Capacitive Angular-position Sensor Research Articles

A MEMS-Based Three-Ring Capacitive Angular Position Sensor With an Absolute Zero Position Feature

A MEMS-Based Three-Ring Capacitive Angular Position Sensor With an Absolute Zero Position Feature

A Multivector Variable Area Type Capacitive Angular Displacement Encoder Based on a Rotating Electric Field and Improvement of Its Demodulation Method

The problems of the traditional type grating angular displacement encoder make it difficult to balance the measurement performance of miniaturization, high precision and high resolution, and the ability to resist vibration and shock is poor. In this article, a multivector variable-area capacitive angular displacement encoder based on a rotating electric field is proposed. Based on the principle of constructing a rotating electric field, a three-layer transmissive sensing structure with upper and lower layers of electrode plates and a modulation rotor as the middle layer is proposed and designed. A single-channel signal is used as the input excitation signal of the capacitive sensor to avoid the problem of mismatching multiple input signals. A pair of quadrature signals reflecting the spatial angle information is obtained through electric field demodulation and differential processing. The phase demodulation method based on the time scale is further adopted to improve the precision of demodulation. Simulations and experiments show that the sensor has a diameter of 50 mm, a thickness of 5 mm, a resolution of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$0.81''$ </tex-math></inline-formula> , an accuracy of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$2.5''$ </tex-math></inline-formula> , and a maximum nonlinear error of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$40''$ </tex-math></inline-formula> . The design is verified to be feasible, which provides a new solution for capacitive angular position sensor design.

Design and Implementation of a System-on-Chip for Self-Calibration of an Angular Position Sensor

In this study, a novel signal processing algorithm and hardware processing circuit for the self-calibration of angular position sensors is proposed. To calibrate error components commonly found in angular position sensors, a parameter identification algorithm based on the least mean square error demodulation is developed. A processor to run programs and a coprocessor based on the above algorithm are used and designed to form a System-on-Chip, which can calibrate signals as well as implement parameter configuration and control algorithm applications. In order to verify the theoretical validity of the design, analysis and simulation verification of the scheme are carried out, and the maximum absolute error value in the algorithm simulation is reduced to 0.003 %. The circuit’s Register-Transfer Level simulation shows that the maximum absolute value of the angular error is reduced to 0.03%. Simulation results verify the calibration performance with and without quantization and rounding error, respectively. The entire system is prototyped on a Field Programmable Gate Array and tested on a Capacitive Angular Position Sensor. The proposed scheme can reduce the absolute value of angular error to 4.36%, compared to 7.68% from the experimental results of a different calibration scheme.

A High-Precision Absolute Angular Position Sensor With Vernier Capacitive Arrays Based on Time Grating

This paper presents a high-precision absolute capacitive angular position sensor based on time grating. The sensor includes two single ring incremental type sensors arranged as outer and inner rings of capacitive arrays forming $N$ and $N-1$ measurement periods, respectively. The outer ring incremental sensor is employed as a fine measurement component to provide high-precision angular displacement values. The phase difference between the two incremental sensors is employed as a coarse measurement component to achieve absolute angular positioning according to a measurement principle similar to that of a vernier caliper. The proposed design is validated and optimized via tests conducted for a prototype sensor with $N\,\,=180$ and a diameter of 154 mm fabricated using standard printed circuit board manufacturing technology. Based on these results, a differential induction electrode structure is proposed to greatly reduce the magnitude of crosstalk interference signals captured by the induction electrodes of one array produced by the lead wires of the excitation electrodes of the other array. The optimized sensor achieves absolute angular position measurements with an original measurement accuracy of ±2” over a full 360° measurement range. The proposed sensor has advantages of low cost, high measurement precision, simple positioning method, and ease of manufacture.

Self-Calibration of Nonlinear Signal Model for Angular Position Sensors by Model-Based Automatic Search Algorithm.

