Shaft Encoder Research Articles

A reduced-order dynamic model for end-effector position control of a flexible robot arm

The dynamic model of a robot arm composed of flexible beams and revolute joints is developed using a Rayleigh-Ritz based substructure synthesis technique and the linear theory of elastodynamics. Low-degree power functions in space variables of each substructure (beam) are adopted as shape functions for the purpose of discretization. Boundary conditions between substructures are then considered in the form of linear constraints of generalized (modal) coordinates and are enforced a posteriori, yielding the assembled dynamic system. This modified approach allows a systematic formulation which is independent of the problem characteristics and analyst's initiative, and allows a simpler reduced-order model with less degrees of freedom than those obtained by other discretization schemes, e.g. the finite element method. Using a pole placement technique and such a model for a robot arm with a revolute joint and a single flexible link carrying a payload, a state feedback controller is designed for the purpose of vibration suppression as well as trajectory tracking. The output signals consist of ''modal'' measurements provided by accelerometers and ''rigid'' angular velocity and position measurements provided by a tachometer and an optical shaft encoder on the motor shaft. Comparison of experimental and simulation results confirms the simplicity and validity of the proposed method.

Laser Velocimeter Measurements in the Pump of an Automotive Torque Converter: Part II—Unsteady Measurements

The unsteady velocity field found in the pump of an automotive torque converter was measured using laser velocimetry. Velocities in the inlet, mid-, and exit planes of the pump were investigated at two significantly different operating conditions: turbine/pump rotational speed ratios of 0.065 and 0.800. A data organization method was developed to visualize the three-dimensional, periodic unsteady velocity field in the rotating frame. For this method, the acquired data are assumed to be periodic at synchronous and blade interaction frequencies. Two shaft encoders were employed to obtain the instantaneous angular position of the torque converter pump and turbine at the instant of laser velocimeter data acquisition. By proper “registration” of the data, visualizing the transient interaction effects between the stator and the pump, and between the pump and the turbine, was possible. Results showed strong cyclic velocity fluctuations in the pump inlet plane as a function of the relative stator-pump position. Typical percent periodic fluctuations in the through flow velocity were 70 percent of the average throughflow velocity. The upstream propagation influence of the turbine on the pump exit plane flow field was seen to be smaller. Percent periodic fluctuations of the throughflow velocity were typically 30 percent. The effect of the stator and turbine on the midplane flow field was seen to be negligible. The incidence angle at the pump inlet fluctuated by 27 and 14 deg for the 0.065 and 0.800 speed ratios, respectively. Typical slip factors at the exit were 0.965 and fluctuated by less than 1 percent.

Laser Velocimeter Measurements in the Pump of an Automotive Torque Converter: Part I—Average Measurements

A torque converter was tested for two turbine/pump rotational speed ratios, 0.065 and 0.800, and a laser velocimeter was used to measure three components of velocity within the pump. Shaft encoders were used to record the instantaneous pump angular position, which was correlated with the velocities. Average flow velocity profiles were obtained for the pump inlet, mid-, and exit planes. Large separation regions were seen in the mid- and exit planes of the pump for a speed ratio of 0.800. Strong counterclockwise secondary flows were observed in the midplane and strong clockwise secondary flows were seen in the exit plane of the pump for all conditions; vorticities were evaluated and are reported. Velocity data were also used to find the torque distribution. For both speed ratios the torque was approximately evenly distributed between the inlet and exit. Finally, slip factors were evaluated at the mid-and exit planes. At the midplane they were approximately the same as for conventional centrifugal pumps; however, at the exit plane the slip factors are larger than for centrifugal pumps.

