Re: [Emc-users] Stepper Musings

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Colin Kingsbury wrote:
> At first I was not really big on this but the more I think about it, the
> more I like it. I play with small, benchtop size machines, where inexpensive
> steppers and drives generally work well and cost half or less of what servos
> would. A pseudo-servo mechanism that simply tripped a fault when something
> bad started to happen would provide most if not all of the benefit that true
> servos would, at much less cost. It's not magic and won't make the machine
> "more precise," but it would make it more fault-tolerant and that seems
> significant. It would also be a unique competitive edge that could bring a
> lot of users over to EMC2.
>
> Is this something that could be done just by wiring a few ~$50 encoders into
> a parport, or would you need more specialized IO hardware?
>   
Each encoder needs 2 pins, so it could be done with an extra parallel 
port.  But, the software encoder
has a speed limit.  If you are running a 25 us (SERVO_PERIOD_NS = 25000) 
then you could keep up
with encoders moving at about 40,000 counts/second, but should leave 
significant margin for noise
and timing fluctuations.  So, maybe 25,000 counts/sec is a good safety 
factor.  Now, if you have
low-res encoders with 200 lines/rev, that gives 800 counts/rev.  That is 
a lot less than a microstepping
drive on a stepper, so you really can't use it for closed-loop control, 
but you can use it for detection
of positioning errors.  Well, let's take a Sherline or Taig as an 
example, they have a 20 TPI leadscrew.
So, 800 counts/rev * 20 TPI gives 16,000 counts/inch of motion.  So, at 
the 25,000 counts/sec safe
limit, this is 1.56 inches/second, or 93 IPM, which is probably 
excessive for one of those machines.
On the other hand, if you wanted to use a 2000 line/rev encoder giving 
8000 counts/rev, you would
be limited to 9.3 IPM.  So, you can do the calculation for the 
SERVO_PERIOD and the machine/
encoder combination and see if a hardware encoder counter is warranted.


Jon