Hydraulic pumps supply fluid to the components in the system. Pressure in the system develops in reaction to the load. It means a pump rated for 30 MPa is capable of maintaining flow against a load of about 30 MPa with a low system pressure loss and leakage. Also, pumps have a power density about 10 times greater than an electric motor in terms of volume efficiency. Common types of hydraulic pumps are listed in Table 2. Therefore, pump design techniques, efficiency, reliability, price, and operating conditions are researched by many groups and industries. Modern pump techniques are desirable as the ‘heart’ of the hydraulic machinery.
Effective approaches, or models for studying pump characteristics, are developed by researchers continuously. Some pioneering work on calculating the cylinder pressure during the trapping period between ports was proposed by Yamaguchi (1966). In his work, the efficiencies of pumps and motors were defined with the compressibility of flow, trapping, and relief grooves. The effect of relief groove design on pump power losses and pump noise can be predicted by using this model. This model has been referred to, and successfully applied in lots of further research (Zeiger and Akers, 1985; Schoenau et al., 1990; Manring and Zhang, 2001; Manring, 2003). A comprehensive study of the design and analysis of hydrostatic pumps and motors is described in (Ivantysyn and Ivantysynova, 2003), which provides the understanding of the working principles of pumps and motors and the design of displacement machines in accordance with the state-of-the-art analysis and modern design methods.
In the last 10 years, pump optimization research has been gradually increased in terms of pump efficiency, power loss, and noise level. Many useful tools have been developed for analyzing and optimizing pump performance. A simulation tool CASPAR for the calculation of the non-isothermal gap flow in the connected gaps of swash plate type axial piston machines has been developed (Wieczorek and Ivantysynova, 2002). It is an effective and useful approach to support the development of the pump. CASPAR allows the improvement of the efficiency of swash plate machines by optimization of the gap macro geometry without lengthy testing (Lasaar and Ivantysynova, 2002). However, the software only considers viscous friction, and elasto hydrodynamic effects are not considered. Using CASPAR, for example, the tribological system formed by the piston/cylinder assembly of swash plate axial piston machines, and the influence of a piston macro and micro geometry variation on energy dissipation can be effectively investigated (Ivantysynova and Lasaar, 2004). Besides CASPAR, some particular tools, such as HYGESim (HYdraulic GEar machines Simulator), are being developed for analyzing gear pump performance with high accuracy of simulation (Devendran and Vacca, 2013). It is a multi-domain simulation model for the detailed analysis of external gear machines. HYGESim was successfully utilized to optimize the design of the grooves on the lateral bushes presented in (Vaseena and Vacca, 2010) and an external spur gear pump shown in (Devendran and Vacca, 2013).
The future research areas and challenges for pump and motor design will include the accuracy of the pump and motor mathematical model (Eaton et al., 2006), novel design, and optimization techniques (Mauck et al., 2000). Also, high bandwidth pump control techniques will be more and more addressed and investigated in further research (Ivantysynova, 2008). The trend in pump design is gradually moving towards high efficiency, low noise, compactness, and light weight.
Hydraulic control valves are designed to route the fluid to the desired actuator in systems. They usually consist of a spool which can slide to a different position in the housing to control the fluid. The spool position may be actuated by mechanical levers, hydraulic pilot pressure, or solenoids which push the spool left or right. They can be catalogued in different ways in terms of their design techniques, functions, and materials. It is widely recognized that the control valve is one of the most expensive and sensitive parts of a hydraulic circuit.
