Implicit Force Control Research Articles

Implicit force and position control to improve drilling quality in CFRP flexible robotic machining

Flexible machining systems are being widely applied in the aircraft and automotive industries to reduce costs and increase productivity. The workpieces for the corresponding applications are bigger and more varied than those for conventional applications. These workpieces can be made of carbon fiber reinforced plastic (CFRP) because it provides a high strength-to-weight ratio. Nevertheless, CFRP has poor machinability owing to its brittleness. To achieve flexible machining, industrial robots are being increasingly adopted, especially for drilling. However, they cannot meet industrial requirements such as machining precision, cost, and productivity. This is because industrial robots have a considerably lower stiffness than conventional computer numerical control machines given their structure consisting of serial links. Consequently, tasks such as drilling using industrial robots may present production defects. Drilling defects generally appear as burrs in the hole entrance and deviations of the desired hole center. In this study, the patterns and causes of such defects during robotic drilling were analyzed, and it was determined that the main defect was caused by the deviation of the tool tip due to the low robot stiffness. The defect was then corrected using implicit force and position control. Experimental results showed that when the defect was corrected, the maximum hole diameter error decreased by 16.68% when compared with a scenario in which no compensation for the defect was made. In addition, the maximum error decreased by 10.83% when compared with drilling that was guided by a pilot hole.

A Robust Hybrid Position/Force Control Considering Motor Torque Saturation

This paper proposes a robust hybrid position/force control (HPFC) system for multi-degree of freedom manipulators (MDoFMs) with torque constraints on each joint. General HPFC systems can control the interactive contact forces and positions of manipulators in various environments. In HPFC systems, to improve the control performance for MDoFMs, motor torque saturation should be considered. Thus, a robust HPFC system that considers torque constraints by predictive functional control (PFC) is proposed in this paper. The proposed method simultaneously handles the response characteristics and torque constraints of actuators by using PFC as joint space position controllers. Additionally, the robustness of actuators against external forces is enhanced, and the model parameter errors are compensated by the disturbance observer technique. Moreover, implicit force control law and inverse kinematics are introduced for the workspace position/force control to implement the joint space position controllers. Consequently, the stability and robustness of the actuators are considered, and suitable position/force control can be performed even if the torque is saturated. The validity of the proposed method is tested with three planar manipulator joints with a uniaxial force sensor.

Implicit force control approach for Safe physical Robot-to-Human object handover

This research focuses on the development of the conceptual frameworks of human-human interaction applied for a robotic behaviour-based approach for safe physical human-robot interaction. The control has been constructed based on understanding the dynamic and kinematic behavioural characteristics of how two humans pass an object to each other. This has enabled a KR-16-KUKA robot to naturally interact with a human so as to facilitate the dexterous transfer of an object in an effective manner. Implicit force control based on Proportional Integral and Fuzzy Logic Control which allows the robot end effector’s trajectory to be moderated based on the applied force in real-time was adopted. The experimental results have confirmed that the quantitative performance of the force-controlled robot is close to that of the human and can be considered acceptable for human-robot interaction. Furthermore, the control based Fuzzy Logic Control was shown to be slightly superior performance compared to Proportional Integral control.

Mixed Data-Driven and Model-Based Robot Implicit Force Control: A Hierarchical Approach

This paper presents a hierarchical data-driven/ model-based control architecture to robot implicit force control aiming at enhancing closed-loop performance. An inner data-driven controller, based on a recursive implementation of virtual reference feedback tuning, and relying on a model-based description of the robot compliance, prevents the deterioration of the force controller performance associated with environment modeling and identification. Then, an outer model predictive controller (MPC) acting as a reference governor selects on-line the optimal reference fed to the inner virtual reference feedback tuning closed-loop system in order to improve the closed-loop performance. The linear-time-invariant (LTI) formulation of the MPC allows to explicitly obtain the MPC control law and further demonstrate closed-loop stability. The effectiveness of the proposed control strategy is experimentally validated on force regulation experiments performed on different environment materials with a six-degree-of-freedom industrial robot equipped with a force/torque sensor.

Implicit Robot Force Control Based on Set Invariance

This paper addresses the problem of robot implicit force control by means of set invariance theory. The proposed control law improves the invariance control approach by avoiding the definition of a nominal stabilizing controller, while simultaneously ensuring the absence of output overshoots with respect to the reference. The resulting set invariance based controller is first introduced for a single-input single-output system and then extended to multi-input multi-output systems, in order to be applied to the robot implicit force control problem. Its effectiveness is experimentally validated and compared to state-of-the-art approaches on a hybrid force/position task performed with a 6 DOF industrial robot equipped with a force/torque sensor.

