Spherical Actuator Research Articles

Development of Electric Flexible Spherical Actuator for Rehabilitation

Development of Electric Flexible Spherical Actuator for Rehabilitation

Position control of 3-DOF spherical actuator with cogging torque compensation

Position control of 3-DOF spherical actuator with cogging torque compensation

Spacecraft attitude maneuver control using two parallel mounted 3-DOF spherical actuators

A parallel configuration using two 3-degree-of-freedom (3-DOF) spherical electromagnetic momentum exchange actuators is investigated for large angle spacecraft attitude maneuvers. First, the full dynamic equations of motion for the spacecraft system are derived by the Newton-Euler method. To facilitate computation, virtual gimbal coordinate frames are established. Second, a nonlinear control law in terms of quaternions is developed via backstepping method. The proposed control law compensates the coupling torques arising from the spacecraft rotation, and is robust against the external disturbances. Then, the singularity problem is analyzed. To avoid singularities, a modified weighed Moore-Pseudo inverse velocity steering law based on null motion is proposed. The weighted matrices are carefully designed to switch the actuators and redistribute the control torques. The null motion is used to reorient the rotor away from the tilt angle saturation state. Finally, numerical simulations of rest-to-rest maneuvers are performed to validate the effectiveness of the proposed method.

An electrorheological spherical joint actuator for a haptic master with application to robot-assisted cutting surgery

This paper presents a spherical joint actuator using an electrorheological (ER) fluid to make a new haptic master which can be applicable to a robot-assisted cutting surgery. The proposed haptic master consists of two actuators for different motions; ER spherical joint device for 3-DOF rotational motion and ER linear device for 1-DOF translational motion. An appropriate size of the haptic master is designed based on the governing equations of the dynamic model and manufactured. The haptic master is then connected to a slave robot for the cutting surgery of a tissue. In the surgical process, the forces required to cut the tissue (a pork in this work) are obtained using a force sensor via experimets and used as the desired forces to be tracked by a slave robot controlled by a proportional-integrative-derivative (PID) controller. The desired repulsive forces are then embodied through the haptic master so that the operator can feel the forces occurred in cutting process (actual forces). In order to validate the effectiveness of the proposed haptic master, the tracking control performances between the desired force and actual force are evaluated in time domain.

3-DOF Outer Rotor Electromagnetic Spherical Actuator

Conventionally, many single-degree-of-freedom (single-DOF) actuators have been used to realize devices with multiple-degrees-of-freedom (multi-DOF). However, this makes their structures larger, heavier, and more complicated. In order to remove these drawbacks, the development of spherical actuators with multi-DOF is necessary. In this paper, we propose a new 3-DOF outer rotor electromagnetic spherical actuator with high torque density and wide rotation angles. The dynamic characteristics are computed employing 3-D FEM and its effectiveness is verified by carrying out measurements on a prototype. Then, in order to realize further high torque density, the electromagnetic pole arrangement is optimized using Genetic Algorithm (GA) and the effectiveness of the optimized stator poles arrangement is verified.

Magnetic field analysis of novel spherical actuators with three-dimensional pole arrays.

Spherical actuator is an electric device that can achieve multiple degree-of-freedom rotary motions in a single joint. Permanent magnet array is a key factor that influences the output performance of electromagnetic spherical actuators. In this paper, a novel three-dimensional (3D) pole array is proposed to improve the system flux density and thus the output performance. Analysis of magnetic field distribution is extremely important for spherical actuators with 3D magnet arrays. Thus, the investigation of magnetic field is conducted in analytical, numerical, and experimental ways. The general solution of magnetic scalar potential in 3D space is formulated analytically based on Laplace's equations and spherical harmonics, and then specific solutions of the magnetic scalar potential and magnetic flux density are obtained by using boundary conditions. Numerical computation is utilized to validate the analytical model and to facilitate the observation of the magnetic field variation. A research prototype and a testing platform of magnetic field have been developed for experimental study. The testing platform can move the probe to any position around the spherical actuator and measure the flux density automatically. Experiments are conducted to obtain the flux distribution. Both numerical and experimental results validate the analytical model well.

