Contact Force Control Research Articles

Dynamic Characteristics of Constrained Manipulators for Contact Force Control Design

The authors present the results of eigenstructure analyses of the dy namics of several manipulator and environment configurations and discuss the implications for successful force controller design. The manipulator configurations considered include single rigid link, sin gle flexible link, two rigid links, and two flexible links. In all cases, the tip of the manipulator is in contact with a deformable environ ment. Particular emphasis is placed on the interaction between the structural stiffness of the manipulator link and the stiffnesses of the contacting regions of the manipulator tip and the environ ment. It is shown that, for given link parameters, this interaction results in distinct changes in the modal structure of the manipula tor/environment system as the effective contact stiffness increases. In particular, the contact force response is dominated by a single mode for a given effective contact stiffness, whereas the control inputs excite all modes except those associated with the contact region vibration. The changes in the modal structure of the manip ulator/environment system predicted by the eigenstructure analysis are verified by experiment for the single flexible link configuration.

高速鉄道用パンタグラフにおける外乱推定と接触力制御

高速鉄道用パンタグラフにおける外乱推定と接触力制御

Contact stability analysis of a one degree-of-freedom robot

The aim of this note is to examine the conditions of stability of a simple robotic task: we consider a one degree-of-freedom (dof) robot that collides with a spring-like environment with stiffness k, the goal being to stabilize the system in contact with the environment. We study conditions on the feedback gains that guarantee quadratic Lyapunov stability of the task with a well-conditioned solution to the Lyapunov equation. It is shown that when the environment's stiffness k grows unbounded, those conditions yield unbounded values of the gains. Motivated by the stability analysis of the impact Poincare map in the perfectly rigid case ( \(k = + \infty\)), we propose an analysis that is independent of k. It enables us to conclude on global asymptotic convergence of the system's state towards the equilibrium point. This work can also be seen as the study of stability of a contact (force control) phase, taking into account the unilateral feature of the constraint.

Robust Decoupling Control of Contact Forces in Robotic Manipulation

Abstract This paper deals with the problem of controlling contact forces in robotic manipulators with general kinematics. The main focus is on control of grasping contact forces exerted on the manipulated object. A visco-elastic model for contacts is adopted. Robustness of the decoupling controller with respect to the uncertainties affecting system parameters is investigated. Sufficient conditions for the invariance of decoupling action under perturbations on the contact stiffness and damping parameters are provided. These conditions are meaningful for several classes of manipulation systems with general kinematics.

The Role of Environment Dynamics in Contact Force Control of Manipulation Robots

This paper draws attention to the role of the environment dynamics in force control of robots interacting with dynamic environment. Traditional control approaches are based on restrictive assumptions, on environment properties and cannot assure stability in general. In this paper, the role of the dynamic environment is emphasized and a contribution to understanding the mechanism of elasto-dynamic interaction is given. In this paper, it has been assumed that the force is stabilized and then local stability conditions of the whole system are studied. The study has been made on basis of a linearization of the closed-loop system.

An actively compliable probing system

A new probing mechanism and an associated active compliance control algorithm have been developed for in-circuit test of a PCB (printed circuit board). Commercially available robotic probing devices are incapable of controlling contact force generated when a rigid probe contacts with a solder joint at high speed. This uncontrollable excessive contact force often makes some defects on the surface of the solder joint, which is plastically deformable over some limited contact force. This force also makes unstable contact motions resulting in unreliable test data. To overcome these problems, we propose a serially connected macro and micro manipulator equipped with active compliance capability for safe and reliable in-circuit test. Moreover, with this probe the depth of penetration onto a solder joint can be regulated after contact. This article describes the design characteristics, modeling, and control scheme of the newly proposed devices. The comparison of conventional passive compliance and force feedback control methods with the proposed control scheme for the contact force control is presented. The results obtained from a series of experiments clearly show the effectiveness of the proposed system.

Multiple-Point Contact Control for Part Assembly. Position Control of Total Contact Force.

In an assembly task such as square pole and its hole, two objects sometimes have four or five contact points. Additionally, they move while maintaining multiple-point contact. We can devise many methods that can give stable multiple-point contact and stable relative movement. We have previously proposed a method that controls simultaneously every amount of force exerted on each contact point. We propose another method in which contact point is added one by one with position control of the total contact force. Compared with the former one, the new method requires only a small number of calculations for trial motions. The contact trial motions induce some kind of relative shift between the two objects. In trials of up to three contact points, the shift is very small. This seems to lessen the shift problem. In an experiment, the new method requires only one time of re-measurement of contact model data for the shift problem.

