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

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

In this paper, a simple angle sensor is presented, which is easy to fabricate and is low cost. The sensor is constructed using square shaped flexible planar coils, bent, and arranged in a cylindrical pattern. The sensor design was verified using appropriate analytical design, and an algorithm to identify the quadrants of the measured angle was developed. The output of the sensor is tolerant toward errors due to the axial displacement of the sensing coil. This makes it suitable for applications where the shaft may be subjected to axial motion (even up to 30% of sensing coil length) in addition to angular motion. The radial space required for the existing through-shaft angle sensors is large. Although capacitive sensors can be employed, their output is sensitive to moisture, dust, and oil. The proposed sensor requires very small radial space (about 1.5mm) and its output is not sensitive to such environmental parameters. A prototype sensor was constructed to verify the sensor operation and to evaluate its performance. The prototype sensor had an rms error of 0.58% and the maximum error was within ±1%. The resolution of the measurement system was estimated to be 0.15°. A detailed error analysis has been conducted and the outcome shows that the output of the sensor has very low sensitivity to the source of errors.

A Flexible, Planar-Coil-Based Sensor for Through-Shaft Angle 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

The invention discloses a time grating linear displacement sensor. The time grating linear displacement sensor comprises a fixed ruler and a movable ruler, wherein the fixed ruler is composed of a fixed ruler base body and two same and parallel sensor units arranged on the upper portion and the lower portion of the fixed ruler base body, wherein each sensor unit comprises exciting coil and induction coils. The exciting coils are formed by sequentially arranging a plurality of forward-wound plane rectangular spiral exciting coils and a plurality of reverse-wound plane rectangular spiral exciting coils along a measurement direction in a staggered mode, and the exciting coils can generate a magnetic field which changes according to sine rules. The induction coils are formed by sequentially arranging a plurality of forward-wound induction coils and a plurality of reverse-wound induction coils along the measurement direction in a staggered mode. The movable ruler and the fixed ruler generate relative movement along the measurement direction, the induction coils which are connected in series in the sensor units together output induction signals of constant amplitudes, phase demodulation processing and high frequency clock pulse interpolation are performed on the induction signals and excitation signals, and linear displacement is obtained after conversion. The time grating linear displacement sensor can double the resolving power of the time grating linear displacement sensor under the condition that the degree of precision of scale marks is not increased.

Time grating linear displacement sensor

The invention discloses a time grating angular displacement sensor, comprising a stator and a rotor The stator is formed by a stator substrate and two same sensing units The two sensing units are parallel and are arranged on the upper end face and the lower end face of the stator substrate in a non-interfering manner The sensing unit comprises an exciting coil and an induction coil Wires are winded on the exciting coil and the induction coil in a lying 8 shaped winding method The induction coils of the two sensing units are connected in series The rotor is formed by an upper rotor substrate, a lower rotor substrate, a support column, and two same magnetic conductive units The two magnetic conductive units are respectively embedded on the upper rotor substrate and the lower rotor substrate and are parallel to each other The magnetic conductive unit is formed by winding a magnetizer or m same magnetizers on the shaft axis of the upper rotor substrate or the lower rotor substrate in equal interval arrangement The initial positions of the magnetizers in the two magnetic conductive units satisfy specific conditions The time grating angular displacement sensor can improve measurement resolution by double under the condition of not increasing reticle precision or number of pole-pairs of a sensor

Time grating angular displacement sensor

The invention discloses an absolute type time-grating linear displacement sensor. The absolute type time-grating linear displacement sensor comprises a fixed ruler and a moving ruler. The fixed ruler is composed of a fixed ruler basic body, a first sensing coil and a second sensing coil. The first sensing coil and the second sensing coil are respectively wound in a horizontally-inverted-eight shape. The moving ruler is composed of a moving ruler basic body, a first excitation unit and a second excitation unit. The first excitation unit is composed of 4a1 first magnetizers and 4a1 first excitation coils. The second excitation unit is composed of 4a2 second magnetizers and 4a2 second excitation coils. The 4a1 first excitation coils are respectively provided with four paths of excitation currents, and the 4a2 second excitation coils are respectively provided with four paths of excitation currents. The moving ruler moves relative to the fixed ruler. The first sensing coil and the second sensing coil respectively output a first sensing signal and a second sensing signal. Phase demodulation processing is carried out on the first sensing signal and the second sensing signal, an obtained phase difference is expressed by the pulse number of an interpolated high frequency clock, and after conversion, the absolute displacement of the moving ruler is obtained. The sensor is capable of realizing absolute type linear displacement measurement, and the resolution and the precision are simultaneously improved.

Absolute type time-grating linear displacement sensor

The invention provides a planar two-dimensional time grating displacement sensor. The planar two-dimensional time grating displacement sensor is composed of fixed array planes and a movable array plane, wherein the fixed array planes are arranged relatively in parallel in an up-down mode. Each fixed array plane is composed of a fixed array plane base body and exciting coils arranged on the surface of the fixed array plane base body. The movable array plane is composed of a movable array plane base body and induction coils arranged on the surface of the movable array plane base body. The single movable array plane is adopted to obtain magnetic signals at different positions of the fixed array planes according to the electromagnetic induction principle, and the magnetic signals (space information) are converted into electric signals (time information) and then processed according to the space-time coordinate transformation theory, so that the planar two-dimensional space displacement is obtained. Two linear displacement sensors do not need to be installed perpendicularly, a binary optical device prepared in a complex process is not needed, a complex optical path design is not needed, the fixed array planes and the movable array plane are prepared in a common semiconductor machining process, and consequently the planar two-dimensional time grating displacement sensor has the advantages of being simple in structure, low in cost, resistant to oil fouling and dust and high in impact vibration capacity.

Tang Qifu

Papers

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

Time grating linear displacement sensor

Time grating angular displacement sensor

Absolute type time-grating linear displacement sensor

Planar two-dimensional time grating displacement sensor