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Fred
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Joined: Tue Jan 15, 2008 2:31 pm Posts: 15344 Location: Home sweet home!

Educate me!! :) I realise the wikipedia has a good page here : http://en.wikipedia.org/wiki/PID_controllerIf anyone would care to comment on anything related to this at all, specifically about real world aspects of limitations and quirks and implementation that would be excellent. In addition, this search could provide some interesting reading for some of you : http://www.google.com/search?hl=en&q=smith%20predictorWhat have you got? Thanks in advance, Admin.
_________________DIYEFI.org  where Open Source means Open Source, and Free means Freedom FreeEMS.org  the open source engine management system FreeEMS dev diary and its comments thread and my turbo truck!n00bs, do NOT PM or email tech questions! Use the forum! The ever growing list of FreeEMS success stories!

Tue Apr 22, 2008 3:05 pm 


thebigmacd
LQFP112  Up with the play
Joined: Thu Apr 10, 2008 5:51 pm Posts: 205

I've got no knowledge on the Smith Predictor, but I deal with PID all the time.
Firstly, 99% of the time, forget about the D part. A PI controller is all you really need to deal with.
Secondly, error = ideal  actual.
The Proportional part of a PI controller is very simple, the farther the value is from ideal, the more output you apply to counteract the error. This component is just Kp * Error, where Kp is the "proportional gain" or constant of proportionality that determines how much output you are going to apply for a given amount of error. If Kp = 1, for every 1 unit of error, the output will be 1. This determines how sensitive to error the controller will be. If Kp is high, the output is going to change a lot for very small variations in the error. Depending on how the output actually affects the system (direct or inverse action), Kp will be positive or negative. The inverse of Kp, 1/Kp is called the Proportional band, and is generally used where the output is bounded in terms of 0% and 100%.
The Integral part of a PI controller is simple as well, you just take a total error of all the samples in the past and multiply them by a constant Ki. This adds an offset to the output that accounts for steadystate constant error once equilibrium is reached with the P portion. Remember, this is not an average, but a SUM of all previous values of error. If the output is physically unable to cause the system to reach the desired value due to a jam or lack of power or a disconnect, this component will "wind up" quite quickly and cause a lot of problems.
For PI control, OUT = Kp*err + Ki*sum_err
Now, in discrete space which is what our microprocessors operate in, this can be greatly simplified by using only the current and previous error sample and not maintaining a sum. I won't go there quite yet.
If you think about the response of a controller to error as a line on a graph in the form y = mx + b, y is the output, m is Kp, x is error, and b is Ki*tot_err.
_________________ Keith MacDonald Control Engineering (Systems) Technologist

Tue Apr 22, 2008 5:13 pm 


AbeFM
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Joined: Sat Feb 16, 2008 12:11 am Posts: 629 Location: Sunny San Diego

