Neural network impedance force control of robot manipulator

The performance of an impedance controller for robot force tracking is affected by the uncertainties in both the robot dynamic model and environment stiffness. The purpose of this paper is to improve the controller robustness by applying the neural network (NN) technique to compensate for the uncertainties in the robot model. NN control techniques are applied to two impedance control methods: torque-based and position-based impedance control, which are distinguished by the way of the impedance functions being implemented. A novel error signal is proposed for the NN training. In addition, a trajectory modification algorithm is developed to determine the reference trajectory when the environment stiffness is unknown. The robustness analysis of this algorithm to force sensor noise and inaccurate environment position measurement is also presented. The performances of the two NN impedance control schemes are compared by computer simulations. Simulation results based on a three-degrees-of-freedom robot show that highly robust position/force tracking can be achieved in the presence of large uncertainties and force sensor noise     

A Robust Position and Force Control Strategy for 7-DOF Redundant Manipulators

This paper is concerned with robust position and contact force control for 7-DOF redundant robot arms. An outer-inner loop controller, called the augmented hybrid impedance control scheme is developed. A 6-DOF force/torque sensor is used to measure the interaction forces. These are fed back to the outer-loop controller that implements either a force or an impedance controller in each of the 6 DOF of the tool frame. The force controller is provided with a force set point, and desired inertia and damping are introduced in the force control loop to improve transient performance. The inner loop consists of a Cartesian-level potential difference controller, a redundancy resolution scheme at the acceleration level, and a joint-space inverse dynamics controller. Experimental results for two 7-DOF robot arms (redundant, dextrous, isotropically enhanced, seven-turning pair robot (REDIESTRO) and Mitsubishi PA10-7C) are given to illustrate the performance of the force control strategy. A successful application of the proposed scheme to a surface cleaning task is described using the REDIESTRO, while position and force tracking experiments are described for the Mitsubishi PA10-7C robot.     

A stable transition controller for constrained robots

This paper addresses the problem of contact transition from free motion to constrained motion for robots. Stability of transition from free motion to constrained motion is essential for successful operation of a robot performing general tasks such as surface following and surface finishing. Uncertainty in the location of the constraint can cause the robot to impact the constraint surface with a nonzero velocity, which may lead to bouncing of the robot end-effector on the surface. A new stable discontinuous transition controller is proposed to deal with contact transition problem. This discontinuous transition control algorithm is used when switching from free motion to constrained motion. Control algorithm for a complete robot task is developed. Extensive experiments with the proposed control strategy were conducted with different levels of constraint uncertainty and impact velocities. Experimental results show much improved transition performance and force regulation with the proposed controller. Details of the experimental platform and typical experimental results are given     

Experimental study of contact transition control incorporatingjoint acceleration feedback

Joint acceleration and velocity feedbacks are incorporated into a classical internal force control of a robot in contact with the environment. This is intended to achieve a robust contact transition and force tracking performance for varying unknown environments, without any need of adjusting the controller parameters. A unified control structure is proposed for free motion, contact transition, and constrained motion in view of the consumption of the initial kinetic energy generated by a nonzero impact velocity. The influence of the velocity and acceleration feedbacks, which are introduced especially for suppressing the transition oscillation, on the postcontact tracking performance is discussed. Extensive experiments are conducted on the third joint of a three-link direct-drive robot to verify the proposed scheme for environments of various stiffnesses, including elastic (sponge), less elastic (cardboard), and hard (steel plate) surfaces. Results are compared with those obtained by the transition control scheme without the acceleration feedback. The ability of the proposed control scheme in resisting the force disturbance during the postcontact period is also experimentally investigated     

Contact transition control via joint acceleration feedback

Stable and controllable transition from free motion to constrained motion is of central importance for robots in contact with the environment in many applications. In this paper, a joint acceleration feedback control scheme of high bandwidth is employed to damp oscillations during the contact transition when the approaching speed does not vanish. In this control scheme, a classical integral force controller is refined by means of joint acceleration and velocity feedback. This is intended to achieve a stable contact transition without need of adjusting the controller parameters adaptive to the unknown or changing environments. Extensive experiments are conducted on the third joint of a three-link direct-drive robot to verify the proposed scheme for the environments of various stiffnesses, including elastic (sponge), less-elastic (cardboard), and hard (steel plate) surfaces. Results are also compared with those by the transition control without the acceleration feedback. The proposed scheme is shown to be promising in terms of robustness, stability and adaptability     

An interaction controller formulation to systematically avoid force overshoots through impedance shaping method with compliant robot base

