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A hard contact is an impact that occurs when the robot contacts a stiff environment with a high velocity. In the pretransition phase of the hard contact, it is important to maintain stability while reducing succeeding force peaks and the duration time. For these purposes, we propose two control methods. One is a suppression controller that suppresses position rebounds in-between the impact time. The other is a flexible-damped joint with a joint damping controller The flexible-damped joint is a new joint type designed to increase the hardware damping. It is controlled by a novel controller, called the joint damping controller, to enhance damping. The major advantage of the proposed methods is a guaranteed stability. The performance of the proposed methods is validated via several comparative experiments. [DOI: 10.1115/1.2719766]

The PID trajectory tracking controller for Lagrangian systems shows performance limitation imposed by inverse dynamics according to desired trajectory. Since the equilibrium point cannot be defined for the control system involving performance limitation, we define newly the quasiequilibrium region as an alternative for equilibrium point. This analysis result of performance limitation can guide us the autotuning method for P1D controller. The quasiequilibrium region is used as the target performance of autotuning PID trajectory tracking controller. The autotuning law is derived from the direct adaptive control scheme, based on the extended disturbance input-to-state stability and the characteristics of performance limitation. Finally, experimental results show that the target performance can be achieved by the proposed automatic tuning method.

Several 3-DOF pure translational parallel manipulators have been reported recently. However, kinematic conditions of such translational parallel manipulators have not been studied deeply. Most previously reported translational parallel manipulators were analyzed by considering constraints on the motion of an independent leg. In addition to the constraints of independent legs, the effect of the constraints on an overall mechanism are analyzed in this paper. The overall kinematic constraints are explained by introducing the restriction of motion, which is a constraint that prohibits a manipulator from moving in a certain region or direction, instantaneously. By introducing it, kinematic constraints can be embedded explicitly in the formulation of rate kinematics, and by analyzing it, constraints for a desired motion can be found. This paper derives constraints for pure translational motion in a systematic way. Necessary and sufficient conditions are suggested and several translational parallel manipulators with their kinematic features are provided.

One of the most important and widely used concepts in the kinematic analysis of robot manipulators is the reciprocal screw. However, there are no general expressions and easy methods to obtain the reciprocal screw in an analytic manner. This paper suggests an analytic formulation of the reciprocal screws of arbitrarily aligned screw systems. Since the reciprocal screws obtained in this paper are represented by the direction vectors and the position vectors of the given screws, we can analyze the relation between the reciprocal screw system and the given screw system easily. With the results, to find a reciprocal screw is to solve an algebraic equation of the corresponding system of screws. In order to show the usefulness of the result, several examples related to the robot manipulator are provided. For a nonredundant serial manipulator, the pseudo inverse of the Jacobian matrix is shown to be equivalent to the wrench matrix obtained by the reciprocity. For a parallel manipulator, a leg is isolated to obtain an independent part from the manipulator and is analyzed analytically. The proposed method can be applied to any arbitrarily aligned screw system. [DOI: 10.1115/1.1539508]

A combined observer is synthesized by unifying the conventional linear state estimator and the perturbation observer to estimate plant uncertainties and disturbances. It enables robust state estimation for uncertain dynamical systems and simultaneously, provides full-state to the perturbation observer under output feedback conditions. The proposed combined observer is very practical since it is given as a recursive discrete-time form with minimal tuning parameters, and it requires no knowledge of the plant uncertainty. A coupled estimation error dynamics is derived, and the related technical issues such as stability and noise sensitivity are addressed. The combined observer setting is also extended to stochastic systems, and the discrete Kalman filter is reformulated by including the perturbation estimate update process. Numerical examples and experimental results validate the proposed schemes. [DOI: 10.1115/1.1540112] Keywords: State Estimator, Discrete Kalman Filter, Perturbation Observer, Robustness