Adaptive Task-Space Control of Flexible-Joint Manipulators

This paper considers the motion control and compliance control problems for uncertain rigid-link, flexible-joint manipulators, and presents new adaptive task-space controllers as solutions to these problems. The motion control strategy is simple and computationally efficient, requires little information concerning either the manipulator or actuator/transmission models, and ensures uniform boundedness of all signals and arbitrarily accurate task-space trajectory tracking. The proposed compliant motion controllers include an adaptive impedance control scheme, which is appropriate for tasks in which the dynamic character of the end-effector/environment interaction must be controlled, and an adaptive position/force controller, which is useful for those applications that require independent control of end-effector position and contact force. The compliance control strategies retain the simplicity and model independence of the trajectory tracking scheme upon which they are based, and are shown to ensure uniform boundedness of all signals and arbitrarily accurate realization of the given compliance control objectives. The capabilities of the proposed control strategies are illustrated through computer simulations with a robot manipulator possessing very flexible joints.

フレキシブル・アームの曲げ・ねじり結合振動と力のロバスト制御

フレキシブル・アームの曲げ・ねじり結合振動と力のロバスト制御

Running Test on Current Collector with Contact Force Controller for High-Speed Railways.

To reduce noise caused by current collectors on high-speed railways, a wing-Shaped low-noise current collector has been developed. This collector head is directly supported by a composite insulator. However, it becomes difficult to maintain the predetermined contact force of the contact strip against the trolley wire. Therefore, the vertical height of those apparatus should be controlled actively. The upper part of the composite insulator is under high voltage. When using sensors only under the insulator, it is difficult to distinguish wire disturbance from lift disturbance. In this study, the difficulty is solved by using H∞ controller to estimate those disturbances in each frequency range. In traveling experiments using full-size prototype equipment on the running vehicle, it was confirmed that the contact force variation due to lift of 68 N at the speed of 112km / h was reduced by about 33% compared with active control without lift compensation, and the wire push-up variation was reduced by about 67%.

Simultaneous control of robot manipulator impedance and generalized force and position

Certain industrial tasks require forces to be applied to objects during task execution. Of these tasks, a subset of them may be completed successfully with only crude control over contact force. A open-loop method of force control is proposed for these applications. This method utilizes the impedance control of Hogan to achieve open-loop control over generalized contact forces and position. The net result is simultaneous control of manipulator mechanical impedance and the generalized position and the force of the robot. An algorithm is proposed which determines the manipulator generalized position inputs, the only signal available to an impedance controller, in order to generate prescribed generalized forces while in contact with fixed objects. A new approach to the establishment of the stability of impedance controls during object contact is given, utilizing the theory of singularly perturbed dynamic systems. Experimental results on a two degree-of-freedom robot confirms the validity of the approach proposed here to control contact forces in an open-loop manner.

Control of Flexible Load Deformations and Environment Contact Forces in Dual-Arm Manipulation

A pseudo-inverse-Jacobian scheme to control deformations of a flexible load while attaining contact force goals with an external environment is discussed in this paper. The flexible load is manipulated by two arms. A kinematic description of the flexible load is obtained from a lumped-parameter model. Coordinated motion of the arms is achieved through kinematic redundancy resolution at the differential kinematic level. The strategy proposed here uses load flexibility to accommodate for position errors that would result in the build-up of internal forces in a rigid-load case. Simulation results are presented that illustrate the success of the method.

Contact Force Control Based on Learning Operation without Switching the Servo Mode at the Instant of the Impact.

We propose a new method of controlling contact force based on iterative learning operation. Most of conventional contact force control techniques must change servo from the position control mode to the force control mode at instant of impact. However, if there exists a time delay during change of servo mode, an unstable phenomenon arises. On contrary, proposed method is based on damping control and change only force command signal at a certain time T0 after impact. The learning controller optimizes force command signal before T0 and realizes dead-beat contact force response after impact. This results in smooth contact operation. We show some simulation and experimental results to verify effectiveness of proposed method.

Dynamics and Control of Intelligent Jig with Function of Manipulation

In order to develop intelligent jigs with a manipulation function, a new kind of mechanism is proposed. We call this mechanism the rotational base and single-jointed finger mechanism (RBSF mechanism). This mechanism does not have enough joints for active control of contact forces. Therefore, arbitrary acceleration and angular acceleration cannot be applied to workparts. To address this problem, kinematics, dynamics, motion planning, manipulation planning, and control of the manipulation by the RBSF mechanism are presented. The feasibility of the generation of object acceleration and angular acceleration is discussed using the method for the linear complement problem. The results of simulations and experiments are presented to show the validity of the proposed theory.

Adaptive hybrid control of manipulators on uncertain flexible objects

This paper presents an adaptive hybrid control approach for a robot manipulator to interact with its flexible object. Because of its flexibility, the object dynamics influence the robot's control system, and since it is usually a distributed parameter system, the object dynamics as seen from the robot change when the robot moves. The problem becomes further complicated such that it is difficult to decompose the robot's position and contact force control loops. In this paper, we approximate the object's distributed parameter model into a lumped 'position state-varying' model. Then, by using the well-known nonlinear feedback compensation, we decompose the robot's control space into a position control subspace and object torque control subspace. We design the optimal state feedback for the position control loop and control the robot's contact force through controlling the resultant torque of the object. We use the model-reference simple adaptive control strategy to control the torque control loop. We also st...

