Rockwell Automation 1747-PT1, D1747NP002 Hand-Held Terminal - Manual (Page 360)

Chapter 26
PID Instruction
26–16
Normally, when the (CV) Output % is changed, the scaled value in the
“Output” location is changed. This value is a number from 0 to 16383
corresponding to the (CV) Output % of 0 to 100. Although the (CV) Output
% is displayed in the control block (word 16), modifying this word in the
manual mode has no effect on the “Output” value. When you are in the
manual mode, the scaled value in the “Output” location can be changed in
either of two ways:
•
Use the Zoom display to change the (CV) Output % in the Run mode, or
•
Write a ladder program that will convert the (CV) Output % to an analog
value and place it into the “Output” (or “Control Variable”) location. An
example of this is shown on page 26–20.
The following paragraphs discuss:
•
Input/Output Ranges
•
Scaling to Engineering Units
•
Zero-crossing Deadband
•
Output Alarms
•
Output Limiting with Anti-reset Windup
•
The Manual Mode
•
Feed Forward
•
Time Proportioning Outputs
Input/Output Ranges
The input module measuring the process variable (PV) must have a full scale
binary range of 0 to 16383. If this value is less than 0 (bit 15 set), then a
value of zero will be used for PV and the “Process var out of range” bit will
be set (bit 12 of word 0 in the control block). If the process variable is >
16383 (bit 14 set), then a value of 16383 is used for PV and the “Process var
out of range” bit is set.
The Control Variable, calculated by the PID instruction, has the same range
of 0 to 16383. The Control Output (word 16 of the control block) has the
range of 0 to 100%. You can set lower and upper limits for the instruction’s
calculated output values (where an upper limit of 100% corresponds to a
Control Variable limit of 16383).
Scaling to Engineering Units
Scaling lets you enter the setpoint and zero-crossing deadband values in
engineering units, and to display the process variable and error values in the
same engineering units. Remember, the process variable PV must still be
within the range 0 to 16383.
Application Notes