User guide

Chapter 26
PID Instruction
26–17
Select scaling as follows:
1. Enter the maximum and minimum scaling values Smax and Smin in the
PID control block. Refer to the control block of the PID instruction on
page 26–9. The Smin value corresponds to an analog value of zero for
the lowest reading of the process variable, and Smax corresponds to an
analog value of 16383 for the highest reading. These values reflect the
process limits. Setpoint scaling is selected by entering a non-zero value
for one or both parameters. If you enter the same value for both
parameters, setpoint scaling is disabled.
For example, if measuring a full scale temperature range of – 73 (PV=0)
to +1156
° C (PV=16383), enter a value of –73 for Smin and 1156 for
Smax. Remember that inputs to the PID instruction must be 0 to 16383.
Signal conversions could be as follows:
Process limit –73 to +1156
o
C
Transmitter output (if used) +4 to +20 mA
Output of analog input module 0 to 16383mA
PID instruction, Smin to Smax –73 to +1156
o
C
2. Enter the setpoint (word 2) and deadband (word 9) in the same scaled
engineering units. Read the scaled process variable and scaled error in
the control block as well. The control output (word 16) is displayed as a
percentage of the 0 to 16383 range. The output transferred to the output
modules is always unscaled.
When you select scaling, the instruction scales the setpoint, deadband,
process variable, and error. You must consider the effect on all these
variables when you change scaling.
Zero-crossing Deadband DB
The adjustable deadband lets you select an error range above and below the
setpoint where the output does not change as long as the error remains within
this range. This lets you control how closely the process variable matches
the setpoint without changing the output.
+DB
SP
–DB
Error Range
Time
Zero-crossing is deadband control that lets the instruction use the error for
computational purposes as the process variable crosses into the deadband
until it crosses the setpoint. Once it crosses the setpoint (error crosses zero
and changes sign) and as long as it remains in the deadband, the instruction
considers the error value zero for computational purposes.
Select deadband by entering a value in the deadband storage word (word 9)
in the control block. The deadband extends above and below the setpoint by
the value you enter. A value of zero inhibits this feature. The deadband has
the same scaled units as the setpoint if you choose scaling.