collision-free configurations between the start and goal config- urations. These methods consist mainly of two classes of algo- rithms, the family of road-map methods that include the visibility graph, the Voronoi diagram, the free-way method and the Roadmap algorithm [5], and the cell-decomposition methods [6]. Continuous explicit methods, on the other hand, consist in basically open-loop control laws. One important family of methods is based on optimal-control strategies [7], whose main disadvantages are their computational cost and dependence on the accuracy of the robot’s dynamic model. Besides planning robot motion, control laws that assure the execution of the plan are required in order to accomplish the robot’s task. Thus, one fundamental research topic focus- es on control techniques. A robot manipulator is a nonlinear, multi-variable system and a wide spectrum of control tech- niques can be experimented here, ranging from the simpler proportional derivative (PD) and proportional integral deriv- ative (PID) control to the computed-torque method [8], and the more sophisticated adaptive control [9] whose details are out of the scope of this survey. Typical industrial robots are designed to manipulate objects and interact with their environment, mainly during tasks such as polishing, milling, assembling, etc. In the control of the interaction between manipulator and environment, the contact force at the manipulator’s end effector is regulated. There are diverse schemes of active force control, such as stiffness control, compliant control, impedance control, explicit force control and hybrid force/position control. The first three schemes belong to the category of indirect force control, which achieves force control via motion control, while the last two methods perform direct force control by means of explicit clo- sure of the force-feedback loop. Readers who wish to study this subject in detail will find an interesting account in [10]. An attractive alternative for implementing force-control laws is the use of passive mechanical devices so that the trajec- tory of the robot is modified by interaction forces due to the robot’s own accomodation. An important example of passive