Ryo Kikuuwe

Proxy-based sliding mode control for accurate and safe position control

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

Admittance and Impedance Representations of Friction Based on Implicit Euler Integration

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

Enhancing haptic detection of surface undulation

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.

Proxy-Based Sliding Mode Control: A Safer Extension of PID Position Control

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.

Fixed-step friction simulation: from classical Coulomb model to modern continuous models

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.

Robot perception of environment impedance

Proposes an identification technique of constraint condition that the environment imposes on the robots 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.

An edge-based computationally efficient formulation of Saint Venant-Kirchhoff tetrahedral finite elements

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.

A Control Framework to Generate Nonenergy-Storing Virtual Fixtures: Use of Simulated Plasticity

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 users 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 users 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.

A Guideline for Low-Force Robotic Guidance for Enhancing Human Performance of Positioning and Trajectory Tracking: It Should Be Stiff and Appropriately Slow

This paper considers the application of a low-force robotic manipulator to guide a human users movements to place a tool (or the users 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 robots 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.

A Sliding-Mode-Like Position Controller for Admittance Control With Bounded Actuator Force

This paper proposes a new position controller that is suitable for the use as the internal position servo of an admittance controller with bounded actuator forces. The new position controller approximately behaves as a proportional-integral-derivative (PID) controller with an acceleration feedforward in normal situations and as a sliding mode controller when the actuator force is saturated. The admittance controller employing the new position controller realizes smooth transitions between saturated periods and unsaturated periods. Moreover, it quickly responds to changes in the applied force even when the actuator force is saturated, leading to better stability and smoothness. The controller was validated through experiments using a robotic manipulator.