Contact transition control with semiactive soft fingertips

We address the problem of controlling contact forces generated when the fingers of a robot close on an object or make contact with surfaces in the robot's environment. The use of controllable fingertip materials is proposed to enhance system performance and avoid contact instability problems that can arise with noncollocated sensors and actuators. We describe a novel fingertip employing an electrorheological fluid and present the results of experiments to evaluate its capabilities when used with a robot contacting a stationary object at various speeds. Having characterized the fingertip behavior as a function of applied voltage, we turn to the question of optimal control, Using a simplified dynamic model of a robot equipped with a controllable electrorheological fingertip, we obtain a piece-wise optimal solution for the fingertip damping as a function of time, to minimize settling time while satisfying constraints on the contact forces.

Independent control of limb position and contact forces in cat posture

1. It has previously been demonstrated that a set of geometric and kinetic parameters are invariant in cats standing at their preferred interfoot distance and weight distribution. Thus the length and the angle of orientation relative to the vertical of each limb axis remain approximately constant when the supporting platform is tilted in the sagittal plane. The direction of the tangential contact forces is similarly constrained in response to horizontal translations. The main aim of the present study is to assess whether or not the control of limb position is independent of the control of the contact forces at the feet. To this end we have examined cat posture under a number of different conditions expressly designed to increase the range of postural variability. We considered that if the specification of limb position is a mere byproduct of the neural control of contact forces (or vice versa), geometric and kinetic parameters would covary interdependently. If instead limb position and contact forces are controlled in parallel and independently of each other, they will tend to follow different laws of variation. 2. Limb position and contact forces were measured in intact cats standing freely on a support platform. In a first series of experiments the pitch angle of the platform was randomly changed, as were the interfoot distance and head orientation. In another series of experiments cats were tilted in the presence of an external load tending to shift the weight distribution. The same load was applied in two different manners: 1) it made contact with a very limited surface of the body, and 2) it was attached by means of a long vest that made contact with most of the trunk and produced abnormal somesthesic cues to the body. 3. The range of different experimental conditions resulted in substantial trial-to-trial variations of the length and orientation of the axis of the limbs, as well as variations of the magnitude and orientation of the net contact forces. We found that the changes of the orientation of the contact force vector are uncorrelated with the corresponding changes of limb orientation, thus providing a first line of evidence in favor of the existence of a separate neural control of geometric and kinetic parameters. 4. Another line of evidence is provided by the specific form of the laws of variation of geometric parameters and tangential forces in different animals. Under normal (unloaded) conditions the values of the limb joint angles tend to covary linearly.(ABSTRACT TRUNCATED AT 400 WORDS)

An Approach to Motion and Force Control of Coordinated Robot Arms in the Presence of Joint Flexibility

A system consisting of several flexible joint robot arms is studied in this paper. The paper addresses the motion and force control problem of such systems. A coordinating controller, consisting of a motion controller, contact force controller, and internal force controller, is designed to distribute the load between the coordinated flexible joint robot arms, and control the motion of the object, the internal force and contact force between the object and the environment, as well generate the desired joint elastic force. The system stability is analyzed based on Lyapunov stability theory. It is shown that with the proposed controller the motion and force control variables can be regulated to track asymptotically their desired trajectories. Simulation results are given to illustrate the performance of the proposed controller.

Control of manipulation with dextrous hands

This paper deals with a technique for the control of contact forces between the links of a robotic hand and an object. The goal is to control the position of the object while minimizing the consumption of power and the danger of slippage. The general problem of tracking arbitrary trajectories of the grasped object is considered first, and a feedback linearization technique is discussed that theoretically solves the problem. This technique is however hardly feasible in practice. An alternative practical controller for the stabilization of the grasp about equilibrium configurations of the object, in spite of external disturbances of bounded intensity, is presented.

Inverse dynamics and feedforward controllers for high precision position/force tracking of flexible joint robots

SUMMARYA nonlinear feedback control based on inverse dynamics is proposed for robots with flexible joints during constrained motion task execution. Based on constrained system formalism, the presented control scheme achieves simultaneous, independent control of both position and contact force at the robot end-effector. The method can be directly applied to robot control, or it can be used as the basis for developing other advanced control strategies. In this paper, a. feedforward controller is considered. Issues related to the practical application of the full inverse dynamics and feedforward control algorithms, such as evaluation of feedback variables, the use of predictors to eliminate the time delay of digital control, and the design of robust controllers, are discussed. The results of extensive numerical simulation are used to show the effectiveness of the proposed controllers and to compare their performances. It is shown that it is possible to achieve high precision tracking if the appropriate predictors are used to eliminate the effect of computational delay of the digital controller. Applying a straightforward approach for robustness of the proposed controllers has additionally improved the trajectory tracking accuracy.