Hydraulic Quadruped Robots: Current Research Landscape and Future Trajectories of Components and Hydraulic Systems

The rapid evolution of hydraulic actuation, additive manufacturing, reverse engineering, and intelligent control technologies has propelled hydraulically actuated quadruped robots into the forefront of advanced robotics research. These robots exhibit exceptional mobility in unstructured terrains such as jungles and mountainous regions, with the ability to carry payloads and move at considerable speeds, making them invaluable for applications ranging from terrain exploration and military logistics to hazardous environment operations. As the demand for high-performance mobile robotic systems continues to grow, the development of hydraulic quadruped robots has become a global research focus, with teams across the world pushing the boundaries of what these machines can achieve—yet significant challenges remain, including disorganized structural layouts, high energy loss, and complex control strategies. A comprehensive study led by researchers from top academic institutions in China has recently synthesized the current state of research in hydraulic quadruped robot components and hydraulic systems, identified the technological gaps between domestic and international development, and outlined clear future directions for the field, shedding light on how to unlock the full potential of these advanced robotic systems.

The history of hydraulic quadruped robot research dates back to the late 1960s, when the US General Electric Company and the US Army Mobility Systems Laboratory collaborated to develop the Walking Truck, a hydraulic quadruped vehicle that marked a groundbreaking milestone in the field. By translating the driver’s hand and foot movements into robotic motion signals, the Walking Truck achieved discrete contact walking, laying the foundational framework for subsequent hydraulic quadruped robot development. Decades later, Boston Dynamics, under the leadership of Professor Marc Raibert, emerged as a pioneer in legged robotics, delivering a series of iconic hydraulic quadruped robots that redefined the industry’s capabilities. Funded by the US DARPA program, BigDog, unveiled in 2006, could carry a 50kg payload and reach a maximum speed of 1.6 m/s, with its 2008 iteration achieving a 2D test speed exceeding 3.1 m/s and demonstrating robust adaptability to slopes, snow, and gravel. Building on BigDog’s success, Alpha Dog and the Legged Squad Support System (LS3) further boosted payload capacity to over 150kg and extended endurance to 32km, integrating onboard computing for camera data processing, target following, terrain perception via detectors and GPS navigation, and obstacle avoidance.

Boston Dynamics addressed the longstanding limitations of speed and noise with the launch of Cheetah in 2012, a robot with an original component design and a spinal structure integrated into its torso that set a still-unmatched speed record for hydraulic quadrupeds at 45.5 km/h on a test bench. WildCat, the outdoor version of Cheetah, featured a highly integrated system, a unique oscillating cylinder for the hip joint, and an embedded hydraulic cylinder for the knee joint, enabling Bound and Gallop gaits at a maximum speed of 25.7 km/h alongside high-speed steering and sudden stops. In 2015, the company introduced Spot, an electro-hydraulic hybrid quadruped robot that traded payload capacity for lower noise and greater motion flexibility, capable of walking and trotting in office spaces, grasslands, and small hills, and undergoing rigorous testing by the US Marine Corps Warfighting Laboratory with a maximum endurance of 45 minutes.

The US military’s heavy investment in quadruped robotics has spurred global research efforts, with the Italian Institute of Technology (IIT) and the Korea Institute of Industrial Technology (KITECH) emerging as leading international players outside the US. IIT’s Dynamic Legged Systems (DLS) laboratory, led by Claudio Semini, began its research with a single leg in the spring of 2007 and unveiled the HyQ robot after three generations of prototype iterations. HyQ, powered by an external hydraulic station, featured a high-performance torque motor for the swing joint and custom hydraulic cylinders for the hip and knee joints, reaching a maximum speed of 2 m/s and resisting lateral impacts to a certain degree. The third-generation HyQ-blues, launched in 2013 with improved hip actuators, hydraulic systems, and electronic components, was later sold to the Agile and Dexterous Robotics Lab at ETH Zurich for deep learning research. IIT’s subsequent HyQ2Max and HyQReal robots, driven by onboard hydraulics with custom oscillating cylinders for hip joints and Integrated Servo Actuators (ISA) co-developed with MOOG for swing and knee joints, demonstrated extraordinary performance—notably the ability to tow a 3-ton aircraft—with significant improvements in load characteristics, dynamic performance, and structural integration. HyQ-Centaur, developed in early 2016, added a hydraulic robotic arm to the quadruped body to perform operational tasks, with the ultimate goal of integrating dual arms for enhanced functionality.

