Analysis

In designing the next major prototype, we looked to reconfigure the mechanical mechanism to properly incorporate the electrical motor as well as increase the output clamping force by increasing the system's mechanical advantage. First a SolidWorks line model, shown in figure 6, was created. This was used to verify the positional analysis section of the Matlab code which was then used in determining the system's mechanical advantage. Once both of these were complete, mechanical advantage as a function of input angle, link lengths, and joint pivot locations could be found and optimized. The graphs produced, one of which is shown in figure 7, easily highlighted that the mechanism's toggle point greatly increased the mechanical advantage of the system. If properly designed, the toggle point could be used to greatly increase the gripping and locking power of the mechanism. Finally, we designed a servo horn that could be fitted to the manufacturer's original horn. This design allowed us to easily attach the servo to the rest of the mechanism using the same screws as the other joints and zip-ties.

Figure 6: SolidWorks Line Model

Figure 7: MATLAB analysis

With the new optimized link sizes suggested from the Matlab code, the Solidworks model was updated to include a more finished device encompassing an electric motor, crank slider, and updated gripper links. Figure 8 shows the model below.

Figure 8: SolidWorks model of prototype

Manufacturing

Acrylic and the laser cutter were again used to build the prototype with varying layers of 1/4th and 1/8th inch pieces. The joints were made of #8 screws and nuts that were available from the Maker Space. A standard size servo was also procured to serve as our input source. To create the jaw, a vertical plate was then glued to 4-bar coupler arms. Finally the servo arm was press fitted into an acrylic cutout and then zip-tied together.

Figure 9: Toggle Point Prototype

Results

This device was designed to take advantage of the system's toggle point for an increase in mechanical advantage. As the servo turned into the closed position, the device reached the toggle point and should have locked around the rope causing the rope to be fixed within the systems jaws. Unfortunately, we found this area of operation to be too unstable and unpredictable. As the jaw was closed it would lock on the toggle point, but the jaws would not necessarily have locked on to the rope. Also, as the jaws we opened, the device had a tendency to change into a crossed orientation and would have to be manually reset.

Aside from these difficulties were able to learn several other lessons that we then incorporated into our next design. We learned that the servo model downloaded from GrabCAD was slightly smaller than the servo we planned to use and that extra material should be removed to allow the wires to be passed through. We also leaned that, while the press fit of the servo horn worked well, the sharp corners for the zip-tie slots created a stress concentration that would eventually lead to cracking of the acrylic, shown in figure 10. This prototype also showed us that conventional screws did not provide us with a consistent surface area needed for smooth operation. Finally, we found that the acrylic sheets were not precisely 1/4 inch. This caused issues because the vertical plate was cut with that assumption and the inconsistency required us to flex the acrylic arms to fit in to the precut slots.

Figure 10: Acrylic Horn cracking at zip-tie indention.

Because of the issues with the toggle point, we decided to return to the parallel 4-bar design of our previous prototype. This change, along with the other lessons learned allowed us to greatly improve upon this prototype.