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One major challenge in fluid power is the improvement and optimization of the efficiency of mobile hydraulic systems. Conventional fluid power systems often exhibit relatively low overall efficiencies caused by inefficiencies in the various components, such as a prime mover, variable displacement pump, valves, fittings, hoses, and actuators. While each component contributes to the losses in the overall system, the pump converts the mechanical shaft energy from the prime mover to energy transmitted hydraulically and is one of the most crucial components impacting overall system efficiency. Using on/off technologies, new pump architectures have enabled the opportunity to increase the efficiency over conventional designs using positive sealing valves in place of conventional port plate designs. This work proposes, investigates, and assesses the development and optimization of a digital variable displacement pump using a novel cam actuation technique on radial piston pumps. The novelty of this work is the development and parameter optimization of a mechanically actuated digital radial piston pump that can achieve high efficiencies from minimum to maximum displacement compared to common conventional variable displacement pump technologies. In this study, a sensitivity analysis is conducted to study the parameters of the system to optimize the pump. The parameters assessed in this study include: the valve bore size, cam transition and compression angles, piston diameter, and dead volume in the pumping chamber. The simulation results show that after optimizing the parameters of the system, the pump in design could reach a maximum efficiency of approximately 93% and was capable of upholding efficiencies above 80% between 30–100% displacement. [ABSTRACT FROM AUTHOR]

Robotic legs, such as for lower-limb exoskeletons and prostheses, have bimodal operation: (1) within a task, like for walking (high speed and low force for the swing phase and low speed and higher force when the leg bears the weight of the system); (2) between tasks, like between walking and sit–stand motions. Sizing a traditional single-ratio actuation system for such extremum operations leads to oversized heavy electric motor and poor energy efficiency at low speeds. This paper explores a bimodal actuation concept where a hydrostatic transmission is dynamically reconfigured using custom motorized ball valves to suit the requirements of a robotic leg with a smaller and more efficient actuation system. First, this paper presents an analysis of the mass and efficiency advantages of the bimodal solution over a baseline solution, for three operating points: high-speed, high-force, and braking modes. Second, an experimental demonstration with a custom-built actuation system and a robotic leg test bench is presented. Control challenges regarding dynamic transition between modes are discussed and a control scheme solution is proposed and tested. The results show the following findings: (1) The actuator prototype can meet the requirements of a leg bimodal operation in terms of force, speed, and compliance while using smaller motors than a baseline solution. (2) The proposed operating principle and control schemes allow for smooth and fast mode transitions. (3) Motorized ball valves exhibit a good trade-off between size, speed, and flow restriction. (4) Motorized ball valves are a promising way to dynamically reconfigure a hydrostatic transmission while allowing energy to be dissipated. [ABSTRACT FROM AUTHOR]

In offshore winch drive applications, determining the required number of pistons in digital displacement motors is critical for minimizing torque ripples. Digital displacement motors have shown promise for improving energy efficiency for offshore operations, such as placing equipment on the seabed or mineral drilling. However, they are known for exhibiting significant torque ripples, which can affect load-handling precision. This paper estimates the required number of pistons for realizing a digital hydraulic winch drive based on information from a commercial winch. The proposed drive employs full-stroke displacement strategies at high speeds and partial-stroke at low speeds. By simulating steady-state operations, this study correlates torque output with position oscillations. The results show that 37 pistons are required to keep position oscillations below a benchmark threshold of 10 mm throughout the drive's operating range to avoid hindering the drive's performance. However, such a high piston count could result in high costs due to the large, expensive valves required for partial-stroke operations. Therefore, this paper suggests an alternative drive topology for future research, which could potentially reduce the number of pistons that are operated with partial strokes. [ABSTRACT FROM AUTHOR]

Digital hydraulics is a technology gaining perceptible growth in fluid power research. The advantages of digital fluid power systems can be realized through improved system efficiencies, energy savings, increased productivity, and system performance compared to traditional fluid power systems. Conventional check valve pumps use differential pressures to deliver pressurized flow to the system. Digital fluid power pumps enable conventional check valve pumps to achieve variable displacements by enhancing the controllability of the inlet and outlet valves through digital hydraulic technologies and techniques. The benefit of this technology is the use of positive sealing check valves with lower leakage losses compared to typical variable displacement pumps, increasing the unit's overall efficiency. The primary focus of prior digital pump/motor research has been on digital actuation using electronic solenoids to actuate or latch the valves. While these electrical systems provide a platform for digital hydraulic techniques, they come with a cost: added energy sources, advanced controls, and expensive data acquisition systems. Research has also shown that minor valve timing inconsistencies can limit the potential energy savings of digital pumps in electrically actuated systems. A system configuration that promotes the advantages of digital hydraulics while mitigating the disadvantages associated with electrical systems is mechanically actuated systems. This work discusses variable cams and their advantages/disadvantages in digital radial piston pump/motor technologies. The significance of this work is the investigation of the digitalization of radial piston pumps through mechanically actuated valving systems, which has yet to be implemented in prior research. This paper evaluates various design concepts for commercializing digital radial piston pumps using mechanically actuated cams. A two-quadrant pump and a four-quadrant pump/motor design are simulated to assess their potential efficiency across the bandwidth of their displacement. The results show that the two systems can achieve relatively high efficiencies across their displacement bandwidth but show room for further improvement by optimizing these systems. This study is the first step in designing an integrated mechanically actuated variable cam system in digital radial piston pumps. [ABSTRACT FROM AUTHOR]

