Control Methods for Bipedal Walking Robots With Integrated Elastic Elements

Control Methods for Bipedal Walking Robots With Integrated Elastic Elements

Sergei Savin (Innopolis University, Russia)
Copyright: © 2019 |Pages: 21
DOI: 10.4018/978-1-5225-7879-6.ch017

Abstract

In this chapter, the problem of controlling bipedal walking robots with integrated elastic elements is considered. A survey of the existing control methods developed for walking robots is given, and their applicability to the task of controlling the robots with elastic elements is analyzed. The focus of the chapter lies with the feedback controller design. The chapter studies the influence that the elastic elements modelled as a spring-damper system have on the behavior of the control system. The influence of the spring-damper parameters and the inertial parameters of the actuator gear box and the motor shaft on the generated control laws and the resulting peak torques are discussed. The changes in these effects associated with motor torque saturation and sensors nonlinearities are studied. It is shown that the introduction of torque saturation changes the way the elastic drive parameters affect the resulting behavior of the control system. The ways to use obtained results in practice are discussed.
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Introduction

Walking robots are one of the central research topics in mobile robotics, due their wide applicability and their potential for natural integration into social and industrial processes, allowing automating a variety of labor-intensive tasks (Luk et al., 2006; Armada et al., 2005; Buschmann 2010). Particular applications for legged robots range from disaster response and to pipe inspection (Bouyarmane et al., 2012; Savin & Vorochaeva, 2017a).

There had been a lot of research focused on particular types of walking robots, including bipedal robots, quadrupeds and multi-legged robots (Hurmuzlu et al., 2004). Bipedal robots present particular interest due to possibility of using anthropomorphic designs and implementing anthropomorphic motions, which can be beneficial in terms of integrating of this type of robots into the existing processes and infrastructure (Ogura et al., 2006, Khatib et al., 2004). Examples of anthropomorphic biped designs can be found in (Kaneko et al., 2008; Kaneko et al., 2011; Khusainov et al., 2015; Hirose et al., 2007). Non-anthropomorphic bipeds can be found in (Xie et al., 2018; Clary et al., 2018). A separate class of humanoid walkers are active exoskeletons, which are inherently anthropomorphic (Yan et al., 2015; Young et al., 2017; Panovko et al., 2016; Jatsun et al., 2018).

One of the benchmarks for the walking robot technology had been DARPA Robotics Challenge (Johnson et al., 2015; Feng et al., 2015), which demonstrated of the progress in the bipedal robotics and simultaneously revealed a number of problems that still need to be addressed. One of these problems is safe interaction between the robot and the environment, including obstacles, tools, people and other robots. Currently used practical solutions to these challenges are to minimize the robot’s interactions with the environment as much as possible, and to act as a stationary manipulator when a manipulation task is present (Atkeson et al., 2015). This leads to stiff and inefficient motions and non-robust control schemes. The capability to operate safely in the scenarios where unplanned contact and collisions are inherently possible has not yet been achieved by humanoid robots.

One of the ways this capability can be achieved is by designing the robots to be less rigid (Pratt, 2002; Robinson et al., 1999). This might allow avoiding damage both to the robot and to the environment in the events of the unplanned contact and collisions. Introduction of elastic elements can also be used for improvement of robots’ energy efficiency in particular regimes of operation (Pratt et al., 1997). However, this type of robot design requires changes in the control strategies employed for bipedal robots.

This chapter looks at the problem of controlling bipedal walking robots with integrated elastic elements. It is a broad class of robots with varying structures. This problem can be partially solved by using existing techniques developed for walking robots with rigid structures. This is true for the algorithms based on the robot’s kinematics and geometry, rather than their dynamics. However, some of the elements of the control pipeline, including feedback controller design and controller parameters tuning require separate analysis.

Key Terms in this Chapter

Elastic Drive: An actuator with an integrated elastic element installed.

Bipedal Walking Robot: A mobile robot with two legs, which primary mode of operation requires these legs to periodically acquire and break contact with the supporting surface.

Control Error: The difference between the desired values of the generalized coordinates and their actual values.

Integrated Elastic Element: A mechanical system acting as a spring and damper, connected to the output shaft of the actuator’s gearbox.

Sensor model: A mathematical model that describes the relations between the sensor output and the actual values of the measured parameters.

Torque Saturation: An effect that manifests in an electric motor being able to produce torques higher than a given threshold values.

Generalized Inertia Matrix: The matrix of the quadratic form of the kinetic energy of the mechanical system, obtained for the given choice of generalized coordinates.

Constrained Linear Quadratic Regulator: A modification of the linear quadratic regulator that takes into account the explicit mechanical constraints.

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