In Hydraulic Robots, How EHA (Electro-Hydrostatic Actuator) Can Stabily be Enhanced, and Energy Consumption be reduced?

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With the swift advancement of artificial intelligence, additive manufacturing, environmental sensing, and human-machine interaction technologies, robots have become pivotal in scenarios such as aerospace, geological exploration, and disaster relief. Robots are increasingly capable of undertaking a variety of complex tasks in extreme environments, either independently or in collaboration with humans. Common propulsion methods for robots include pneumatic, electric, hydraulic, and hybrid drives. Among these, hydraulic drive systems are widely utilized within robot systems due to their high load capacity, stability, and power density advantages. Such systems find application in rescue robots, exoskeleton robots, legged robots, and humanoid robots.

In hydraulic-driven robot systems, actuators, acting as the robot's "muscles," directly perform work and are crucial for the robot's response to its environment and initiation of action. Therefore, research into hydraulic robot actuators is particularly important.

Hydraulic robot actuation systems can be categorized into two types based on the control object: valve-controlled systems and pump-controlled systems.

In valve-controlled systems, hydraulic pumps usually maintain a stable output flow and pressure, controlling the robot's actions through servo valves. Conversely, in pump-controlled systems, the hydraulic pump's output flow and pressure are directly adjusted to meet the varied action requirements of the robot.

Valve-controlled systems, due to the overflow losses from relief valves and throttling losses from electro-hydraulic servo valves, suffer from low efficiency and significant heating issues, rendering them unsuitable for hydraulic robots with high endurance requirements.

Pump-controlled systems, on the other hand, can adjust their output based on actual demand, reducing the output of the hydraulic pump under lower load conditions without the throttling losses associated with servo valves. This substantially reduces energy consumption, achieving energy-saving effects.

Despite the complexities in control algorithms and slower response times, pump-controlled systems continue to be widely utilized across various fields, such as construction machinery and industrial production, due to their output flexibility and efficiency.

Among the many configurations of pump-controlled actuation systems, Electro-hydrostatic actuators (EHAs) have garnered widespread attention for their high integration, energy efficiency, and power density advantages. In addition to inheriting the benefits of pump-controlled systems, EHAs also offer the advantages of distributed electro-hydraulic power systems.

Compared to traditional centralized systems powered by a single source driving multiple actuators, EHAs (Electro-hydrostatic actuators) serve as independent drive devices in hydraulic robots, eliminating the need for bulky centralized oil sources and extensive hydraulic piping. This innovation not only miniaturizes the robot's size and weight but also allows for energy to be supplied on-demand at each output location, significantly reducing the robot's energy consumption and further enhancing its mobility.

Electro-Hydrostatic Actuator HYC-EHA1kW-L50kN/100mm

Additionally, EHAs offer advantages such as easy maintenance, low noise, cost-effectiveness, and good interchangeability, ensuring that the failure of other joint actuators does not affect the independent operation of EHAs.

Fundamentally improving the energy efficiency and reliability of actuator systems, EHAs represent a highly attractive technological concept for the next generation of hydraulic robots. Currently, EHAs are primarily used in high-output power applications such as aircraft, naval ships, and automotive suspension systems.

The development of EHAs for small-sized robots with a high force-to-weight ratio is actively underway, making the application of EHA technology in joint actuation for hydraulic robots a mainstream trend in the development of high-performance robots. Therefore, the question arises: how can EHAs be applied to hydraulic robots to enhance operational stability and achieve energy reduction?

«——【·Robot Applications·】——»

Characterized by high power density, high energy utilization, and strong anti-interference capabilities, Electro-Hydrostatic Actuators (EHAs) have been employed as actuators in a variety of robot prototypes, including articulated robots, wearable robots, and legged robots. They serve in numerous application scenarios, such as manufacturing, medical devices, and military transport.

Electro-Hydrostatic Actuator HYC-EHA1.6kW-L6kN/170mm

Articulated robots are comprised of multiple drive components that form joint connections, allowing each joint to rotate within a certain range. This enables the robot to perform complex movements and posture adjustments.

