Direct-drive Actuators Research Articles

Robust Adaptive Control of a Bimanual 3T1R Parallel Robot With Gray-Box-Model and Prescribed Performance Function

An elaborated bimanual 4-degrees-of-freedom (DOF) parallel robot is designed with large translational workspace, light moving mass and direct-drive actuation. The transient response greatly affects the throughput when using this robot for high-speed pick-and-place applications. However, this cannot be properly taken care by existing black-box model-based control schemes. Meanwhile, it is tedious to derive its dynamic model analytically and such a model is inapplicable for the real-time control. In this work, a gray-box-model-based control structure is proposed, with the retained inertial dynamics are directly derived by the principle of virtual work and the other parts are estimated by adaptive neural networks. This ensures calculation efficiency and integrity of the dynamics. Moreover, a prescribed performance function is constructed to ensure the specified tracking requirements both in transient and steady states. On the basis, the robust integral of signum of error term is incorporated to compensate the structural and unstructural uncertainties, which further improves the robustness during high-frequency motion. Comparative real-time experiments have been performed on the actual robot with attainment of the predefined performance and higher tracking accuracy.

Stall Torque Performance Analysis of a YASA Axial Flux Permanent Magnet Synchronous Machine

There is a trend to go towards low gear-ratio or even direct-drive actuators in novel robotic applications in which high-torque density electric motors are required. The Yokeless and Segmented Armature Axial Flux Permanent Magnet Synchronous Machine is therefore considered in this work. In these applications, the motors should be capable to deliver high torque at standstill for long periods of time. This can cause overheating of the motors due to a concentration of the losses in a single phase; hence, it becomes necessary to derate the motor torque. In this work the influence of the slot/pole combination, the addition of a thermal end-winding interconnection and the equivalent thermal conductivity of the winding body on the torque performance at standstill will be studied both experimentally via temperature measurements on a prototype stator, and via a calibrated 3D thermal Finite Element model. It was found that both a good choice of the slot/pole combination and the addition of a thermal end-winding interconnection have a significant influence on the torque performance at standstill, and allow up to 8% increase in torque at standstill in comparison to a reference design.

Improved Sliding Mode-Active Disturbance Rejection Control of Electromagnetic Linear Actuator for Direct-Drive System

The electromagnetic linear actuator is used as the core drive unit to achieve high precision and high response in the direct-drive actuation system. In order to improve the response performance and control accuracy of the linear drive unit, an improved sliding mode-active disturbance rejection control (ISM-ADRC) method was proposed. A motor model was established based on improved LuGre dynamic friction. The position loop adopts the improved integral traditional sliding mode control based on an extended state observer, and the current loop adopts PI control. The stability of the system is verified based on the Lyapunov theory. A nonlinear dilated state observer is used to effectively observe the electromagnetic linear actuator position and velocity information while estimating and compensating the internal and external uncertainty perturbations. At the same time, the saturation function sat(s) is used to replace the sign(s) and introduce the power function of the displacement error variable. The improved integral sliding mode control law further improves the response speed and control accuracy of the controller while reducing the jitter inherent in the conventional sliding mode. Simulation and experimental data show that the proposed improved sliding mode-active disturbance rejection control reduces the 8-mm step response time of the electromagnetic linear actuator by 21.9% and the steady-state error by less than 0.01 mm compared with the conventional sliding-mode control, while the system has 49.4% less adjustment time for abrupt load changes and is more robust to different loads and noise.

High-performance magneto-rheological clutches for direct-drive actuation: Design and development.

