The second iteration of the overall CAD design has been improved in four main ways. First, the Watt 6-bar mechanism is actuated by a crank-rocker. Second, the new crank-rocker is timed mechanically by means of a Geneva mechanism. Third, a detailed and improved human-exoskeleton interface is detailed. Fourth, the nut and bolt fasteners from the previous design are replaced with shafts, bearings, spacers, and retaining rings to ensure a more robust and frictionless interface between linkages.
As can be seen from Figure 1, the Watt 6-bar mechanism now has an additional linkage attached to the bicep. This linkage is the middle link of a four-bar crank-rocker mechanism. The four-bar crank-rocker is comprised of the body as ground, the upper circular gear as the driver link, the middle linkage attaches to the bicep, and the bicep linkage becomes the driven link.
Figure 1: Side View of Prototype 2 Exoskeleton
The purpose of this additional crank-rocker was to simplify the actuation of the system. In the previous iteration of the design, the exoskeleton had to move between two discrete angles. This required starting, stopping, and changing positions of the motor during each cycle of motion. This means a more complicated control system and the need for either a damper or compliant element to absorb the momentum change of the system. In the new design with the crank-rocker, the motor merely has to rotate the upper gear (or driven link) continuously in order to rotate the bicep linkage the appropriate range of motion.
The next important addition to this design is the Geneva mechanism. A partial rendering of the Geneva mechanism is represented in the CAD model (shown in Figure 1) as the two circular gears. Although not depicted accurately, the Geneva gears work by fixing the bottom gear to the motor. For every 360 degrees of motion of the bottom gear, the upper Geneva gear will rotate 180 degrees. As a result, for a full cycle of the exoskeleton, the motor must rotate 720 degrees. The first phase occurs from 0 to 180 degrees of the motor and extends the exoskeleton to its near-toggle position. From 180 to 360 the exoskeleton stays extended. From 360 to 540 degrees the exoskeleton returns to its initial position. Finally, from 540 to 720 the exoskeleton again rests for a half revolution. The purpose of this additional mechanism was to provide more realistic timing to the basketball trainee. We do not want continuous motion. This could be accomplished through a control system of the motor. However, since this mechanism is designed to be marketed as a relatively inexpensive toy, the hope was to make a mechanical timing system in order to avoid additional (expensive) electrical components.
The next addition to the design was the exoskeleton interface with the human body. In the first iteration, most of the design emphasis was placed upon the kinematics of the linkages and less upon the interface with the wearer. In this design, we developed a exoskeleton spine that will serve numerous purposes. First, it will connect to the exoskeleton linkages, thus providing a ground for the mechanism. Second, it will provide a place to attached the motor and circuits to control the mechanism. Finally, velcro straps will be used to securely attach the exoskeleton spine to the users shoulders and torso. The spine component is shown in Figure 2.
Figure 2: Back View of Exoskeleton
The last addition to Prototype 2 was the detailed and thought-out BOM and assembly plan. In the previous design, the linkages were secured with nut and screws. This was adequate for an initial prototype, but added far too much friction to allow for a small motor to actuate all of the mechanisms. In this SolidWorks design, every component of the mechanism was detailed before assembly. In particular, thought was placed with regards to friction reduction. As a result, the current SolidWorks model details predefined holes, shafts, bearings, retaining rings, and screws to ensure an easy assembly and relatively frictionless mechanism. The interface between bearings, shafts, retaining rings, and linkages are shown in Figure 3.
Figure 3: Transparent View of Bearing Interface
In general, the shafts are press-fit into one linkage and the inside diameter of the bearing. The bearing is press fit into the other link. A spacer is placed over the shaft and between both linkages to ensure no contact. Finally, retaining rings are placed on both sides to ensure that the shaft and both linkages are axially secured.