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Initial Design - Risk Reduction:
The 4-Bar Linkage conceptThe vertical bar and thigh bar in the image are both fixed in position so the ground link of the 4-Bar linkage is an invisible bar that runs from the knee to the crank. This concept offers a simple single degree of freedom while requiring motion input from a motor in only one direction. This is most advantageous for the simple fact that the motor will have to run at least 1,000,000 revolutions per test, and requiring the motor to only run in one direction reduces wear greatly.By varying the lengths of the coupler and crank, as well as the crank position the required ranges of motion for tests can be attained. However, when brought to full human leg size the lengths of the coupler and crank became cumbersome. In addition, in order to adjust the test range, we felt that too many settings would have to be adjusted in the system which would be inconvenient for DJO.
The Slider Crank conceptThis concept was born from the discussions of how to optimize the 4-bar linkage to have a smaller space footprint. This design still takes advantage of a motor running in only one direction but uses less material and physical space than the 4-bar design. We went from sketches directly to an acrylic model with this design, and the CAD model is made with .25" acrylic in mind. Also of note is that for our model, because of the limitations of the materials the crank is in the same plane as the "leg" itself, but due to the nature of a slider crank, it would not have to be for later designs.This concept has more moving parts than the 4-bar design, and thus more precise tolerances in order to reach the desired ranges of motion. Despite this, the adjustments between test ranges are much more manageable requiring less physical changes of the overall system.
Proposed Design: CAD and Construction
Having decided to move forward with the slider crank actuation method a full scale drawing was created in SolidWorks. Hollow 6061 aluminum square tubing was chosen for lightweight components that still provide plenty of strength. The crank and pusher links have to be swapped out with separate sets in order to achieve the various ranges of motion. These linkages can easily be swapped with a few screws and will be color coded as sets so that there will be no confusion as to how to attach them to the knee cycle tester. Below are some videos that highlight the three test ranges of motion.
Pulling Slider Crank
In order to save space and reduce motor torque requirements, the slider crank was redesigned. This allows the brace to assist the motor in opening the leg while gravity assists the motor in closing the leg.
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