Final Design

The Knee Brace Cycle Tester

Knee Brace Cycle Tester

The cycle tester achieves all desired test ranges of motion at a frequency that can be varied from 0.5-1.5 Hz.  The leg covering properly supports the soft knee braces during testing.  While a cycle test is being performed the tester records the brace resistive force for each cycle.  In addition, the linkage structure of the cycle tester is modular in design.

Actuation

A slider crank was designed in order to bend the knee through the desired rotation angles.  The slider crank is a single degree of freedom system that allows the motor to operate in a single direction.  Avoiding making the motor constantly switch directions during its operation puts less strain on it and prolongs its life. Especially since a typical fatigue test easily eclipses 1,000,000 cycles. 

Leg Linkages moving from 0 to 90 degrees.

The slider crank takes advantage of the weight of its component linkages to bend the knee, and the resistive force of the knee brace to straighten the knee.  Motor strain was further reduced by utilizing these two contradicting forces.  In order to achieve the three different ranges of motion, crank and coupler sets were created that can easily be interchanged.  The linkage components were machined from 6061 Aluminum tubing for strength while still remaining lightweight.

Leg Shape

An anthropomorphic leg covering was designed to prevent soft knee braces from collapsing into the mechanical joint structure.  The covering had to accommodate and enclose the femur and tibia links while still being able to achieve the same ranges of motion.

Leg Covering

The leg covering was 3D printed at DJOs facilities.  3D printing was chosen over the alternative methods primarily due to the availability of 3D printing resources at DJO.  If the leg covering should ever break due to material fatigue, DJO has immediate access to new leg coverings.  This is also beneficial should DJO ever require a modification to the leg shape, or a whole new joint structure.

Sensors

A force sensor was placed in the calf portion of the leg covering in order to record the brace resistive force each cycle. 

Force Sensor Placed in Leg Covering

The measured force from the force sensor was then logged onto a micro SD card with an Arduino UNO microcontroller.  In addition to logging the brace resistance, the microcontroller monitored the cycle count through a photosensor.

Enclosure

An external enclosure for the cycle tester was designed both to support the cycle tester and its components as well as safely constrain the moving leg.  80/20 extruded t-slot aluminum was used for this framework for its modular advantages as well as remaining lightweight and compact.

Results and Performance

The cycle tester successfully detected and recorded a decrease in performance of a soft knee brace as it decayed. 

A soft knee brace was tested in the 0-90 range and every 500 cycles it was cut to simulate wear.  The cycle tester successfully measured a decrease in performance of the brace as the wear became more prominent.