Re: Low-cost controller for 10 small DC servo motors with quadrature encoder
- From: RMDumse <rmd@xxxxxxxxxxxxx>
- Date: Mon, 8 Sep 2008 10:05:31 -0700 (PDT)
On Sep 8, 5:46 am, Albert Goodwill <albertgoodw...@xxxxxxxxx> wrote:
Thank you for the information.
I have just checked your web site with for your IsoPod(TM) and
ServoPod(TM) product line.
They sound FANTASTIC!!!
In my opinion, nothing else comes close for motion control.
The IsoPod(TM), ServoPod(TM), have two built in hardware quadrature
inputs that only do quadrature decoding. They share pins with some of
the timers. There are 16 timers, 14 with pins. The 16-bit timers are
programmable and can do the function of quadrature as well. They can
be concatenated to give a full 32-bits of quadrature.
So you can do the 6 channels of quadrature in hardware by using the
two built-ins and four timer channels. You still have a couple timers
left over for other purposes. By hardware, I mean, once set up, you
only have to read them when you want the answer. There is no software
burden on the processor to have these channels coming in. I have some
code examples for setting up 32-quadrature decodes. I use 6 channels
input on the Panavision Back Pan Plus movie crane, so I know it works
and works well. In fact, I even use the other timers to pull the
velocity of the signal by doing pulse width timing on all channels.
I'm not absolutely certain IsoMax(TM) can handle that many channels of
PID. It rather matters how often you want updates. Perhaps at 100
updates per second, I'd think there is a good chance of operating in
purely high level.
Then NMIH-0030 is not my favorite H-bridge because the PWM upper limit
is low, in the audio range. I much prefer the NMIH-0050, which is more
robust, and we've tested at higher rates. Depends on your motors.
Coreless are low inductance and need higher PWM frequencies or they
will be operating in the resistive range, which is pure waste of
power.
Have you looked into Locked Anti Phase (LAP)? With a single PWM line
you can send both direction and power. At 50% the wheel is locked, at
25% it is half power one way, and at 75% it is half power the other
way. Again, we've set up the NMIN-0050 to be able to operate off of
LAP adding an inverter to the board. Since it isn't my favorite, I
don't remember how the NMIN-0030 is set up for LAP.
But either LAP, or complementary, or PWM on one line or another at a
time, you have to control the direction of the motor, and may have to
reverse a line when crossing zero, if you use, say, a PWM line for
power and a port line for direction. Only LAP and complementary PWM
take care of themselves that way.
Yes, it is also possible to decode RC pulse streams. The timers are
again wonderfully configureable, and can be used in a processor hands-
off mode to measure the width of a pulse. Being able to reliably pull
a string of pulse widths out though, may depend on getting back to the
stream often enough, and that may depend on how loaded your processor
is with the PID routines. Again I have some code examples of pulling
the pulse width on a single servo channel, and we've got support in
the language for doing that measurement.
Randy
.
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