On Thu, Aug 30, 2001 at 06:38:23PM +1000, Roman Black wrote:
> Byron A Jeff wrote:
> >
> > After years of development ;-) I've finally have my prototype robotic lawnmower
> > platform to a usable state. The most valuable lesson I learned is that
> > everything in the drive path must be metal, clamped, and use lock washers.
> >
> > Next on the agenda is motor control for the drive motors. Bob Blick's
> > discrete h-bridge is a natural for the project. But of course you end up
> > with 8 power transistors, plus another one for the cutting motor.
>
> And you end up wasting a lot of energy on the
> forward losses of the bridge transistors, see below.
>
> > I wanted to get some ideas of the best way to heatsink a bunch of power
> > transistors.
>
>
> Hi, great to see the robot lawnmower taking shape!
> If you haven't put too much work into the drive motor
> h-bridges, I would suggest using the more old fashioned
> system, ONE transistor with PWM to drive the motor,
> and a cheap relay to reverse the motor wires.

Hmmm. I was thinking about the relay route. I got bogged down in trying to
figure out a simple control box for the beast.

PWM isn't an issue. I plan to run full bore all the time.

The relay control problem was associated with the fact that I have two
independant drive motors. The H-Bridge had the control infrastructure that
I wanted: forward, reverse, stop for each wheel. I could then turn that into
my true interface: forward, reverse, spin left, spin right (it's a circular
base) and stop.

I will go and rethink how to do this with DPDT relays and the transistor.

In fact now that I think about it, it may be able to be done with just 3
relays.


>
> You don't really need h-bridge complexity or cost,
> and with 5 amp motors you mentioned, the relay system
> is much more efficient, relay contacts have very little
> heat or waste. You can also optimise the ideal ONE
> transistor or FET for the motor, with a very low on
> voltage (Vce sat) and good base drive circuit.
> This is harder to do with a h-bridge. With a battery
> powered mower robot you probably want to keep it as
> efficient as possible.

True.

>
> And you only need 2 large heatsinks, one for each
> transistor. :o) You don't really need high speed
> control of engine braking, which is one area that
> the h-bridge might be better.

Agreed. I will go and rethink the problem.

BAJ

--
http://www.piclist.com hint: PICList Posts must start with ONE topic:
[PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads