Rockwell Automation 1747-PT1, D1747NP002 Hand-Held Terminal - Manual (Page 356)

Chapter 26
PID Instruction
26–12
For the SLC 500 PID instruction, the numerical scale for both the process
variable (PV) and the control variable (CV) is 0 to 16383. To use
engineering units, such as PSI or degrees, you must first scale your analog
I/O ranges within the above numerical scale. To do this, use the Scale (SCL)
instruction and follow the steps described below. Refer to the Analog I/O
Modules User Manual, catalog number 1746–NM003 for more information.
Scale your analog input by calculating the slope (or rate) of the analog input
range to the PV range (0 to 16383.) For example, an analog input with a
range of 4 to 20mA has a decimal range of 3277 to 16384. The decimal
range must be scaled across the range of 0 to 16383 for use as PV.
Scale the CV to span evenly across your analog output range. For example,
an analog output which is scaled at 4 to 20mA has a decimal range of 6242 to
31208. In this case, 0 to 16383 must be scaled across the range of 6242 to
31208.
Once you have scaled your analog I/O ranges to/from the PID instruction,
you can enter the minimum and maximum engineering units that apply to
your application. For example, if the 4 to 20mA analog input range
represents 0 to 300 PSI, you can enter 0 and 300 as the minimum (Smin) and
maximum (Smax) parameters respectively. The Process Variable, Error,
Setpoint, and Deadband will be displayed in engineering units in the PID
Data Monitor screen. Setpoint and Deadband can be entered into the PID
instruction using engineering units.
The following equations show the linear relationship between the input value
and the resulting scaled value.
Scaled value = (input value x slope) + offset
Slope = (scaled max. – scaled min.) / (input max. – input min)
Offset = scaled min. – (input min. x slope)
Use the following values in an SCL instruction to scale common analog input
ranges to PID process variables.
Parameter 4 to 20mA 0 to 5V 0 to 10V
Rate/10,000 12,499 10,000 5,000
Offset –4096 0 0
Use the following values in an SCL instruction to scale control variables to
common analog outputs.
Parameter 4 to 20mA 0 to 5V 0 to 10V
Rate/10,000 15,239 10,000 19,999
Offset 6242 0 0
PID and Analog I/O Scaling