2) Design Process Group 13

When discussing the demands that would be placed on our robot in order to get it to climb a ladder, we realized that we would need to design a mechanism that would be able to interact with the rungs of the ladder. A big part of this would involve inserting an end effector between rungs, a motion of the mechanism to drive the vertical motion of the body, and retracting from between the rungs so that the end effector would not inhibit the robot’s further vertical movement.

In our first major iteration of the robot, we pursued a design that employed four fourbar mechanisms to drive the vertical motion. Because we wanted to prevent the robot from falling off the ladder, it followed that there must always be an end effector engaged with the ladder. To accomplish this, we arranged four fourbars on our robot (one in each corner) and had two arms perfectly out of phase with the other two. To eliminate any concern of arm collision or interference, we conceived of a design that allowed for each set of arms to operate in its own plane by offsetting the lower arms. All of the fourbar mechanisms and arms would be driven off of a single motor that would actuate the driveshafts.

Figure 2.1: Image of the first design iteration of the ladder-climbing robot.

Although our first design largely accomplished our goals, we realized that the horizontal offset of the arms would be difficult to produce outside of CAD.  Thus, we pursued a second design that eliminated the need for the awkward step in the chassis walls. In this new design, we moved one set of arms to the inside of the chassis, thereby removing the chance that the arms would interfere with each other. We attached all four fourbar assemblies to a single hex shaft to drive all of them at the same time. We would drive the hex shaft with a DC motor that would be stepped down to achieve about 5 rpm. Again, the fourbar arms would be perfectly out of phase to ensure the motion that we wanted.

Figure 2.2: Image of the second major CAD iteration of the robot.

Unfortunately for our second major iteration, we discovered a critical flaw: when the fourbar assemblies go through their cycles, links would interfere with the main drive shaft, thus rendering the design inoperable. This prompted our third and final major design, the “Gamma'' iteration. Learning from this mistake with the driveshaft, we once again split the fourbar assemblies into two groups, with a hex shaft for each group. Each hex shaft would drive two fourbar assemblies. We designed a pair of inner walls to mount one pair of arms, and left the other pair of arms on the outside of the chassis to avoid interference and collision problems. The hex shafts were mounted in their respective positions with bushings or bearings, and both were driven by the same DC motor stepped down to about 5 rpm. The gears were 3D-printed to ensure that they fit our needs. To reduce manufacturing time, the fourbar linkages and the majority of the body were designed to be laser-cut. All non-driven joints were to be constructed from nut/bolt pairs, with spacers as needed.

Figure 2.3: Image of the Gamma Design iteration of the robot CAD.