Radial Position Control Research Articles

Design of suspension control system for bearingless induction motor using fuzzy-I controller

In the bearingless induction motor (BIM), the rotor of BIM is supported by the magnetic force, the radial position control technique is studied in this paper, an approximation and simplification are used to create a linearized model of suspension system. The objective of this paper is to design a control system that overcomes the limitations of the traditional controller and to improve the control performance and achieve high stability, even if it is exposed to external disturbance and internal disturbance (changes in load or speed). In order to solve this problem, the proposed controller is based on the fuzzy-I controller that combines the positive features of both fuzzy logic and the integration. In comparison with the conventional PID, the results show that the fuzzy-I controller reduces the peak-to-peak rotor deviation by 23% under effect of external disturbance force and 31% under effect of speed variation and 9% under effect of load variation.

Application of Active Disturbance Rejection in a Bearingless Machine with Split-Winding

In this paper it is proposed the displacement control of a bearingless induction machine (BIM) with split winding and optimized drive structure using Active Disturbance Rejection Control (ADRC). Considering that the BIM is a multivariable, nonlinear, and time-varying system with coupled variables, advanced control techniques can be useful in order to make the system operate efficiently and with good dynamic performance. The ADRC considers the total disturbance, composed of unmodeled dynamics, nonlinearities, uncertainties, and load variations, as an extended state and estimates it in real-time through a state observer. This increases the overall robustness of the control system to disturbances of different natures. The application of the ADRC technique on the radial position control of the BIM used in this work showed that a Linear version of ADRC is not able to compensate for radial load disturbances but this drawback can be solved by the use of a nonlinear observer in the ADRC structure. Besides that, both control versions of the ADRC were able to make stable the naturally unstable radial displacement of the machine’s rotor.

Magnetic suspension performance and energy consumption characteristics of a maglev pediatric VAD using an ultra-compact axial gap type self-bearing motor with different number of actively controlled axes

An ultra-compact ventricular assist device with an axial gap type self-bearing motor has been developed for use in pediatric circulatory support. The developed self-bearing motor has double stator structure to actively control five degrees of freedom (DOF) of levitated impeller postures and to enhance motor torque capacity with small device size which is 22 mm in outer diameter and 33.6 mm in total height, respectively. This study investigated magnetic suspension stability and energy consumption characteristics during pump operation with 3-DOF (z, θx, θy) control and 5-DOF (x, y, z, θx, θy) control. The axial position and inclination angles were sufficiently stabilized independently with the radial position control. The radial position control can be effective to suppress the radial whirling oscillation at resonance, however power consumption increased by up to 0.4-0.9 W compared to in the case of the motor utilizing 3-DOF control.

A Novel Driving Scheme for Three-Phase Bearingless Induction Machine with Split Winding

In order to reduce the costs of implementing the radial position control system of a three-phase bearingless machine with split winding, this article proposes a driving method that uses only two phases of the system instead of the three-phase traditional one. It reduces from six to four the number of inverter legs, drivers, sensors, and current controllers necessary to drive and control the system. To justify the proposal, this new power and control configuration was applied to a 250 W machine controlled by a digital signal processor (DSP). The results obtained demonstrated that it is possible to carry out the radial position control through two phases, without loss of performance in relation to the conventional three-phase drive and control system.

Plasma flux expansion control on the DIII-D tokamak

A new controller has been developed with help of the flexible divertor poloidal-field coil set of the DIII-D tokamak, to aid in the precise control of the flux expansion in the scrape-off layer. The single-input multiple-output architecture ensures flexibility through a complementary set of orthogonal actuator direction to guarantee minimum effect on existing controlled variables, e.g. radial and vertical position control of the X-point. A non-linear free-boundary simulation code (GSevolve) is used for simulating the closed-loop response and for verifying the implementation of the control algorithm on the DIII-D plasma control system. First results of the experimental commissioning of the new controller during 2020 DIII-D campaign are also presented.

Radial position control for magnetically suspended high‐speed flywheel energy storage system with inverse system method and extended 2‐DOF PID controller

To achieve high-precision position control for the active magnetic bearing high-speed flywheel rotor system (AMB-HFRS), a novel control strategy based on inverse system method and extended two-degree-of-freedom (2-DOF) proportional–integral–derivative (PID) controller is proposed in this study. First, the inverse system method is employed to decouple the AMB flywheel rotor system with strong non-linear and coupling, into four independent subsystems. Subsequently, extended 2-DOF PID controllers are used to regulate the decoupled subsystems, to obtain good performances of disturbance rejection and tracking simultaneously. In the extended 2-DOF PID controller, a differential signal is produced by a velocity observer to improve the ability of against noise. Finally, the characteristics of stability, tracking, disturbance rejection and robustness of the control strategy proposed are analysed in theory, and its ability and effectiveness to control the radial position of the AMB-HFRS are further studied by simulations and experiments. It is shown that the control strategy proposed can keep the AMB-HFRS suspending stably from static state to the speed of 24,000 rpm, has the advantages of good tracking, high disturbance rejection, strong robustness and good ability of against noise.