Robotic arms and hands are typical examples of articulated robots. Such robots can execute tasks such as grasping, transporting, and assembling objects.

As a high-load independent drive device, the Electro-Hydrostatic Actuator (EHA) boasts the advantages of a distributed electro-hydraulic power system, making it suitable for the joint actuators of robots. This capability allows for the broader application of articulated robots in fields such as industrial production.

Wearable Robots: Wearable robots are robotic systems that directly interact and integrate with humans, enhancing physiological and locomotive abilities through the wearing of robotic devices.

Exoskeleton robots represent the most typical category of wearable robots, currently employed in fields such as industry, military, medical, and rehabilitation. The development of wearable robots
necessitates actuation systems that are compact, highly energy-efficient, and capable of sustaining heavy loads. Furthermore, for physical interaction, the robot's ability to control force is essential, and reverse drivability is a key characteristic for achieving force control.

Electro-Hydrostatic Actuator HYC-EHA1kW-L50kN/50mm

Due to the direct force transmission between the hydraulic pump and actuator in EHAs, without being interrupted by servo valves, EHAs possess bidirectional force control, endowing them with reverse drivability. The development of EHAs provides substantial support and enhancement for wearable robots.

Scholars have proposed a highly integrated, lightweight, and reversible EHA design applied to prosthetics. This EHA system has a total weight of 2.4kg, a maximum output torque of 15N·m, and a motion range of 110 degrees. It meets the biomechanical torque and speed requirements of the knee joint, capable of tracking the actual knee trajectory, simulating viscoelastic behavior, and regenerating power during braking.

Compared to other powered prosthetic solutions, the highly integrated and lightweight EHA significantly reduces power consumption and enhances endurance capabilities.

Legged robots: Legged robots capitalize on the advantage of discrete foot placement for flexible terrain adaptability, finding widespread application in terrain exploration, military equipment, and material transport.

These robots place high demands on mobility and load-bearing endurance, where the higher energy efficiency of EHAs plays a crucial role in the application of legged robots.

Researchers have developed a hydraulic pump-controlled cylinder drive system for a quadruped robot, utilizing a separated EHA structure. This structure comprises an electric motor pump located in the robot's body and hydraulic cylinders positioned in the leg joints. This drive unit can reduce the energy consumption of the drive system, decrease the system's heat generation, and enhance the robot's endurance and limb flexibility.

In application scenarios such as wearable robots and legged robots, EHAs exhibit two major characteristics: high power density and energy efficiency, which to a certain extent enhance the robot's output capabilities and endurance time.

How to integrate EHAs into appropriate joints to further improve the overall performance of the robot is a question worth investigating. Moreover, breakthroughs in new components, configurations, and control algorithms will further expand the application range of EHAs.

Electro-Hydrostatic Actuator HYC-EHA0.6kW-L20kN/100mm

«——【·System Configuration·】——»

The Electro-Hydrostatic Actuator (EHA) is a highly integrated and independent closed hydraulic system.

Taking a typical EHA configuration as an example, the system consists of a controller, drive circuitry, motor, hydraulic pump, accumulator, check valve assembly, relief valve assembly, mode switching valve, actuating cylinder, integrated valve block, pressure sensor, and displacement sensor.

The motor rotates in a forward or reverse direction based on the drive control signals emitted by the controller and drive circuit, driving the hydraulic pump to provide the system's flow and pressure, thereby actuating the movement of the actuator cylinder.

The accumulator establishes the return oil pressure for the EHA's closed system, preventing cavitation due to too low inlet pressure at the hydraulic pump. The one-way valve group serves as a replenishing oil circuit for the pump's low-pressure chamber, ensuring that high-pressure oil does not flow back into the hydraulic pump through the leakage port. The relief valve provides an overload protection circuit for the EHA.

The mode-switching valve is used to facilitate mode transitions of the EHA actuator; when the actuator malfunctions, this valve isolates the actuator cylinder from the pump source. Compared to valve-controlled systems, the pump-controlled system of the EHA does not require components such as servo valves and coolers, offering a simpler, more compact structure along with a higher power density.