Digital hydraulics is a discrete technology that integrates advanced dynamic system controls, digital electronics, and machine learning to enhance fluid power systems' performance, overall efficiency, and controllability. A mechanically actuated inline three-piston variable displacement digital pump was previously proposed and designed. The inline three-piston pump incorporates complex mechanical and hydraulic subsystems and highly coupled mechanisms. The complexity of the utilized subsystems poses challenges when assessing the viability of the conceptual design. Therefore, this work focuses on designing, developing, and implementing a collaborative virtual platform involving a digitized module showcasing the internal mechanical structure of the digital pump utilizing mixed reality (MR) technology. MR technology is acknowledged as the forthcoming evolution of the human–machine interface in the real–virtual environment utilizing computers and wearables. This technology permits running simulations that examine the complexity of highly coupled systems, like the digital pump, where understanding the physical phenomenon is far too intricate. The developed MR platform permits multiple users to collaborate in a synchronized immersive MR environment to study and analyze the applicability of the pump's design and the adequacy of the operated mechanisms. The collaborative MR platform was designed and developed on the Unity game engine, employing Microsoft Azure and Photon Unity Networking to set up the synchronized MR environment. The platform involves a fully interactive virtual module on the digital pump design, developed in multiple stages using Microsoft's Mixed Reality Tool Kit (MRTK) for Unity and deployed in the synchronized MR environment through a HoloLens 2 MR headset. A research study involving 71 participants was carried out at Purdue University. The study's objective was to explore the impact of the collaborative MR environment on understanding the complexity and operation of the digital pump. It also sought to assess the effectiveness of MR in facilitating collaboration among fluid power stakeholders in a synchronized digital reality setting to study, diagnose, and control their complex systems. Surveys were designed and completed by all 71 participants after experiencing the MR platform. The results indicate that approximately 75% of the participants expressed positive attitudes toward their overall MR platform experience, with particular appreciation for its immersive nature and the synchronized collaborative environment it provided. More than 70% of the participants agreed that the pump's collaborative MR platform was essential for studying and understanding the complexity and intricacy of the digital pump's mechanical structure. Overall, the results demonstrate that the MR platform effectively facilitates the visualization of the complex pump's internal structure, inspection of the assembly of each of the involved subsystems, and testing the applicability of the complicated mechanisms. [ABSTRACT FROM AUTHOR]

Offshore winch drives require high energy efficiency and control precision, making digital displacement motors an attractive solution due to their high efficiency and potential controllability. However, the response time and the realized energy efficiency of these motors are heavily dependent on the chosen displacement control strategy, especially at low-speed operation. This paper considers various displacement control strategies to investigate whether digital displacement motors are a viable solution for offshore winch drive applications. The motor specifications are derived based on the requirements of a commercial offshore winch drive system. The analysis reveals that various displacement control strategies should be used across the drive's operational speed range to ensure both satisfactory performance and high efficiency. Full-stroke and partial-stroke strategies are optimal for speeds above 28 rpm and 20 rpm, respectively, but unsuited for lower-speed operation. For speeds below 20 rpm, an improved sequential-stroke strategy is therefore presented. The proposed displacement control strategy provides instantaneous motor response and maintains high energy efficiency, although its robustness is slightly reduced at higher operating speeds above 20 rpm.