I messed with this a bit in college, and have been reading up on it some recently having no end of issues with the MegaSquirt's impersonation of the PID controller. I remember never having serious trouble setting one up before, but suddenly they are the worst thing I've ever used. Ztransform is french for "boot in your neither regions". Just ask Jean, he's from Canada, he probably knows French. thebigmacd wrote: Now, in discrete space which is what our microprocessors operate in, this can be greatly simplified by using only the current and previous error sample and not maintaining a sum. I won't go there quite yet. I'm curious, why is this easier? Something like: Int_Term = Previous_Error + Ki * Current_Error verses Int_Term += Ki * Current_Error thebigmacd wrote: For PI control, OUT = Kp*err + Ki*sum_err
Ok, just to be very clear here.... You didn't mean OUT += Kp*err + Ki*sum_err This is driving me SO batty. The way I wrote it is how the MS does it. They say it makes sense, but I don't buy it! If you have a simple P controller, it integrates. If I'm approaching my target, my output gets bigger not smaller, ensuring massive overshoot. The other way to look at it is this: AbeFM wrote: Quote: PID controller theoryNote: This section describes the ideal parallel or noninteracting form of the PID controller. For other forms please see the Section "Alternative notation and PID forms". The PID control scheme is named after its three correcting terms, whose sum constitutes the manipulated variable (MV). Hence: where Pout, Iout, and Dout are the contributions to the output from the PID controller from each of the three terms, as defined below. Proportional termThe proportional term makes a change to the output that is proportional to the current error value. The proportional response can be adjusted by multiplying the error by a constant Kp, called the proportional gain. The proportional term is given by: Where * Pout: Proportional output * Kp: Proportional Gain, a tuning parameter * e: Error = SP âˆ’ PV * t: Time or instantaneous time (the present) I guess I don't know what this "ztransform" thing is. But using wiki's terms: However, using the '+=', you get something differnet, and for discussion, let's again drop the I, D terms: MV(t) = MV(t1) + Kp*E(t) + Ki (=0) + Kd(=0) MV(t) = MV(t1) + Kp*E(t) MV(t) = MV(t2) + Kp*E(t1) + kp*E(t) MV(t) = (Sum over i = 0 to t) Kp*E(t) MV(t) = kp * (Sum over i = 0 to t) E(t) That looks an aweful lot like: which, to my mind, makes it the integral term. For that matter, the I term becomes a double integral, and the D term cancels since it's the integral of a derivative (or think about adding something with a different sign every time? Sorta a stretch there). Anyway, I'm going to go try it on the bench, but I must be missing something really big in this "ztransform" thing, because the P term should be timescale insensitive to first order. More there than I needed, but you end up with, towards the end, an infinite sum over time. What does all this mean? It means when your idle is low, or your boost is low, your control valve continues to open even as you approach the target. It ensures you'll never get into the 'steady offset' case where a purely proportional controller should be. And it ensures that tuning it is an utterly black art. The recommendation of the folks who wrote that code: If you half the time period, completely retune the controller, there is no way to just adjust the values to get the same response. A TRUE PID controller would mean you either wouldn't change anything (absolute time reference) or you would leave Kp alone, and only half/double the Ki and Kp terms if you only considered time in loopexecution steps. I guess the intelligent thing to do is to recompile my own version of the code with the proper math in there, tweak all the constants and scale factors (of numbers like 100,000) they put in to make it function, and see how it goes. As it is, it would minimally need a lot of prediction. Of course, since the control doesn't go negative, I'd have to define a 'neutral' position, I think. So instead of OUT = sum(error_terms), it would be like OUT = sum(error_terms) + 50%, though really were it me, I'd choose that number to be the steady state number that works (28% for me), but I think I'm making this more complicated than it needs to be. Or, I'm terribly misunderstanding something.

Tue Apr 22, 2008 7:00 pm 


jbelanger
LQFP144  On Top Of The Game
Joined: Sat Feb 23, 2008 8:58 pm Posts: 387

8InchesFlacid wrote: Ztransform is french for "boot in your neither regions". Just ask Jean, he's from Canada, he probably knows French. Yeah, French is my first language. But the accent here is not the same as in France (both in French and English) so you'd be more likely to get a Dtransform here than a Ztransform. Now back to the serious discussion...

Tue Apr 22, 2008 8:11 pm 


Fred
Moderator
Joined: Tue Jan 15, 2008 2:31 pm Posts: 15344 Location: Home sweet home!

LOL :) At least french canadians don't blow up green peace boats... ;)
_________________DIYEFI.org  where Open Source means Open Source, and Free means Freedom FreeEMS.org  the open source engine management system FreeEMS dev diary and its comments thread and my turbo truck!n00bs, do NOT PM or email tech questions! Use the forum! The ever growing list of FreeEMS success stories!

