2 - Preliminary simulation of intelligent force control

In this paper, we propose an adaptive nonlinear strategy for the motion and force control of manipulators with flexible joints. Our approach provides force control when in contact and robust motion control in its absence, all without the need for a control switch. This self-tuning behaviour for mixed contact/non-contact scenarios results from a unified formulation of force and motion control, with an integral transpose-based inverse kinematics core and adaptive update laws to cope with the manipulator flexibility and the contact stiffnesses. The global boundedness of all signals and the asymptotic stability of this controller are guaranteed via Lyapunov analysis. Finally, we validate its applicability experimentally by using low-cost hardware in a realistic mixed-contact scenario, demonstrating low computational demand.

Two different control schemes, position model referenced control (PMRC) and force model referenced control (FMRC), are developed for a simplified two robots model with one interacting axis. The dynamic stability of the position (or force) control under the FMRC (or PMRC) is analyzed. Results indicate that PMRC, although providing better dynamic performance of the position control, may degrade the dynamic performance of the force control. However, FMRC can provide better dynamic performance of both position and force control.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

A hydraulic drive unit (HDU), applied in each joint of a hydraulic driven legged robot, usually adopts the impedance control outer loop based on the hydraulic position or the force control inner loop during the motion process. When the hydraulic control adopts closed-loop force control, its control performance can directly determine the control performance of the impedance control outer loop. Therefore, it is significant to design a high-accuracy force control method aimed at HDUs. In this paper, aiming at the above research concept, the force control system of the HDU is introduced first, and the lack of the force control performance is studied under different working conditions on the HDU performance test platform. Second, a model-based variable input controller (MVIC) is designed to improve the force control performance. Finally, the control performance of the MVIC is verified on the HDU performance test platform. The experimental results show that the MVIC can greatly improve the force control accuracy under different working conditions and that the controller has good robustness. The above research results can provide important reference and lay the experimental foundation for force-based impedance control.

In this paper, the problem of force feedback control of kinematically defective manipulators (KDMs) is considered. KDMs are robot manipulators that have fewer joints than the dimension of the space in which their end-effector moves. It is well known that controlling the end-effector velocity of an n-joint KDM can be easily solved by appropriately selecting n components of the output twist, thus squaring the control problem. On the contrary, we show that such a component selection approach is not appropriate in general to solve the force control problem for KDMs. In particular, for advanced force control applications, such as comanipulation, where the contact geometry is not known in advance, the selection of the wrench components leads to a lack of passivity, which in turn may induce instability. This instability does not arise from the system dynamics. Rather, it can be viewed as a new form of kinematic instability. Moreover, by formulating the problem in the joint space, we show how to properly design a stable force controller for KDMs subject to arbitrary external forces applied to their end-effector. Furthermore, we propose several implementations for pure force control and damping control. Experimental results with a kinematically defective laparoscopic comanipulator illustrate these propositions.

An experimental and theoretical comparison is made of force control performance with different types of innerloop joint servoing techniques. The problem of disturbance rejection and sensitivity to plant dynamics variations (robustness) is addressed. Position, velocity, strain gauge derived joint torque, and current servos are designed and implemented on a specially instrumented industrial robot, and the end-effector force feedback performances achieved are compared. Joint strain derived torque servoing is found to provide the best overall robust force control performance. Experimental results of the robust hard-on-hard contact achieved with the novel force controller implementation based on joint torque sensing are provided. Conclusions are drawn on the force control performance achievable on a geared robot given the joint servoing technique. >

A dynamic force controller based on momentum is implemented to the redundant system in this paper. With this method, the centrifugal and Coriolis terms are eliminated from the system and precise and stable force control is realized. Firstly, force control based on momentum is described. This controller does not have any singular points and is able to be applied to dynamic state. A null space controller for the dynamic controller is proposed and is constructed in the angular momentum dimension. Then, position and force vector is united into work space using equivalent mass matrix. Several simulation results are shown to confirm the validity of the proposed method.

