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In lugnet.robotics.rcx, Elizabeth Mabrey wrote:
> I just confirmed that some RCX happens to allow various output power to
> different output ports. In my case, the port A tends to produce more power
> thru motor and the port C does. I then switched the two motors. The
> condition stays the same. Therefore, the difference is not on the motors.
> Any input will be greatly appreciated!
> ----------------------------------------------------------------------------
> --------
> Best Regards,
> Elizabeth Mabrey
Sounds like more friction in the drive mechanism to one wheel than the other.
I've had the same problem with tank-driven robots. I was using the fast 9V
motors geared down, but they were under-powered for the application. The
friction in the drive trains was too great for the motor power. I think the
motors supplied with the RCX kit permit only the size and weight of robot from
the kit. Anything bigger requires two motors per port, which the RCX can
handle, though the batteries might not last as long. I decided for my robot
that 12V train motors would supply enough power - 8 watts each instead of 3
watts for RCX kit motors.
The RCX uses identical motor driver ICs that would have very little variation in
ratings, certainly not enough to vary the speed that much.
A solution is to change the way the robot is driven. Use one motor to do
forward and backward drive to both wheels and the other motor to do differential
drive. If the second motor is static, the robot can only go in a straight line.
A few very successful battle robots in UK robot wars use this technique.
Use differential gears to add the two motor drives together for one wheel and
subtract them for the other. You can also use two motors in the forward drive
to give more power to straight movement, enabling you to gear it up relative to
the differential drive for faster straight movement and slower spinning.
Most of the friction problems can then be eliminated by avoiding worm gears and
running the wheels at less than motor speed. Then the RCX kit motors can drive
a larger robot.
Mark
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