Multi-machine analysis of termination scenarios with comparison to simulations of controlled shutdown of ITER discharges

To improve our understanding of the dynamics and control of ITER terminations, a study has been carried out on data from existing tokamaks. The aim of this joint analysis is to compare the assumptions for ITER terminations with the present experience basis. The study examined the parameter ranges in which present day devices operated during their terminations, as well as the dynamics of these parameters. The analysis of a database, built using a selected set of experimental termination cases, showed that, the H-mode density decays slower than the plasma current ramp-down. The consequential increase in fGW limits the duration of the H-mode phase or result in disruptions. The lower temperatures after the drop out of H-mode will allow the plasma internal inductance to increase. But vertical stability control remains manageable in ITER at high internal inductance when accompanied by a strong elongation reduction. This will result in ITER terminations remaining longer at low q (q95 ~ 3) than most present-day devices during the current ramp-down. A fast power ramp-down leads to a larger change in βp at the H–L transition, but the experimental data showed that these are manageable for the ITER radial position control. The analysis of JET data shows that radiation and impurity levels significantly alter the H–L transition dynamics. Self-consistent calculations of the impurity content and resulting radiation should be taken into account when modelling ITER termination scenarios. The results from this analysis can be used to better prescribe the inputs for the detailed modelling and preparation of ITER termination scenarios.

Study on Consequent-Pole Type 5-DOF Self-Bearing Motor

Magnetic bearings can support rotating shafts without any contact and have been used in special conditions such as in cryogenic, vacuum, and clean environments. In this paper, consequent-pole type 5-DOF self-bearing motor is proposed. By using this motor structure, the rotor radial and tilt position control is easily possible because the forces and torques don’t depend on the rotational angle of the rotor. The three-dimensional magnetic field analysis using the finite element method is carried out. Based on these analysis results, the experimental setup was made and the motor performances are confirmed. Though the further improvement of the motor efficiency is necessary, the consequent-pole type 5-DOF self-bearing motor satisfies the target motor torque and shows the feasibility as an artificial heart pump.

Radial position control of a magnetically suspended rotor system in a direct-driven spindle using inverse system scheme

Direct-driven spindles have no mechanical transmission trains and gears, and are the key actuators for computerized numerical control machine tools. These magnetically suspended rotor systems are required to provide fast response and high precision. However, these systems are non-linear and strongly coupled. The traditional proportional, integral, derivative (PID) control method has been widely used for such systems owing to its relative simple realization. However, the tracking, disturbance rejection and robustness properties of the controlled plant may not be satisfied. To solve these problems, this paper presents a decoupling control strategy based on an inverse system scheme and combines it with the internal model control method to guarantee system robustness to the parameter uncertainty and external disturbance. By introducing an inversion of the magnetically suspended rotor system into the original system, a new pseudolinear system is developed. It can be shown that this addition effectively eliminates the influence of the unmodelled dynamics, and improve the accuracy and robustness of the whole system. The simulation and experimental results show that when compared with the traditional control scheme, the proposed control scheme provides good decoupling and robustness performance for the magnetically suspended rotor system in different operating conditions.

Rotor Radial Position Control and its Effect on the Total Efficiency of a Bearingless Induction Motor With a Cage Rotor

This paper presents a control method for the rotor radial position control of a two-pole 30 kW 3000 r/m cage induction machine that has an additional four-pole stator winding to produce the radial force on the rotor. The controller is used to stabilize the machine, to remove vibration caused by mass-unbalance and rotation, and to produce static force to counter gravity. Both measurement results and finite element computations are shown. In the measurements, the rotor is on a long flexible shaft supported by external bearings. The proposed control strategy allows the machine to run at all operation points (speed, torque) up to the rated values, including at the first critical speed caused by the first natural bending frequency of the rotor shaft. The power consumption of the radial position control and the total effect on efficiency is analyzed. Both rotor eccentricity and the produced four-pole field affect the losses of the machine. The additional power consumption and the total efficiency are presented as a function of the radial force produced.

Advanced Radial Position Control of a Recording Beam for Super Multilayer Disc with Separated Guide Layer

We have developed an advanced radial position control system for a super multilayer disc with a separated guide layer to realize stable recording on the plane recording layers located far from the guide layer. The developed system consists of an advanced sled servo system and a specially designed optics based on the advanced push–pull method. Recorded signals on all recording layers were evaluated by using integrated-maximum likelihood sequence error estimation (i-MLSE) as the evaluation indicator. As a result, we confirmed sufficient recorded signal qualities and feasibility at the quad-speed continuous recording with a constant track pitch of 0.32 µm, the same as that of Blu-ray disc (BD). We believe that our proposed concept is the most practical solution for the next-generation optical disc system and our control system is essential to realize stable multilayer recording with a constant track pitch.

