measuring strain and voltage with scxi 1000/1521B/1317/PCI-6280

[AaScWi]

Hello,

I'm new to labview.....  I want to use one task to measure the strain and voltage from the 1521B module.  I am using the DAQ assistant vi.  LabView8.2 and DAQmx 8.6

First I added the AI/strain channels and it works.  Then I attemp to add AI/voltage channels and recieve the following error.  Error- 200128.  Measurement: property must have the same value for all repeated physical channels.  What am I doing wrong?  The DAQ measures the same signal either way and I would like to use the builtin strain configurations.

basically I would like to display the strain and the voltage at the same time.  Again new to labview...Appreciate the help.

Aaron 

[devchander]

Before answering your original query,

something i would like to ask:

Do u want measure voltage on a DAQcard 6280's spare AI channel? or do you want to measure voltage on the SCXI 1521B's channel itself??

ps: both are possible, while measuring voltage on DAQ cards's AI channel is more straight forward, measuring voltage on SCXI 152, needs some settings to be changed

[AaScWi]

I would like to measure the voltage on the 1521/B channels. 

[NI-Bongo]

Hi Aaron,

The error you are seeing is occurring because you cannot use the same physical channel for different measurement tasks, such as strain and voltage. There are a few ways you can go about doing this.  If you have another channel available and are using a full bridge configuration, you can add wires from the differential signal output of your strain gauge to an adjacent channel to do the voltage measurement, thus using two different physical channels. Another option, which you can use if you do not have additional analog inputs available or are using a configuration other than full bridge is to use a Custom Voltage with Excitation task.  This will allow you to provide the necessary excitation for your strain gauge, but still read the raw voltage from it.  The strain measurement can then be added by configuring this scaling in software manually.  That is essentially what the strain task does under the hood. If you let me know what type of bridge you are using, I can give you further advice about the appropriate way to go about the measurement.

Regards,

[Matthew_W]

 Hi Aaron,

 Just to add a little bit to the good advice above.

 If your goal is to see the voltage returned from the strain measurement and the strain values then I would do that this way:
   1. Use a Custom Voltage With Excitation task. This will allow you to configure for a bridge but the values returned to Labview are in volts.
   2. Perform your own scaling from Voltage to Strain or use the Convert Strain Gauge Reading VI's (search the block diagram for Convert Strain Gauge Reading).

Here's a great starting point for Getting Started with DAQmx. This portal covers all things DAQmx, you'll find your questions answered there, here on the forums or in the NI-DAQ Help that is installed to:
   Start->Programs->National Instruments->NI DAQ->NI-DAQmx Help

The SCXI-1520 can be used to read voltages from sources other than a strain gauge. Here's a good KB that talks about the task configuration to do so.

 Have a great morning,

 MatthewW
 Applications Engineer
 National Instruments




Message Edited by Matthew W on 02-01-2008 08:02 AM