This paper proposes a new variable gain sliding mode filter augmented by variable windowing for achieving smooth and reactive response over a broad range of input frequencies. The proposed filter can be seen as a synergistic combination of Kikuuwe et al.'s [1] sliding mode filter with varying gain and sliding surfaces and a novel varying-length moving-window algorithm. In all schemes, the estimated input speed is employed for rendering the filter parameters between low and high settings. The discrete-time algorithm of the proposed filter does not suffer from chattering due to implicit (backward) Euler method. The effectiveness of the proposed filter in achieving better trade-off between noise attenuation and signal preservation is validated in both simulation and experimental scenarios by using the velocity signal obtained by differentiation of quantized position data.

This paper proposes a position controller for commercial hydraulic excavators. It is constructed by combining a proportional-derivative (PD) controller and a sliding-mode controller as a differential algebraic inclusion and also is integrated with a recently-proposed hydraulic actuator model. The use of the PD control is intended to make the controller insensitive to the deadtime in the hydraulic system, which is typically 0.1 s to 0.6 s in commercial excavators. The use of the sliding-mode controller combined with the actuator model is for handling the saturation of the actuator force, which may happen when the target position is not close enough to the current position and when the relief valves open. Moreover, this paper extends the controller to deal with the effect of the regenerative pipelines, which are embedded in commercial excavators to realize efficient operations but act as a source of disturbance on the controller. This paper also shows an analysis that can be used for tuning the controller parameters. The proposed controller was validated with simulations and experiments using a 13-ton class excavator, in which some set-point control tasks and trajectory-tracking tasks were performed.Note to Practitioners—This paper proposes a position controller for hydraulic excavators. The controller was validated with the boom and arm actuators of a 13-ton class commercial excavator, with trajectory-tracking and set-point control tasks. Most of the controller parameters can be set referring to available specifications of the hydraulic circuit, such as the set pressures of the relief valves and the cross-sectional areas of the chambers. There are three parameters (the proportional gain, the derivative gain, and the time constant of the convergence) that should be carefully tuned, but their physical interpretations are relatively straightforward, and they can also be tuned along our guideline using pole locations of a particular transfer function. It has been shown that the proposed controller properly works despite the existence of the deadtime in the hydraulic systems, which are typically 0.1 s to 0.6 s. An extended version of the proposed controller handles the effects of the regenerative pipeline, which exists in some hydraulic systems, e.g., the arm actuator, for efficient operation and is not accounted for in the original version of the controller. The accuracy of the proposed controller will be further improved by combining it with better means to estimate external forces by using additional sensors, such as pressure sensors installed to actuator chambers.

This paper presents a task-space admittance controller applicable to redundant manipulators equipped with torque sensors. It extends Kikuuwe's (2019) torque-bounded admittance controller (TBAC), which allows for imposing explicit limits on the joint actuator torques without causing unsafe behaviors such as oscillation and overshoots. The proposed controller enforces that the end-effector follows predefined task-space dynamics as long as the joint torques are unsaturated and the configuration is away from singularities. The behavior in the nullspace, which arises from the redundant degrees of freedom and singular configurations, is governed by predefined joint-space dynamics. The task-space and joint-space dynamics are combined through a newly proposed continualized pseudoinverse, which employs the singular value decomposition. Results of experiments using a seven-degree-of-freedom Kinova Gen3 robot illustrate the validity of the proposed admittance controller in various scenarios, including the case where the robot is fully stretched.

The simulation of static friction, and especially the effect of stiction, is cumbersome to perform in discrete-time due to its discontinuity at zero velocity and its switching behavior. However, it is essential to achieve reliable simulations of friction to develop compliant torque control algorithms, as they are much disturbed by this phenomenon. This paper takes as a base an elastoplastic model approach for friction, which is free from chattering and drift. It proposes two closed-form solutions that can be used to reliably simulate the effect of stiction consistently with the physics-based Stribeck model. These solutions consider the nonlinearity and velocity dependency, which are main characteristics of lubricated joints. One is directly inspired by the Stribeck nonlinear terms, and the other is a simplified rational approximation. The reliability of this simulation method is shown in simulation, where the consistency and stability are assessed. We also demonstrate the accuracy of these methods by comparing them to experimental data obtained from a robot joint equipped with a high gear reduction harmonic drive.