This paper presents an improved design, complete analysis, and prototype development of high torque-to-mass ratio Magneto-Rheological (MR) clutches. The proposed MR clutches are intended as the main actuation mechanism of a robotic manipulator with five degrees of freedom. Multiple steps to increase the toque-to-mass ratio of the clutch are evaluated and implemented in one design. First, we focus on the Hall sensors’ configuration. Our proposed MR clutches feature embedded Hall sensors for the indirect torque measurement. A new arrangement of the sensors with no effect on the magnetic reluctance of the clutch is presented. Second, we improve the magnetization of the MR clutch. We utilize a new hybrid design that features a combination of an electromagnetic coil and a permanent magnet for improved torque-to-mass ratio. Third, the gap size reduction in the hybrid MR clutch is introduced and the effect of such reduction on maximum torque and the dynamic range of MR clutch is investigated. Finally, the design for a pair of MR clutches with a shared magnetic core for antagonistic actuation of the robot joint is presented and experimentally validated. The details of each approach are discussed and the results of the finite element analysis are used to highlight the required engineering steps and to demonstrate the improvements achieved. Using the proposed design, several prototypes of the MR clutch with various torque capacities ranging from 15 to 200 N·m are developed, assembled, and tested. The experimental results demonstrate the performance of the proposed design and validate the accuracy of the analysis used for the development.

Force Fight and Its Elimination in a Prototype of a Redundant Direct-Drive Avionic Actuator

Nowadays, in aeronautical applications, rigorous reliability requirements are high constraints for embedded systems. Therefore, material redundancy is adopted to ensure safety whatever operation conditions may be. In the design of a direct-drive actuator for a specific application, redundancy is taken into account using a compound of four three-phase permanent magnet synchronous machines (PMSM). Each two electric machines are mounted in series sharing the same rotor with a common through shaft. The stators of each two electric machines are designed and manufactured as a six-phase machine. For each machine, power supply, sensors, and control loops are independent. In the characterization of the designed actuator and in the validation of its specific control in healthy operation mode and in faulty operation mode, taking into account the force fight phenomenon between the motor lanes, a specific test bench is developed. The aim of this paper is to highlight this phenomenon in faulty operation mode and to develop an easily implementable monitoring architecture, in order to eliminate it. Finally, with the proposed approach of monitoring, the force fight phenomenon is eliminated in faulty operation mode keeping the same performance as in healthy operation mode.

Design and Experimental Investigation of BLDC Motor for Aircraft Electromechanical Actuator

Among the electric motors used in the aircraft industry, brushless direct-current (BLDC) ones have the highest power density and efficiency. This paper presents the BLDC motor design and electromagnetic analysis for use in direct-drive aircraft electromechanical actuator. An experimental BLDC motor prototype has been made to test the output. Finite-element analysis (FEA) has been applied to finalize and investigate the characteristics of the BLDC motor design. Then, FEA output has been compared to experimental results in order to validate the latter. The high specific torque has been attained by the 12-slot, 8-pole configuration. Segmenting the rotor magnetic pole has reduced loss to eddy currents in permanent magnets, while double-layer concentric winding has reduced loss to copper and more effective dissipated heat. The BLDC motor presented herein has short response time and high rotor rotational angle precision thanks to the high torque to rotor moment-of-inertia ratio. For evaluating BLDC motor performance assessment, experiments have been run stand-alone, i.e. with no integration in a gearless EMA. Rotational speed vs torque, as well as the holding torque vs phase current at a given BLDC motor rotor position fit well the FEA output at a maximum difference of 10%. Post-test residual magnetization readings of the permanent magnets proved the BLDC motor operable.

Electromechanical Actuation for Morphing Winglets

As a key enabler for future aviation technology, the use of servo electromechanical actuation offers new opportunities to transition innovative structural concepts, such as biomimicry morphing structures, from basic research to new commercial aircraft applications. In this paper, the authors address actuator integration aspects of a wing shape-changing flight surface capable of adaptively enhancing aircraft aerodynamic performance and reducing critical wing structural loads. The research was collocated within the Clean Sky 2 Regional Aircraft Demonstration Platform (IADP) and aimed at developing an adaptive winglet concept for green regional aircraft. Finite Element-based tools were employed for the structural design of the adaptive device characterized by two independent movable tabs completely integrated with a linear direct-drive actuation. The structural design process was addressed in compliance with the airworthiness needs posed by the implementation of regional airplanes. Such a load control system requires very demanding actuation performance and sufficient operational reliability to operate on the applicable flight load envelope. These requirements were met by a very compact direct-drive actuator design in which the ball recirculation device was integrated within the screw shaft. Focus was also given to the power-off electric brake necessary to block the structure in a certain position and dynamically brake the moveable surface to follow a certain command position during operation. Both the winglet layout static and dynamic robustness were verified by means of linear stress computations at the most critical conditions and normal mode analyses, respectively, with and without including the integrated actuator system.

