Absolute Encoder Research Articles

Calibration of rotary encoders with different interfaces by means of a dynamic goniometer

Rotary optical encoders are widely used in all areas of industry, robotics, and special equipment. The accurate measurement of their characteristics is a necessary task for their successful implementation. We attempt to find a unified approach to determining the accuracy of rotary encoders. The proposed approach is based on statistics and code reliability, which characterizes the probability that the measured code corresponds to the specified code. To determine the code reliability, one has to measure the turn angle with a goniometric device and compare the measured values with the encoder codes. The paper considers the dynamic goniometer for determining the error of absolute angle encoders, which allows for highly accurate measurements in a wide range of rotation rates. The measurement is made at certain moments of time, depending on the electrical interface of the encoder. In the case of a parallel interface, the time points are determined by the moment when the encoder code changes. This data collection allows for obtaining a general population. In the case of an encoder, with the sequential approach to reading the code, it is not possible to obtain the general population in a wide range of rotation rates. We analyze the above-mentioned case in detail and present experimental data obtained with a dynamic goniometer and the results of code reliability calculations.

New UHV angle encoder for high resolution monochromators, a modern spare part for the Heidenhain UHV RON 905

There are a large number of soft X-ray grating monochromators for high-resolution synchrotron radiation experiments in operation worldwide. At BESSY II eighteen of the monochromators currently in use are equipped with Heidenhain UHV RON905 angle encoders. Those angle encoders have successfully been in operation for decades. Today, this type of encoder has become a legacy product and repairs are getting expensive. Therefore, we have developed a new angle encoder, a mechanically compatible drop-in replacement of the RON905. A mechanically fitting prototype, based on RENISHAW absolute encoders, was tested on a high precision angle drive test bench. Fourier analysis of the encoder data allowed us to determine the precision for different angle ranges and indicates a better precision for the new angle encoder. Furthermore, we will introduce an on-line, in-situ method using electron/absorption spectroscopy to improve system accuracy with plane grating monochromators in collimated light using the newly developed encoder.

The Series Elastic Gripper Design, Object Detection, and Recognition by Touch

Abstract In recent years, robotic applications have been improved for better object manipulation and collaboration with human. With this motivation, the detection of objects has been studied with a series elastic parallel gripper by simple touching in case of no visual data available. A series elastic gripper, capable of detecting geometric properties of objects, is designed using only elastic elements and absolute encoders instead of tactile or force/torque sensors. The external force calculation is achieved by employing an estimation algorithm. Different objects are selected for trials for recognition. A deep neural network (DNN) model is trained by synthetic data extracted from standard tessellation language (STL) file of selected objects. For experimental setup, the series elastic parallel gripper is mounted on a Staubli RX160 robot arm and objects are placed in pre-determined locations in the workspace. All objects are successfully recognized using the gripper, force estimation, and the DNN model. The best DNN model is capable of recognizing different objects with the average prediction value ranging from 71% to 98%. Hence, the proposed design of the gripper and the algorithm achieved the recognition of selected objects without the need for additional force/torque or tactile sensors.

Time-Sequential Variational Conditional Auto-encoders for Recommendation

In this study, we propose a method for adding time of action information to a Variational Auto-encoder (VAE)-based recommendation system. Since time of action is an important information to improve the accuracy of recom-mendation, many methods have been proposed to use the information of time of action, such as purchase or reviewof a product, for recommendation. And VAE-based recommendation systems have been reported to be more accu-rate and robust for small data sets compared to traditional deep learning-based recommendation systems. Existingresearch on introducing time information into VAEs includes a method of weaving information on the order in whichproducts are preferred by passing the encoding layer consisting of RNN, but the time information of the productpreferred is not considered. If the absolute time information is not taken into account when recommending a product,for example, when a temporary boom causes many users to prefer a particular product, it may be judged to be a pref-erence based on the user’s preferences, which may adversely affect the recommendation results. Based on the aboveproblems, this study examines a VAE-based recommendation system to improve the recommendation accuracy byadding time information of each action to the input information, and finally proposes Time-Sequential VAE (TSVAE)and confirms its accuracy. In addition, to verify how to add time information to improve the accuracy, we conductedexperiments using multiple models with and without absolute time information and different encoders of time intervalinformation, and evaluated the accuracy.