Digital hydraulics is a novel alternative to proportional or servovalve-controlled systems in fluid power engineering, providing hydraulic systems with high-energy efficiency, good controllability, and insensitivity to contamination. Switched inertance hydraulic converters (SIHCs) are new digital hydraulic devices that can adjust flow and pressure by digital switching instead of throttling the flow. In this paper, an efficient closed-loop control system is proposed for SIHCs subject to time-varying loading conditions in which the load pressure and/or flow varies with time. The control system is designed to operate SIHCs at optimized switching frequencies and ratios that maximize system efficiency when the load varies. With the proposed controller, the SIHC can effectively adapt to the time-varying load and has achieved up to 10% efficiency improvement and up to 65% pressure ripple reduction without affecting the system's dynamic responses, compared with using a nonoptimized controller. The work shows the feasibility and advantages of simultaneously controlling the switching ratio and switching frequency of SIHCs with a time-varying load. As time-varying loading conditions are commonly found in hydraulic applications, the research outcomes constitute an important aspect in the design and development of highly efficient SIHCs and their practical use in hydraulic machinery. [ABSTRACT FROM AUTHOR]

Valve switching is one of the most important power losses in digital hydraulics. In this paper, a dynamic Zero-Flowrate-Switching (ZFS) control method is developed for hydraulic/pneumatic systems. The proposed control strategy is to actively reduce the flowrate passing through the valve to zero before switching off the valve. To realize it, an accessory line, in which a RLC oscillator is applied to generate a sinusoidal flowrate, is allocated parallel to the main control line for flowrate regulation. The pressure, with high frequency and small amplitude wave outputs from a regular piston pump, is used to drive the RLC oscillator. To evaluate the performance of this strategy, mathematical models for switching power loss and pressure pulses are developed and characteristic analysis is conducted. The results show that the energy loss of the system when applying the dynamic ZFS controller is reduced to 16% compared to that of a normal hydraulic system without a dynamic ZFS controller; moreover, pressure pulses in a dynamic ZFS system are much more minor than those in a Hard-Switching system. [ABSTRACT FROM AUTHOR]

Digital hydraulics is a discrete technology that integrates advanced dynamic system controls, digital electronics, and machine learning to enhance fluid power systems’ performance, overall efficiency, and controllability. A mechanically actuated inline three-piston variable displacement digital pump was previously proposed and designed. The inline three-piston pump incorporates complex mechanical and hydraulic subsystems and highly coupled mechanisms. The complexity of the utilized subsystems poses challenges when assessing the viability of the conceptual design. Therefore, this work focuses on designing, developing, and implementing a collaborative virtual platform involving a digitized module showcasing the internal mechanical structure of the digital pump utilizing mixed reality (MR) technology. MR technology is acknowledged as the forthcoming evolution of the human–machine interface in the real–virtual environment utilizing computers and wearables. This technology permits running simulations that examine the complexity of highly coupled systems, like the digital pump, where understanding the physical phenomenon is far too intricate. The developed MR platform permits multiple users to collaborate in a synchronized immersive MR environment to study and analyze the applicability of the pump’s design and the adequacy of the operated mechanisms. The collaborative MR platform was designed and developed on the Unity game engine, employing Microsoft Azure and Photon Unity Networking to set up the synchronized MR environment. The platform involves a fully interactive virtual module on the digital pump design, developed in multiple stages using Microsoft’s Mixed Reality Tool Kit (MRTK) for Unity and deployed in the synchronized MR environment through a HoloLens 2 MR headset. A research study involving 71 participants was carried out at Purdue University. The study’s objective was to explore the impact of the collaborative MR environment on understanding the complexity and operation of the digital pump. It also sought to assess the effectiveness of MR in facilitating collaboration among fluid power stakeholders in a synchronized digital reality setting to study, diagnose, and control their complex systems. Surveys were designed and completed by all 71 participants after experiencing the MR platform. The results indicate that approximately 75% of the participants expressed positive attitudes toward their overall MR platform experience, with particular appreciation for its immersive nature and the synchronized collaborative environment it provided. More than 70% of the participants agreed that the pump’s collaborative MR platform was essential for studying and understanding the complexity and intricacy of the digital pump’s mechanical structure. Overall, the results demonstrate that the MR platform effectively facilitates the visualization of the complex pump’s internal structure, inspection of the assembly of each of the involved subsystems, and testing the applicability of the complicated mechanisms.

Digital hydraulics is a technology gaining perceptible growth in fluid power research. The advantages of digital fluid power systems can be realized through improved system efficiencies, energy savings, increased productivity, and system performance compared to traditional fluid power systems. Conventional check valve pumps use differential pressures to deliver pressurized flow to the system. Digital fluid power pumps enable conventional check valve pumps to achieve variable displacements by enhancing the controllability of the inlet and outlet valves through digital hydraulic technologies and techniques. The benefit of this technology is the use of positive sealing check valves with lower leakage losses compared to typical variable displacement pumps, increasing the unit’s overall efficiency. The primary focus of prior digital pump/motor research has been on digital actuation using electronic solenoids to actuate or latch the valves. While these electrical systems provide a platform for digital hydraulic techniques, they come with a cost: added energy sources, advanced controls, and expensive data acquisition systems. Research has also shown that minor valve timing inconsistencies can limit the potential energy savings of digital pumps in electrically actuated systems. A system configuration that promotes the advantages of digital hydraulics while mitigating the disadvantages associated with electrical systems is mechanically actuated systems. This work discusses variable cams and their advantages/disadvantages in digital radial piston pump/motor technologies. The significance of this work is the investigation of the digitalization of radial piston pumps through mechanically actuated valving systems, which has yet to be implemented in prior research. This paper evaluates various design concepts for commercializing digital radial piston pumps using mechanically actuated cams. A two-quadrant pump and a four-quadrant pump/motor design are simulated to assess their potential efficiency across the bandwidth of their displacement. The results show that the two systems can achieve relatively high efficiencies across their displacement bandwidth but show room for further improvement by optimizing these systems. This study is the first step in designing an integrated mechanically actuated variable cam system in digital radial piston pumps.

