Encoder Resolution Research Articles

Analysis of Accuracy and Measuring Range of Dual Absolute Encoder System

As small-sized absolute encoders are currently being installed in small and high-power motors, a dual absolute encoder (DAE) system comprising two absolute encoders can now be constructed. The two absolute encoders in the DAE system are installed on a motor shaft and an output shaft; further, the two shafts are connected by a reduction mechanism. The DAE system will replace an ordinary dual encoder system in the future, because of its high accuracy and extended measurement range. In this paper, an analysis of the accuracy and measuring range of DAE system is presented. Index numbers that are calculated from readings of absolute encoders have a one-to-one correspondence with the number of rotations of the two shafts. Their positions are calculated based on the index number and extended greatest common factor ( EGCF ) of 1 and the inverse of reduction ratio. The error in the index number is investigated and maximum error tolerance of an absolute encoder that guarantees correct position estimation is proposed. Experiments are conducted separately in narrow and wide measurement ranges. Each range exhibits different types of errors, small random and large periodic errors. The large periodic errors are compensated using harmonic function model. The small random errors are utilized to calculate the maximum error tolerance and minimum resolution of the absolute encoder at the output shaft. The experimental results demonstrate that the final error decreases and the measurement range is extended to 1/ EGCF in the DAE system compared with the ordinary dual encoder system.

Performance Analysis of Disturbance Estimation Techniques for Robust Position Control of DC Motor

In this paper, the problem of high-performance robust position control of a DC motor based on the uncertainty and disturbance estimator (UDE) is presented. The UDE based sliding mode controller (SMC) is proposed which makes the system insensitive to the effect of Coulomb’s friction, parameter variations, and variable external load in DC motor. Fair comparison between UDE based SMC and existing classical technique is carried out by making PID as a robust controller by using Q-filter design. The different filter designs provide different estimates of nonlinearity, uncertainties, and external disturbances at a different resolution of position encoders. The proposed method guarantees robustness and ultimately eliminates the chattering phenomenon which is generally occurred in SMC.

A Time-Efficient Method for Determining an Optimal Scaling Factor and the Encoder Resolution in the Multichannel FXECAP-L Algorithm with Evolving Order for Active Noise Control

Affine projection (AP) algorithms have demonstrated faster convergence speed than conventional least mean square (LMS) algorithms, thus providing an attractive solution in the active noise control (ANC) field. However, the AP algorithms demand high computational cost, restricting their practical use in real-time ANC applications. Recently, a multichannel filtered-x error-coded affine projection-like (FXECAP-L) algorithm with evolving order has been proposed to reduce the computational burden by maintaining the convergence speed of AP algorithms. In order to obtain an efficient and robust FXECAP-L algorithm with evolving order, the scaling factor and encoder resolution need to be adjusted manually, which is a time-consuming and costly effort that must be carried out by expert designers. To reduce these costs and efforts, we introduce, for the first time, a strategy for automatic adjustment of the scaling factor and encoder resolution that benefits the rapid development of practical ANC applications. To demonstrate its practical use, we applied the proposed strategy for controlling the noise in an acoustic duct. The practical results demonstrate the automatic adjustment of the FXECAP-L algorithm which maintains high convergence speed at the expense of a small compromise in terms of processing time.

Research on Speed Oscillation of Ultra-Low Speed Turntable in Aerospace Due to Limited Resolution Encoder

Abstract A lot of motor direct-drive turntables used in aerospace operate at ultra-low speed and require high speed precision. Thus, the limited encoder resolution becomes critical to the speed estimation of the turntable system. In this paper, The speed observation process in feedback channel is mathematically modeled and the speed oscillation induced by the observation error is fully analyzed. To get the high speed precision under limited encoder resolution, a novel speed estimator based on tracking differentiator (TD) is proposed, and related parameter tuning method is discussed. Finally, numerical simulations and related experiments were carried out, which showed the effectiveness of the speed estimator algorithm.