Overview of thrust ripple suppression technique for linear motors

Direct-drive actuators, linear motors are widely used in many industrial and military applications, particularly in high-end manufacturing due to advantages of high force density, rapid dynamic response, and low thermal losses. Permanent magnet linear synchronous motors (PMLSMs) can dramatically improve the dynamic and static performance of the motion system. However, as one of the most critical sources of error, the thrust ripple of linear motors can deteriorate performance and even excite the mechanical resonance. Thrust ripple suppression technology had received broad interest and has been researched extensively. Therefore, this paper summarizes different types of thrust ripple suppression methods and their principles are analyzed in detail. Firstly, structural optimization methods are introduced to suppress the thrust ripple and increase the precision of the thrust. Secondly, control methods are described to decrease the velocity fluctuation caused by the thrust ripple. Thirdly, a combination of structural design and control method is presented to compensate the ripple, meaning high order harmonic components are eliminated by permanent magnets (PM) skewing technology and low order harmonic are compensated by a linearization observer. Finally, conclusions are made regarding thrust ripple suppression technology and the future trend is proposed.

Design and Analysis of Linear Motor Driving Force Used by Direct-drive Actuator

Design and Analysis of Linear Motor Driving Force Used by Direct-drive Actuator

Detent Force Compensation for PMLSM Systems Based on Structural Design and Control Method Combination

As direct-drive actuators, permanent-magnet linear synchronous motors (PMLSMs) are widely used in high velocity and high precision applications. The detent force, however, can deteriorate the performance and even excite the mechanical resonance. This paper focuses on a novel detent force compensation scheme for PMLSM systems through a combination of structural design and control method. First, due to the bandwidth constraint of the control system, eliminating high frequency ripples is unfeasible; skewed permanent magnets (PMs) considering an optimal skewing length are designed to suppress high order harmonic components. Second, based on the model of PMLSM with skewed PMs, a linearization observer is derived and applied independently to the velocity controller for further diminishing low-order harmonic components. To facilitate implementation in the digital control system, a discretization method taking account of estimated errors is designed. Through the online calculation, the estimated detent force is injected to the control system in a feedforward way. To tune the proposed scheme properly, the convergence of the algorithm is analyzed by utilizing Lyapunov stability theory. Simulation studies are performed to prove the effectiveness of the proposed method, and experiments are provided to confirm the theoretical analysis and simulation results.

Computationally Efficient Thermal Analysis of a Low-Speed High-Thrust Linear Electric Actuator With a Three-Dimensional Thermal Network Approach

Permanent-magnet synchronous linear motors (PMSLMs) are more and more frequently used as all-electric direct-drive actuators in those applications where a force needs to be developed along a fixed direction. In this paper, an accurate 3-D thermal model of a PMSLM is derived through a lumped-parameter network approach which exploits all the symmetries in the actuator structure to maximize the spatial density of nodes. Numerically efficient techniques are then proposed to solve the thermal network analytically. Some experimental validations are finally presented based on the thermal testing of a laboratory prototype.

