Impedance is considered to be one of the primary controlled-properties in general biomechanical systems, with the relationship between redundant actuation and impedance modulation an important issue. Primarily, this work deals with the modeling and analysis of the spring-like impedance properties inherent in the musculoskeletal structures of biomechanical systems. A stiffness model for biomechanical systems is derived, and a load distribution methodology is investigated which attempts to explain how biomechanical systems simultaneously control their impedance properties, motion, and internal loading by utilizing redundant actuation. Through examples of the human arms, the presence and the necessity for excess-redundant actuation is illustrated in terms of spring-like impedance modulation.

As a robot loses some of its capabilities due to component failure, fault-tolerant controllers isolate the faulty unit and complete the task with minimum performance loss. However, as the robot undergoes a failure and accumulates large errors, controller torque requirements usually increase and may even exceed actuator capacities. This problem can be avoided in redundant serial manipulators if the redundancy of the system is exploited to redistribute the actuator torques. This is accomplished by implementing the controller in the end-effector space. The computed torque method and PID feedback control are used in the controller. When torque demand on an actuator exceeds the allowable limit, that actuator is isolated and the required load is distributed among the remaining actuators. This controller not only avoids the saturation problem, but also allows the robot to eliminate errors and track the original trajectory. Application of the method is demonstrated on a four degree-of-freedom serial manipulator.>

A null space damping method is proposed which solves the stability problem commonly encountered in existing local joint torque optimization techniques applied to redundant manipulators. The damped joint motion is quite stable and globally outperforms undamped techniques in the sense of torque minimization capability. In addition, simulation results show that the resulting damped joint motion becomes conservative after an initial transient stage for cyclic end-effector trajectories, while undamped pseudo-inverse solutions are reported to never lead to conservative motion. Three undamped and damped joint torque optimization algorithms are considered and discussed with comparison to the previous literature. The effectiveness of the proposed null space damping method is demonstrated by computer simulation.>

Previous implementations of robot force control seldom produced satisfactory results, and researchers in the past have experienced significant instability problems associated with their force controllers. When a manipulator is constrained to an environment (force-controlled), geometric stability due to the manipulator configuration and the force-controlled direction is shown to be a significant factor in overall system stability. This exploratory study points out a rather intuitive, geometrically based stability and analyzes the phenomenon both analytically and graphically. Sequential joint self-motion algorithms for kinematically redundant manipulators are suggested for reduced transitional impact and greater stability in the ensuing force-controlled operation.>

The authors derive the object-based effective stiffness of antagonistically actuated, backdrivable, parallel linkages and show how to use this result to synthesize the generalized input loads required to create a desired system stiffness. They investigate the open-loop stability characteristics of such antagonistically actuated systems as a simple four-bar linkage, a dual-arm system, and a multifingered hand. In addition, they compare object-based and joint-based load distribution schemes in terms of their effect on system stability and input load requirements for an overconstrained planar mechanism. These investigations show an extreme interdependence between system geometry, actuation mode, and stability.>

A modeling procedure for a completely general kinematic system and a stiffness formulation technique for antagonistically actuated systems are given, in a format which is directly applicable to the design of high-stiffness robotic manipulator controllers. The formulation is developed in terms of kinematic influence coefficients. This involves some generalization of an existing modeling technique so that hybrid manipulator systems (combinations of parallel and serial manipulator systems) can be systematically treated. Antagonistic stiffness, which is developed extensively, is seen to be very promising for the design and control of future manipulators with high precision requirements under various operational disturbances.>

For an object being rigidly grasped and manipulated by multiple robotic mechanisms, the internal loading characteristics at a common coordinate set within the object are considered. It is demonstrated that representation of internal forces and moments in these common coordinates give insight into force and load distribution schemes developed previously. In particular, it is shown how internal loads may be created in some end-effector force distribution schemes even when no component in the null-space of the grasp matrix is included. It is further shown that a particular pseudoinverse of the grasp matrix, which can be shown to be consistent with the kinematic constraints, may be used to eliminate this situation. The case of a two-arm system is used to illustrate the concepts introduced.>

A novel approach for the control of task-space stiffness characteristics in systems consisting of a superabundance of kinematically dependent inputs is proposed. When there are more input actuations than operational degrees of freedom, internal preloads can be generated that produce effective restoring forces in the face of displacement or disturbances imposed on the system. Examples of this excessive actuation can be found in certain modes of structurally overconstrained parallel manipulators and in antagonistically structured serial manipulators. The input loads are synthesized offline (prior to operation) and entered as a feedforward component so that the desired effective loads on objects are obtained, and (simultaneously) significant disturbances at the task level can be largely rejected in an open-loop fashion. This reduces the burden and shortcomings of standard feedback schemes. Moreover, a layered feedback scheme is used to compensate for small perturbations and unmodeled dynamics. The open-loop task-based stiffness control scheme as applied to structurally parallel mechanism/robotic linkage systems is investigated. The scheme's applicability as a programmable active compliance device is also discussed.>

The issue of distributing the task among the multiple robot arms while considering the manipulator dynamics is considered. The forces and moments required to move an object are distributed in such a way that extra degrees of freedom within the system may be used to satisfy or optimize criteria related to the manipulator dynamics. A method to perform such subtasks is introduced, and examples of possible criteria noted. It is expected that such techniques will produce trajectories which will be more desirable for the individual arms, dynamically, since the dynamics are considered in the task distribution.>