This paper presents a review of the latest advances in the field of capacitive, inductive (eddy current), and magnetic sensors, for measurement of absolute displacement. The need for accurate displacement and position measurement in the micrometer, nanometer, and subnanometer scales has increased significantly over the last few years. Application examples can be found in high-tech industries, metrology, and space equipment. Besides measuring displacement as a primary quantity, absolute displacement sensors are also used when physical quantities such as pressure, acceleration, vibration, inertia, etc., have to be measured. A better understanding of the commonalities between capacitive, inductive, and magnetic displacement sensors, as well as the main performance differences and limitations, will help one make the best choice for a specific application. This review is based on both theoretical analysis and experimental results. The main performance criteria used are: sensitivity, resolution, compactness, long-term stability, thermal drift, and power efficiency.

Advances in Capacitive, Eddy Current, and Magnetic Displacement Sensors and Corresponding Interfaces

Human Activity Recognition using embedded sensors has lately made renowned development and is drawing growing attention in numerous application domains including machine learning, pattern recognition, context awareness, and human-centric sensing. Due to the lacking of a prominent analysis of this topic that can acquaint concomitant communities of the research avant-garde, there are still vital perspectives that, if pleaded, would create a vital turn in the way of interaction among people and mobile devices. In this paper, we have presented a comprehensive survey along with the prevailing state of various challenges of human activity recognition based on wearable, environmental, and smartphone sensors. Firstly, we have shown numerous factors to be considered for the data pre-processing part regarding noise filtering and segmentation methods. Besides, we have made a list of sensing devices, sensors, and applications that can be used for collecting activity data along with a discussion on sensor position and requirements. Moreover, we have made a comprehensive analysis of some benchmark datasets, which includes information about sensors, attributes, activity classes, etc. Finally, we have shown an analysis of activity recognition approaches on some of the benchmark datasets based on existing works.

Challenges in Sensor-based Human Activity Recognition and a Comparative Analysis of Benchmark Datasets: A Review

Wireless inductive sensors have been deployed in the position and displacement sensing for a wide variety of applications ranging from manufacturing processes, medical systems, automotive, and aerospace industries, to civil infrastructures. Here, we propose and experimentally demonstrate an ultrasensitive wireless displacement sensing technique based on the concept of parity-time (PT) symmetry or space-time reflection symmetry, first explored in quantum mechanics and later extended to wave physics. This PT-symmetric telemetric sensing system comprises an active – RLC tank (stationary reader) and a passive RLC tank (movable tag), which are linked in a wireless manner via inductive coupling. Specifically, such a non-Hermitian electronic system obeying the PT symmetry, when operated around the exceptional point (EP), can achieve drastic frequency responses and high sensitivity, well beyond the limit of conventional fully passive wireless displacement sensors. The proposed telemetric sensing system shows great potential for contactless, high-precision measurement of displacement, lift-off distance, position, and vibration, particularly suitable for emerging industrial and healthcare Internet-of-Things (IoTs).

High-Sensitivity Wireless Displacement Sensing Enabled by PT-Symmetric Telemetry

This paper presents a critical review of advancements made in the area of non-contact angle sensing in the last two decades. Angular position sensing is an essential function in most of the engineering systems associated with industries such as automotive, robotics, process, and aircraft. The review shows significant progress that has been achieved in the performance aspects of angle sensors, overcoming the challenges introduced by new and specific applications. Progress is also visible in effectively utilizing the emerging allied technologies on the sensing and electronics field for the development of high-end angle sensors. As a part of the review, the important sensing techniques utilized for the angle sensors are identified and carefully looked at the advantages and limitations of those. Besides, the potential areas for improvements are also suggested. The important techniques employed to design and develop the angle sensors are based on capacitive, inductive, magneto-resistive, and optical schemes. Signal conditioning circuits, analog or digital, or a combination of it, are necessary to derive the best from the sensing elements. They play a vital role in the overall performance of the sensor. The review addresses the advancement in the sensing element, methodology, and signal conditioning aspect in detail. In this review, the important angle sensors reported are evaluated against the performance parameters such as range, resolution, linearity, accuracy, sensitivity, power consumption, complexity, cost, manufacturability, and reliability.

Technologies and Applications of Angle Sensors: A Review

In this paper, we present a new, efficient displacement sensor using core-less planar coils that are magnetically coupled. The sensor consists of two planar stationary coils and one moving coil. The mutual inductance between the stationary coils and the moving coils are measured, and the displacement is computed. The sensor design was validated using numerical computation. Two prototype sensors of different dimensions were fabricated and tested. The first prototype sensor developed has a measurement range of 70 mm and an R.M.S. error of 0.8% and the second sensor has a measurement range of 56 mm and an R.M.S. error in measurement of 0.9%. The signal output from the sensor is made tolerant to errors due to variations in the vertical position of the moving coil. The new sensor is low in cost, easy to manufacture, and can be used in a number of industrial displacement sensing applications.

