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 $|e|_{\rm max}$ and a performance indicator $\rho$  (the ratio of $|e|_{\rm max}$ 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.

A Survey on Control of Hydraulic Robotic Manipulators With Projection to Future Trends

This letter shows accurate and autonomous creation of free-form trenches using a walking excavator. We present hardware extensions and modifications for full automation, a mapping approach specifically tailored to excavation, environment collision-free trajectory planning on these maps, an arm controller aware of various limits and an improved state machine that enables the execution on real hardware. Furthermore, previous work about excavation planning and the design of a single soil-independent dig cycle is extended and transferred from simulation to hardware. The entire system is tested on a four-segment, piecewise-planar trench, and a free-form curved trench. Both shapes were successfully excavated with unprecedented accuracy.

/pdf/autonomous-free-form-trenching-using-a-walking-excavator-1yfd6pgxvi.pdf

Autonomous Free-Form Trenching Using a Walking Excavator

This letter presents a novel planning and control approach for autonomous excavation that is independent of the soil composition and goes beyond a single dig. In contrast to the existing work, which is mostly based on position trajectories for the bucket motion, we define a single dig cycle directly by an end-effector force–torque trajectory. This trajectory applied to different types of soil, resulting in bucket motions that are used by expert operators to adapt to different terrain. Thereby, we can overcome the limitations of current approaches that suffer from the fact that soil interaction forces are dominant and immensely hard to predict or estimate. The end-effector forces and motion of the redundant excavator arm are controlled using a hierarchical optimization approach. Additionally, a large-scale iterative planner is proposed to consecutively execute single digs until a desired ground geometry is achieved. The shown simulation results include multidig excavations that precisely recreate the desired ground shapes. Practical tests are to follow.

Planning and Control for Autonomous Excavation

A common disadvantage of multi-legged robots is that they often lack the manipulation capability. To overcome this limitation, an arm can be added to the body of the multi-legged robot, to perform manipulation tasks and provide assistance for locomotion. First, we proposed an attachment configuration of the arm for a multi-legged robot that provide a uniform workspace in front, below and above the base robot trunk. Second, an integrated control framework promises to keep the mobility and the balance of the mobile platform and provides precise manipulation capability of the arm incorporating a payload estimation scheme. Finally, we verify an integrated control framework with experimental results of a static and walking mobile platform while moving the arm.

Towards a multi-legged mobile manipulator

Energy-efficient and high-precision control of hydraulic robots

This paper presents the realization of the worldwide first automated walking excavator chassis. To this end, the authors build a new generation of high-performance hydraulic valves with integrated pressure feedback to achieve fast and accurate cylinder force tracking. This allows automatically adapting the legs to uneven terrains and optimally shaping the ground reaction forces in order to change the orientation and height of the cabin. Due to the contact redundancy, automated balancing is implemented as a contact force optimization problem, including constraints on contact forces and joint torques. The corresponding prioritized optimization problem can be simplified by using a quasi-static approximation of the system dynamics and a complexity reduction due to the kinematic structure of the legs. Our approach considers the unknown configuration and load of the cabin, arm, and bucket as system disturbances, whereby gravitational effects are approximated as well as possible. It is tested in a Gazebo simulation and validated in different experiments by using a prototype walking excavator machine. The proposed method revolutionizes operator control of these versatile but complex multipurpose vehicles: Instead of manual and coordinatively very demanding cylinder position adjustment, the operator can command simple high-level commands like a cabin pose. Furthermore, it significantly reduces peak forces in the cylinders and at the contact points, which causes less damage to the mechanics and the ground.

/pdf/force-control-for-active-chassis-balancing-3o6bxfuyh3.pdf

Force Control for Active Chassis Balancing

This paper presents the successful implementation of force control strategies on a 12 ton walking excavator to optimize the ground reaction force distribution for better stability, less terrain damage and to reduce the operation complexity. Using cleverly arrange standard industrial valve components to separately control in- and out-flow of the hydraulic cylinders, we achieve accurate and fast joint torque control purely based on pressure feedback. On the full system level, we realize an automated force distribution to adjust the center of pressure and to level out the cabin of the machine. While the operator has still full control over the excavator, this assistance system guarantees permanent ground contact and ideal force distribution among the all four wheels independent of the level of the terrain. The proposed method significantly improves operability of the walking excavator in rough terrain.

Towards optimal force distribution for walking excavators

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Force Control for Active Chassis Balancing

Towards optimal force distribution for walking excavators