Newsgroups: comp.robotics
Path: brunix!news.Brown.EDU!noc.near.net!howland.reston.ans.net!torn!newshub.ccs.yorku.ca!newshub.ariel.yorku.ca!cs911225
From: cs911225@ariel.yorku.ca (KEN E WILLMOTT)
Subject: H bridge driver circuit
Message-ID: <1993Sep12.155344.273@newshub.ariel.yorku.ca>
Sender: news@newshub.ariel.yorku.ca (USENET News System)
Organization: York University, Toronto, Canada
Date: Sun, 12 Sep 1993 15:53:44 GMT
Lines: 49

>>
>>I have a suggestion. Well, a few actually. One, leave some small
>>caps on the motor, to suppress noise from the motor itself.
>>Then right near your power driver module, add a filter.
>>
>>...more comments...

>>
>>	|                                         |
>>	d                                         d
>>	|                                         |
>>	*---())()(---*---- m -----*----()()())----* 
>>	|            |            |               |
>>	d            |            |               d
>>	|            =            =               |
>>	|            |            |
>>	             gnd          gnd
>>

>Hold the phone, Ken. Logic problem here. You've just created a
>low pass filter to the INPUT of your motor - where you are feeding
>the PWM signal. The PWM signal is composed of may harmonics of the
>base frequency of the signal, and you have just cut them off to the
>motor. So, what the motor sees is a chopped sinusoidal at the frequency
>of your PWM signal because you cut off the the 'shaping' information of 
>the signal. I don't think the motor is going to respond to THIS signal 
>like you want it to.

>I must be missing something here, 'cause I have a feeling I am about
>to get a real warm butt from all the flaming I am about to receive.... :-)

>Greg

>---------------------------------------------------------------------------
>Greg Campbell                      Idacom - an operation of Hewlett-Packard
>email: gregc@idacom.hp.com    Voice: (403) 462-4545     Fax: (403) 462-4869
>---------------------------------------------------------------------------


Flames? Naaaah. But I must explain. It is true that the PWM signal
is composed of harmonics, *BUT*, in this circuit, it includes a
DC offset. You're probably thinking of the effects of filtering
a pulse train that is centred on zero. It is that DC offset that
ends up as (almost) pure DC to drive the motor, after filtering.
Does that do the trick?

	-Ken

	-Ken Willmott