Self-calibration of a variable-line-spacing grating for an absolute optical encoder with a Fizeau interferometer

The principle of the self-calibration method for the evaluation of a planar scale grating having a constant pitch is extended to realize the evaluation of the pitch distribution of a planar scale grating having variable line spacings (VLSs) along the X- and Y-directions. In the conventional self-calibration method, the wavefronts in the zeroth-order diffracted beam and the first-order diffracted beams observed by a Fizeau interferometer arranged in the Littrow configuration were employed to evaluate the pitch deviation of a scale grating. The arithmetic operation with the wavefront data realizes the evaluation of the pitch deviation over a large area in a short time, while cancelling the influence of the out-of-flatness of a scale grating. Meanwhile, theoretical equations in the conventional self-calibration method cannot be directly applied to the evaluation of a VLS grating due to its unique properties of the pitch distribution. In this paper, major modifications are thus made to the conventional theoretical equations for deriving the pitch distribution of a VLS grating. To verify the performance of the newly proposed method, the pitch distribution of a VLS grating employed in a commercial planar absolute encoder is evaluated in experiments.

High-Performance Control of Surface PM Synchronous Motor by Power Factor Angle-Based Control of Stator Voltage Vector

A new high-performance control method for surface PM synchronous motor is presented here. In this technique, orientation and amplitude of stator voltage vector are directly controlled to achieve stable operation and optimal dynamic performance. Through voltage vector position control, stator current is aligned at nearly $$\angle {90^{\circ }}$$ angle from rotor direct axis. This condition resembles with that of vector control and ascertains fast transient response. Orientation of stator voltage is controlled with the help of measured rotor position and estimated power factor angle. To acquire position information, an absolute optical encoder is used. The amplitude of voltage vector is varied proportionally with rotor frequency to optimize input current. Performance of the drive is comparable to existing vector-controlled drives with position sensors and better than available sensorless drives. Furthermore, the developed drive is simple and cost effective since closed-loop current control or any kind of position observers is not used. Extensive simulation and experimental studies are carried out to evaluate the performance of the drive. The results are found satisfactory.

Absolute rotary encoder system based on optical sensor for angular measurement

A goniometer is a device used to measure the angular range of motion of various human joints and muscle groups, such as the elbow, knee or waist. However, the majority of mechanical goniometers are heavy, provide inaccurate measurements and are affected by dust or stains. Electronic and optical fiber-based goniometers demonstrate better capability than their mechanical equivalents; however, due to their higher costs and complicated setup requirements, their usage is not viable. Optical-based encoder patterns were GC-ARE previously proposed and discussed in some studies. Such optical-based encoder patterns achieve the same angular resolution as traditional Gray code encoder patterns with many shortcomings, such as a high error rate and low tolerance, which arise as a result of the encoding method. This paper presents a design for a reliable and non-intrusive phase shift code absolute rotary encoder (PSC-ARE), which is based on an optical sensor, providing a low-cost, high-precision absolute rotary encoder system. An inexpensive optical mouse sensor is used to capture the code image and the phase shift code transforming time-domain data into frequency domain, which is used to decode the corresponding absolute angle. The PSC-ARE proposed in this study shows a performance improvement over conventional designs owing to the reduced interference from dust and other environmental particles and provides lower sensitivity to calibration errors. Furthermore, the proposed design is not only more compact and portable than the traditional goniometer, and it can also provide a longer lifespan as no stepper motor is included. Finally, the proposed design demonstrates high tolerance to “unintentional defocus” effects and noise effects.

Low-cost non-concentric diffraction-based encoder

In this paper, we propose a novel low-cost optoelectronic sensor to measure angular displacements from 0 to 360° utilizing diffraction principles that avoid the need for complicated software and hardware designs. The arrangement of the sensor includes a cylinder attached to a rotary element in a non-concentric design containing a fixed laser that emits light to the wall of the cylinder which is made of a high-frequency diffraction grating film. The non-concentric rotational movement of the cylinder produces a variation of the distance between the orders of the projected diffraction pattern, allowing measurements of the angular position of the shaft. The mathematical design of the encoder was validated through simulations demonstrating its viability and reliability of the proposed equations. An experimental prototype was built to verify, via interferometric techniques, the resolution in the order of 0.03 [arcsec], therefore showing the potential of a low-cost diffraction optical absolute encoder with an outstanding cost-benefit ratio.

