sensors Article A Capacitive 3-Axis MEMS Accelerometer for Medipost: A Portable System Dedicated to Monitoring Imbalance Disorders Michał Szermer * , Piotr Zaj ˛ac,Piotr Amrozik, Cezary Maj, Mariusz Jankowski, Grzegorz Jabło ´nski , Rafał Kiełbik, Jacek Nazdrowicz, Małgorzata Napieralska and Bartosz Sakowicz Department of Microelectronics and Computer Science, Lodz University of Technology, 93-005 Lodz, Poland; [email protected] (P.Z.); [email protected] (P.A.); [email protected] (C.M.); [email protected] (M.J.); [email protected] (G.J.); [email protected] (R.K.); [email protected] (J.N.); [email protected] (M.N.); [email protected] (B.S.) * Correspondence: [email protected]; Tel.: +48-42-631-2722 Abstract: The constant development and miniaturization of MEMS sensors invariably provides new possibilities for their use in health-related and medical applications. The application of MEMS devices in posturographic systems allows faster diagnosis and significantly facilitates the work of medical staff. MEMS accelerometers constitute a vital part of such systems, particularly those intended for monitoring patients with imbalance disorders. The correct design of such sensors is crucial for gathering data about patient movement and ensuring the good overall performance of the entire system. This paper presents the design and measurements of a three-axis accelerometer Citation: Szermer, M.; Zaj ˛ac,P.; dedicated for use in a device which tracks patient movement. Its main focus is the characterization Amrozik, P.; Maj, C.; Jankowski, M.; Jabło´nski,G.; Kiełbik, R.; of the sensor, comparing different designs and evaluating the impact of the packaging and readout Nazdrowicz, J.; Napieralska, M.; circuit integration on sensor operation. Extensive testing and measurements confirm that the designed Sakowicz, B. A Capacitive 3-Axis accelerometer works correctly and allows identifying the best design in terms of sensitivity/stability. MEMS Accelerometer for Medipost: Moreover, the response of the proposed sensor as a function of the applied acceleration demonstrates A Portable System Dedicated to very good linearity only if the readout circuit is integrated in the same package as the MEMS sensor. Monitoring Imbalance Disorders. Sensors 2021, 21, 3564. https:// Keywords: MEMS accelerometer; ASIC readout circuit; portable system; imbalance disorders doi.org/10.3390/s21103564 Academic Editor: Daniel Arvidsson 1. Introduction Received: 16 April 2021 Research into microelectromechanical systems (MEMS) is one of the most dynamically Accepted: 20 May 2021 Published: 20 May 2021 developing branches in microelectronics [1–4]. The devices can be found almost every- where, from smartphones and the internet-of-things to clothes and many other applications. Publisher’s Note: MDPI stays neutral Most importantly, MEMS accelerometers are used in GPS-aided navigation systems [5–7], with regard to jurisdictional claims in and military [8], automotive [9], aerospace [10] and medical devices [11–14]. Thanks to published maps and institutional affil- their miniature size they can also be easily used in healthcare applications. For example, iations. two devices, SwayStar [15] and VertiGuard [16], can be used to monitor the gaits of patients by examining their movement. We propose a similar system developed in cooperation with the Medical University of Lodz [17,18] for patients with imbalance disorders. Medipost is a small, compact device which can be easily mounted on the back of the patient’s belt. It is supplied by a Li-Ion battery and can be used at home. Medipost communicates via Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. Bluetooth with a smartphone [19], on which any received data is preliminarily processed This article is an open access article by a dedicated application. Next, the data is transmitted to a PC located in a medical centre, distributed under the terms and where the patient’s movement can be monitored by a doctor and the proper treatment conditions of the Creative Commons administered. Apart from monitoring movement, the system can also prevent serious Attribution (CC BY) license (https:// injury by triggering a warning when the patient is about to fall. creativecommons.org/licenses/by/ On its basic level, Medipost uses MEMS inertial sensors and dedicated integrated 4.0/). readout circuits for measuring the acceleration and angular velocity of certain parts of Sensors 2021, 21, 3564. https://doi.org/10.3390/s21103564 https://www.mdpi.com/journal/sensors Sensors 2021, 21, x FOR PEER REVIEW 2 of 16 Sensors 2021, 21, 3564 2 of 16 On its basic level, Medipost uses MEMS inertial sensors and dedicated integrated readout circuits for measuring the acceleration and angular velocity of certain parts of the patient’s body, which allows precise evaluation of their movement and potential move- the patient’s body, which allows precise evaluation of their movement and potential ment-related health issues. movement-related health issues. However, the correct operation of the entire system requires the proper design of an However, the