Reading Small voltages

Hi guys I am a mechanical and aerospace engineering major working on a very large project, part of which involves building a static test stand for high powered rocket motors. I am reading thrust output using a custom built load cell. The load cell uses two strain gauges in a half wheatstone bridge. The output is being amplified by a 741 op amp. At the moment the bridge is not balanced but I plan on using a potentiometer to do this. I am using an Arduino Mega to read the output voltage but am having problems getting the sensitivity I need to read the voltage increase when the load cell is loaded. Any advice on how to set up the circuit or to increase sensitivity?

What is the voltage range you are getting out of the bridge now?

Once you balance the bridge, the output should go from zero to some max, right?

Can you just change the 741 gain to output 0..5V for the force range you want?

Otherwise add another 741 or other OpAmp with the needed gain.

It's also possible to use a lower Vref and change the Arduino Analog input range.

Let us know how this works out...

Hello
I do not know how your 741 is configured but I would suggest if not already using a differential amplifier to
minimise common mode effects .. In the old days we used strain gauges in a bridge that had somewhere of about 20 millivolts full scale, and amplified it to 5 volts full scale..The actual ranges and scales were determined by our customers..The 741 would not be my first choice in opamps..
There are many very good instrumentation amplifiers available today.
Goodluck,
jolphil

I used a 741 back in 1972, they were not new then but were reasonable. By today's standards a 741 is so totally rubbish that you would do well to replace it.
Assuming it is operating on a split 12V supply you should take precautions not to expose the arduino to anything outside the 0 to 5V range. So that includes negative voltages as well as over voltages. If you replaced the 741 with a modern single rail, rail to rail 5V op amp you would not need to do this.

Thanks for the input everyone. I connected my circuit to a dmm and checked the output voltage from the bridge (unbalanced) and am getting around 29 mV and when measuring the output from the op amp I am getting around 2.47 V. This is about what I want but there are still errors. When measuring the bridge voltage there is a voltage increase when the load cell is loaded but when measuring the op amp output there is no discernible change in voltage when loading the cell. I then connect the output from pin six of the op amp to a analog input pin on the Arduino board and print to serial the output voltage is a constant 2.47 V but again no change when loading the cell. I am running out of time on this project and am very worried at this point. My code is below.

/*
Load Cell data collection program
*/
const double ArduinoVoltage = 5.00; // CHANGE THIS FOR 3.3v Arduinos
const double ArduinoResolution = ArduinoVoltage / 1024;

int vPin = 0; // Assign input pin from load cell circuit

void setup()
{
Serial.begin(9600);
pinMode(vPin, INPUT);
}

void loop()
{
double val = 0.0; // Variable for storing vPin input
double voltage = 0.0; // Variable for storing voltage input
val = analogRead(vPin); // Read the voltage from vPin
voltage = val*ArduinoResolution;
Serial.println(voltage);
delay(100);
}

Have you also connected the ground of your amplifier supply to the ground of the arduino?

The bridge and the op amp have separate voltage sources. The arduino is providing a 5 volt excitation voltage to the bridge and the op amp has a 9 v external excitation voltage.

http://cerulean.dk/words/?page_id=42

paco

I am getting around 2.47 V. This is about what I want but there are still errors. When measuring the bridge voltage there is a voltage increase when the load cell is loaded but when measuring the op amp output there is no discernible change in voltage when loading the cell.

I assume you need more gain in your op-amp, or another gain stage as Terry suggested. But, you need to design your circuit so that you don't amplify that ~2.5V bias (or feed-in a lower bias and amplify it along with the signal).

I am running out of time on this project and am very worried at this point.

On a "big project", isn't there an electronics enginner available to help? And once you get this working, I'd build two of 'em. The last thing you need is for a custom ond-of-a-kind gizmo failing and shutting everything down!

I have tried various levels of gain from 100 to 10000 but still no improvement. I would also like to balance the bridge by attaching a potentiometer in parallel with one of the bridge resistors but no matter what potentiometer I use I can not get the unloaded bridge to read zero volts. As far as electrical engineers go I am on my own. I have mechanical and aerospace professors to ask questions to but no electrical.

zach_ina_box:
I have tried various levels of gain from 100 to 10000 but still no improvement. I would also like to balance the bridge by attaching a potentiometer in parallel with one of the bridge resistors but no matter what potentiometer I use I can not get the unloaded bridge to read zero volts. As far as electrical engineers go I am on my own. I have mechanical and aerospace professors to ask questions to but no electrical.