A Linear Compensation Method for Improving the Accuracy of an Absolute Multipolar Magnetic Encoder

This paper proposes a linear compensator algorithm to improve the performance of an absolute multipolar magnetic encoder (AMPME). An AMPME is an absolute magnetic rotary encoder that uses a multipolar magnet (MPM) to increase the resolution. The resolution can be dramatically increased in proportion to the number of poles in the MPM. However, various hardware problems that occur during the encoder manufacturing process degrade the AMPME performance. Also, harmonic components occur in the raw data due to various problems, such as the resistance error of the analog circuit, magnetic field overlap between the magnets, position error between the sensor and magnet, and pole-pitch difference. In particular, during the magnetization process of the MPM, the pole-pitch difference becomes a problem when the sizes of each pole are not uniform. This problem causes harmonic components that reduce the absolute position accuracy. To solve these problems, this paper proposes a linear compensation method. The proposed linear compensator consists of two parts. The first part is the enhanced ratiometric linearization for phase calculation and calibration. The second is the phase compensator for removing the phase difference via the pole-pitch difference of the MPM. The linear compensator improves various parameters by precomputing the offset, amplitude, and phase corrections. After compensating for sinusoidal signals, the linear compensator applies appropriate parameters at the appropriate times. This method is faster, easier to set up, and more accurate than the conventional method. Furthermore, this method is experimentally verified against the existing harmonic rejection method. Experimental results are provided to verify the effectiveness of the proposed method.

Absolute Position Acquisition for Linear Synchronous Motor With Passive Mover

The winding segmented permanent magnet linear synchronous motor has restrictions on the last absolute position acquisition when the power goes from off to on, and the traditional absolute position acquisition systems depend on complex battery and calibration systems. Therefore, a true absolute magnetic encoder based on the Vernier principle with error rejection and decoding technology is studied in this paper. First, the output signal of the magnetic encoder with noise was subjected to the adaptive ellipse parameter correction algorithm with recursive least squares (RLS), and the source output signal predicted and reconstructed. Furthermore, the RLS was optimized by the speed weighting factor and the forgetting factor to reduce the computational pressure and the iterative process. Second, one improved double second-order generalized integrator(DSOGI) decoding technology was investigated by extracting and separating the symmetric-positive sequence components in the unbalanced signal, and, using the phase-locked loop technique for tracking calculation, accurate speed and angle obtained. The adaptive ellipse parameter fitting algorithm effectively reduced the problem of overshoot and steady-state error of the DSOGI, which provides a practical and theoretical basis for the future application of the magnetic encoder. This trajectory arrangement not only avoids the influence of accumulated error on the accuracy of encoder, but it also solves the contradiction between high resolution and small volume, and the advantages over the special decoding chip IC-Haus were also verified by comprehensive experimental results.

The Probe: Design and Development of a Large Hexapod Robot

A hexapod robot has six legs; as such it can always walk in a statically stable way. In addition, people do not often see large-legged robots in real life. For these reasons, we design and develop a large hexapod robot to impress people at retail stores. The developed hexapod robot, named Probe, has a height of 2.1m, width of 3.3m, and mass of 480kg. The actuator is customized with a frameless BLDC motor, a harmonic drive gear, an electromagnetic brake, a multi-turn absolute magnetic encoder, heat sinks, etc. To firmly secure the frameless motor, we use a clamping ring and suggest the design of a tapered shaft hub and insert. We design an interface circuit board to acquire sensor data and a brake switch board to engage and release brake via an Elmo driver’s signal. Additionally, an analytic solution to the kinematic equations of Probe’s legs is described. After development of the Probe, Two Probes are deployed at two different stores in South Korea and they conduct scenario motions during business hours. In future, the design of the foot tip will be modified because the urethane cover beneath foot tip wears while the Probe is walking.