KITECH, in collaboration with Hyundai Rotem, a renowned name in South Korea’s railway vehicle industry, developed the p2, a 16-degree-of-freedom (DOF) quadruped robot designed to mimic the skeletal structure of a horse with four DOFs per leg, all driven by hydraulic motors for a compact overall structure. However, the excessive DOFs per leg presented challenges for overall control and high-speed motion, requiring a supporting frame for operation. The team’s qRT-1 (12 DOFs, all driven by linear cylinders) and qRT-2 (with rear leg units replaced by wheels, achieving a speed of 1.3 m/s in Trot gait and a climbing angle of 20°) further refined the design, leading to Jinpoong—their flagship hydraulic quadruped robot, powered by a fuel engine-driven pump with a combination of linear and oscillating cylinders, capable of stable walking in unknown environments.

In Japan, Professor Shigeo Hirose of the Tokyo Institute of Technology has been researching quadruped robots since 1976, developing over a dozen models from the spider-like Kumo-I to the Titan series, which have become important experimental platforms for the field. The hydraulically actuated TITAN-XI robot has been successfully applied to concrete slope reinforcement, performing tasks such as deep hole drilling and anchor insertion on steep slopes, autonomously crossing 15° ramps, avoiding obstacles, and providing operational assistance in rugged and steep terrain. With three DOFs per leg, TITAN-XI can withstand a load of 5200kg on flat ground, estimate navigation steps based on terrain information, and be controlled via a remote operating system with attitude sensors feeding back positional data.

In China, research on hydraulic quadruped robots started relatively late, but significant progress has been made in recent years, with the Robot Research Center of Shandong University standing out as a domestic trailblazer. The team began its research with variable stiffness control of a single leg in 2008 and achieved a breakthrough in 2010 with the development of SCalf-1, a simple 12-DOF hydraulic quadruped robot prototype powered by an external hydraulic station that reached a maximum walking speed of 1.8 m/s—China’s first hydraulic quadruped robot to achieve high-speed Trot gait walking, which passed its first phase acceptance in 2013. Funded by the second phase of the National High Technology Research and Development Program (863 Program), the team continued to advance basic theories and key technologies, developing SCalf-2 with an integrated onboard hydraulic power system for untethered walking and SCalf-3 with an integrated environmental perception system for autonomous walking. Currently, the team is actively developing all-hydraulic and electro-hydraulic hybrid quadruped robots, with integrated robotic arms for operational tasks; the dual-arm hydraulic quadruped robot has completed basic functional verification, achieving object grasping via whole-body control theory with a payload of around 20kg and an improved speed limit.

The 863 Program has also supported research teams at Harbin Institute of Technology (Li Mantian’s team), National University of Defense Technology (Ma Hongxu’s team), Beijing Institute of Technology (Wang Junzheng’s team), and Shanghai Jiao Tong University (Gao Feng’s team), all of which have developed hydraulic quadruped robot prototypes and achieved basic robotic functions in a relatively short time. In recent years, more domestic institutions have joined the research fray: the “Tieniu” hydraulic quadruped robot from the North Vehicle Research Institute won first place in the legged robot category at the “Kuayue Xianzu” Challenge launched by the Chinese army in 2016; Huazhong University of Science and Technology developed two generations of HustDog hydraulic quadruped robots, making progress in structural optimization and compliant control; research teams at Harbin University of Science and Technology, Jiangsu Jicui Intelligent Manufacturing, Yanshan University, the Hefei Institute of Intelligent Machinery, and Zhejiang University (Jin Bo’s team) have also developed quadruped prototypes or conducted in-depth research on hydraulic single-leg control, with some yet to be publicly unveiled. Despite these achievements, domestic hydraulic quadruped robots still face a notable technological gap with international counterparts in key areas such as motion speed, core component performance, system design and integration technology, and motion control algorithms, highlighting the need for long-term investment in basic research and innovative design.