Inertia identification for the speed observer of the low speed control of induction machines

This paper presents a control method for induction machines in a low speed range with an instantaneous speed observer and inertia identification. When a low resolution incremental-type encoder is used for the speed detection, only the average speed in the interval of the encoder pulses is observed, and it usually makes the speed controller unstable at the very low speed range. This paper, therefore, proposes a low speed control method with a speed observer which is implemented by the disturbance observer using a low precision shaft encoder. Furthermore, to improve the performance of the speed controller, the authors perform the identification of the inertia which is estimated by the periodic test signal. They show that this proposed method is superior to the conventional method by comparing simulation and experimental results.

Experiments on orientation recovery and steering of an autonomous mobile robot using encoded magnetic compass disc

This paper describes the experiments on orientation recovery and steering of an autonomous mobile robot which uses an encoded magnetic compass disc as an orientation sensor. The mobile robot developed for experiments in this work is highly cost-effective and has two independent position-sensing systems. The one is an odometric sensor installed on the passive castor wheel, and the other consists of an optical incremental shaft encoder using magnetic compass disc. The experiments have been performed on two cases: (1) line path tracking test in a slippery environment and (2) orientation steering test in a circular path. Experimental results show that, while line path tracking in a slippery environment, the orientation of the mobile robot is recovered successfully with an acceptable position shift after two-time recovery action. For the circular path test case, the strayed angle after 90/spl deg/ orientation steering is only about 0.30/spl deg/. It is shown that the system designed is simple and applicable to mobile robot navigation.

Inertia moment identification in the average speed‐type instantaneous speed observer

AbstractIn recent servomotor control systems using highspeed microprocessors, it is difficult to obtain accurate speed information by using a low‐resolution shaft encoder because encoder pulses are very sparse in the low‐speed range. To overcome this problem, the instantaneous speed observer is proposed, system stability is improved, and strong disturbance rejection control is realized. However, when the inertia moment changes, the estimated instantaneous speed has error because the observer uses the nominal value of the inertia moment to calculate the instantaneous speed. For this reason, the identification method of the inertia moment for the “position input‐type” speed observer is proposed but it operated at about 50 rpm speed command. This paper proposes some novel identification methods for the “average speed‐input type” instantaneous speed observer and identifies the inertia moment in the very low speed range below 1 rpm.

Superharp — A wire scanner with absolute position readout for beam energy measurement at CEBAF

The CEBAF superharp is an upgraded beam wire scanner which provides absolute beam position readout using a shaft encoder. Superharps allow for high precision measurements of the beam's profile and position ( Δx ∼ 10 μm). The Hall C endstation at CEBAF will use three pairs of superharps to perform beam energy measurements with 10 −3 accuracy. The three pairs are installed at the beginning, the mid-point and the end of the Hall C arc beamline. Using superharps in conjunction with a dual sensor system: the direct current pick-up and the bremsstrahlung detectors, beam profile measurements can be obtained over a wide beam current range of 1 ∼ 200 μA.

Chaotic behavior of the Zeeman Catastrophe Machine

In this paper, a computer simulation will first demonstrate the catastrophe-theoretic features and rich dynamical behavior of the Zeeman Catastrophe Machine. Furthermore, the salient features of the simulated behavior will be demonstrated with the aid of a shaft encoder interfaced to a PC. The period-doubling route to chaos will be noted in both the simulation and experiment.

Optimised digital positional encoder patterns

Several patents for shaft encoders are briefly reviewed. A method is described by which two bits per track can be encoded in a manner which gives a complete Gray code output, thus offering the possibility of simpler and more compact encoders.

A microcontroller-based real-time speed measurement for motor drive systems

This paper describes the development of a digital real-time speed control system based on a single-chip microcontroller, Intel 80C196KC. The speed measurement is achieved with accuracy and high resolution over the entire speed range. Calibration shows that ± 0·04% measurement accuracy is obtained based on a 360 pulses/rev shaft encoder. The developed system is characterized using both simulation and practical tests. The system performance has been assessed using both laboratory and industrial test rigs. Typical results are given and discussed in this paper.