Redundant and Flexible Pseudorandom Optical Rotary Encoder

Optical encoders are mainly used in modern motion servo systems for high-resolution and reliable position and velocity feedback. Pseudorandom optical rotary encoders are single-track and use a serial pseudorandom binary code to measure absolute position. The realization and analysis of such a rotary encoder with advanced code scanning and error detection techniques, as well as an improved redundancy in operation, are presented. A presented serial code reading solution uses two phase-shifted code tracks and two optical encoder modules. So, the realized encoder, hybrid in nature, provides “output on demand” and more or less reliable position information using very efficient error checking. Compared to a standard absolute encoder, this encoder requires a smaller code disc, facilitates installation, has greater flexibility in operation, and is less sensitive to external influences.

복수개의 엔코더를 이용한 다회전 앱솔루트 엔코더

A multi-turn absolute encoder is a sensor that can measure the multi-turn rotation angle of a shaft. Recently, multi-turn absolute encoders have been studied and developed for robots. In particular, the compact hollow joint structure used in cooperative robots requires a hollow multi-turn encoder with high performance and space efficiency. In this paper, we present an absolute encoder system model that consists of multiple encoders, and propose a method to compute the multi-turn rotation angle of the system. Using the proposed method, a multi turn encoder system was designed and developed for the hollow joint of the robot. The method succeeded in measuring the absolute rotation angle with a resolution of ±0.004 rad through 84 turns of the shaft.

Machine learning-based method for linearization and error compensation of a novel absolute rotary encoder

The main objective of this work is to develop a miniaturized, high accuracy, single-turn absolute, rotary encoder called ASTRAS360. Its measurement principle is based on capturing an image that uniquely identifies the rotation angle. To evaluate this angle, the image first has to be classified into its sector based on its color, and only then can the angle be regressed. Inspired by machine learning, we built a calibration setup, able to generate labeled training data automatically. We used these training data to test, characterize, and compare several machine learning algorithms for the classification and the regression. In an additional experiment, we also characterized the tolerance of our rotary encoder to eccentric mounting. Our findings demonstrate that various algorithms can perform these tasks with high accuracy and reliability; furthermore, providing extra-inputs (e.g. rotation direction) allows the machine learning algorithms to compensate for the mechanical imperfections of the rotary encoder.

Adaptive feedforward control of a collaborative industrial robot manipulator using a novel extension of the Generalized Maxwell-Slip friction model

Collaborative industrial robots often use strain-wave transmissions which display a highly nonlinear behavior. In particular, the friction torque depend on the load torque and the hysteresis characteristics were recently found to depend on the joint angular position.This paper presents a novel extension of the Generalized Maxwell-Slip friction model to describe said phenomena in a combined framework. The method overcomes the discontinuity around zero velocity of existing models. Experiments on the Universal Robots UR5e manipulator show superior performance in terms of torque prediction accuracy and tracking performance of the proposed method.An adaptive feedforward friction compensator is proposed based on the extended Generalized Maxwell-Slip friction model to compensate the time-variations of the Coulomb and viscous friction due to, respectively, wear and mispredictions of the lubricant temperature. The adaptive estimator relies on the sensing hardware readily available in the joints of the Universal Robots manipulators, i.e. two absolute rotary encoders; one at each side of the transmission, current sensing for the electric actuator, and a temperature sensor. Results show a considerable reduction of the torque prediction error and tracking error.

An absolute surface encoder with a planar scale grating of variable periods

An absolute surface encoder for measurement of absolute in-plane position with a mode-locked femtosecond laser and a planar scale grating having variable periods along the two orthogonal directions is proposed. In the proposed method, the absolute in-plane position is measured through observing the spectra of the coupled groups of the positive and negative first-order diffracted mode-locked femtosecond laser emanated from a single point on the planar scale grating. Due to the dispersive effect of the planar scale grating having variable periods, each of the diffracted modes has a unique angle of diffraction associated with the absolute position information. Therefore, the absolute in-plane position can be detected by obtaining the peak frequencies in the optical spectra of the coupled positive and negative first-order diffracted beams received by dual detector units. The dual-detector configuration of the optical head also contributes to improving the robustness of the absolute position measurement against the angular error motion of the planar scale grating. Furthermore, interference signals capable of being used for the improvement of the absolute position measurement can be generated under the dual detector configuration. Results of the numerical calculations for the theoretical verification of the proposed method, fabrication of a scale grating having a variable period (VLS grating) with interference lithography, design and development of the optical head, as well as the results of basic experiments by using the developed optical head and the VLS grating, are reported.

