An approach for force control of redundant robots under unknown environment

Mitigating collision is a fundamental issue in contact problems, and is required to ensure the safety of a robotic cell. Research into the contact problem between robots and their environment is divided into two parts: one uses the environmental contact model and parameter estimation, the other uses the robot force control method. There are two main problems with this research method. One is that the two research levels are not effectively combined to form a complete solution for force control in practice. The other problem is that research on excessive contact force in the collision phase has not been studied in depth for force control. In this paper, a sensing-executing bionic system is proposed that combines environmental detection and robotic force control based on the way an ant functions. The bionic system clearly explains the process from environment detection to robot control, which can provide guidance when designing a new robot control system. An adaptive switching control algorithm is proposed to mitigate the collision force in the collision phase. From the simulation results, the collision force is significantly reduced due to the implementation of adaptive switching control. Finally, a new self-sensing device is designed which can be integrated into the robot control device. However, as there are no additional sensors or computational complexity in the system, the effectiveness of the circuit and superiority of the adaptive parameter update must be verified by experimentation.

To achieve force control of an industrial robot, this paper proposes an I-PD force control system based on an instantaneous state observer. The structure of the proposed system is based on a resonance ratio control system and a feedback signal of the reaction force response. In this paper, the gain of a pseudo derivation is designed as the feedback gain. From the results of the designed gains, the feedback gain Kf is a negative value. Hence, the resonance ratio control system becomes equivalent to a state feedback. For the stability of the proposed I-PD force control system based on the acceleration control system and a state feedback, a new analysis method is required. This paper analyzes an open-loop of the proposed system considering the bandwidth of the observer. The results of an analysis show that an observer with a wide bandwidth is required. Therefore, the parameters of proposed system using the instantaneous state observer are designed based on the coefficient diagram method. The effectiveness of the proposed method is confirmed by performing numerical simulations based on the model of an industrial robot arm. The results show that the proposed method is effective for the stable force control of an industrial robot arm.

Soft robots are increasingly being explored and developed in various settings that demand safe and adaptable interactions between robots and their environments. In addition, soft robots exhibit passive compliant behavior and generate continuous deformations when engaging with the environment. This imposes challenges on achieving active, on-demand interaction force control, especially when feedback force-sensing devices are not available. Consequently, there is a need to explore new model-based force control paradigms for soft robots. In this article, we propose a (quasi-)static force control approach for soft robots based on compliance modeling, avoiding the necessity for feedback control loops or extensive training data collection. The proposed approach can deliver contact force control along three Cartesian axes when the robot is actuated into various configurations. The compliance matrix is derived from the robot configuration, which allows the calculation of desired deflection displacements needed to generate on-demand forces. The resulting force control is achieved by solving inverse kinematics problems based on these deflection displacements. The efficacy of our proposed controller is validated through experiments with both one- and two-segment pneumatic-driven soft continuum robots. The results demonstrate effective static force control performance, with mean control errors below 5% of the desired peak forces.

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.

2 - Preliminary simulation of intelligent force control

A quantum mechanical canonical transformation due to Bogoliubov and Tyablikov (BT), applied previously to eliminate a single Coriolis term coupling two nearly degenerate vibrational degrees of freedom of a polyatomic molecule [J. Chem. Phys. 94, 461 (1991); 95, 1884 (1991)], is generalized. First, we show how to use an angular momentum dependent BT transformation to effectively decouple two vibrational degrees of freedom Coriolis coupled by two components of the angular momentum. This is accomplished by choosing the molecule-fixed axes so that, in the rotated frame, there is only one Coriolis term. Redefining the orientation of the molecule-fixed axes at equilibrium and using the BT transformation enables us to move large off-block matrix elements into vibrational blocks so that we can use second order perturbation theory to treat strong Coriolis coupling. Second, we develop a mixed BT transformation-perturbation theory method to calculate energy levels for molecules for which both Coriolis and centrifugal coupling are present. The method is tested on a two-mode model of formaldehyde using an ab initio Hamiltonian. Third, we combine our BT transformation method and conventional contact transformation theory to derive effective Hamiltonians for nearly degenerate vibrational states coupled by Coriolis and centrifugal terms without resorting to a reduced dimensional model. It is very difficult to use perturbation theory after having applied a BT transformation. We circumvent this problem by first using standard perturbation theory to decouple modes whose zeroth-order energies are well-separated and then applying a BT transformation for the strongly coupled modes. The theory is applied to an experimentally determined Hamiltonian for formic acid.

So far, a considerable amount of research has been devoted to flexible arms, especially to their modelling, position control and vibration control. Meanwhile very few research has addressed the problem of force control, since it is one of the most important functions of the robot tasks. In this paper, we present a two-degree-of-freedom control system for the force control of flexible cartesian robots. Use of a two-degree-of freedom control system guarantees the independent design of the feedforward and feedback control characteristics. The feedforward controller is designed using inverse dynamics, while the design of the stabilized feedback controller is achieved by Lyapunov's direct method. Numerical simulation and experimental results confirm that faster precise force control can be realized with the proposed control system.

