16.3: Design Process

The initial CAD for this project was borrowed from the research paper "An open-source anthropomorphic robot hand system: HRI hand" by Hyeonjun Park and Donghan Kim as a jumping-off point to test how the mechanism behaved before the kinematic analysis was complete. Notably, the authors of this paper chose to actuate each digit individually, while we aim to actuate them all at the same time.

Table 1: Link Lengths

Figure 1: Drawing of Linkage

The CAD as given in the form of step files in the research paper was taken and altered to fit with the M2 and M3 screws we had available. The lengths of the links were altered slightly as described above in Table 1 to achieve a slightly more acute gripping angle at the end-effector. The availability of the CAD in the paper was a very useful tool as it allowed us to experiment before the analysis code was completed.

Figure 2: Initial CAD Borrowed from Research Paper and Altered

Initially, we hoped to 3D print only the fingertip and palm base of the hand (Links 1 and 6), and use laser-cut wooden pieces as the rest of the links. However, upon assembly, we realized we failed to account for the disparity between the listed thickness of the wood and the actual thickness. We found that there was no sold thickness of wood that matched, and so we opted to switch to 3D printing so as to not have to manually change the thickness of each piece in CAD. Despite this failure, it was still a worthwhile incident because we were able to roughly assemble the pieces and see that the mechanism did indeed work.

Figure 3: Prototype No. 1 - Failed due to lack of link thickness tolerancing

This design looked impressive, but it had several problems. First, it had a limited range of motion. As the mechanism is mirrored on both sides for stability, there needs to be a long bolt that crosses through joint O_4. This ended up interfering with the motion of the finger by blocking the four-bar from extending further once it hit the bolt. This resulted in a limited range of motion and a high flexion distance from the finger to the palm, which would mean we could only really hold large objects. Another problem we faced was the difficulty of assembly, which took at least an hour and a half because of all the small bolts and the lack of tolerancing on the 3D-printed holes. Finally, the hole sizes were not uniform which led to there being a bit of play.

Figure 4: Prototype No. 2 - Failed due to lack of ROM of end-effector

We decided that this design did not meet our needs, and went back to the drawing board. We decided to simplify the design to make manufacturing and assembly easier and also to remove the bolt at joint O₄ to increase our range of motion. This process was quick since we simply followed the same 3D sketch as the original and made extrudes in the assembly to get a skeleton of fundamentally the same linkage.

This prototype had several benefits over the previous one. First, it was better toleranced and assembled more easily since we standardized all the bolt sizes to M3. Secondly, it had a greater range of motion due to the lack of a bolt going across the middle connecting Link 2 to Link 4, which was previously hitting the mechanism and stopping it. Finally, it was overall simpler to manufacture and assemble. Going forward, this is the basic design we will be using for our final product.

Figure 5: Initial Design 2, simplified linkages

Figure 6: Prototype No. 3 - Updated prototype with simpler linkage design, higher ROM

The next step for our project will be to design a more comprehensive ground link that will incorporate five of these fingers into a hand. A special case is the thumb; we will simply reduce it's DOF from that of an actual thumb to 1, and treat it as a shorter finger. We plan on using springs in parallel to connect each finger to the linear actuator located on the back of the palm. The fingers will be extended when the actuator is unextended, keeping the springs in tension. When the actuator extends, it will allow the spring to compress, thus releasing the force on the actuation joint on L₂ allowing the fingers to close. We will select springs with sufficient stiffness such that there will still be force on the fingers when they are fully flexed, which will provide the force needed to grasp whatever object is in the palm. Due to the natural compliance of the springs, the fingers will be able to grasp objects of differing widths.

Table 2: Projected Bill of Materials for Final Project