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User Manual
GFK-1503F
This operation, with the Manual Command tracking CV in Automatic mode and setting CV
in Manual mode, provides a bump-less transfer between Automatic and Manual modes. The
CV Upper and Lower Clamps and the Minimum Slew Time always apply to the CV output in
Manual mode and the integral term is always updated. This assures that when a user rapidly
changes the Manual.
Command value in Manual mode, the CV output cannot change any faster than the slew
rate limit set by the Minimum Slew Time, and the CV cannot go above the CV Upper Clamp
limit or below the CV Lower Clamp limit.
14.3.3
Time Interval for the PID Function
The start time of each PLC sweep is used as the current time when calculating the time
interval between solutions of the PID function. The times and time intervals have a
resolution of 100 microseconds. When an application uses multiple PID functions, all of
them use the same time value.
The PID algorithm is solved when the current time is equal to or greater than the time of the
last PID solution plus the Sample Period or 10 milliseconds; whichever is larger. If the Sample
Period is set for execution on every sweep (value = 0), the PID function is restricted to a
minimum of 10 milliseconds between solutions. If the sweep time is less than 10
milliseconds, the PID function waits until enough sweeps have occurred to accumulate an
elapsed time of 10 milliseconds. For example, if the sweep time is 9 milliseconds, the PID
function executes every other sweep, and the time interval between solutions is 18
milliseconds. If a specific PID function is executed more than once per sweep (by referencing
the same reference array location in multiple PID function blocks), the algorithm is solved
only on the first call.
The longest possible interval between executions is 65,535 times 10 milliseconds, or 10
minutes, 55.35 seconds.
14.4
PID Algorithm Selection (PIDISA or PIDIND) and
Gain Calculations
The PID function supports both the Independent Term (PID_IND) and ISA standard
(PID_ISA) forms of the PID algorithm. The Independent Term form takes its name from the
fact that the coefficients for the proportional, integral and derivative terms act
independently. The ISA algorithm is named for the Instrument Society of America (now the
International Society for Measurement and Control), which standardized and promoted it.
The two algorithms differ in how words 6 through 8 of the reference array are used and in
how the PID output (CV) is calculated.
The Independent term PID (PID_IND) algorithm calculates the output as:
PID Output = Kp * Error + Ki * Error * dt + Kd * Derivative + CV Bias
where Kp is the proportional gain, Ki is the integral rate, Kd is the derivative time, and dt is
the time interval since the last solution.
PID Built-in Function Block
Chapter 14
Jan 2020
261
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