This work addresses the end-effector trajectory-tracking force and motion control of a three-dimensional three-link robot considering measurement noises. The last two links of the manipulator are considered as structurally flexible. An absolute coordinate approach is used while obtaining the dynamic equations to avoid complex dynamic equations. In this approach, each link is modeled as if there is no connection between the links. Then, joint connections are expressed as constraint equations. After that, these constraint equations are used in dynamic equations to decrease the number of equations. Then, the resulting dynamic equations are transformed into a form which is suitable for controller design. Furthermore, the dynamic equations are divided as pseudostatic equilibrium and deviation equations. The control torques resulting from the pseudostatic equilibrium and the elastic deflections are obtained easily as the solution of algebraic equations. On the other hand, the control torques corresponding to the deviations are obtained without any linearization. Encoders, strain gauges, position sensors and force and moment sensors are required for measurements. Low pass filters are considered for the sensors. For the crossover frequencies of the sensors, low and high values are chosen to observe the filtering effect on the robot output.

Series Elastic Actuators (SEAs) have several benefits for force controlled robotic applications. Typical SEAs place an elastic element between the motor and the load, increasing shock tolerance, allowing for more accurate and stable force control, and creating the potential for energy storage. This paper presents the design of a compact, lightweight, low-friction, electromechanical linear SEA used in the lower body of the Tactical Hazardous Operations Robot (THOR). The THOR SEA is an evolutionary improvement upon the SAFFiR SEA [1]. Design changes focused on reducing the size and fixed length of the actuator while increasing its load capacity. This SEA pairs a ball screw-driven linear actuator with a configurable elastic member. The elastic element is a titanium leaf spring with a removable pivot, setting the compliance to either 650 or 372 [kN/m]. The compliant beam is positioned parallel to the actuator, reducing overall packaging size by relocating the space required for spring deflection. Unlike typical SEAs which measure force through spring deflection, the force applied to the titanium beam is measured through a tension/compression load cell located in line with each actuator, resulting in a measurable load range of +/−2225 [N] at a tolerance of +/−1 [N]. A pair of universal joints connects the actuator to the compliant beam and to the robot frame. As the size of each universal joint is greatly dependent upon its required range of motion, each joint design is tailored to fit a particular angle range to further reduce packaging size. Potential research topics involving the actuator are proposed for future work.

ABSTRACTThe paper proposes a novel method for the nonlinear redundantly actuated parallel robot based on force/position hybrid control structure. In order to solve the limitation of making a compromise for internal model controller, a two-DOF fractional order internal model control algorithm combining the internal model control principle and the fractional order theory is proposed for the position branch of the parallel robot redundantly actuated. This algorithm can realise the adjustment of the dynamic performance and anti-interference of 6PUS-UPU respectively. Aiming at the big force control error fractional order internal model, fuzzy control theory and the fractional order internal model controller are integrated into a new controller-fuzzy fractional order internal model(FFOIM) force control algorithm. Then Admas/Matlab simulation results demonstrate that the proposed algorithm can further reduce the driving force error of the system, and also retain the strong anti-interference of fractional order internal model controller.

This paper proposes a novel parallel robot - Virtual Ground Robot (VGR) - that is driven by three Series Elastic Actuators (SEAs) to interact with a human. The proposed Virtual Ground Robot provides a virtual ground on which a human can stand on and interact in three directions: the pitch, the roll and the height directions. The most significant features of the proposed VGR are that 1) it is driven by RFSEAs (Reaction Force-sensing Series Elastic Actuator), and thus it can provide precise forces and torques, 2) the size of the VGR is small enough for a human to stand on with ease, and 3) it can generate torque/force large to support a weight of a human. Taking advantage of RFSEAs utilized in the proposed VGR, Spatial Force control algorithm is proposed in this paper. In order to design this controller, the motions of VGR are defined in the task space, the joint space and the RFSEA level. Based on the Kinematics, force control of VGR in the task level, which is named Spatial Force Control is designed and verified using experiments.

Force control of industrial robots: The contact problem

This paper introduces a conceptual design of a hybrid mechatronic system consisting of a rigid robotic arm and a flexible link. The hybrid system represents an intermediate step towards a new type of instruments to be employed in the field of robotic surgery. These instruments are characterized by high distal structural flexibility allowing for safer interaction with the tissue. This paper shows that despite of the flexibility it is possible to accurately control the interaction forces between the robot and the soft and delicate tissue. To this end, a force control method is suggested, featuring quasi-static system model that is resolved in real-time inside the control law. The paper describes the overall system architecture, explains the basic concept behind the force control law and includes experimental validation of the force control of the hybrid mechatronic system. Conclusions are drawn and some indications for further work are formulated.

The authors present a robust force control which adapts to wide parameter variances. The disturbances and the parameter variations of motors and the environment are compensated by the force-based disturbance observer discussed. The force controller is based on the controller of the second derivative of a force. In order to realize stable force control for a wide variation of stiffness, identification control based on the least-squares method is introduced. The parameters of the force controller and the disturbance observer are adjusted according to the identified stiffness. Simulation and experimental results show that the adaptive system works well to stabilize force response. >

In this paper a dynamic tracking control of mobile robot using neural network global fast sliding mode (NN-GFSM) is presented. The proposed strategy combines two control approaches, kinematic control and dynamic control. The laws of kinematic control are based on GFSM in order to determine the adequate velocities for the system stability in finite time. The dynamic controller combines two control techniques, the GFSM to stabilize the velocities errors, and a neural network controller in order to approximate a nonlinear function and to deal the disturbances. This dynamic controller allows the robots to follow the desired trajectory even in the presence of disturbances. The designed controller is dynamically simulated by using Matlab/ Simulink and the simulations results show the efficiency and robustness of the proposed control strategy.