Robo-Rats Locomotion: Pivot Drive

Pivot drive is a unique type of locomotion used by one of the Robo-Rat groups during the previous course (pivot drive is my name for the system).  The pivot drive system is composed of a two parts: 1) a four-wheeled chassis with non-pivoting wheels and, 2) a rotating platform which can be raised or lowered:

The wheels are the platform are driven by the same motor, although the platform is geared to rotate slowly.  When the platform is raised, the wheels will translate the robot in a straight line--the platform will spin but as it is not touching the ground it has no effect.  When a turn is required, the robot stops the drive motor and activates the motor which lowers the platform.  Once the platform is in the down position, the drive motor is activated.  Now the drive motor spins the robot since the wheels are off the ground.  When the robot has rotated to the desired heading, the drive motor is stopped and the platform is raised.  Now the robot can translate again using the drive motor.

This design produces mechanically guaranteed straight-line motion which is a real advantage for odometry.  However, the cost is the complex design necessary to raise and lower the platform while coupling power to the platform drive shaft.  A simpler design (but requiring more control hardware) would use three motors: one to drive the wheels, one to rotate the platform, and one to raise/lower the platform.  Fortunately, the raising/lowering of the platform would not require complicated position sensing, contact switches at the end points of travel would suffice.  The motor drive circuitry could be a couple of relays or transistors, as speed control is not necessary.

Motors:

2 - One to drive the wheels & rotating platform and one to raise/lower the platform.

3 - One to drive the wheels, one to rotate the platform, and one to raise/lower the platform.

Pros:

Control - Separate actuation of translation and rotation make control much easier.  Straight-line motion is guaranteed mechanically--there is no need for interrupt-based control as in the case of the differential drive method.

Cons:

Complexity - The 2-motor system is quite complex, the 3-motor system is easier to build but would require extra position sensing electronics.

Versatility - In this system translation and rotation are mutually exclusive, unlike the synchro drive or the dual differential drive.  This is not a big negative for the Robo-Rat competition, but for general use the inability to make arbitrary radius turns could be constraining.

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Last modified: 04/04/01 22:30