correct operation of the entire system requires the proper design of accelerationan acceleration sensor. sensor. Therefore, Therefore, in this in paper this paperwe present we present and characterize and characterize a custom a customaccel- erometeraccelerometer designed designed specifically specifically for the Medipost for the Medipost device. The device. paper The is organized paper is organized as follows: as Sectionfollows: 2 presents Section2 thepresents operation the operation principle principleand the design and the of design the 3-axis of the accelerometer; 3-axis accelerome- Sec- tionter; 3 Section describes3 describes the simulations, the simulations, as well asas well the measurement as the measurement results results of the ofsensor; the sensor; and Sectionand Section 4 presents4 presents our Conclusions. our Conclusions. 2.2. MEMS MEMS Accelerometer Accelerometer Design Design and and Manufacturing Manufacturing 2.1.2.1. Capacitive Capacitive Accelerometer Accelerometer Operation Operation Principle Principle and and Design Design WeWe designed designed a a3-axis 3-axis accelerometer, accelerometer, consis consistingting of threethree independentindependentcapacitive capacitive single- sin- gle-axisaxis accelerometers, accelerometers, which which operates operates by measuring by measuring changes changes in capacitance in capacitance caused caused by applied by appliedacceleration. acceleration. The accelerometer The accelerometer itself consists itself ofconsists a seismic of massa seismic hanging mass on hanging springs overon springsa substrate over a [20 substrate]. Combs [20]. are Combs attached are to attach the mass,ed to formingthe mass, a forming movable a partmovable of the part sensor. of theThe sensor. same numberThe same of number parallel of combs parallel is in combs turn connected is in turn to connected a frame which to a frame forms which a fixed formspart ofa thefixed sensor, part thusof the forming sensor, two thus capacitors forming with two movablecapacitors and with fixed movable plates on and both fixed sides platesof the on seismic both sides mass. of The the concept seismic ofmass. a single-axis The concept accelerometer of a single-axis is shown accelerometer in Figure1. is In shownthe sensor in Figure described 1. In the in this sensor paper, described two accelerometers in this paper, have two beenaccelerometers designed according have been to designedthis principle: according one to operating this principle: in the one X-axis oper andating another in the in X-axis the Y-axis. and another in the Y-axis. Anchor Fixed Spring Fingers Seismic Mass d-x Ctop Ctop d-x Fixed Fixed Comb Comb d+x Cbottom Cbottom d+x Spring Movable Fingers Anchor FigureFigure 1. 1.TheThe operation operation principle principle of the of thecapacitive capacitive accele accelerometerrometer acting acting in the in X the and X andY axes Y axes(not to (not scale).to scale). AsAs shown shown in in Figure Figure 1,1, the design consists ofof twotwo capacitors:capacitors: CCtop andand C bottombottom. .When When accelerated,accelerated, the the mass mass moves, andand oneone of of the th capacitancese capacitances increases increases while while the otherthe other decreases de- creasesdue to due the to displacement the displacement of the of fingers. the finger Thes. difference The difference between between these these capacitances capacitances can be canmeasured, be measured, thus the thus magnitude the magnitude of acceleration of acce canleration be derived. can be Thederived. value The of the value capacitances of the capacitancesCtop and Cbottom Ctop andafter C accelerationbottom after acceleration is applied is is applied given by: is given by: 11 11 C == #nS ,, C ==#nS (1)(1) top −d − x bottom d++ x where S is the surface of a single finger, d is the distance between fingers, x is the displace- ment caused by acceleration, n is the number of fingers which form a single capacitor plate and # is the electrical permittivity of the material between the fingers. Sensors 2021, 21, x FOR PEER REVIEW 3 of 16 where S is the surface of a single finger, d is the distance between fingers, x is the displace- ment caused by acceleration, n is the number of fingers which form a single capacitor plate and ε is the electrical permittivity of the material between the fingers. As the principle of operation of such a sensor is well described in literature [21], we have only presented the most important equations necessary to understand the accel- erometer’s behaviour; these are given in Appendix A of this paper. Although we have also designed a Z-axis accelerometer, it has to be emphasized that it has a slightly different design to the X- and Y-axis accelerometers described above [22]. left right Sensors 2021, 21, 3564While it is also constructed with two combs, which form C and C capacitors (Figure 3 of 16 2), the design of the seismic mass is asymmetric to obtain a capacitance change upon ac- celeration. In this design, the mass with movable fingers rotates in the XZ plane, which is presented in Figure 2.