PIDLoop¶
The PIDLoop has multiple constructors available. Valid syntax can be seen here:
// Create a loop with default parameters
// kp = 1, ki = 0, kd = 0
// maxoutput = maximum number value
// minoutput = minimum number value
SET PID TO PIDLOOP().
// Other constructors include:
SET PID TO PIDLOOP(KP).
SET PID TO PIDLOOP(KP, KI, KD).
// you must specify both minimum and maximum output directly.
SET PID TO PIDLOOP(KP, KI, KD, MINOUTPUT, MAXOUTPUT).
// remember to update both minimum and maximum output if the value changes symmetrically
SET LIMIT TO 0.5.
SET PID:MAXOUTPUT TO LIMIT.
SET PID:MINOUTPUT TO LIMIT.
// call the update suffix to get the output
SET OUT TO PID:UPDATE(TIME:SECONDS, IN).
// you can also get the output value later from the PIDLoop object
SET OUT TO PID:OUTPUT.
Please see the bottom of this page for information on the derivation of the loop’s output.

structure
PIDLoop
¶ Suffix Type Description LASTSAMPLETIME
scalar
decimal value of the last sample time KP
scalar
The proportional gain factor KI
scalar
The integral gain factor KD
scalar
The derivative gain factor INPUT
scalar
The most recent input value SETPOINT
scalar
The current setpoint ERROR
scalar
The most recent error value OUTPUT
scalar
The most recent output value MAXOUTPUT
scalar
The maximum output value MINOUTPUT
scalar
The maximum output value ERRORSUM
scalar
The time weighted sum of error PTERM
scalar
The proportional component of output ITERM
scalar
The integral component of output DTERM
scalar
The derivative component of output CHANGERATE
scalar
(/s)The most recent input rate of change RESET
none Reset the integral component UPDATE(time, input)
scalar
Returns output based on time and input

PIDLoop:
LASTSAMPLETIME
¶ Type: scalar
Access: Get only The value representing the time of the last sample. This value is equal to the time parameter of the
UPDATE
method.

PIDLoop:
INPUT
¶ Type: scalar
Access: Get only The value representing the input of the last sample. This value is equal to the input parameter of the
UPDATE
method.

PIDLoop:
SETPOINT
¶ Type: scalar
Access: Get/Set The current setpoint. This is the value to which input is compared when
UPDATE
is called. It may not be synced with the last sample.

PIDLoop:
ERROR
¶ Type: scalar
Access: Get only The calculated error from the last sample (setpoint  input).

PIDLoop:
MAXOUTPUT
¶ Type: scalar
Access: Get/Set The current maximum output value. This value also helps with regulating integral wind up mitigation.

PIDLoop:
MINOUTPUT
¶ Type: scalar
Access: Get/Set The current minimum output value. This value also helps with regulating integral wind up mitigation.

PIDLoop:
ERRORSUM
¶ Type: scalar
Access: Get only The value representing the time weighted sum of all errrors. It will be equal to
ITERM
/KI
. This value is adjusted by the integral windup mitigation logic.

PIDLoop:
PTERM
¶ Type: scalar
Access: Get only The value representing the proportional component of
OUTPUT
.

PIDLoop:
ITERM
¶ Type: scalar
Access: Get only The value representing the integral component of
OUTPUT
. This value is adjusted by the integral windup mitigation logic.

PIDLoop:
DTERM
¶ Type: scalar
Access: Get only The value representing the derivative component of
OUTPUT
.

PIDLoop:
CHANGERATE
¶ Type: scalar
Access: Get only The rate of change of the
INPUT
during the last sample. It will be equal to (input  last input) / (change in time).

PIDLoop:
RESET
()¶ Returns: none Call this method to clear the
ERRORSUM
andITERM
components of the PID calculation.

PIDLoop:
UPDATE
(time, input)¶ Parameters: Returns: scalar
representing the calculated outputUpon calling this method, the PIDLoop will calculate the output based on this this basic framework (see below for detailed derivation): output = error * kp + errorsum * ki + (change in input) / (change in time) * kd. This method is usually called with the current time in seconds (TIME:SECONDS), however it may be called using whatever rate measurement you prefer. Windup mitigation is included, based on
MAXOUTPUT
andMINOUTPUT
. Both integral components and derivative components are guarded against a change in time greater than 1s, and will not be calculated on the first iteration.
PIDLoop Update Derivation¶
The internal update method of the PIDLoop
structure is the equivalent of the following in kerboscript
// assume that the terms LastInput, LastSampleTime, ErrorSum, Kp, Ki, Kd, Setpoint, MinOutput, and MaxOutput are previously defined
declare function Update {
declare parameter sampleTime, input.
set Error to Setpoint  input.
set PTerm to error * Kp.
set ITerm to 0.
set DTerm to 0.
if (LastSampleTime < sampleTime) {
set dt to sampleTime  LastSampleTime.
// only calculate integral and derivative if their gain is not 0.
if Ki <> 0 {
set ITerm to (ErrorSum + Error * dt) * Ki.
}
set ChangeRate to (input  LastInput) / dt.
if Kd <> 0 {
set DTerm to ChangeRate * Kd.
}
}
set Output to pTerm + iTerm + dTerm.
// if the output goes beyond the max/min limits, reset it and adjust ITerm.
if Output > MaxOutput {
set Output to MaxOutput.
// adjust the value of ITerm as well to prevent the value
// from winding up out of control.
if (Ki <> 0) and (LastSampleTime < sampleTime) {
set ITerm to Output  min(Pterm + DTerm, MaxOutput).
}
}
else if Output < MinOutput {
set Output to MinOutput.
// adjust the value of ITerm as well to prevent the value
// from winding up out of control.
if (Ki <> 0) and (LastSampleTime < sampleTime) {
set ITerm to Output  max(Pterm + DTerm, MinOutput).
}
}
set LastSampleTime to sampleTime.
set LastInput to input.
if Ki <> 0 set ErrorSum to ITerm / Ki.
else set ErrorSum to 0.
return Output.
}