Tue Apr 22, 2008 8:15 pm 


thebigmacd
LQFP112  Up with the play
Joined: Thu Apr 10, 2008 5:51 pm Posts: 205

Okay here goes: PID ProgrammingThe Ztransform of 1 times a function is 1, or Z^0 The Ztransform of the integral of a function is 1 / (1  Z^1) The Ztransform of the derivative of a function is (1  Z^1)^2 / (1  Z^1) This gives us a transfer function of: Output/Error = Kp * 1 + Ki * (1 / (1  Z^1)) + Kd * ((1  Z^1)^2 / (1  Z^1))We then multiply through to remove the bottom term of the fractions. Multiply through by (1  Z^1) Output/Error * ((1  Z^1)) = Kp * (1  Z^1) + Ki + Kd * (1  Z^1)^2 Output/Error * 1  (Output/Error * Z^1) = Kp  (Kp * Z^1) + Ki + Kd (1  2Z^1 + Z^2) Output/Error * 1  (Output/Error * Z^1) = Kp  (Kp * Z^1) + Ki + Kd  (2 * Kd * Z^1) + (Kd * Z^2) Output/Error = (Kp + Ki + Kd)  ((Kp + 2 * Kd) * Z^1) + (Kd * Z^2) + (Output/Error * Z^1)
Wherever something is multiplied by one, we can replace this with Z^0, because anything ^ 0 = 1. Output/Error * Z^0 = (Kp + Ki + Kd) * Z^0  ((Kp + 2 * Kd) * Z^1) + (Kd * Z^2) + (Output/Error * Z^1)
We then multiply through by Error Output * Z^0 = (Kp + Ki + Kd) * Error * Z^0  ((Kp + 2 * Kd) * Error * Z^1) + (Kd * Error * Z^2) + (Output * Z^1)
See a pattern? Write it out by hand and see. In Z space, Z^0 means NOW, Z^1 means ONE AGO, and Z2 means TWO AGO. We can transfer this to a program by indicating N TIMES AGO by and integer number. 0 = now, 1 = one ago, 2 = two ago, etc. Code: OUTPUT[0] = (Kp + Ki + Kd) * Error[0]  (Kp + 2 * Kd) * Error[1] + Kd * Error[2] + OUTPUT[1]
We add lines before and after to shift the data to the proper array location and we are done. As stated in the info above, Kd, Ki, and Kp change depending on the sample period. We can add the last output value by using += rather than using an array index. Code: Error[2] = Error[1] //Shift data by one time constant Error[1] = Error[0] //Shift data by one time constant Error[0] = Actual  Requested //Calculate error NOW Alpha = Kp + Ki + Kd //Calculate Alpha constant Beta = Kp + 2 * Kd //Calculate Beta constant Gamma = Kd //Calculate Gamma constant OUTPUT += Alpha * Error[0]  Beta * Error[1] + Gamma * Error[2]
Instant PID control. PI ProgrammingIf you start with PI only: Output/Error = Kp * 1 + Ki * (1 / (1  Z^1))
You end up with: Output/Error = (Kp + Ki)  Kp * Z^1 + Output/Error*Z^1 Output = (Kp + Ki) * Error * Z^0  Kp * Z^1 + Output * Z^1
Which gives you the code Code: Output[0] = (Kp + Ki)*Error[0]  Kp*Error[1] + Output[1]
Which then simplifies to Code: OUTPUT += (Kp + Ki)*Error[0]  Kp*Error[1]
In MS format, the would be Code: OUTPUT += (Kp + Ki)*Error_Now  Kp*Error_Last
This is how a PI controller should be written. P ControlOutput/Error = Kp * Z^0Output = Kp * Error * Z^0Code: Output = Kp * Error[0]
This one is pretty straightforward. General rule of thumb is is there is only a proportional term, you only look at the current state [0]. If there is an integral term, you look at the last state [1] as well. If there is an integral term, you have to look two time ago [2]. Don't forget to shift your table values at the proper time and in the correct order!
_________________ Keith MacDonald Control Engineering (Systems) Technologist

Wed Apr 23, 2008 4:52 am 


AbeFM
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Joined: Sat Feb 16, 2008 12:11 am Posts: 629 Location: Sunny San Diego

