Subject:
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R/C cars as robot chassis
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Newsgroups:
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lugnet.robotics.handyboard
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Date:
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Tue, 12 Nov 1996 17:44:23 GMT
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Original-From:
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Chuck McManis <cmcmanis@freegate.net+NoMoreSpam+>
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Viewed:
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1550 times
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Here is a rule of thumb to keep handy when converting R/C cars to robots;
o If the R/C car used either NiCd batteries, or it had a NiCd battery
pack, the motors will draw too much current for the Handyboard.
This is strictly true of any controller that uses the L293D or TI equivalent. The
ideal motor for the L293D is either a "lego" motor or a converted R/C servo.
Both are low current motors, that achieve torque by using a lot of gearing and
thus make for slow (but much more stable) robots.
If you do convert a chassis that uses motors driven by NiCds, then build a
new H-bridge to control them. You can do this with either MOSFET or BJT
transistors in a switching ( full staturation) configuration. If you use BJTs (like
TIP120's for example) you may need to raise the voltage slightly to compensate
for the drop the transistors introduce (nearly 1.2 volts in some cases)
To get an idea of the amount of current you could draw, measure the coil
resistance of the motors (connect an ohmmeter across the leads of
the motor) This is the 'pure resistive' load of the motor when the shaft
is not turning (ie it is 'stalled', which also occurs when you apply power
to start it, but it has not yet started rotating). Typical resistances will
be one ohm or so, however I've seen R/C car motors with resistances
lower than that. For a 7.2V NiCd pack (5 cells) the 1 ohm resistance
means you will draw 7.2 amps on stall from the batteries, actually it
will be closer to 8 amps on a new battery pack) Be sure your h-bridge
can handle that current.
--Chuck
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