Electro-hydrostatic Actuator Research Articles

Design, Control and Evaluation of the Electro-Hydrostatic Actuator, PREHydrA, for Gait Restoration Exoskeleton Technology

A preliminary design for the PREHydrA (passive return, electro-hydrostatic actuator) concept; a high force density, remote actuator, is tested to requirements for wearable robotics technology intended for gait restoration; one of the most demanding tasks for wearable robotics. While the concept offers good wearability properties, it has never been used in wearable robotics for full support. This work shows that the combination of electro-hydrostatic actuators with a return force and series elastic element offers a good alternative to other actuation types. Custom and small commercial components are used in a design for the knee joint. An experimental setup with a pendulum representing a swinging lower leg was used to show force and angle tracking performance. The results of a maximum zero force (-400-1100 N actuator force range) tracking mean absolute error of 61 N (6.79 Nm joint torque error) at 5.5 Hz excitation and a full swing (70°) within 0.35 s (0.8 m/s actuator velocity), match or exceed current state of the art exoskeleton actuation and control and show that the PREHydrA concept is very well suited for application in exoskeleton technology, especially when the cylinder design is optimized.

Research on Fault Diagnosis Method of Electro-Hydrostatic Actuator

Electrohydrostatic actuator is a type of actuator that uses hydraulic energy as the energy transmission carrier, which has the advantages of small size and high power. Since it is commonly used in harsh conditions such as strong vibration, high pressure, and heavy loads, condition monitoring and fault diagnosis of its hydraulic system are particularly important. This paper proposed a novel fault feature extraction method and applied to fault diagnosis of electrohydrostatic actuator. Firstly, the pressure signal of the hydraulic system is decomposed at multiple scales to obtain the center frequency of its maximum energy intrinsic mode component, and the feature data set is constructed based on the statistical features of the time domain. Then, a fault identification model of hydraulic system based on support vector machine is established. Finally, the fault classification and identification results of the hydraulic system are outputted. After a variety of method comparisons, the method proposed in this paper has a fault time ratio accuracy of 96.7%, which provides a basis and a new way for the fault diagnosis of the hydraulic system.

ПЕРСПЕКТИВЫ РАЗВИТИЯ ПРИВОДОВ РУЛЕВЫХ ПОВЕРХНОСТЕЙ ГРАЖДАНСКОГО САМОЛЕТА

The main problems arising in the development of systems of actuators with remote control for promising aircraft are considered. The tasks of the fly-by-wire control system for actuators of the aircraft are determined. The directions of development of improving the mass-dimensional and dynamic characteristics, improving the design and layout of circuit solutions, control systems and improving operational characteristics, which provide an increase in the quality and safety of the aircraft operation, are given. A concept for the development of actuators for the control surfaces of a passenger aircraft has been formulated, including the main directions of scientific and experimental work carried out by technical design companies, scientific organizations and manufacturers of hydraulic units both in Russia and abroad. The improvement of weight and size characteristics is possible due to the energy perfection of actuators and ensuring synchronization of their operation, the use of autonomous actuator circuits, and pumping stations of variable capacity. The issues of improving the dynamic characteristics of actuators involve the problems of increasing speed, accuracy, stability, and controllability. When considering promising design and layout schemes of actuators, the features of the use of electrohydrostatic actuators, integrated layout, electrohydraulic control and the advantage of an autonomous electrohydraulic actuator are shown. The section devoted to control systems characterizes fly-by-wire system, the use of Load Sensing (LS)-regulation of volumetric hydraulic machines, issues of performance management and integration with an intelligent computer control system. When considering the performance characteristics of perspective actuators, the possibilities of reducing heat losses, problems of increasing reliability and resource, and features of bench testing are highlighted. New directions for the development of actuators are considered, such as the use of fiber-optic control wiring instead of electric, as well as the use of plasma technologies for control surfaces.

