Scaling??

I posted a program using an analog input (see other posts) on a MicroLogix 1000 and someone suggested I scale the input before using the limits. In the SLC help files, it says to use this instruction to convert 4-20mA input to a PID variable or use SCL to scale an analog input to control an analog output. My question is, do I really need to scale this? I'm not using analog out, nor is the input used for PID. Also, if I use the SCL, do I put it in both rungs ahead of the limits, or just the first rung? Thanks for all help in advance!

The scale function simply ranges the data.

It converts 2's complement numbers to a range of your choosing that have some "real world meaning".

You just enter the Raw value high and low limits, and the high and low limits of your desired engineering value. You also have to enter the source(your input card data addresssomething like I:2.0) and the destination of the scaled data.(usually an Integer word like N7:0)

Example
Raw low = 0
Raw high = 32767
Engineering low = 0
Engineering high = 100
If the input card sees 0 SCP writes 0 to the output
If the input card sees 32767 SCP writes 100 to the output
And proportionaly every other value in between.

SCP can be used for a LOT of other purposes than just scaling for PID or and output card.

I hope that helps a little. The instruction help file in Logix500 is pretty good at explaining this too.

I am sure I said something the regulars will dispute, but that's the BASIC nature of the instruction I think.

By the way you can put the instruction anywhere you want unless you write a lot of "scan dependant logic". (not likely)

I usually put my scaling all together in one routine if there is a lot of it.

You should only need one scale function per input.

Good Luck

It isn't necessary to scale the input data, just very convenient.

Us humans think in terms of Engineering Units. If we are controlling a flow, for example, we know we want 700 gpm and that 200 gpm is low and 1500 gpm is high. When you are testing and debugging a program it is faster and simpler to have all of your comparisons and limts and so on in these units. Otherwise, everytime you log on and look at the input data of 15283 you have to grab a calculator and convert it to a meaningful number.

This may not be important if you have only one analog input, but if you have a lot of them it is very important.

ok...let me see if I understand this correctly. Hypothetically, rung one has the SCL instruction. Source is the 0-10VDC analog input (my case, it's I:0.5). Rate is (scaled max-min / input max-min), or 3.05 in my case. Not sure about offset...but the destination is N7:0.

Next rung has GRT & LES ahead of B3:3/0. GTR is Source A N7:0 Source B is '3'. LES is Source A N7:0 Source B is '7'. B3:3/0 will only energize if if the voltage is between 3&7 VDC, correct?

One rung down has a GRT ahead of B3:3/1 and the GRT is Source a N7:0 Source B is '7'. B3:3/1 will energize when N7:0 is at 7+ VDC. Correct, or am I missing something? Please help!!!

Not quite, unless you are really measuring voltage. Scale the input to a real term ie if 0V = 0 gallons per minute flow and 10 Volts = 1000 gallons a minute then scale the analog to this. That way your 7 volts is equal to a flow of 700 gallons per hour. N7:0 will read as 700 and when you look at your comparisons you can work in real engineering units.
Regards Alan Case

RSL said:

.....
SCP can be used for a LOT of other purposes than just scaling for PID or and output card.
....

Click to expand...

Too bad the ML1000 doesn't support the SCP instruction, just the SCL instruction which is not as flexible (meaning you've got to do some of the work yourself or using some other instructions to set it up).