NI-9234/Sound and Vibration Assistant: Input signals being zeroed (tared) automatically

[KFod]

Hello everyone,

I have a voltage source and a force sensor as inputs into a 9234 DAQ module.

The software zeroes both signals every time they change. For example, if the voltage being read by the DAQ increases to a constant value, the software will plot an increase in voltage, followed by a slow return to zero. Similarly, if I increase the force applied on the force sensor and hold it at a constant value, the program will plot an increase in force, followed by a return to zero.

Here is a graphical representation of the problem, which uses the force sensor as an example:

The sensors are passive. The force sensor is a PCB 208C02, and the voltage source is from an LDM-1000 signal conditioner, which recieves signal from an MHR 250 LVDT. The changes in displacement by the LVDT core are interpreted by the signal conditioner, which translates the position to a voltage value that can be read by the DAQ.

Thank you in advance.

[Henrik_Volkers]

And what is the question/problem?

The force sensor has a 1 mHz Highpass, so it will go to zero 🙂

[KFod]

Thank you for your help. This does not explain what I am seeing with the LVDT, however.

Before using the NI-9234, I checked the output voltage from the signal conditioner with a multimeter. Sure enough, the voltage remained at a constant nonzero value for any LVDT position that is not at zero. If I were to move the LVDT core and keep it still, for example, the voltage would change and remain at a constant value correlating with the core position.

The problem here is that either the DAQ or the NI Sound and Vibration Assistant is zeroing the voltage signal after every displacement, in the same manner as is displayed in the picture in my initial post.

Do you know if there is a way to stop this from happening?

[AnalogKid2DigitalMan · Accepted solution]

I would guess you have the 9234 set to AC Coupling mode, whereas you should be using DC Coupling.

-AK2DM

[Henrik_Volkers]

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@AnalogKid2DigitalMan wrote:

I would guess you have the 9234 set to AC Coupling mode, whereas you should be using DC Coupling.

-AK2DM

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😄

 https://www.youtube.com/watch?v=Zk5Il6KQrd8

[Henrik_Volkers · Accepted solution]

Additionally the IEPE input only allow the 9234 AC input with a 0.5 Hz -3dB cutoff ...

So to get a dF/dl  (delta Force to delta length) it's not a bad idea to set both to AC (maybe add an additional  1mHz 1or2 order highpass to the length for better long term match)

To get an absolute length you can read the LVDT with a 3. channel in DC mode.