Model Predictive Control for Transparent Teleoperation Under Communication Time Delay

Prior efforts in bilateral teleoperation under communication delay have mainly yielded control algorithms that sacrifice performance in order to guarantee robust stability. In contrast, this paper proposes a multimodel predictive controller that can enhance the teleoperation transparency in the presence of a known constant delay. Separate controllers are designed for free motion/soft contact and contact with rigid environments, with switching between these mode-based control laws occurring according to the identified contact mode. Performance objectives such as position tracking and tool impedance shaping for free motion/soft contact, as well as position and force tracking for contact with rigid environments, are incorporated into a multi-input/multi-output state-space representation of the system dynamics. New Artstein-type state and measurement transformations are proposed to generate delay-free dynamics suitable for output-feedback control, based on the original dynamics with delays in various input and output channels. The application of the continuous-time linear quadratic Gaussian control synthesis to the resulting mode-based delay-free dynamics yields control laws that guarantee closed-loop stability and enhanced performance in each phase of teleoperation. The robustness of the mode-based controllers with respect to parametric uncertainty is analyzed. Experimental results with a single-axis teleoperation setup demonstrate the effectiveness of the proposed approach     

Force Sensing Using Kalman Filtering Techniques for Robot Compliant Motion Control

Robot motion can be classified into free and contact motion. In practical tasks, however, the motion may transit from free motion to contact motion and vice versa. In such tasks, a position controller is designed in free motion. A compensator is then added in the force feedback loop to help the system reach the desired target impedance when the end-effector is in contact with the environment. To obtain accurate contact force, a Kalman filter is used to extract contact force from the wrist force sensor signal which contains inertial force of the end-effector also.     

接触作业型飞行机械臂系统的力/位置混合控制

针对飞行机械臂系统移动接触作业问题,使用了一个力/位置混合控制框架,用以控制飞行器系统持续可靠地接触外部环境同时保持一定大小的接触力,并实现在接触过程中的期望轨迹跟踪.首先将作业空间分成2个子空间--约束空间和自由空间,并分别进行力控制和位置控制.对于力控制问题,证明闭环无人机系统是一个类弹簧-质量-阻尼系统,然后在约束子空间中设计逆动力学控制器来实现接触力控制.自由飞行空间中的运动控制依靠轨迹规划和位置控制器来实现.最后,开发了基于六旋翼飞行机器人的单自由度飞行机械臂系统,在飞行状态下进行接触墙面并跟踪倾斜直线轨迹的实验.结果显示本文所使用方法能够保证在平稳移动的同时控制期望的接触力.     

Stable Switching PD Control Using Hunt-Crossley Model for a Press Platform

This study proposes a switching proportional-derivative (PD) controller for a press platform for inspecting the surfaces of a wind turbine blade. We use the Hunt-Crossley model to represent the probe shape or the nonlinearity of the inspection platform. This model consists of a nonlinear spring and a damper; therefore, it is more accurate than linear spring-damper models. We prove the global asymptotic stability of a PD force feedback controller and a PD position feedback (force) controller. However, both the controllers suffer from implementation-related problems. Specifically, the PD force feedback controller makes the impact force large, and the PD position feedback controller cannot easily measure small position changes when the platform contacts the surface. These problems of each controller are solved by switching the two controllers. The PD force feedback control and PD position feedback control are used when the platform is in the contact and noncontact states, respectively. We prove that the proposed switching PD force/position feedback controller is globally asymptotically stable. Further, simulations show satisfactory performance resulting from stable switching between the two control schemes.

     

Nonlinear adaptive control of master–slave system in teleoperation

This paper is devoted to the nonlinear control design problem to achieve stability of master–slave manipulators in teleoperation system and its transparency in the sense of motion/force tracking. Nonlinear adaptive controllers are bilaterally designed for both master and slave sites to guarantee the stability of whole system and motion tracking performance. Global boundedness of the overall adaptive system and asymptotic motion (velocity/position) tracking are established. Especially, the concept of “virtual master manipulator” is introduced to increase degree of freedom of control design for force tracking performance. The resulting force tracking error depends only on the acceleration of the designed virtual master manipulator. Accurate dynamic parameters of manipulators, their acceleration information as well as models of human operator and environment are not required in the control design. Another important feature of our approach is the relaxation for the trade-off between motion and force tracking performances.     

