AVR-Chat

That is true about the glitch, I had a tendency to do a "one wire" 
approach and just enable the bridge, that enable will eliminate the 
glitch.  When playing with LA I didn't see high currents, I just noted 
that it was the _same_ current no matter what the motor speed was.  I 
did like the responsiveness of LA over SM though you do lose some of the 
resolution, all things kept the same, but you didn't have that low 
magnitude "dead band" on start up either.  All things in trade off.

DLC

Larry Barello wrote:
> 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
> 
> 
> 
>  
> Yahoo! Groups Links
> 
> 
> 
>  
> 
> 
> 

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Dennis Clark    TTT Enterprises
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