P11011: Modular Motion Tracking Knee Flexion Unit V1
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Concept Testing and Comparison

Knee Flexion Sensor Testing (Reproducibility)

Flex Sensor

Flex Sensor - Bent at End (Near Wire)
Degrees Bent Output Voltage - Trial 1 (V) Output Voltage - Trial 2 (V) Output Voltage - Trial 3 (V)
0 2.116 1.98 2.09
20 2.102 1.93 2.09
40 1.958 1.03 1.48
60 1.51 0.659 1.02
80 1.297 0.818 0.813
100 2.43 0.592 2.27
120 2.14 2.22 2.12
140 2.16 1.998 1.9
160 2.03 1.97 2.31
180 2.215 2.35 1.31
Flex Sensor - Bent at Tip (End of Sensor)
Degrees Bent Output Voltage - Trial 1 (V) Output Voltage - Trial 2 (V) Output Voltage - Trial 3 (V)
0 2.08 2.28 2.24
20 3.45 2.95 3.91
Flex Sensor - Bent at Middle
Degrees Bent Output Voltage - Trial 1 (V) Output Voltage - Trial 2 (V) Output Voltage - Trial 3 (V)
0 2.26 2.3 2.16
20 2.93 3.22 2.86
40 3.27 3.21 3.04
60 2.89 3.05 2.95
80 3.6 3.12 3.48

From the tables shown above, it can be seen that the Flex Sensor is very unreliable. It varies a lot from trial to trial, and would not be a good choice to reproduce results everyday for the Knee Flexion device.


Potentiometer

Potentiometer
Degrees Rotated Output Resistance - Trial 1 (Kohms) Output Resistance - Trial 2 (Kohms) Output Resistance - Trial 3 (Kohms)
0 97.59 97.458 97.493
20 96.76 96.929 97.478
40 87.123 88.65 88.605
60 78.003 78.73 88.605
80 66.68 69.09 71.375
100 63.43 62.74 64.76
120 57.61 58.036 58.337
140 53.225 52.238 52.616
160 48.87 48.154 46.183
180 44.09 43.623 41.591

As can be seen from the above table, the potentiometer is a very reliable device with little variability from trial to trial. This device is fairly easy to set-up and use, and would be a good choice to use for the Knee Flexion device.

Calculating Expected Potentiometer Lifetime

From the paper Expected Values for Steps per Day in Special Populations, the mean steps per day for a poststroke patient is approximately 4000 steps/day. Since the knee flexion device will only measure the motion of one knee, the number of rotations the device will experience will be 2000 steps/day. For a minimum cycle count of 100k cycles prior to failure, the device will last for at least 50 days of daily use. For a minimum cycle count of 1 million cycles, the device will last for at least 500 days or approx 1 year and 3 months of use on a daily basis.

If the device is being used in the clinic, it will be used by approximately 15 patients per week for 20 weeks out of the year. Dr. Mowder approximated that each patient will take 100 steps per session (50 steps per leg). At 50 steps/session, 15 patients per week, a 100k minimum cycle device will function for 133.3 weeks or 6.67 clinic years. A 1 million cycle device will last for 1333.3 weeks or approximately 66.7 clinic years.


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