Ratiometric-Linearization-Based High-Precision Electronic Interpolator for Sinusoidal Optical Encoders

Electronic interpolation is the key technology for further improving the measurement resolution of optical encoders. In this paper, an electronic interpolator based on the ratiometric linearization conversion method is presented. The proposed method converts the sinusoidal encoder signals into a nearly perfectly linear output signal through mathematical manipulation that only involves basic operations of addition, subtraction, multiplication, and division. Thus, the displacement can be precisely determined using a simple linear equation. Furthermore, quadrature interpolation pulses are also generated from the linear output signal by using the amplitude subdivision method. Since the linearization procedure is based on the ratiometric operation, interpolation accuracy is independent of amplitude fluctuation of the encoder signals. Theoretical analysis shows that the nonlinear error of the proposed interpolator is below ±0.0034 rad over a signal period of 2 π rad, which corresponds to an interpolation error of ±0.0108 μ m for a linear optical encoder with a pitch of 20 μ m. In the experiment, the proposed strategy is successfully implemented within a field programmable gate array, and applied to a 20 μ m-pitch optical encoder. Experiments are performed to demonstrate the effectiveness of the proposed method.

Motion Signals With Velocity Jumps: Velocity Estimation Employing Only Quantized Position Data

This letter addresses the problem of causal velocity estimation of motion signals exhibiting sudden velocity changes. It is explicitly assumed that only sampled quantized position data are available, and that no dynamic system model is at hand, to allow for an effective solution with minimal a priori information. Our solution consists of a finite-response filter with adaptive window length and a velocity jump detection algorithm. A velocity jump is detected when the difference between sampled position data and predicted position is larger than a parametric threshold. The specific value for the threshold is obtained via an analytical formula that depends on sampling time, maximum allowed variation in acceleration (away from velocity jumps), and encoder resolution. The developed method is first demonstrated on simulated quantized data corresponding to a bouncing motion and its performance compared to that of other existing non-model-based methods. Its effectiveness is furthermore illustrated by means of trajectory tracking results on an experimental setup experiencing impacts.

High resolution extensometer based on optical encoder for measurement of small landslide displacements

There are many ways to measure landslide displacement using sensors such as multi-turn potentiometer, fiber optic strain sensor, GPS, geodetic measurement, ground penetrating radar, etc. The proposed way is to use an optical encoder that produces pulse signal with high stability of measurement resolution despite voltage source instability. The landslide measurement using extensometer based on optical encoder has the ability of high resolution for wide range measurement and for a long period of time. The type of incremental optical encoder provides information about the pulse and direction of a rotating shaft by producing quadrature square wave cycle per increment of shaft movement. The result of measurement using 2,000 pulses per resolution of optical encoder has been obtained. Resolution of extensometer is 36 μm with speed limit of about 3.6 cm/s. System test in hazard landslide area has been carried out with good reliability for small landslide displacement monitoring.

Polymer Optical Fiber for Angle and Torque Measurements of a Series Elastic Actuator's Spring

Series elastic actuators (SEAs) can provide low output impedance, bandwidth close to the human movement, and direct measurement of torque through the spring deflection. These advantages enable the application of SEAs in human active orthosis and exoskeletons. However, conventional technologies to measure the spring deflection are bulky, inhibit natural pattern of movement or are sensitive to misalignments. This paper presents the application of polymer optical fiber (POF) as a sensor to measure the spring deflection to overcome some of the issues of conventional technologies, since it is compact, lightweight, and have electromagnetic immunity. Furthermore, the spring is employed to validate a torque sensor based on POF stress-optic effects. Results show high linearity of both sensors and mean squared errors below the encoder resolution employed as a reference on dynamic measurements.

Estimation of disturbance torque for ballscrew drive system with using high resolution encoder

Ballscrew drive system has high mechanical efficiency and is able to get high gear ratio easily. But it also has non liner friction elements like rolling friction and viscous friction, so it makes ballscrew control accuracy worse. For that reason, it is proposed that inserting disturbance observer into control system and compensating disturbance torque. Generally, rotary encoder is used for position sensor in ballscrew control, but its resolution affects accuracy of drive control and disturbance observer. In this paper, we propose a speed control system using AC servo motor ball screw drive system, compare driving characteristics in low speed range when using low resolution encoder and high resolution encoder as position sensor. Also we examine effects for driving characteristics by using and compensating disturbance observer. These verifications were carried out by simulation and actual machine.