As the primary power source for hydraulic quadruped robots, the onboard hydraulic power system is responsible for energy supply and distribution, and is also the main source of noise and heat generation—its design rationality directly determines the overall motion performance of the robot. The prime movers driving the hydraulic pumps that deliver high-pressure oil to actuators fall into two main categories: internal combustion engines (ICEs) and electric motors, each with distinct advantages and disadvantages. ICEs feature a relatively complex structure (requiring exhaust pipes, among other components), with difficult-to-control rotational speeds leading to high noise and low energy efficiency, but their high fuel energy density enables long endurance times. Electric motor-driven systems, by contrast, offer a simple design, low noise, and relatively low heat generation, but the low energy density of batteries requires larger sizes for the same endurance, posing challenges for structural design and lightweighting.

Hydraulic quadruped robot systems are classified into open and closed circuits based on their hydraulic system type. Open circuits require oil tanks with a capacity several times the required flow rate for heat dissipation and impurity precipitation, significantly increasing system weight. Closed circuits feature a more compact structure with oil circulating in sealed pipelines, but suffer from poor heat dissipation and thus require additional oil cooling equipment, as well as a positive-pressure oil tank to increase the pump suction port pressure and prevent cavitation. While pump-controlled systems offer a higher efficiency upper limit—adapting to load demands via variable speed or displacement, avoiding throttling valve energy loss, and delivering significant energy-saving effects—their response speed is inferior to valve-controlled systems, making most research focus on improving the efficiency of valve-controlled systems.

The valve-controlled system is the most commonly used hydraulic system for hydraulic quadruped robots, adopted by BigDog, Scalf, Tieniu, and others. In this system, an engine drives a hydraulic pump to supply pressure oil to the entire system, which is distributed to each actuator via a valve block; a servo valve adjusts the spool position to supply the flow required by the actuators; and an accumulator provides instantaneous large flow for specific working conditions. To address the inefficiencies of traditional valve-controlled systems, the Unmanned System Research Center of the North Vehicle Research Institute and Shang Jianzhong’s team at the National University of Defense Technology have proposed a high-low pressure two-stage hydraulic system that uses on-off valves and a controller to switch flow rates under different working conditions, significantly improving valve-controlled system efficiency. Equipped with a small-capacity accumulator for energy storage and pressure fluctuation reduction, and a large-flow variable displacement main pump for continuous oil supply, this system drastically reduces heat generation from hydraulic valves under low-load, high-flow conditions and improves the overall hydraulic system energy utilization rate by 50%. However, speed mutations occur during high-low pressure switching, posing new challenges for robot control strategies.

Recognizing the fast response, low cost, and strong pollution resistance of high-speed on-off valves, many researchers have proposed replacing traditional servo or proportional valves with high-speed on-off valves and digital valves. E. Guglielmino et al. proposed using high-speed on-off valves to form a Hydraulic Buck Converter (HBC) for quadruped robot control, which was shown to save 75% energy compared to conventional valve-controlled systems, with lower costs and better robustness to oil contamination—though the complex structure results in larger volume and mass. Shang Jianzhong’s team also proposed replacing proportional valves with high-speed on-off valves, using PWM (Pulse Width Modulation) signals to adjust the valve duty cycle for independent load port flow control, achieving one-way throttling at the load port and eliminating unnecessary throttling loss. Peng Peng et al. of the Hong Kong Polytechnic University applied different digital valve hydraulic systems to robots, finding that the hydraulic system efficiency was significantly improved compared to servo valve systems when the robot operated in Walk and Trot gaits. Hydraulic accessories, an important component of the onboard power system, play a critical role in ensuring robot performance: accumulators provide instantaneous flow, reduce pressure pulsation and shock; oil quality, temperature, and pressure sensors safeguard oil quality and provide early temperature and pressure warnings during the robot’s high-dynamic motion. These accessories are numerous and account for a large proportion of the total weight—for example, BigDog’s air cooler weighs 4.36kg, 1.5 times the weight of the hydraulic pump, with the weight of accumulators, filters, and flowmeters also being non-negligible.