I think we all need to see a complete schematic of your bridge/op-amp/arduino connections to be really able to help you troubleshoot your circuit effectively, otherwise we can only throw out ideas that may or may not be helpful to you.

As far as how to implement a manual 'zeroing' function on your bridge there are several ways to accomplish this. An adjustable offset voltage can be added to a summing input circuit using a buffer output op-amp stage.

If you do rather wish to implement it directly on the bridge circuit the usual method is to wire a small value pot (relative to the size of the two resistors in the reference leg of the bridge, say 10%) in series at the junction of the two fixed resistors in the reference leg side of the bridge, and then wiper of the pot becomes one of the two take off points going to the differential input op-amp. A decent 10 turn trim pot is a much better choice then a single turn pot for this kind of application. That make sense?

Lefty

zach_ina_box:
The bridge and the op amp have separate voltage sources. The arduino is providing a 5 volt excitation voltage to the bridge and the op amp has a 9 v external excitation voltage.

So the grounds are not connected together then. That is your problem.

Attached is an image of my circuit. Sorry for the quality (I made it in Paint). I talked to a friend of mine who is a senior electrical engineering major and he could find nothing wrong with my circuit and still didn’t get the voltage output of the amp to change when loading the cell. But he did give me the contact info of a professor of his that may be able to help so I plan on talking to him tomorrow.

Compare your circuit to this proper circuit:

http://cerulean.dk/words/wp-content/single-supply-bridge-amplifier.gif

That circuit is a quarter bridge with an INA125 instrumentation amp which has 16 inputs while I am using a half bridge with a 741 op amp which has 8 inputs. While its not the same circuit it has given me some ideas to debug my circuit.

zach_ina_box:
That circuit is a quarter bridge with an INA125 instrumentation amp which has 16 inputs while I am using a half bridge with a 741 op amp which has 8 inputs. While its not the same circuit it has given me some ideas to debug my circuit.

Full Bridge circuits (four branches count them) are best interfaced with a true differential instrumentation op-amp, that is their main purpose in life. There are others instrumentation op-amps available, even some with 8 pins. Also, you are using the word inputs when you mean pins, as there is a big difference between those two words. Also a 741 has been an obsolete chip for many decades now, you probably couldn't pick a worst device to read a precision low level device like a load cell. And your circuit is a full bridge not a half bridge.

Here is a very good article on instrumentation/sensor bridges with both good theory and practical information. You will build a better functional circuit if you take the time to read through it.

Lefty

You are correct about the instrumentation amplifier, I should have used a better amp than the 741 but I am out of time thus I must use what I have and make it work. A bridge is classified by how many active elements are in the bridge, mine has two thus is a half bridge. The most frustrating part is that we utilized the same configuration, only with a quarter bridge, and a 741 op amp to measure pressure in a pressure vessel in a lab and it worked fine. The difference being that we measured the output using a very sensitive multimeter and I no longer have that luxury.

zach_ina_box:
You are correct about the instrumentation amplifier, I should have used a better amp than the 741 but I am out of time thus I must use what I have and make it work. A bridge is classified by how many active elements are in the bridge, mine has two thus is a half bridge. The most frustrating part is that we utilized the same configuration, only with a quarter bridge, and a 741 op amp to measure pressure in a pressure vessel in a lab and it worked fine. The difference being that we measured the output using a very sensitive multimeter and I no longer have that luxury.

Well I would very much dispute your definition of various bridge configurations. How many of the resistance elements have fixed values Vs variable values has no bearing on if the circuit configuration is in fact a classic four element Wheatstone bridge configuration or not, as yours most certainly is. You are certainly allowed to have your own personal definition of circuit configurations, but just don’t expect to be able to communicate to others clearly what you mean. :wink:

Lefty

Well I would very much dispute your definition of various bridge configurations.

Yes I agree with that.
If you are just going to make up definitions expect people to miss understand you.

What I can't understand is that you say you are out of time and you will have to make what you have work. Why not use the time you will waste on the futile taks of trying to get it to work, just doing it properly!