The MyoRobot: A novel automated biomechatronics system to assess voltage/Ca2+ biosensors and active/passive biomechanics in muscle and biomaterials

We engineered an automated biomechatronics system, MyoRobot, for robust objective and versatile assessment of muscle or polymer materials (bio-)mechanics. It covers multiple levels of muscle biosensor assessment, e.g. membrane voltage or contractile apparatus Ca2+ ion responses (force resolution 1µN, 0–10mN for the given sensor; [Ca2+] range ~ 100nM–25µM). It replaces previously tedious manual protocols to obtain exhaustive information on active/passive biomechanical properties across various morphological tissue levels. Deciphering mechanisms of muscle weakness requires sophisticated force protocols, dissecting contributions from altered Ca2+ homeostasis, electro-chemical, chemico-mechanical biosensors or visco-elastic components. From whole organ to single fibre levels, experimental demands and hardware requirements increase, limiting biomechanics research potential, as reflected by only few commercial biomechatronics systems that can address resolution, experimental versatility and mostly, automation of force recordings. Our MyoRobot combines optical force transducer technology with high precision 3D actuation (e.g. voice coil, 1µm encoder resolution; stepper motors, 4µm feed motion), and customized control software, enabling modular experimentation packages and automated data pre-analysis. In small bundles and single muscle fibres, we demonstrate automated recordings of (i) caffeine-induced-, (ii) electrical field stimulation (EFS)-induced force, (iii) pCa-force, (iv) slack-tests and (v) passive length-tension curves. The system easily reproduces results from manual systems (two times larger stiffness in slow over fast muscle) and provides novel insights into unloaded shortening velocities (declining with increasing slack lengths). The MyoRobot enables automated complex biomechanics assessment in muscle research. Applications also extend to material sciences, exemplarily shown here for spider silk and collagen biopolymers.

Development of a reflective optical encoder with submicron accuracy

Signal distortion is a key issue that limits the measurement resolution and accuracy of optical encoders. In this paper, an optical encoder based on generalized grating imaging using a two-dimensional index grating is presented. The general expression of intensity distribution for generalized grating imaging including the relative displacement between the scale grating and the reading head is derived, and the formation of the signal distortion of the optical encoder is analyzed. Then, a two-dimensional index grating, which consists of multiple grating tracks with defined offsets, is proposed to suppress the dominant third and fifth order harmonic signals. The operating principle of the two-dimensional index grating is explained in detail and a reflective optical encoder is developed. In the experiment, approximately ideal Lissajous figure of the encoder signals is obtained. Fourier analysis of the encoder signals shows that both the third and fifth order harmonic distortions are below 0.6%. Experimental results show that the interpolation error of the optical encoder is within ±0.18μm, and the accuracy is better than ±0.3μm over 255 mm travel range with a maximum variation of 0.136 μm.

Dual displacement resolution encoder by integrating single holographic grating sensor and heterodyne interferometry.

A novel encoder with dual displacement resolution is developed by integrating a multiple-grating-scale holographic displacement sensor and a heterodyne interferometer. With suitable arrangement of the measurement system, two effective grating pitches (0.41 μm and 10.62 μm) can be obtained and the theoretical sensitivities of them are 0.9 °/nm and 0.036 °/nm. Meanwhile, the best resolution of the proposed method can be estimated of 0.3 pm and 7.4 pm, respectively. Furthermore, displacement errors of the proposed method can be better than 0.2% for 1 mm displacement measurement. The experimental results showed that the proposed encoder provided high sensitivity, high resolution, and well against environmental disturbance.

SegNet: A Deep Convolutional Encoder-Decoder Architecture for Image Segmentation.