This paper proposes a new admittance controller that realizes safe behavior even under torque saturation. The new controller is analytically equivalent to a conventional admittance controller as long as the actuator torque is not saturated, but is free from unsafe behaviors such as snapping back, oscillation, or overshoots, which may happen with conventional admittance controllers after torque saturation. The new controller is described by a differential algebraic inclusion, and can be understood as a conventional admittance controller expanded with an additional algebraic loop through a normal-cone operator. Its continuous-time representation involves a nonsmooth, set-valued function, but its discrete-time implementation is free from set-valuedness and given as a closed-form algorithm as a result of the use of implicit (backward) Euler discretization. The controller is tested with one joint of an industrial manipulator equipped with a force sensor.

This paper proposes an external force estimator including an elastoplastic friction model with improved static friction behavior. Hayward and Armstrong's elastoplastic friction model is one of the simplest model representing friction phenomena with compliance. This model however produces non-zero output force in the static friction state, which results in steady-state error in external force estimation. This paper improves the estimation accuracy by applying a friction model with the output force being reduced in the static friction state. The proposed estimator was validated through experiments with an actuator system comprised of a ball screw and a timing belt. The experimental result shows that the estimation accuracy is improved by the proposed estimator.

An efficient method to compute the joint friction torques as well as the external contact forces in forward dynamics simulation of a robot manipulator is proposed. The existence of friction inside mechanical gears substantially affects behaviors of the robot, and hence, should be taken into account. The strict form of the equation of motion disables distinguishing the internal friction torques from the external forces, which causes an increase of the degree of indeterminacy and the computation cost. A semi-implicit approach in which the internal friction torques and the external contact forces are alternatively computed is proposed based on an assumption that the net joint actuation torque does not much vary in a short interval. It significantly reduces the computation cost at a comparative accuracy with a completely implicit method.

High-gain proportional-integral-derivative (PID) position control involves some risk of unsafe behaviors in cases of abnormal events, such as unexpected environment contacts and temporary power failures. This paper proposes a new position-control method that is as accurate as conventional PID control during normal operation, but is capable of slow, overdamped resuming motion without overshoots from large positional errors that result in actuator-force saturation. The proposed method, which we call proxy-based sliding mode control (PSMC), is an alternative approximation of a simplest type of sliding mode control (SMC), and also is an extension of the PID control. The validity of the proposed method is demonstrated through stability analysis and experimental results.

This article describes a computationally efficient formulation and an algorithm for tetrahedral finite-element simulation of elastic objects subject to Saint Venant-Kirchhoff (StVK) material law. The number of floating point operations required by the algorithm is in the range of 15% to 27% for computing the vertex forces from a given set of vertex positions, and 27% to 38% for the tangent stiffness matrix, in comparison to a well-optimized algorithm directly derived from the conventional Total Lagrangian formulation. In the new algorithm, the data is associated with edges and tetrahedron-sharing edge-pairs (TSEPs), as opposed to tetrahedra, to avoid redundant computation. Another characteristic of the presented formulation is that it reduces to that of a spring-network model by simply ignoring all the TSEPs. The technique is demonstrated through an interactive application involving haptic interaction, being combined with a linearized implicit integration technique employing a preconditioned conjugate gradient method.

This paper presents a modular architecture of simulation software that is capable of properly capturing Coulomb friction in mechanical systems. The presented architecture is built upon Kikuuwe et al.’s discrete-time friction model, which is based on the implicit Euler method. The paper reformulates the model into a form that can be integrable through the standard fourth order Runge-Kutta method. Some examples based on the presented architecture are presented.

