This paper presents the design of the hydraulically actuated quadruped robot HyQ2Max . HyQ2Max is an evolution of the 80 kg agile and versatile robot HyQ. Compared to HyQ, the new robot needs to be more rugged, more powerful and extend the existing locomotion skills with self-righting capability. Since the robot's actuation system has an impact on many aspects of the overall design/specifications of the robot (e.g., payload, speed, torque, overall mass, and compactness), this paper will pay special attention to the selection and sizing of the joint actuators. To obtain meaningful joint requirements for the new machine, we simulated seven characteristic motions that cover a wide range of required behaviors of an agile rough terrain robot, including trotting on rough terrain, stair climbing, push recovery, self-righting, etc. We will describe how to use the obtained joint requirements for the selection of the hydraulic actuator types, four-bar linkage parameters, and valve size. Poorly sized actuators may lead to limited robot capabilities or higher cost, weight, energy consumption, and cooling requirements. The main contributions of this paper are: 1) a novel design of an agile quadruped robot capable of performing trotting/crawling over flat/uneven terrain, balancing, and self-righting; 2) a detailed method to find suitable hydraulic cylinder/valve properties and linkage parameters with a specific focus on optimizing the actuator areas; and 3) to the best knowledge of the authors, the most complete review of hydraulic quadruped robots.

Design of the Hydraulically Actuated, Torque-Controlled Quadruped Robot HyQ2Max

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

Robots with legs and arms have the potential to support humans in dangerous, dull or dirty tasks. A major motivation behind research on such robots is their potential versatility. However, these robots come at a high price in mechanical and control complexity. Hence, until they can demonstrate a clear advantage over their simpler counterparts, robots with arms and legs will not fulfill their true potential. In this paper, we discuss the opportunities for versatile robots that arise by actively controlling the mechanical impedance of joints and particularly legs. In contrast to passive elements such as springs, active impedance is achieved by torque-controlled joints allowing real-time adjustment of stiffness and damping. Adjustable stiffness and damping in real-time is a fundamental building block towards versatility. Experiments with our 80 kg hydraulic quadruped robot HyQ demonstrate that active impedance alone i.e. no springs in the structure can successfully emulate passively compliant elements during highly dynamic locomotion tasks running, jumping and hopping; and that no springs are needed to protect the actuation system. Here we present results of a flying trot, also referred to as a running trot. To the best of the authors' knowledge this is the first time a flying trot has been successfully implemented on a robot without passive elements such as springs. A critical discussion on the pros and cons of active impedance concludes the paper. This article is an extension of our previous work presented at the International Symposium on Robotics Research ISRR 2013.

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Towards versatile legged robots through active impedance control

This paper reports the design and control of a fast torque-controlled hydraulic humanoid robot, TaeMu. The robot has 15 active joints that are all driven by hydraulic servocylinders with an external hydraulic power supply. The 1377-mm tall robot has a three-axis active torso used for static and dynamic balancing. It weighs 72.3 kg including a dummy weight stand for the arms. Its lightweight design with carbon-fiber-reinforced plastic allows the legs to have a similar mass distribution to that of human legs. We present the details of the hardware design including the hydraulic actuator selection, mechanism, and control systems, as well as the passivity-based controller design for joint torque control and whole-body motion control. We present experimentally obtained results, such as speed testing, basic torque control testing, full-body compliant balancing with attitude regulation/tracking, and balanced full-squat motions. The experimentally obtained results and estimated joint specification imply the high potential of the robot to perform human-like motions if the researchers can invent proper control algorithms.

Design and Experimental Evaluation of a Fast Torque-Controlled Hydraulic Humanoid Robot

Demonstrating acrobatic behavior of a humanoid robot such as flips and spinning jumps requires systematic approaches across hardware design, motion planning, and control. In this paper, we present a new humanoid robot design, an actuator-aware kino-dynamic motion planner, and a landing controller as part of a practical system design for highly dynamic motion control of the humanoid robot. To achieve the impulsive motions, we develop two new proprioceptive actuators. The actuator’s torque, velocity, and power limits are reflected in our kino-dynamic motion planner by approximating the configuration-dependent reaction force limits. For the landing control, we effectively integrate model-predictive control and whole-body impulse control by connecting them in a dynamically consistent way to accomplish both the long-time horizon optimal control and high-bandwidth full-body dynamics-based feedback. With the carefully designed hardware and control framework, we successfully demonstrate dynamic behaviors such as back flips, front flips, and spinning jumps in our realistic dynamics simulation.

The MIT Humanoid Robot: Design, Motion Planning, and Control For Acrobatic Behaviors

This paper reports on development of a fast torque-controlled hydraulic humanoid robot Tae-Mu. We present details on the hardware including the actuation, mechanical design, control and sensing systems. The robot has 13 active joints which are all driven by hydraulic servo cylinders with an external hydraulic power supply. The robot is installed with a three-axis active torso, which are utilized for static and dynamic balancing. The height is 141 cm, and the weight is 72.3 kg including a dummy weight stands for the arms. The lightweight design with carbon-fiber-reinforced plastic allowed the legs have the similar mass distribution to human legs. We present experimental results including basic torque control, balancing against external push, balanced squat motions and compliant balancing on a moving platform. The attached experimental video and graphs demonstrates that the fabricated robot has a high potential as a research platform for human sensory-motor control because of its compliance, high torque, speed, range of motion, safety and durability.

Development of a fast torque-controlled hydraulic humanoid robot that can balance compliantly

The paper reports on a hydraulic robotic leg, a research platform suitable for exploring high-performance legged locomotion. We propose to use hydraulic linear actuators combined with lightweight links made out from carbon-fiber-reinforced plastic so that we can maximally enjoy their innate high load-to-weight ratio. The robot is designed so as to have a one-to-one mass ratio between the actuators and other parts. Based on the hydraulic servo actuator dynamics, the paper describes the details of velocity and force control of the robot joints, along to our passivity-based force control framework. Details on the hardware including the mechanisms, microcontrollers, and simulators are also described. Finally, the paper provides experimental results on zero-force tracking control, gravity compensation, task-space impedance control, and jumping.

Daisuke Suewaka

Papers

Design and Experimental Evaluation of a Fast Torque-Controlled Hydraulic Humanoid Robot

Development of a fast torque-controlled hydraulic humanoid robot that can balance compliantly

Lightweight hydraulic leg to explore agile legged locomotion