AVR-Chat

On Sun, 2005-09-04 at 16:29 -0600, Kevin wrote:

> > I'm trying to find more data about the R/C throttle control.
> > I'm looking for a 25,000 RPM motor to use on a PCB drilling machine
> > and these seem small enough and high speed enough to do the job. 
> > Unfortunatly, the combination of motor and ESC can run upwards of
> $1750 !
>   I doubt that you'll want an RC controller for that kind of 
> operation, they aren't all that reliable - which is OK for an RC car, 
> not so OK for a precision milling operation.  Also, for a mill you'll
> want motor feedback so that the bit can keep a constant speed up under
> changing loads - An RC motor driver won't do that.
> > Any comments or suggestion are welcome.
>    I recommend Googling for motor controller designs with feedback
> that use PID or something close to that to control the motor speed.

   Naw.  PID is too complex for this sort of thing, and gross overkill.
All he needs is a simple PWM controller, with speed feedback as an
option.  Using a brushless DC motor, or a small three pahse A/C
induction motor, speed control probably isn't a big issue, since the
speed is locked to the basic frequency of the power waveform. If the
load increases, and the frequency of the power waveform doesn';t
change,m the motor will just draw more current as needed to maintain
sync with the power waveform.

   There are several PWM speed control system projects for brushless DC
motor controls on avrfreaks.net undee the projects list,. and I know
Atmel has at least three Applications notes on such things.  Other
microcontorller vendors also have numerous applications notes on this.

   While it is arguable whether, for a small drilling application, A/C
induction motors and Space Vector Pulse Width MOdulation might be a bit
of overkill, there is that route to consider as well.  (I frankly don't
know whether the Space Vector approach is beneficial for brushless DC
motors, but I suspect not)  I do know that PWM will offer far better
power efficiency and far lower cost of power control components than any
other approach to brushless DC motor or A/C induction motor speed
control.  And PWM is much more inherently "computer friendly" than other
approaches, both at the controller level, and at the level of
inte5rfacing to PCs for control and monitoring of systems.

Tom