electronics Article Magnetic Elastomer Sensor for Dynamic Torque and Speed Measurements Valentin Mateev * and Iliana Marinova   Citation: Mateev, V.; Marinova, I. Magnetic Elastomer Sensor for Dynamic Torque and Speed Measurements. Electronics 2021, 10, 309. https://doi.org/10.3390/ electronics10030309 Academic Editor: Raed A. Abd-Alhameed Received: 30 December 2020 Accepted: 23 January 2021 Published: 28 January 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). Department of Electrical Apparatus, Technical University of Sofia, 1156 Sofia, Bulgaria; iliana@tu-sofia.bg * Correspondence: vmateev@tu-sofia.bg; Tel.: +359-2-965-2257 Abstract: In this paper is proposed a dynamic torque, rotational speed, and shaft position sensor. It is built of magnetic elastomer coating directly applied over a rotating shaft. The sensor is used for precise measurements of changes in torque and speed, and it is usable at high rotational speeds, directly on the device shaft. The sensor is based on magnetic elastomer material deformation and the corresponding change in magnetic field amplitude and direction. The proposed sensor design is simple and can acquire reliable readings for a wide range of rotational speeds. Sensor design consists of the following: magnetic elastomer coating with nanoparticles, in which, elastomer is used for a sensing convertor; magneto-resistive linear field sensor; and microprocessor unit for calibration and control. Numerical and experimental test results are demonstrated and analyzed. Sensor implementation aims to meet magnetic mechatronic systems’ specific requirements. Keywords: torque sensing; nano-magnetic elastomer; magnetic paint; rotational speed sensor; flexible sensor; printable sensor; flexible electronics; printable electronics 1. Introduction The precise measurement of static and dynamic mechanical torques is of great impor- tance for many applications. Static torque measurements are required at non-rotational shafts or for those rotating with low peripheral speeds. Typical applications are in sensors for electrical machines, e.g., blocked rotor testing [1], automobile steering wheel sensors [2], robotics arm joints [3–6], leaver and beam deformation sensors [6], etc. Dynamic torque sensing is much more complicated mainly because of the required deviation of immobilized sensor output and rotating shaft. Dynamic torque sensors’ principles of operation are based mainly on electric-tensoresistive strain gauges [2], acoustic interference [7,8], magnetic permeance and reluctance effects [9,10], and optical polarization [10,11]. Nevertheless, in these existing technologies, the connection problem with the rotating sensing element is not satisfactorily solved, especially for transient modes. Depending on the torque sensing element’s location, two principal types of design architectures are employed [12,13], the first one uses a directly attached sensor element on the rotating shaft, where readings are transmitted to the information channel by wire brushes, inductive coupling, or even by wireless radio transmitters. Most of the electric- tensoresistive strain gauges are connected this way. Electric power supply is provided to the directly attached sensor elements and corresponding electronic modules, e.g., wire brush contact or inductive coupling connection [4,14,15]. The second principal torque sensor design architecture type uses an emitted physical field quantity (acoustic, optical light, electromagnetic) from a static (non-rotating) emitter toward the rotating shaft where specially designed sensing material passively or actively modulates the input field flux quantity. The modulated signal is received and encoded by a non-rotating receiver [14–19]. The second sensor design architecture type eliminates the direct connection problems of attaching wires or optical fibers to the rotating shaft under observation. This way, the sensing equipment impact over the testing process is minimal. However, open-field interaction is much more vulnerable to outer noise Electronics 2021, 10, 309. https://doi.org/10.3390/electronics10030309 https://www.mdpi.com/journal/electronics