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We were provided with a much larger and more powerful brushed DC motor, but had avoided using it up to this point due to its size and weight. In order to minimize both of these parameters, we removed the drive's encoder, associated electronics, and all-metal gearbox. We then printed a set of reduction gears using spur gears from our gear generator to decrease speed and increase torque at the gearbox output. After several quick iterations, we found that a reduction of 36.75:1 (8:24, 8:28, 8:28) produced approximately 15 RPM with more than adequate torque to drive our entire robot. 15 RPM results in one step every ~2 seconds, which is ideal for our robot with its sliding counterweight.

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Our first design was a four-bar mechanism intended to carry the motor's battery pack as weighta counterweight. The problems with this concept were that the battery pack did not weigh enough to balance the robot and that traditional bar mechanisms are smooth and continuous, whereas our counterweight needed movement that was sharp and discontinuous. Ideally, our counterweight would sit motionless above the grounded foot, then move quickly to the other foot during the brief period of time where fraction of a second that both feet were in contact with the ground. We produced a four-bar mechanism that covered the desired range of motion but could not produce the desired time trajectory. Also it did not have the structural strength and stiffness required to hold a sufficiently sized counterweight mass.

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