Clipping Data; Trying to Scale +/- 5v to +/-3kip lbs

[OKors]

I'm trying to use Signal Express to scale a +/- 5v signal to +/- 3000lbs. It's currently outputting a waveform but clips the data at +/- 60lbs.

Here's some info on my setup:

DAQ= SCXI-1000 chassis, SCXI-1302 50-pin terminal block, and feedthrough panel.

Test Setup= MTS-407 2-channel servo/hydraulic controller controlling a hydraulic actuator w/ 3kip load cell. 

The load cell is pre-configured (scale and gain) within the 407, so the full scale of the device (in my case it's the load cell) is relative to 10volts of output signal (+/- 5volts). In other words, when I monitor the load signal coming out of the 407 it's always +/- 5volts; -5volts = -3000lbs / +5volts = +3000lbs.

How I've configured my channel within Signal Express:

I've created a basic Acquire block; Add Step> Acquire Signals> DAQmx Acquire> Analog Input> Voltage...

Under the STEP SETUP tab- I've set my Signal Input Range to + 5 Max and -5 Min (reason being is because that's the range of the voltage signal coming out of the 407- or 10v full scale).

TERMINAL CONFIGURATION- I've selected RSE (what signal I do have tends to look a little more realistic with RSE selected).

CUSTOM SCALING- I've used the TABLE scaling function to define my range of pre-scaled to scaled values.

Prescaled= Volts, Scaled= "Load"

My table looks like so:

Prescaled Values           Scaled Values

          0                                0

          5                                3000

         -5                              -3000

While using the 407 to run a basic sine wave I see the load plot but it clips just above +60 and just below -60. I do see some realistic data within the waveform that IS viewable. The ACTUAL loads are roughly +485/-640, this is what the 407 displays.

Please understand, I'm really new at using this hardware and software. If the information provided is inadequite or confusing please ask questions and I'll try to be more specific.

Thank you!

Message Edited by OKors on 05-29-2009 11:58 AM

[OKors]

What there's a timelimit on when you can edit your post???

Something I was going to add; here are a couple things I have tried:

• Selected all three Terminal Configurations; RSE seems the most realistic. Attatched a ground wire between the 407's ground lug and the SCXI-1302 block (no change with any Configurations)

• Tried to rescale the Table using Shunt Values from the 407 and a DVM to monitor actual voltages. In other words, I applied a shunt to the load cell using the 407 and used the shunted load value from the 407 as my Scaled Value (1300.6lbs @ 100k positive shunt) (-1263lbs @ 100k negative shunt). While monitoring a DVM I used those signal voltages as Pre-Scaled values.

• Tried using Slope / Offset scaling (data was either non existant or something very unrealistic)

[Chris_W.]

Hi Okors,

There are a couple of things that first come to mind, but I am a bit confused about your set still.

First of all, what are the specific connections you are making between the 407 and the 1302 connector block? I have found a data sheet for the 407 HERE, but I just want to make sure that this is the correct device. So, in your post, when you mentioned... "when I monitor the load signal coming out of the 407 it's always +/- 5volts," what were you referring to? I thought that you are monitoring the output of a load cell? Or do you have one of the two connections on the front of the 407 connected to the 1302 and the 407 is reading the load cell signal and passing it through to the 1302?

The reason that I ask, is if you are actually reading ±5V from the load cell into your SCXI/DAQ setup, then we can pretty much be sure that the issue is with the scaling in Signal Express. 

The other thought that I had, was that it would be great to verify what the actual output of the load cell is. If you can confirm in the raw data that it does go from -5V to +5V then, it may be an issue with the connection or again with the scaling. Another thing that verifying this voltage output of the load cell could tell us is whether the load cell is receiving the proper excitation voltages. If the excitation to the load cell is not sufficient then it might be limiting the output of that sensor. 

And one last thing that I thought to try was in your step setup in Signal Express, you could use a linear scale instead of a table scale and just just the y=600x as the linear equation for scaling. This should work the same as described in your table scale, but it would interesting to see if you have any change. 

Also to verify what connections you should be making with your signals, a great resource is THIS developer zone tutorial. Specifically, section number 3, on measuring floating signals.

A little more information on some of this would be great.

Chris W

Message Edited by Chris W. on 06-01-2009 02:14 PM

[OKors]

Thanks for the reply, I think I've figured it out. Ultimately it ended up being a scaling issue along with not entering in the proper values for the Signal Input Range under Voltage Input Setup. I thought this was the min/max of the analog signal coming into the SCXI so I entered +/- 5volts, when it is infact the desired full scale of your SCALED signal. So I needed to enter +/- 3000 since I was working with a 3kip load cell.

Now I have a question regarding how SE handles sample rates which I'll address in another topic.

Thanks!