Some recent studies have addressed a class of human-machine coordination employing ldquovirtual fixtures,rdquo which are computer-generated walls or guides presented through haptic devices for assisting precise path tracing and for preventing the entry to specified regions. This paper presents the concept and control algorithms of a new class of virtual fixtures that is based on simulated plasticity. The plasticity-based virtual fixtures act as hard walls as long as the user's force is smaller than a predetermined yield force, but the user can deviate from the fixtures by intentionally producing a force larger than the yield force. As a characteristic of plasticity, the proposed virtual fixtures do not store elastic energy; the reaction force from the fixture almost always opposes the user's motion to decelerate the motion. Thus, the plasticity-based virtual fixtures are expected to be useful in some applications where safety is an utmost priority. This paper presents control algorithms for realizing the concept of plasticity-based virtual fixtures, addressing technical challenges in treating discontinuous nature of plasticity in discrete-time systems. The algorithms were demonstrated through experiments using impedance-type and admittance-type haptic devices.

This paper considers the application of a low-force robotic manipulator to guide a human user's movements to place a tool (or the user's hand) at a predetermined position or move it along a predetermined trajectory. This application is potentially useful, e.g., skill training for humans, rehabilitation, and human-machine coordination in the manufacturing industry. A proportional-derivative (PD)-type position control can be used for this application, but the parameters for the controller should be appropriately chosen for enhancing the human performance of positioning and trajectory tracking. We hypothesize that the robot's position control should be stiff and appropriately slow, i.e., the proportional gain should be high and the time constant (the ratio of the derivative gain to the proportional gain) should be appropriately large. Such characteristic has been difficult to be realized in ordinary PD position control because it requires direct high-gain velocity feedback. However, our recent technique, which is proxy-based sliding mode control (PSMC), is capable of producing such a hypothetically preferred response and allows us to empirically validate the hypothesis. The results of experiments using two distinctly different robotic devices supported the hypothesis, showing that the time constant should be set around 0.1 s rather than 0.01 and 0.5 s.

This paper presents an approach for producing virtual fixture based on simulated plasticity, which can be used for assisting precise manual manipulations performed by human users through haptic interfaces. The fixture acts as a guide to help path-tracing tasks and as a wall for preventing a tool from entering a specified region, but the user can move against the fixture by intentionally producing a force larger than a predetermined yield force. The advantage of the proposed virtual fixture is that it is always passive and it acts as a hard fixture when the user's force is smaller than the yield force. The algorithm was demonstrated through experiments using an impedance-type haptic interface.

High-gain PID position control, which is widely used with industrial robots, involves some risks in cases of abnormal events, such as unexpected environment contacts and temporal power failures. This paper proposes a new position control method to achieve both accurate, responsive tracking during normal operation and smooth, overdamped recovery from a large positional error after abnormal events. The proposed method, which we call proxy-based sliding mode control, is a modified version of sliding mode control adapted to discrete-time systems, and also is an extension of PID control. The validity of the proposed method is demonstrated through experiments

High-gain PID position control, which is widely used with robots having complex dynamics, involves some risks in cases of abnormal events, such as unexpected environment contacts, temporal power failures, and wrong position commands from a higher-level controller. We previously proposed a proxy-based sliding mode control (PSMC) scheme, which is an extension of PID control that ensures slow, moderate, overdamped recovery from a large positional error at abnormal events without sacrificing tracking accuracy during normal operation. Its weakness is that the velocity is not bounded. This paper proposes a velocity-bounding PSMC (VB-PSMC) scheme, which is an extension of PSMC to impose an arbitrary magnitude limit on the velocity. The VB-PSMC method can be used as a lowest-level position servo that is safer than PSMC and much safer than conventional PID control. The advantage of VB-PSMC was demonstrated through implementation experiments

Modeling of friction force is cumbersome because of its discontinuity at zero velocity. This paper presents a set of discrete-time friction models for the purpose of haptic rendering and virtual environment construction. These models allow friction to be treated as an admittance-type or impedance-type element of a virtual environment. They are derived from implicit Euler integration of Coulomb-like discontinuous friction and linear mass-spring-damper dynamics, and have closed-form expressions. They include rate-dependent friction laws, and their extension to multidimensional cases is easy in most practical cases. The validity of the models is demonstrated through numerical examples and implementation experiments

