Design

We began the projecting by researching several mechanisms that could function as the rope gripper. Knowing that we wanted the device to be actuated by an electric motor, we knew that the mechanism would have to be able to transform the rotary motion of our actuator into a gripping movement. Figure 3, from the Mechanism Atlas, shows a 8-bar robot gripper mechanism. This mechanism utilizes two mirrored 4-bar mechanisms in parallel that are linked together with a single slider. In the initial stage of our project this designed looked promising because the simultaneous and parallel motion of both arms of the device would allow a user to align its center with the rope and have the arms close upon it providing griping strength.

Figure 3: Robot gripper mechanism 1

In order to determine if this design was viable, we decided to design and build a prototype of the mechanism as a proof of concept. To determine appropriate link lengths and to help find any unforeseen issues with manufacturing and assembly, we first created a model of our mechanism using SolidWorks, shown in figure 4. From this model we were able to actuate the mechanism and identify and correct some issues before manufacturing. For example, our initial model did not account for the height of the screw heads that we were going to use for the mechanism's joints. By creating this simulation and watching it move were able to see that the device needed pass though areas that would allow for uninhibited movement of the joint during operation. Finally, we decided to apply an adhesive foam on to the jaws of the device. It was thought that this foam would conform to the rope when the device was closed. This deformation would increase the contact area on the rope and increasing the device's maximum frictional force and ultimately gripping strength.

Figure 4: Initial mechanism SolidWorks model

Manufacturing

To manufacture the mechanism, we used 1/8th inch acrylic and a laser cutter to cut the material to the desired length that were designed in the CAD files. A combination of bolts and nuts were then used to replicate the pin joints. The completed mechanism is shown in below in figure 5.

Figure 5: Proof of concept prototype.

Results

The initial results of this prototype were promising. Even before the optimization of link lengths, the device was easily actuated, lightweight, and provided considerable gripping strength. Also, the proof of concept mechanism highlighted several action items that would need to be addressed in the upcoming prototypes for improved operation. Firstly, a rotational to linear motion mechanism such as a crank slider would need to be added to properly use a motor input. Also, in order to provided the most mechanical advantage, the device's link lengths would need to be optimized. Additionally, in order to provide a better grip of the rope, a proper griper would need to be added. The foam added to the contact area did conform to the rope, but failed to provided significantly more grabbing abilities. Lastly, while bolts allow for quick prototyping, the bolts rotated with the links as the device was used. This rotation would eventually loosen the nut and cause the device to fail.

1 Williams, B. (n.d.). An Atlas of Structures, Mechanisms, and Robots. Retrieved December 4, 2015, from http://www.ohio.edu/people/williar4/html/PDF/MechanismAtlas.pdf

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