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The initial Fourbar mechanism was designed to have a quick return in order to get out of the way of the curb. It's grounded points were placed on the axles of the major and minor wheels of the wheelchair. This was done for simplicity in mounting and ease of attachment.
Kinematics
This deviceInitial variations of this mechanism were based off of quick return fourbar mechanisms and iterated to minimize the input displacement needed for the return to occur (ie, minimize the quick return time). A representation of this can be found on the prototyping page.
Problems
This version's major flaws included a lack of curb clearance and the resultant angle of the wheelchair through travel. The footprint of the pushbar for this mechanism was made large in order to time the contact with the ground and to provide more stability for the CG. However, this meant that during the quick return travel, the foot intersected with the curb. Re-timing the system to avoid this meant that the wheel would not be the proper distance behind the curb. This version also took the user through around 20 degrees of deviation from the horizontal, providing too much deflection than the 6-10 degrees sought after.
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A demonstration of the mechanism's position analysis can be seen in the above video.
The mechanical advantage of the system during lift (phase 3) can be seen in the following image. The mechanical advantage stays above 4 and peaks at approximately 17 at the beginning. The motion of the lift implies that as the person is being lifted (up to 80 degrees in theta 2) will only need to assert 33 pounds of force as the mechanical advantage stays above 6. The mechanical advantage tapers off only once the lift motion transitions to horizontal translation which requires much less input force.
