PI controller algorithm

On Apr 28, 4:26=A0pm, cerr <ron.egg...@gmail.com> wrote:
> > Read this:http://www.wescottdesign.com/articles/Sampling/pidwophd.html
>
> This article is really helpful and for sure the best of all I read
> today (and that was quite a few).
> Thank you for that, I rewrote the code and came up with below which
> should look better than what I had before:
>
> typedef struct
> {
> =A0 signed float iState;
> =A0 float iMax,
> =A0 =A0 =A0 =A0 =A0 =A0 iMin;
>
> =A0 float iGain,
> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 pGain;
>
> }SPid;
>
> SPid CntrlData;
> CntrlData.iGain=3D1;
> CntrlData.pGain=3D1;
> CntrlData.iMax=3D50;
> CntrlData.iMin=3D0;
>
> signed int8 error=3D0;
> int8 target=3D150;
>
> error =3D adcval() - target; //adcval() will return 8bit value
> CntrlLEDs(&CntrlData, error);
> //-----------------------------------------------------------------------=
-- ----
> signed float CntrlLEDs(Spid *pid, signed int8 error)
> {
> signed float pTerm,iTerm;
>
> /** P **/
> pTerm =3D pid->pGain * error;
>
> /** I **/
> pid->iState +=3D error;
>
> if (pid->iState > pid->iMax)
> =A0 pid->iState=3Dpid->iMax;
> else if (pid->iState < pid->iMin)
> =A0 pid->iState=3Dpid->iMin;
>
> iTerm =3D pid->iGain * pid->iState;
>
> return pTerm+iTerm;
>
> }
>
> Thanks again for your help and your article sure is a keeper!

Well, I'll use above PI controller for now, built it into my software
but can't test it yet as i don't have the hardware available yet...
any one know of a free controller tune application where i an enter my
possible values and maybe even let it run with my code...?

Thanks,
Ron

On Apr 29, 3:40=A0am, Paul <p...@pcserviceselectronics.co.uk> wrote:
> In article <ipclva$qq...@speranza.aioe.org>, not.going.to...@seen.com
> says...
>
>
>
> > Hi Ron,
>
> > On 4/28/2011 1:32 PM, cerr wrote:
> > > I need to resolve following problem:
> > > I have an led panel with an opto transistor on it that measures the
> > > light emitted/reflected from the target. I want to control the emitte=
d
> > > light to be equal to a certain setpoint from what the opto transistor
> > > measures. Does that make sense? I'm using an 18F pic to control the
> > > whole circuit and I wanted to come up with a PI if not a PID
> > > controller algorithm that controls my LEDs. I've been reading about
> > > PID controllers but am not 100% sure about some things. So far I've
> > > come up with below code:
>
> ......
>
> > Also, depending on what you are controlling and how it is likely
> > to *fail*, you might want to put some anti-windup protection on
> > your integral term to keep "Integrator" from becoming *so*
> > large (positive or negative) that it dominates the loop's
> > performance for an inordinately long time. =A0E.g., imagine using
> > "doubles" for these variables. =A0Now, imagine the detector is
> > obstructed for 10,000 iterations of your control loop. =A0In
> > that case, you are likely to see a big "Error" *and* accumulate
> > it repeatedly -- regardless of what your control is trying to
> > do to compensate for it! =A0When the detector is eventually
> > unobstructed, the "Output" is way out of whack for a long
> > time (until the integrator "clears").
>
> As a general rule for any electronics or algorithm (software or
> hardware) that is measuring light level from photodetectors to imaging
> sensors should alwasy cope with the error conditions first of
>
> a) The output is always minimum (obstruction/lens cap)
>
> b) The output is always maximum (pointed at sun or bright light)
>
> c) The output of the sensor is disconnected, or failed.
>
> At start up I always ignore at least the FIRST two samples to determine
> what the ambient condition is. In your case I would also every once
> in a while do a detection cycle with the emitter OFF to get an ambient
> condition. This is not used in control loop but tests the sensor is
> working if ambient is not min or max. Then with emmitter ON your error
> must be at least a minimum away from ambient for operations to be
> useable.

