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Re: Yes, there is a difference.
Posted by Dave... at 2/8/2001 9:03 PM EST
Engineer on team #5, Robocards, from Melvindale, MI and AFL (Alcoa Fujikura Ltd).
In Reply to: Yes, there is a difference.
Posted by Al Skierkiewicz on 2/7/2001 9:11 AM EST:
DC motors operate by inducing a magnetic field created by the armature within the magnetic field of the permanent magnets in the housing. As any elementary science teacher can attest to, a wire wrapped around a nail with DC current running through it will create a directional magnetic field, and by changing the polarity of the wires, the magnetic field will change 180° (N becomes S and vice versa).
The armature is wrapped with magnet wire from the inside out, and so running current in opposite directions will not only change the direction of the magnetic field from the armature, but will interact differently with the magnetic field from the permanent magnets, which does not get to change its directional field.
I imagine that a DC motor which does not use permanent magnets but rather windings in the housing to create the second magnetic field is not as susceptible to power loss in forward vs reverse. Both fields would change 180° any time the polarity was reversed, keeping roughly the same power in either direction.
We simply limit the power to the stronger drive motor by a trial and error method in the programming. If the robot steers to the right, we decrease the maximum power available to the left drive motor (e.g. p1_y = p1_y MAX 228) until the thing goes straight. Crude, but effective and easy to change.
My two cents worth.
: I have checked with Raul and Jim B and they HAVE experienced a variation in forward/reverse speed. In past years a thorough investigation showed as much as 10% variation in speed from unit to unit and forward/reverse. As Joe has suggested, (we agree) getting drivers to invest time behind the wheel is the best way to overcome the problem. I am still having a problem in analyzing why this occurs. Production variations can explain unit to unit changes but not for/rev. Any other input?
: Al
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