As the basic driving units of hydraulic quadruped robots, each active joint of the robot’s legs is driven by 2 to 4 servo actuators, whose performance directly determines the robot’s load capacity and dynamic performance. Servo actuators are divided into linear and rotary drive types, with linear cylinders being the most widely adopted for driving linkages to output torque and achieve joint swing of individual legs. BigDog, the pioneer of modern hydraulic quadruped robots, uses integrated servo linear cylinders to drive each joint, integrating a MOOG E024 valve, a uniaxial tension-compression sensor, an LVDT displacement sensor, and a low-friction hydraulic cylinder. This compact structure meets the robot’s requirements for limited installation space and high output force, enabling the robot to climb 35° slopes, walk with a 45kg payload, and reach a maximum speed of 6.4 km/h. Integrating the electro-hydraulic servo valve with the hydraulic actuator shortens the distance between them, allowing heat generated by the valve-controlled cylinder unit to be quickly returned to the oil tank to maintain actuator performance, reducing oil volume expansion in hydraulic pipelines to improve actuator dynamic response, and facilitating oil pipe layout with co-located inlet and outlet ports on the same side.

Domestic research teams at Shandong University, National University of Defense Technology, Harbin Institute of Technology, Beijing Institute of Technology, Jiangsu Jicui Intelligent Manufacturing, and KITECH in South Korea have adopted a similar actuator structure to BigDog, with differences only in sensor principles, using nozzle flapper servo valves for high-frequency response control and tension-compression and displacement sensors to provide output force and position feedback signals for closed-loop control, enabling good robot motion performance. IIT’s DLS laboratory’s HyQ uses Hoerbiger off-the-shelf hydraulic cylinders with only a tension-compression sensor mounted on the piston rod for output force measurement; the servo valve is installed on the torso, and joint position information is measured via encoders on each joint rotating shaft—an actuator principle also adopted by Huazhong University of Science and Technology’s team. Shanghai Jiao Tong University’s “Xiaoxiang” quadruped robot features a self-designed hybrid drive servo linear cylinder “Hy-Mo”, which highly integrates a motor, spool, valve sleeve, and cylinder barrel; the servo motor directly drives the spool to control the opening degree, achieving high-frequency actuator control. This integrated design reduces the weight of structural components, results in a more compact shape, and offers high pollution resistance. Shang Jianzhong’s team at the National University of Defense Technology designed a new type of hydraulic cylinder with a variable effective piston area, which improves hydraulic system efficiency by controlling the connection between multiple plunger chambers and the main hydraulic system via on-off valves; adjusting the effective area balances the different actuator load pressures, reducing throttling loss and system energy consumption significantly.

With the growing demand for high-speed hydraulic quadruped robots, higher requirements have been placed on components, driving the pursuit of advanced integration and lightweighting. After the initial design of HyQ, IIT’s DLS laboratory began focusing on component-level research, collaborating with MOOG (US) and the University of Bath (UK) to design a highly integrated intelligent hydraulic actuator (ISA). All components, including the servo valve, relief valve, displacement sensor, and pressure sensor, are integrated onto a titanium alloy cylinder barrel manufactured via additive manufacturing, enabling pressure sensing, position detection, and closed-loop control of the actuator. A two-stage hydraulic servo valve and optimized flow channels with lower pressure loss support higher dynamic robot motion. The publicly available ISAv2 and ISAv5 versions have been applied to single-leg test benches, HyQ upgrades, HyQReal, and HyQ2Max—with ISAv5 powering the towing of a 3-ton aircraft, demonstrating exceptional power output—though the actuator’s market price exceeds 150,000 RMB, a significant barrier to widespread adoption.

Building on years of engineering experience, the Robot Research Center of Shandong University proposed a cylinder-leg integration scheme, integrating the thigh structural components and knee joint hydraulic cylinder to achieve lightweighting and realizing pipe-free knee joint drive via internal flow channels. This design reduces the single actuator weight by 30% and the single leg weight by 10%, delivering remarkable lightweighting effects, albeit with a doubled cost. Similarly, Professor Kong Xiangdong’s team at Yanshan University researched integrated cylinder-leg joints and developed an integrated drive unit, which reduces the hip and knee joint weights by 11% and 7% respectively while achieving a higher output power than HyQ2Max’s leg joints. Professor Xu Bing’s team at Zhejiang University adopted non-metallic material substitution for lightweighting, completing three generations of carbon fiber hydraulic cylinder iterations and exploring the entire process of “model design-simulation verification-automated processing-post-processing”. The carbon fiber hydraulic cylinder integrates displacement and tension-compression sensors, reducing weight by 40% compared to steel cylinders of the same specification—simulations of a quadruped prototype in Trot gait show significant improvements in overall motion performance and energy consumption with this weight reduction.