Nowadays, light-weight manipulators are widely adopted in many applications requiring manipulation/interaction with compliant/fragile objects. Reduced inertia and controlled compliance, indeed, make such manipulators particularly attractive when compliant mountings (or mobile platforms) are adopted and contact force overshoot may compromise the application. The here presented work proposes the design of a force-tracking controller for interaction tasks allowing to systematically avoid any force overshoot for lightweight robots mounted on compliant bases. The developed algorithm allows to compensate for the compliant robot base dynamics that affects the interaction. The control gains are calculated to track a target force reference through the estimation of the robot base state and the interacting environment stiffness. Closed-loop stability and control gains calculation are described. The control law has been validated in a probing task involving a compliant robot base and a compliant environment to show the obtained performance.     

Generalized impedance control of robot for assembly tasks requiringcompliant manipulation

To perform assembly tasks requiring compliant manipulation, the robot must follow a motion trajectory and exert an appropriate force profile while making compliant contact with a dynamic environment. For this purpose, a generalized impedance in the task space consisting of a second-order function relating the motion errors and interaction force errors is introduced such that the contact force can be commanded and controlled. With generalized impedance control, the robot can behave with a desired dynamic characteristic when it interacts with the environment. To ensure the success of the assembly, a strategy during task planning which takes into consideration the interrelation between motion and force trajectories as well as contact compliance is introduced. The generalized impedance control method is applied to the prismatic joint of a selective compliance assembly robot arm (SCARA) robot for inserting a printed circuit board (PCB) into an edge connector socket. Depending on the progress of the parts joining operation, various amount of interaction forces are generated which have to be accommodated. It is demonstrated that an assembly strategy which consists of a sequence of carefully planned target impedance can enable the task to be executed in a desirable manner. The effectiveness of this approach is illustrated through experiments by comparing the results with those obtained using a well-established position control scheme as well as the original impedance control method     

A unified approach to position and force control by fuzzy logic

This paper proposes a unified control strategy of position and force. The authors' technique is based on impedance control with fuzzy logic and realizes the smooth shift from position control to force control and vice versa. At first, a robust impedance controller based on a disturbance observer is shown, and the method to unify the position and the force control through one controller is described. Next, an algorithm to estimate the dynamic characteristics of the environment is shown, and a force tracking control using the estimated parameters is proposed. Finally, the unified control algorithm of position and force based on fuzzy logic is established. The validity of this method is confirmed by several experimental results     

Design of a robot gripper with force feedback control

The design of a two-jaw robot gripper using a D.C. servomotor with optical encoder is described. Force control of the gripper is achieved using armature current sensing as a means of detecting motor torque and hence the force applied between the gripper jaws.The motor was modelled mathematically to obtain the second order equation relating armature current to the applied motor voltage. This relationship was used to develop a proportional plus integral digital control algorithm enabling force feedback to be achieved.Two-way communication between the ASEA robot controller and the IBM-XT gripper controller enables a range of gripping forces to be accessed throughout the robot programme cycle, any one of which may be selected at a particular time. Anticipated problems associated with the thermal drift and transient responses have largely been overcome by repeatedly recalibrating the gripper throughout a work shift.     

Robust neural force control scheme under uncertainties in robotdynamics and unknown environment

The original impedance function is known to lack robustness due to unknown robot dynamic model and the environment. In order to improve that result, a new impedance function is derived which specifies a desired force directly. This results in a new robust robot force tracking impedance control scheme, which employs a neural network as a compensator to cancel out all uncertainties. The proposed neural force control scheme is capable of making the robot track a specified desired force as well as of compensating for uncertainties in environment location and stiffness, and in robot dynamics. Separate training signals for free-space motion and contact-space motion control are developed to train the neural compensator online. The design of the training signals is justified. Simulation studies with a three-link rotary robot manipulator are carried out and the results show excellent force tracking performance     

Robust PID control of fully-constrained cable driven parallel robots

In this paper dynamic analysis and robust PID control of fully-constrained cable driven parallel manipulators are studied in detail. Since in this class of manipulators cables should remain in tension for all maneuvers in their workspace, feedback control of such robots becomes more challenging than that of conventional parallel robots. In this paper, structured and unstructured uncertainties in dynamics of the robot are considered and a robust PID controller is proposed for the cable robot. To ensure that all cables remain in tension internal force concept is used in the proposed PID control algorithm. Then, robust stability of the closed-loop system with proposed control algorithm is analyzed through Lyapunov direct method and it is shown that by suitable selection of the PID controller gains, the closed-loop system would be robustly stable. Finally, the effectiveness of the proposed PID algorithm is examined through experiments on a planar cable driven robot and it is shown that the proposed control structure is able to provide suitable performance in practice.     