For joint tracking control of flexible robots, this paper presents a two-step design of controller: the bandwidth modulation with modal feedback approach. First, we focus on the design of rigid part's motion controller considering the bandwidth of the rigid subsystem. We investigate the relationship between macro joint tracking performance and vibration suppression capability using the bandwidth parameter. After adjusting the bandwidth of rigid motion, the composite control, which is the second step consisting of rigid and flexible sub-controllers, is applied like singular perturbation approach. As the flexible sub-controller, we propose a direct modal feedback controller that is very simple, but effective to suppress the vibration. The validity and effectiveness of the proposed method are verified by experiments using a POSTECH 3-D flexible robot. [DOI: 10.1115/1.1514240]

Model based disturbance compensating methods such as disturbance observer, robust control with adaptive algorithm, enhanced internal model control are well known control structures for robust motion controller which can satisfy desired performance and robustness of high-speed/high-accuracy positioning systems. In this paper, these are analyzed in the unified framework and their design method is proposed. To do this, a generalized disturbance attenuation framework named RIC (robust internal-loop compensator) structure is introduced. Through parameterization based on RIC, it is shown that there are inherent equivalences in their structures and the proposed RIC gives a general design framework for the model based disturbance compensating methods. Through simulation, the proposed method is verified. [DOI: 10.1115/1.1468995] Keywords: Two-Loop Structure, Robust Internal-Loop Compensator, Disturbance Observer, Unified Framework

A robust tracking controller for mechanical systems is proposed by combining the robust perturbation compensator which effectively attenuates the perturbation to the plant with the nominal tracking controller designed using the sliding surface. This approach enables a smooth sliding mode in tracking control loop without chattering problem. A unified view is given on a class of perturbation observers. Three kinds of equivalent expressions for the perturbation of a plant is described. In terms of the equivalents, we propose the feedforward perturbation observer (FFPO), the feedback perturbation observer (FBPO), and the sliding mode perturbation observer (SMPO). Successively, by hierarchically adopting these three observers to attenuate the residual perturbation, the hierarchical perturbation compensator (HPC) is constructed with stability analysis. The adaptive and integral property of the HPC greatly enhances the robust performance with minimal control effort. The actuator saturation issue is also considered. Experimental results demonstrate the effectiveness of the proposed controller. [DOI: 10.1115/1.1468996] Keywords: Smooth Sliding Mode Control, Perturbation Observers, Robust Performance, Residual Perturbation, Feedforward Compensation, Hierarchical Compensation

An efficient algorithm to simulate the workspace for a n-link redundant manipulator with rotating base is developed. The unique feature of the algorithm is its ability to automatically generate the binary angle selection code for the given manipulator. This code system makes it possible to avoid repetitions in calculating positions and plotting arcs for the workspace and thus saves considerable computing and graphic time. Two types of workspace are possible using this algorithm. Numerical examples including the workspace of human arm motion and that of a 20-link system are demonstrated to illustrate the effectiveness of the algorithm.

A robust high-speed motion controller is proposed. The proposed controller consists of the proximate time optimal servomechanisms (PTOS) for high-speed motion, disturbance observer (DOB)for robustness, friction compensator, and saturation handling element. In the proposed controller, DOB basically provides the chance to apply PTOS to nondouble integrator systems by drastically reducing disturbances as well as unwanted signals due to difference between real system and the double integrator model. But, in DOB-based systems, if control input is saturated due to control input of PTOS and/or DOB, overall system stability cannot be guaranteed, which is first found and analyzed in this paper. To solve this problem, robust stability and internal stability conditions of DOB-based system are derived. It is also shown that DOB could violate the internal stability, when the control input is saturated. Eventually, a simple saturation handling element is inserted to maintain internal stability of overall system. Also, we explain that our two saturation handling methods, i.e., Additional Saturation Element (ASE) and Self Adjusting Saturation (SAS) are the equivalent solutions of saturation problem to maintain internal stability. The stability and performances of the proposed controller are verified through numerical simulations and experiments using a precision linear motor system. [S0022-0434(00)01103-5]