Friction force is cumbersome in numerical simulations due to its discontinuity at zero velocity. Fixed-step simulation techniques are especially desirable for control purposes, such as haptic friction rendering and friction compensation. Previous techniques have difficulties especially in numerical robustness and extensibility to multidimensional cases. This paper proposes two discrete-time friction models that can be used in fixed-step simulations. They can be used in multidimensional space, and can capture arbitrary velocity-dependent friction phenomena. The first one is a discrete-time discontinuous model. This model determines the friction force so that the velocity reaches zero in finite time by using the values of the mass and the timestep size. The second one is a discrete-time continuous model, which is a serial coupling of the discontinuous model and a linear viscoelastic element. This model is useful for haptic rendering because it is formulated as a velocity-input, force-output system. The second model can be extended into a more sophisticated friction model, which exhibits a hysteresis behavior in the presliding regime. Results of numerical simulations and an experiment are presented.

Human-machine cooperative systems have been proposed. Most of them consider carrying, positioning and assembling tasks. Precise tracking tasks are also required in factories. However, an assist system for tracking task has not been studied well. In this paper, we aim to realize the human-machine system that assists a tracking task. In order to achieve the goal, the virtual friction field is proposed. Isotropic and anisotropic friction fields are presented and they are applied to the rotating object which should be tracked. The tracking task experiments are carried out and it is shown that the virtual guide of the friction improves the tracking performance effectively.

This paper introduces a device for enhancing detection of surface undulation through active touch. This device, which we call a “tactile contact lens,” is composed of a sheet and numerous pins arranged on one side of the sheet. Experimental results show that a small bump on a surface can be detected more accurately through this device than by bare finger and than through a flat sheet. A mathematical analysis of this phenomenon suggests two possible explanations for this phenomenon. One lies in the lever-like behavior of the pins. The pins convert the local inclination of the object surface into the tangential displacement of the skin surface. The second is the spatial aliasing effect resulting from the discrete contact. Due to this effect, the temporal change in the skin surface displacement is efficiently transduced into the temporal change in the skin tissue strain. The results of this analysis are then discussed in relation to other sensitivity-enhancing materials, tactile sensing mechanisms, and tactile/haptic display devices.

When the end-effector of a robot is slid on a flat surface, the stiffness matrix obtained by linear fitting of position and force contains information not only on the stiffness coefficient and on the normal direction, but also on the friction coefficient. This paper proposes a new method for extracting information on those properties from the stiffness matrix provided by the impedance perception, which the authors previously proposed. The proposed method can be implemented as an encapsulated module for perception, which is separated from any control and planning methodologies. Therefore, this can be used for both autonomous and remote controlled robots, and even for monitoring a manipulation tasks performed by humans. Results of preliminary experiments are presented.

In a previous paper, we proposed the impedance perception technique, by which the stiffness matrix that constrains the motion-force relation of the robot's end-effector is estimated on-line, and the uncertainties of the estimates are evaluated. Based on this technique, this paper proposes a method of extracting information on local properties of a cylindrical curved surface, including normal direction, primary directions, curvature, and stiffness and friction coefficients, from the stiffness matrix obtained under the situation where the end-effector is slid on the surface. This technique can be implemented as an encapsulated perception function independent from control strategies, and thus it can be used for both autonomous and remote-controlled robots, and for direct monitoring of human manipulations. Results of preliminary experiments are presented.

Proposes an identification technique of constraint condition that the environment imposes on the robot's end-effector, based on position and force sensing during arbitrary manipulation. In the proposed method, the impedance that constrains the motion of the end-effector is estimated on-line; the uncertainties of the estimates are evaluated; and discontinuous changes of the impedance are detected. This method can be installed on robots as human-like perception of impedance, and can be used for monitoring human demonstration. Results of preliminary experiments are presented.

The purpose of this study is to establish a method of virtual-reality-mediated motion/force teaching from human teacher to human "teachee". For effective teaching, position and force information of the teacher's action have to be communicated to the teachee. Position information is easy to display, but force information is difficult to display by conventional haptic devices. In this paper, we propose a new type of haptic interface with fingertip presser which could make it possible to display both position and force information. The teacher's finger force of pushing a surface is displayed as a mechanical pressing force onto the teachee's fingertip, and position is displayed by active multijoint linkage. The results of basic experiments show that the proposed method is effective as a media of motion/force teaching, but reveal some limitations.