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Figure 11 - Joint hole size calibration with shoulder blots.
Figure 12: Final model
Analysis
Using the same program as before, the kinematics of the mechanism were analysisanalyzed. This showed the location of the toggle point and at what angle we should install the servo motor should initially be installed. The mechanical advantage of the system as a function of input angle is plotted below in figure 13.
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Once the mechanics of the system were in place, the servo control program had to be built. Because our system needed to be attached to the body and somewhat mobile, we decided that a headless system (no monitor, keyboard, or mouse) would be most effective. Because of our previous experience, we decided to control our servo with a Raspberry Pi and a bread board. The bread board layout, shown below in figure 14, contains two switches. The first switch is used to change the desired position of the motor (open or closed), while the second signals the Pi to shutdown. This shutdown switch prevents memory card corruption that could be caused by abrupt power removal. Finally, an LED is added to signal that the servo control program is running. The servo is then attached to the Pi's PWM (pulse width modulation) pin to control the motor's position, detailed in figure 15. Next, the servo is powered by a six volt battery pack and grounded to the Pi.
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The code pictured below, in figure 16, shows the servo control program. Every two seconds the Raspberry Pi checks to see the status of the switches and adjusts the position of the servo accordingly. If the shutdown switch is closed the computer cleans up all GPIO pins and begins to shutoff. Because of the lack of a real time clock on a Raspberry Pi, the position of the servo cannot be checked more frequently than once every two seconds. High demand frequency loops cause can the sensitive timing of pulse widths, and thus the servo, to become unstable. Due to the nature of our project, quick changes in the jaw position are not necessary and a two second delay is acceptable. Finally, to make this a headless system, the control program must be executed when the computer boots up. This is done with a simple cron job that also gives defines the servo the open and closed position.
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By building off of the lessons learned from the manufacturing of the previous prototypes, we were able to apply several design considerations into the final prototype. Carrying over the tolerance considerations of the laser cut acrylic, several in contact pieces were cut just oversized and then sanded to allow for a smooth and dynamic motion. Rather than using zip ties to attach the electric servo motor, we cut sandwiching acrylic pieces to link the motor to the crank slider. The use of shoulder bolts rather than standard threaded bolts kept the motion smooth and robust. Also, in order to lessen the effects of friction on the many links comprising the system, we used motor oil to grease the links and provide for a smooth operation. Finally, Velcro straps were added to the device to allow it to be attached to the inside of the user's hand for use. The links below contains a videos that demonstrates the device.
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