Refined Modeling and Characteristic Analysis of Electro-hydrostatic Actuator

Refined Modeling and Characteristic Analysis of Electro-hydrostatic Actuator

전기-유압 구동 모듈을 적용한 건설기계용 조향 시스템 해석 모델 개발

종래의 건설기계는 내연 기관의 동력으로 유압 펌프를 구동시켜 생성되는 유압 에너지로 작업을 수행하였다. 그러나 지구 온난화에 의한 환경 규제를 만족시키기 위해서 내연기관에서 배출되는 배기가스를 저감시키거나 내연기관을 전기모터로 대체하여 궁극적으로는 zero-emission을 만족하는 친환경 기술이 개발되고 있다. 본 논문에서는 전기 에너지를 활용하는 전기 구동 건설기계의 조향 시스템에 전기모터로 유압 펌프를 구동하는 전기-유압 구동 모듈을 적용하였으며, 그 성능을 예측하기 위한 해석 모델을 개발하였다. 개발된 해석 모델은 시험 데이터와 비교하여 타당성을 검증하였으며, 전기-유압 구동모듈의 조향 시스템에 적용 가능성을 확인하였다. 향후, 개발된 모델은 설계 변수의 민감도 해석을 위한 기초 모델로 활용할 예정이다.

A Sliding Mode Control Strategy for an ElectroHydrostatic Actuator with Damping Variable Sliding Surface

Electro-hydrostatic actuator (EHA) has significance in a variety of industrial tasks. For the purpose of elevating the working performance, we put forward a sliding mode control strategy for EHA operation with a damping variable sliding surface. To start with, a novel sliding mode controller and an extended state observer (ESO) are established to perform the proposed control strategy. Furthermore, based on the modeling of the EHA, simulations are carried out to analyze the working properties of the controller. More importantly, experiments are conducted for performance evaluation based on the simulation results. In comparison to the widely used control strategies, the experimental results establish strong evidence of both overshoot suppression and system rapidity.

A Low-Cost Miniature Electrohydrostatic Actuator System

Hydraulic linear actuators dominate in high power applications but are much less common in low power (<100 W) systems. One reason for this is the cost: electric actuators in this power range generally exhibit lower performance but are also much less expensive than hydraulic systems. However, in recent years, some miniature hydraulic components have been mass produced, driving down prices. This paper presents the application of these low-cost components, together with a novel very low-cost 3D-printed valve to create an electrohydrostatic actuator. Capable of very high power and force density, this system is competitive on cost with lower-performing electric actuators. This paper presents models for the system’s performance, as well as experimental validation data.

Active Fault-Tolerant Control Strategy for More Electric Aircraft under Actuation System Failure

The aircraft hydraulic system is very important for the actuation system and its failure has led to a number of catastrophic accidents in the past few years. The reasons for hydraulic loss can be leakage, blockage, and structural damage. Fortunately, the development of more electric aircraft (MEA) provides a new means of solving this difficult problem. This paper designs an active fault tolerant control (AFTC) method for MEA suffering from total hydraulic loss and actuation system failure. Two different kinds of scenarios are considered: leakage/blockage and vertical tail damage. With the application of the dissimilar redundant actuation system (DRAS) in MEA, a switching mechanism can be used to change the hydraulic actuation (HA) system into an electro-hydrostatic actuation (EHA) system when the whole hydraulic system fails. Taking account of the gap between HA and EHA, a degraded model is built. As for vertical tail damage, engine differential thrust control is adopted to help regain lateral-directional stability. The engine thrust dynamics are modeled and the mapping relationship between engine differential thrust and rudder deflection is formulated. Moreover, model reference control (MRC) and linear quadratic regulator (LQR) are used to design the AFTC method. Comparative simulation with the NASA generic transportation model (GTM) is carried out to prove the proposed strategy.