In offshore winch drive applications, determining the required number of pistons in digital displacement motors is critical for minimizing torque ripples. Digital displacement motors have shown promise for improving energy efficiency for offshore operations, such as placing equipment on the seabed or mineral drilling. However, they are known for exhibiting significant torque ripples, which can affect load-handling precision. This paper estimates the required number of pistons for realizing a digital hydraulic winch drive based on information from a commercial winch. The proposed drive employs full-stroke displacement strategies at high speeds and partial-stroke at low speeds. By simulating steady-state operations, this study correlates torque output with position oscillations. The results show that 37 pistons are required to keep position oscillations below a benchmark threshold of 10 mm throughout the drive’s operating range to avoid hindering the drive’s performance. However, such a high piston count could result in high costs due to the large, expensive valves required for partial-stroke operations. Therefore, this paper suggests an alternative drive topology for future research, which could potentially reduce the number of pistons that are operated with partial strokes.

Digital hydraulic drives are known for their superior efficiency, power density, and robustness. Such advantages make them an attractive alternative (to electric drives) for actuation of exoskeleton devices. This work presents development of a prototype for such a digital hydraulically driven knee exoskeleton and its experimental testing. The device uses two miniature hydraulic cylinders and a novel mechanism to translate the linear motion to rotary motion. The device is controlled via a passive control method in the stance phase and a simplified model predictive control method in the swing phase. In this work, the design of the exoskeleton device is optimized with respect to compactness and weight. Next, the features of the design are further refined to ensure that the device is able to support the operational loads. This design is then realized into a prototype with a mixture of inhouse manufactured parts and procured components. Finally, via experimental tests, the performance of the design and the control strategy are investigated. Certain drawbacks related to valve size and overall weight are observed in the prototype, which will be addressed in future studies. [ABSTRACT FROM AUTHOR]

The switched inertance hydraulic converter is a sub-domain of digital hydraulics which relies on digital switching to adjust pressure or flow instead of the dissipation of power by throttling, providing an energy-efficient alternative to conventional proportional or servo valve-controlled systems. The high-speed switching valve is a key component to realise the digital switching and its switching characteristics has significant effects on the performance of switched inertance hydraulic converters. In this article, the switching characteristics of a high-speed rotary valve are investigated. The switching orifice area of the valve is calculated based on the movement of the valve components with a consideration of the design of the valve body and leakage. This is validated using the computational fluid dynamics model and in experiments. The valve is theoretically modelled considering the switching orifice, leakage, transition throttling and compressibility effect, and the results are validated by computational fluid dynamics and experimental tests. The valve is able to deliver a flow rate of 40 L/min at a pressure drop of 10 bar and switch at the maximum frequency of 317 Hz, with a switching transition time of about 1 ms, which shows promising performance for the use in digital hydraulics. The theoretical and computational fluid dynamics models can assist the design and optimisation of digital high-speed rotary valves, which can be very useful for understanding, analysing and optimising the characteristics and performance of switched inertance hydraulic converters in digital hydraulics. [ABSTRACT FROM AUTHOR]

Valve switching is one of the most important power losses in digital hydraulics. In this paper, a dynamic Zero-Flowrate-Switching (ZFS) control method is developed for hydraulic/pneumatic systems. The proposed control strategy is to actively reduce the flowrate passing through the valve to zero before switching off the valve. To realize it, an accessory line, in which a RLC oscillator is applied to generate a sinusoidal flowrate, is allocated parallel to the main control line for flowrate regulation. The pressure, with high frequency and small amplitude wave outputs from a regular piston pump, is used to drive the RLC oscillator. To evaluate the performance of this strategy, mathematical models for switching power loss and pressure pulses are developed and characteristic analysis is conducted. The results show that the energy loss of the system when applying the dynamic ZFS controller is reduced to 16% compared to that of a normal hydraulic system without a dynamic ZFS controller; moreover, pressure pulses in a dynamic ZFS system are much more minor than those in a Hard-Switching system.

