SCXI-1140 noisy signal

[pascal2007]

My application:

I connected a Function generator (to generate some functions: sinusoidal or square) with the BNC cables to a SCXI-1140 (via the terminal block SCXI-1305. The chassis is a SCXI-1000).

I use the National Instrument cable to connect the SCXI to the Data Acquisition Card (PCI6052E) in my computer.

I use Labview 6.1 to read the signal “Waveform Graph”.

I control the movement of 2 axes by 2 motors (one stepper and one magnetic).

I want to use hot wires to take measurement.

Problem:

When I turn OFF the 2 motors, from Labview, the signal I read is clear and no noise.

When I turn ON one or both motors, from Labview, the signal I get has a noise.

I tried 3 type of BNC cables (I do not know if they are double shielded or no!!!): the first is from Dantec Dynamics cable (bedea # 1084 RG 58, made in Germany), the second has this note M17/028 RG058  EBD, the third has this note 03554 Amphenol Canada RG-58 C/U. Unfortunately the problem still exist, I have a noise in my signal.

I tried on the SCXI 1305, the 2 positions of the switches position (G: Disconnects ground-reference resistor, F: Floating source). The problem still exist, I have a noise in my signal.

When I connect the function generator to the oscilloscope, I do not have any problem for both situations: Motors ON or OFF. The signal is clear and without noise.

Question: What is the problem?

Thank you in advance,

[samantham]

Hello,

Lets first rule out any programming issues.  I am interested in knowing the Input Configuration.   When you are programming in LabView, are you selecting the channel from the PCI-6052E (Dev#ai#) or from the SCXI 1140 (SC#Mod#).  The SCXI 1140 limits you to differential.  This will help eliminate common noise that may be coming from the motors. 

You also mentioned that you have tried switching in-between the G and F mode.  Please refer to the NI Developer Zone I have attached below … specifically Table 1.  If you are switching in-between the floating and grounded signal source and using differential, it relates to the first column of the table.  Wiring to AI Sense is already taken care of in the terminal block with the switch.  However, it seems that you understand this concept.

The noise could either be coming from the output lines you are using to control the motor or the motor itself.  If it is the output lines from the board then you have some control over the situation.  Shielded wires should help with the noise.  On the other hand, if it is the motor itself you can try moving the motor further away from the function generator and its lines leading to the CPU. 

There is a simple test you can run to figure out which scenario is happening.  Disconnect the output wires from the motor itself.  Run the LabView application where you are still outputting to the motors.  If the noise is present when you set the motors to ON, then it is the output wires.  If the noise is not present then it is the motor.

Field Wiring and Noise Considerations for Analog Signals

[pascal2007]

Hi Samantha M,

First thank you for your reply.

To be honest I do not know if I select the channel from the PCI-6052 or from the SCXI. Can you be more explicit because I am new in Labview? How I can choose to select the device from PCI-6052 or from the SCXI? I send you my program for help.

I ordered a double shielded cable. I will come back to you if there any problem when I receive it.

I think the problem is the motor because when I turn it OFF, no noise and when I turn it ON, I get a big noise.

Thank you again.

[samantham]

Hello,

I wanted to make sure that the physical channel you are sampling from in LabView is from the SCXI 1140.  This would mean that the LabView channel names looks like “SC#Mod#”, where the # will be the SCXI chassis number in the Measurement and Automation Explorer (MAX) and the slot number where the 1140 located.  On the other hand, if you have LabView looking at the PCI-6052E instead of the SCXI … the channel name would look like “Dev#ai#”, where the Device number is the one in MAX for the 6052E and the ai# is the channel you are wiring into.  MAX is defaulted to install on your desktop. 

The DAQ Assistant or low level DAQmx functions are two ways you could currently be acquiring an analog input using the DAQmx driver.  With the DAQ Assistant, the selection of the channel is located in the window that opens up for setting up the task.  I have included a screen shot of this window.  If you are using the DAQMX functions, then please open up the example titled Acq&Graph Voltage Int-Clk.vi located under the Help menu.  Explore the block diagram and front panel by looking at the “Physical Channel” control.

Message Edited by samantham on 12-13-2007 11:59 AM