This study proposes a novel model-based automatic search algorithm to realize the self-calibration of nonlinear signal model for angular position sensors. In some high-precision angular position sensors, nonlinearity of the signal model is the main source of errors and cannot be handled effectively. By constructing a signal flow network framework and by embedding a modeling search network, the parameters of the nonlinear signal model can be searched, and the calibration signal can be obtained. The convergence of the network search process was analyzed. The relationship between the optimization threshold and the convergence accuracy was also studied in simulations. Compared with the maximum angular error reduction to 47.42% after the calibration with simplified model that ignores signal nonlinearities, the proposed scheme was able to reduce this error to 0.0025% in simulations. By implementing the technique in a capacitive angular position sensor, the experimental results showed that the maximum angular error was reduced to 1.63% compared to a reduction of 86.02% achieved with the simplified model calibration. The effects of the search network order and layer number on the calibration accuracy were also analyzed, and the optimal parameters under experimental conditions were obtained. Correspondingly, the proposed scheme is able to handle calibration of nonlinear signal model and further improve sensor accuracy.

A Novel Single-Excitation Capacitive Angular Position Sensor Design.

This paper presents a high-precision capacitive angular position sensor (CAPS). The CAPS is designed to be excited by a single voltage to eliminate the matching errors of multi-excitations, and it is mainly composed of excitation electrodes, coupling electrodes, petal-form sensitive electrodes and a set of collection electrodes. A sinusoidal voltage is applied on the excitation electrodes, then the voltage couples to the coupling electrodes and sensitive electrodes without contact. The sensitive electrodes together with the set of collection electrodes encode the angular position to amplitude-modulated signals, and in order to increase the scale factor, the sensitive electrodes are patterned in the shape of petal-form sinusoidal circles. By utilizing a resolver demodulation method, the amplitude-modulated signals are digitally decoded to get the angular position. A prototype of the CAPS is fabricated and tested. The measurement results show that the accuracy of the sensor is 0.0036°, the resolution is 0.0009° and the nonlinearity over the full range is 0.008° (after compensation), indicating that the CAPS has great potential to be applied in high-precision applications with a low cost.

A capacitive angular sensor with flexible digitated electrodes

Purpose – This paper aims to present a prototype of a capacitive angular-position sensor which exploits advantages of flexible/printed electronics. The novelty of the sensor is that the capacitor structure is placed at the circumference of the rotor and stator, that it posses two channels (capacitor structures) electrically shifted for p/4 and that the rotor is common for both channels. The electrodes of the sensing capacitor are digitated, providing a triangular transfer function. Design/methodology/approach – This sensor prototype consists of two flexible inkjet-printed silver electrodes forming a cylindrical capacitor structure. One of them is wrapped around the stator and another is wrapped around the rotor part of a simple mechanical platform used to precisely adjust the angular displacement. Findings – The capacitance as a function of angular position was measured using an inductance capacitance impedance (LCZ) Meter, and results are presented for a full-turn measurement range. The experimental results are compared with analytical ones and very good agreement has been achieved. Originality/value – The proposed capacitive sensor structure can be used as an absolute or an incremental encoder with different resolutions, and it can be applied in automotive industry or robotics.

Capacitive Angular-Position Sensor With Electrically Floating Conductive Rotor and Measurement Redundancy

This paper presents a capacitive angular-position sensor with a contactless electrically floating conductive rotor and an interface electronic circuit that is designed to maximize the performance/cost ratio. The sensor includes two separate and independent measurement sections that sense the same angle and provide redundancy in critical applications. The electronic interface is based on a relaxation oscillator that, for each of the two sections, measures an appropriate quantity that relates capacitance ratio to angular position and provides a dc output voltage that varies ratiometrically with respect to the supply voltage. The sensor was built in a version with /spl plusmn/11/spl deg/ measuring range for each section. Experimental tests showed a linearity better that 1% of the span.