A Robust Adaptive Iterative Learning Control for Trajectory Tracking of Permanent-Magnet Spherical Actuator

This paper presents a robust adaptive iterative learning control (ILC) algorithm for 3-DOF permanent magnet (PM) spherical actuators to improve their trajectory tracking performance. The dynamic model of a PM spherical actuator is a multivariable nonlinear system with interaxis coupling terms. Uncertainties such as modeling errors, loads, and external disturbances exist in the model unavoidably, which will affect the performance, including the precision of the control system. Hence, to compensate for these uncertainties, a new hybrid control scheme that consists of a proportional–derivative (PD) feedback control with varying gains, a PD-type ILC with adjustable gains, and a robust term is developed. The new control law combines the advantages of simplicity and easy design of the PD control, the effectiveness of the ILC to handle model uncertainties and repetitive disturbances, and the robustness of the robust term to random disturbances. In addition, to expedite the convergence rate, the gains in the PD feedback control and the PD-type ILC are adaptively adjusted according to the iteration times. It is shown that the system tracking error approaches zero as the number of iterations increases. To demonstrate the proposed algorithm and verify the established theoretical results, both simulations and experiments are conducted. The results have shown that the proposed control algorithm can effectively compensate for various uncertainties and can thus improve the trajectory tracking performance of spherical actuators.

Research on the Relative Coordinate Transformation and Relative Position Detection System of 3-DOF Spherical Actuator

Abstract The attitude detection of 3-DOF actuator is an important technology of 3-DOF spherical actuator control. For the space attitude change of the stator and the traditional detection method is invalid, the relative coordinate transformation is proposed in this paper, through the analysis of the coordinate systems of the spherical actuator. The transformed relative coordinate data is proved by using MATLAB platform. The traditional coordinate transformation and the relative coordinate transformation are compared in this paper. Using the relative coordinate transformation algorithm, the relative position detection system based on the MEMS six-axis sensor is designed. Using two sensors to detect both the attitude of the stator and the rotor, the relative position detection system is realized. The data received from sensor transformed by this method is used to verify the feasibility.

Development of Hybrid Type Flexible Pneumatic Cylinder for Considering Less Air Consumption

Inexpensive rehabilitation devices that can be used at home are required because of a lack of PT and welfare workers. In the previous study, the low-cost portable rehabilitation device using a flexible spherical actuator that consists of flexible pneumatic cylinder was proposed and tested. However, a compact and high power compressor that supplies air pressure to pneumatic actuator has not been developed yet. In particular, the heat generated by compressing air prevents to miniaturize it. To realize a home rehabilitation, the small-sized compressors or less air consuming flexible actuators are required. In this study, a hybrid type flexible pneumatic cylinder driven by electric motors and air pressure is proposed and tested. The concept, the construction and the operating principle of the proposed actuator were described. The position control using the tested actuator is also carried out.

Magnetic field modeling based on geometrical equivalence principle for spherical actuator with cylindrical shaped magnet poles

Spherical actuator is a device that can achieve multiple degree-of-freedom (DOF) rotary motions in a single joint. It has a bright application prospect on moment gyros and satellite systems. Cylindrical shaped PM pole is widely used because of easy fabrication and pattern of magnetization. However, its topology and structure cause difficulties in accuracy magnetic modeling. Thus, the structure optimization and performance design become complicated. This paper proposes a novel magnetic field method for spherical actuator with cylindrical shaped PM poles based on geometrical equivalence principle. For convenient expression of magnetic field, three types of approximated dihedral cone shaped magnet poles are put forward based on different equivalent principles. On the basis of optimal equivalent principle, accurate harmonic model is set up with the magnetic field formulated. In addition, the analytical result is confirmed by numerical simulation from different dimensions. A research prototype with 8-PM-poles outer rotor and 24-coils stator is assembled and an experimental setup is established. The experimental results show experimental curves, analytical curves and FEM curves match with each other well which proves the accuracy of the proposed model. Thus, the indirect method of modeling magnetic field for spherical actuator with cylindrical shaped PM poles can be an effective tool. The magnetic field results can be the foundation for further study.

