Installation manual

Publication 1747-RM001G-EN-P - November 2008
9-8 Proportional Integral Derivative Instruction
When set for STI mode, the PID executes and 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. Set the STI period in word S:30. 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
The loop update (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 S:30. The valid range is 0.01 to
10.24 seconds. See page 11-12 for help entering STI setpoint.
Deadband
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.
Table 9.7 Loop Update Parameter
Tuning Parameter
Descriptions
Address Data Format Range Type User
Program
Access
Loop Update Word 13 word (INT) 1 to 1024 control read/write
Table 9.8 Deadband Parameter
Tuning Parameter
Descriptions
Address Data
Format
Range Type User Program
Access
DB - Deadband Word 9 word
(INT)
0 to 32,767 control read/write