Torque Sensor 4: Torque Sensor Build Log – Initial Prototype

Torque Sensor 1: Mechanicals of Materials

Torque Sensor 2: Strain Gauge & Wheatstone Bridge

Torque Sensor 3: Instrumentation Amplifier

Torque Sensor 4: Torque Sensor Build Log 

Updated: 5/20/2016 – Initial Page Creation

Alright, the last three pages I talked about a bunch of theory and stuff that I needed to learn/review in order to build a torque sensor. This page will cover my first attempt at building a torque sensor. Luckily for me, all of the numbers I used in the previous examples were from the actual components I have – I bought 20 250 ohm strain gauges on eBay for about $18, I am using a 3/8″ socket extension with the diameter and alloy from the Torque Sensor 1 page, and I have a Teensy 3.2 that runs on 3.3V. So I set about super gluing the strain gauges to the socket extension, as close to 45° as possible, then I used hot glue to stop the small wires from breaking or shorting to the extension, and some electrical tape and solder to finish up the assembly. Here are some pictures from that whole ordeal.

I simply connected a ratchet to the socket extension, and another ratchet to the other end (I have some 8 point sockets, super useful) and twisted. I made a super simple Arduino program to measure the voltage and output the serial data:

int sensorPin = A0; 
float sensorValue = 0; 

void setup() {
 Serial.begin(9600);
 while(Serial.available()==0);
}

void loop() {
 sensorValue = (float) map(analogRead(sensorPin),0,1023,0,3300)/1000.0; 
 Serial.print("Voltage = \t");
 Serial.print(sensorValue,4);
 Serial.println("\t V");
 delay(10);
}

I copy-pasted the output into Excel and graphed it:

Now the voltage isn’t centered at 1.6V like I want – I think this may have to do with the fact that I used the analog ground for the torque sensor and not the op amp, and I also didn’t connect the analog ground and digital ground… I don’t know too much about how the whole analog ground thing works to be honest. But either way, this is proof that the torque sensor works!

Future Steps:

I lack the fundamental ability to simply call a project “finished”. That being said, I have some things I would like to do with this before I call it quits. First, I want to actually make an instrumentation amplifier like I wrote about. That shouldn’t be too difficult. Next, I want to make some sort of apparatus so I can accurately lay the strain gauges at 45° so they are relatively accurate. An adjustable gain (basically just using a potentiometer instead of a resistor), and impedance matching with the instrumentation amplifier seem fun. I also want to get this off of a breadboard and implement a simple LCD display – an electronic torque sensor for super cheap! I will post more about these as I work on them. Thanks for reading!