Development of Control Method for Outer‐Rotor Spherical Actuator

SUMMARYConventional multi‐degree‐of‐freedom drive systems need many single‐degree‐of‐freedom actuators, but this makes the structure larger and heavier. The development of a spherical actuator should reduce the number of actuators, but the low torque and narrow rotation range are still problems. We developed a 2‐degree‐of‐freedom spherical actuator with an outer rotor that can produce a higher torque and wider rotation angle. In this paper, we propose a control method; we confirmed its usefulness by performing dynamic analysis using the three‐dimensional finite element method and by taking measurements with a prototype.

Closed-loop magnetic bearing and angular velocity control of a reaction sphere actuator

This article presents the first closed-loop magnetic bearing and angular velocity experimental results of a reaction sphere actuator for satellite attitude control. The proposed reaction sphere is a permanent magnet spherical actuator whose rotor is supported by magnetic bearing and can be torqued electronically about any desired axis. The spherical actuator is composed of an 8-pole permanent magnet spherical rotor and of a 20-pole stator with electromagnets. The electromechanical model of the reaction sphere is summarized together with procedures to estimate the rotor magnetic state, the back-EMF voltage, and the rotor angular velocity, which are all fundamental ingredients for controller design. Dynamic controllers are developed to levitate the rotor inside the stator (magnetic bearing) and to control the rotor angular velocity. The magnetic bearing is based on a state-space controller with reduced-order displacement velocity estimator whereas the angular velocity controller is a simple proportional controller with a dedicated angular velocity estimator. The developed control algorithms are experimentally validated using the developed laboratory prototype showing the ability of simultaneously levitating the rotor while rotating it about a given arbitrary axis.

Equivalent energized coil model for magnetic field of permanent-magnet spherical actuators

Magnetic field modeling is extremely important for electromagnetic (EM) driven spherical actuators. This paper proposes a novel mathematic modeling method based on equivalent energized coil and Biot–Savart law to formulate the complex magnetic field distribution in three-dimensional (3D) space. The energized coil model is employed as an equivalent substitution of cylindrical permanent magnet (PM) poles. The magnetic field distribution of single energized coil is then formulated analytically. The complete magnetic field model of the actuator with multiple cylindrical PM poles is thus achieved from linear superpositions. Compared with other conventional approaches, as there are no omission of high order of harmonic terms, shape approximation of magnet poles and assumption of evenly distributed flux field, it helps to improve the modeling accuracy. Furthermore, this method is more generic for other flux field applications. It is available for both PM and EM poles, and theoretically could be implemented for other magnet shapes. The computational time may increase for complex magnet shapes and distribution patterns. The proposed method is applied to the spherical actuator with novel 3D magnetic pole array that helps to improve the actuator torque output. Numerical computation is conducted to validate the derived analytical magnetic field model. It shows that the analytical model fits with the result from finite element method (FEM) closely. A research prototype and an automatic experimental platform have been developed. Experiment is thus conducted to measure the magnetic field distribution of the spherical actuator. The data comparison shows that the analytical model matches the experimental measurement result well. The developed model can be employed for subsequent study of torque formulation and control implementation.

Development of a Novel Spherical Actuator With Two Degrees of Freedom

This paper describes the design and implementation of a novel spherical actuator that can generate two tilt-motion degrees of freedom. Voice coil motors were adopted as actuators in a spherical actuator to utilize their simple driving principle and constant torque coefficient characteristic. The sensing and guiding mechanism was built inside the proposed spherical actuator and enabled compactness and ease of connection to other application systems. The actuator was designed using a design optimization framework to obtain high torque. A prototype actuator was manufactured with optimally designed parameters, and its performance was evaluated.