Well, I have a software low passfilter after I read thwe adc and i let
it swing it on bootup, meaning i loop 20 * FILTER in order to get a
valid value, I determined that 20x is a pretty good value to get a
decent start value with a filter like this:
MyVal =3D (MyVal * FILTER + ADCVal) / (FILTER + 1) and i'm currently
using a FILTER value of 10.

> If you sampling regime is 50 or 60 Hz you should be able to fit a test
> sample inbetween sampling times without effecting the sampling times
> without affecting the sampling loop.

Yep, that's actually the plan, in between the LED-on cycles, I'm
sampling the ambient brightness which is fed back to the "main-brain"
of the system.

Thanks for your contribution too!
roN

In article <16c21154-6e25-4c2a-bf76-
57afd41477ac@n2g2000prj.googlegroups.com>, ron.eggler@gmail.com says...
> 
> On Apr 29, 3:40&#4294967295;am, Paul <p...@pcserviceselectronics.co.uk> wrote:
> > In article <ipclva$qq...@speranza.aioe.org>, not.going.to...@seen.com
> > says...
> >
> >
> >
> > > Hi Ron,
> >
> > > On 4/28/2011 1:32 PM, cerr wrote:
> > > > I need to resolve following problem:
> > > > I have an led panel with an opto transistor on it that measures the
> > > > light emitted/reflected from the target. I want to control the emitted
> > > > light to be equal to a certain setpoint from what the opto transistor
> > > > measures. Does that make sense? I'm using an 18F pic to control the
> > > > whole circuit and I wanted to come up with a PI if not a PID
> > > > controller algorithm that controls my LEDs. I've been reading about
> > > > PID controllers but am not 100% sure about some things. So far I've
> > > > come up with below code:
> >
> > ......
> >
> > > Also, depending on what you are controlling and how it is likely
> > > to *fail*, you might want to put some anti-windup protection on
> > > your integral term to keep "Integrator" from becoming *so*
> > > large (positive or negative) that it dominates the loop's
> > > performance for an inordinately long time. &#4294967295;E.g., imagine using
> > > "doubles" for these variables. &#4294967295;Now, imagine the detector is
> > > obstructed for 10,000 iterations of your control loop. &#4294967295;In
> > > that case, you are likely to see a big "Error" *and* accumulate
> > > it repeatedly -- regardless of what your control is trying to
> > > do to compensate for it! &#4294967295;When the detector is eventually
> > > unobstructed, the "Output" is way out of whack for a long
> > > time (until the integrator "clears").
> >
> > As a general rule for any electronics or algorithm (software or
> > hardware) that is measuring light level from photodetectors to imaging
> > sensors should alwasy cope with the error conditions first of
> >
> > a) The output is always minimum (obstruction/lens cap)
> >
> > b) The output is always maximum (pointed at sun or bright light)
> >
> > c) The output of the sensor is disconnected, or failed.
> >
> > At start up I always ignore at least the FIRST two samples to determine
> > what the ambient condition is. In your case I would also every once
> > in a while do a detection cycle with the emitter OFF to get an ambient
> > condition. This is not used in control loop but tests the sensor is
> > working if ambient is not min or max. Then with emmitter ON your error
> > must be at least a minimum away from ambient for operations to be
> > useable.
> 
> Well, I have a software low passfilter after I read thwe adc and i let
> it swing it on bootup, meaning i loop 20 * FILTER in order to get a
> valid value, I determined that 20x is a pretty good value to get a
> decent start value with a filter like this:
> MyVal = (MyVal * FILTER + ADCVal) / (FILTER + 1) and i'm currently
> using a FILTER value of 10.

You have heard of 'Garbage In Garbage Out' If all the readings are due 
to pointing at a bright light, so reading is out of normal range, your 
filter will still produce an out of output.

> > If you sampling regime is 50 or 60 Hz you should be able to fit a test
> > sample inbetween sampling times without effecting the sampling times
> > without affecting the sampling loop.
> 
> Yep, that's actually the plan, in between the LED-on cycles, I'm
> sampling the ambient brightness which is fed back to the "main-brain"
> of the system.

Do you do any the reading is meaningless do NOT process, flag error
states.
 
> Thanks for your contribution too!
> roN

-- 
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