Allen Bradley CPT Instruction

Ones_Zeros · Mar 23, 2017

Hey Everyone
I was wanting to get some advise on the math below on this expression
for a SCP instruction

(75.0-10.0)*(Suction_Pressure-4000)/16000+0.0 = 10.37

My transmitter is scaled +75 to -10
how would i change the above expression to include the -10?

the result should come out to around 0.2

thanks

janner_10 · Mar 23, 2017

You can download a scp AOI from the knowledge base for ladder if you don't have fb.

Ones_Zeros · Mar 23, 2017

I'm sorry i meant to say CPT instruction
been long day

I have the SCP add on instruction downloaded but I would have to take the process down to download it into the controller

i apologize

PapaSmiff · Mar 23, 2017

Well, I solved for Suction_Pressure and got 4255.26 - is this correct for your system?

Math-wise, I saw two options:
1. Change (75.0-10.0) to (75.0+10.0). This gave a result (with the above Suction_Pressure) of about 1.35.
2. Change the 0.0 at the end to 10.0. This assumes that the original expression can be written ((75.0-10.0)*(Suction_Pressure-4000)/16000)+0.0. This, of course, would give a result of 0.37.

Do either of these look like they would work for you?

Ones_Zeros · Mar 23, 2017

Math-wise, I saw two options:
1. Change (75.0-10.0) to (75.0+10.0). This gave a result (with the above Suction_Pressure) of about 1.35.
2. Change the 0.0 at the end to 10.0. This assumes that the original expression can be written ((75.0-10.0)*(Suction_Pressure-4000)/16000)+0.0. This, of course, would give a result of 0.37.

Yeah,
the end result I was looking for was around 0.2

raw suction_pressure value = 6553

(75.0-10.0)*(Suction_Pressure-4000)/16000+0.0 = 10.37

(75.0-10.0)*(6553-4000)/16000+0.0
so would i change the +0.0 at the end of the equation to -10?

thanks

PapaSmiff · Mar 23, 2017

Yep, I went back and did it over again. If I read your original formula correctly (that is, it can be written with the extra parenthesis as I did), the +0.0 at the end of the formula can be changed to -10 to provide an answer of 0.37.

Again, this is just me looking at math. I may not be looking at your process correctly.

Mickey · Mar 23, 2017

The -10 is your scaled min,see text..

Ones_Zeros · Mar 23, 2017

Hello Mickey,

(((Scaled Max-Scaled Min)/(Max Raw Input-Min Raw Input))*(Raw Input-Min Raw Input))+Scaled Min = Scaled Value

Since my scaled Min is -10 & scaled Max is 75
Would I still enter this in the expression as 75-10 or 75 - (-10) ?

(75-10)*(6553-4000)/16000-10 = value

Thanks

kamenges · Mar 23, 2017

The total expression would be:

((75-(-10))*((Input-4000)/16000))+(-10)

Keith

rdrast · Mar 24, 2017

1: You can import AOI's online, unless there are versioning differences. If there are, open the AOI in a demo program offline, create a new one in the online program, and copy every thing over, manually if necessary.

2: This is simple math. If you want to see how it works, plug it into Excel or something similar, and play with the numbers.

Ones_Zeros · Mar 24, 2017

Seems I'm have issues inputing my data into the formula to get the correct value.
The final value should be around 0.2 to 0.3

(((Scaled Max-Scaled Min)/(Max Raw Input-Min Raw Input))*(Raw Input-Min Raw Input))+Scaled Min = Scaled Value

the data is
scaled Max = +75
scaled min = -10
Raw In = 6546
Max raw input = 16000
Min Raw input = 4000

the final value should be around 0.2 to 0.3

i just want to make sure i plug my values into this expression correctly

Thanks for the help

Ron Beaufort · Mar 24, 2017

The final value should be around 0.2 to 0.3

where did you get those numbers? ...

the scaled value that I get for an input of 6546 comes out to 8.034 and change ...

but then again - maybe the formula that you're giving here is not a "standard" y=mx+b ... I'll take a look at it when I get some spare time ...

ELake20 · Mar 24, 2017

The math doesn't lie...

You have provided incorrect scaling data to achieve what you are looking for

Ones_Zeros · Mar 24, 2017

the data is
scaled Max = +75
scaled min = -10
Raw In = 6546
Max raw input = 16000
Min Raw input = 4000

the final value should be around 0.2 to 0.3

here is what i know & what normally we have scaled the setting on the
1769-IF8 analog card
the transmitter is scaled +75 to -10
so this should be my scaled max = +75 and min = -10
the input Max = 16000 & Min = 4000 (based on the scaling setting on the 1769-IF8 analog input card) I might have these wrong
the Raw input 6546 is the actual 4-20ma raw value coming into the card from the transmitter

the actual read out on the transmitter is somewhere around 0.2 to 0.3
so based on the scaling on the transmitter i know this is what my final value needs to be after working out the expression

I may not have these values correct since the expression is not working out correctly
what ever i need to change on the expression, please let me know

thanks alot i appreciate you guys looking over this

dmargineau · Mar 24, 2017

Are you sure the transmitter is actually displaying a true value?

You can't quite presume the transmitter is "good" by default; instrumentation needs to be 'digitally" calibrated but not before empirically proven to be true.

Measure (using dedicated means) the respective physical variable: Pressure -> Manometer; Temperature -> Thermometer; Distance -> Micrometer/Caliper, etc.

After empirically establishing the present process variable value, measure the sensor's electrical output signal value corresponding to the present physical value (mA meter for current devices or V meter for voltage devices).

Only after the two steps had been completed and proven to be accurate one should go to the next step and acquire the true digital value within the controller application.

Allen Bradley CPT Instruction

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