• 1. A novel dual-magnetic circuit actuated fast-switching valve (DAFV) is designed. • 2. A multi-stage excitation control algorithm (MSEC) is proposed. • 3. The mathematical model of DAFV is established in detail. • 4. The key influence parameters of the DAFV characteristics are obtained. • 5. The DAFV and MSEC are verified to improve flow accuracy and reduce power consumption. Solenoid type fast-switching valve is the commonly used control element in digital hydraulics, and its magnetic circuit is completely composed of soft magnetic materials. The current research primarily focuses on reducing the response time of the valve, but it neglects the influence of the relationship between the opening and closing response times on the control accuracy. However, it faces the issue of opening and closing processes asymmetry aggravated by coil severe heating and various operating pressures, which would lead to flow errors and restrict control accuracy. Hence, this paper proposed a novel dual-magnetic circuit actuated fast-switching valve (DAFV) with a multi-stage excitation control algorithm (MSEC). The DAFV utilizes a permanent magnet (PM) to maintain the open state without electrical, avoiding large temperature rises. The variable excitation control of MSEC is achieved by precisely adjusting the duty cycle of both the delay and movement stages, ensuring stability and symmetry in response time. Among in, the duty cycles are set online for each stage to adapt to the operating pressure and target response time. The experiment shows that the response times of opening and closing almost remain the same at approximately 1.16 ms under different operating pressures. The relative flow error is reduced to within 4.1 %, while the maximum temperature rise is 15 ℃. [ABSTRACT FROM AUTHOR]

This paper describes a new type of digital hydraulic valve run by two servo motors. Digital hydraulics is a cuttingedge technology, which saves more exhausted energy than conventional hydraulic valves. It includes conventional valves, but its working principle is different. Similar or different size valves constitute a digital hydraulic valve assembly. When the assigned valves are opened, a certain amount of flow is obtained from the output of the valve assembly. To control a digital hydraulic valve, Pulse Number Modulation (PNM) Control technique is used for equal valve flow rates, while Pulse Code Modulation (PCM) is used for different valve flow rates. Valves are exerted by independently launched electric coils. Previous studies used controller board and external power booster circuits for coils. In this study, a new type of digital hydraulic valve is designed, manufactured, and tested with the PNM method. The studied valve body has two different valve groups. Every group includes 16 equal valves and 1 camshaft rotated by 1 servo motor. The servo motors are controlled by a PLC. The calculated performance index is found to be 5.1ms which is similar to the results of previous studies. The experimental results showed that the cam and servo motor controlled digital hydraulics is applicable to variable speed control hydraulic systems. [ABSTRACT FROM AUTHOR]

This paper describes a new type of digital hydraulic valve run by two servo motors. Digital hydraulics is a cutting-edge technology, which saves more exhausted energy than conventional hydraulic valves. It includes conventional valves, but its working principle is different. Similar or different size valves constitute a digital hydraulic valve assembly. When the assigned valves are opened, a certain amount of flow is obtained from the output of the valve assembly. To control a digital hydraulic valve, Pulse Number Modulation (PNM) Control technique is used for equal valve flow rates, while Pulse Code Modulation (PCM) is used for different valve flow rates. Valves are exerted by independently launched electric coils. Previous studies used controller board and external power booster circuits for coils. In this study, a new type of digital hydraulic valve is designed, manufactured, and tested with the PNM method. The studied valve body has two different valve groups. Every group includes 16 equal valves and 1 camshaft rotated by 1 servo motor. The servo motors are controlled by a PLC. The calculated performance index is found to be 5.1ms which is similar to the results of previous studies. The experimental results showed that the cam and servo motor controlled digital hydraulics is applicable to variable speed control hydraulic systems.

This investigation utilized binary-coded, parallel-connected on-off valves that can achieve high flow rates with fewer valves while addressing flow peak challenges. By considering temperature and refining modeling techniques, the study rectifies certain limitations observed in previous research, such as neglecting temperature, imprecise valve orifice flow coefficients, and absent flow pattern visualization, thereby enhancing flow prediction accuracy. The results for the ML_CFD-based model suggest that although extrapolation challenges exist in rarely data-driven systems, the proposed approach exhibits errors under 5 % across diverse metrics, attributable to the effectiveness of well-constrained overparameterized models and the segmented structure of digital flow control units. On the other hand, while the simplified Hybrid Analytical model shows minor deviations and offers easier implementation, it encounters constraints when processing data beyond its pre-tuned coefficient of discharge values. • Introduced binary-coded, parallel-connected on-off valves for high flow rates in DFCUs. • Enhanced flow prediction accuracy by considering temperature and precise valve orifice flow coefficients. • ML_CFD-based model exhibits errors under 5 % across various metrics. • Simplified Hybrid Analytical model offers easier implementation but has constraints with pre-tuned coefficients. • Combined CFD, Analytical modelling, and machine learning for robust flow rate prediction in DFCUs. [ABSTRACT FROM AUTHOR]

In this paper, the challenges regarding the application of a switched control approach to a digital displacement machine (DDM) are discussed. The system under consideration is initially presented. Subsequently, the reasons for considering the design and application of the specific, switched controller are discussed. Finally, the challenges are presented, which could function as future research.

