PID-filter routine (2nd try).

[HankMcSpank]

Cool Henrik, I think I may have a use for this

Ok, this is a whole new area to me, here's what I think I've grasped wrt how this works ....

You have a desired value (setpoint), you take an incoming ADC reading & subtract it from your desired value....the PID sub routine then gets called & it returns a value of pid_out ?

Am I right so far?!!

Ok, what to do with pid_out?

For my intended use, I want to control a 256 step digital SPI pot (which alters the magnitude of an analog signal - an AGC in essence)...I have a 'desired value' ADC sample (a simple 8 bit ADC sample of the wiper from a std analogue pot ittingt between VCC & Gnd) that I want the signal to 'hit', but obviously the digital pot needs to either increment or decrement towards getting the signal to meet the desired value - I'm wondering how I translate the "Direction = pid_Out.15" to be increment/decrementof thedigital pot ...or does it not matter - ie will the value of pid_out just be a value that I can write direct to the digital pot?

What do I need to tweak with this bit...

Main_Loop:
    gosub adc
    pid_Error = Setpoint - ADValue      'Calculate the error
    Gosub PID                          'Result returned in pid_Drive
    Direction = pid_Out.15             'Set direction pin accordning to sign
    pid_Out = ABS pid_Out              'Convert from two's comp. to absolute
how to control the  digital pot here?!!!
    Pause 10                           'Wait....
    Goto Main_Loop                  '...and do it again.

I’m controlling my digital pot like thus…

write_VR1:    
    LOW CS    
    SSPBUF = VR1_Select   'put the command byte into SSPBUF
    gosub letclear
    SSPBUF = VR1_Position   'put the data byte into SSPBUF
    gosub letclear
    high CS
    RETURN
    
    letclear:  
    IF PIR1.3 = 0 Then letclear  ' wait for SPI interupt flag  
    PauseUs 10         ' 25uS fudge factor  
    PIR1.3 = 0        ' clear buffer full status
    Return

I’m figuring it’s just a case of something like GOSUB write_VR1 & then within there having the data byte be pid_out?

Hand holdng warmly accepted!

[HenrikOlsson]

Hello,

You have a desired value (setpoint), you take an incoming ADC reading & subtract it from your desired value....the PID sub routine then gets called & it returns a value of pid_out ?

Am I right so far?!!

Yes, or the other way around depening on the polarity of the feedback signal in relation to the polarity of the drive signal. So
pid_Error = Setpoint - ActualValue
or
pid_Error = ActualValue - SetPoint

Ok, what to do with pid_out?

That is really up to you ;-)

I'm wondering how I translate the "Direction = pid_Out.15" to be increment/decrementof thedigital pot ...or does it not matter - ie will the value of pid_out just be a value that I can write direct to the digital pot?

Yes, from my understanding of your application your input signal (setpoint) ranges from 0 to 255 and you want the output of the filter to range from 0 to 255 as well. The direction bit in this case is set when the filter wants negative output, not decreasing but negative, something you can't get with your setup.

So, what you need to do is to clamp the output to 0 with something like:

If pid_Out.15 THEN pid_Out = 0

Also, to clamp the output to a maximum of 255 you should set pid_Out_Clamp to 255. Or, which is actually better because it gives the filter more "resolution", set it to 1024 and then divide the final output by 4 before writing it to the pot.

Good luck!
/Henrik.