Hirohiko Arai

Planning walking patterns for a biped robot

Biped robots have better mobility than conventional wheeled robots, but they tend to tip over easily. To be able to walk stably in various environments, such as on rough terrain, up and down slopes, or in regions containing obstacles, it is necessary for the robot to adapt to the ground conditions with a foot motion, and maintain its stability with a torso motion. When the ground conditions and stability constraint are satisfied, it is desirable to select a walking pattern that requires small torque and velocity of the joint actuators. We first formulate the constraints of the foot motion parameters. By varying the values of the constraint parameters, we can produce different types of foot motion to adapt to ground conditions. We then propose a method for formulating the problem of the smooth hip motion with the largest stability margin using only two parameters, and derive the hip trajectory by iterative computation. Finally, the correlation between the actuator specifications and the walking patterns is described through simulation studies, and the effectiveness of the proposed methods is confirmed by simulation examples and experimental results.

A high stability, smooth walking pattern for a biped robot

Biped robots have better mobility than conventional wheeled robots, but they tip over easily. In order to walk stably in various environments such as rough terrain, up and down slopes, or regions containing obstacles, it is desirable to adapt to such ground conditions with a suitable foot motion, and maintain the stability of the robot by a smooth hip motion. We propose a method to plan a walking pattern consisting of a foot trajectory and a hip trajectory. First, we formulate the constraints of a foot trajectory, and generate the foot trajectory by 3rd order spline interpolation. By setting the values of constraint parameters, it is easy to produce different types of foot motion. Then, we formulate a hip trajectory using a 3rd order periodic spline function, and derive the hip trajectory with high stability. Finally, the effectiveness of the proposed method is illustrated by simulation examples.

Collision-Free Trajectory Planning for a 3-DoF Robot with a Passive Joint

This paper studies motion planning from one zero-velocity state to another for a three-joint robot in a horizontal plane with a passive revolute third joint. Such a robot is small-time locally controllable on an open subset of its zero-velocity section, allowing it to follow any path in this subset arbitrarily closely. However, some paths are “preferred” by the dynamics of the manipulator in that they can be followed at higher speeds. In this paper, the authors describe a computationally efficient trajectory planner that finds fast, collision-free trajectories among obstacles. The planner decouples the problem of planning feasible trajectories in the robot’s six-dimensional state space into the computationally simpler problems of planning paths in the three-dimensional configuration space and time scaling the paths according to the manipulator dynamics. This decoupling is made possible by the existence of velocity directions, fixed in the passive link frame, which can be executed at arbitrary speeds. Results of the planner have been implemented on an experimental underactuated manipulator. To the authors’ knowledge, it is the first implementation of a collision-free motion-planning algorithm for a manipulator subject to a second-order nonholonomic constraint.

Balance control of a piped robot combining off-line pattern with real-time modification

Since a biped robot tends to tip over easily, stable and reliable biped walking is a very important achievement. In this paper, we propose a balance control method based on an off-line planned walking pattern with real-time modification. First, a method of generating a highly stable, smooth walking pattern is presented. Then, a method of real-time modification consisting of body posture control, actual zero moment point control and landing time control based sensor information is proposed. By combining the proposed off-line walking pattern with real-time modification, the biped robot can walk smoothly and adapt to unknown environments. The effectiveness of the proposed method is confirmed by dynamic simulator such as walking on unexpected irregular rough terrain, soft ground and in environments in the presence of disturbances.

Human-robot cooperative manipulation using a virtual nonholonomic constraint

A robotic assistance system for handling long objects that are difficult to manipulate with only one point of support is presented. The robot grasps one end of the object and helps the human operator to carry at the other end. Such cooperative manipulation in a horizontal plane is considered here. The control method proposed uses a virtual nonholonomic constraint. The movement of the object is constrained as if it were being carried on a wheel attached to the object. This method can prevent the object from slipping sideways and simplify the carrying operation. The experimental results show that an operator can easily handle a long object when aided by the robot.

Assist system for carrying a long object with a human-analysis of a human cooperative behavior in the vertical direction

Deals with an assist system for carrying a long object with a human operator. When we carry such an object, we often grasp both ends and move it cooperatively. Our purpose is to establish how to design the assist system which can achieve such a task. It is difficult to apply conventional control laws. On the other hand, humans can achieve such a task. Therefore, we measure the human cooperative behaviors and analyze them to find the cooperative rules. Based on the rules, we propose a control law of the assist system. Furthermore, we construct a prototype system and verify the validity of the control law.

Feedback control of a 3-DOF planar underactuated manipulator

Feedback control of a manipulator with a passive joint which has neither an actuator nor a holding brake is investigated. The manipulator has three degrees of freedom in a horizontal plane, with the third joint being passive. The dynamic constraint on the free link is 2nd-order nonholonomic. A trajectory for positioning is composed of simple translational and rotational trajectory segments. The trajectory segments are stabilized by nonlinear feedback, considering the motion of the center of percussion of the free link. Simulation results show the effectiveness of the feedback control.

Development of power assist system with individual compensation ratios for gravity and dynamic load

This paper present the design concept of a power assist system. In such system, when the controller is designed without considering the maximum torque of the actuators, the actuators can sometimes become saturated, resulting in a loss of stability and manoeuvrability. We propose a method for dealing with this problem. The load force is divided into gravitational and dynamic component, and each component is attenuated by an individual ratio. These ratios are determined considering the maximum power of the operator and the actuators.

Design of a power assist system with consideration of actuator's maximum torque

This paper proposes a control method for a power assist system which attenuates the load force. In the system, the question of how to select a power assist ratio is important. This ratio must be selected with consideration of the maximum torque of each actuator used in the system, otherwise the actuator saturation may occur and cause the lack of the manoeuvrability and instability. To avoid such saturation problems, we propose a new control method which, after dividing an operated load into gravity load and dynamic load, selects a power assist ratio for the dynamic by considering the remaining actuator torque after the ratio for the gravity load is determined basing on the operators capability. The control law is formulated for a single axis power assist system and the effectiveness of the method is confirmed by experiments.

Robotic metal spinning-shear spinning using force feedback control

Metal spinning is a plasticity forming process that forms a metal sheet or tube by forcing the metal onto a rotating mandrel using a roller tool. This is a study on metal spinning applying robot control techniques such as force feedback control with the aim to develop flexible and intelligent forming processes, and to expand a new application area for robot control. An experimental setup was developed for gathering basic data on the forming process. Some results of preliminary experiments are presented. The influence of the clearance between the roller and mandrel is also discussed. The author proposes applying hybrid position/force control for shear spinning, which is free from fine adjustment of the clearance. The effectiveness of the proposed method was experimentally verified.