AVR-Chat

Dennis: Solving the glitch problem was the intended use for the ENABLE line
(and coasting).  If you are seeing large current draw with LA then your PWM
rate is way too slow.  That is the big problem with LA, IMO, is that pwm
rates of 60-120khz are needed with small, low inductance motors.

On my paper, www.barello.net/Papers/Motion_Control I relate the difference
between cheap aircraft servo motors (gutted) and high quality coreless (I
think) MAXON printer motors with PWM @ 15khz (TI 754410): The cheap motors
with iron cores had ~15ma peak to peak ripple while the MAXON motors had
150ma - about 1/2 the stall current!

So, it all depends (as usual)...

My other presentation (H-Bridge.pdf) has some graphs and pictorial
descriptions of what is happening with the current in an H-Bridge driven
motor situation.

In my experience, Synchronous-rectification (i.e. driving the motors
on-shorted) gives similar responsiveness and linearity as the LA approach,
with the advantage of 1/2 the ripple.  

If your hardware (or s/w) is very clever, you can alternate the side of the
H-Bridge doing the rectification (shorting) from cycle to cycle to double
the switching frequency seen by the inductance for a given PWM rate and cut
your ripple in half again.  That requires a PWM for each side and adjusting
the phase between them to adjust the duty cycle.

Cheers!

-----------
Larry Barello
www.barello.net


| -----Original Message-----
| From: AVR-Chat@yahoogroups.com [mailto:AVR-Chat@yahoogroups.com] On Behalf
| Of Dennis Clark
| Sent: Monday, August 28, 2006 11:49 AM
| To: AVR-Chat@yahoogroups.com
| Subject: Re: [QUARANTINE] [AVR-Chat] Re: PWM for reversible DC motors
| 
| Any H-bridge (usually) can be drived locked antiphase.  My biggest
| problem with locked-antiphase is power up glitch.  You have to have the
| PWM at the center duty cycle or the motors turn.  Which means that when
| you start up and your line is low, your motor takes off full bore until
| the PWM comes on.  The other problem I have with locked antiphase is
| that you pull the same current no matter what the motor speed, this is
| wasteful of power on a battery powered system.  I have seen that locked
| antiphase is MUCH more responsive to PWM change.
| 
| DLC