A Radial Position Control Method of Bearingless Motor Based on d - Q -Axis Current Control

This paper presents a radial position control method of bearingless motor (BELM) based on a d-q-axis current control and an integrated winding arrangement. The BELM performs the functions of a motor as well as magnetic bearing. Rotor is suspended with noncontact by suspension force. In a conventional BELM, there are two kinds of stator windings such as motor and suspension windings. The motor winding outputs the rotational torque, and the suspension winding outputs the suspension force. To simplify the structure of conventional BELM, integration of two windings is proposed in this paper. By integrating the windings, the structure of BELM becomes simple, and also, the torque and force productions are increased. The d-axis stator current controls the suspension force, and the q-axis current controls the developed torque of the motor. In the case when there is less number of slots in the machine, the actual suspension force cannot follow the command. In order to make the actual suspension force follow the command, a compensation method is also proposed in this paper. A simulation study is done for the proposed radial position control based integrated winding BELM using finite-element analysis software. Furthermore, a prototype machine of the proposed BELM based on d-q-axis current control is built to confirm the performance of the motor and the magnetic suspension in real time. Rotor stabilization with magnetic levitation is confirmed using the prototype machine.

Linearized models for a new magnetic control in MAST

The aim of this work is to provide reliable linearized models for the design and assessment of a new magnetic control system for MAST (Mega Ampère Spherical Tokamak) using rtEFIT, which can easily be exported to MAST Upgrade. Linearized models for magnetic control have been obtained using the 2D axisymmetric finite element code CREATE L. MAST linearized models include equivalent 2D axisymmetric schematization of poloidal field (PF) coils, vacuum vessel, and other conducting structures. A plasmaless and a double null configuration have been chosen as benchmark cases for the comparison with experimental data and EFIT reconstructions. Good agreement has been found with the EFIT flux map and the experimental signals coming from magnetic probes with only few mismatches probably due to broken sensors.A suite of procedures (equipped with a user friendly interface to be run even remotely) to provide linearized models for magnetic control is now available on the MAST linux machines.A new current driven model has been used to obtain a state space model having the PF coil currents as inputs. Dynamic simulations of experimental data have been carried out using linearized models, including modelling of the effects of the passive structures, showing a fair agreement. The modelling activity has been useful also to reproduce accurately the interaction between plasma current and radial position control loops.

FPGA based Fuzzy Logic Controller for plasma position control in ADITYA Tokamak

Tokamaks are the most promising devices for obtaining nuclear fusion energy from high-temperature, ionized gas termed as Plasma. The successful operation of tokamak depends on its ability to confine plasma at the geometric center of vacuum vessel with sufficient stability. The quality of plasma discharge in ADITYA Tokamak is strongly related to the radial position of the plasma column in the vacuum vessel. If the plasma column approaches too near to the wall of vacuum vessel, it leads to minor or complete disruption of plasma. Hence the control of plasma position throughout the entire plasma discharge duration is a fundamental requirement. This paper describes Fuzzy Logic Controller (FLC) which is designed for radial plasma position control. This controller is tested and evaluated on the ADITYA RZIP control model. The performance of this FLC was compared with that of Proportional–Integral–Derivative (PID) Controller and the response was found to be faster and smoother. FLC was implemented on a Field Programmable Gate Array (FPGA) chip with the use of a Very High-Speed Integrated-Circuits Hardware Description-Language (VHDL).

Solenoid-free startup experiments in DIII-D

A series of DIII-D experiments was performed to investigate the potential for initiating plasma current using only poloidal field coils located outside the DIII-D central solenoid, i.e. ‘solenoid-free’. Plasma current to 166 kA was achieved using 2–3 MW of electron cyclotron (EC) heating and was limited by coil and power supply constraints. Flux conversion to plasma current was similar to standard DIII-D startup with some degradation at higher plasma current associated with stray fields and vertical stability issues. In preliminary solenoid-free experiments, neutral beam (NB) current drive (CD) levels were small and attributed to reduced CD efficiency associated with low electron temperature produced by the low current, low confinement plasma. Lack of plasma radial position control also contributed to a reduction of NBCD. Similarly, ECCD was small owing to low plasma temperature and outside EC launch which is required in the solenoid-free scenario. Synergistic experiments were carried out using standard solenoid initiated plasmas in order to study noninductive CD in limited, Lmode plasmas, typical of that generated by solenoid-free startup. While substantial noninductive current can be driven, self-sustaining levels of noninductive current have not yet been achieved with our present six-source co-injection NB system combined with EC and fast wave systems. At low plasma current and high levels of localized EC heating, substantial MHD is generated and this was seen to severely limit plasma performance. Although further optimization is possible in the limited plasma regime, full noninductive, steady-state operation may require diverted plasma with H-mode quality confinement. Discharges obtained during the solenoid-free campaign are compared with results of previous DIII-D campaigns aimed at achieving a steady state, noninductive CD solution.