A contactless capacitive angular-position sensor

This paper presents an absolute capacitive angular-position sensor with a contactless rotor. The sensor is mainly composed of three parts: the capacitive sensing element, a signal processor, and a microcontroller. The electrically floating rotor can be either conductive or dielectric. For the dielectric material, we chose plastic, and for the conductive rotor, we chose aluminum. The sensing element has a redundant structure, which reduces mechanical nonidealities. The signal processor has a multicapacitance input and a single output, which is a period-modulated square-wave voltage. The microcontroller acquires output data from the processor and sends them to a PC, which calculates the rotor position. Theoretical analysis, supported by experimental results, show that the sensitivity to mechanical nonidealities of the sensing element is higher in the case of a conductive rotor. The resolution of the capacitive angular-position sensor over the full range (360/spl deg/) was better than 1. The measured nonlinearity was /spl plusmn/ 100 and /spl plusmn/ 300 for the dielectric and the conductive rotor, respectively.

Digital capacitive angular-position sensor

A capacitive angular position sensor capable of covering 360° is presented. It is made up of four quadrants, each of which consists of a parallel plate capacitor whose higher potential plate (HPP) has a specific shape. A grounded semi-circular plate, whose angular position is measured, moves between these capacitors. When these capacitors are supplied with sinusoidal voltages of mutually shifted phases in a bridge system, the phase of the output voltage of the bridge changes with the mechanical angle. Simulation results show that this relationship can be linearised if the shape of the HPPs is modified. A simple proto-type has been developed in which the shaping of the HPPs gives satisfactory improved results. The phase of the output voltage is measured digitally and the relationship between the angular position and the digital output is found to be linear, with a linearity error ±0.5% and a resolution of 3 arc minutes.

Virtual rotor grounding of capacitive angular position sensors

This paper presents a new approach for the design of capacitive angular position and angular speed sensors. The idea is to obtain nonconductive grounding of the rotatable part of the sensor. This approach allows the use of conductive rotor blades fixed to a nonconductive shaft. A simple prototype has been developed to show the theory of operation. Model calculations and experimental results show that virtual grounding allows neglecting the rotor-ground resistance in a wide range.

The application of the capacitor's physics to optimize capacitive angular-position sensors

The distributions of the potential and charge density in a capacitive angular-position sensor with three electrode disks in an axis-symmetrical configuration have been found by creating a suitable physical model and solving Laplace's equation. The influences of geometrical parameters on the nonlinearity, which originates from the electric-field-bending effect, are discussed in detail for a capacitive angular-position sensor. The approximately analytical results are verified by a numerical analysis and are found to be very useful to predict the influences of geometrical nonidealities. The calculations are experimentally verified by using a novel, absolute angular-position sensor with a nonlinearity of less than /spl plusmn/17 and /spl plusmn/50 over measurement ranges of 15/spl deg/ and 90/spl deg/, respectively.

A microcontroller-based self-calibration technique for a smart capacitive angular-position sensor

A self-calibration technique for capacitive position sensors, based on the use of a microcontroller is presented. This technique offers a good trade-off between the memory size of the microcontroller and the algorithmic complexity. The application of this technique for self-calibration of the inaccuracy caused by the geometrical errors and pattern errors in a smart capacitive angular-position sensor is discussed. This self-calibration technique doesn't need any accurate reference. Experimental results show that the self-calibration is relatively effective in reducing the influences of the geometrical errors and pattern errors of the sensor. The reduction of the inaccuracy of the smart capacitive angular-position sensor amounts to more than a factor of two.

An accurate low-cost capacitive absolute angular-position sensor with a full-circle range

A novel high-performance smart capacitive angular-position sensor with a full-circle measurement range is presented. The sensor is mainly composed of three parts: the capacitive sensing element, a signal processor and a microcontroller. Using an appropriate algorithm, the effects of the main undesired influences are eliminated or strongly reduced. The measurement range covers the full-circle range (360/spl deg/). The structure of the sensing element is optimized to reduce the influence of the electric-field-bending effect and the mechanical errors. The signal processor has a multicapacitance input and a single period-modulated output, and includes a nearly linear capacitance/period converter. The microcontroller acquires output data from the processor, calculates the positions and optionally communicates with the outside digital world. The resolution of the smart capacitive angular-position sensor is 1.5 arcsec, and the nonlinearity is less than /spl plusmn/58 arcsec over the 0-360/spl deg/ range for a measurement time of 140 ms.