TEAM MEMBERS
Wendy Siu
Mark Holland
Zichong Li
Introduction
The overall goal of this project was to create a mechanism able to climb a ladder. We decided to theme our project as a monkey climbing a ladder. The monkey would have two arms that pulled itself up the ladder and a tail that acted as a counterbalance for stability.
Initial Design
There were The two main kinematic mechanisms to design which were the climbing interesting mechanisms are the arm mechanism and the tail mechanism.
The Climbing Mechanism
We needed the climbing mechanism to have a specific motion. The cycle of motion was to reach in between the rungs, pull down, move back behind the rungs, reach up, and repeat.
Figure 1. Link Lengths
The lengths of each link in the monkey arm were labeled as in Figure 1. The changing length of R3 is the result of the radius of the rotary wheel.
MATLAB Code
The Matlab Code produced the following two graphs shown as Figures 2 and 3. The first graph shows the length of R3 versus theta. The second graph shows the position of the point P for each full rotation of the rotary wheel. The code can be found separately under this page.
Figures 2 and 3. R3 length vs. Theta, and Position of point P.
From the resulting position plot in Figure 3, the tip of the monkey arm, point P, would have 3 inches of vertical clearance. The ladder was then designed off of this clearance. The motion of point P emulates the back and forth climbing motion that we desired. The two arms needed to be 180 degrees out of phase so that two arms could be used instead of four. This also meant that only one arm would hold the weight at once.
Figure 4. Two arms linked together by one axle.
The Tail Mechanism
The tail mechanism was initially the same as the climbing mechanism because we thought that the back and forth motion generated would produce a swinging motion.
Figure 5. First Internal Design
Part Implementation
The rotation shoulder joint was modeled as a linear bearing with rotation provided by a plastic turntable. Two of these joints were used in the robot.
Figure 6. Rotating Shoulder Joint
A worm gearbox was used to generate as much torque as possible. The gearbox provided a gear ratio of 336:1. The motor’s stall torque was 4.9 mN*m with a speed of 5040 rpm. With this gear ratio, the stall torque was 14.58 in*lbs, and the new speed was 15 RPM. We calculated our torque requirements to be 12.25 in*lbs based on the weight of the parts, so this motor and gearbox was sufficient.
Figure 7. Worm Gearbox. Figure 8. Assembled worm gearbox with rotary wheel attached
The rotary wheel, ball bearings, and rod eyes were used together to connect the gearbox’s axle to movement of the arms. The arm, a ¼” diameter hardened steel rod used specifically for the linear bearing, was threaded through the rod eye and held in place using ¼” O-rings. The bearing was press fit into the rotary wheel, and the rod eye was attached to the inner part of the ball bearing. The angular portion of the arm was made by bending metal tubing around the rod.
Figure 9. Parts used in Arm to Wheel Connection
Figure 10. Arm Implementation Concept
A set of bevel gears were connected to the gearbox’s axle to provide motion for the tail mechanism. The gear was attached to a aluminum mounting hub which was attached to the axle via set screw.
Final Design Concept
For the final design, we kept most of the initial design concepts except for the tail mechanism. The tail mechanism was changed to a simple rocker mechanism as shown in Figure 12.
Figure 12. Simple Rocker Mechanism.
We chose to use this design rather than the arm mechanism design because this mechanism would produce a true rocking motion, as opposed to the climbing motion of the arm mechanism. The implementation of the tail mechanism involved the use of bevel gears to switch planes to the backside of the monkey to swing to tail. The rocking motion of the tail was synchronized with the motion of the arms - the tail would swing to the side of the pulling arm to balance the weight of the monkey. The implementation can be seen in Figure 13.
Figure 13. Tail Implementation
The bevel gear was attached to the housing using a bolt and pin clip. The pin joint on the bevel gear was similarly implemented as the arms. This joint again used a ball bearing, a rod eye, and O-rings to hold the rod in place. The rotating turntable served as a rotating, grounded joint. The final joint connected the wooden rod to the tail, and was implemented using two rod eyes and held in place using O-rings.
After initial testing of the climbing mechanism, we found that the robot weight was imbalanced. This imbalance caused the monkey to tilt backwards which made its arms ineffective. Wheels were added to the side of the robot to stabilize it against the ladder. These wheels were taken from pulleys. The ladder rungs were spaced 3" apart, and the rung spacing was determined by the using the MATLAB code and through initial testing.
Figure 14. Ladder. Figure 15. Close up of Pulley Wheels.
Finally, the internals of the final build are shown in Figure 16.
Figure 16. Internals of Monkey Robot
The arm mechanism is a variation of an inverted crank slider, while the arm mechanism is a crank and rocker mechanism.
GIF Files
Climbing the Horizontal Ladder
Tail Mechanism
Arm Mechanism
Conclusions