In teleoperation tasks, communication delays between master and slave sides negatively affect the stability and transparency of closed loop system, and make it difficult to maintain a desired contact force. In order to improve the performance of contact force control in teleoperation under large time delays, a shared compliant control method is proposed in this paper. On the master side, the operator issues a desired contact force command according to the feedback of the contact stiffness identified on line besides motion commands. And on the slave side, a local contact force controller is designed using an adaptive Smith predictor, so as to shares control load with the operator. Experimental results show that this method can improve the force control performance, lower the difficulty of operation, and help the operator complete contact tasks with proper forces.In teleoperation tasks, communication delays between master and slave sides negatively affect the stability and transparency of closed loop system, and make it difficult to maintain a desired contact force. In order to improve the performance of contact force control in teleoperation under large time delays, a shared compliant control method is proposed in this paper. On the master side, the operator issues a desired contact force command according to the feedback of the contact stiffness identified on line besides motion commands. And on the slave side, a local contact force controller is designed using an adaptive Smith predictor, so as to shares control load with the operator. Experimental results show that this method can improve the force control performance, lower the difficulty of operation, and help the operator complete contact tasks with proper forces.

In this paper an intelligent prediction algorithm for robot force control is reviewed. The algorithm is not only applied to impedance controller, but also hybrid position/force controller. The experimental platform with open architecture controller is developed to test the force tracking effect when the environmental change in curvature and stiffness is taken into account. Using the system an easily open architecture is achieved in hardware and software, which is able to deal with the implementation of force tracking tasks of unknown contact environment performed by robot manipulators. The force tracking experiments for the irregular surface in different control model, different tracking velocity, different desired force, different contact stiffness are executed. The comparative experimental results show validity of the proposed method.

This article presents the contact force control approach for a quad-rotor system to perform tasks of interacting with the environment. The hovering capability of the quad-rotor system allows the force in the altitude direction to be regulated by realizing the impedance function. To obtain the better force control performance, inherent and external disturbances to the quad-rotor system are suppressed by designing the acceleration-based disturbance observer (AbDOB). Force tracking impedance control is applied to regulate the contact force to the environment. Simulation studies of force tracking control for changing a light bulb on the ceiling are performed to evaluate the feasibility of the proposed force control task for a quad-rotor system.

A new approach to the interaction control of systems with unknown dynamics has been developed by combining natural admittance control (NAC) with time delay control (TDC). The proposed nonlinear controller does not require specific knowledge of the system's dynamics or the environment nor does it necessitate the estimation of system parameters, except inertia, rendering it easy to implement. It rejects unmodeled dynamics, nonlinearities and disturbances without difficult characterization process while preserving the desired dynamics. The simulation results demonstrate not only good external disturbance rejection, robustness to parameter changes and insensitivity to noise, but also demonstrate good trajectory following providing good rejection of internal Coulomb friction. This novel control technique is also used to develop a nonlinear bang-bang impact controller (NBBIC) for stabilization of a robot manipulator upon collision with a stiff environment. It uses the above-mentioned NAC/TDC control algorithm with negative force feedback to absorb impact forces and stabilize the system. This control input alternates with zero when no environment force is sensed due to loss of contact. This alternation of control action repeats until the impact transient subsides and steady state is established. After impact transient, proportional-derivative force control is used. It is shown via simulations that NBBIC provides stable interaction between the robot with severe nonlinear joint friction and a stiff environment and achieves rapid response while minimizing force overshoots.

The four-channel bilateral system that is a hybrid of position and force in the acceleration dimension based on a disturbance observer can be divided into two modal spaces: common and differential modal spaces. In the four-channel bilateral system, bilateral position control is implemented in the differential modal space, and bilateral force control is implemented in the common modal space, respectively. On the other hand, In the three-channel system, unilateral position control is implemented in the slave side, and bilateral force control is implemented in the common modal space. This paper analyzes the performance of position control and force control in four-channel and three-channel systems under time delay. According to the analysis, this paper proposes a method of velocity difference damping that utilizes the velocity difference between the robot and the robot model. This method improves the performance and stability of bilateral force control in the common modal space without deteriorating the position control. The validity of the proposed control system is confirmed by some experimental results.

