Publication 1766-RM001A-EN-P - October 2008
342
Process Control Instruction
Time Mode (TM)
The time mode bit specifies when the PID is in timed mode (1) or STI
mode (0). This bit can be set or cleared by instructions in your ladder
program.
When set for timed mode, the PID updates the CV at the rate specified in
the loop update parameter (PD10:0.LUT).
When set for STI mode, the PID updates the CV every time the PID
instruction is scanned in the control program. When you select STI,
program the PID instruction in the STI interrupt subroutine. The STI
routine should have a time interval equal to the setting of the PID “loop
update” parameter (PD10:0.LUT). Set the STI period in word STI:0.SPM.
For example, if the loop update time contains the value 10 (for 100 ms),
then the STI time interval must also equal 100 (for 100 ms).
Loop Update Time (LUT)
The loop update time (word 13) is the time interval between PID
calculations. The entry is in 0.01 second intervals. Enter a loop update
time five to ten times faster than the natural period of the load. The
natural period of the load is determined by setting the reset and rate
parameters to zero and then increasing the gain until the output begins to
oscillate. When in STI mode, this value must equal the STI time interval
value loaded in STI:0.SPM. The valid range is 0.01…10.24 seconds.
Tuning Parameter
Descriptions
Address
Data
Format
Range
Type
User Program
Access
TM - Time Mode
PD10:0.TM
binary
0 or 1
control
read/write
TIP
When using timed mode, your processor scan time should be at least
ten times faster than the loop update time to prevent timing
inaccuracies or disturbances.
Tuning Parameter
Descriptions
Address
Data Format Range
Type
User Program
Access
LUT - Loop Update Time
PD10:0.LUT
word (INT)
1…1024
control
read/write
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