Subject:
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Re: Add motor driver to RCX
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Newsgroups:
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lugnet.robotics.rcx
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Date:
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Thu, 5 Aug 2004 21:32:12 GMT
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Viewed:
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4274 times
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In lugnet.robotics.rcx, John Barnes wrote:
> In lugnet.robotics.rcx, Steve Hassenplug wrote:
>
> > > John Barnes (HiTechnic) has available a motor amplifier which draws it's power
> > > from an external source, and can control 4 motors from 1 RCX ouptut. I haven't
> > > tried one, but you might find it helpful: http://www.hitechnic.com/Motoramp.htm
> >
> >
> > 1) I'm not sure these are currently available
> > 2) They only give you independent control of two motors per RCX output (but you
> > can connect motors together)
> > 3) They won't work with the 8366 motors (they were designed before those were
> > available)
>
> The amplifiers definitely won't work.
>
> I am bringing a demo 'bot to BrickFest which uses the buggy motors with an RCX.
> The RCX actually drives regular technic gear motors which drive a cam shaft
> which operates the buttons on the 9v (6 x AA) battery box. So in a sense, the
> gear motors, cams and battery box buttons are a mechanical amplifier. And by
> golly, the resulting 'bot has some serious pep! The first time I tried it on the
> kitchen floor (only non carpet floor in the house), it nearly killed the cat!
> Took off at amazing speed and destroyed itself against the 'fridge. Since then,
> I have geared the buggy motors down - not to get more torque, but - to slow it
> down enough to where I can catch it!!!!
>
> So while we all sit and ruminate on how to pass the elusive float state to an
> external H bridge chip, you can always try what I did and risk life and limb in
> the process ;)
>
> JB
I've drawn a circuit to control more powerful motors (sadly in Visio format at
the moment, though I my convert to jpeg later to post it). Since the H-bridges
have flywheel diodes across the transistors, this will short the motor when the
bridge is turned off, so no float state is possible.
However, the same scheme can drive 6 motors from the RCX, two at a time at full
speed or four at a time with the speeds of two motors in a pair adding up to
full speed. The sceme could be expanded to drive 9 motors one at a time.
There are three modules, an opto-isolator module, a logic module and an H-bridge
module.
Using one of each to begin with:
Two opto-isolators with their LEDs back to back give the direction and speed
signal: 0,0 for off, 0,1 for forwards and 1,0 for backwards.
In a logic module these signals A and B are turned into A.!B and !A.B by four
NOR gates on a 4001 chip. CMOS chips are used since they drive MOSFETS easily.
The two signals drive opposing pairs of power MOSFETs in an H-bridge. The
MOSFETs will dissipate I*I*RDSON watts.
I intend to run 1 Amp load continuously, so will assume 2 Amp load, therefore to
get 0.5W of heat (probably low enough to do without a heatsink) I need RDSON to
be 0.125 ohms or less. I'll look for one in a TO220 package, such as a BUZ10,
which is rated at 70W, VDSmax 50V, IDmax 20A, I/P capacitance 550pF (affects
turn-off time for reversing), RDSON 0.08 ohms. Running at 1 Amp continuously
(maximum train load on my layout) I'll get 0.08W of heat. 2 Amps will give
0.32W.
In six-motor configuration:
Use one opto-module per output (3)
Use one logic module per opto module (3)
Use three additional logic modules, with a 4081 AND gate chip on each.
Use 6 H-bridges.
Outputs A and B drive motors. Output C switches between the 3 pairs of motors,
feeding into the AND gates. The logic module for output C requires one extra
NOR gate to decode a !A.!B state, to enable pair A on 0,0, pair B on 0,1 and
pair C on 1,1.
If output C is set to forwards at speed 5, it will give 0,1 for 6/8 of the time
and 0,0 for 2/8, so motor A1 will go at speed 1 and motor A2 at speed 5.
The nine-motor configuration would use output A to drive the motors and outputs
B and C (3 states each = 9 combinations) to switch between motors. 3-input AND
gates would be required.
I intend to get hold of the components to build three single drivers and one
6-motor driver, and start experimenting. That's 36 MOSFETs and 12
opto-isolators, so about $100!
If a single one works on breadboard I'll make a veroboard one. I'll cut 9V
leads on half to make all the connections, so there's no modification to the RCX
of any other plastic parts.
If you want to use one for an RC Car, check the MOSFET calculations for the
power and voltage drop required. RDSON of 0.125 ohms at 2 Amps will drop 0.25V
across each MOSFET, losing 0.5 Volts, so the power supply should be 9.5 volts to
compensate. Voltage drop will rise linearly with current and heat dissipated
with current squared. If RDSON is too high, mounting heatsinks on your car will
slow it down :-)
For high power I suggest BUZ11 MOSFETS, with RDSON of 0.03 ohms, giving 0.27W of
heat and 0.18V dropped at 3 Amps. These cost 1.8 times as much though.
Mark
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Message is in Reply To:
 | | Re: Add motor driver to RCX
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| (...) The amplifiers definitely won't work. I am bringing a demo 'bot to BrickFest which uses the buggy motors with an RCX. The RCX actually drives regular technic gear motors which drive a cam shaft which operates the buttons on the 9v (6 x AA) (...) (20 years ago, 5-Aug-04, to lugnet.robotics.rcx)
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