For example, a high-speed on-off valve is essential for modern digital fluid power systems. Control using high-speed and high flowrate on-off valves has been proposed as a way to significantly improve the efficiency of fluid power systems over a conventional valve control approach in switched inertance hydraulic systems (SIHS) (Pohl et al., 2001; Scheidl et al., 2008a; 2008b; Johnston, 2009; Pan et al., 2014a; 2014b). This will be described in Section 2.2. Cao et al. (2005) suggested that energy can be lost in an on-off valve system as a result of valve transition times, fluid compressibility, pressure drops across the on-off valves, and hysteresis in the accumulator bladder, fluid friction, and leakage. A technology for the smart-design of the high bandwidth on-off valve is desirable. A four-way rotary valve was developed by Brown et al. (1988) with a desired switching frequency of 500 Hz. Winkler (2004) later presented a high flowrate and low resistance poppet valve that used multiple metering edges to provide a flowrate of 45 L/min at a pressure drop of just 0.5 MPa. The valve spool was driven by a solenoid with a rise time of approximately 1 ms. Tu et al. (2007; 2012) described a fluid driven pulse width modulation (PWM) on/off valve based on an unidirectional rotary spool. The valve was expected to give a flowrate of 40 L/min with a low-pressure drop (0.62 MPa), and high speed of 2.8 ms (transition time at 100 Hz PWM frequency). A seat type valve, which switches on and off within 1 ms at a nominal flowrate of 100 L/min at 0.5 MPa pressure drop, was also developed by Winkler and Scheidl (2007). Kudzma et al. (2012) proposed a 3-way linear-acting high-speed valve which enables low flow resistance (65 L/min at 1.0 MPa) and fast switching speed of 0.69 ms, low leakage and very high flow gain. Such high-speed valves give opportunities for improvements in the efficiency of modern digital hydraulic systems (Kogler and Scheidl, 2008).
More generally, modern valve modeling and design approaches have developed rapidly in the last few decades.
Poppet type metering valves have many advantages, including low leakage, and an economical design and control design is challenging as the metering element is not hydrostatically balanced (Fales, 2005). It is also found that minor geometric differences in the poppet valve can result in very different valve characteristics. In particular, there is a very strong dependence upon the width of the contact area between the poppet and its seat (Johnston et al., 1991). Numerical simulation of fluid flow in poppet valves was studied in (Vaughan et al., 1992), and nonlinear and linear models for the dynamic analysis and design of the poppet valve were proposed in (Fales, 2006; Muller and Fales, 2008; Opdenbosch et al., 2009). These offer effective tools for investigating the performance of poppet valves. Also, newly developed materials were used in the valve development in order to improve valve performance (Tao et al., 2002; Wang et al., 2011). Moreover, various control schemes for controlling hydraulic poppet valves and compensation have been proposed to achieve high bandwidth, fast response, and robustness (Opdenbosch et al., 2004; 2008; Xiong et al., 2015). As can be seen, the poppet valve is a very typical hydraulic component and is widely used in industry. The research of the poppet valve can help researchers and engineers get a better understanding of the operation and physical phenomena of the valve and offer clever industrial solutions.
High-speed solenoid valves have been widely used for their good performance, fast response, and high energy efficiency. A nonlinear dynamic model of a high-speed direct acting solenoid valve was presented in (Vaughan and Gamble, 1996). The solenoid was modeled as a nonlinear resistor/inductor combination, with inductance parameters that change with displacement and current. The spool assembly was modeled as a spring/mass/damper system. This model can accurately predict the spool displacement of a proportional solenoid valve to a voltage input. It enables the development of a new valve from design to performance evaluation before the manufacture of a prototype. In the meantime, methodologies for nonlinear modeling, parameter determination, and performance evaluation of high performance solenoid valves are studied to speed up the design and optimization (Khoshzaban Zavarehi, 1999; Sohl and Bobrow, 1999; Reuter et al., 2010).
New techniques are also investigated for the development of hydraulic actuators that use hydraulic power to facilitate mechanical operation. For example, piezoelectric material has been used in actuators in terms of the usage of the piezoelectric effect (Sirohi and Chopra, 2003). Also, various adaptive control schemes for manipulating hydraulic actuators have been proposed. Earlier research focused primarily on linear control theory. A robust adaptive controller applied to hydraulic servo systems for noncircular machining was introduced in (Tsao and Tomizuka, 1994), and another robust adaptive control scheme was devised in (Plummer and Vaughan, 1996) for the control of hydraulic servo-systems. For a good performance and a high bandwidth control of a hydraulic actuator, a modern motion controller was required (Bobrow and Lum, 1996).