Optimizing the De-Bruijn Code of Rotary Optical Encoders Preventing From the Photocurrent Blooming

The De-Bruijn sequences could provide appropriate circular arrangements to achieve a single-track absolute rotary optical encoder in the servo motor application. One example used popularly is the maximum length sequences. High-resolution single-track encoders need a sensor array to obtain the position signal simultaneously. Nevertheless, the individual photocurrents would interference by other adjacent photocurrents, like the photocurrent blooming. The circumstances would increase the bias noise for the digital binarization. The concept in this article would try to find the optimal De-Bruijn sequences to reduce the photocurrent blooming interference intrinsically without signal process remodification. Three different types of the 9-bits codes, the adjacent De-Bruijn code, the quarter De-Bruijn codes, and the optimal De-Bruijn codes, are analyzed, manufactured, and experimented in the actual encoders. Based on the open-factor of the eye-diagram, the adjacent De-Bruijn code has 0.328 and would be the worse signal quality in De-Bruijn sequences. The quarter De-Bruijn codes have 0.386 of the open-factor with 17.7% improvement to adjacent De-Bruijn code. Finally, the optimal De-Bruijn codes have 0.436 of the open-factor with 32.9% improvement to the traditional M-code. The fact would mean the possible ability of 3-bits progress from 9-bits of adjacent De-Bruijn code to 12-bits of the optimal De-Bruijn codes under the same signal-to-noise ratio.

Measurement and compensation of errors in absolute encoder using dual absolute encoder system

The dual absolute encoder (DAE) system is a measurement system for a motion control system that comprises two absolute encoders and one reduction mechanism. In terms of accuracy and measuring range, DAE is superior to ordinary dual encoder systems, which comprise one incremental encoder, one absolute encoder, and a reduction mechanism. In this study, we focus on the error measurement and compensation using DAE system. There have been many studies to measure and compensate for the errors in measuring systems. However, this typically demands high precision equipment to measure the errors, and attaching and detaching the equipment affects the errors. We measured the errors in the absolute encoder by using the DAE systems solely. The measured errors are modeled using harmonic functions and compensated for by using the modeled errors. The experimental results reveal that the maximum errors and mean absolute errors decrease by one-seventh and one-twelfth, respectively, after error compensation. As a result, we can compensate the errors in the encoders with the encoder system, itself. Additionally, the modeled errors in a DAE system are observed to remain constant even when changing the reduction ratios between two absolute encoders. Once the errors are measured and modeled in the DAE system, the modeled errors can be applied to a DAE system with a different reduction ratio for error compensation.

Temperature invariant and high precision absolute rotary encoder using photocells on visible light spectrum

This paper introduces temperature invariant absolute rotatory encoder using photocells on the visible light spectrum. A multivariate function is proposed to remove error arising on photocells through temperature variance and a small change in light intensity. Multiple experiments are conducted in the controlled environment to prove proposed multivariate function. Encoder introduced in this paper is more scalable than grey code encoder available in the market. Dynamic auto calibration is also proposed to make encoders more reliable with time.

An Effective Method to Improve the Accuracy of a Vernier-Type Absolute Magnetic Encoder

This article proposes a method to improve the accuracy of a vernier absolute magnetic encoder. The encoder consists of a master and a nonius multipolar magnetic track. Sinusoidal signals from the master and nonius tracks are used to infer the absolute information. Unfortunately, these signals are contaminated by nonideal factors such as different amplitudes, dc-offsets, phase shifts, and random noise. Moreover, harmonics existing in the encoder signals distort the vernier principle and significantly affect the accuracy of the encoder. To address these problems, the present article proposes an efficient method with three main parts. The first is an observer phase-locked loop (OPLL), which is used to estimate the phase and eliminate the nonideal factors. The second is nonlinear phase compensation, which is used to correct the vernier principle that deviated due to the existing harmonics. Finally, a pole pitch compensation method is introduced to modulate the master phase angle from the OPLL to eliminate the harmonic distortion. The proposed method can eliminate the nonideal factors, harmonic distortion and improve the accuracy of the encoder. All the proposed methods were implemented on an ARM STM32F407ZG. The experimental results confirm the validity of the proposed method for practical applications.