Experimental research on tribological characteristics of TiAlN coated valve plate in electro-hydrostatic actuator pumps

Moving towards high power density, high dynamic responsibility, and high reliability, the electro-hydrostatic actuators (EHA) have put forward great demand on the piston pumps. As a typical pump control system, EHA pumps need to maintain high efficiency in a wide range of rotational speed and pressure to implement the function of position servo. High speed and wide speed range bring great challenge to the cylinder block/valve plate interface, which has to fulfill simultaneously a bearing and sealing function. Cermet coatings are beneficial to improve the friction and wear performance of the sliding surfaces. In this study, a dedicated frictional test apparatus of cylinder block/valve plate interface has been built which could accurately simulate the working conditions of EHA pumps in a wide operational condition. Equations of the friction coefficient are established based on the combination of experiments and friction theory. When pairing with the tin-bronze cylinder block, friction coefficients of TiAlN coated and nitrided valve plate are calculated by these equations. Additionally, the wear status of these two friction pairs are evaluated by a 3D measuring laser microscope. In the wide speed range of 500–10000 rpm, the decrease percentage of friction coefficient and the wear rate for TiAlN coated valve plate are 52.55%–64.9% and 12–14% of the nitrided one, respectively.

A survey on electro hydrostatic actuator: Architecture and way ahead

The nascent development in the field of the hydraulic actuator is the integration of various cutting edge technology like mechanical, hydraulic, control and electrical the result is the multidisciplinary final product which is high-performance Electro Hydrostatic Actuator (EHA). This is an independent direct actuation method which is also known as “power-by-wire” (PBW) technology. This technology is design to replace the cumbersome leak-prone centralized hydraulic system with a compact and reliable actuation system. This technology originally developed for flight control application but due to enhance reliability and increasing popularity currently, it is being used in many industrial applications for different purposes. This paper attempt to bring out the component level comprehensive insight of EHA and consolidate all available literature of this field in the form of literature survey. These papers also talk about the pros and cons of the system and various control methods to curb noted disadvantages. Most of the existing research literature considers the EHA with an asymmetrical cylinder which is widely used in aircraft. However, in industrial application, the asymmetrical cylinder is most suited and predominately used. The control of asymmetrical cylinder poses a greater challenge vis-a-vis symmetrical cylinder due to differences in the area of two-chamber. This paper also tries to throw some light on the methods of controlling the cylinders. Finally, this paper concludes by finding research gaps and further scope of improvements and application of EHA in armoured recovery vehicles.

Comparative study of force control methods for bipedal walking using a force-sensitive hydraulic humanoid

ABSTRACT This paper describes an experimental comparison of force control methods, including resolved acceleration control (RAC), joint position control (JPC), and resolved viscoelasticity control (RVC), focusing on the biped walking performance. In previous studies, we reported a stable biped walking using the JPC and RVC methods on a hydrostatically driven humanoid, Hydra. All joints of Hydra are actuated by electro-hydrostatic actuators, showing the high backdrivability and force-controllability. However, we did not realize the RAC on Hydra, which is a standard method in state-of-the-art humanoid robotics. It is the first time that we have reported the RAC method realized a biped walking on Hydra. Moreover, the comparison between three methods is presented.

Experimental Study on Critical Design of Electro-Hydrostatic Actuators Small in Size and Light in Weight

Actuation systems for robots and other machines used in critical applications is an area that requires further research. In such applications, a machine works in a human environment and physically interacts with humans. Reliability and backdrivability are still insufficient in current systems. An electro-hydrostatic actuator has the potential advantage of high reliability by nature and high backdrivability in mechanical simplexity when it is designed to be small and light. This study provides a theoretical investigation of the methods for evaluating internal leaks and other mechanical losses, such as Coulomb and viscous friction, and experimentally evaluates two types (trochoid and involute gear) of prototyped hydraulic pumps.

Active Load-Sensitive Electro-Hydrostatic Actuator for More Electric Aircraft

In the flight control system, using an electro-hydrostatic actuator (EHA) instead of the currently used relatively mature electro-hydraulic valve-controlled actuator, there are three prevailing concerns, namely heating, size, and stiffness. This paper proposes a novel principle EHA, called active load-sensitive EHA (ALS-EHA), which can actively realize the adaptive adjustment of pump displacement with load pressure. Its principle analysis and mathematical modeling based on the direct load-sensitive EHA (DLS-EHA) configuration is done to obtain the relationship between motor current and hydraulic reduction ratio. Then, its stiffness characteristics are analyzed, especially the influence of hydraulic reduction ratio on impedance at low frequencies combined with investigating the power matching of ALS-EHA. A comparative experiment between the developed ALS-EHA and the EHA with fixed pump displacement and variable motor speed (EHA-FPVM) was carried out. The results reveal that the proposed ALS-EHA can reduce the motor heating and its displacement tracking error is smaller near zero speed owing to its higher impedance from the lower hydraulic reduction ratio under heavy load conditions.

