Mike Mansheim wrote:

>
> The simplest control system for something like this is:
>
> if      (speed < desired speed) increase pwm output
> else if (speed > desired speed) decrease pwm output
>
> This will oscillate around the desired speed, which often is perfectly
> acceptable.  How much it oscillates depends, among many other things, on
> how much you change the pwm output each time.  In your example, you make
> the change in pwm output "proportional" to the difference between the
> actual speed and the desired speed, which can be a useful enhancement -
> although it will still always oscillate around the desired speed.
> But it is not a "classical" proportional control, where the pwm output
> each time this calculation is done is a function *only* of the difference
> between the speed and the desired speed, and has no "memory" - i.e. it is
> completely unrelated to the previous pwm output.  The integral term is
> what adds this memory.
> A couple of practical notes if you use this approach:
>
> - it can be useful to add a hysteresis term to create a window in which
>   no change is made to the pwm.  e.g.:
>
>   if      (speed < (desired speed - hysteresis)) increase pwm output
>   else if (speed > (desired speed + hysteresis)) decrease pwm output
>
> - I realize you were probably picking numbers out of the air for your
>   example, but just in case, don't feed 1100 to a 10 bit pwm register.
>   It will max out at 1023, if you've set the pwm frequency correctly
>   relative to the clock; less otherwise.
>

I thought it might oscillate, and if it's very narrow it'll probably be
acceptable.
Sorry for the figures - they were just an example - never thought to
make them more accurate to the PWM module.

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