Depending on the control method, EHAs can be categorized into three types: Fixed Pump Variable Motor (FPVM), Variable Pump Fixed Motor (VPFM), and Variable Pump Variable Motor (VPVM), each with its own advantages. The FPVM configuration is simple and highly reliable but has a lower bandwidth. The VPFM offers a quicker dynamic response than FPVM, but its overall efficiency is relatively lower. In the VPVM configuration, both the pump's displacement and the motor's speed can be adjusted simultaneously, combining the advantages of the other two configurations. However, it has lower reliability and requires higher control technology expertise.

Hydraulic robots have multiple requirements for EHAs to achieve high-performance motion, including reliability, energy efficiency, and dynamic response. The FPVM configuration, known for its simplicity, high efficiency, and ease of control, has been widely adopted in the aerospace industry, representing a relatively mature and mainstream configuration with substantial practical engineering application experience. Therefore, for robotic applications, the EHA configuration takes the FPVM as a reference, making improvements to the conventional configuration to meet the requirements for quick response and energy-efficient operation in robots. This can specifically include power regulation configurations, dual-rod EHA configurations, load-sensitive configurations, and energy recovery configurations.

Electro-Hydrostatic Actuator HYC-EHA0.4kW-L20kN/100mm

Rapid Response Configuration: Robots require the swift injection of instantaneous energy during explosive movements, while they need to maintain position in a stationary state. The dynamic response of EHAs is constrained by the high rotational inertia of servo motors and bi-directional pumps, making it challenging to meet the motion demands of robots. Therefore, various research teams have proposed a rapid response configuration to address this critical issue.

A scholar proposed an EHA configuration utilizing a power regulator. This power regulator consists of a high-pressure accumulator and a proportional valve. The system rapidly and flexibly stores and releases hydraulic energy based on varying operational conditions, enabling the EHA to achieve quick responses and expand its frequency range.

A hydraulic lock is a type of solenoid valve that locks the actuator based on the operating mode and system status, allowing the actuator's output position to remain fixed, maintaining rigidity, and reducing energy consumption during loading. Given the requirements for power density, robots utilizing EHAs often employ asymmetrical hydraulic cylinders, making the dynamic response of the EHA dependent on the direction of the load. When the direction of the load changes, the response time of the piston rod's extension and retraction significantly varies.

Based on position-controlled single-rod EHA, add a force-controlled dual-rod EHA. Since the force-controlled dual-rod EHA compensates for all external forces, the position-controlled single-rod EHA can operate as if it were under no load. This configuration demonstrates excellent position control performance. Moreover, the system can be reconfigured as a redundant system, allowing for safe operation in emergencies in case of failures.

Energy-efficient Configuration: The average efficiency of mobile hydraulic robots is low, and their energy consumption is high, making the enhancement of their endurance capabilities particularly critical.

Electro-Hydrostatic Actuator HYC-EHA23kW-L100kN/460mm

Compared to traditional valve-controlled systems, pump-controlled EHA systems do not suffer from the overflow losses of relief valves and the throttling losses of servo valves, fundamentally increasing energy efficiency. However, the FPVM configuration of EHA also faces issues such as the motor overheating under increased loads and the fixed-displacement pump supplying more energy than needed. Since EHA operates as a closed hydraulic system, it leads to challenges in dissipating the heat generated by the EHA's motor under heavy load conditions.

Researchers have proposed an active load-sensitive configuration for EHAs.

Its distinctive feature involves adding a Pressure Following Valve (PFV) between the pump's load-sensitive pressure point and the variable angle mechanism of the piston pump swashplate.

Electro-Hydrostatic Actuator HYC-EHA0.4kW-L52kN/50mm

By actively controlling the electric current input to the PFV to adjust the displacement of the EHA pump, and further regulating the motor speed, a two-degree-of-freedom cooperative control over the pump's output flow is achieved. This allows the EHA to actively increase the pump's displacement for quick response needs, and to decrease the pump's displacement in static heavy load conditions, thereby addressing issues related to motor heating and dynamic performance.

