Masayuki Kawai

Development of a real-time fluid simulator for an interactive virtual environment: Improvement of density feedback in SPH

In this research, we developed a real-time fluid simulator for a haptic interface to display a virtual environment including 3-dimensional fluid by using smoothed particles hydrodynamics (SPH). SPH is a type of particle method and is suitable for a fast computation; however, it is difficult to incorporate sufficient fluid incompressibility in SPH for simulating a real-time process. This is because SPH was originally developed to study compressible fluid dynamics. In this paper, we propose a new control method using nonlinear feedback to compensate for elements that interfere with incompressible motions and propose its implementation method that does not significantly increase the computation time. Finally, simulations are performed to compare the proposed method with a conventional method, and the effectiveness of the proposed method is quantitatively demonstrated from the standpoints of computation time and incompressibility.

Power Assist Control Based on Motion Estimation of Wearer’s Forearm Using Motion Sensor

Powered exoskeletons are becoming popular especially in fields of nursing care and agriculture. We have been developing a powered exoskeleton that is supposed to be used in case of a nuclear hazard. Conventional powered exoskeletons typically use electromyographs (EMG), force switches, or force sensors on the wears feet. Unfortunately, the former suffers from the decline in measuring accuracy or slips of EMG sensor by sweating. The force-switch-based assist control method has a difficulty in speedy assist control of walk because it needs a few steps of a walk for the recognition. The force-sensor-based assist control method realizes a rapid assist control but the assistive motion is defined beforehand in general. We propose a new approach for power assist controller based on user motion prediction using 9 axis motion sensor. The motion sensor detects the angle and angular velocity of the wears limb and estimates the appropriate angle of the powered attachment according to the estimated motion. This report conducts experiments with one degree of freedom powered arm to evaluate the proposed method by EMG sensors and a force sensor.

Leg control based on human motion prediction using motion sensor for power assist suit without binding knee

Recently, power assist suits which support human physical activities have been researched. We are developing a power assist suit for workers in a nuclear power plant. If a disaster happens, the workers have to wear heavy radiation protective equipment. The power assist suit is supposed to support the wearer so that it reduces the load of the radiation protection equipment during an operation. This paper proposes a power assist control method based on motion prediction using 9-axis motion sensors. The power assist suit enables rapid power assist because the motion sensor can detect the start of walking motion in real time. A motion database consists of angles and angular velocities of wearers chest, lower limbs, and joint angles of the power assist suit. The power assist suit recognizes the motion of the wearer based on the database. Then, it assists the wearer based on the estimation of the motion of the wearer. We conduct experiments to evaluate the proposed method.

Adaptive Parameter Adjustment of a Real-Time SPH Simulation for Interactive Virtual Environments: Application to Parallel Processing

In this research, we develop a real-time fluid simulator, which uses smoothed particle hydrodynamics (SPH), for virtual environments including 3-dimensional fluid. SPH is a type of particle method and easy to control computational time by reducing or increasing the number of particles, however, it is difficult to change the number of particles while maintaining the same volume of fluid and the stability during a simulation. In this paper, we propose a new method to automatically adjust the number of particles, mass, radius in kernel functions and gains of density feedback for a real-time process while maintaining the stability. This paper also proposes to apply the method to a system using parallel processing with multi-thread programming. Finally, simulations are performed to estimate the effectiveness of the proposed method for parallel processing.

Optimal passivity design of a virtual coupling including FIR-type fractional derivatives for a haptic interface

This paper describes a haptic interface with a virtual coupling, including fractional derivatives. A haptic interface is a force feedback technology in virtual reality that takes advantage of the human sense of touch. In a haptic interface, virtual impedance, that is called virtual coupling (VC), is commonly used between the virtual and real objects to calculate reaction force. VC generally consists of a virtual stiffness and damper, but the stiffness has to be set low in a system with long sampling periods. In order to increase the virtual stiffness, this paper considers a VC including fractional derivatives approximated by an FIR approximation. First, we theoretically analyse effects of a single fractional derivative term by using the passivity analysis, but the result shows that its effects depend on the occurred frequency. This paper, therefore, proposes a method to combine multiple fractional derivative terms in a VC and a method to optimize parameters in each fractional derivative term. Finally, experiments are performed to measure the maximum value of the stiffness to illustrate the effects of the proposed method.

Compensation of Hydraulic Drag for an Underwater Manipulator Using a Real-Time SPH Fluid Simulator: Application in a Master-Slave Tele-operation

This paper discusses a control method of a tele-operation system for an underwater manipulator robot. Since fluid drag affects the dynamics of the underwater manipulator and degrades the accuracy of the control, it is necessary to decrease the influence of water. Generally, in order to compensate the fluid drag, theoretical approaches are used, but the applications are restricted by the simple modeling of the robot. Instead, we adopt a method to compensate the drag using a real-time fluid simulation. By using the simulation, the method can be applied to a robot with a complicated shape. For the method, we develop a real-time fluid simulator using smoothed particle hydrodynamics. The simulator calculates the fluid drag with the position of the underwater manipulator, and the drag is fed back to the controller to eliminate the real fluid drag. Finally, experiments have been performed to evaluate the effectiveness of the proposed method.