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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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