Scholars have proposed an energy recovery configuration based on the FPVM, leveraging the four-quadrant operating principle of EHA to enable energy recovery under auxiliary load conditions.

When the output speed direction of the hydraulic cylinder aligns with the force direction, and the system operates under auxiliary load conditions, high-pressure oil flows out of the hydraulic cylinder and into the hydraulic pump. The hydraulic pump then converts into a hydraulic motor, reversing and driving the motor to rotate, thus allowing the potential energy of the auxiliary load to be recovered by the motor unit, achieving the function of energy recovery.

The various configurations of EHAs determine the functionalities that hydraulic robots can achieve at a system level. Improved configurations extend the classic models from different perspectives based on the specific working conditions of robots, thereby providing targeted, more flexible, and efficient power output methods. This enables robots to excel in various tasks and working environments.

Electro-Hydrostatic Actuator HYC-EHA1kW-L47kN/300mm-DS

«——【·Hardware Components·】——»

The main components of an EHA include the motor, hydraulic pump, hydraulic cylinder, and integrated valve block. Technological breakthroughs in these core components are crucial for enhancing the performance of EHAs used in robots.

Motor: Serving as the power source, the motor is one of the most critical components of an EHA. In EHAs, the motor is directly connected to the pump via a shaft, rotating and driving the pump upon receiving input signals from the controller. To ensure that the EHA performs efficiently, stably, and reliably in terms of control, the motor must meet various requirements, such as a wide range of speed and torque, rapid response capabilities, high control precision, and compact size and weight.

When applied to robots, the motor of an EHA often operates under varying working conditions. Therefore, the motor needs to provide an appropriate range of speeds and torques and be capable of adjusting its output across a broad speed range to meet the robot's load capacity and action needs at different speeds. At the same time, the output of the hydraulic pump needs to be adjusted in real-time according to the robot's action requirements, necessitating that the motor can respond quickly and adjust its output power in a short period.

Furthermore, the robot's need for precise force control and speed control requires the motor to be capable of precise manipulation. Therefore, the motor must be equipped with high-precision controllers and control algorithms to ensure the accuracy and stability of the output flow. Moreover, robotic applications place higher demands on the size and weight of the EHA motor to ensure the compactness and lightweight nature of the robot system, which aids in enhancing the performance and endurance of the robot system.

Hydraulic Pump: As the heart of the EHA, the hydraulic pump converts the mechanical energy of the motor into hydraulic energy, providing high-pressure oil for the hydraulic cylinder.

Common types of EHA pumps include screw pumps, gear pumps, and piston pumps. EHAs utilizing a pump-controlled system can achieve reverse driving, offering an actuator with impact resistance for robots that interact with the environment. To enhance the reverse driving capability and force sensitivity of the EHA, minimizing the friction in the hydraulic pump is particularly crucial.

The pump, by utilizing the viscous friction of the fluid, transforms mechanical energy into hydraulic energy, eliminating mechanical contact between components to minimize static friction to the greatest extent. This enhances the actuator's reverse drivability and torque controllability, aiding in the achievement of smoother force control in robots. However, under the same motor driving conditions, the output power of a screw pump is only about 30% of that of a gear pump, resulting in lower efficiency.

In wearable exoskeleton robots, gear pumps are often chosen for their simple structure and compact size. The development of miniature accumulators integrated within the pump aims to reduce the system's volume and weight. Robots utilizing EHA require high output pressure from the pump, and a critical factor in ensuring this output pressure is minimizing the pump's internal leakage.

The application extends to robotic hands driven by clustered EHAs. By improving the stiffness of the crescent gear pump components, internal leakage in the pump was reduced by 93%. The pump utilizes a high-rigidity ceramic pump casing, effectively preventing leakage caused by deformation of the pump casing under high pressure. The further design included a ball-bearing structure capable of precisely adjusting the gap between the gear and pump casing, to minimize internal leakage.

