Strain Gauges

Hello, I am new here and was wondering if anyone had done this or know of a way.

And yes, before you ask, I have been searching for over 2 hours on the right way to do this. I am a Mechanical Engineer Student and am building a platform to get data from a strain gauge.

Here is my problem, I setup the strain gauge in a Wheatstone bridge configuration. I have a it properly setup (verified by an electronics professor) but am still getting nothing.

Here is the setup (sorry for the bad pic)

I get voltage from both Analog Readings and measure the difference. The problem is that when I get my outputs, and then try and find the difference, I can never get the voltage to change.

Voltage from analog 1 = 3.91V Voltage from analog 2 = 3.91V Difference = 0.0V

I know the strain gauges work. I can put my voltmeter on a1 and a2 and get a positive or negative reading when stress is applied to one of the strain gauges.

Do I need to some how amplify the output or what do I need to do?

void setup() {
    Serial.begin(9600);
}

void loop() {
  
    int sensorValue;
    int sensorValue2;
    float volts;
    float volts2;
    float difference;

    sensorValue = analogRead(0);
    volts = sensorValue * 5.0 / 1023;            // convert AD units to volts
    sensorValue2 = analogRead(1);
    volts2 = sensorValue2 * 5.0 / 1023;            // convert AD units to volts
    difference = volts-volts2;

    Serial.print("?f");
    Serial.print(volts);
    Serial.print("V 1st\n");
    delay(1000);
    
    Serial.print("?f");
    Serial.print(volts2);
    Serial.print("V 2nd\n");
    delay(1000);
    
    Serial.print("?f");
    Serial.print(difference);
    Serial.print("V Diff\n");
    delay(1000);

}

Hi there.

We have been working quite a bit with Arduino and Load Cells (strain gauges) I have written a little guide to it here: http://christian.liljedahl.dk/guides/arduino-and-load-cell (Scroll down for the tech and the code)

Generally you need an amplifyer. We are using a little IC instrumentation amplifier, but you can do it with cheap opamps too.

kind regards Christian Liljedahl

If you can measure the voltage difference with a voltmeter, how large is the difference? Volts or millivolts? If the change is millivolts, then you need an amplifier.

Is your arduino ground connected to the ground on the circuit you showed?

Yes, the ground is connect to the arduino ground.

I have several op amps just can't figure out how to get them to work. Any help would be greatly appreciated.

Thanks.

Well you can get a overview here: http://en.wikipedia.org/wiki/Instrumentation_amplifier

Basically you need to work out the millivolt scale your sensors are working with (0 to some max millivolts vs max weight or strain), set the gain of the amplifier to convert that range to say a 0-1vdc output voltage. Then utilize the analog reference command (http://arduino.cc/en/Reference/AnalogReference ) to utilize the internal regulated voltage reference, usually 1.1vdc. This gives you the best resolution/stablity without having to go to high quality instrumentation components.

http://arduino.cc/en/Reference/AnalogReference

Lefty