DEWBOT IX Caterpillar Drive
A number of concepts were considered and evaluated. The Caterpillar Drive approach was selected because it was perceived that:
- The climb time could be relatively short, allowing significant scoring by shooting Frisbees;
- the climbing mechanisms would not interfere with a shooter; and
- the concept was consistent with a low center-of-mass robot.
The ConceptThe concept of caterpillar drive was to "simply" drive up the pyramid's corner pole: a slick, powder-coated 1½" steel tube at a 60° incline from grade, interrupted by two horizontal corners extending 2½" from the corner pole's surface. So this becomes a drive-train problem.
In order to cross the two horizontal corners, three (3) drive wheels oriented along the robot's drive axis will engage the corner pole. Each of these three drive wheels may be independently disengaged and lifted away to allow a portion of the robot to cross over a horizontal corner; only one drive wheel will ever be disengaged at any given time while climbing. Each wheel has a "claw" to hold it to the pole and to also provide the wheel with the normal force needed for traction. The claws terminate with bearings so as not to impede the climb with friction. Only the drive wheels and the bearings at the claw ends actually contact the pole during climbing.
Skids on either side of the drive axis will rest against at least one pyramid horizontal at all times to prevent the robot from rotating around the pole. Since the robot chassis is shorter (27.75") than the distance between horizontal pyramid members (34.5"), the skids need to extend during climbing to insure that at least one pyramid horizontal is in contact with the skids.
We started by looking at the geometry, power and normal force requirements for climbing the pyramid's corner pole. It turned out that at a linear climbing velocity of 6 in/s (which was as fast as anyone was comfortable with), either a single CIM or MiniCIM could provide the necessary power. Traction would require that we increase the normal force between the robot wheel and the pole by at least 100 lbf above the force provided naturally by gravity.
Taking this analysis a step further, we adapted the team's drive-train mathematical model to the pyramid climb. We ultimately settled on a single CIM motor reduced by a 54.4:1 ratio driving 1.1 inch effective diameter wheels. Terminal drive speed is calculated to be 4.6 in/s. The CIM should run at 26.9 amps during the climb; pretty close to optimum efficiency for the motor.
- The bottom of each caterpillar leg has a driven bowtie wheel and a claw assembly. The bowtie wheel drives the robot up the pyramid corner pole, while the claw clamps the robot securely to the pole and also provides the necessary normal force to provide the wheels with the traction needed to climb the pole. The claws contact the pole with ball bearing rollers, so as not to provide friction inhibiting the climb.
- Claws are closed around the pole using a 2" bore x 2" stroke pancake pneumatic cylinder. At 60 psig working pressure, these cylinders provide 162 lbf on retraction. Each caterpillar claw is driven by its own pancake cylinder. Torsion springs open the claws when the pancake releases.
- Each caterpillar leg is mounted to the chassis via a pivot point at the top which allows the leg to swing back and upwards to clear the pyramid's horizontal corners. 2.5" of clearance is needed. The front and middle caterpillar legs are retracted and extended via a 1-1/16" bore x 4" stroke pneumatic cylinder. The rear leg is held down by a latex spring and is raised passively by the pyramid corner when the robot drives over it.
Power source & transmission
- The caterpillar drive is powered by a single CIM motor with an AndyMark Toughbox Nano (am-0518) 12.75:1 gearbox. To reduce mass, the two 50T steel gears were replaced with aluminum equivalents. Further reduction is achieved by type 35 chain & sprockets in two stages: 24:15 and 24:9, yielding an overall 54.4:1 reduction from CIM to wheels.
- The transition between the first sprocket reduction (24:15) and the second (24:9) is via 3/8" steel drive shafts (one per caterpillar leg) which transfer power from the right to the left side of the caterpillar drive unit. These drive shafts also serve as the pivot/swing axes for the caterpillar legs. By utilizing the power transmission axles as the leg pivot axes, chain length problems during caterpillar leg retraction and extension are avoided.
- The 24T sprocket at the end of the second sprocket reduction is mounted on and drives the ½" Al wheel axle at the bottom end of each caterpillar leg.
- A pair of skids serve to stabilize the robot during climbs by engaging and resting on at least one pyramid horizontal member, thereby preventing the robot from rotating around the pole as it climbs.
- Skids are 1.5" OD x 1.0" ID x 27.5" Acetal (Polyoxymethylene) tubes secured to the chassis using 80/20 1010 stand-offs. Acetal was selected for its mechanical toughness and extremely low coefficient of friction. Skids are positioned so that only one skid at a time typically contacts a pyramid horizontal while climbing (typically the left, battery side), but with skids on both sides, robot rotation in both directions is limited.
- Since the skids are 7" shorter than the interval between horizontal pyramid members (34.5"), skid extensions are provided to bridge the gap. The skid extensions are 15" lengths of 1.0" OD acetal rod axially connected to the rod of a 3/4" bore x 8" stroke pneumatic cylinder. Both acetal extension and pneumatic cylinder are housed coaxially within the skids. When actuated, the skid extensions provide an additional 8" of skid length, sufficient to bridge from one horizontal to the next. The forward ends of both the skids and the extensions are angled at 45° to facilitate alignment.
- We wanted self-centering wheels and were initially inspired by boat trailer rollers. These rollers had been used by some teams to successfully climb the tower at the end of Breakaway. We called these bowtie wheels.
- Besides self-centering, the wheels need to have an adequate coefficient of friction against the pyramid's powder-coated surfaces, a tough requirement as we had no samples of the pyramid surface and saw this for the first time at Hatboro-Horsham. We understood that the pyramid surface was likely to be fairly slippery.
- Our bowtie wheels went through several generations of development:
- Saw-cut purchased neoprene boat trailer rollers
- 40-durometer polyurethane rod cut on a lathe
- Neoprene cut on a lathe
- Molded 80-shore hardness polyurethane using a formulation containing a mold release
- Molded polyurethane using the same formulation without mold release
- A composite molded "superwheel" with the above polyurethane and internal polymer skeleton
- Lathe-cut wheels had rough, friable surfaces with inadequate friction coefficients (although this point is confounded by the subsequent discovery that the 4-40 bolts securing these wheels to the axles had sheared). The neoprene wheels also tended to leave a lot of rubber on the pyramid surface. Inadequacy of the neoprene wheels lead us to the decision not to climb at Hatboro-Horsham. The molded polyurethane wheels have a durable, smooth, frictive surface and demonstrated their ability to climb the pyramid without slipping.
- The wheels are secured to the ½" Al drive axle via specially modified ½" Al collars which fit into recesses in both ends of the wheels. The collars are bolted to the wheel (originally with (2) 4-40 x 7/8" FHCSs and later with (4) 6-32 x 7/8" FHCSs after the 4-40s were found to shear). The collars are then clamped to the axle, making a secure connection.
- We'd like to thank and acknowledge FRC Team 357 (Royal Assault), for their inspiration and guidance in molding polyurethane wheels.
- Induction type proximity sensors are mounted in front of each of the three legs to sense the leg's approach to a horizontal corner.
- Each leg's return to a lowered position is sensed by a limit switch.
Two levels of state machines control the caterpillar drive. The lower level of state machines controls the sequence of actions for raising and lowering individual legs. The higher level of state machines, seen below in the state machine diagram, controls when to raise or lower individual legs, extend the skids, and turn the caterpillar drive motor on or off, based on sensor inputs and hard-coded timing. The first three states have operator controls that determine the state of the caterpillar drive. Once the first horizontal rung of the pyramid is sensed by the front leg's proximity sensor, however, the state of the caterpillar drive is determined solely by the signals from sensors and hard-coded timing.