Custom Scaling Goes wrong

[vtttt]

Hi,

I am using SignalExpress to collect the data from a wire pot. I have calibrated the pot and constructed the linear relationship between votage and displacement. However, when I put the slope and interception into the custom scaling, the reading is wrong. Something interesting is that, with out changing anything, I read the votage without scaling, and calculate the displacement by the same equation ( y= ax+b), it is right! Can anyone help me out here? Meeting with professor is approaching........

Thank you so much.

Vttt

[USBarnes]

Vttt,

Could you provide more information about the settings of your task? Is it a basic analog input task? Is the custom scale you are referring to the one that you can select/create from the Step Setup>>Configuration page in the DAQmx Acquire Step? As some alternatives to this, since your meeting is approaching, you might try using some other steps from the processing section like the Scaling and Conversion, Arithmetic, or Formula steps.

[OKors]

Couple questions:

By "wire pot" I'm assuming you mean a String Pot? Or wire based displacement transducer?

If so:
How are you plugging this into your system? Typically you'd have to set this up as a bridge device.
Are you calculating the slope on your own or from some calibration documentation?
If calculating on your own how are you determining the y=mx+b values? You're looking for the slope which is 'm' in the equasion so your formula should be m=y/x (y=full scale range of the device, x=sensor output at full scale). Put your offset (Y) in later.

Another option; if you getting voltage readings you can always build a custom "Map Range" or "Table" scale. But you'll need some other device like a tape measure to determine the output voltage at the specified distance.

of course all that is assuming you're using a string pot.
Make and model of your sensor would be helpful.

[vtttt]

Hi Sir:

Yes, string pot it is.

I did 'custom voltage with excitation' in signal express, and specified it as 'full bridge I'.

I have the slope right. But the 'absolute value' is wrong. For example, if I get y=10x+1 from calibration, that means y=11 inches cosponding to x=1 volt. However, when I use  'custom scaling' module in signalexpress, it spits out a 13 inches reading, which is 2 inches wrong. But the voltage is truly 1, I checked it by change the acquiring value back to 'volts', i.e., without any 'custom scaling'.

The slope is right, I guess if I zero it every time, it could work. But it is weird, and makes me worry about the results.

The spring pot is 'celesco' brand, pt101 seris.

Thank you so much.

[OKors]

what happens when you leave the string pot fully retracted and enter a zero value for the y-intercept? What's your offset and min/max readings then?

Have you done a physical measurement on the string pot with a tape measure to see how many inches of actual wire are exposed when fully extended? Quite often these string pots will have extra wire on the spool.

• Get your slope entered and leave the y-intercept at zero, run some data and check for any sensor offset (should not be much).
• Zero out this offest in your custom scale. Keep in mind whatever static 'offset' you see displayed in your data you need to enter the OPPOSITE in your y intercept. In other words, if you have 0.5" of offset with the sensor at rest (or connected to your test article) you need to enter "-0.5" in the y-intercept.
• With the string pot zero'd at its fully retracted position, use a tape measure to physically measure X amount of string and verify this to what Signal Express is showing.
  
  Just to be clear; you're using 1 volt of excitation???

[vtttt]

I used a fixed frame belonging to the lab, which string pot could be attached to. Sorry I do not have my camera with me. When they are attached, I used 1 inch from the bottom of that "frame" as '1 inch' in my calibration, and '2 inches' from the bottom as '2 inches'. Without detaching them, this '1 inch' place should cospondding to ' 1 inch' in SignalExpress. I will try to show some pics later. 

Since the slope is right, and it is all I need. Probably it is not a big problem.......

I used 10v excitation. 

Thank you so much.