Neural network learning of robot arm impedance in operational space

Impedance control is one of the most effective control methods for the manipulators in contact with their environments. The characteristics of force and motion control, however, is determined by a desired impedance parameter of a manipulator's end-effector that should be carefully designed according to a given task and an environment. The present paper proposes a new method to regulate the impedance parameter of the end-effector through learning of neural networks. Three kinds of the feed-forward networks are prepared corresponding to position, velocity and force control loops of the end-effector before learning. First, the neural networks for position and velocity control are trained using iterative learning of the manipulator during free movements. Then, the neural network for force control is trained for contact movements. During learning of contact movements, a virtual trajectory is also modified to reduce control error. The method can regulate not only stiffness and viscosity but also inertia and virtual trajectory of the end-effector. Computer simulations show that a smooth transition from free to contact movements can be realized by regulating impedance parameters before a contact.     

Development of a high speed and precision wire clamp with both position and force regulations

This paper presents the mechanism and robust control of a monolithic wire clamp to achieve fast and precision operations for strong and robust micro device packaging. The wire clamp is piezoelectrically actuated and a two-stage flexure-based amplification was designed to obtain large and parallel jaw displacements. The grasping forces of the wire clamp were evaluated based on finite element analysis (FEA), and the force measurement was presented. The wire clamp was manufactured using wire EDM technique and the position and force transfer functions were obtained based on the frequency response approach. The position/force switching control strategy was employed to regulate the motion position and grasping force, and the position/force switching controller composed of a PID controller for position control and a sliding model controller (SMC) for force control was designed. Experimental tests were carried out to investigate the performance of wire clamp with the position/force switching controller during the grasping and releasing operations. The results show that the wire clamp exhibits good performance and demonstrate that high speed and precision grasping operations can be realized through the developed wire clamp and the control strategy.     

Global regulation of a planar robot arm striking a surface

Considers the problem of modeling and controlling the impact of a two-degree-of-freedom planar robot arm against an infinitely rigid and massive surface. For this case study, the basic equations describing the motion of the robot arm are derived for noncontact and contact conditions. A control scheme is proposed on the basis of a reduced-order observer that is able to asymptotically estimate the impact-induced forces and to allow their asymptotic compensation when the robot arm is in contact with the surface. The resulting control system ensures the global asymptotic regulation of the position of the arm with an assigned impact-induced force. The case study is completed by a simulation test     

Contact control for advanced applications of light weight arms

Many applications of robotic and teleoperated manipulator arms require operation in contact and noncontact regimes. This paper deals with both regimes and the transition between them with special attention given to problems of flexibility in the links and drives. This is referred to as contact control. Inverse dynamics is used to plan the tip motion of the flexible link so that the free motion can stop very near the contact surface without collision due to overshoot. Contact must occur at a very low speed since the high frequency impact forces are too sudden to be affected by any feedback generated torques applied to a joint at the other end of the link. The effects of approach velocity and surface properties are discussed. Force tracking is implemented by commands to the deflection states of the link and the contact force. This enables a natural transition between tip position and tip force control that is not possible when the arm is treated as rigid. The effects of feedback gain, force trajectory, and desired final force level are of particular interest and are studied. Experimental results are presented on a one-link arm and the system performance in the overall contact task is analyzed. Extension to multi-link cases with potential applications are discussed.     

Dynamics and Noncollocated Model‐Free Position Control for a Space Robot with Multi‐Link Flexible Manipulators

In this paper, both the dynamics and noncollocated model‐free position control (NMPC) for a space robot with multi‐link flexible manipulators are developed. Using assumed modes approach to describe the flexible deformation, the dynamic model of the flexible space robotic system is derived with Lagrangian method to represent the system dynamic behaviors. Based on Lyapunov's direct method, the robust model‐free position control with noncollocated feedback is designed for position regulation of the space robot and vibration suppression of the flexible manipulators. The closed‐loop stability of the space robotic system can be guaranteed and the guideline of choosing noncollocated feedback is analyzed. The proposed control is easily implementable for flexible space robot with both uncertain complicated dynamic model and unknown system parameters, and all the control signals can be measured by sensors directly or obtained by a backward difference algorithm. Numerical simulations on a two‐link flexible space robot are provided to demonstrate the effectiveness of the proposed control.     