The electro-hydraulic actuator (EHA) is a new high-performance actuation system that combines the benefits of conventional hydraulic systems and direct-drive electrical actuators. It originally was developed for the aerospace industry and expanded applications into many other hydraulic industries. This device eliminates the need for separate hydraulic pumps and tubing, simplifying system structures and improving safety and reliability. EHA is a very promising device that has attracted a lot of research interest from its design to control (Alleyne and Liu, 2000; Habibi and Goldenberg, 2000; Liu and Alleyne, 2000; Niksefat and Sepehri, 2000) and the nonlinearities, system identification, model uncertainties, and disturbances (Ling et al., 2011; Lin et al., 2013). According to recent applications of EHA in Airbus A380 aircraft, engineers still face some unique problems with EHA (Van den Bossche, 2006), which include the performance and life of the pump, efficiency of the electric motor, reliability of the power electronics, and heat rejection problem. On Aug. 29, 2005, the A380 n°1 flew for the first time simulating a dual hydraulic system failure, the control surfaces being driven by the EHA and electrical back-up hydraulic actuator. The aircraft operated with no significant difference in the servo control hydraulic system (Van den Bossche, 2006).
Two examples of fluid power systems are presented in this section to give a general idea of modern fluid power techniques and applications. Technical details can be found in related references.
In 2011 Prof. Scheidl and his colleagues presented a Forward Look article titled ‘Is the future of fluid power digital?’ (Scheidl et al., 2011) which is a response to Achten’s Forward Look article ‘Convicted to innovation in fluid power’ in 2010 (Achten, 2010). Scheidl et al. (2011) believed that digital fluid power is a new branch of fluid power, which offers high potential for innovative solutions. And successful digital fluid power applications need new components, a comprehensive understanding of the system and novel control strategies or principles.
The digital hydraulic system has several advantages compared with continuous or analogue technologies, such as higher efficiency, precision, robustness, and reliability. It also offers new functionality that is impossible with existing fluid power systems, such as sensorless incremental actuation and digital control for multiple units’ arrangements. Today, digital fluid power applications have gradually entered into our industries and market. It is certain that they will become much more digital and provide new opportunities for fluid power engineering in the future (Scheidl et al., 2011).
Hydraulic switching control is a sub-domain of digital hydraulics (Scheidl et al., 2013). An SIHS is a typical example of digital fluid power systems. It performs analogously to an electrical ‘switched inductance’ transformer, and is one possible approach to raise efficiency (Brown, 1987; Scheidl et al., 2008b; Johnston, 2009). This technique makes use of the inherent reactive behaviour of hydraulic components. A fluid volume can have a capacitive effect, whilst a small diameter line can have an inductive effect (Johnston, 2009). High-speed switching valves are needed to achieve the sufficient switching frequencies (Pan et al., 2014a; 2014b). It uses a fast switching valve to control flow or pressure, and is potentially very efficient as it does not rely on dissipation of power by throttling.
Fig. 3 shows two basic modes of SIHS, a flow booster and a pressure booster, which are configured by reversing the inlet and outlet connections in a three-port SIHS (Johnston, 2009). They consist of a long, small diameter ‘inertance’ tube and a high-speed switching valve with one common port, two switched ports, and an accumulator. The common port is connected alternately to the high pressure supply port and then the reservoir, operating cyclically and rapidly such that the high and low pressures are opened alternately. The delivery port might connect to a loading system, and includes an accumulator or other capacitance in order to maintain a constant loading pressure.
Fig. 3Schematic diagram of SIHS: (a) Flow booster; (b) Pressure booster (Reprinted from ( Pan et al., 2014b ), Copyright 2014, with permission from SAGE)
Q HP : flowrate from the high pressure supply port; Q LP : flowrate from the low pressure supply port; Q DELIVERY : delivery flowrate; Q RETURN : return flowrate; Q SUPPLY : supply flowrate
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When the valve is connected to the high-pressure supply port, flow passes from the high pressure supply to the delivery port and the fluid accelerates in the inertance line. When the valve is open to the reservoir, fluid is drawn from the reservoir to the delivery port by the momentum of the fluid in the inertance tube (Pan et al., 2014a; 2014b). As long as the valve is switched quickly, the delivery flow will only reduce slightly due to a small deceleration of fluid velocity when connected to the low-pressure supply, remaining a constant delivery flow. The physical characteristics of the SIHS provide the opportunity for high efficiency.
