PID control is hard.  We’ve got a tutorial that I don’t understand, but there is something much simpler that’s drop dead easy to implement in SPLat or any other programming language.

PI control is the way.  When we’ve got some time I’ll draw some pictures and explain more about it, but our customers have used this for things as diverse as hot air temperature regulation and concrete truck barrel speed control.  It works and isn’t hard to get running quickly.

Here’s a graph collected from the hot air blower.  In this instance:

• Kc = 12
• Ti = 100
• Limit = 70

Colour explanation:

• Green = Temperature requested by the user
• Red = Measured temperature
• Pale Blue = The drive (%) sent to the heater.  The heater control is not fully proportional, it only has 8 steps, about 12.5% each.
• Gray = The “reset” value – note it clips at 70 to prevent “wind-up”.

For your own use, you’ll have to mess around tuning the kfKc and kfTi values, but it’s nowhere near as hard as doing a PID.

;algorithm constants
kfKc                    fEQU  12.0
kfTi                    fEQU  100.0
kfLimit                 fEQU  70.0                    ;prevent windup

;runtime values
fTarget                 defFLOAT                      ;target value
fMeasured               defFLOAT                      ;current measured value
fOutput                 defFLOAT                      ;current value to output

;algorithm vars
fReset                  defFLOAT
fError                  defFLOAT


;===============================================================================
;DESCRIPTION:
; This subroutine is called on a regular tick interval to calculate the next
; output value.
;PARAMETERS:
;  -> Nil
;RETURNS:
; <-  Nil
;===============================================================================
subDoPIAlgorithm:
   ; Error  = Setpoint - Measured
   fRecallQ       fTarget                             ;get the target value
   fRecallW       fMeasured                           ;get the actual value
   fSub                                               ;calc Target - Actual
   fStore         fError

   ; Reset  = Reset + Kc/Ti * Error
   fLoadQ         kfKc
   fMul
   fWtoQ
   fLoadW         kfTi
   fDiv
   fRecallQ       fReset
   fAdd
   fLoadQ         kfLimit                             ;limit integral to..
   fGoIfWltQ      _THChkResetLow
   fSwap
_THChkResetLow
   fLoadQ         0.0
   fGoIfWgtQ      _THSaveReset
   fSwap                                              ;..prevent "wind-up"
_THSaveReset
   fStore         fReset

   ; Output = Kc * Error + Reset
   fLoadW         kfKc
   fRecallQ       fError
   fMul
   fRecallQ       fReset
   fAdd

   ; limit output to 0 - 100%
   fLoadQ         0.0
   fGoIfWgeQ      _THTestMax
   fSwap
_THTestMax
   fLoadQ         100.0
   fGoIfWleQ      _THDoDrive
   fSwap

_THDoDrive
   fStore         fOutput
   Return