Digital hydraulics is a recently introduced concept in the field of hydraulics. Some of the solutions used in digital hydraulics have been used for a long time in classical hydraulics, but not at the level at which they can be applied now. The development of the field of digital hydraulics was possible due to the technological advances that the field of electronics has experienced. At present, digital hydraulics intends to offer simpler devices, with less demanding requirements than those of servovalves and with a high reliability and energy efficiency. An intelligent system implies a system that is equipped with various sensors and transducers, connected to a Programable Logic Controller (PLC) that gathers all the information and transmits it to a software interface, from where the user can remotely monitor system parameters, can change their values and generate various reports. Another feature of an intelligent system is the fact that an automatic adjustment loop can be made, which allows it to adapt its own parameters according to needs. In the present paper, the authors started from a digital hydraulic switching valve that they upgraded to become an intelligent hydraulic equipment, which can be controlled, tracked and parameterized remotely via a PLC with Ethernet port and application software. [ABSTRACT FROM AUTHOR]

Hydraulic hybrid drivetrains, which are fluid power technologies implemented in automobiles, present a popular alternative to conventional drivetrain architectures due to their high energy savings, flexibility in power transmission, and ease of operation. Hydraulic hybrid drivetrains offer multiple environmental benefits compared to other power transmission technologies. They provide heavy-duty vehicles, e.g., commercial transportation, construction equipment, wagon handling, drilling machines, and military trucks, with the potential to achieve better fuel economy and lower carbon emissions. Despite the preponderance of hydraulic hybrid transmissions, state-of-the-art hydraulic hybrid drivetrains have relatively low efficiencies, around 64% to 81%. This low efficiency is due to the utilization of conventional variable displacement pumps and motors that experience high power losses throughout the drive cycle and thus fail to maintain high operating efficiency at lower volumetric displacements. This work proposes and validates a new methodology to improve the overall efficiency of hydraulic hybrid drivetrains by replacing conventional pump/motor units with their digital counterparts. Compared to conventional pump/motors, the digital pump/motor can achieve higher overall efficiencies at a wide range of operating conditions. A proof-of-concept digital pump/motor prototype was built and tested. The experimental data were integrated into a multi-domain physics-based simulation model of a series hydraulic hybrid transmission. The proposed methodology permits enhancing the overall efficiency of a series hydraulic hybrid transmission and thus allows for energy savings. Simulating the system at moderate load-speed conditions allowed achieving a total efficiency of around 89%. Compared to the average efficiency of the series hydraulic hybrid drivetrains, our simulation results reveal that the utilization of the state-of-the-art digital pump enables improving the total efficiency of the series hydraulic hybrid drivetrain by up to 25%.

This article concerns high accuracy positioning control with switching optimization for an equal coded digital valve system. Typically, pulse number modulation control cannot realize micro-positioning due to the characteristics of step-wise flow variation, therefore, a new position controller consisting of a model-based pulse number modulation and a differential pulse width modulation strategy is proposed to control the position of a hydraulic cylinder at high and low velocity cases, respectively. In addition, in order to solve several problems caused by the pulse number modulation and differential pulse width modulation, such as increased number of switchings and large difference among number of switchings of valves, a switching optimization consisting of a switching cost function, a circular buffer and a circular switching method is proposed. An adaptive weight of the switching cost function is proposed for the first time to reduce the total number of switchings under different pressure differences and its design criterion is presented. A circular buffer and a new circular switching method are used to improve the degree of equal distribution of switchings when the pulse number modulation and differential pulse width modulation are used, respectively. Comparative experimental results indicated that the average and the minimum positioning error for the proposed controller are only 10 and 1 μm, respectively. The number of switchings and the degree of equal distribution of switchings are significantly optimized. Moreover, the pressure fluctuations caused by the proposed controller remain acceptable. [ABSTRACT FROM AUTHOR]

This article studies a digital hydraulic servo drive driven by a variable speed electric servomotor. Digital displacement control is implemented by using a two-port digital hydraulic power management system having six pistons and 18 on/off control valves. The first port of the digital hydraulic power management system controls the cylinder speed, while the second port is connected to a hydraulic accumulator. The peak power is taken from the accumulator, and the electric servomotor supplies only the average power into the system. An experimentally validated simulation model is used, and the results show a combination of adequate controllability and excellent energy efficiency. The estimated reduction in the size of the electric motor is 57%. [ABSTRACT FROM AUTHOR]