Further summarization addresses the challenges of high-speed rotation in axial piston pumps, including cavitation, flow pressure fluctuation, tilting motion of rotating components, and pump heating issues, with detailed solutions presented. The pump converts the mechanical energy from the motor into hydraulic energy, which is then transferred to the actuating elements.

When applied to hydraulic robots, the pump is required to have a high-power density and quick response speed.

Electro-Hydrostatic Actuator HYC-EHA0.3kW-L3kN/100mm

Performance-wise, this is primarily demonstrated at high pressures, where increasing pressure helps reduce the system's size and weight. Structurally, the focus is on cartridge designs, which allow for better integration with other components and significantly reduce the pump's size ratio.

«——【·Control Algorithms·】——»

EHA is a nonlinear, strongly coupled complex system, where control technology plays a crucial role in its performance. Due to the inertia of servo motors, the control performance of pump-controlled EHAs has traditionally been considered inferior to that of valve-controlled systems where flow is regulated by servo valves.

The goal of researching control technologies is to enhance the control precision, response speed, and robustness of EHAs, further reflecting on the robot's capabilities in position control, force control, and compliant control regarding the external environment.

The application of EHAs necessitates accurate tracking and control of the hydraulic cylinder's position.

Various control algorithms have been proposed to improve the position-tracking performance of EHAs, which exhibit parameter uncertainty and nonlinearity, such as PID control, sliding mode control, and state feedback control.

The PID control algorithm, due to its simple structure and ease of implementation, is widely used in EHA control. An adaptive fuzzy PID controller is used to control the permanent magnet synchronous motor of the EHA. Compared to traditional PID controllers, EHAs controlled with fuzzy PID show stable performance and strong anti-interference capability, managing to return to a specified position after experiencing load disturbances. However, the rise time with fuzzy control strategies is longer, necessitating an increase in motor speed to reduce this rise time.

Addressing the issue of leaks in hydraulic systems, scholars have proposed a state equation based on the relative speed between the motor side and the load side. By designing a full-state feedback controller using the state equation, stable position tracking with high feedback gain was achieved, demonstrating good robustness.

Electro-Hydrostatic Actuator HYC-EHA1kW-L130kN/50mm

«——【·Conclusion·】——»

Based on the analysis of EHA's applications in robots, this discussion has covered three aspects: system configuration, hardware components, and control algorithms. It has introduced the application of EHA in various types of robots, including articulated robots, wearable robots, and legged robots. The use of EHA can significantly enhance hydraulic robots in terms of power density, stability, and energy consumption.

EHAs hold a broad application prospect in hydraulic robots. With continuous technological advancements and innovations, EHAs are set to evolve towards high-performance components, high power density, high precision control, and efficient energy management. This development will provide hydraulic robots with more efficient actuation outputs and quicker response capabilities, ensuring precise motion control and endurance to meet the growing demands of robot applications and the development requirements for energy conservation and environmental protection.

The integration of hydraulic and electric technologies in EHAs will become a significant direction for development in the field of hydraulic robots.

He Jun

Specialized in the Casting & Machining Industry with 20+ experience ★ Focus on Providing fluid couplings, Axial piston micropump & EHA, motion solutions, checkweigher solutions ★ Founder at Jaalink.

In Hydraulic Robots, How EHA (Electro-Hydrostatic Actuator) Can Stabily be Enhanced, and Energy Consumption be reduced?

None

«——【·Robot Applications·】——»

«——【·System Configuration·】——»

«——【·Hardware Components·】——»

«——【·Control Algorithms·】——»

«——【·Conclusion·】——»

He Jun

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In Hydraulic Robots, How EHA (Electro-Hydrostatic Actuator) Can Stabily be Enhanced, and Energy Consumption be reduced?

None

«——【·Robot Applications·】——»

«——【·System Configuration·】——»

«——【·Hardware Components·】——»

«——【·Control Algorithms·】——»

«——【·Conclusion·】——»

He Jun

Learn More About He Jun

Want to Know More About Our Products

Talk To Our Expert

Choose Jaalink For Smooth Connections

About Your Brands

Products

Quick Link

Contact Us