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Author Topic: strange torque sensor noise issue  (Read 1911 times)

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strange torque sensor noise issue
« on: March 21, 2008, 03:43:12 PM »
I am having a strange issue with my torque sensor.

Basically I have a torque sensor with a long metal rod attached. When force is applied to the rod, torque is applied that is measured by my electronics setup.

It had some noise, but after adding filter caps to the power supplies and amplifier all the noise went away.

Doing this above, everything works fine . . . but now is where it gets strange . . .

The rod now goes into contact with de-ionized water (about 180k ohms per inch). In this water I have a bunch of my waterproofed servos (that in theory at least have no electrical contact with the water) and waterproof AC lighting.

The water is entirely still and not moving, nor is anything in the water moving either. Nothing has changed from above except contact with the water.

Now the torque sensor outputs huge amounts of noise (like 20% noise)!

Attempting to correct this I ground the ground wire for the torque sensor, but no change. No additional filter capacitors made a difference.

Anyone with ideas on what is going on?

I'll try grounding the metal rod, but other than that I'm out of ideas . . .

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Re: strange torque sensor noise issue
« Reply #1 on: March 26, 2008, 02:40:31 PM »
Problem solved . . . but not explained . . .

The noise only happens when water touches the metal rod. I then placed a grounded wire in the water, but to no affect. Then I taped the grounded wire to the metal rod, and the noise goes away! Amazing!

Makes no sense, considering the sensor supposedly has a chassis ground wire, but it doesn't do anything . . .

(and the force sensor company, despite me spending ~$1k for that sensor, refused to answer my emails . . . >:()

Offline AndrewM

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Re: strange torque sensor noise issue
« Reply #2 on: March 26, 2008, 02:57:56 PM »
You said you had servos in the water?  Is it possible that the servos are giving off a low frequency RF field from the motors that the metal rod is picking up?  I say "low frequency" only because lower frequencies travel better through liquid than air.  If you take the servos out of the equation, just the metal rod in the water, does it still pick up noise?

EDIT:  Just reread your original post.  The water proof AC lighting would be my guess for low frequency, the 60hz humm, especially if it was a flourescent light.
« Last Edit: March 26, 2008, 03:00:25 PM by AndrewM »
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Re: strange torque sensor noise issue
« Reply #3 on: March 27, 2008, 06:25:15 AM »
But that doesn't really explain why putting the grounded wire in the water didn't do anything . . .

So I ran more tests . . .

The water is de-ionized - my multimeter says 1.8 megaohms per inch when placed in the water.

Whether the servos are powered or not, receiving a signal or not, no noise enters the system.

However, I found a decent amount of noise enters the system when I turn on the incandescent lighting . . . interesting . . .

Offline AndrewM

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Re: strange torque sensor noise issue
« Reply #4 on: March 27, 2008, 07:16:22 AM »
The light is likely producing a 60Hz RF field in your water.  The water itself is not drawing noise voltage, just acting as a low frequency RF carrier, so a grounding wire inside the water is only going to act as an antenna and ground the RF it picks up.  Think of it like this:  your light is a radio tower, your grounding wire is an antenna on a radio; it will pick up the signal from the radio tower but not all of it.  Just because you turn on your radio and tune to the local station does not mean the guy next door can't tune to the same station.  So the metal rod also picks up the same signal as noise.

You can shield the light with a ferrocage, which would make the light pointless to have.  Switch the light to a DC bulb and powersupply.   Or do what you already did and ground the metal rod.
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