Ultrasound-guided three-dimensional needle steering in biological tissue with curved surfaces

In this paper, we present a system capable of automatically steering a bevel-tipped flexible needle under ultrasound guidance toward a physical target while avoiding a physical obstacle embedded in gelatin phantoms and biological tissue with curved surfaces. An ultrasound pre-operative scan is performed for three-dimensional (3D) target localization and shape reconstruction. A controller based on implicit force control is developed to align the transducer with curved surfaces to assure the maximum contact area, and thus obtain an image of sufficient quality. We experimentally investigate the effect of needle insertion system parameters such as insertion speed, needle diameter and bevel angle on target motion to adjust the parameters that minimize the target motion during insertion. A fast sampling-based path planner is used to compute and periodically update a feasible path to the target that avoids obstacles. We present experimental results for target reconstruction and needle insertion procedures in gelatin-based phantoms and biological tissue. Mean targeting errors of 1.46±0.37mm, 1.29±0.29mm and 1.82±0.58mm are obtained for phantoms with inclined, curved and combined (inclined and curved) surfaces, respectively, for insertion distance of 86–103mm. The achieved targeting errors suggest that our approach is sufficient for targeting lesions of 3mm radius that can be detected using clinical ultrasound imaging systems.

Implicit force control for industrial robots in contact with stiff surfaces

The implicit force control design for an industrial robot with elastic joints, in contact with a stiff surface, is discussed in this paper. First, it is shown that the dominant dynamics of the system are characterized by a low-frequency mode arising from the interplay between the integral action of the PID position controller and the deformation of the transmission. A simple compensation of these linear dynamics allows recovery of an almost algebraic relation between torque commands and reaction forces, typical of an explicit force control strategy. Then it is shown that an integral controller on the force error outperforms a PD controller in terms of stability of the closed-loop system, in particular as far as the robustness with respect to the contact stiffness is concerned. This result extends to the implicit force control a similar finding previously obtained for the explicit force control. Experimental results, gathered on the industrial robot COMAU SMART 3S, are also reported.

Linear Quadratic Frequency-Shaped Implicit Force Control of Flexible Link Manipulators

An implicit force control scheme for flexible link manipulators is considered in this article. The control output is composed of a feedforward and a feedback term. The feedforward torque component compensates the underlying rigid arm dynamics along the desired trajectory. The feedback component regulates the joint coordinate error perturbations. The minimization of a linear quadratic frequency-shaped cost functional yields the time-varying feedback controller gains. The frequency shaping dependence is included to eliminate undesirable effects associated with control and observation spillover. The proposed control scheme is employed in simulation studies on a two link rigid-flexible manipulator. Sufficient conditions for spatial tracking bounds and temporal gain switching bounds for the linearized rigid body error dynamics are provided. The effects of the linearized unmodelled error dynamics related to the flexible states on these bounds are also examined.

A Predictive Control Application to Force Control of Robotic Manipulators

This paper presents a force control strategy for robotic manipulators in the presence of non-rigid environments. The approach combines two distinct control methodologies in a force control scheme: the well known implicit force control, namely impedance control, and a predictive control algorithm. The predictive controller generates the necessary target positions and velocities to the impedance controller, which results in a desired force profile. The performance of the control scheme was tested in a two degree-of-freedom PUMA 560 robot, which end-effector is forced to move along a frictionless surface located on the horizontal plane. The simulation results obtained reveal an accurate force tracking performance for the overall force control scheme, especially in the presence of non-rigid environments.

Implicit Force Control for Industrial Robots in Contact with Stiff Surfaces

The implicit force control design for an industrial robot with elastic joints, in contact with a stiff surface, is discussed in this paper. It is shown that the dominant dynamics of the system is characterized by a low-frequency mode related to the interaction between the integrator of the PID position controller and the elasticity of the transmission. It is also shown that an integral controller on the force error outperforms a PD controller in terms of stability of the closed loop system. Experimental results, gathered on the industrial robot COMAU SMART 3S, confirm the theoretical analysis.

Robot assisted knee surgery

Discusses establishing a force control strategy incorporating active motion constraint. The following subjects are considered: problems with conventional surgery; robot assisted surgery; control strategy; representing the motion constraint; design of the desired position; force control strategy; implicit force control; modified damping control; experimental results. >

Design of a Force Controller Based on Frequency Characteristics.

This paper proposes a method to design a force controller based on frequency characteristics. Generally, force control algorithms are classified into two types, the explicit force control algorithms such as Hybrid position/force control and the implicit force control algorithms such as impedance control. The explicit force control algorithms control the contact force directly, implicit force control algorithms control the manipulator motion-force relation. Recently, the equivalence of these two types of algorithms has been shown by A. A. Goldenberg (1992), and R. Volpe and P. Khosla (1993). In this paper, we propose a method to design an explicit force controller based on the equivalence to the impedance controller, so that the resultant control system has the desired frequency characteristics. The method is applied to a manipulator with one degree of freedom and experimental results illustrate the effectiveness of the proposed method.