SGS - Manufacturing and Assembly

Overview:

We relied heavily on our lessons learned from previously building the crank-slider. We planned to use laser-cut acrylic for our links, drilling and reaming the holes to ensure they fit precisely and would be perpendicular to the face of each link. We iterated the design of the housing to allow for easy disassembly in case a part needed to be adjusted or replaced, with a final result that was portable and sturdy. The final prototype relies on external power: a USB charger to power the Arduino and a typical 120 V wall outlet for the motor power supply.

Robotic Linkage:

We began by building a SolidWorks model of each link and combining them into an assembly (Figure 10). This allowed us to plan to avoid any collisions of the links, shafts, and casing, as well as qualitatively validate the results of our link length optimization. To keep the links from interfering with one another, we used varying lengths of shaft to space them into three distinct layers. These shaft lengths were also kept conservative in total length to ensure all links move in the same plane. An additional geometric constraint was detected: one of the grounded joints would have to be fixed by shafts on the opposite side of linkage from the motor connection and other ground joint. To address this, we decided to use the front-facing, clear "display" acrylic sheet to act as the ground link for that joint. The other ground link, which would serve as an opaque background for the linkage, was made out of nicer hobbyist plywood. The holes for the shaft, bearing, and motor shaft in the ground links were drilled out in the same manner as those for the linkage. The side profile view of the link layers and their position between the ground link sheets is shown in Figure 8 below.

Figure 8. Spacing of the link layers in between the acrylic and plywood ground link sheets.

Housing:

The construction of the housing was done entirely with wood, using wood screws to mount the heavy motor to a pine 2" x 4" and smaller machine screws to fix the power supply and Arduino to a third sheet of plywood. The legs that separate the acrylic and plywood were cut out of a 2" x 4" into uniform cubes, and the longer legs to make room for the motor, power supply, and controller were cut in a similar way. An important step to making these pieces uniform and aesthetically pleasing was to run the original 2" x 4" first through a jointer and second through a planer to produce clean, flat faces that meet at sharp 90° corners. The cuts were made using a table saw with a setup to ensure they were of uniform length. Long (5.5") screws were used to hold the housing together. We drilled out the holes after first using wood glue to connect the long and short legs to the middle plywood piece, and could then use nuts and washers to squeeze everything together and hold it in place. Views of the housing with motor, power supply, and controller are shown in Figures 9 and 11.

The 2" x 4" used to mount the motor in place was cleaned up similarly using the jointer and planer, and was cut to length of the housing using a miter saw. To allow the motor wires to be connected to the controller to receive power, a small channel was cut into the side of the 2" x 4". The motor was mounted by piloting four holes and using wood screws to firmly hold the motor's Delrin mounting block to the wood. Finally, the 2" x 4" with motor attached was fixed to the bottom plywood sheet using four more wood screws that were also piloted.

Figure 9. Side view highlighting placement of Arduino, power supply, and motor. On the left of the 2" x 4", the Arduino can be seen in front of the power supply. On the right, the motor is shown as attached to the center 2" x 4".

Table 2. Final parts list.

Figure 10. Initial CAD assembly using the optimized link lengths.

Figure 11. Isometric view of the completed robotic linkage and housing.

Figure 12. Channel cut into motor-mounted 2" x 4" to allow easy connection to controller wires.