Multivariable feedforward-feedback motion control of a constrained two-link manipulator with a flexible forearm

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.

This paper is concerned with a direct teaching/playback method for a hybrid position and force control in a robot manipulation. By adjusting the impedance of the robot appropriately, a teaching operator can manipulate the robot tip easily in free space and constrained space by appling force to a teaching lever mounted at the tip of the robot. The operational force applied by the operator and the contact force acting on the smoothly curved constrained surface are stored in the computer as the teaching data. A given task in the playback process is to make a smooth contact with a constrained surface from the free space and to apply a specified forces while the tip of the robot is moving along the constrained surface. In the playback, the contact force is possible to follow the stored values of the contact force or the different values programmed after the teaching. To keep accuracies of the force control even when the location of the constrained surface is slightly changed from the teaching process, the well-known “hybrid control technique” is applied to obtain control laws. The method is applied to a 2-link vertical articulated direct-drive hydraulic robot. The experimental results affirm the validity of the proposed method.

In this study, the stable detection principle of a MEMS hardness sensor with “reference plane” structure is theoretically analyzed and demonstrated with experimental results. Hardness measurement independent of contact force instability is realized by the optimum design of the reference plane. The fabricated devices were evaluated, and a “shore A” hardness scale (JIS K 6301 A) was obtained as the reference in the range from A1 to A54 under a stable contact force. The contact force dependence on hardness sensor signals was effectively reduced by 96.6% using our reference plane design. Below 5 N contact force, the maximal signal error of hardness is suppressed to A8. This result corresponds to the detection capability for fat hardness, even when the contact force is unstable. Through experiments, stable detection of human body hardness has been demonstrated without any control of contact force.

SUMMARYThis paper addresses the problem of contact force control for multiple contacts distributed over multiple links in a robot. This is of importance when performing complex tasks in unstructured environment, particularly in humanoid robot applications. The proposed multicontact control framework provides a new way of defining the operational space coordinates, which facilitates the specification of multiple contact control. The contact force space on multiple links is constructed as an operational space for the highest priority task. Motion control, given lower priority, can be executed using the rest of degree of freedom within the null-space of the force control. The dynamic control structure, then, provides a means to control each contact force and motion independently. This dynamic decoupling enables each contact force controller to utilize linear control theories. In particular, the contact force controllers adopt full state feedback control and estimation methods to produce robust performance with respect to modeling and parameter uncertainties. The effectiveness of the multiple contact control framework was demonstrated using a PUMA560 manipulator, with multiple contacts on the end-effector and third link. The demonstrated tasks involved controlling each of the contact forces with null-space motion.

Constrained motion of a planar rigid-flexible manipulator is considered in this study. a dynamic model with zero tip deformation constraint of constrained rigid-flexible 608

Experimental evaluation of theoretical contact forces in the cat patellofemoral joint

This paper investigates the role of precise estimation of contact points in force control. This analysis is motivated by scenarios in which robots make contacts, either voluntarily or accidentally, with different parts of their body. Control paradigms that are usually implemented in robots with no tactile system, make the hypothesis that contacts occur at the end-effectors only. In this paper we try to investigate what happens when this assumption is not verified. First we consider a simple feedforward force control law, and then we extend it by introducing a proportional feedback term. For both controllers we find the error in the resulting contact force, that is induced by a hypothetic error in the estimation of the contact point. We show that, depending on the geometry of the contact, incorrect estimation of contact points can induce undesired joint accelerations. We validate the presented analysis with tests on a simulated robot arm. Moreover we consider a complex real world scenario, where most of the assumptions that we make in our analytical derivation do not hold. Through tests on the iCub humanoid robot we see how errors in contact localization affect the performance of a parallel force/position controller. In order to estimate contact points and contact forces on the forearm of the iCub we do not use any model of the environment, but we exploit its 6-axis force/torque sensor and its sensorized skin.

