Design I
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).
Figure 1: Linkage Mechanism for Design I Prototype
In the design process of this mechanism, it was assumed from the requirements that a 100 Newton clamping force on the rope would be enough to fully drive the mechanism to the toggle position. The free body diagram of this condition is shown in Figure 2. Using this assumption, the mechanical advantage calculation described earlier in the Design Requirements and Theory section was used to find required input torque from the servo motor. The results of this analysis is shown in Figure 3.
Figure 2: Free body diagram of rope force interaction with one half of the cam cleat
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 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.
The main impression obtained from the kinematic analysis of this first design is that the motor torque requirement was very low. Most servo motors could provide the expected required input of less than 0.5 Nm.
Design II
The next design iteration included the integrated motor rather than just simulating it previously. Since general cleat and four bar features stayed the same, the relative kinematics presented for the first design also apply to this case. However, there were some design changes to obtain Design II shown in Figure 4. First, L3 in Design I was very large, so this was eliminated in Design II to make the device more compact. Also, the motor was integrated with this design, but the tie rod was not used to reduce complexity of the system. Alternatively, the team chose to impart a direct torque on L2 using the servo motor. Lastly, the grip operation was blind when reaching and grabbing for ropes due to the orientation of the cleat sliders. This was rotated 90 degrees in the new design, which allowed the user to have a different hand position that was more natural for rope gripping.
Figure 4a Top: Design II in the closed position, Bottom: Design II in the open position
Figure 4b Animation of Prototype II Rotating 90° CW
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.
Figure 5: The torque input requirement of Design II within the working range of the mechanism. The plot is made with respect to L2 on the x-axis.
Even though the torque output required by the motor is higher in the new design, it is still well within the maximum output of the servo motor chosen for the product of 1.7 Nm. If we had chosen a weaker servo motor, the original Design I would have been the only feasible option. Figure 6 shows the mechanism in its completed form.
Figure 6: Complete Design II assembly with cover over the mechanism
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 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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