Newsgroups: comp.robotics
Path: brunix!uunet!gumby!wupost!micro-heart-of-gold.mit.edu!news.media.mit.edu!fredm
From: fredm@media.mit.edu (Fred G Martin)
Subject: motor driver for large current loads
Message-ID: <1992Oct20.165301.9903@news.media.mit.edu>
Keywords: Miniboard
Sender: news@news.media.mit.edu (USENET News System)
Organization: MIT Media Laboratory
References: <1992Oct19.214046.267@news.dfrf.nasa.gov> <1992Oct20.162818.9394@news.media.mit.edu>
Date: Tue, 20 Oct 1992 16:53:01 GMT
Lines: 42

Whoops, I missed the keyword "PWM" from your description of
motor-driving requirements on my first quick read and reply.

Well obviously, relays won't do.  Or will they?  Here's a control idea
I first saw in an article by Karl Lunt.  It's ideal for controlling
large motors using PWM, when you don't want to break the bank paying
for the H-bridge electronics required for all that current.

The concept is to use a single MOSFET (or HEXFET, or whatever) to do
the PWM, and a DPDT relay to flip the polarity when you want to drive
your motor backward:

			     -/			 o---------+
		           -/			           |  +---+
MOTOR SUPPLY POS.---------/    o----------o-------         |  |   |
			     |               /   o----+--- |--+   |
	      PWM control----+	             /        +    |
			              +----- /   o----+    |    MOTOR
                        	      |      /             |
		        	      |   o-------         |      |
              Relay control-----------+   |      o---------+------+
					  |
					  |
MOTOR SUPPLY NEG.-------------------------+

In the diagram, the electronic switch is represented by a simple
switch, controlled by the "PWM control" line.  The DPDT relay reverses
polarity to the motor depending on the "Relay control" line.

The nice thing about this circuit is that you only use one expensive
50 amp electronic switch, and you still get what you want:
bidirectional PWM control.  The assumption that you don't need to swap
directions rapidly, but this is reasonable, given your application
(wheelchair control).

The circuit is normally wired so that the unpowered state of the relay
maps to the robot (wheelchair) going forward, thus saving on relay
coil juice and relay lifetime!

Hope this helps.

	-Fred