Rotary actuators are favored by some research teams for their compact structure, though their power density is lower than linear drives, making them mostly suitable for swing joints to achieve compact designs. IIT’s MiniHyQ uses a single-vane oscillating cylinder from France’s DS Dynatec, providing a torque output of 60 N·m with a swing range of up to 220°; an absolute encoder measures real-time joint position, and a torque sensor measures output torque for position and force control. HyQ2Max’s dual-vane oscillating cylinder integrates an additively manufactured intelligent valve, encoder, and torque sensor, with a swing range of 100°, a rated output torque of 170 N·m, and a mass of only 2.1 kg—notably, the servo oscillating cylinder remains stable under an external impact of 250 N·m, supporting the robot’s 3-ton aircraft towing capability. KITECH’s 16-DOF hydraulic quadruped robot p2 features all joints driven by servo oscillating cylinders, achieving a motion speed of 0.8 m/s; the team’s flagship Jinpoong robot uses a high power-to-weight ratio oscillating cylinder from South Korea’s KNR for its swing joint, controlled by a servo valve from the UK’s Star Hydraulics, with a swing range of ±70° and an output torque of up to 400 N·m at 21 MPa, and an estimated single unit mass of around 5.0 kg based on the company’s mature product power-to-weight ratio. Professor Xu Bing’s team at Zhejiang University proposed a new type of plunger oscillating cylinder based on the concept of plunger hydraulic cylinders, which achieves oscillation via two linear cylinders and a special-shaped cam—this compact structure delivers an output torque of up to 180 N·m, and simulations of a quadruped robot show a 30% reduction in average flow compared to traditional linear cylinder drives under the same working conditions. The team also innovatively designed a spiral oscillating cylinder, which drastically reduces starting friction while achieving a high force-to-weight ratio of 600 N·m/5.2 kg, close to that of the team’s self-developed dual-vane oscillating cylinder, with plans to apply it to the team’s next-generation quadruped prototype.

The development of hydraulic quadruped robots is inseparable from advanced control strategies, with four mainstream methods currently widely used in the field: Central Pattern Generator (CPG), Virtual Model Control (VMC), Model Predictive Control (MPC), and Whole Body Control (WBC), each with unique characteristics, advantages, and limitations. Proposed by H. Kimura et al., CPG is a control method based on biological neural regulation mechanisms, widely applied to quadruped robot control by engineering simulating the animal motion control area CPG to construct multiple periodic neural oscillators on the robot system. Coordinating the output of these oscillators generates joint trajectories with rhythmic motion, replicating biological neural center control and reflexes. V. Barasuol et al. of IIT proposed a task-space CPG-based trajectory generation method that adjusts the foot end trajectory and body posture according to different terrains, with its effectiveness verified on the HyQ robot; the team later realized flying trot gait on HyQ based on this method. Domestic research on CPG-based control for hydraulic quadrupeds is mainly concentrated at Harbin Institute of Technology, where Wang Tingting et al. established a multi-layer CPG neural network model to achieve stable rhythmic motion control of robot joints, with tests on simulation platforms and physical prototypes demonstrating strong robustness in generating rhythmic motion in unknown complex environments independently. Yang Yuanyuan et al. constructed a multi-layer CPG neural network with reflex coordination, conducting in-place stepping and walking gait experiments via simulation and prototyping. CPG algorithms offer an inherent advantage in trajectory generation and gait switching, effectively solving the motion coupling problem of multi-joint legs in low-speed quadruped robots, but they are complex with numerous parameters; designing a reasonable and practical rhythmic motion function is challenging, and the mechanism of interaction between algorithm parameters and robot dynamic characteristics is unclear, making parameter adjustment difficult.

First proposed by Jerry Pratt of the MIT Leg Laboratory for biped robot control, Virtual Model Control (VMC) is based on the idea of serially connecting the controlled object with a “spring-damper” virtual model. When a deviation occurs between the controlled object and the desired target value, the virtual model generates a corresponding virtual force, which is adjusted to keep the controlled object following the target value. I. Havoutis et al. of IIT used the VMC method to equate one robot leg to a virtual “spring-damper” model, achieving Trot gait motion at a maximum speed of 0.7 m/s by actively controlling leg impedance via model parameter adjustment. Domestic teams at Shandong University and National University of Defense Technology have conducted in-depth research on this method, with experimental verification on the physical hydraulic robots Scalf and Tieniu respectively. VMC features a simple and intuitive concept without involving complex dynamics, and in theory can make the robot’s motion approach the dynamic characteristics of any virtual model (or component)—the key to its success lies in selecting a virtual model that conforms to the kinematic laws of the quadruped robot itself.

