DEWBOT V Drive Train
Executive Summary
A 6 Wheel Drive (6wd) drive-train with the chassis oriented wide was selected for this year’s robot on the basis of providing superior maneuverability on a low-friction surface.
Wheelbase is 20.25” (long) x 33.25” (wide). Center of Mass (for the Robot) is about 9” above the Playing Surface and about 2” aft of chassis center.
Drive-train is Tank, with independent Left and Right drives. For each side, Middle wheels are direct-driven by custom gearboxes. Front and Rear wheels are slave-driven from the middle wheels via independent chains, minimizing the impact should the robot lose a chain.
Direct-drive gearboxes are based on AndyMark Shiftboxes modified to: 1) add an additional axle and reduction for direct drive; and 2) disable the shift mechanism (unnecessary for Lunacy). (1) CIM motor is used per drive. Overall reduction ratio (CIM to Wheel) is 24:1, providing a maximum speed of 5.9 ft/s and more than adequate torque for turning.
A novel 7th Wheel is provided at the center-rear of the chassis, driven independently via a direct Globe Motor. The 7th Wheel is beneficial for 1) rotating (spinning) the robot in-place; and 2) braking. The 7th Wheel is oriented perpendicular to the 6wd and may be engaged or disengaged by the Pilot/Driver. When disengaged (default), the 7th Wheel is raised off the playing surface and the robot drives in 6wd Tank mode using dual joysticks. When the Pilot/Driver holds down the trigger on either drive joystick, the 7th Wheel is lowered via a pneumatic cylinder to contact the playing surface. This also takes weight off the (4) wheel 6wd wheels so that the robot is able to pivot rapidly on (3) wheels, Front Left, Front Right and 7th. When engaged, the (3) drives are controlled in Arcade mode using the triggered joystick. Releasing the trigger returns the 7th Wheel to its default raised and inactive position (and the driver joysticks to 2-joystick Tank mode).
Development
Inadequate drive-train performance in our 2008 robot inspired the team to prototype and test several drive-train/chassis designs during the summer of 2008. Observations led the team to understand mathematically the mechanics of steering and driving. A drive-train mathematical model was developed. Based on these experiences, a hypothetical 6wd drive-train / chassis design was developed which, the team believed, would offer excellent drive-train performance under a wide range of potential game conditions. This hypothetical design included custom gearboxes directly driving the middle wheels, with slave chain drive for the front and rear wheels.
Rover Wheel / Regolith friction coefficients were determined experimentally. These coefficients are, naturally, far lower than those of past experience. Notably, transverse direction friction coefficients (both static and kinetic) are somewhat higher than in-line.
The low friction coefficients raise challenges for a drive-train. Turning torque calculations reveal that many chassis configurations will have difficulty steering. These calculations indicated that a 6wd drive-train oriented wide (front & back are the long sides) would provide good steering performance.
A wide orientation reduces front-to-back stability due to the shorter wheel-base. We considered this an acceptable trade-off, as the low-friction surface will keep acceleration low (no wheelies), the trailer should help stabilize the robot (front-to-back) and there are no ramps to climb. Still, we endeavored to keep center-of-mass low and centered. Wheel-base was made as long as possible within the short side dimension.
Drive-train modeling showed no benefit to using (2) CIM motors per side, so (1) was employed. For control good on the slippery surface, a modest top speed (~6 ft/s) and single-gear drive was selected.
All 6 drive wheels are on the same plane (no attempt made to lower middle wheels).
Bearing blocks for front & rear wheels are mounted on 80/20 rails, enabling easy and fast adjustment of chain tension.
Frame
Chassis frame is MIG-welded Aluminum.
(2) forms were created to assist in accurately welding the frame.
Specifications
Drive-train type: 6 wheel drive (Tank Drive) alternative 7th wheel 3 wheel drive pivot mode
Motors: (1) CIM each side (1) Globe driving 7th Wheel
Gearbox: 24:1 reduction single-speed direct drive (½” keyed shaft)
Transmission: Direct drive of Middle & 7th Wheels Slave Chain Drive for Front & Rear Wheels Type 35 Steel Chain & 22-tooth sprockets
Wheels: Rover Wheels (6” diam)
Wheel-Base: Length: 20.25 in Width: 33.25 in (to wheel centerlines)
Max Speed: 5.9 ft/s
