Matching Drill Motors and Fischer-Price Motors

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Posted by Tim Gates at 2/14/2001 9:33 AM EST


Engineer on team #288, RoboDawgs, from Grandville High School and X-Rite/Delphi.



A 4 to 1 gear ratio brings the no load RPM range of the two motors within reasonable range of each other. A 3.6 to 1 might be better, but we used a 4 to 1. This leaves the FP motor with a slightly higher RPM than the drill motor. We ran both motors individually on the same load and measured the RPM over the entire range of the speed controller. Unfortunately the load wasn't enough to keep the RPM under the inherent balance of Counter Electromotive Force(CEMF) and Electromotive Force(EMF), the balance between the volatge applied to the armature and the voltage induced into the armature as it spins through the flux from the field magnets. However, if you apply excessive load to the drive train and lock rotor, there is no CEMF and both motors are contributing the maximum torque possible to the system. This torque is limited by the DC resistance of all current paths involved from the battery to the armature. As you lessen the load towards the baseline friction of the two motors, the two motors continue to contribute torque to the system until one of them hits the RPM where its CEMF opposes EMF enough to limit rpm. At that point if you increase RPM without increasing the current through the armature, the CEMF will exceed EMF and the motor becomes a load on the system. I haven't completed enough test yet to prove it, but I intuitively believe that a 130lbm on carpet is enough load to keep the RPM below the situation described above. The other consideration is regenerative braking. When the speed controller output drops to nothing and the robot is coasting, the motors become generators. I believe the jumper on the speed controller determines how that current is handled.