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> Need Help With A Difference Amplifier., Piezo sensor to null loadcell vibrations
RIOTPACK
Posted: June 10, 2009 01:17 am
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Hi.

I am working on a project to cancel out ambient factory vibrations present on my load cells signal line by placing a bi morph piezo sensor on the loadcell and putting it through a difference amplifier.

The problem is the loadcell has a DC offset under load (of course) and I figure nothing will be cancelled because the loadcells AC vibration is superimposed on the DC which is shifting.

So far I have drawn up a schematic which is outlined below.

Piezo Sensor - Opamp - 100hz bessel filter

Loadcell - Inamp - 100hz bessel filter

Then both feed into a difference amplifier with mixing controls, the piezo signal is attenuated below the load cells signal so it cannot place an illegal signal on the line.

Am I missing something here to do with the DC offset? I can post schematics if it would help.

Regards,

Mike
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Sch3mat1c
Posted: June 10, 2009 02:36 pm
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If the two sensors produce equal signals in response to ambient noise, with one producing that important DC offset which is your load signal, then a simple subtraction and, if you don't need the bandwidth, filter on the output to cut down higher frequency noise (especially noise which gets through as a result of phase shift between the two sensors) should be all you need. Is it not working?


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RIOTPACK
Posted: June 11, 2009 05:56 am
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Unfortunately I'm getting 50Hz mains hum after the piezo amplifier and the vibration may be around 50Hz so cannot notch it out.

I am running a virtual ground as I have to use the standard 10v single rail load cell supply - So I have 2x10k resistors in series with 2x1u caps with the centre tapped.

At the moment its not going well at all - maybe something in my design.

Can I post the schematics here?

The thing is I am working with signals of around 10-100mv so any noise can mess things up.

This post has been edited by RIOTPACK on June 11, 2009 05:58 am
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Sch3mat1c
Posted: June 11, 2009 12:28 pm
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Say, if you're using a single ended supply, what's signal ground referred to? If your output is referred to 0V, you'll have an offset you can't do anything about (without adding a negative rail). You'll also get half supply noise in the output. Signal ground has to be referred to the same ground that everything else is, the virtual ground.

How well shielded are the cells? Piezo sounds high impedance. Ambient noise could couple in nicely that way.

To post the schematic you'll have to upload it somewhere, e.g. photobucket.

Tim


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tekwiz
Posted: June 11, 2009 07:52 pm
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What kind of bandwidth do you require for your load cell's normal signals? cool.gif


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RIOTPACK
Posted: June 12, 2009 01:11 am
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Signal ground is 1/2 the supply rails which is split using .1% 10ppm 10k resistors with 1u caps across them. I can compensate for minor drifts due to uneven loading by changing the precision resistor if need be.

That offset has to remain because the loadcell is a Wheatstone bridge and will be referenced to 1/2 supply naturally, so when I place the signals through a difference amplifier they will have the same offset.

The sensor is on a small standalone board with a shield side on the PCB, The piezo sensors impedance is 1M but I have put a 1M in series at the sensor end to lower the line impedance, I want to stay shielded and not have to learn common mode noise rejection over twisted pair.

I am cutting the ac component off at 100Hz on both sensors, I think this is high enough to not subtract valid signals.

This project is to eliminate having to develop a DSP algorithm to filter the noise, or just using a basic cap to smooth the loadcells signal.

I will upload the schematic and post back.

Thankyou for your comments.

This post has been edited by RIOTPACK on June 12, 2009 01:12 am
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RIOTPACK
Posted: June 12, 2009 01:22 am
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I think I found the answers.

Thanks for your help.

This post has been edited by RIOTPACK on June 17, 2009 12:57 am
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