Re: Single Servo circuit

[garrett beaubien <garrettb77-at-yahoo.com>]

Hello all,

> 
> Nice job of drawing and packaging your project. 
> This is a very well
> thought out and executed combination of EMC software
> modification, PC
> interface board, and servo system.  It will provide
> a low cost way for many
> of us to experiment with simple amplifiers and DC
> motors and see how well
> the EMC can be tuned for this.

Thank you.


> 
> I'm wondering what kind and size motor you are using
> and the ppr of the
> encoder?  A system that I looked at recently
> required more than 2k ppr for
> smooth motor control.
> 

I'm using a small pittman gearhead motor with integral
encoder.  I don't have any specs on the motor, but
it's about 4" x 1.5".  The encoder has 500 lines.  The
counter chip can do the quadrature X4 multiplication
to give 2000 "steps" per rev.  For the system you
looked at, was the 2000 the line count? or was it
after the quadrature  multiplication? 

> Were your choices of PID and FFn trial-and-error or
> did you use a
> tuner?  What are your impressions of the accuracy,
> stability, and such of
> the amp/motor/hardware that you use?  
> 


Very much trial-and-error.  In fact I'm quite sure
they are not even the "best" values :) 
I don't think that the Analog portion or the circuit
is "up-to-snuff".  I also think that the gearhead
causes problems due to the backlash.  The accuracy of
the system does not seem to be terrible.  On the emc
display it holds position within a few thousandths of
an inch (I haven't checked what this translates to in
degrees of shaft rotation).  The steady state error
improves if I add just a teensy bit of integral, add
too much and the motor runs away.  I'm just playing
with an unloaded motor right now, and if I add almost
any derivative, the motor oscillates.  I wonder if the
backlash of the gears causes this? 

I think the next thing I will do is use the amp in
torque mode.  That is control current rather than
voltage.  I have read that this setup is easier to
tune.

Thanks,
Garrett.

__________________________________________________
Do You Yahoo!?
Yahoo! Messenger - Talk while you surf!  It's FREE.
http://im.yahoo.com/