms. In a non-stressed state, the gauge typically has a resistance of a few hundred ohms. (b) Building undergoing vibration. Strain gauges can be used to measure and record deflections vs time. Image credit: (c) Aluminum beam with strain gauge elements wired in Wheatstone Bridge configuration found in the lab. The 2 gold/tan elements labeled 1 and 2 at right are 2 out of 4 total strain gauge elements configured as WB. 5