Installation manual

Publication 1747-RM001G-EN-P - November 2008
Proportional Integral Derivative Instruction 9-23
PID and Analog I/O Scaling
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 below.
1. 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.
2. 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 are 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)