In this paper, a new continuum-based pantograph/catenary model based on the absolute nodal coordinate formulation (ANCF) is proposed and used to develop an effective method to control the contact force which arises from the pantograph/catenary interaction. In the proposed new model, only one ANCF gradient vector is used in the formulation of the pantograph/catenary contact conditions, thereby allowing for using the proposed approach for both fully parameterized and gradient deficient ANCF finite elements. A three-dimensional multibody system (MBS) model of a pantograph mounted on a train is developed using a nonlinear augmented MBS formulation. In order to take into account the catenary large deformation, ANCF finite elements are used. The contact between the pantograph and the catenary system is ensured using a sliding joint constraint whereas the contact between the rail vehicle wheels and the train track is modelled using an elastic contact formulation. In addition to the use of the new MBS approach to model the pantograph/catenary interaction, the contact force between the pantograph and the catenary is computed using a simpler lumped parameter model which describes the pan-head and the plunger subsystem dynamics. In order to reduce the standard deviation of the contact force without affecting its mean value, a control actuator is used between the pan-head and the plunger. To this end, three types of control laws for the control action are designed to improve the contact quality both in the transient phase and in the steady state phase of the pantograph/catenary interaction. The first control law proposed features a feedback structure whereas the second and the third control strategies employ a feedback plus feed-forward architecture. In order to demonstrate the effectiveness of the proposed method, the results of a set of numerical simulations with and without the controllers are presented.

This study introduces a reduced-order leg dynamic model to simplify the controller design and enhance robustness. The proposed multi-loop control scheme tackles tracking control issues in legged robots, including joint angle and contact-force regulation, disturbance suppression, measurement delay, and motor saturation avoidance. Firstly, model predictive control (MPC) and sliding mode control (SMC) schemes are developed using a simplified model, and their stability is analyzed using the Lyapunov method. Numerical simulations under two disturbances validate the superior tracking performance of the SMC scheme. Secondly, an Nth-order linear active disturbance rejection control (LADRC) is designed based on a simplified model and optimization problems. The second-order LADRC-SMC scheme reduces the contact-force control error in the SMC scheme by ten times. Finally, a fourth-order LADRC-SMC with a Smith Predictor (LADRC-SMC-SP) scheme is formulated, employing each loop controller independently. This scheme simplifies the design and enhances performance. Compared to numerical simulations of the above and existing schemes, the LADRC-SMC-SP scheme eliminates delay oscillations, shortens convergence time, and demonstrates fast force-position tracking responses, minimal overshoot, and strong disturbance rejection. The peak contact-force error in the LADRC-SMC-SP scheme was ten times smaller than that in the LADRC-SMC scheme. The integral square error (ISE) values for the tracking errors of joint angles θ1 and θ2, and contact force f, are 1.6636×10−28 rad2⋅s, 1.7983×10−28 rad2⋅s, and 1.8062×10−30 N2⋅s, respectively. These significant improvements in control performance address the challenges in single-leg dynamic systems, effectively handling disturbances, delays, and motor saturation.

Catheter-tissue contact force is a determinant of radiofrequency (RF) ablation lesion effectiveness. However, ablation on a beating heart is subject to force variability, making it difficult to optimally deliver consistently durable and transmural lesions. This work evaluates improvements in contact force stability and lesion reproducibility by using a catheter contact-force controller (CFC) during lesion delivery in vitro and in vivo. Using a sheath and force-sensing catheter, an experienced operator attempted to maintain a constant force of 20 g at targets within the atria and left ventricle of a pig manually and using the CFC; the average force and contact-force variation (CFV) achieved using each approach were compared. Ablation lesions (20 W, 30 seconds, 17 mL/min irrigation) were created in bovine tissue samples mounted on a platform programmed to reproduce clinically relevant motion. CFC-assisted lesions were delivered to stationary and moving tissue with forces of 5 to 35 g. Mimicking manual intervention, lesions were also delivered to moving tissue while the CFC was disabled. Resultant lesion volumes were compared using two-way analysis of variance. When using the CFC, the average force was within 1 g of the set level, with a CFV less than 5 g, during both in vitro and in vivo experiments. Reproducible and statistically identical (P = .82) lesion volumes proportional to the set force were achieved in both stationary and moving tissue when the CFC was used. CFC assistance maintains constant force in vivo and removes effect of motion on lesion volume during RF lesion delivery.