We present a novel and practical deep fully convolutional neural network architecture for semantic pixel-wise segmentation termed SegNet. This core trainable segmentation engine consists of an encoder network, a corresponding decoder network followed by a pixel-wise classification layer. The architecture of the encoder network is topologically identical to the 13 convolutional layers in the VGG16 network [1] . The role of the decoder network is to map the low resolution encoder feature maps to full input resolution feature maps for pixel-wise classification. The novelty of SegNet lies is in the manner in which the decoder upsamples its lower resolution input feature map(s). Specifically, the decoder uses pooling indices computed in the max-pooling step of the corresponding encoder to perform non-linear upsampling. This eliminates the need for learning to upsample. The upsampled maps are sparse and are then convolved with trainable filters to produce dense feature maps. We compare our proposed architecture with the widely adopted FCN [2] and also with the well known DeepLab-LargeFOV [3] , DeconvNet [4] architectures. This comparison reveals the memory versus accuracy trade-off involved in achieving good segmentation performance. SegNet was primarily motivated by scene understanding applications. Hence, it is designed to be efficient both in terms of memory and computational time during inference. It is also significantly smaller in the number of trainable parameters than other competing architectures and can be trained end-to-end using stochastic gradient descent. We also performed a controlled benchmark of SegNet and other architectures on both road scenes and SUN RGB-D indoor scene segmentation tasks. These quantitative assessments show that SegNet provides good performance with competitive inference time and most efficient inference memory-wise as compared to other architectures. We also provide a Caffe implementation of SegNet and a web demo at http://mi.eng.cam.ac.uk/projects/segnet.

Accuracy Assessment of an Industrial Actuator

A commercial linear actuator equipped with a 0.1 μm resolution encoder was used as a contact displacement sensor with adjustable force. The accuracy of the position reading of the actuator was evaluated from experimental data taking into account the uncertainty contributions. The tests consisted of length measurements of grade 0 steel gauge blocks. Measurements with different values of contact force were performed to assess its influence. A statistical analysis of the experimental data was performed to support the accuracy assessment. Systematic effects were identified and corrected. An expanded uncertainty (k=2) lower than 1 μm was estimated.

Development of a combined magnetic encoder

Purpose As a type of angular displacement sensor, the Hall-effect magnetic encoder incorporates many advantages. While compared with the photoelectric encoder, the magnetic encoder nevertheless has lower precision and lower resolution. So, the purpose of this essay is to find a way to increase the precision and resolution of the magnetic encoder. Design/methodology/approach By combining a single-pole magnetic encoder and a multi-pole AlNiCo magnet, the precision and resolution of this combined magnetic encoder are increased without increasing its volume or complicating its structure. A special algorithm system is developed to ensure faithful encoding and decoding. Findings Tests show that the combined magnetic encoder, with a diameter of 67.12 mm (including shaft) and thickness of 6.9 mm, has a precision of ±6′, compared with a 15-bit photoelectric encoder and a static resolution of ±0.6′. Originality/value This new kind of magnetic encoder could be used in specialized fields which need high-precision servo-control systems that are small, have ultra-low-speed and high-speed ratios and are non-oil-polluting or shock-resistant.

Weight-Transducerless Rollback Mitigation Adopting Enhanced MPC With Extended State Observer for Direct-Drive Elevators

The permanent magnet traction machine has become the tendency of modern gearless elevators, because of small volume, high efficiency, and good dynamics. However, the direct-drive elevator car will slide during the electromagnetic brake releasing at elevator startup, if the speed controller cannot generate a suitable electromagnetic torque to trace the change of the uncertain disturbance on traction sheave. To attenuate the car sliding of the gearless elevator installed with an ordinary resolution encoder, a weight-transducerless rollback mitigation strategy adopting an enhanced model predictive control with extended state observer (ESO) for speed loop is proposed. The ESO is used to overcome the mismatch of predictive model caused by the braking torque and the unknown car load. Both the stability and the parameter selection of the observer are analyzed. Additionally, a nonlinear tracking differentiator is used to process the raw speed signal generated by the conventional speed differential sampling instead of the low-pass filter, which can overcome the vibration of the elevator car when the brake releases. Finally, experimental results verify that the proposed control strategy can achieve shorter sliding distance and smaller sliding speed for the direct-drive elevator installed with an ordinary-resolution encoder.

Development of Precise Encoder Edge-Based State Estimation for Motors

We developed and implemented an algorithm named Precise Encoder Edge-based State Estimation for Motors (PEESEM) . Instead of conventional periodic encoder sensing to obtain the quantized motor position, we detect the encoder edge’s time and value precisely. Then, we use the edge-time Kalman filter (ETKF) containing predictions at edge time and periodic sampling time, and update at edge time. Only the first edge in each sampling interval is utilized to reduce the computation time at high motor speed. The proposed algorithm guarantees far more accurate state estimation with low encoder resolution and uncertainty on motor parameters. Performance of the proposed algorithm is validated through simulations and implementation on a two-wheeled mobile robot (TMR).

