Problem with using global virtual channel and custom scaling

[Steve_Lee]

Hi All,

I'm trying to creat global virtual channel and custom scaling for the measurement. I created a global virtual channel for analog voltage input. And I used custom scaling for the relation between measured voltage and torque. The table for pre-scaled values and scaled values can be seen from attachment.

Voltage      Torque

1.222           0
1.00          200
1.11          100
1.36         -100
1.51         -200

For positive torque, it works fine. But for negative torque measurement, labview stops giving real-time torque after -60. It should be able to go upto -200. I measured the voltage and it go upto 1.40V. Therefore, the question is why labview does not use the scale table since the measured voltage is in the range of table.

Any suggestion is highly appreciated.

Steve

[Ali M]

Hello Steve,

To better answer your question, how are you testing these values in LabVIEW? What results do you get when you use Measeurement & Measurement Explorer (MAX)?

Best regards,

Ali M

Applications Engineer

National Instruments

[Steve_Lee]

Hi Ali,

In labview, I use preconfigured virtual channel as the input and I also check it by directly running this virtual channel in data neighborhood in MAX. They give same result, which do not show negative torque less than -60.

I also check analog voltage of this channel for my usb-DAQ in device and interface of MAX and it shows voltage more than 1.36V.

Since 1.36V is still in the range of custom scaling table, it's supposed to give torque around -100 but it did not.

Thank you for looking into this issue.

Steve

[Ali M]

Hello Steve,

I recreated your custom scale in MAX and was able to successfully acquire expected values. I also tested 1.36V and obtained -101.536458, 1.40V: -127.578125, 1.45V: -160.130208, 1. 49: -186.171875 and so on. I would recommend trying the following steps for testing purposes. In MAX: create a Global Virtual Channel (GVC) voltage input task and create the custom scale (under Custome Scaling>Create new>Table). Also set the "Display Type" fom Chart to Table. Create an analog output task (1 sample on demand). Set a value in the AO task (e.g. 1.36 then select "Apply Value") > run the task; next run the GVC task (make sure that your hardware is also connected correctly e.g. wire AO0 to AI0). Change the input voltage values of your AO and observe the readings in the GVC task. If you have questions regarding these steps, feel free to let me know, the GVC readings should correspond to your scale.

Best regards,

Ali M

Applications Engineer

National Instruments

[Steve_Lee]

Hi Ali,

I did try your method. Generate analog voltage output to a global virtual channel configured as analog voltage input. Without using custom scale, the voltage shows in GVC matches the one generated. However, once using custom scale, GVC does not go beyond -62. It looks like the scale itself has problem but I couldn't tell. Thanks.

Steve

[Ali M]

Hello,

I tested the GVC with your custom scale in LabVIEW, and also observed correct results. Since I cannot reproduce this behavior; my recommendations would be to review the steps provided in my previous post starting with a fresh GVC and table in MAX and also post screenshots of your GVC, custom table, AO task and AI task obtained from MAX.

Best regards,

Ali M

Applications Engineer

National Instruments

Message Edited by Ali M on 06-04-2010 05:39 PM