Stable impact and contact force control by UAV for inspection of floor slab of bridge

ABSTRACT

This paper describes the contact force control on an unmanned aerial vehicle (UAV) developed to inspect the floor slabs of bridges. Our UAV is equipped with a three degree-of-freedom manipulator on top of the UAV body. To control the UAV for stable contact with the slab surface, the impact force should be considered. The impact force is modeled based on Hertzian contact stress. The control strategy of the UAV is cascade control separated into attitude control and position-force control. The attitude, position and force feedback are PID control. The force feedback is integrated into the position feedback seamlessly, and the output of the force feedback is added to the desired end-point position of the manipulator. This paper focuses on contact of the UAV and the floor slab. Therefore, the UAV is modeled considering the impact force in the vertical direction. The control method in the vertical direction is described, and then the altitude control and the contact force control are assessed. The altitude of the UAV was controlled with a 0.45?[sec] delay during ascending and 1.76?[sec] during descending. The UAV could control the contact force with mean error 1.61 ± 1.08?[N] while the desired contact force was 3?[N].     

Adaptive control of bilateral teleoperation system with compensatory neural-fuzzy controllers

This paper presents two adaptive neural-fuzzy controllers equipped with compensatory fuzzy control in order to adjust membership functions, and as well to optimize the adaptive reasoning by using a compensatory learning algorithm. To the first controller is applied compensatory neural-fuzzy inference (CNFI) and to the second compensatory adaptive neural fuzzy inference system (CANFIS). Each controller is incorporated into a two channel bilateral teleoperation architecture involving force-position scheme, which combines the position control of the slave system with force reflection on the master. An analysis of stability and transparency based on a passivity framework is carried out. The resulting controllers are implemented on a one degree of freedom teleoperation system actuated by DC motors. The experimental results obtained show a fairly high accuracy in terms of position and force tracking, under free space motion and hard contact motion, what highlights the effectiveness of the proposed controllers.

     

Adaptive Control For Robotic Manipulators Executing Multilateral Constrained Task

This paper presents a model‐based adaptive control in task coordinates for robotic manipulators executing multilateral constrained tasks The controller works based on the concept of orthogonality between force and motion in the subspaces derived from the constraints. The control gains are independently adjustable in each subspace. The friction force, depending on the contact force, is compensated adaptively. Asymptotic convergence for both force and motion tracking errors is guaranteed by the Lyapunov‐Like Lemma. Experimental results obtained using a 3 D.O.F. robot are given.     

Stability guaranteed teleoperation: an adaptive motion/force control approach

An adaptive motion/force controller is developed for unilateral or bilateral teleoperation systems. The method can be applied in both position and rate control modes, with arbitrary motion or force scaling. No acceleration measurements are required. Nonlinear rigid-body dynamics of the master and the slave robots are considered. A model of the flexible or rigid environment is incorporated into the dynamics of the slave, while a model of the human operator is incorporated into the dynamics of the master. The master and the slave are subject to independent adaptive motion/force controllers that assume parameter uncertainty bounds. Each parameter is independently updated within its known lower and upper bounds. The states of the master (slave) are sent to the slave (master) as motion/force tracking commands instead of control actions (efforts and/or flows). Under the modeling assumptions for the human operator and the environment, the proposed teleoperation control scheme is L/sub 2/ and L/sub /spl infin// stable in both free motion and flexible or rigid contact motion and is robust against time delays. The controlled master-slave system behaves essentially as a linearly damped free-floating mass. If the parameter estimates converge, the environment impedance and the impedance transmitted to the master differ only by a control-parameter dependent mass/damper term. Asymptotic motion (velocity/position) tracking and force tracking with zero steady-state error are achieved. Experimental results are presented in support of the analysis.     

QFT-based design of force and contact transition controllers for a flexible link manipulator

This paper deals with the modeling, system identification and robust control of flexible link manipulators that are required to perform contact task operations. For a single flexible link (SFL) manipulator in contact, two infinite dimensional models are developed and dynamic differences with respect to the force sensing devices are examined. Generalized orthonormal basis functions (GOBFs) are adopted for system identification and new algorithms are developed that improve the identification of resonant systems. The identification results, combined with estimated measures of model uncertainties, are directly used in the design of robust controllers. For the contact transition control, a switching condition is proposed based on robust position and force controllers. The stability of the switching controller is examined using a piecewise quadratic Lyapunov approach. Both simulation and experimental results are presented showing the effectiveness of the proposed technique.     

A position/force control for a robot finger with soft tip and uncertain kinematics

We consider the position and force regulation problem for a soft tip robot finger in contact with a rigid surface under kinematic and dynamic parametric uncertainties. The reproducing force is assumed to be related to the displacement through a nonlinear function whose characteristics are unknown, but both the actual displacement and force can be directly measured. Kinematic uncertainties concern the rigid surface orientation and the contact point location. Kinematic parameters involved in the contact point location concern the length from the last joint to the contact point and the rest of the link lengths in the general case. An adaptive controller with a composite update parameter law is proposed, and the asymptotic stability of the force and estimated position errors under dynamic and kinematic uncertainties is shown for the planar case. Simulation results for a three‐degrees‐of‐freedom planar robotic finger are presented. © 2002 Wiley Periodicals, Inc.     

