Alt-BEAM Archive

Message #05858



To: beam@sgiblab.sgi.com
From: Bruce Robinson Bruce_Robinson@bc.sympatico.ca
Date: Wed, 25 Aug 1999 21:10:08 -0700
Subject: [alt-beam] Re: New Guy Questions


Ben Hitchcock wrote:
>
> ... For the rear legs, I used the H-bridge chip
> that came with the servo. This was very small,
> and works extremely well. Of course, I hooked
> it up with both inputs high the first time, and
> it smoked. ...
> ... After that I rewired my microcore to be active
> low, instead of active high.

And Tom Mairs wrote:

> I prefer H-bridges, but you have to be careful
> not to let both sides go active at the same time
> (smoke!) .

Let us not overlook the original H-bridge design Mark Tilden described.
He used a 74HC139 chip to prevent smoking a bridge. (It doesn't matter
whether the inputs are active high or active low).

This is a decoder chip: it takes two inputs, and for each of the four
possible input combinations, it makes ONE output go low (the others
remain high). If you use an H-bridge design that triggers on a low
signal, you can use it directly with this chip. If your H-bridge design
triggers on high signals, then you'll have to use a couple of inverters
on the 74HC139 outputs.

Another point -- the '139 chip is a dual device -- it has two
independent decoders, so you can manage two H-bridges with one chip.

The original Tilden paper can be found at:

http://people.ne.mediaone.net/bushbo/beam/BiomechMotorBridges.html

and it's an excellent tutorial on the H-bridge -- too often overlooked
in my opinion.

Regards,
Bruce



5859 Thu, 26 Aug 1999 15:10:34 +0000 [alt-beam] Re: New Guy Questions beam@sgiblab.sgi.com "Ben Hitchcock"
>From: Bruce Robinson

>Let us not overlook the original H-bridge design Mark Tilden described.
>He used a 74HC139 chip to prevent smoking a bridge. (It doesn't matter
>whether the inputs are active high or active low).
>
>This is a decoder chip: it takes two inputs, and for each of the four
>possible input combinations, it makes ONE output go low (the others
>remain high). If you use an H-bridge design that triggers on a low
>signal, you can use it directly with this chip. If your H-bridge design
>triggers on high signals, then you'll have to use a couple of inverters
>on the 74HC139 outputs.
>
>Another point -- the '139 chip is a dual device -- it has two
>independent decoders, so you can manage two H-bridges with one chip.



Ahh yes, but I fitted ALL the electronics into the servo cases. All except
the batteries. I simply didn't have room for another chip in there. Also,
if you are going to use a 74HC139 chip to protect the H-bridge, then why not
just use that (or similar) chip as the motor driver?

I also used the 5K pot in the rear servo for centering - alleviating the
need for messy springs or end stops. The front servo is angled such that
gravity centres that servo. I had to pull the pot out of the front to make
room for the microcore.

Ben



5860 Thu, 26 Aug 1999 14:02:13 -0300 [alt-beam] Re: Newbie question alt-beam@egroups.com michael.hirtle@ns.sympatico.ca (Michael Hirtle) u can reverce a motor with an H-bridge

mr_squishee@hotmail.com wrote:

> Hi,
> I'm new to BEAM and stuff like this in general and what I wanted to
> know is, How do you reverse a motor?
> Thanks,
> Steven M
>
> ------------------------------------------------------------------------
>
> eGroups.com home: http://www.egroups.com/group/alt-beam
> http://www.egroups.com - Simplifying group communications



Home