The objective of this
capstone design project was to
integrate the the undergraduate training in
Mechanical Engineering, and apply this knowledge to the design of a
mechanical system. We chose to invent, design and fabricate a
stair-climbing mechanism that would climb a simple stairway. An extended
objective was to make the mechanism capable of walking up and down an
inclined plane.
Figure
1. Stair Climbing Mechanism: Front view
Project
Discussion
The
work on this project was distributed over two semesters. The first
semester was spent in problem search, selection of a specific
problem, prior art study, concept generation and evaluation, modeling
and analysis, followed by
detailed design. Once the design was complete, detailed part and
assembly drawings were generated for submission to the machine shop.The next stage in the project was carried out in the following
semester. This involved fabrication, assembly and testing.
We
decided to build a three-legged robot to ensure transverse stability.
Since there was no mechanism to shift the CG of the system, the option
of a two-legged robot was ruled out. The two outer legs were coupled and
always moved together. The kinematics of human motion was closely
studied and was then exploited to define the motion geometry for the
stair climbing mechanism. A kinematic design analysis was performed
keeping in mind the motion of each limb and interference avoidance with
the steps. Based on this analysis, limb lengths were decided. Each leg
had a lower and upper limb. Once the kinematics was taken care of, the
focus was shifted to the actuation strategy. Two DC motors was mounted
on lower limbs, one for the outer legs and one for the inner leg. The
motor shaft was attached to the respective upper limb. When the first
motor (mounted on say leg.1) was turned on, the lower limb of leg.1
remained stationary, pressed against the step, while the upper limb
rotated forward, thus lifting the other leg (leg.2). To avoid
interference with the steps, the first motor also lifted the second leg
(leg.2) from the ground by means of a pulley and strings. The first
motor would stop as soon as the second leg rested on the next higher
step. At the very same instance, the second motor would turn on and
repeat a similar process. It was envisioned that by means such
synchronization, the mechanism would repeatedly scale steps.
Figure
2. Stair Climbing Mechanism: Rear view and Side view
Performance
and Conclusion
Despite
a very serious effort, this mechanism failed to meet the final
objective. It could not scale any steps, nor could it walk up or down on
an incline. There were some very harsh lessons learnt from this
exercise, since an year’s labor yielded no significant results. In
retrospect, it was realized that the project had been poorly planned and
managed. There were some serious technical flaws as well. Despite the
kinematic design being sound and innovative, the dynamic analysis was inadequate. There was no sensing or feedback control. The
motion synchronization relied on crude on-off switches. This was a very
ambitious project that required better technical expertise and project
management skills to be successful. Regardless of the failure, this was an
immense learning experience in many regards (e.g. to save costs on
gearbox, we pulled out the gear train from old mechanical clocks).