Model Predictive Control (MPC) has been applied to single-leg, biped, and quadruped robots, with its basic principle being to predict the system state over a future time domain based on a known predictive model and the current state, convert the control problem into an optimization problem including predicted values according to a specific performance index, and apply only the first value of the solved result sequence to the controlled object. M. Neunert et al. proposed a whole-body nonlinear model predictive control (NMPC) method using a whole-body dynamic model including contact dynamics, with contact positions, sequences, and times all solved via an optimization solver; the controller can run on an actual quadruped robot at a frequency of 190 Hz. O. A. V. Magana et al. estimated unknown terrain via a convolutional neural network (CNN) to provide safe foothold positions, which are fed to the MPC controller to optimize ground reaction forces—this control strategy was validated via simulation on HyQReal. Chen Teng et al. of Shandong University researched the MPC method, verifying the stable operation of Trot, Bound, Gallop, and other gaits as well as terrain robustness via Webots simulation. The application of MPC mainly depends on balancing model accuracy and computing power; while computers and optimization algorithms have advanced significantly in recent years, the high computing power required for high-dynamic motion remains the main limitation of MPC application.

Whole Body Control (WBC), a relatively new control concept, enables multi-task functionality under robot environmental constraints via simple adjustments, with several types including QP-based WBC, null space projection (NSP)-based WBC, hierarchical QP (HQP)-based WBC, and a combination of QP and NSP. B. Aceituno et al. used a mixed-integer convex optimization (MICP) formulation to simultaneously plan contact positions, gait patterns, and motion, enabling HyQ to move on a variety of complex terrains. S. Fahmi et al. adopted a QP-based WBC method considering the robot’s overall dynamics, drive limits, joint limits, and contact interactions, with experimental verification on HyQ under different terrain conditions and gaits. C. Mastalli et al. used a QP-based WBC method with terrain modeled as a cost map; this control framework couples body motion, gait planning, and terrain mapping, also verified on HyQ to achieve body motion tracking while satisfying dynamic and torque limits and preventing slipping. G. Raiola et al. proposed a general motion framework without centroid trajectory planning based on an HQP-based WBC method, which can achieve both high-speed and static gaits, with simulation and experimental verification on different simulated terrains. Current WBC research mainly focuses on task priority allocation, controller robustness, and solution efficiency; WBC enables multi-task robot motion, but its disadvantage is the large number of optimization solution calculations required, with real-time control remaining a key challenge.

Despite the significant progress in hydraulic quadruped robot research, challenges such as high noise, low energy conversion efficiency, and disorganized structures continue to restrict performance improvement. The future development of hydraulic quadruped robots will focus on four core directions: high-speed and high-pressure onboard power systems, component integration and lightweighting, advanced heat dissipation/energy-saving/noise reduction systems, and the application of intelligent algorithms—these directions will collectively unlock the high dynamic performance of hydraulic quadruped robots and accelerate their industrial application.

First, the high-speed and high-pressure development of onboard power systems will fully exploit the high power density advantages of hydraulic systems. Most existing hydraulic quadruped robots operate at a pressure level of less than 21 MPa and a pump speed of around 6000 r/min; analysis of aircraft hydraulic systems shows that increasing pressure from 21 MPa to 28 MPa reduces weight by 2.46% and volume by 13.8%, while an increase to 35 MPa reduces weight by 12.2% and volume by 28.3%. For a 12-DOF hydraulic quadruped robot, a pressure increase from 21 MPa to 28 MPa is estimated to reduce hydraulic cylinder weight by 8% and the overall hydraulic system weight by 5%, delivering significant lightweighting effects. High-speed development also offers remarkable lightweighting benefits: increasing the pump speed from 1500 r/min to 10000 r/min reduces the hydraulic pump mass by 88%. The rapid development of upstream and downstream components such as high-speed motors and high-frequency response control valves has made the high-speed and high-pressure development of onboard hydraulic systems feasible, and will further drive the integration and modularization of these systems.

