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On-line Master/Slave Robot System Synchronization with Obstacle Avoidance

Rogelio de J. Portillo-Vélez
Centro de Investigación y de Estudios Avanzados del Instituto Politécnico Nacional (Cinvestav-IPN), Av. Instituto Politécnico Nacional
#2508, San Pedro Zacatenco, 07360. México D.F.

Carlos A. Cruz-Villar
Centro de Investigación y de Estudios Avanzados del Instituto Politécnico Nacional (Cinvestav-IPN), Av. Instituto Politécnico Nacional
#2508, San Pedro Zacatenco, 07360. México D.F.

Alejandro Rodríguez-Ángeles
Centro de Investigación y de Estudios Avanzados del Instituto Politécnico Nacional (Cinvestav-IPN), Av. Instituto Politécnico Nacional
#2508, San Pedro Zacatenco, 07360. México D.F.


In this work, it is proposed a controller for the synchronization of master/slave robotic systems. The aim of the proposed controller is to provide autonomy to the slave robot, via obstacle avoidance capability. The controller includes two terms. The first term is a PID controller, which is mapped through the task Jacobian from the task space to the robot joint space. The second term is the on-line solution of an optimal control problem (OCP), which considers the dynamic model of the slave robot. The performance index of the OCP pursues three objectives. The first goal is synchronization of master/slave end-effector position. The second goal is to keep the joint positions of the slave robot within feasible limits. The third goal is the obstacle avoidance of the whole arm at the slave robot side. Experimental results show the effectiveness of our proposal when tested on a writing task.


Synchronization, Optimization, Redundant Manipulators, Obstacle Avoidance.

>>Full text
Rogelio DE J.
PORTILLO-VÉLEZ, Carlos A. CRUZ-VILLAR, Alejandro RODRÍGUEZ-ÁNGELES, On-line Master/Slave Robot System Synchronization with Obstacle Avoidance, Studies in Informatics and Control, ISSN 1220-1766, vol. 21 (1), pp. 17-26, 2012.

1. Introduction

Controlled synchronization of robots is a well developed research area [1]. This kind of synchronization has potential applications to production processes, and master/slave systems. Typically, master-slave robotic systems perform synchronization tasks in hazardous and unstructured environments, which demand the slave robot to work in situations which might not be predicted a priori. These situations include information delays or changes in the environment as moving obstacles, which may hinder the task at the slave robot side.

The obstacle avoidance problem has been approached considering the path planning problem [2] and the control problem [3]. Depending on the knowledge of the obstacle trajectory, on-line or off-line approaches are considered. If a robot works in environments where obstacle trajectory is unknown, then off-line path planning schemes become useless. Therefore, schemes for on-line controlled synchronization of master/slave robotic systems must be adopted when the obstacle trajectory is not known in advance. Moreover, in a master/slave system, the operator guides the end-effector position of the slave robot through the master robot. However, the operator is not aware about the control of the rest of the slave robot structure, [4]. This fact imposes constraints on both, the human operator and the slave robot performance. Thus, increasing the autonomy of the slave robot greatly improves the performance of the master/slave robotic system.

Several approaches to achieve controlled synchronization of master/slave robotic systems have been adopted in the past. One approach is the direct control, where the motion of the slave robot is directly synchronized with the motion of the master robot [5]. Other approach is the supervisory control approach, where the operator monitors the task execution, and the slave robot has higher autonomy [6]. The shared control approach considers the direct control together with a local sensor based controller for the slave robot [7], [8]. Notice that, none of the cited approaches considers obstacle avoidance capabilities.

Some techniques have been explored in order to provide autonomy to the master/slave robotic systems. In [9], the robot is provided with sensor systems to acquire necessary information about the environment. On the other hand, it is well known that kinematic redundancy of a robot, i.e. the presence of additional degrees of freedom (DOF) to perform a task [10], allows rendering autonomy by performing additional goals, as energy consumption optimization or obstacle avoidance, see [11], [12], [13] and [14].

In this work, it is considered a master/slave robotic system commanded by a human operator. The master robot works in a structured environment, i.e. a well defined one. The slave robot synchronizes with the end-effector position of the master robot in an unstructured environment, where unexpected objects may appear and collide with the slave robot structure. Then, exploiting the slave robot redundancy, its autonomy is increased while avoiding obstacles.

To achieve three objectives; the master/slave synchronization, the obstacle avoidance and robot joint limits avoidance at the slave side, an on-line optimal controller is proposed. The proposed controller simultaneously performs trajectory tracking and position time varying obstacle avoidance, while only instantaneous obstacle position is required. In this work, such a position is obtained via a CCD camera, to on-line locate a repulsive potential field around the obstacle. However, visual servoing approach is not followed as the robot closed loop feedback is performed by using the optical encoder attached to each joint actuator.

The design of the synchronization controller is considered as a dynamic optimization problem, which is on-line solved by means of the gradient flow approach [15]. Therefore, the state derivatives with respect to the optimizing controller input (sensitivities) are computed by on-line solving a set of adjoint differential equations.

Four features are distinguished in this work: i) consideration of the dynamic model of the slave robot in the controller design, ii) the inverse kinematic models of both robots are not required, iii) the fact that the operator is not aware of the obstacle presence, indeed, the controller does not know the obstacle trajectory but the instantaneous position, and iv) the controller considers joint limits avoidance at the slave robot side, in order to ensure feasible slave robot configurations.

The rest of the paper is organized as follows. The synchronization problem is stated in section 2. In section 3, the necessary mathematical models are provided. Section 4 describes the proposed synchronization controller. A case study is presented in section 5. In section 6 experimental results are discussed. Section 7 closes the paper with the conclusions.


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