SCC-SG24 & SCC68 (Torque transducer)

[ATMA]

HW: PCI6229, SCC68, SCC-SG24, TRT-25 (Transducer techniques, torque transducer) using Virtual TEDS from ni.com
SW: LV 8.2.1, DAQmx 8.3

Hi guys, with the 'rig' above i'm trying to extract the torque of a motor.

Based on what i know, the SCC-SG24 will detect the small change of voltage in the bridge of the transducer, amplify it can send it to my DAQ through the SCC68 connector block. The TEDS data will be used to properly scale my data so that i'll get an accurate Torque vs Sample graph. I have attached the screenshot from MAX and it shows the Amplitude vs Sample of my transducer. As you can see the values are very small, in the mV range. I've tried changing the max/min input limit to 200m/-200m but it doesn't allow me to do that. And how do i convert this voltage measurement to torque?

[ATMA]

This are the settings in MAX

[John E]

Hi,
The SG24 Module is a strain gauge module that is meant to take in very small voltage measurements.  It then amplifies those voltages so they can be more accurately digitized, then converts them to digital signals along with the scaling information.  Thus the computer accurately stores the values as small voltages.  You may wish to look through this manual: http://www.ni.com/pdf/manuals/371073b.pdf for steps on properly setting up the SG24.  Especially page 26 which highlights how to Convert Voltage Measurements to Units of Strain.


Regards

[ATMA]

Sorry for the late reply, i've been away.

I've already read the manual. But i'll go through the equation part again.

One thing, on my SG24 strain gauge there are 2 potentiometers. What are they for? Are they for offset nulling?

If they are indeed for offset nulling, how do i use them? Even with my transducer at rest, my DAQ displays erratic non-static voltages. Do i adjust this R39 R40 (pg24 of the guide) until it becomes zero?

Message Edited by ATMA on 08-28-2007 11:24 PM

[ATMA]

I've managed to get a consistent (albiet noise-like) output from my torque transducer.

I've attached images from MAX and my VI results, both are when my transducer is under no strain.

1) I noticed that both their values are approximately close, -0.35 to -0.39. When i try turning my transducer with a torque wrench, 7 out of 10 times the resultant value displayed in the waveform has a difference of approximately same as that value compared to the wrench value.  Is this my offset?

If so is there a reason why not 10 out of 10 times, what happened to the other 3 times when i get erronous result?

I've tried playing with the potentiometer on SG24 to reduce this value but no luck.

2) Are these the proper waveforms?

3) In the Y-axes of both the images, one is Amplitude another is TEDS. Based on the manual, i can convert every sample mathematically into torque before displaying the waveform to get torque as my Y-axis. What is the equation/relationship between torque and the sensor output voltage? I've tried googling for it but no luck.

[hoffiz]

Hi Atma,

Doing and average of the signal coming from the torque transducer will help better understand what is happening because the torque transducer can be very sensitive and that is why your readings oscillates between -0.35 and -0.39.  In the manual page 23 says that (in the picture) those are the offset nulling resistor for AIX.

If no strain is apply to the torque transducer then yes change the resistor until your signal is in average in 0.  For the other three times it can be noise, it can be poor connections it will also depends on your torque transducer specs, this is a useful link for basics in torque measurements http://zone.ni.com/devzone/cda/ph/p/id/255, Another very good resource for strain measurements in general is http://zone.ni.com/devzone/cda/tut/p/id/3642 What are the values the other three times when it does not match the wrench?  How those the images look when you apply strain? Let’s say you apply a know strain how does the measurement looks like?

JaimeF

Message Edited by Jaime F on 08-29-2007 03:54 PM

[SasidharUppuganti]

Dear Atma,

My name is Sasidhar. I am Biomedical grad student at the University of Akron. I am working in the spine biomechanics lab on a project on the same lines as yours. I need your guidance in this regards. I have an AC servo motor that I intend to move clockwise and counter-clockwise to a limit of +/- 10 Nm with a safety factor of 2. The HW and SW being used are,

HW: PXI-7352 Motion controller, UMI-7764 Motor interface board, PXI-6251 M-DAQ, SCC68, TRT-100 (100 in-lb, i.e 9.04 Nm) Transducer techniques torque cell using DPM-3 (Digital Panel meter) Transducer techniques, and Virtual TEDS from ni.com

SW: LV 8.5

Can you guide me through the connection in between TRT-100 and M-DAQ PXI-6251 via SCC-68. Also reading the torque cell data on PC.

Thank you.

Sasi.