Generally, the valve controlled electro-hydraulic systems are able to achieve high motion tracking precision, but suffer from large amount of throttling losses. As an alternative, although the direct pump controlled systems are highly energy efficient, it is far more complicated to attain high tracking precision. In order to achieve both objectives of high precision tracking performance and high energy efficiency, a kind of novel pump and valve combined electro-hydraulic system is developed in this paper. The basic working principles are that the pump supplies the majority amount of flow driving the actuator in a highly efficient way, while the valves are involved to ensure the tracking precision. The controller with four parts is designed to handle the challenges from not only the common nonlinear properties of the hydraulic system, but also the additional flexibilities brought by the hardware combination. Two sets of comparative experiments are conducted to show the advantages of the proposed method by tracking both slow and fast motion reference trajectories. The results indicate that the proposed system is able to realize the same high level of tracking precision as the valve control method, but consumes about 35%-47% less of energy.

The rapid industrial development in the fields, such as robotics and high-tech manufacturing, has put forward much higher requirements for the application of high-performance electrohydraulic system, which should not only achieve high control accuracy but also work in a highly energy-efficient way. As we know, a valve control method could achieve good accuracy, but the low energy efficiency is its essential drawback. Conversely, a pump control method could achieve good energy efficiency, but the achievable accuracy is the drawback. Since neither individual valve control nor individual pump control method can meet the dual requirements, a novel idea of advanced valves and pump coordinated hydraulic control design is developed in this article. Within the control details, the high-performance adaptive robust control law is synthesized to handle the nonlinearities and uncertainties of the electrohydraulic system, and the optimized command generation approach for the valves and the pump is developed to attain the high level of energy efficiency while considering the control performance. Essentially, the control command of the pump is based on the desired model compensation to provide the majority amount of flow in an energy-efficient way, and control commands of the valves are generated directly by tracking errors to guarantee the control accuracy. The theoretical performance of the overall system control design is analyzed. Comparative experiments are conducted, and impressive results are obtained, showing that the proposed system could achieve even better tracking accuracy than the typical valve-controlled system, while the energy efficiency reached a high level. The advantages of the advanced system control design can be verified both in theory and experiments.

Hydraulic manipulators are usually applied in heavy-load and harsh operation tasks. However, when faced with a complex operation, the traditional proportional-integral-derivative (PID) control may not meet requirements for high control performance. Model-based full-state-feedback control is an effective alternative, but the states of a hydraulic manipulator are not always available and reliable in practical applications, particularly the joint angular velocity measurement. Considering that it is not suitable to obtain the velocity signal directly from differentiating of position measurement, the low-pass filtering is commonly used, but it will definitely restrict the closed-loop band-width of the whole system. To avoid this problem and realize better control performance, this paper proposes a novel observer-based adaptive robust controller (obARC) for a multi-joint hydraulic manipulator subjected to both parametric uncertainties and the lack of accurate velocity measurement. Specifically, a nonlinear adaptive observer is first designed to handle the lack of velocity measurement with the consideration of parametric uncertainties. Then, the adaptive robust control is developed to compensate for the dynamic uncertainties, and the close-loop system robust stability is theoretically proved under the observation and control errors. Finally, comparative experiments are carried out to show that the designed controller can achieve a performance improvement over the traditional methods, specifically yielding better control accuracy owing to the closed-loop band-width breakthrough, which is limited by low-pass filtering in full-state-feedback control.

Direct-driven electro-hydraulic systems have a wide range of applications owing to their advantages of energy-savings and relatively high control flexibilities in comparison with classic variable displacement pump-controlled hydraulic systems. However, the control accuracy is limited by the inherent nonlinear hydraulic dynamics. Additionally, the pump flow rate may become nonlinear at low pump speeds, causing large pressure-related flow deviations, thereby limiting the improvement of motion control accuracy. Unfortunately, the pump flow nonlinearity has been ignored or over-simplified without effective modeling in most studies so far. To improve the control accuracy of direct-drive hydraulic systems, a high-precision control strategy must be designed to deal with the nonlinear characteristics and resolve the issue of nonlinear pump flow at low speeds. This study proposes an adaptive robust motion control strategy for a servo direct-driven electro-hydraulic system, using adaptive pump flow rate model compensation. A backstepping integrated direct/indirect adaptive robust controller is designed to deal with the dynamic nonlinearities and uncertainties, which guarantees the stability of the entire hydraulic system. Furthermore, a parameterized polynomial fitting modeling strategy is proposed to accurately describe the nonlinear characteristics of the pump flow rate. Therefore, the uncertain parameters are adjusted in real-time, achieving satisfactory parameter estimations and model compensation for asymptotic motion tracking. Theoretical proof and comparative experiments demonstrate the advantages of the proposed control strategy with adaptive polynomial fitting model compensation for high-precision motion control.

The independent meter-in and meter-out valves control approach makes the two chamber pressures of the hydraulic actuator completely controllable, and offers possibilities for energy conservation. However, the entire flow rate is throttling controlled by the metering valves, which leads to the essential difficulty to realize the high level of energy efficiency further. On the other hand, although the direct pump controlled system might be an option with high energy efficiency, the poor dynamic response and complicated controller design restricts its applications in precise motion occasions. To further improve the energy efficiency as well as guarantee the high precision position tracking performance, a novel hardware configuration which combines the independent metering valves and the direct pump control method is proposed in this paper, which is essentially different from our previous works and other independent metering researches. The motion control of the single-DOF cylinder actuator is studied to verify the effectiveness and advantages of the proposed system. During the tracking process, the pump is controlled by the reference velocity feedforward method and provides the majority amount of the flow rate without throttling losses, while the rest amount is accurately controlled by the independent metering valves in the closed-loop for high precision tracking. Since the hydraulic system is highly nonlinear with uncertainties and disturbances, the high-performance adaptive robust control approach is applied in the closed-loop controller. Besides, the mode selection method and the flow distribution law are involved to deal with the enhanced flexibilities brought by the proposed configuration. Comparative experiments are carried out and the results show the proposed combined hydraulic system achieves the same level of position tracking precision as the independent metering system with less than a half of total energy consumption.

The independent metering system decouples the meter-in and meter-out flows of the chamber, giving extra control flexibility and can reducing energy consumption while keeping motion control accuracy. The control performance of the independent metering system is significantly determined by the valve characteristics.Valve nonlinearities, such as deadband, nonlinear flow gain, and leakage, may restrict the control accuracy. To address the issue and improve the system control precision, the nonlinearities of the valve need to be further modeled. In this article, a nonlinear valve flow model is proposed to accurately depict the relationships between the flow rate, pressure drop, and input voltage. The valve deadband is compensated by the static inverse function, while a certain polynomial projection function is used to mimic the nonlinear flow curve outside the deadband. To deal with uncertainties and nonlinearities in the independent metering system, an adaptive robust control approach is synthesized. The least-squares estimation law is adopted to online adapt the valve parameter and other system parameters. Comparative experiments were carried out to illustrate the effectiveness of the proposed approach. The results show that the valve flow nonlinearities are further captured by the proposed model, and the independent metering system performed a higher level control precision.