Oh well that sounds nice.  I don't have any PID yet (just my crude P),=20
but am working on that now.  From what I read though, it seems like each=20
time I move it back it would take a handful of cycles for the I term to=20
grow enough to move the motor where it needs to be, so would that=20
introduce a noticeable delay to get the motor into position?

While I'm here, what's a good method to tune?  From Wikipedia's info=20
here ( http://en.wikipedia.org/wiki/PID_controller ), I'm going to try=20
the Ziegler-Nichols method, but is there anything else you'd recommend?

Thanks,
-Neil.



On 12/30/2014 9:56 PM, Randy Dawson wrote:
> Hi Niel,
>
> If you have integral working in your algorithm, this will take care of it=
..
>
> As long as a position error exists, the I term will increase in value for=
 each timestep and increase the PID output to turn on your motor against th=
e spring.
>
> In the other direction, without the spring opposing, the I term is 'out o=
f the loop', and the proportional term does it all by itself (or maybe not,=
 for a tight seal)
>
> What you are going to see, is P slew the motor until resistance is met, t=
he longer its not at the desired position, output increases, closing agains=
t the resistance.
>
> I have used this on servo valves with rubber seats, where I always have t=
o give the output term an increase, the longer it remains out of position.
>
> Randy
>
>> Date: Tue, 30 Dec 2014 21:37:48 -0500
>> From: picdude3@narwani.org
>> To: piclist@mit.edu
>> Subject: [EE] Asymmetrical motor/servo PID control?
>>
>> Hi all,
>>
>> I'm trying to make what is essentially a servo for a one-off project for
>> a friend... I'm driving a window motor (I can control speed
>> bidirectionally) with a controller/driver and there's a potentiometer
>> attached for feedback.  For now, I'm using an Arduino to get it running,
>> but I'll get a custom PCB made for a PIC-based circuit later.  BUT...
>> there's a one-sided force (spring) on the load the motor is moving.
>>
>> For an initial test I tried setting the motor speed proportional to how
>> far off it was from it's target, but when it gets close, the spring load
>> is too much for it.  Changing the force so it's a bit more at close to
>> target position helps, but I think I'm just going to use PID.  My
>> concern is that with the spring force acting only one way, the PID
>> parameters would be different from one side to the next.  Essentially to
>> stay in a fixed position, I'd need a continuous non-zero speed on the mo=
tor.
>>
>> How is this (easily) handled?  Do I do separate cases for one direction
>> vs. the other?  Or do I empirically determine how much speed to send it
>> to counter the spring force and add that value before sending to the
>> motor controller?  Of course, this would assume the spring it mostly
>> constant force for the full range of movement.  But what if it's not?
>>
>> Cheers,
>> -Neil
>>
>> --=20
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