MEMS Inertial Sensor Assembly for Vestibular Prosthesis

M.S. Weinberg, C. Wall III

Abstract:	Over 90 million Americans will seek medical attention for dizziness, a malfunction of the inner ear, at least once in their lifetime. Relying on the low weight and excellent performance available in 1997 Draper MEMS inertial instruments, we developed and successfully tested a prototype vestibular prosthesis. Balance-impaired patients who were unable to stand unaided stood with the prosthesis. Multiple-axes tilt modules are now being tested at three sites. While previous papers focused on patient test results and medical diagnoses where the prosthesis could be advantageous, this paper documents the engineering details required for detecting vertical with MEMS instruments. Thus far, experimental protocols have concentrated on front-to-back body sway using single-axis tilt modules consisting of one gyroscope and one linear accelerometer. A digital processor low-pass filters the accelerometer out-put, high-pass filters and integrates the gyroscope output, and combines the two filtered signals to obtain a wide bandwidth tilt estimate. The digital processor also drives small vibrators called tactors mounted on the torso to convey the tilt information to the patient. Based on an inverted pendulum model and demonstrated in patient testing, 0.03 Hz was selected as the filter break point to separate gravity from angular (180-deg phase shift from tilt) and lateral acceleration. The third-order high-pass filter greatly reduces the effects of gyro drift and initial miscalibration. An on-patient, 1-s calibration mathematically aligns the tilt indicator null to the patient's comfortable vertical. The vertical tilt estimation algorithms have been extended to a 6-degree-of-freedom (6-DOF) instrument assembly operating through large angles. To be useful, the balance prosthesis should estimate the vertical within 0.1 to 1 deg. Until applications become better defined, the best available, small sensors will be used. A 1-milligravity (mg, 0.0098 m/s2) accelerometer bias shift causes a 0.001-rad (0.06-deg) tilt error. Since the bias is calibrated at instrument turn-on, it must be maintained for roughly 16 h. One-mg stability represents the best MEMS sensors, while 5 to 50 mg is typical of the much less expensive sensors used for automotive applications. With appropriate filter design, the gyro contributes only transient errors during startup and normal operation. With the 0.03-Hz filters, 360 deg/h/ gyro white noise results in 0.1-deg tilt error. If the gyro bias changes by 80 deg/h in less than 10 s, a 0.1-deg tilt error follows for roughly 10 s. The tilt magnitude and time that causes a balance-impaired subject to fall are not known. Short-term bias stabilities of 50 deg/h are satisfied by high-performing MEMS gyros, while lower cost instruments are less stable.
Published in:	Proceedings of the 59th Annual Meeting of The Institute of Navigation and CIGTF 22nd Guidance Test Symposium (2003) June 23 - 25, 2003 Hyatt Regency Hotel Albuquerque, NM
Pages:	240 - 249
Cite this article:	Weinberg, M.S., Wall, C., III, "MEMS Inertial Sensor Assembly for Vestibular Prosthesis," Proceedings of the 59th Annual Meeting of The Institute of Navigation and CIGTF 22nd Guidance Test Symposium (2003), Albuquerque, NM, June 2003, pp. 240-249.
Full Paper:	ION Members/Non-Members: 1 Download Credit Sign In