You must be good, you used italics AND bold (not just holding down shift). Seriously, thanks for the nice write up. Here's what I noticed, aside from the obvious that Engineers start with 'the z transform blah blah' instead of what a z transform is or why you'd do it... But it's ok, I think I managed to make some sense of it anyway. And again, thanks for putting it out explicitly! thebigmacd wrote: Code: OUTPUT[0] = (Kp + Ki + Kd) * Error[0]  (Kp + 2 * Kd) * Error[1] + Kd * Error[2] + OUTPUT[1]
As I did in other places, I want to look at what happens to just the P term, if that doesn't make sense, I think something's broken. Code: OUTPUT[0] = (Kp + Ki + Kd) * Error[0]  (Kp + 2 * Kd) * Error[1] + Kd * Error[2] + OUTPUT[1]
set Ki = Kd = 0
OUTPUT[0] = Kp * Error[0]  Kp * Error[1] + OUTPUT[1] (but output1 depends on history....) = Kp * (Error[0]  Error[1] + Error[1]  Error[2]) + OUTPUT[2] (for the dense, we'll go another step) = Kp * (Error[0]  Error[2] + Error[2]  Error[3]) + OUTPUT[4] (carried to infinite we get) = Kp * Error[0] + OUTPUT[initialposition]
Now THAT'S exactly what I've been saying all along, that the P term should be purely proportional to the current error, plus some initial 'best guess' term, chosen to be the steady state position you think it the best guess (I'd probably use the valve opening for idle that I normally set it at). Quote: We add lines before and after to shift the data to the proper array location and we are done. As stated in the info above, Kd, Ki, and Kp change depending on the sample period. We can add the last output value by using += rather than using an array index. Code: Error[2] = Error[1] //Shift data by one time constant Error[1] = Error[0] //Shift data by one time constant Error[0] = Actual  Requested //Calculate error NOW Alpha = Kp + Ki + Kd //Calculate Alpha constant Beta = Kp + 2 * Kd //Calculate Beta constant Gamma = Kd //Calculate Gamma constant OUTPUT += Alpha * Error[0]  Beta * Error[1] + Gamma * Error[2]
Instant PID control. I'll have to look, I think what happened is the MS folks are using Alpha = KP Beta = KI Gama = KD Or something like that, I have to look closer. But they are doing something way wrong. The MS does this: Code: rpm_error = (int)targ_rpm  (int)outpc.rpm;
Kp = ((long)rpm_error * flash5.pwmidle_Kp);
tmp1 = ((long)(Kp) * (int)(flash5.pwmidle_open_duty  flash5.pwmidle_closed_duty)) / ((long)pg5_ptr>pwmidle_target_rpms[PWMIDLE_NUM_BINS  1] * 1000); tmp1 += IACmotor_pos;
if (tmp1 < flash5.pwmidle_min_duty) { tmp1 = flash5.pwmidle_min_duty; } else if (tmp1 > flash5.pwmidle_open_duty) { tmp1 = flash5.pwmidle_open_duty; } IACmotor_pos = tmp1; IACmotor_last = IACmotor_pos; which boils down to: Code: error = target  actual; Kp_error = error * Kp; tmp1 = Kp_error; tmp1 += OUTPUT; OUTPUT = tmp1;
which in turn boils down to: Code: error = target  actual; OUTPUT += Kp * error;
That's an integral. Quote: PI ProgrammingIf you start with PI only: Output/Error = Kp * 1 + Ki * (1 / (1  Z^1))
(dot dot dot) In MS format, the would be Code: OUTPUT += (Kp + Ki)*Error_Now  Kp*Error_Last
Looks like I started responding before reading everything you said, so yes we're saying the same things. My questions now is how do I convince the MS guys they are wrong? I was going to go through their code for all three terms, but wow... it's hard to read through. The more I look at it the less I care, it's wrong, so... what more do I need to know? If I felt the guys on that board wanted to make it work right, instead of just BE right, I'd go through the trouble of fixing it. FYI it's: Code: rpm_error = (int)targ_rpm  (int)outpc.rpm;
pwmidle_error_sum += rpm_error;
if (pwmidle_error_sum > 1000) { pwmidle_error_sum = 1000; } if (pwmidle_error_sum < 1000) { pwmidle_error_sum = 1000; }
pwmidle_deriv = rpm_error  pwmidle_last_error; pwmidle_last_error = rpm_error;
Kp = ((long)rpm_error * flash5.pwmidle_Kp); Ki = ((long)pwmidle_error_sum * flash5.pwmidle_Ki); Kd = ((long)pwmidle_deriv * flash5.pwmidle_Kd);
if (IdleCtl == 6) { tmp1 = (((long)((Kp + Ki  Kd) / 100 ) << 8) * (int)(flash5.pwmidle_open_duty  flash5.pwmidle_closed_duty)) / ((long)pg5_ptr>pwmidle_target_rpms[PWMIDLE_NUM_BINS  1] * 2550); } else { tmp1 = ((long)(Kp + Ki  Kd) * (int)(flash5.pwmidle_open_duty  flash5.pwmidle_closed_duty)) / ((long)pg5_ptr>pwmidle_target_rpms[PWMIDLE_NUM_BINS  1] * 1000); }
tmp1 += IACmotor_pos;
if (IdleCtl == 7  IdleCtl == 8) { if (tmp1 > (int)flash5.pwmidle_min_duty) { tmp1 = flash5.pwmidle_min_duty; } else if (tmp1 < (int)flash5.pwmidle_open_duty) { tmp1 = flash5.pwmidle_open_duty; } } else { if (tmp1 < flash5.pwmidle_min_duty) { tmp1 = flash5.pwmidle_min_duty; } else if (tmp1 > flash5.pwmidle_open_duty) { tmp1 = flash5.pwmidle_open_duty; } }
DISABLE_INTERRUPTS; IACmotor_pos = tmp1; IACmotor_last = IACmotor_pos; ENABLE_INTERRUPTS; The nice part about the Ztransform is it's mathematically very cheap. I should really boil it down and make sure it does what I want (I think so), and that it takes into account all history (not just the past two time points). Leaving actual processing cycles aside, PID looks like this (to me): error = target  actual time_loopstart = get_current_time()delta_t = time_loopstart  time_lastloopP_contribution = Kp * error I_contribution += Ki * error * delta_tD_contribution = Kd * (error_last  error) / delta_terror_last = error time_lastloop = time_loopstartoutput = P_contribution + I_contribution + D_contribution That assumes the loop gets run regularly. If not, you need to correct by the time since the last time you were run.shown underlined.

