Rockwell Automation 1747-PT1, D1747NP002 Hand-Held Terminal - Manual (Page 351)

Chapter 26
PID Instruction
26–7
•
Scaled setpoint minimum (Smin) (control block word 8) – If the
setpoint is to read in engineering units, then this parameter corresponds to
the value of the setpoint in engineering units when the control input is
zero. Range: –16383 to +16383.
•
Scaled setpoint maximum (Smax) (control block word 7) – If the
setpoint is to read in engineering units, then this parameter corresponds to
the value of the setpoint in engineering units when the control input is
16383. Range: –16383 to +16383.
Note: Smin – Smax scaling allows you to enter the setpoint in
engineering units. The deadband plus error will also be displayed in
engineering units. The process variable PV will still be expected to be
within the range 0 –16383. That is, Smin – Smax scaling provides a full
resolution PID calculation.
•
Mode (control block word 0, bit 0) – STI is the default condition.
TIMED indicates that the PID updates its output at the rate specified in
the loop update parameter (word 13); STI indicates that the PID updates
its output every time it is scanned. When you select STI, the PID
instruction should be programmed in an STI interrupt subroutine, and the
STI routine should have a time interval (STI period S:30) equal to the
setting of the PID “loop update” parameter (word 13). For example, if
the loop update time contains the value 10 (for 100 ms), then the STI time
interval must also equal 10.
•
Output limit (control block word 0, bit 3) – Select YES if you want to
limit the output to minimum and maximum values:
Output YES
Output Limiting Selected
NO
Output Limiting Deselected
min The value you enter will be the minimum
output percent that the control variable
CV will obtain.
If CV drops below this minimum value,
the following will occur:
•
CV will be set to the value you
entered, and
•
the output alarm, lower limit LL bit
will be set.
The value you enter will determine when
the output alarm, lower limit bit is set.
If CV drops below this minimum value,
the output alarm, lower limit LL bit will be
set.
max The value you enter will be the maximum
output percent that the control variable
CV will obtain.
If CV exceeds this maximum value, the
following will occur:
•
CV will be set to the value you
entered, and
•
the output alarm, upper limit UL bit
will be set.
The value you enter will determine when
the output alarm, upper limit bit is set.
If CV exceeds this maximum value, the
output alarm, upper limit UL bit will be
set.