Digital pump development is another possible future trend in hydraulic systems. As most hydraulic system loads need variable flow for proper operation, it is conventional to control the flow using valves which alter the flow at the expense of energy loss. The digital displacement technique has been proposed for improving hydraulic system energy for transferring energy between mechanical and fluid power (Rampen et al., 1994; Ehsan et al., 2000; Linjama and Huhtala, 2009). Digital pump/motors aim to increase the efficiency and range of operation of fluid power systems by minimizing leakages, friction losses, and compressibility losses. Digital pumps can be implemented similarly to digital valves (Linjama, 2011). The fixed displacement pump is controlled by the high-speed switching on-off valve, which enables the flowrate to continuously switch between the main system and the reservoir. More generally, arranging digital pumps on the same axis and controlling by individual switching on-off valves, the flowrate can be modulated by using different control coding strategies. For the piston pump, the digital control technique is to control each piston of the pump independently by using switching on-off valves. It can be catalogued to the ‘pure pump’ and ‘pump-motor’ modes in terms of piston functions including pump, idle, and motor modes. The motor mode requires continuous switching of the control valves (Linjama, 2011). Artemis Intelligent Power Ltd. started research in the development of piston digital pump/motors in the 1980s and the first publications were in 1990 (Rampen and Salter, 1990). This technique enables innovative solutions for mobile equipment systems (Wadsley, 2011). The piston digital pump/motors research has also been investigated in Purdue University recently (Merrill and Lumkes, 2010; Merrill et al., 2013). The advantages and disadvantages of the different operating strategies and the design trade-offs for digital pump/motors were studied.
The MIT Leg Laboratory is well known for its milestone work of Prof. Marc Raibert, who showed in the 1980s that robotic running could be accomplished using appropriate control strategies and algorithms (Pratt, 2000). His 3D biped is shown in Fig. 4a. Later, Prof. Raibert left MIT to form Boston Dynamics which is an engineering and robotics design company that is best known for the development of BigDog, as shown in Fig. 4b. These two robots are hydraulically driven.
Fig. 4Typical hydraulic driven robots
(a) Marc Raibert’s 3D biped (Image courtesy of Marc Raibert and the MIT Leg Laboratory, 1984) (Reprinted from (Pratt, 2000), Copyright 2000, with permission from IEEE); (b) BigDog-Boston Dynamics (http://en.wikipedia.org/wiki/Boston_Dynamics)
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Boston Dynamics aims to build unmanned legged robots with rough-terrain mobility superior to existing wheeled and tracked vehicles. The ideal robot would travel anywhere a person or an animal could go using their legs, run for many hours at a time, and carry its own fuel and payload. It would be smart enough to negotiate terrain with a minimum of human guidance and intervention (Raibert et al., 2008). BigDog has onboard systems that pro-vide power, actuation, sensing, controls, and communications. The power supply is a water-cooled two-stroke internal combustion engine that delivers about 15 hp (1 hp=746 W). The engine drives a hydraulic pump which delivers high-pressure hydraulic oil through a system of filters, manifolds, accumulators, and other plumbing to the robot’s leg actuators. It has about 50 sensors which measure the attitude, acceleration of the body, and motion and force of the actuators and also monitor BigDog’s homeostasis. It has successfully performed different locomotion gaits, such as walking, trotting, and bounding, carried up to 154 kg payload and hiked for 2.5 h (Raibert et al., 2008).
Hydraulic Quadruped (HyQ) is a versatile hydraulically powered quadruped robot, which is developed for use as a platform to study the high dynamic motions and the navigation performance of the robot in the Italian Institute of Technology, Italy. Fig. 5 shows the photograph of the HyQ Leg and the CAD model of the robot body with the onboard hydraulic system. The design, component specifications, and experimental validation of the HyQ were described in (Semini et al., 2011).