Design of Direct Drive Electro-Hydraulic Actuator Based on Internet of Things Technology

Direct drive electro-hydraulic actuator is a new type of hydraulic transmission with easy maintenance and high efficiency, which has been widely used in municipal engineering (such as water pipe network, gas pipe network), nuclear power plants, and other occasions. For these applications, we designed the electro-hydraulic valve actuator based on Internet of things technology (IOT for short). This paper introduces the mechanical structure and the working principle of the direct-drive electro-hydraulic valve actuator, analyzes IOT architecture. Detailed design was carried on according to the IOT architecture and related experiments have been done. The actuator can automatically, accurately control valve opening, be monitored by PC remotely, display device real-time information through LCD and be operated by touch screen.

Control of a Novel Linear-Rotary Actuator for High-Speed Pick-and-Place Application

A direct-drive electromagnetic actuator is designed with integration of linear and rotary modules in a common shaft. The two-degree-of-freedom motion is driven by two sets of single-phase coils enclosed by dual-magnet and rotary Halbach array accordingly, greatly enlarging the force/torque constants while minimised the moving mass. Moreover, with integral vacuum sensing and actuation, it is extremely suitable for high-speed pick-and-place (PnP) application. Thus, in the second part of this paper, a typical PnP process is designed with sequential motion of linear and rotary axes with s-curve command. It is also proved that the proportional-integralderivative (PID) controllers are capable to stabilise the closed-loop dynamics even with presence of various non-linearities and disturbances in both axes, such as friction, position dependent DC gain, gravity force and counter gravity spring. The experiment test verifies that this 2-DOF actuator is capable to PnP more than 8000 integrated-circuit chips per hour with simple decentralised PID plus feedforward control.

A fault-tolerant triple-redundant voice coil motor for direct drive valves: Design, optimization, and experiment

A direct drive actuator (DDA) with direct drive valves (DDVs) as the control device is an ideal solution for a flight actuation system. This paper presents a novel triple-redundant voice coil motor (TRVCM) used for redundant DDVs. The TRVCM features electrical/mechanical hybrid triple-redundancy by securing three stators along with three moving coils in the same frame. A permanent magnet (PM) Halbach array is employed in each redundant VCM to simplify the system structure. A back-to-back design between neighborly redundancies is adopted to decouple the magnetic flux linkage. The particle swarm optimization (PSO) method is implemented to optimize design parameters based on the analytical magnetic circuit model. The optimization objective function is defined as the acceleration capacity of the motor to achieve high dynamic performance. The optimal geometric parameters are verified with 3D magnetic field finite element analysis (FEA). A research prototype has been developed for experimental purpose. The experimental results of magnetic field density and force output show that the proposed TRVCM has great potential of applications in DDA systems.

Development of a Spherical Air Bearing Positioning System

In line with the general trend of the automation, spherical motion systems are increasingly required in applications such as robot joints, manipulators, orientation control systems, and angular alignment systems. In this paper, a spherical air bearing positioning system (SABS) aiming at providing highly precise rotational motions in two degrees of freedom is designed. The SABS mainly consists of direct-drive voice coil actuators and pneumatic bearing. This paper presents the mechanical structure and control system of the SABS. The model of the SABS is identified based on adaptive control concepts. To eliminate the measurement noise, a noise filter is designed on the basis of the model. Then, an observer-based PID controller is designed and implemented on a dSPACE development card. The experiment results show that the designed controller can achieve higher precision and better tracking performance of about ten times compared to that from a traditional PID controller.

Force Ripple Compensation in Linear Motors Based on Closed-Loop Position-Dependent Identification

Permanent-magnet linear synchronous motors can serve as direct-drive actuators for machine tools and production machines, whenever high dynamic performance are required. Since compliance in transmission elements is eliminated, higher control bandwidths can be achieved, when compared to traditional solutions based on conversion from rotational to linear motion. Like their rotational counterparts, these motors are, however, affected by a ripple, in the form of parasitic and undesired force pulsations. In this paper, a compensation scheme of force ripple is proposed, based on a simple and compact model of the disturbance. The compensation requires a preliminary identification stage that simply consists of a couple of controlled motions at constant and very low velocity. The compensation term is then applied in parallel to the position/velocity controller, as a function of the linear position of the motor. A robustness analysis is performed based on a convenient formulation of the closed-loop system, as well as on a Lyapunov argument. Experiments that demonstrate the effectiveness of the method are reported, even when compared to a traditional force disturbance observer.