One way to realize inertia in energy saving hydraulic switching converters is a mechanical oscillator connected to a piston. Its two basic advantages over the use of fluid in an inductance pipe are higher compactness and a better decoupling of inductance and capacitance; these are opposed by a more complex valve system, which raises costs if electric control is applied. This paper presents and studies an oscillating mass converter with pure hydraulic control. It features a pressure control function and constitutes a step-up converter. A simple model is established to elucidate the basic properties of the function principle under idealized conditions. The complete system with the hydraulic control concept is studied by an elaborate dynamical model. It is shown that the converter is able to operate in the intended way under the conditions of the mathematical model. A potential application for a load sensing type meter out control of a cylinder drive is sketched.

Despite being widely used in several applications, commercially available spool valves, both servovalves and proportional valves, are inefficient components because they cause high power consumption due to the large pressure drops across the metering orifices. A recent research field aims at substituting spool valves with on/off valves having high switching frequency (changing state between open and closed in a few milliseconds or less) and producing low pressure drops, in order to make the so-called digital hydraulics possible. In spite of the advantages that it could provide, digital hydraulics does not have significative industrial applications yet, because of the difficulty in manufacturing such high frequency on/off valves. Hence, this paper performs a feasibility study of an on/off poppet-type valve actuated directly by a commercially available ring stack, which is a multilayer piezo-actuator capable of generating very high actuation forces needed for this application. Modulation of the average flow can be achieved by changing the duty cycle of the pulse width modulation (PWM) signal driving the piezostack. An inertance tube could also be used to smooth flow pulsation. The simulations obtained using a detailed Simulink model show that high switching frequency and very effective flow modulation can be obtained with this valve architecture along with low pressure drops and high flow rates, thus making it potentially suitable for digital hydraulics. The disadvantages of this single stage architecture are the large dimensions of the piezo stacks, and the high current generated because of both the high capacitance of the piezo stack and the high frequency switching. However, large-scale production of these components could help to reduce the costs, and the simulations show that limiting the maximum current to 10 A still provides good regulation.

ITER-RH system is used to exchange the divertor's 54 cassette assemblies in the vessel. Water hydraulics and servo valves are currently used in the task but concerns exist with robustness and availability of suitable components. An alternative option is to use digital technology where on/off valves and intelligent control is used to produce proportional output. So far, no proper high-pressure on/off valves existed in the market and therefore a new concept was developed and tested with promising results. A mock-up of the remote handling system was used as test bench. The system was simulated in order to dimension the valve system and to tune the valve controller. A complete valve package with 16 prototype on/off valves and a manifold was manufactured and long-term tests of 60 working days was conducted. A new level of performance was demonstrated throughout the tests as tracking accuracies were approximately ten times better than with servo valve. Although some design faults where detected and system had faulty components, tracking accuracy was very good. Tracking and positioning capability of digital valve system exceeds all requirements and seems applicable for ITER RH application. [ABSTRACT FROM AUTHOR]

Recent advancement in controllability of digital hydraulic is similar to the performance of a proportional/servo hydraulic system, and several studies show that digital hydraulic will be an alternative for proportional/servo hydraulic. In this paper, tip point tracking of a single-link arm is taken as the subject of the study. Here, the single-link arm is controlled by a digital hydraulic system, which is established with parallel-connected on/off valves. In order to attain stepwise flow control, the pulse code modulation technique is used. By referring to the previous work, the control signal for the trajectory tracking is calculated by taking account of cylinder chamber pressure and velocity. But in this study, the required volume flow rate for trajectory tracing is taken into account for generating control signal. This approach improves the performance of the digital hydraulic system at lower velocity tracking and also reduces the computational complexity. The analysis is conducted with the proposed algorithm for 4-bit and 5-bit digital flow control units and tip point response of single link is presented. The results show that the 5-bit system has significantly better performance than the 4-bit system. In addition, the analysis is conducted with different stroke lengths such as 200, 100 and 50 mm for studying the behaviour of the system at lower velocity tracking. Better controllability is achieved at lower velocity tracking, and the results obtained with the proposed algorithm have nearly 2% tracking error. [ABSTRACT FROM AUTHOR]

The article presents laboratory testing of an experimental digital hydraulic cylinder model with three surfaces that is powered by constant pressure and flow and achieves steps of force and speed actively controlled. The digital hydraulic cylinder has the piston surface divided into three annular, functionally independent areas, and by selecting the combinations of active areas, seven stages of forces or speeds result, that can be controlled by a dedicated distribution block and a dedicated computer system. [ABSTRACT FROM AUTHOR]

