I'm playing around with EMC, trying to control a small brushed DC motor.
The parallel port controls an H-bridge circuit which feeds power to
the motor. The motor has a 256-line encoder on the motor shaft, which
I read through the parallel port.
The fastest thread my PC can run reliably is about 40 KHz, so that's
how fast I'm reading the encoder lines. This really puts a damper
on my top motor speed.
Let L be the number of lines on the encoder. Let R be the speed of
the motor in revolutions per minute.
(4 * L) is the number of quadrature transitions per revolution.
(R / 60) is the number of revolutions per second.
(4*L) * (R/60) is the number of quadrature transitions per second.
F = 2 * (4*L) * (R/60) is the minimum sampling frequency for reliable
operations (according to Nyquist, as I understand it).
If F = 40 KHz and L = 256, then R = 1170 RPM. Experimentally, I can push
the speed a bit higher than this before it starts missing transitions and
losing position. I guess that factor of two (Nyquist) is conservative?
Anyway, this is much slower than the top speed of the motor. I think
there are two ways to deal with this situation:
The prefered way is to read the encoder lines much faster, say 10x.
This is hard over a parallel port, but easily doable on dedicated
hardware like the Pluto-P or similar.
The alternative, if option 1 isn't possible, is to limit the motor
speed. That is what I want to ask about in this email.
I've determined that with no load on the motor shaft, stepping the
PWM duty cycle from 0 to 100% causes the motor to accelerate from a
stand-still to the 1200 RPM limit in about 75 us.
My plan for trying to limit the motor shaft speed is to run a two-level
PID controller. The first PID controller tries to achieve the target
position using encoder.position as feedback. The output from the
position PID controller goes not to the pulse generator, but to the
command for the velocity PID controller. The velocity PID controller
uses encoder.velocity as feedback, and *its* output goes to pwmgen.value.
I'll run the velocity PID controller in a relatively fast thread (no
more than 75 us period), but the position PID controller can run at the
usual rate of 1 KHz or so.
Does this seem feasible? Or am I barking up the wrong tree?
--
Sebastian Kuzminsky
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