Strain gauge (BF-350) + hx711 (always zero)

Hi everybody,

I am working on a project, which objective is to find the force applied to a point at a carbon fiber wing bar.

I am using two strain gauges BF-350, 2 resistances (330ohms), arduino and a HX711.

But the problem is because is just reading zero´s. I don´t know if the problem is the strain gauge, or the resistance, or the Wheatstone half bridge.
I did a simulation on Ansys, and the strain at the point i want to measure is about 0,03mm.

I am sending images about the Wheastone bridge that we create for that, the code and strain gauge installed at the wing bar,"







#include "HX711.h"

#define DOUT  3
#define CLK  2

HX711 scale;

float calibration_factor = -7050; //-7050 worked for my 440lb max scale setup

void setup() {
  Serial.begin(9600);
  Serial.println("HX711 calibration sketch");
  Serial.println("Remove all weight from scale");
  Serial.println("After readings begin, place known weight on scale");
  Serial.println("Press + or a to increase calibration factor");
  Serial.println("Press - or z to decrease calibration factor");

  scale.begin(DOUT, CLK);
  scale.set_scale();
  scale.tare(); //Reset the scale to 0

  long zero_factor = scale.read_average(); //Get a baseline reading
  Serial.print("Zero factor: "); //This can be used to remove the need to tare the scale. Useful in permanent scale projects.
  Serial.println(zero_factor);
}

void loop() {

  scale.set_scale(calibration_factor); //Adjust to this calibration factor

  Serial.print("Reading: ");
  Serial.print(scale.get_units(), 1);
  Serial.print(" lbs"); //Change this to kg and re-adjust the calibration factor if you follow SI units like a sane person
  Serial.print(" calibration_factor: ");
  Serial.print(calibration_factor);
  Serial.println();

  if(Serial.available())
  {
    char temp = Serial.read();
    if(temp == '+' || temp == 'a')
      calibration_factor += 10;
    else if(temp == '-' || temp == 'z')
      calibration_factor -= 10;
  }
}

On your HX711 board, is the "E-" connected to GND?
(Disconnect everything and measure for 0Ω from E- to GND.)

I'd love to help, but you need to provide a schematic showing how the sensors are connected to your arduino.

Do you mean this?

image

Hi,

I tried that, but that-s no difference.

Do u think may be the strain gauge is wrong? For my objetive is not corret?

Or maybe is the weld?

What is the source for that Wheatstone configuration?

The bridge arrangement appears to be consistent with this:


from Wheatstone Bridge Circuit | Strain Gauge | HBM

That's not a strain. Strain does not have units of length. Strain is elongation divided by length, so it is, nominally, unit-less. Since it is often such a small number when expressed as meters per meter, it is often referred to in terms of um / m, or microns per meter, or micro-strain.

One way to test whether your bridge is working is remove everything from it, apply some voltage to E+ and E-, and use your DMM to measure the resulting voltage across A+ and A-, for no load and full load. Compare the measured voltage to the voltage calculated using the properties of the strain gauges, configuration, and applied voltage.

One way to test your HX711 is to ditch the calibration part, output raw readings for no load and full load, and see if those match the expected output (obtained by calculation).

No, the whole circuit. You have missed off the important bit with the strain guages and power supply.

Any luck?
I'm having the same issue.

Those two resistances are high precision and low temperature coefficient I hope? 0.1% metal film is a good starting point. Carbon 5% resistors will not do here.