Remediation of time-delay effects in tokamak axisymmetric control loops by optimal tuning and robust predictor augmentation

Abstract It is sometimes incorrectly assumed that, because superconducting tokamaks already have significant intrinsic or imposed sources of control delay, introducing extra delays/lags into the axisymmetric control loops will have negligible detrimental impact on the plasma control. This study exposes and quantifies the detrimental effects imposed by time delays/lags in the control loop in superconducting tokamaks, using as an example the plasma current control and radial position control in a vertically stable circular plasma in the KSTAR tokamak. Delays and lags in the power supplies, data acquisition, and vessel structure are taken into account. Optimal tuning of PID controllers in combination with an ohmic-flux control strategy is proposed as a possible method for remediating the negative effects of time delays/lags. In addition, an augmentation of the control loop by the introduction of a robust predictor has been proposed to improve the performance of the time-delayed closed-loop system when the amount of delay/lag in the loop is unknown. The Nyquist dual locus technique based on the Argument Principle in complex theory is employed to assess stability of the optimally tuned closed-loop system in the presence of time delays.

Nonlinear Feedback Control of a Bearingless Brushless DC Motor

The demands on bearingless drive configurations concerning performance as well as costs are high. The proposed bearingless brushless DC motor consists of five concentrated coils in a symmetrical arrangement, which generate radial forces and motor torque simultaneously in interaction with a permanent-magnet-excited disk-shaped rotor. Additionally, tilting deflection and the axial position of the rotor are stabilized passively by means of magnetic reluctance forces. Thus, system costs can be reduced significantly compared to a conventional bearingless motor setup, which actively stabilizes all 6 DOF. Due to the nonlinearity of the plant, the use of linear control design methods alone is not suitable for achieving a high operation performance. This paper introduces a novel radial position and motor torque control algorithm for a bearingless brushless DC motor based on the theory of feedback linearization. Thereby, the combined model of translatory and rotatory dynamics can be split into independent linear systems by means of a nonlinear change of system coordinates and a static-state feedback. Experimental results demonstrate the effectiveness of the proposed approach.

Direct Control of Radial Displacement for Bearingless Permanent-Magnet-Type Synchronous Motors

In recent years, research has been focused on bearingless motors to suspend the rotor stably by controlling rotor radial displacement. Without studying the relationship between the radial suspension force and radial displacement, the traditional control method for rotor radial displacement has some drawbacks such as the complexity of computation and time-consuming tuning. In order to overcome these drawbacks, a novel approach to control the rotor radial displacement was proposed based on the relationship between the radial displacement and radial suspension force. The rotor flux orientation is adopted to decouple the electromagnetic torque and radial suspension force. This approach directly controls the rotor radial displacement of bearingless permanent-magnet-type synchronous motors. In addition, a control system was designed by applying this approach. The simulation and experimental results show that the proposed control strategy is effective in realizing stable operation in rotor radial position control.

Nanopositioning and Nonlinearity Compensation for Step Imprint Lithography Tool

In this paper, the motion mode and nanopositioning accuracy in the step imprinting lithography process are presented, and the positioning errors different from the traditional errors, such as the gap error existing in the hinges of the stage structure and the random error produced during the process of the stage position adjustment, are analyzed. To avoid and eliminate these nonlinearity errors, radial basis function-proportional integral derivative and position control algorithms are introduced into the macro- and microdriving processes, respectively. The innovation of this driving method is that the motion locus is monotone, nonoscillatory, and a multistep approaching target, which eliminates the root of the random error by single direction driving mode and avoids the backlash error through preloading function. Driving experiments of different motion ranges prove that this nonlinearity compensation is very effective and the positioning accuracy during the step imprinting process can be improved up to 10-nm.

Magnetic control of plasma current, position, and shape in Tokamaks: a survey or modeling and control approaches

Plasma current, position and shape control systems and modeling approaches were reviewed. The literature was divided into three categories, namely plasma radial position and current control, vertical stabilization of elongated plasmas, and plasma shape control. The plasma current and radial position control problem can be easily solved using a filament plasma model - the Shafarov lumped parameter equation and two separate PID controllers. At most, a MIMO controller can be used if decoupling is desired. Stabilization of the plasma vertical motion requires new control strategies in which the control algorithm is changed on the basis of an estimate of the measurement accuracy. For controlling the shape during the plasma formation and start-up phase, controllers based on magnetic flux control are useful when control requirements are not stringent.