Primary control of linear motion by variable speed electric motors driving a hydraulic cylinder via a constant displacement pump is an established and successful concept with a frequent use in industry. One problem arises when low or zero motion speed has to be realized under high pump pressure conditions. Such load scenarios occur frequently in certain pressing processes, e.g., for sintering or veneering. Most pumps have a lower speed limit, below which critical tribological conditions occur which impair lifespan and efficiency. In addition, pump speed control and pump fluctuation suffer from the mixed lubrication conditions in such an operation range. For a circumvention of such low speed pump operation, a digital valve control concept is presented and studied in this paper. Valve control is used in load holding phases with low speed. Pressure is provided by an accumulator which is charged by the pump in short charging cycles at reasonable pump speeds. It is shown that the mean control error during load holding phase lies within the desired band and the fluctuations of the control force are lower than those of the pump control. In addition, the unfavorable pump operation conditions can be avoided via digital control.

This paper reviews recent developments in digital switched hydraulics particularly the switched inertance hydraulic systems (SIHSs). The performance of SIHSs is presented in brief with a discussion of several possible configurations and control strategies. The soft switching technology and high-speed switching valve design techniques are discussed. Challenges and recommendations are given based on the current research achievements. [ABSTRACT FROM AUTHOR]

This study proposes a novel digital hydraulic valve system using multiple equal size on/off valves and a circulating switching control, with an aim to increase the resolution and the linearity of the digital hydraulic valve systems. The solution is founded on the equal coded valve system concept which represents a recent development in the digital hydraulic valve technology. The circulating switching control algorithm is used to overcome nonlinearities occurring in the typical noncirculating switching control and to decrease the operating frequency of single on/off valves. As a result, a substantial improvement in tracking control performance is demonstrated with 8+8 parallel connected valves. The results verify that compact, robust, and high-performance valve control can be realized. [ABSTRACT FROM AUTHOR]

Hydraulic systems are prevalent in agricultural machinery and equipment and can be found transmitting power for vehicle drive wheels, powering attachments, and controlling motion (booms, steering, load height, etc.). Agricultural applications of fluid power have advanced in terms of capability and efficiency, but opportunities remain for significant improvements in efficiency, noise reduction, and reliability. The average system-level hydraulic efficiency of current mobile agricultural machines is only 21.1%. Because nearly all hydraulic systems use pumps to convert engine power to fluid power, improving the efficiency of the pumps (and motors when used as actuators) significantly impacts the system efficiency. This work examines the impact of using more efficient digital pump/motors to improve the overall efficiency of agricultural equipment, such as tractors, harvesters, planters, fertilizers, sprayers, and attachments. Maintaining higher pump/motor efficiency throughout the operating range is the central principal for the energy savings. Currently used variable-displacement pumps have low efficiencies at low displacement levels due to constant losses that do not scale with the power produced. Digital pump/motors minimize these inefficiencies because energy losses scale more closely with the power produced. Experimental results indicate an average efficiency of 85% when operating at 20% to 100% displacement. This efficiency is 15% to 20% higher on average than with current variable-displacement axial piston pumps. This study demonstrates that achieving this improvement in the efficiency of the pump/motors used in tractors and harvesters alone would conservatively save $61.7 million worth of energy annually for end users in the U.S. agricultural sector. [ABSTRACT FROM AUTHOR]

A cylinder with a continuously variable piston area is considered by many an impossible goal. This paper presents solutions for digital hydraulic cylinders with certain discrete values of multiple surfaces, which, selected by well established rules, can control the force and speed output values although they are supplied with constant pressure and flow. Also hereinafter we present test diagrams for digital pumps and digital switching directional control valves, as well as a test stand model for multiple-area cylinders, their testing methodology and mathematical modeling of a three-area digital cylinder. [ABSTRACT FROM AUTHOR]

The switched inertance hydraulic system (SIHS) is a novel high-bandwidth and energy-efficient digital device which can adjust or control flow and pressure by a means that does not rely on throttling the flow and dissipation of power. An SIHS can provide an efficient step-up or step-down of pressure or flow rate by using a digital control signal. In this article, analytical models of an SIHS in a four-port high-speed switching valve configuration are proposed, and the system dynamics and performance are investigated theoretically and experimentally. The flow responses, system characteristics, and power consumption can be predicted effectively and accurately by using the proposed models, which were validated by comparing with experiments and with numerical simulation. The four-port configuration is compared with the three-port configuration, and it is concluded that the former one is less efficient for valves of the same size, but provides a bi-direction control capability. As bi-direction control is a common requirement, this constitutes an important contribution to the development of efficient digital hydraulics. [ABSTRACT FROM AUTHOR]