The new LLNL AMS sample changer

The Center for Accelerator Mass Spectrometry at LLNL has installed a new 64 position AMS sample changer on our spectrometer. This new sample changer has the capability of being controlled manually by an operator or automatically by the AMS data acquisition computer. Automatic control of the sample changer by the data acquisition system is a necessary step towards unattended AMS operation in our laboratory. The sample changer uses a fiber optic shaft encoder for rough rotational indexing of the sample wheel and a series of sequenced pneumatic cylinders for final mechanical indexing of the wheel and insertion and retraction of samples. Transit time from sample to sample varies from 4 to 19 s, depending on distance moved. Final sample location can be set to within 50 microns on the x and y axis and within 100 microns in the z axis. Changing sample wheels on the new sample changer is also easier and faster than was possible on our previous sample changer and does not require the use of any tools.

A current‐activated sediment trap

A current‐activated sediment trap is described that collects particles based on preselected current speed and direction criteria. The collector incorporates a steep‐walled funnel, a motor‐driven carousel with sample cups, an absolute optical shaft encoder, and an acoustic current meter. A field trial of the trap was carried out in a shallow marine embayment over 3 d where it correctly positioned cups in response to tidal changes in current speed and direction. Particle organic content and sedimentation rates calculated over hourly intervals were similar to measurements with a standard sequential sampling trap over several hours when the two traps were deployed simultaneously.

High-performance Control of a Servomotor using a Low Precision Shaft Encoder. Instantaneous Speed Observer and Identification of the Inertia Moment.

Due to the recent development of DSP (Digital Signal Processors), we can control a servomotor with a very short sampling period, e. g. 100 [μs]. The speed information is usually calculated from the increased pulse number of a shaft encoder within one sampling period. However, in this simple method, the speed resolution in lower speed range is easily lost. If we use a longer sampling period for maintaining the speed resolution still high, the system will be unstable. To overcome this problem, we proposed the instantaneous speed observer to estimate the instantaneous speed at every 100μs sampling point and greatly improved the system stability. Also, we realized the disturbance rejection control using the disturbance observer inherently included in the speed observer. However, this speed observer needed the exact value of the inertia moment. In this paper, we propose an identification method of the inertia moment by applying some adaptive methods. We implemented the basic least square method, the recursive least square method and the fixed trace method, and compared their identification performances. We realized the stable operation of the speed observer. Further, we designed STR (Self Tuning Regulator) type speed controller by feeding back the identified inertia moment for further performance improvement. The proposal in this paper is one suggestion to combine the robust and adaptive methods in servo motor control.

A method of mobile robot localisation by fusion of odometric and magnetometric data

This paper presents the dead reckoning localisation technique of the robot ROMANE (“Robot Mobile Autonome de Navigation en Exterieur”) which is conceived to move outdoors on level roads without traffic. Vehicle localisation with incremental shaft encoders on the wheels is improved by integration of a magnetic compass into the robot system. Fusion of odometric and magnetometric data by Kalman filtering increases the accuracy of the estimated robot heading. Consequently the uncertainty of the vehicle position is reduced. To evaluate our localisation method we present some experimental results of trajectory estimations.

An instructional robotics and machine vision laboratory

The US Naval Academy has assembled a laboratory consisting of 16 identical stations in support of several robotics and machine vision courses. Each station has an 80386-based microcomputer, a five degree-of-freedom robot arm, a video-rate vision system, and a speech synthesis system. The robot arms are capable of teach-pendant operation or control via the attached computer. Programs written in the robot control language can be downloaded into, stored in, and run from robot RAM. For greater flexibility, move instructions can be generated by the controlling computer (in any desired computer language) and transmitted to the arm for execution via an RS-232 link. Each joint of the robot (plus the gripper) has a dedicated microprocessor for closed-loop servo operation utilizing incremental drive-motor shaft encoders and micro-switches for "home" locations. The vision system includes an RS170-compatible video camera, a PC-compatible frame grabber board, and a video monitor. This configuration permits the utilization of 2D and 3D vision feedback in the robot control process. The laboratory is used in support of undergraduate courses that cover such topics as robot kinematics and task planning, elementary machine vision, and artificial neural networks.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Performance characteristics of a multiple-sample, cesium-sputter, negative ion source

