How can I use negative in scale function?

narky · Dec 3, 2013

Hi, I'm using thermistor as my analog input. Thermister is a negative coefficient and AB scaling seem to only accept positive(this could also be my limited brain cells). I was hoping to use N7 through SCP function but it wont accept Min > Max. SCL slope rates seem to only accept positive decimal number so now I'm stuck. Can you please help?

dmargineau · Dec 3, 2013

What platform

LC, ML, SLC?
What do you mean by a "negative coefficient"?...

If, for example, the "thermister" "puts" out 32767 at Min. Temperature and 0 at Max. Temperature then make Input Min. =32767 and Input Max.=0 while Scaled Min.=32 (Deg.F)and Scaled Max.=212 (Deg.F) if following the water states temperatures for example.

bernie_carlton · Dec 3, 2013

If you already have determined the scale and offset then just do the math individually. Don't worry about SCP or SCL. They don't do anything magic that can't be done by a couple of math operations or even a CPT instruction.

narky · Dec 3, 2013

dmargineau said:
What platformLC, ML, SLC?
What do you mean by a "negative coefficient"?...
If, for example, the "thermister" "puts" out 32767 at Min. Temperature and 0 at Max. Temperature then make Input Min. =32767 and Input Max.=0 while Scaled Min.=32 (Deg.F)and Scaled Max.=212 (Deg.F) if following the water states temperatures for example.

Hi, this for ML. SCP won't let me do as above. Min can't be more than Max. Additional SCL won't let me put negative decimal for slope. I'll try to use integer file and see if I can indirect SCL to it.

narky · Dec 3, 2013

bernie_carlton said:
If you already have determined the scale and offset then just do the math individually. Don't worry about SCP or SCL. They don't do anything magic that can't be done by a couple of math operations or even a CPT instruction.

Thk, didn't thought about CPT instruction. Will try that tonight.

Mickey · Dec 3, 2013

Curious, how are you connecting the thermistor to your processor?

A transmitter with a 4-20ma output would be a better choice.

http://www.acromag.com/catalog/997

https://www.google.com/#q=thermistor+transmitter

narky · Dec 3, 2013

Mickey said:
Curious, how are you connecting the thermistor to your processor?

A transmitter with a 4-20ma output would be a better choice.

http://www.acromag.com/catalog/997

https://www.google.com/#q=thermistor+transmitter

This is only as class project. Its a used car temp sensor that I found in the garage. Instead of using it as 4-20, I connect resistor in parallel and then use 0-5vdc as signal. This way the signal is almost in straight line linear. It calibrated it with water and seem accurate. Now I just have to get PLC to convert to *F,lol.

Mickey · Dec 3, 2013

See picture below the SCP should work.

Your scaling values may differ. How do you have the analog input's raw data configured?

Which ML are you using?

steves0118 · Dec 3, 2013

Equations

These are the equations that I use to scale inputs in the attached Logix 5000 Add-On Instruction. Negative coefficients (Rate) shouldn't be a problem.

Rate = (Scaled_Max - Scaled_Min) / (Input_Max - Input_Min)
Output = (Raw_Input_Unscaled - Input_Min) * Rate + Scaled_Min

narky · Dec 3, 2013

Mickey said:
See picture below the SCP should work.

Your scaling values may differ. How do you have the analog input's raw data configured?

Which ML are you using?

Thank you so much. It works like a charm!!!. Temp is exact but I'm expecting it to offset about 1% near top/bottom.

Also just found out that this 1500 doesn't have CPT nor advance maths support. I could use LN function to do 3 point equation to make this less than 1% error. Now are these maths for SLC version???

THK for all helps. Now I got to put it on Excel for class presentation.

Mickey · Dec 3, 2013

Now are these maths for SLC version???

In the SLC5/03,04 and 05, yes.
In the SLC5/01 and 02 no.

They are also in the ML1400.

How can I use negative in scale function?

narky

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