A novel architecture of electro-hydrostatic actuator with digital distribution

To address the flow imbalance rapidly of the asymmetric Electro-Hydrostatic Actuator (EHA), this paper presents a novel architecture of asymmetric EHA with the Digital Distribution (EHA-DD). It also improves the flow nonlinearity and control accuracy of the pump, especially in the low-speed condition. The digital distribution with two High-Speed on–off Valves (HSVs) not only balances flow instead of the check valves, but also replaces the pump at the low-speed to control output flow by adjusting the PWM signal. The pump and HSVs are the crucial components to control flow output. Firstly, the flow calculation models of the pump and the duty ratio inversed model of the HSV are obtained through experimental tests and the identification method. To get the control input signal for the required flow, the pump speed and PWM duty ratio for the HSVs are inversed to compensate for flow output. Further, a multimode digital flow distribution control method based on pump speed, mainly including the pump-controlled mode for large flow demand and valve-controlled mode for little flow demand, is proposed to control the accurate flow output actively. The step extension experiments based on the flow calculation models are conducted on the EHA-DD prototype under elastic, opposite varying load. The results demonstrate that the EHA-DD realizes little position error by accurate flow control, and it is also beneficial to improve the service life of the pump.

Viscous Loss Analysis of the Flooded Electro-Hydrostatic Actuator Motor under Laminar and Turbulent Flow States

The electro-hydrostatic actuator (EHA) is one of the most prevalent types of power-by-wire (PBW) actuation systems. With the increase in EHA power density, using the pump’s leakage oil to cool the motor has been gradually adopted to solve the problem of excessive motor temperature. However, the viscous friction loss caused by the liquid viscosity will seriously affect the heat dissipation effect and dynamic performance of the motor. To calculate the motor viscosity loss accurately, a novel calculation method is proposed in this paper. Using the energy gradient theory, the relationship between the fluid flow state and the rotation speed is analyzed. In addition, the lumped parameter model of viscous loss is established by using the conservation of momentum theory and computational fluid dynamics (CFD) simulation. A test rig is designed to test the viscous friction loss for various rotation speeds, and the test results show a good agreement with the theoretical analysis. The present results demonstrate the effectiveness of the lumped parameter model and provide a better calculation method for wet motor viscosity loss calculation.

Research on Power Matching and Energy Optimal Control of Active Load-Sensitive Electro-Hydrostatic Actuator

Electro-hydrostatic actuator (EHA) is an important type of power-by-wire (PBW) which is a highly integrated closed volume control system with the advantages of high reliability, high efficiency and easy maintenance. However, under heavy load conditions, a large amount of heat the EHA motor produces affects the system’s working time and even the system life, which restricts the widespread application of EHA systems. In order to solve the problem of motor heating and the contradiction between high dynamics and high efficiency of the EHA system, a novel active load-sensitive principle and structure of EHA system is proposed based on the load-sensitive principle. The active load-sensitive EHA (ALS-EHA) is a dual control variable system consisting of a motor pump main loop and a load-sensitive loop. As for the latter, the system load pressure is introduced into a special designed and optimized pressure fellow valve (PFV). The valve outlet is connected to the plunger pump swash plate variable mechanism. The EHA pump displacement is actively adjusted by controlling the current input to PFV. The two-degree-of-freedom cooperative control for the output flow of the pump can be implemented by adjusting the motor speed and the pump displacement. An energy optimal robust control law based on fuzzy and disturbance compensation is presented for the control architecture. The simulation results show that the ALS-EHA system based on the control law proposed in this paper can effectively reduce the motor heat while ensuring the dynamic performance of the system.

