Hydraulic systems represent a crucial part of the drivetrain of mobile machines. The most important drivers of current developments, increasing energy efficiency and productivity, are leading to certain trends in technology. On a subsystem level, working hydraulics are utilizing effects by improving control functions and by maximum usage of energy recovery potential. Independent metering and displacement control, partly in combination with hybrid concepts, are the dominating approaches. Traction drives gain advantage from optimized power split transmissions, which consequently are being used in a growing number of applications. On the level of components, increase of efficiency and dynamics as well as power density are important trends. Altogether, design of systems and components is more and more based on modular concepts. In this sense, among others, sensors and control elements are being integrated to actuators; electric and hydraulic technology is being merged. In order to achieve maximum efficiency and performance of the entire machine, control of hydraulics has to include the whole drivetrain and the entire mobile machine in its application. In modern words, mobile hydraulic systems are a part of cyber physical systems.

The paper presents a study of a highly integrated hydraulic powertrain for mobile working machines. The highlight of this new concept is the replacement of separated drive and working pumps in conventional systems with only one displacement pump. After a comparison of different system architectures, the appropriate operating and control strategies for the chosen topology were applied and optimized. With the help of simulations in AMESim, a proof of the function of the hydraulic circuit and the controlling has been established. In addition, the energy saving potential of the integrated hydraulic system is also considered, by comparing the energy consumption of the new and the conventional powertrain in different duty cycles.

<p>The electrification trend affecting off-road vehicles is paving the way toward dedicated electrified hydraulic actuation systems. Although traditional centralized fluid power architectures are still utilized in many applications for low cost, power density, and reliability, nowadays emission policy results in an increasing interest in developing electro-hydraulic actuator (EHA) solutions. EHAs enable non-throttling actuation and energy recuperation during overrunning loads, leading to higher transmission efficiency and lower fuel consumption. These features in energy efficiency make EHAs competitive in meeting emission regulations compared to conventional hydraulic solutions.</p> <p>The key challenge in developing the EHA solution comes from the high cost and space requirements, especially for the adoption of self-contained EHAs in mobile applications. In this study, two architectures for the EHA are proposed, a closed-circuit architecture and an open-circuit one, to determine the most practical and efficient configuration. The most effective open-circuit architecture with distributed concepts is further investigated for implementation, which requires less modification of the mechanical structures and performs more efficiently than the closed-circuit alternative. The proposed EHA is driven by an electro-hydraulic unit (EHU) consisting of a variable-speed electric motor and a fixed-displacement hydraulic pump, which is relatively cost-effective. A novel hydraulic configuration is proposed, which allows the EHA to cover full-speed operating ranges in four quadrants. </p> <p>To verify the EHA design, the behavior of the proposed system should be predicted prior to costly experiments and demonstrations. For this purpose, an integrated simulation model is developed based on the lumped parameter approach in the Amesim environment. The model includes the electric system, the hydraulic system, and the mechanism to be implemented, which are capable of flexible analysis of functionality, efficiency, and thermal performance.</p> <p>In this work, a dedicated test rig for EHA testing is developed. The test rig can help verify EHA performance, test the control algorithm, and diagnose errors before implementing the system on real applications. The experimental results from the test rig also validate the simulation model. An independent load drive of the test rig allows testing all possible  loading conditions of the proposed EHA, thus demonstrating the energy performance in four quadrants. Thermal behavior is investigated with long duty cycles to determine the need for additional cooling equipment. After the validation of the hydraulic configuration, a power electronics setup is added to the test rig, which allows to drive the EHA system with the novel designed EHUs. Validation on the test platform paves the way for implementation in a vehicle. </p> <p>As a final step, the proposed EHA system is implemented in a reference vehicle, a Case New Holland TV380 skid steer loader. A novel designed EHU is adopted to drive the system for technology demonstration. The energy savings capacity of the EHA is investigated in comparison to the baseline measurements of the traditional open-center hydraulic architecture. The impressive savings from the reduction of throttling losses and energy recovery guide the possible commercialization of such EHAs in mobile hydraulic applications. The controller design of the implemented EHA system is investigated with the aim of improving the dynamic performance, e.g., reducing damping oscillation. Basic power management strategies are also studied to integrate EHA with the power train of current hydraulic machines. Regarding future work, based on this research but not within the scope of this study, the proposed EHA system can be adopted with different types of prime movers, such as axial piston machines as the hydraulic part of an EHU. Furthermore, the design approach proposed in this study can help resize the EHA system for other applications with different loading conditions and power requirements, and the energy savings capability can be further investigated. With this, a comprehensive market analysis will be performed for the commercialization of EHA. </p>