Closed loop speed controller

[Ryan7777]

Thanks for the fast reply Darrel!

1. The PID code is for a servo right? I see that it has direction control and I'm guessing when the setpoint and the ADC value match, it's going to stop the motor and fight to keep it in that position?

2. what do I change in the code to make this thing run as a speed control?

[Darrel Taylor]

That's what the example does, but the PID routine itself can be used for MANY different things.

In Henrik's example, it's controlling "Position". But if the numbers presented to the PID are representative of the Speed, then it will adjust the output until the Speed is matched.

It doesn't really matter what it is, temperature, humidity, speed, acceleration ...

If you can read it, then you can make a "Setpoint" within that range.
The PID loop adjusts the output until the "Reading" and the "Setpoint" are the same.

It does it in a way that is as fast as possible while minimizing overshoot and rejecting spurious influences as much as it can.

All you need to do, is find a way to read the speed of the motor, and a way to control the speed of the motor, which I think you already have.
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[HenrikOlsson]

Hi,
Just as Darrel says it can be used to control "anything". Basicly you feed it the Error and it returns the amount of Drive it thinks is neccesary to reduce the Error to zero based on the P- I- & D-gains and a couple of other paramters. It doesn't know, or care, if it's position, speed or whatever.

pidError = Setpoint - CurrentSpeed	'Calculate error
Gosub PID				'Call PID. Result returned in pid_Out

There are a couple of things you need to concider. Since you're driving the motor in one quadrant only. There may be occations when the output of the PID routine is "negative" ie. it want's to actually reverse the motor. Obviously you can't do that with only one transistor so you need to clamp the output from the PID routine to positive drive only, perhaps something like this:

pid_Error = Setpoint - CurrentSpeed   	'Calculate error
Gosub PID                           	'Result returned in pid_Out                           
 If pid_out.15 then                 	'If "negative drive" is called for...
   pid_out = 0                       	'..set output to 0...
   pid_Ei = 0                        	'And keep the I-term from accumulating..
  
 Else                               	'and if positve drive is called for....

   pid_Out = ABS pid_Out             	'...set it to absoulte value of pid_Out
 Endif

Since you want to controll the speed from zero all the way up I think you should concider more than a single a pulse per rev. Otherwise it can get really hard to get good control in the lower speedrange.

Another thing that is important in digital PID control is constant sampling rate. Ideally you set up a timer interrupt to run at whatever rate you need. What rate to run at depends on the timeconstant of the load and you may need to experiment with a bit but for a basic motorcontrol I'd say somewhere between 100-1000Hz.

/Henrik Olsson.

[Ryan7777]

yeah, that was one thing i was worried about, when my speed goes below the clamp setting.. and it doesnt have to go to complete zero, I can handle that in other ways. my final output shaft is 173RPM, I might add more magnets to the main motor shaft, before the gear reduction just to be safe, i'll have to see what i can get away with. I'm not quite sure i know what your saying about sampling rate? I was going to use timer1 as a counter to get the tach of the motor, which people typically want as RPMs for a display and would count for a second, but are you saying i should count for less then a second? or as much as possible? and also, im sure the number i get for the speed isnt going to be 1:1 with my ADC (the speed setpoint), should i be adjusting something to scale those, or? I don't have everything on hand i need at the moment for a bench test and experiment, I'll have to order a few more things (fets, a rectifier and cap for the motor). i'd like to have a handle on code things before hand tho! thanks again.

[HenrikOlsson]

Hi,
About the sampling rate. The motor and whatever is attached to it has a certain mechanical time constant. If you apply a voltage to it it it takes a certain amount of time for it to "respond". If you sample a lot slower than this time constant you won't be able to get good control.

I'll take a shot at an example:
If we are controlling the temperature in our swimming pool (which probably have a very large time constant) perhaps we should read the actual temperature and calculate a new "drive-value" for the heater once every hour or even slower than that. But if we're controlling the position of the mirror in a laser-scanner we may need to read the actual position and calculate a new "drive-value" tens of thousands of times per second.

Whatever the rate is (once a day or once every uS) it needs to be constant because the final output is based on the error itself (P) the magnitude and amount of time the error has been present (I) and the rate of change of the error (D).

It all depends on how "stiff" you want the control-loop to be. You may get away with a sampling rate of 1s if you can live with very a slow response. If you need to "instantly" respond to the change in load you need to sample much much faster than that.

There was a really good article available on the web called PID without a PhD explaining this a lot better than I can but I can't seem to find it. If you look around a bit you may be able to find it.

EDIT: Here it is.

HTH
/Henrik Olsson

[Acetronics2]

Hi, Enrik

You should browse the Parallax site : in their applications ... there's all your link shows ... but for PICs AND PbP ( or compatible) written.

http://www.parallax.com/Portals/0/Do...eb-PC-v1.0.pdf

see also here : http://www.emesystems.com/BS2math5.htm#Smoothing

" Hope it can help " ... as we use to say !!!

Alain