Modeling and Analysis of a 3-DOF Spherical Momentum Exchange Actuator for Spacecraft Attitude Maneuver

AbstractThis paper describes a prototype and analytical studies of a three-degree of freedom (3-DOF) spherical momentum exchange actuator for three-axis attitude control. Its moving part, without any gimbal structures cascaded, can be directly driven by a stator electromagnetic field instead of servomotors, which sets it apart from conventional control moment gyros. First, the motion of equation and torque exchange model are developed. Analysis shows that the proposed actuator possesses a considerable torque amplification effect. Then a two-level torque-sharing strategy tailored for this device is presented to decouple the command torque into reaction flywheel/gimbal torque and further assign them to each of the stator coils. The physical limit of the rotor’s tilt range is investigated and an n-step incremental Euler eigenaxis rotation is resorted to as the solution. Numerical simulation results present a desirable dynamic response, revealing that the single device has the potential to provide limited thr...

Development and Control of Flexible Spherical Actuator for Portable Rehabilitation Device

This study aims at developing a potable rehabilitation device which can be safe to use during holding it. In our previous study, a novel flexible pneumatic cylinder that can be used even if it is deformed by external force has been developed. In this paper, a portable rehabilitation device using the flexible spherical actuator that consists of two ring-shaped flexible pneumatic cylinders is proposed and tested. The low-cost control system using four small- sized quasi-servo valves and an embedded controller is also developed. The spherical actuator is also improved so as to apply to the portable rehabilitation device. In addition, the attitude measuring system for attitude control of the device using a tiny embedded controller and two accelerometers is constructed and tested. The attitude control of the device using the measuring system is executed. As a result, the portable rehabilitation device that it is possible to give the rehabilitation motions to patients with attitude control can be realized. 

遺伝的アルゴリズムを用いた三自由度球面アクチュエータの磁極配置最適化

The development of multiple degree-of-freedom actuators is expected in robotics and industrial machinery fields. There remain, however, some problems in terms of the torque density, control method, and sensing systems. Under the circumstances, we are studying on a 3-degree-of-freedom outer rotor spherical actuator and its control method. We have also confirmed the characteristics employing 3D-FEM. In this paper, genetic algorithm is employed to search for the optimal stator pole arrangement which has the maximum torque constant (Instead, minimum mean current with a constant torque). Then, the effectiveness of the optimization is verified through the comparison of the mean current controlled to generate a constant torque between the optimized and the initial models.

Development of Flexible Displacement Measuring System Using Wire-type Linear Encoder for Flexible Spherical Actuator

This study aims at developing a potable rehabilitation device which can be safe to use while holding it with human hands. In the previous study, the novel flexible pneumatic cylinder that can be used even if it is deformed has been developed. The flexible spherical actuator using two ring-shaped flexible pneumatic cylinders was proposed and tested as a portable rehabilitation device. The attitude control system using a tiny embedded controller, four small-sized quasi-servo valves and two accelerometers was also proposed and constructed. In the next step, it is necessary to recognize the relative position between both stages to prevent both hands to contact each other. In this paper, a flexible pneumatic cylinder with wire-type linear encoder is proposed and tested. The compact sealing mechanism in the cylinder is also proposed. In addition, the measuring system using embedded controller is proposed and tested. As a result, it is confirmed that the tested sensor can measure the displacement of the cylinder.

Development of Portable Rehabilitation Device Using Flexible Spherical Actuator with Built-in Embedded Controller and Valves

Development of Portable Rehabilitation Device Using Flexible Spherical Actuator with Built-in Embedded Controller and Valves

Experimental Verification of Feedback Control of a 2-DOF Spherical Actuator

To drive conventional motor drive systems that have multiple degrees of freedom (DOF), such as industrial robot arms, the number of actuators employed has to be equal to the number of DOF. This causes the system to be large and heavy, and also have a lower overall efficiency. To solve these problems, the development of spherical actuators, which can rotate around multiple axes, is being conducted. In general, multi-DOF actuators have many poles and coils, and the currents of the coils have to be controlled individually, making the control system complicated. In this paper, we propose a feedback control method for a 2-DOF actuator that enables the currents of the coils to be easily determined. The effectiveness of the control method is verified through experimental verification.