Robust force control with a feed-forward inverse model controller for electro-hydraulic control loading systems of flight simulators

A hybrid control strategy for an electro-hydraulic control loading system (EHCLS) of a flight simulator in the presence of a control mechanism kinetic parameter perturbation is proposed to improve the force tracking accuracy and guarantee robust stability of the EHCLS system. A double-loop model of the EHCLS, including the control mechanism and the hydraulic mechanism, is established and analyzed from the force-displacement impedance perspective. A force closed-loop parameter model of the EHCLS is identified by a recursive-least-squares (RLS) algorithm and its inverse model is designed using a zero phase error compensation technology to expand the frequency bandwidth of the force closed-loop system of the EHCLS. A μ theory of robust control is employed to design a stable controller for enhancing robust stability of the EHCLS in the presence of uncertainties of the inner loop, the control mechanism and the high frequency disturbance force. Simulation and experimental results show that the proposed hybrid control approach can greatly improve the control performance of the EHCLS by expanding the frequency bandwidth of the force closed-loop system and enhancing stability of the EHCLS, which can decrease displacement output response error of the EHCLS from 10.34% to 3.1%.     

Adaptive incremental sliding mode control for a robot manipulator

An adaptive incremental sliding mode control (AISMC) scheme for a robot manipulator is presented in this paper. Firstly, an incremental backstepping (IBS) controller is designed using time-delay estimation (TDE) to reduce dependence on the mathematical model. After substituting IBS controller into the nonlinear system, a linear system w.r.t. tracking errors is obtained while TDE error is the disturbance. Then, the AISMC scheme, including a nominal controller and an SMC, is developed for the resulted linear system to improve control performance. According to the equivalent control method, the SMC in the AISMC scheme is to handle TDE error. To receive optimal control performance at the sliding manifold, an LQR controller is selected as the nominal controller. The SMC is designed based on positive semi-definite barrier function (PSDBF) since it prevents switching gains from being over/under-estimated, and two practical problems are addressed in this paper: A new PSDBF is designed and conservative (large) setting bounds affecting tracking precision and/or system stability are avoided; An improved PSDBF based SMC is developed where the PSDBF and an adaptive parameter are used simultaneously to regulate switching gains, and the system is still stable when sliding variable occasionally exceeds the predefined vicinity. Moreover, finite-time convergence property of the sliding variable is strictly analyzed. Finally, real-time experiments are conducted to verify the effectiveness of the proposed control method.     

A learning scheme for low-speed precision tracking control of hybrid stepping motors

Servo control of the hybrid stepping motor is complicated due to its highly nonlinear torque-current-position characteristics, especially under low operating speeds. This paper presents a simple and efficient control algorithm for the high-precision tracking control of hybrid stepping motors. The principles of learning control have been exploited to minimize the motor's torque ripple, which is periodic and nonlinear in the system states, with specific emphasis on low-speed situations. The proposed algorithm utilizes a fixed proportional-derivative (PD) feedback controller to stabilize the transient dynamics of the servomotor and the feedforward learning controller to compensate for the effect of the torque ripple and other disturbances for improved tracking accuracy. The stability and convergence performance of the learning control scheme is presented. It has been found that all error signals in the learning control system are bounded and the motion trajectory converges to the desired value asymptotically. The experimental results demonstrated the effectiveness and performance of the proposed algorithm.     

An experimental study on improving hybrid position/force control of a robot using time delay control

Robot position/force control has been a difficult task owing to the interaction between a robot and the environment. In addition to the dynamic instability, the interaction causes the following problems: (1) the dynamic coupling effect of the robot; (2) positional disturbance due to the uncertain surface of the environment with which the robot is in contact; and (3) vibration at steady state. To solve these problems, time delay control (TDC), well known for its robustness to plant uncertainties and disturbances, has been used as a baseline control law for hybrid control. In conjunction with TDC, the following three ideas were also used. (i) To reduce the dynamic coupling, a more accurate mass matrix was used instead of the constant mass matrix. (ii) To reject positional disturbance from the environment, force derivatives instead of position derivatives were used in the TDC law. (iii) Finally, to reduce the amplitude of the vibration at steady state, a novel scheme was adopted to enhance the resolution of A/D conversion for the force sensor. Experiments showed obvious improvements in the quality of the hybrid control, thereby clearly demonstrating the effectiveness of TDC with the proposed ideas.     

