Oh so you can split.  I think I'll experiment with this.

Thanks,
-Neil.


On 12/30/2014 10:10 PM, Bob Blick wrote:
> Hi Neil,
>
> Either use a different multiplier for your integral term or different
> limits for integral growth, based on polarity.
>
> Cheerful regards, Bob
>
> On Tue, Dec 30, 2014, at 06:37 PM, Neil wrote:
>> 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
>> motor.
>>
>> 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

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