Design and Development of a Planar Linear Variable Differential Transformer for Displacement Sensing

This paper introduces a noncontact, low-cost, and reliable inductive sensor for angular displacement measurement. It is suitable to work in harsh industrial environments. The sensor has simple structure consisting of three key parts: a ferromagnetic stator, which is a pure plane; a ferromagnetic rotor, which has face slots; and four layers of planar copper coils, including primary and secondary coils. Primary coils are supplied with two orthogonal 4000-Hz ac, and secondary coils output a signal whose phase is proportional to angular displacement. Primary coils are designed with sinusoidal shape, so that magnetic field between the stator and rotor has approximately sinusoidal distribution, and finally linearity between the phase variation of output signal and angular displacement is well helped by this design. The structure and working principles of the sensor are explained in detail. Moreover, a sensor model was simulated to verify the feasibility of the sensor working principles and a sensor prototype was designed for actual experiment. At last actual experiment results are given and analyzed, showing that the sensor prototype has achieved accuracy better than ±12 arcsec in the range of 0°-360°, and this kind of sensor may have a better performance by improving the layout of primary coils and front-end signal-process circuit.

An Inductive Angular Displacement Sensor Based on Planar Coil and Contrate Rotor

This paper presents a sensing mechanism of a rotary magnetic encoder based on the time grating approach. The relationship between spatial displacement and time standard is built by the orthogonal alternating magnetic field in the rotational motion. Time pulses serve as measurement standard and are counted to measure the angular displacement. Therefore, this displacement detection method is called time grating. The alternating magnetic field is generated using the magnetic ring rotating with the shaft. Pairs of tunnel magneto resistance sensors are separated by 90° electrically angles to induce the alternating magnetic field and output orthogonal voltage signals. The travelling wave would be generated by signal processing, which has the phase shift with the rotating shaft. Thus, the rotation angle is measured by counting the time pluses corresponding to the phase difference. A rotary magnetic encoder has been fabricated to evaluate the proposed method. Experimental results indicate that the measurement has high resolution and stability.

Sensing Mechanism of a Rotary Magnetic Encoder Based on Time Grating

This paper presents a non-contact inductive linear displacement sensor which is based on planar coils realized by the printed circuit board (PCB) technique. The sensor mainly consists of a ferromagnetic stator, a ferromag-netic mover, several excitation coil-PCBs, and a pickup coil-PCB. Excitation coil-PCBs are embedded in the mover and generate quadrature magnetic fields both in temporal- and spatial-domains. The stator and mover confine the magnetic fields to propagate between them. Pickup coil-PCB is located between the stator and mover to receive the magnetic fields and outputs a signal whose phase is proportional to the displacement of the mover. Simulations of a 3-D sensor model and final experiment of a prototype are introduced and both verify the feasibility of this sensor. In order to overcome inexact quadrature in spatial domain of magnetic fields caused by asymmetric mover structure, a simple but effective compensation method realized in temporal domain is employed by the prototype. Experimental results show that the prototype has original measurement error of less than 50 $\mu \text{m}$ in the range of 0 –208 mm, and moreover, the error is still of great regularity so that the sensor may have better performance if there is digital calibration for measurement data or improvement in sensor structure.

An Inductive Linear Displacement Sensor Based on Planar Coils

Accurate rotor position is essential for a permanent magnet synchronous motor (PMSM) to execute vector control Due to the big volume, the resolver and the encoder as standard position sensors cannot be used in the space-limited application The sensor-less methods whose characteristic are sensitive to the parameter of the motor cannot be used in mass-produce Therefore, this paper presents tunnel magnetoresistance (TMR) sensors as a sensing element embedded into the PMSM to detect the magnetic field applied by the built-in magnets and high-frequency time pulses served as a measurement standard to measure the position TMR sensors excited by the altering voltage are integrated with the PMSM without taking up extra space Besides, the rotation position is measured by counting the time pulses corresponding to the phase shift of the TMR sensor’s output signal, which is a simple and effective method to detect the position Related design aspects include the corresponding signal circuit, and the sensing element is introduced in detail in this paper Experimental results prove the validity of the proposed method, and multi-group TMR sensors are used to get higher detection accuracy Although developed for the PMSM, the presented embedded position detecting approach can be used for where the space is limited or the other types of actuators to make it integration

Embedded Position Detection for Permanent Magnet Synchronous Motor With Built-In Magnets

This paper presents an inductive linear displacement sensor with bilateral sensing units, which consists of a stator with excitation coils, two movers with pickup coils and a mover base. Two movers have the same structure and work on two sides of the stator to realize bilateral sensing. Bilateral sensing units contribute to reducing the negative influence from the gap change between the movers and stator on the measurement performance of the sensor, because bilateral mover structure is able to maintain the sum of two side gaps constant and thus reach good magnetic coupling with the stator. Comparison experiments of the prototypes in different working gaps have been implemented and verified that the sensor with bilateral sensing units has better repeatability and accuracy than that with unilateral sensing unit. Based on the regular errors shown in the measurement results of the proposed bilateral sensing sensor in comparison experiments, this paper also presents the detailed analysis about the sources of some dominating error components. Accordingly, a sensor prototype with improved designs for reducing measurement errors was developed. Final experimental results of the improved sensor prototype show that the accuracy is better than $15~\mu \text{m}$ and the repeatability is about $7~\mu \text{m}$ with less than $1.2~\mu \text{m}$ resolution in the range of 0mm~288mm.

Donglin Peng

Papers

An Inductive Angular Displacement Sensor Based on Planar Coil and Contrate Rotor

Sensing Mechanism of a Rotary Magnetic Encoder Based on Time Grating

An Inductive Linear Displacement Sensor Based on Planar Coils

Embedded Position Detection for Permanent Magnet Synchronous Motor With Built-In Magnets

An Inductive Linear Displacement Sensor With Bilateral Sensing Units