Fig. 5Hydraulic Quadruped (HyQ) developed in Italian Institute of Technology, Italy (Reprinted from ( Semini et al., 2011 ), Copyright 2011, with permission from SAGE )
(a) Photograph of the HyQ Leg; (b) CAD model of the robot body with the onboard hydraulic system
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The John Deere forestry walking machine is a prototype harvester used in the logging process, as shown in Fig. 6. The walking harvester was designed by John Deere’s research and development unit in Finland in the 1990s (Available from http://forestindustry.com/feature-article/world-s-first-prototype-walking-forest-machine-displayed-john-deere-pavilion). It aimed to create a machine that has the maximum working stability and minimum impact on challenging terrain, such as soft, sloping, and uneven conditions. The sensors in the six articulated legs react automatically to different terrain, while the computer control system controls all walking functions including the direction of movement, the travelling speed, the step height, and gait and the ground clearance. Each foot of the machine has a larger contact area than a conventional wheel, which spreads the load and decreases the amount of ground disturbance, while tires may leave grooves that channel rainwater and create erosion. The Timberjack measuring and control system is used to control the harvester head and the Timberjack Total Machine Control system is used to regulate the machine loader and engine functions in order to optimize machine operation. The operator-friendly controls are incorporated in a single joystick (Available from http://www.theoldrobots.com/Walking-Robot2.html). This innovative machine provides insight into new technologies that are being developed and could possibly change the future of the logging industry and pave the way for future developments in productive and environmentally friendly machines.
The researchers of the Korea Institute of Industrial Technology developed the hydraulic actuated quadruped walking robot qRT-2, which is the front drive machine with hydraulic linear actuators and wheeled back legs, as shown in Fig. 7a. The qRT-2 weighs about 60 kg and can carry over 40 kg of payload. It successfully demonstrated the trotting gaits at a speed up to 1.3 m/s on an even surface and walking at 0.7 m/s on an uneven and ramped surface (Kim et al., 2008). They also developed the 4-leg walking robot P2 in 2010 and investigated the hydraulic flow consumption during the robot’s walking behaviour (Kim et al., 2010). A power optimization scheme on the minimization of the hydraulic flow consumption was applied in P2. Simulated and experimental results show very promising performance. All joints of the robotic legs are actuated by small size hydraulic rotary actuators, as shown in Fig. 7b.
Fig. 7Quadruped walking robots developed by Korea Institute of Industrial Technology
(a) A quadruped walking robot, qRT-2 (Reprinted from (Kim et al., 2008), Copyright 2008, with permission from IFAC); (b) A quadruped walking robot, P2 (Reprinted from (Kim et al., 2010), Copyright 2010, with permission from VDE VERLAG GMBH Berlin)
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In China, a hydraulic quadruped walking robot called ‘Baby Elephant” was developed by Gao’s research team (http://www.china.org.cn/china/2012-11/08/content_27042373.htm) in Shanghai Jiao Tong University. The robot weighs about 130 kg and its length, width, and height are 1.2 m, 0.5 m, and 1 m, respectively. It has 12 degrees of freedom for 4 legs controlled by hydraulic actuators. The lithium battery was applied as the power supply for the 10 kW high-performance motor which was used to drive the hydraulic pump (Chen et al., 2013). It can carry a heavy load and walk through the uneven terrain with a good stability. An energy storing mechanism was introduced in this robot, and the simulated and experimental results show its high efficiency (Chen et al., 2013). More investigation and experiments are continuing for the control strategies and gaits optimization. Another hydraulically actuated quadruped bionic robot was developed by the Robotics Centre of Shandong University. It aims to develop a high dynamic quadruped robot which is able to work in complex terrains with good adaptability. It can walk with an average speed of 1 m/s and a maximum speed of 1.8 m/s without a heavy load and 0.4 m/s with a load of 80 kg (Li et al., 2011). The research team in the Beijing Institute of Technology investigated the energy consumption of the designed quadrupedal robot and the results have been published in (Sha et al., 2013).
Good research outputs and achievements have been obtained in the last 20 years. Techniques such as terrain sensors, sophisticated computing, power systems, advanced actuators, and effective dynamic controls are still needed to meet the goals of ideal robot development.
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