Second, component integration and lightweighting will be the core of improving robot performance. As the basic units of robots, component performance directly determines overall robot capabilities; the advancement of additive manufacturing technology has injected new vitality into hydraulic components, with outstanding products such as IIT and MOOG’s co-developed ISA and Domin servo valves achieving improved lightweighting via integration and new manufacturing processes. Future integrated design—including motor-pump, valve-valve block-actuator, and actuator-structural component integration—will drive the next generation of hydraulic quadruped robot development, reducing weight while enabling a more compact and streamlined design. In addition, the lightweighting of accessories such as accumulators and pressurized oil tanks is worthy of in-depth research, which can be achieved via non-metallic material substitution and conformal manufacturing.

Third, the development of new heat dissipation, energy-saving, and noise reduction systems will address the key pain points of hydraulic quadruped robots. Current oil coolers used in quadruped robots are large and heavy, restricting torso design, while their heat dissipation capacity directly affects robot performance—accurately estimating system heat generation has become a research focus. The application of new cooling methods such as semiconductor refrigeration and heat pipes will maintain system temperature rise within a preset range, ensuring normal robot operation. To address the low efficiency of hydraulic quadruped robots, researchers have proposed numerous solutions including high-low pressure supply systems; future research will focus on innovative energy-saving circuit design based on in-depth analysis of rhythmic working conditions with alternating high and low output, recovering partial energy via energy storage components such as accumulators and supercapacitors, and integrating multi-sensor strategies to achieve load prediction and reduce system output. The noise of hydraulic quadruped robots mainly comes from the engine and oil cooler fan, with a noise level of up to 80 dB, close to that of an aircraft cockpit; replacing the engine with a battery-high power density motor combination can reduce noise, but the resulting endurance problem requires further research. Notably, heat dissipation, energy saving, and noise reduction are not independent issues, and the optimal solution must be selected from a system-level perspective.

Finally, the application of advanced intelligent algorithms will elevate the autonomy and adaptability of hydraulic quadruped robots. Simple intelligence such as leader-follower navigation and basic load prediction has become a thing of the past; the future of hydraulic quadruped robots lies in human-like thinking and learning capabilities, enabling evolution during training and further improvement of autonomous control decision-making. In unknown environments and unforeseeable scenarios, these robots will exhibit advanced intelligence, transforming from passive learning to active learning and realizing intelligent self-emergence; group coordination and self-organization mechanisms will ensure the smooth completion of complex tasks, expanding their application scope in military, industrial, and civilian fields.

In summary, hydraulic quadruped robots represent the pinnacle of advanced robotics technology, integrating high-frequency response servo valves, low-friction servo cylinders, and intelligent control algorithms. The global research community has made significant progress in robot prototyping, hydraulic system design, servo actuator development, and control strategy research, with domestic Chinese teams catching up rapidly while still facing a technological gap with international leaders. The future development of hydraulic quadruped robots will be driven by the synergistic advancement of upstream and downstream industries, with ultra-lightweighting achieved via highly integrated structural design, high-efficiency onboard power systems developed to support high burst output, and advanced control algorithms applied to enhance environmental adaptability and high dynamic performance. As research in high-speed/high-pressure systems, component integration, energy-saving/noise reduction technology, and intelligent algorithms continues to advance, hydraulic quadruped robots will move beyond simple low-speed walking to achieve human-surpassing high-speed running and high burst motion capabilities, ultimately realizing large-scale industrial application and playing an irreplaceable role in national defense, logistics, disaster relief, and other critical fields.

Author Affiliation & Publication Information This research was conducted by Zong Huaizhi, Zhang Junhui, Zhang Kun, Huang Hsinpu, Zhang Fu, Wang Ximeng, Xu Bing from the State Key Laboratory of Fluid Power & Mechatronic Systems, Zhejiang University, Hangzhou 310027, China, and Lu Zhenyu from the School of Mechatronic Engineering, Nanchang University, Nanchang 330031, China. The study was published in Chinese Hydraulics & Pneumatics, Volume 45, Issue 8, August 2021, with the DOI: 10.11832/j.issn.1000-4858.2021.08.001.

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