Hi Aram,
I'm certainly grateful that this is not my application because I
suspect there are a lot of compromises to be made.
Assumptions:
a. the system doesn't need accuracy in both directions; i.e. is not
backlash sensitive
b. it really does not have to be direct driven
c. it is possible to use some kind of linear viscous dampener(to
shove the balls against the groove in the
ball screw).
With those assumptions in place you are not limited to having to buy
the perfect (high res) encoder.
There is a gear train (say 200:1) between the driving motor and the
ball screw. Stick-slip can be minimized by the torque/power of the
gear train.
Under these condition you could even use a small stepper motor; I'm
not a fan of steppers but they do have
their place. Or you could use a small servo motor with a decent
resolution encoder attached and maybe even a
dual sensor ... e.g a linear scale to sense primary position. You are
getting down in the region where the
periodic error in the ball screw may show up but only if you need
perfect linear motion.
This is off the top of my head but even so someone can now refine
it. ;-)
Good luck.
Dave
On Apr 20, 2008, at 3:20 PM, [EMAIL PROTECTED] wrote:
> hi
> you said that "Clearly higher feed-rates are more difficult for the
> PID
> loop. "
> what about tunup problem with very slow motion? i mean feed with
> 0.0001
> per minute. i use ball screw 5 mm per revolution
> thanks
> aram
>
>
>> Hi all,
>>
>> I got my x-axis servo motor mounted today and ran some tests.
>> There is a video and some hal-scope screen shots here:
>> http://www.anderswallin.net/2008/04/x-axis-test/
>>
>> This is a system with no motor current, motor voltage, or velocity
>> feedback. An m5i20 generates PWM output for pico-systems DC-brush
>> amps
>> and 4000 pulse/rev encoders provide feedback.
>> This might make the system a bit different to tune from other systems
>> which run another loop on current/voltage/velocity.
>>
>> I found FF1 worked quite predictably and reduced the cruise-phase
>> error
>> to very little. Only P and FF1 was needed for that. (2nd pic on page)
>>
>> Then I added a lot of I-gain which helped a bit for the acc/dec
>> phases.
>>
>> With FF2 I became confused on a higher level (3rd pic on page). It
>> seems
>> to either fix the acc. phase or the dec. phase but not both.
>>
>> The last pic shows moves at different feed-rates. All of them show
>> around 16 to 25 count spikes during the acc/dec. phases.
>> Presumably at
>> the moment when acceleration changes and jerk is infinite. How
>> much have
>> other people been able to reduce these spikes?
>>
>> regards,
>>
>> Anders
>>
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