Read Strain Gauges

[kakam]

Hi,

I’m working with strain gauge (electrical resistance) in area of monitoring of civil structures and i would like make a simple and low cost data acquisition system (to make demonstration) using the parallel port of PC.

Does somebody know any circuit that converts the analog signal provided by a full whetstone bridge with the active strain gauge to digital signal readable in parallel port (or similar)?

Thanks.

 

[ericgibbs]

Hi,

I’m working with strain gauge (electrical resistance) in area of monitoring of civil structures and i would like make a simple and low cost data acquisition system (to make demonstration) using the parallel port of PC.

Does somebody know any circuit that converts the analog signal provided by a full whetstone bridge with the active strain gauge to digital signal readable in parallel port (or similar)?

Thanks.

As you may know the signal from the bridge has to be amplified before you can convert it into a digital signal.

What level of resolution in the digital signal are you looking for? [how many bits?]

I use a Analog Devices AD7705 dual 16bit, for load cells,[it has a PGA built in] and connect the output to a PIC,
display the data on a LCD and also transmit via RS232 to a PC.

 

[kakam]

The resolution isn’t the most important question, however if it is possible I would like to experiment the higher resolution possible (weighting the price also).

Other question is what I make with AD7705. Can I direct connect the output digital signal to serial port of my PC?

If I understood with only the AD7705 I can read the analog signal of the strain gauge in serial port of PC with 16 bit of resolution. Correct? (sorry my lack of knowledge in electronic area).

Thanks.

 

[ericgibbs]

The resolution isn’t the most important question, however if it is possible I would like to experiment the higher resolution possible (weighting the price also). Usually, the higher the resolution the higher the cost.
Other question is what I make with AD7705. Can I direct connect the output digital signal to serial port of my PC? No, thats not possible

If I understood with only the AD7705 I can read the analog signal of the strain gauge in serial port of PC with 16 bit of resolution. Correct? Yes that is correct

Please decide the resolution you require, it is important, there is no point in specifying a resolution higher than whats required.

Example: on your bridge [say, 10mv/Volt sensitivity, excitation say, 10Vdc] what would the minimum and maximum values
represent as an user output?

 

[kakam]

I was thinking what level of resolution I want:

If it is possible, the next situation would be perfect:
Excitation voltage levels of around 3 and 10 Vdc
Sensitivity in the bridge: about 2 ppm
Range: -8000; 8000 ppm

I think 16 bit of resolution is need. Correct?
Please help me if anything is wrong.

Thanks.

 

[ericgibbs]

I was thinking what level of resolution I want:

If it is possible, the next situation would be perfect:
Excitation voltage levels of around 3 and 10 Vdc
Sensitivity in the bridge: about 2 ppm
Range: -8000; 8000 ppm

I think 16 bit of resolution is need. Correct?
Please help me if anything is wrong.

Thanks.

Hi,
with 16bits you will get 65535 points, as you require +/- this works out to +/-32768 points.

If I follow your Sensitivity/Range figures, +/-8000/2 = +/-4000 points [ total span is 8000 points [1FFFhex], which is 13bits.
As ADC's normally come in even bit steps, a 14bit ADC would be suitable ie: +/-8192 points.

Check the sums and see if the 14bit ADC meets your requirement.

 

[kakam]

Hi ericgibbs,

You are totally correct.

With 16384 point (14 bits) I can read the range -8000,8000 with request resolution (about 1.95 ppm). I agree with you.

But, could you give me some more help about your circuit and how it processes the data transmit (digital signal provided by Analog Devices AD7705) to RS232?

One more time, thanks a lot.

 

[ericgibbs]

hi,

Goto www.analog.com and download the datasheet for the AD7952 14bit ADC,
it has a number of addressing methods, choose one to suit your requirement.

Also download the datasheet for the AD623 instrumentation opa.

You can program the PIC to read the AD7952 and send the data to a PC.

 

[kakam]

Thank you.