A multiple-sample, cesium-sputter, negative-ion source which permits sample changes without disruption of on-line tandem electrostatic accelerator operations is described. The source is equipped with provisions for remotely selecting and moving into the beam position any one of 60 samples by means of stepping motors equipped with absolute shaft encoders. A spherical-sector cesium ionizer is used to produce the cesium beam for sputtering the sample material. The source is equipped with a three-element electrode system which has been designed to increase the perveance for cesium ion beam generation and to improve negative ion beam extraction from the source. Details of the mechanical design features and optics of the three-element electrode system are presented. Data pertinent to source operation and the dependence of negative ion yields on certain source operational parameters are also given.

LDV Measurements in a Centrifugal Slurry Pump: Water and Dilute Slurry Flows

A two-component LDV system was used to investigate single and two-phase flow in a representative section at the impeller outlet of a centrifugal slurry pump. Measurements were performed at Qn and 0.5 Qn with water and dilute slurry flows (C = 0.04 and 0.16 percent by volume). The solids consist of 0.8 mm glass beads. Particle Reynolds number is 65. The point liquid and solid velocities were determined with a Doppler signal amplitude discrimination approach. A rotary shaft encoder was used to represent the velocity as a function of the impeller angular position, and to investigate the effect of a finite number of blades on the flow. The data on liquid and slip velocity distributions show large scale flow structures which dominate the two-phase turbulent flow. Overall, solid particles have a larger radial velocity than the carrier fluid at the impeller outlet, but they lag the fluid in the circumferential direction.

A multiple-sample, cesium-sputter, negative ion source

A multiple-sample, cesium-sputter, negative-ion source which permits sample changes without disruption of on-line tandem electrostatic accelerator operations is described. The source is equipped with provisions for remotely selecting and moving into the beam position any one of 60 samples by means of stepping motors equipped with absolute shaft encoders. A spherical-sector cesium ionizer is used to produce the cesium beam for sputtering the sample material. The source is equipped with a three-element electrode system which has been designed to increase the perveance for cesium ion beam generation and to improve negative ion beam extraction from the source.

Instantaneous Speed Observer with Improved Disturbance Rejection Performance based on Higher Order Dynamics.

We propose the instantaneous speed observer for high performance control of servomotors which use inexpensive low precision shaft encoders. Due to the recent development of DSP (Digital Signal Processors), we have been able to control servomotors with a very short sampling period. We usually obtain the speed information from the increased pulse number of a shaft encoder in a sampling period. However, in this simple method, we lose the speed resolution easily. If we get accurate speed information by clocking the period of encoder pulse, the system will be easily unstable in the lower speed range because a long deadtime is inserted in the feedback loop. To overcome these problems, we propose the novel estimation technique of the instantaneous speed by utilizing the armature current together with the encoder information. As this observer inherently includes the disturbance observer. We can use it for the effective disturbance rejection control. We implement this system by using a DSP, and prove through laboratory experiments that it can greatly improve the system stability and achieves the excellent disturbance rejection performance.

Vector control of induction motor without shaft encoder

A method of introduction motor drive that requires no shaft encoder is presented. This system has both torque and speed controls that are performed by vector control. The vector-control scheme is based on a rotor-flux speed control, which is performed by torque-producing current and rotor flux, derived from the stator voltages and currents. If it is possible to obtain both precise torque-producing current and rotor flux, this system will have a good torque and speed performance because of the direct control of the torque and rotor flux. The characteristics of the vector control employing stator voltages and currents usually deteriorate as the speed gets lower because the calculated rotor flux depends on the stator residence, and it is difficult to calculate rotor flux at standstill. The rotor-flux estimator is improved to reduce the stator-resistance influence and to make it possible to calculate rotor flux at standstill.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>