Naoyuki Takesue

Development of ER Brake and its Application to Passive Force Display

Force display systems are important in virtual reality and other applications. While conventional force displays are active systems with actuators and as such may become inherently dangerous, passive force displays are effective methods for assuring safety. In this paper, we developed a brake using electrorheological (ER) fluid, and showed a passive force display system using ER brakes with two degrees of freedom. We also discuss basic control experiments of the system.

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.

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.

Passive force display using ER brakes and its control experiments

Force information is often required for tele-operation systems and virtual reality. Conventional force displays are active systems with actuators. This, however, is inherently active, so that it may become a danger. Consequently, passive force display is an effective method for assuring safety. The authors developed a brake using ER (electrorheological) fluid and passive force display using ER brakes. They discuss two degree of freedom passive force display and basic control experiments.

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.

Development and experiments of actuator using MR fluid

Magnetorheological (MR) fluids are materials that respond to an applied magnetic field with a change in their rheological behavior. Though they are functionally similar to electrorheological (ER) fluids, MR fluids exhibit much higher yield strengths for applied magnetic fields than ER fluid for applied electric fields. The devices using MR fluids have an ability to provide high-torque, low-inertia, safe device and simple interface. In this study, the authors develop an actuator using MR fluid that consists of an input part, an output part and an MR-fluid clutch between them. The output part can be simply a cylinder or a disk, and thereby made extremely lightweight. In order to investigate the characteristics of the developed MR actuator, the experiments are examined.

Precise position control of robot arms using a homogeneous ER fluid

Semi-closed-loop control of a robot achieves only the control of the angular position of the motor, so it is not clear whether the end-effector is precisely positioned or not. Closed-loop control would improve positioning accuracy, but industrial robots which have elasticity in their driving systems might become unstable easily because of the noncollocated positional relationship between sensors and actuators. This study, which is aimed at realizing high-speed and high-precision closed-loop control, uses an intelligent electrorheological fluid (ER fluid). In the next section, a brief introduction is given with respect to two types of ER fluids and the development of ER dampers. Following this is a description of the experimental apparatus, which consists of an ER damper and a one-link robot arm. A discussion of its mathematical model follows. In addition, the effects of an ER damper to the resonance/antiresonance characteristics of 2-inertia systems are discussed. Next, the design of the hierarchical control system is discussed. Firstly, the design of the inner loop controller for motor velocity control is presented. As an outer loop controller, an H/sup /spl infin// controller is obtained by using a mixed sensitivity design method of robust control theory. Then it is shown that the ER damper makes the design of a high gain controller possible while maintaining robust stability. Finally, the effectiveness of the proposed control method is demonstrated by experiments.

Improvement of response properties of MR-fluid actuator by torque feedback control

Magnetorheological (MR) fluids are substances that respond to an applied magnetic field with a change in their theological behavior. Though they are functionally similar to electrorheological (ER) fluids, MR fluids exhibit much higher yield strengths for the applied magnetic fields than ER fluids for the applied electric fields. The devices using MR fluids have an ability to provide high-torque, low-inertia, a safe device and simple interface. In this study, we report on an actuator developed using the MR fluid, which consists of an input part, an output part and an MR fluid clutch between them. First, the basic experiments are examined to investigate the characteristics of the actuator. Next, the torque control system of the MR-fluid actuator is proposed. Finally, the closed-loop control experiments were carried out and it is confirmed that the torque-feedback control is effective for improving the response properties of MR actuators.

Analytic and experimental study on fast response MR-fluid actuator

Magnetorheological (MR) fluids are materials that change their rheological behavior in the presence of an applied magnetic field. Devices using MR fluids have an ability to provide high torque, low inertia, be a safe device and have a simple interface. We have developed an MR-fluid actuator, which consists of an input component, an output component and an MR-fluid clutch between the two. In this paper, we investigate the dynamic characteristics of an MR-fluid actuator and aim to improve its response. Transient magnetic analyses are examined and experimental results are shown in the case of the proposed approaches. Finally, we realize a much faster MR-fluid actuator.