The system treated in this paper is a model of a moulding press for a fabrication process of LSI packages. Performance of force control is important to obtain sufficient quality of the moulding. Considering uncertainty caused by flow characteristics of a servo valve and mechanical stiffness of a frame of the press machine, a method of designing a robust force controller by applying H infinity control theory is proposed. Performance of the robust controller is examined using an experimental setup of a water-hydraulic servo system

In this paper, an asymmetric cable-driven mechanism is proposed for accurate force control of exoskeleton systems with a compact structure. Inspired by the fact that the required forces in human motions are not symmetric in many cases, a spring-actuator type cable-drive mechanism is adopted, which enables a compact cable routing structure. The joint is connected with the exoskeleton frame through a rotary series elastic mechanism to transmit the desired force to the human user. High performance in force control is achieved by advanced control algorithms, which combine a proportional and differential (PD) controller optimized with a linear quadratic (LQ) method with a disturbance observer (DOB) and a zero phase error tracking (ZPET) feedforward filter. The proposed system was tested for the elbow joint. Experimental results confirmed that the proposed system was able to generate and deliver accurate force to the human user even with external disturbances and modeling uncertainties introduced by human motions.

This study is devoted to sensorless adaptive force/position control of robot manipulators using a position-based adaptive force estimator (AFE) and a force-based adaptive environment compliance estimator. Unlike the other sensorless method in force control that uses disturbance observer and needs an accurate model of the manipulator, in this method, the unknown parameters of the robot can be estimated along with the force control. Even more, the environment compliance can be estimated simultaneously to achieve tracking force control. In fact, this study deals with three challenging problems: No force sensor is used, environment stiffness is unknown, and some parametric uncertainties exist in the robot model. A theorem offers control laws and updating laws for two control loops. In the inner loop, AFE estimates the exerted force, and then, the force control law in the outer loop modifies the desired trajectory of the manipulator for the adaptive tracking loop. Besides, an updating law updates the estimated compliance to provide an accurate tracking force control. Some experimental results of a PHANToM Premium robot are provided to validate the proposed scheme. In addition, some simulations are presented that verify the performance of the controller for different situations in interaction.

Many current robotic applications are limited by the industry state of art of the manipulators control algorithms. The inclusion of force and vision feedbacks, the possibility of cooperation between two or more manipulators, the control of robots with irregular topology will certainly enlarge the industrial robotics applications. The development of control algorithms to this end brings the necessity of using open-architecture controllers. Generally the robotic controllers are developed for position control, without accomplishing integrally the requirements of tasks in which interactions with the environment occur. Therefore, this is currently one of the main research areas in robotics, e.g., in (Abele et al., 2007) is presented the identification of characteristics to an industrial robot to execute machining applications. To consider this interaction the robot controller has to give priority to the force control time response, because in the instant of end-effectors contact with the surface, several forces act on the system. Depending on the speeds and the accelerations involved in the process, damages or errors can occur. To avoid these effects, compliances are inserted in tool or in surface of operation. A new reference model for a control system functional architecture applied to openarchitecture robot controllers is presented. Where, this model is applied for integrally development of a five-layer based open-architecture robotic controller for interaction tasks, which uses parallel and distributed processing techniques, avoiding the necessity of compliance in system, allowing a real-time processing of the application and the total control of information. This architecture provides flexibility, the knowledge of all the control structures and allows the user to modify all controller layers. The used controller conception aims to fulfill with the following requirements: high capacity of processing, low cost, connectivity with other systems, availability for the remote access, easiness of maintenance, flexibility in the implementation, integration with a personal computer and programming in high level. 5