Wed Apr 23, 2008 6:47 pm 


AbeFM
Post Whore!
Joined: Sat Feb 16, 2008 12:11 am Posts: 629 Location: Sunny San Diego

You know, thinking about this some more, I don't see why the simple version is so much more computationally expensive. The worst thing I see in it is dividing by the amount of elapsed time.
Otherwise it's a couple of multiplications and a couple of additions and the odd subtraction or two. Maybe there's some floating point issue adding a lot of time to it if the constants are less than unity, but that would be the same as the ztransform case.
The nice part about doing it the 'simple' way is nothing depends on time at all, you pick constants that work and until you get to extremes there shouldn't be an effect when changing the loop time  very handy if you only get around to running it when you get around to it.

Wed Apr 23, 2008 9:31 pm 


thebigmacd
LQFP112  Up with the play
Joined: Thu Apr 10, 2008 5:51 pm Posts: 205

8InchesFlacid wrote: You must be good, you used italics AND bold (not just holding down shift). Seriously, thanks for the nice write up. Here's what I noticed, aside from the obvious that Engineers start with 'the z transform blah blah' instead of what a z transform is or why you'd do it... But it's ok, I think I managed to make some sense of it anyway. And again, thanks for putting it out explicitly! The reason we use the Ztransform is because discrete sampling is just that...discrete samples taken and processed periodically. When you take discrete samples, the analog data between samples is lost. Laplace transforms work for analog because they assume the sample frequency is infinite. As the sample frequency is reduced from infinity, the Laplace transform becomes more and more inaccurate. The critical characteristic of Ztransforms is that they inherently contain a sample period component that completely changes the K constants. Into time space, the translation is Z = A*e^sTAs you can see, if the sample time T is changed, the resultant function is different. In the process of getting a function into Z space, this constant T is factored out and included in the constants, such as Kp, Ki, Kd in our case. The specific math is eluding me at this time, but this is why Ki and Kd change with sample time.
_________________ Keith MacDonald Control Engineering (Systems) Technologist

Wed Apr 23, 2008 9:34 pm 


thebigmacd
LQFP112  Up with the play
Joined: Thu Apr 10, 2008 5:51 pm Posts: 205

8InchesFlacid wrote: You know, thinking about this some more, I don't see why the simple version is so much more computationally expensive. The worst thing I see in it is dividing by the amount of elapsed time.
Otherwise it's a couple of multiplications and a couple of additions and the odd subtraction or two. Maybe there's some floating point issue adding a lot of time to it if the constants are less than unity, but that would be the same as the ztransform case.
The nice part about doing it the 'simple' way is nothing depends on time at all, you pick constants that work and until you get to extremes there shouldn't be an effect when changing the loop time  very handy if you only get around to running it when you get around to it. The only issue is that if you allow sample period to increase, the analog stuff that happens inbetween can start to do some funky things with your accuracy. Zspace isn't perfect in this respect. The long and the short of it is PID actually sucks for any system with higher than 2ndorder response. Which is pretty much anything that does not stick to a 2ndorder differential equation. This is why we use tables for fuel; engine VE is not linear in the mathematical sense.
_________________ Keith MacDonald Control Engineering (Systems) Technologist

Wed Apr 23, 2008 9:39 pm 


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