The physical upper bound on pose repeatability of general serial manipulators based on joint resolution

The accuracy of a serial robot to perform a recurring manipulation task is an important parameter to characterise its operational capability. In online programming, for example, one moves the end effector manually to all desired or necessary poses for a given task and stores the associated joint angles of the manipulator. The robot can then automatically perform this task by executing the sequence of joint angles and thus returning to the poses very accurately. This high repeatability of a typical industrial robot is due to the high resolution of the encoders that are used to measure the robot's joint angles. This finite resolution of the angular encoders imposes a natural limit on a robot's repeatability as the robot's control system operates on these discrete estimates of the angular positions. Even for typical modern industrial robots, that utilise high-resolution encoders, it is worthwhile to consider this issue in detail as one can obtain interesting information regarding the structural properties for a robot that are invariant regarding the resolution of its encoders. For this purpose, we provide a systematic method for computing the upper limit of the estimation error for position and orientation for a desired pose and posture of the robot. The knowledge about the end effector repeatability can be used for optimisation such as manipulator placement. Simulation studies that utilise a general 6R serial robot are used to illustrate this methodology and to analyse the diverse accuracy characteristics of its up to 16 different postures for a given task.

An Approach of Electronic Subdividing Method for Measuring Straightness and Displacement of a Precision Linear Stage Simultaneously

Optical encoders are commonly used in modern positioning systems. The accuracy and resolution of the optical encoders are always restricted by generated sinusoidal signals and the assembly technique. In this study, an electronic signal subdividing system is developed. This system is based on FPGA in combination with A/D and D/A converting circuits. Subdividing algorithm improves the segmenting signal amplitude method. Furthermore, we also construct a laser encoder for measuring straightness error and displacement of a linear stage simultaneously. The laser encoder consisting of the stainless steel bar and the sensor are developed for two-axis (X- and Z-axis) position measurement. The two dimensional sinusoidal array on the stainless steel bar are machined by ultrasonic elliptical vibration cutting system. The stainless steel bar has a three dimensional micro-structured surface, which is a superposition of periodic sinusoidal waves in the X- and Z-directions with spatial wavelengths of 350 µm and amplitudes of 0.5 µm. The laser-based two-axis position sensor is used to detect local slope profiles of the grid surface, and the displacement and straightness error could be extracted from the X- and Z-axis sensing signal. The sensing signal is processed by FPGA subdividing system. In addition, the proposed subdividing method is verified by the performances and effects of measuring results.

Robot-aided assessment of wrist proprioception.

Impaired proprioception severely affects the control of gross and fine motor function. However, clinical assessment of proprioceptive deficits and its impact on motor function has been difficult to elucidate. Recent advances in haptic robotic interfaces designed for sensorimotor rehabilitation enabled the use of such devices for the assessment of proprioceptive function. This study evaluated the feasibility of a wrist robot system to determine proprioceptive discrimination thresholds for two different DoFs of the wrist. Specifically, we sought to accomplish three aims: first, to establish data validity; second, to show that the system is sensitive to detect small differences in acuity; third, to establish test-retest reliability over repeated testing. Eleven healthy adult subjects experienced two passive wrist movements and had to verbally indicate which movement had the larger amplitude. Based on a subject's response data, a psychometric function was fitted and the wrist acuity threshold was established at the 75% correct response level. A subset of five subjects repeated the experimentation three times (T1, T2, and T3) to determine the test-retest reliability. Mean threshold for wrist flexion was 2.15°± 0.43° and 1.52°± 0.36° for abduction. Encoder resolutions were 0.0075°(flexion-extension) and 0.0032°(abduction-adduction). Motor resolutions were 0.2°(flexion-extension) and 0.3°(abduction-adduction). Reliability coefficients were r T2-T1 = 0.986 and r T3-T2 = 0.971. We currently lack established norm data on the proprioceptive acuity of the wrist to establish direct validity. However, the magnitude of our reported thresholds is physiological, plausible, and well in line with available threshold data obtained at the elbow joint. Moreover, system has high resolution and is sensitive enough to detect small differences in acuity. Finally, the system produces reliable data over repeated testing.