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Chapter 8 — Motion Control

Wan Kyun Chung, Li-Chen Fu and Torsten Kröger

This chapter will focus on the motion control of robotic rigid manipulators. In other words, this chapter does not treat themotion control ofmobile robots, flexible manipulators, and manipulators with elastic joints. The main challenge in the motion control problem of rigid manipulators is the complexity of their dynamics and uncertainties. The former results from nonlinearity and coupling in the robot manipulators. The latter is twofold: structured and unstructured. Structured uncertainty means imprecise knowledge of the dynamic parameters and will be touched upon in this chapter, whereas unstructured uncertainty results from joint and link flexibility, actuator dynamics, friction, sensor noise, and unknown environment dynamics, and will be treated in other chapters. In this chapter, we begin with an introduction to motion control of robot manipulators from a fundamental viewpoint, followed by a survey and brief review of the relevant advanced materials. Specifically, the dynamic model and useful properties of robot manipulators are recalled in Sect. 8.1. The joint and operational space control approaches, two different viewpoints on control of robot manipulators, are compared in Sect. 8.2. Independent joint control and proportional– integral–derivative (PID) control, widely adopted in the field of industrial robots, are presented in Sects. 8.3 and 8.4, respectively. Tracking control, based on feedback linearization, is introduced in Sect. 8.5. The computed-torque control and its variants are described in Sect. 8.6. Adaptive control is introduced in Sect. 8.7 to solve the problem of structural uncertainty, whereas the optimality and robustness issues are covered in Sect. 8.8. To compute suitable set point signals as input values for these motion controllers, Sect. 8.9 introduces reference trajectory planning concepts. Since most controllers of robotmanipulators are implemented by using microprocessors, the issues of digital implementation are discussed in Sect. 8.10. Finally, learning control, one popular approach to intelligent control, is illustrated in Sect. 8.11.

Gain change of the PID controller

Author  Wan Kyun Chung

Video ID : 25

The control architecture of the PID tracking controller is introduced. Moreover, according to the gain change, the performance variations of the PID controller implemented in the digital control system are shown.

Safe human-robot cooperation

Author  Fabrizio Flacco, Torsten Kröger, Alessandro De Luca, Oussama Khatib

Video ID : 757

A real-time collision avoidance approach is presented for safe human-robot coexistence. The main contribution shown in this video is a fast method to evaluate distances between the robot and possibly moving obstacles (including humans), based on the concept of depth space. The distances are used to generate repulsive vectors that are used to control the robot while executing a generic motion task. The repulsive vectors can also take advantage of an estimation of the obstacle velocity. In order to preserve the execution of a Cartesian task with a redundant manipulator, a simple collision-avoidance algorithm has been implemented, where different reaction behaviors are set up for the end-effector and for other control points along the robot structure. Reference: F. Flacco, T. Kröger, A. De Luca, O. Khatib: A depth space approach to human-robot collision avoidance, Proc. IEEE Int. Conf. Robot. Autom. (ICRA), Saint Paul (2012), pp. 338-345

Virtual whiskers - Highly responsive robot collision avoidance

Author  Thomas Schlegl, Torsten Kröger, Andre Gaschler, Oussama Khatib, Hubert Zangl

Video ID : 758

All mammals but humans use whiskers in order to rapidly acquire information about objects in the vicinity of the head. Collisions of the head and objects can be avoided as the contact point is moved from the body surface to the whiskers. Such a behavior is also highly desirable during many robot tasks such as for human-robot interaction. This video shows the use of novel capacitive proximity sensors so that robots can sense when they approach a human (or an object) and react before they actually collide with it. The sensors are flexible and thin so that they feature skin-like properties and can be attached to various robotic links and joint shapes. In comparison to capacitive proximity sensors, the proposed virtual whiskers offer better sensitivity towards small conductive as well as non-conductive objects. Equipped with the new proximity sensors, a seven-joint robot for human-robot interaction tasks demonstrates the efficiency and responsiveness in this video. Reference: T. Schlegl, T. Kröger, A. Gaschler, O. Khatib, H. Zangl: Virtual whiskers - Highly responsive robot collision avoidance, Proc. IEEE/RSJ Int. Conf. Intel. Robot. Syst. (IROS), Tokyo (2013)

JediBot - Experiments in human-robot sword-fighting

Author  Torsten Kröger, Ken Oslund, Tim Jenkins, Dan Torczynski, Nicholas Hippenmeyer, Radu Bogdan Rusu, Oussama Khatib

Video ID : 759

Real-world sword-fighting between human opponents requires extreme agility, fast reaction time and dynamic perception capabilities. This video shows experimental results achieved with a 3-D vision system and a highly reactive control architecture which allowfs a robot to sword fight against human opponents. An online trajectory generator is used as an intermediate layer between low-level trajectory-following controllers and high-level visual perception. This architecture enables robots to react nearly instantaneously to the unpredictable human motions perceived by the vision system as well as to sudden sword contacts detected by force and torque sensors. Results show how smooth and highly dynamic motions are generated on-the-fly while using the vision and force/torque sensor signals in the feedback loops of the robot-motion controller. Reference: T. Kröger, K. Oslund, T. Jenkins, D. Torczynski, N. Hippenmeyer, R. B. Rusu, O. Khatib: JediBot - Experiments in human-robot sword-fighting, Proc. Int. Symp. Exp. Robot., Québec City (2012)

Different jerk limits of robot-arm trajectories

Author  Torsten Kröger

Video ID : 760

This video displays the motions of a 6-DOF industrial- robot arm controlled in joint space. The first reference trajectory is not jerk-limited. The second trajectory features a joint jerk limit of 400 deg/s^3 for all six joints, and the third trajectory has a jerk limit of 20 deg/s^3 for all robot joints.

Sensor-based online trajectory generation

Author  Torsten Kröger

Video ID : 761

The video shows the movements of a position-controlled 6-DOF industrial-robot arm equipped with a distance sensor at its end-effector. The task of the robot is to draw a rectangle on the table, while the force on the table is controlled by a force controller which acts only orthogonally to the table surface. The dimensions of the rectangle are determined by the obstacles in the robot's environment. If the obstacles are moved, the distance sensor triggers the execution of a new trajectory segment which is computed within one control cycle (1 ms), so that it can be instantly executed.