Design of Electrical/Mechanical Hybrid 4-Redundancy Brushless DC Torque Motor

This article presents a novel electrical/mechanical hybrid 4-redundancy brushless DC torque motor (BLDCM), which has found applications in direct drive actuators (DDA) of aerial vehicles. This motor is characteristic of electrical/mechanical hybrid 4-redundancy by securing two stators along with two rotors on the same shaft. Each stator contains two sets of windings embedded in separated slots. Furthermore, compared to the scheme of two electrical dual-redundancy BLDCMs linked by a mechanical transmission, the proposed motor has lower volume and lighter weight, eliminates the nonlinearity caused by the gap of the mechanical transmission, moderates the requirement for the torque balance between redundancies and reduces the cogging torque by shifting the magnets. Both magnetic circuit calculation and the finite element analysis (FEA) of the electromagnetic field are conducted to optimize the design process. A prototype motor has been produced and tested. The results indicate that its performances comply with the requirements presented by designers. Moreover, the position frequency response of the prototype in the DDA's unloaded situation has also demonstrated the fulfillment of desired dynamic characteristics.

A Gas-Actuated Anthropomorphic Prosthesis for Transhumeral Amputees

This paper presents the design of a gas-actuated anthropomorphic arm prosthesis with 21 degrees of freedom and nine independent actuators. The prosthesis utilizes the monopropellant hydrogen peroxide as a gas generator to power nine pneumatic type actuators. Of the nine independent actuators, one provides direct- drive actuation of the elbow, three provide direct-drive actuation for the wrist, and the remaining five actuate an underactuated 17 degree of freedom hand. This paper describes the design of the prosthesis, including the design of small-scale high-performance servovalves, which enable the implementation of the monopropellant concept in a transhumeral prosthesis. Experimental results are given characterizing both the servovalve performance and the force and/or motion control of various joints under closed-loop control.

Development of a high-performance force display for material science

It is important to know the inter-atomic bonding state in material for a new material design and analysis. Although the bonding states can be simulated based on the inter-atomic potential, it is difficult to understand them immediately in three-dimensional space. To understand the inter-atomic bonding state intuitively, it is effective to use force display systems. A high-performance force display system for material science is then developed. The force display consists of a parallel mechanism with direct-drive linear servo actuators. The mechanism is called a fixed linear type parallel mechanism. Since the actuators are fixed on the base, high-speed motion of the manipulator is possible, and the mechanism has a high load capacity and back drivability. These characteristics lead to a high-performance force feedback control. In the current paper, the design and prototyping are described. In addition, it is applied for the force display experiment of inter-atomic bonding state, and the effectiveness is demonstrated.

Passivity-based control for flux regulation in a variable reluctance finger gripper

A flux regulation method with passivity-based control (PBC) for a variable reluctance (VR) finger gripper is presented. The performance and stability of the proposed control scheme is verified through mathematical derivations and computer simulation. These verifications indicate that the direct-drive VR finger gripper control has robustness despite certain parameters variations and modelling uncertainties. A novel two-finger gripper based on VR driving principle is also introduced. This kind of gripper has certain advantages over the conventional permanent magnet voice coil gripper, the most notable advantages are the ease of construction, high robustness and very low cost. The work described is the first of its kind in carrying out passivity based control for a direct-drive VR actuator. In the study, the flux and torque behaviour of a mutually-coupled VR gripper was investigated, and its state dynamic equations were derived. Then its flux and torque models were constructed. A similar two-finger VR gripper was fabricated and its characteristics were measured. The PBC controller is designed to regulate the flux linking through the magnetic circuit of the VR finger gripper. Computer simulation and hardware implementation were carried out. The two sets of results compare favourably with each other. Both sets of result show that the system is highly robust and has excellent trajectory tracking performance.