Nonlinear Position Control with Nonlinear Coordinate Transformation Using Only Position Measurement for Single-Rod Electro-Hydrostatic Actuator

In existing methods, full-state feedback is required for the position tracking of single-rod Electro Hydrostatic Actuators (EHAs). Measuring a full state is not always possible because of cost and space limitations. Furthermore, measurement noise from pressure sensors may degrade the control performance. We propose an observer-based nonlinear position control with nonlinear coordinate transformation while only using position measurement to improve the position tracking of single-rod EHAs. The proposed method comprises a position controller and an observer. We propose a nonlinear coordinate transform for the controller design. The desired force is designed for the position tracking and boundedness of the internal state. The position controller is designed to track the desired state variables for the EHAs. Meanwhile, a nonlinear observer is proposed in order to estimate a full state using only the position measurement. The stability of the closed-loop system is investigated via an input-to-state stability property. The performance of the proposed method is validated via both simulations and experiments.

Development of 3-DOF wrist mechanism for electro-hydrostatically driven robot arm

Electro-hydrostatic actuator (EHA) has backdrivability and is suitable for a robot system that cooperates with human. However, its design methodology to miniaturize and to generate high pressure has not been established. We develop a robotic arm, all joints of which are actuated by EHAs. This paper reports the development of the vane motor type EHA and a wrist mechanism using the EHA. We employ a single vane motor for a wide motion range. In order to suppress internal leakage, we carefully set the clearance of the EHA and made the rigid casing by using ceramics disks. It is confirmed that the internal leakage of the new EHA is significantly reduced compared to that of the previous model. The wrist mechanism has three degrees of freedom, consisting of three vane motors in series. The flow path between a hydraulic pump and a vane motor is designed using metal pipings, swivel joints and manifolds are presented for a compact wrist mechanism. We validated the fundamental performance of the developed wrist mechanism through experiments.

A novel pump-valve coordinated controlled hydraulic system for the lower extremity exoskeleton

In order to reduce the weight and improve the energy efficiency of the lower extremity exoskeleton, a novel pump-valve coordinated controlled (PVCC) hydraulic system is presented. This hydraulic system only uses one electro-hydrostatic unit (EHU) and two valves to drive two hydraulic cylinders at the hip and knee of the lower extremity exoskeleton. The PVCC hydraulic system has the advantage of high energy conversion efficiency of the electro-hydrostatic actuator (EHA), which consists of one EHU and one hydraulic cylinder. To meet the requirements of the moment and speed of each joint of the exoskeleton, the proportional valve and on-off valve are added to adjust the flow into two hydraulic cylinders. The performance of EHU is tested by some hydraulic experiments, and the performance of the PVCC hydraulic system is analyzed by AMESim. The results show that the novel hydraulic system can only use one EHU to drive two hydraulic cylinders simultaneously under the premise of meeting the functional requirements of the exoskeleton.

High precision position control of electro-hydrostatic actuators in the presence of parametric uncertainties and uncertain nonlinearities

Characterized by high power-to-weight ratio, modularity and energy efficiency, electro-hydrostatic actuators (EHAs) have been successfully applied to aircrafts and submarines, where high precision and repeatability are in high demand. The position tracking performance, however, can be inevitably affected by parametric uncertainties and uncertain nonlinearities. Model inaccuracy or system variations normally require a large loop gain to achieve robust performance, which leads to over-design. Leakage in the fluid power system decreases steady-state accuracy, and friction in the actuator degrades the transient performance or even causes stick-slip motion at low speeds. Furthermore, the system may exhibit limit cycle (or hunting) due to Stribeck friction and integral action. This paper proposes a robust high precision position control strategy incorporating leakage and friction compensation for EHAs. Quantitative feedback theory (QFT) is applied to design a robust controller that satisfies the prescribed performance specifications without over-design, considering model inaccuracy and system variations. The internal leakage is subsequently compensated based on experimental data instead of incorporating an integrator in the controller; hence, limit cycle is avoided, and response speed is improved. Friction in the actuator is identified based on the LuGre friction model and compensated through an observer in the loop. Friction variation and load fluctuation are considered to be output disturbances to be suppressed by the QFT controller. The QFT controller with leakage and friction compensation scheme is verified through experiments on a typical EHA. Both the steady-state and transient position tracking performances are greatly improved.