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The first prototype consisted of a four bar linkage that opened and closed the jaws of a cam cleat. Velcro straps were utilized about the wrist to attach the device to the user. As shown in Figure 1, the design is meant to be motorized to enable a powered gripper for the impaired user. A tie rod was going to be used to to attach a servo motor horn to link 3 (L3).
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Figure 3:The working range of the mechanism where the rope will likely be compressed is shown in red. All plots are made with respect to the angle of L2. Top Left: Relative angular velocities of L3 and L4 if L2 is moving at 1 rad/s. Top Right: Mechanical Advantage spikes to infinity near the toggle position. Bottom Left: Tie rod force input required on L3 to generate required 100 N output compression force on the rope. Bottom Right: Torque required by the motor assuming the tie rod is connected to a 5mm radius servo horn.
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The design change of making the linkage system more compact did not affect the kinematics significantly. However, the torque input required by changing the servo motor's input method from a tie rod to direct input on L2 had a significant impact. Figure 5 shows that the torque input requirement is much higher for the working range of where the rope contacts the cam cleat.
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Another key factor with this final design was the addition of a case over the mechanism. This would prevent any unwanted snags with the linkage while operating complex movements during sailing. Figure 5 6 also shows the chosen actuation method for this design. A Hall effect sensor was embedded into the top corner of the 3D printed plastic, and when rising edge signals are generated and sent to a microcontroller, the device can open and close via the servo motor on command. Having a magnetic ring attached to the thumb of the user allows him or her to tap the corner of the device and cause the servo to move to the opposite of its current state, which could be open or closed. The toggle switch logic was designed to work the same physically for both opening and gripping on the rope.
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