An aerial manipulator system consisting of a quadrotor UAV and a one-DOF manipulator is introduced for contacting operations in the air. As the system is interacting with external environment to do some operations, there will exist contact force. According to the practical aerial contact manipulation features, it is known that an aerial manipulator usually performs operations as the UAV platform is in hover flight or nearby. To realize controlling the force, the hovering UAV acting as a spring-mass-damper system is firstly proved. Then, based on the derived model and the impedance control algorithm, a contact force control method is presented. That is, the position error is controlled in order to control the contact force ultimately. The actual flight experiments show that the developed system is enable to realize the contact force control without a force sensor.

This paper presents a control scheme to maintain consistent contact force using a handheld robot during soft tissue interaction with imaging instruments such as probe-based confocal laser endomicroscopy (pCLE). The handheld robot incorporates a 6-DOF miniature micromanipulator and a force sensor for the control. Given force set for clearly visualizing tissue on contact, the handheld robot can maintain such a tiny force by actively controlling its tool tip using the micromanipulator. It allows keeping locating the tool tip within the depth-of-focus region of the imaging instrument regardless of actual force on the soft tissue engaged by hand motion delivered to the handpiece of the robot. The contact force is regulated primarily by controlling the depth of the tool tip from the tissue surface. This procedure is accomplished by controlling the position of the tool tip via closed-loop force control. To validate the control scheme, we evaluated force tracking performance at the tool tip given a sinusoidal force input with an amplitude of 0.1 N for various frequencies, while firmly fixing the robot. As a result, the handheld robot could maintain the target force with an RMS error of less than 7.8 mN. The handheld task holding the robot on top of a single contact point was also evaluated, which yields an RMS error of 22.6 mN along the Z-axis of the sensor. Finally, the robot-aided scanning over the surface of the soft tissue was performed using the handheld robot, and the error was significantly reduced by 31.6% on average compared to unaided operation.

A compliant landing strategy for a trotting quadruped robot on unknown rough terrains based on contact force control is presented. Firstly, in order to lower the disturbance caused by the landing impact force, a landing phase is added between the swing phase and the stance phase, where the desired contact force is set as a small positive constant. Secondly, the joint torque optimization of the stance legs is formulated as a quadratic programming (QP) problem subject to equality and inequality/bound constraints. And a primal-dual dynamical system solver based on linear variational inequalities (LVI) is applied to solve this QP problem. Furthermore, based on the optimization results, a hybrid motion/force robust controller is designed to realize the tracking of the contact force, while the constraints of the stance feet landing angles are fulfilled simultaneously. Finally, the experiments are performed to validate the proposed methods.

A new method of generating an optimal approach velocity of a manipulator to its environment is presented in order to control the collision forces properly. First, analysis of a contact motion shows the necessity of contact control not only after the collision, as in previous studies, but also before it. Next, using a model of the force-controlled end-effector and its environment, forces generated at the contact are formulated as outputs of an autonomous system of which the initial condition is determined by the approach velocity. The optimal approach velocity is defined as the velocity which minimizes a performance index : the integration of the square deviation of contact forces from an equilibrium forces in the control of contact forces. A proportional relation between the optimal approach velocity and the contact force reference is derived analytically based on a mass-damper-spring model of the force-controlled end-effector and its environment. Results in simulation and experiment are presented to prove the effectiveness of the method.

Evaluation of a musculoskeletal model with prosthetic knee through six experimental gait trials