This paper presents the recent advancements in the control of multiple-degree-of-freedom hydraulic robotic manipulators. A literature review is performed on their control, covering both free-space and constrained motions of serial and parallel manipulators. Stability-guaranteed control system design is the primary requirement for all control systems. Thus, this paper pays special attention to such systems. An objective evaluation of the effectiveness of different methods and the state of the art in a given field is one of the cornerstones of scientific research and progress. For this purpose, the maximum position tracking error |e|_max and a performance indicator ρ (the ratio of |e|_max with respect to the maximum velocity) are used to evaluate and benchmark different free-space control methods in the literature. These indicators showed that stability-guaranteed nonlinear model based control designs have resulted in the most advanced control performance. In addition to stable closed-loop control, lack of energy efficiency is another significant challenge in hydraulic robotic systems. This paper pays special attention to these challenges in hydraulic robotic systems and discusses their reciprocal contradiction. Potential solutions to improve the system energy efficiency without control performance deterioration are discussed. Finally, for hydraulic robotic systems, open problems are defined and future trends are projected.

In this paper, a force-sensorless high-performance contact force/motion control approach is proposed for multiple-degree-of-freedom hydraulic manipulators. A rigorous stability proof for an entire hydraulic manipulator performing contact tasks is provided for the first time. The controller design for the manipulator is based on the recently introduced virtual decomposition control approach. As a significant novelty, the end-effector contact force is directly estimated from the manipulator's cylinder pressure data, which provides a practical solution for heavy-duty contact force control without engaging fragile force/torque sensors. In the experiments, the proposed controller achieved a force control accuracy of 4.1% at a desired contact force of 8000 N while in motion. This can be considered a significant result due to the hydraulic actuators' highly nonlinear behaviors, the coupled mechanical linkage dynamics, and the complex interaction dynamics between the manipulator and the environment.

In challenging robotic tasks, high-bandwidth closed-loop control performance of the system is required for successful task completion. One of the most critical factors inhibiting the wide-spread use of closed-loop contact control applications has been the control system stability problems. To prevent unstable system behavior, the need for rigorously addressed manipulator dynamics is substantial. This is because the contact dynamics between a manipulator and its environment can be drastic. In this paper, a novel Cartesian space impedance control method is proposed for hydraulic robotic manipulators. To address the highly nonlinear dynamic behavior of the hydraulic manipulator, the system control is designed according to the subsystem-dynamics-based virtual decomposition control (VDC) approach. The unique features of VDC (virtual power flow and virtual stability) are used to analyze the interaction dynamics between the manipulator and the environment. Based on the desired impedance parameters and stability analysis, an explicit method to design the control gains for the proposed impedance control law is developed. The L2 and L∞ stability is guaranteed in both free-space motions and constrained motions. Experimental results demonstrate that the hydraulic robotic manipulator is capable of adjusting its dynamic behavior accurately in relation to the imposed target impedance behavior. This provides compliant system behavior, which is needed in many dynamically challenging robotic tasks.