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accelerometers with SCB-68


I am running a test with two accelerometers to measure vibration in a cantilever.

I am using the SCB-68 connector block. The accels tie in to the connector block....1 line has signal, the other is a ground. I have one accel tied in to Channel 0 and the other tied in to Channel 1.

When I run the test, I get the impulse spike from the accel on the hammer. The hammer has an accel in it and is used to impart a force onto the beam. The other accel is mounted on the beam and it provides the typical transient response curve. So far, so good...

The problem, however, is that the curve (when there are no inputs) is not flat, but rises slightly, as if the units are not grounded properly.

Has anyone seen this or offer a suggestion? I believe all the connections are set up properly.

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I think I have eliminated the rise issue.

Another question: All of the grounds on the SCB68 appear to be tied to the same sink. I am getting some weird drift (around -0.125 volt) on one of the accels. When I read with a DMM I see only zero.

Has anyone experienced a similar condition with the SCB68??

Cheers! Adzel
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I have used the SCB-68 with accelerometers in the past and did not notice this type of behavior. However, I also used the DAQ card in differential mode instead of single ended. You may want to try that to see if it solves your problem (assuming you are not already doing so). You can still tie one of your inputs to ground, if your sensor needs this.

You have probably already done this, but check to make sure your connectors are clean and the input wires are clamped well. I have been burned by accidentally clamping the wire insulation, pinching through it, and getting a flaky contact.
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Make sure you read the section of the manual of the DAQ card referring to input setup (differential, NRSE, etc). For differential you may have to add 'bias' resistors to the + and - inputs to analog ground for best input response.
"It’s the questions that drive us.”
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Gray and Kid,

I think I have eliminated the issue. It was a combined software (AI Config settings for differential) and wiring problem.

Thanks very much!
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