PID Filter Coefficient

[JESmitter]

Thanks for the reply Henrik,

I thought that I might have to disable that feature in Simulink. The PID Tuner should still be able to generate the optimum P, I, and D gains.

I can add noise filtering (a few R's and C's) to my thermocouple amplifier that senses the flame temperature and provides the feedback signal.

If you or anyone comes across a way to provide some first order filtering in the digital domain for a PIC16F1937 for this application, I would love to see it!

Thanks again,

Jacob

[HenrikOlsson]

Take a look at Tracy Allens pages on math (and other stuff) routines for the BasicStamp, specifically this page when digital filtering is covered. I've adopted one of the routines for PBP. I believe, at one point, I stuck that routine to the output of the PID filter but I've never posted it and I don't think I've done/tested anything with it. I'll leave you with the filter itself for you to what you want with.

'****************************************************************
'*  Name    : LowPassFilter.pbp                                 *
'*  Author  : Henrik Olsson                                     *
'*  Notice  : Copyright (c) 2010 Henrik Olsson 2010             *
'*          : All Rights Reserved                               *
'*  Date    : 2010-09-01                                        *
'*  Version : 1.0                                               *
'*  Notes   : Adopted from Tracy Allens BS2Math pages.          *
'*          :                                                   *
'****************************************************************

'--Variables accessed from "outside"--
LowPassFilter_Constant VAR BYTE
LowPassFilter_Input VAR WORD
LowPassFilter_Output VAR WORD

'--Variable used internally
Filter_Sign VAR BIT
Filter_Value VAR WORD
Filter_Z VAR WORD
Filter_Y VAR WORD



'--User must set LowPassFilter_Constant to 0-9 and then GOSUB InitLowPassFilter before
'--calling the actual filter routine. 

pid_LP_Filter_Init:
Select Case LowPassFilter_Constant
CASE 0
 Filter_Value = 65535
Case 1
 Filter_Value = 32768
Case 2
 Filter_Value = 21846
Case 3
 Filter_Value = 16384
Case 4
 Filter_Value = 13108
Case 5
 Filter_Value = 10923
 Case 6
 Filter_Value = 9363
 Case 7
 Filter_Value = 8192
 CASE 8
 Filter_Value = 7282
 Case 9
 Filter_Value = 6554
End Select
RETURN

LowPassFilter:

If LowPassFilter_Constant = 0 then
    LowPassFilter_Output = LowPassFilter_Input
    Goto LowPassFilterDone
ENDIF

'Max difference between current filter output and a new input to the filter
'is 2048 so it's recomended to all input values clamped to +/-1024 or unexpected
'results may occur,
 
'Complete filter executes in 174-177 cycles----

'Multiply input by 16 (shift left by 4) then subtract the current output (times 16) to get the difference. (47 cycles)
Filter_Z = LowPassFIlter_Input << 4
Filter_Z = Filter_Z - Filter_Y

'Get the sign of the difference then convert to ABSolute value (7 cycles).
Filter_Sign = Filter_Z.15               ' sign of the difference          (4 cycles) 
Filter_Z = abs Filter_Z                 ' magnitude of the difference     (3 cycles)  

'Add then divide. Basically this is the same as A=(A+1)/2, A=(A+2)/3, A=(A+3)/4 etc.
'All in all 52 cycles.
Filter_Z = Filter_Z + LowPassFilter_Constant  '(52 cycles including ** operation on next line
Filter_Z = Filter_Z ** Filter_Value

'Now the value is scaled and needs to be added to or subtracted from the output. (8 or 10 cycles)
If Filter_Sign then
  Filter_Y = Filter_Y - Filter_Z
ELSE
  Filter_Y = Filter_Y + Filter_Z
ENDIF     

Filter_Sign = Filter_Y.15           ' sign of total output  (4 cycles)

'Get absoulte value and divide by 16 to scale down the value to the same scale
'as the original input value. Then re-apply the sign. (50 or 52 cycles)
LowPassFilter_Output = (ABS Filter_Y) >> 4

If Filter_Sign then LowPassFilter_Output = -LowPassFilter_Output    '(4 or 7 cycles)

LowPassFilterDone:
RETURN