Fall 2016 Prosthetic Hand: Evolution of Solid Works Modeling
Design and Manufacturing Engineer – Wilson Mach
After CDR, the hand was fully 3D printed for testing. We included the FSR’s on the four fingers and cut out rubber grooves to wear under each finger to hold down the FSR’s and provided grips for each finger to hold on to the items we picked up for testing. At this point I began to model the wrist for rotation. The model would need to accommodate a stepper motor with a shaft to lock in the hand for rotation. This would occur from the motor’s shaft with the motor being fixed to the arm. Since the motor shaft was only 0.5 inch, I used another hub to extend the shaft to the center of mass for more stability. The model was 3D printed for testing and it worked as expected with just the stepper motor installed. However, when we assembled the entire hand with the stepper motor for testing the rotation, the cup in this case was not able to lock in place when we rotated it to a 90 degree. The problem we noticed was that the motor did not have gears inside that would lock or fix the motor on heavy objects. It would not work for the drink; the cup would tilt over when it was unbalanced since there was no locking mechanism in the motor.
Therefore, we needed to change to different motors such as a servo or a bigger DC motor that had geared mechanism to prevent the shaft from rotating in the opposite direction when we turned to a certain degree. With not much time available I needed to model both housings for the servo and DC motor. Both models were printed for installation and rotation purpose to test with the entire hand installed. We tested both motors and they both worked, but we decided to choose the servo over the DC motor because of the increased degree of freedom. Although the servo only rotates for a 180 degrees range, that was enough for us to grab the drink and burger.
We then began to find a solution for the wiring, since there would be 21 wires coming from the hand to connect to the PCB near the end of the arm. The tangling and rotation of the wires would cause problems. After some research we found a 24 wires slip ring that would do the job. A slip ring allowed for the rotation of the wires from one end while keeping the opposite end fixed. It seemed to meet our needs on this project.
With these two parts in mind I designed a model to hold these two together and allow rooms for the wirings to get from the hand to the PCB. I tried modeling two different setups, one in parallel and one in series. After these designs, we had help from the arm group to touch up the design. The series method was deemed to work better since we needed the length of the arm to be longer and placing the servo and slipring in parallel will make the wrist look out of proportion with the hand. After modeling the arm, I needed to remodel the palm to make room for wires and housing for the servo to be connected. I needed to purchase another hub that was 1 inch long to allow for installation to the hand and for rotation locking, since the servo only had a 2 mm gear on the tip. With that gear, it was capable of locking onto a 1 inch hub. The model I made connected both palms together and allowed the servo to be inserted to the bottom palm for rotation.
Figure 1 – Back of Palm with Stepper Motor Housing and Extension Rod for Strudiness
The goal of this design was for the stepper motor to be centered with the half inch shaft installed through the bottom, Then, a hub was added in on the other end to lock into the motor with the palm. The hub would have long screws that extend to the near center of the palm and would have a fastener on the end to lock in those screws.
Figure 2 – Square Shaped Wrist for the Stepper Motor for Connection to the Palm
The original design was to house a square stepper motor within a square wrist connection.
Figure 3 – Round Wrist Shape for the Stepper Motor
Per customer request, a round wrist was designed to more closely resemble a human wrist. This was still designed to house the square stepper motor.
Figure 4 – Forearm with Servo and Slipring in Series
This model was designed to house both the servo (in front) that would connect to the hand and the slip ring (in back) that would connect to the PCB.
Figure 5 – Final Version of Top Palm
The top palm was modeled to be locked in with the bottom palm, the square hole was for the small servo wires from the thumb to be enclosed. The connection to the arm would have a square spacing for wires to go through.
Figure 6 – Final Version of Bottom Palm
The bottom palm was modeled to connect to the top palm. The servo hub was incorporated with screws to lock in and make enough space for the motors at the same time.
Figure 7 – Forearm piece with servo and slip ring
This forearm piece was designed as a two piece product, connecting at the center of the forearm, with left and right pieces. This was the final piece used for the project. It allowed the wires to go through the spacing on the bottom of the servo.