Robust control of positioning systems with a multi-step bang-bang actuator

A nonlinear control scheme for preventing the limit cycle due to the nonlinearity of the multi-step bang-bang actuator in mechanical position control systems is proposed. A linearized model, sinusoidal input describing function (SIDF), for a multi-step bang-bang actuator is introduced to compensate the nonlinearity of the multi-step bang-bang actuator. Using that model, an H_∞ robust controller for position control systems with a bang-bang actuator is proposed by loop shaping techniques with normalized coprime factorization stabilization to address the robustness. The proposed scheme requires a smaller deadband as a result of compensating the nonlinearity of the bang-bang actuator. A single-axis servo system is implemented in order to verify the proposed control scheme experimentally. Experimental results show that the controller can satisfy the special interest, silent contact switching of the actuator.     

Hybrid control of a three-pole active magnetic bearing

The design and implementation of the hybrid control method for a three-pole active magnetic bearing (AMB) is proposed in this paper. The system is inherently nonlinear and conventional nonlinear controllers are a little complicated while the proposed hybrid controller has a piecewise linear form, i.e., linear in each sub-region. A state-feedback hybrid controller is designed in this study and the unmeasurable states are estimated by an observer. The gains of the hybrid controller are obtained by the LQR method in each sub-region. To evaluate the performance, the designed controller is implemented on an experimental setup. The experimental results show that the proposed method can efficiently stabilize the three-pole AMB system. The simplicity of design, domain of attraction, uncomplicated control law and computational time are advantages of this method over other nonlinear control strategies in AMB systems.     

Development and hybrid force/position control of a compliant rescue manipulator

Performing search and rescue tasks in the ruins after disasters demand rescue robots with slender and compliant structure to accommodate the complicated configurations under debris. This paper presents the structural design and system composition of a novel tendon-sheath actuated compliant rescue manipulator with slender and flexible body. The proposed robot can drill into the narrow space where rescuers and traditional rigid robots cannot get in because of size limitation or toxic environment. The self-sensing calibration, dynamic modeling, and hybrid force/position control trajectory of the compliant gripper with integrated position and force monitoring capabilities are analyzed and discussed. With the aim of regulating the gripper displacement and clamping force during operation, a hybrid force/position control strategy is proposed based on a cascaded proportional-integral-derivative (PID) controller and a fuzzy sliding mode controller (FSMC). Experimental setups mainly consisting of servo motor, tendon sheath transmission components, compliant gripper, and real-time control system are established to calibrate the strain gauge sensors and identify the dynamic model parameters. Further experimental investigations involving force tracking experiments, position tracking experiments, and object grasping experiments are carried out. The experimental results demonstrate the effectiveness of the developed self-sensing approach and control strategies during rescue operation.     

Multi-hierarchy interaction control of a redundant robot using impedance learning

The control of robots with a compliant joint motion is important for reducing collision forces and improving safety during human robot interactions. In this paper, a multi-hierarchy control framework is proposed for the redundant robot to enable the robot end-effector to physically interact with the unknown environment, while providing compliance to the joint space motion. To this end, an impedance learning method is designed to iteratively update the stiffness and damping parameters of the end-effector with desired performance. In addition, based on a null space projection technique, an extra low stiffness impedance controller is included to improve compliant joint motion behaviour when interaction forces are acted on the robot body. With an adaptive disturbance observer, the proposed controller can achieve satisfactory performance of the end-effector control even with the external disturbances in the joint space. Experimental studies on a 7 DOF Sawyer robot show that the learning framework can not only update the target impedance model according to a given cost function, but also enhance the task performance when interaction forces are applied on the robot body.     

Adaptive impedance control of a haptic interface

Teleoperation enables an operator to manipulate remote objects. One of the main goals in teleoperation research is to provide the operator with the feeling of the telepresent object and of being present at the remote site. In order for this to happen, a master robot must be designed as a bilateral control system that can transmit position commands to a slave robot and reflect the interaction force. A newly proposed adaptive impedance algorithm is applied to the force control of a haptic interface that has been developed as a master robot. With the movement of the haptic interface for position command generation, the impedance between an operator and the haptic interface varies dynamically. When the impedance parameters and the dynamics of the haptic interface are known precisely, many model based control theories and methods can be used to control the interface accurately. However, due to the parameters’ variations and the uncertainty in the dynamic model, it is difficult to control the interface precisely. Therefore, this paper proposes a new adaptive impedance control algorithm and experimentally verifies the effectiveness of the algorithm for control of the haptic interface.