Dynamic Analysis and Control Synthesis of Grasping and Slippage of an Object Manipulated by a Robot

Grasping an object by a cooperating system such as multi-fingered hands and multi-manipulator robotic system has received much attention. Research has focused on analysis of force-closure grasps and the synthesis of optimal grasping, when there is no slipping condition. Although the control system is designed to keep the contact force in the friction cone and avoid the slipping condition, slippage can occur for many reasons. In this research, dynamics analysis and control synthesis of a manipulator moving an object on a horizontal surface using the contact force of an end-effector are performed considering the slipping condition. Equality and inequality equations of frictional contact conditions are replaced by a single second-order differential equation with switching coefficients in order to facilitate the dynamic modeling. Accuracy of this modeling is verified by comparing the results of the model with those of SimMech. Using this modeling of friction, a set of reduced order form is obtained for equations of motion of the system. A new method is proposed to control the object motion and the end-effector undesired slippage based on the reduced form. Finally, performance of the method is evaluated both numerically and experimentally.     

Experiments in contact control

This article describes the implementation, experimentation, and application of contact control schemes for a 7-DOF Robotics Research arm. The contact forces and torques are measured in the sensor frame by the 6-axis force/torque sensor mounted at the wrist, are compensated for gravity, and then are transformed to the tool frame in which the contact task is defined and executed. The contact control schemes are implemented on the existing robot Cartesian position control system at 400Hz, do not require force rate information, and are extremely simple and computationally fast. Three types of contact control schemes are presented: compliance control, force control, and dual-mode control. In the compliance control scheme, the contact force is fed back through a lag-plus-feedforward compliance controller so that the end-effector behaves like a spring with adjustable stiffness; thus the contact force can be controlled by the reference position command. In the force control scheme, a force setpoint is used as the command input and a proportional-plus-integral force controller is employed to ensure that the contact force tracks the force setpoint accurately. In the dual-mode control scheme, the end-effector approaches and impacts the reaction surface in compliance mode, and the control scheme is then switched automatically to force mode after the initial contact has been established. Experimental results are presented to demonstrate contact with hard and soft surfaces under the three proposed control schemes. The article is concluded with the application of the proposed schemes to perform a contact-based eddy-current inspection task. In this task, the robot first approaches the inspection surface in compliance control until it feels that it has touched the surface, and then automatically levels the end-effector on the surface. The robot control system then transitions to force control and applies the desired force on the surface while executing a scanning motion. At the completion of the inspection task, the robot first relaxes the applied force and then retracts from the surface. © 1996 John Wiley & Sons, Inc.     

On the decoupling of position and force controllers in constrained robotic tasks

In hybrid control of robot manipulators separate controllers are designed for force and position errors control. Controllers are designed either in task or joint space and their outputs combine to provide input torque to the manipulator. Position and force controllers performance in a constrained robotic task is affected by their interaction to a degree dependent on the controller's ability to reject disturbances. Ideally, decoupling of the two control loops is desired to achieve the best performance in position and force directions. In this article, analysis of control loop interactions is performed for contact and noncontact phases, and controller design requirements are developed to achieve maximum decoupling. Design requirements involve output subspace of each controller leading to control discontinuities for contact and noncontact phases. In the noncontact phase, satisfaction of design requirements leads to a fully linearized and decoupled system. When in contact with the constraining surface, design requirements eliminate disturbances in the force loop, but minimize disturbances in the position loop to an extent dependent on force loop performance. Known hybrid control schemes analysis is performed to reveal existence of control loop interactions in these schemes. Confirmation of theoretical analysis is done through simulation of a three revolute planar manipulator. © 1998 John Wiley & Sons, Inc.     

Rolling Contact Motion Generation and Control of Robotic Fingers

Dexterity in human hand is connected with the fingertip rolling ability. In this work we consider rolling motion of spherical robotic fingertips as one of the control objectives together with the set point position control and force trajectory tracking. The generation of a rolling motion trajectory is proposed and a control solution is designed which achieves prescribed transient and steady state tracking behavior. The proposed control law is structurally and computationally simple and does not utilize the dynamics of the robot model or its approximation. A simulation of a five degrees of freedom robot show excellent contact rolling performance even at cases of adverse friction conditions while alternative controllers lead to contact sliding. Experiments with a KUKA LWR4 + are performed to validate the proposed method.