Kenji Nagaoka

Impedance-based contact control of a free-flying space robot with a compliant wrist for non-cooperative satellite capture

This paper presents the impedance-based contact control of a free-flying space robot utilizing a compliant wrist for non-cooperative satellite capture operation. An open loop impedance control law based on contact dynamics model is introduced to realize a desired coefficient of restitution defined between a manipulator hand of a space robot and a contact point on a free-flying target. The coefficient of restitution and the damping ratio are expressed as a function of contact and impedance parameters; and hence, the impedance parameters are tuned by setting a desired coefficient of restitution and a desired damping ratio. The collision experiment using twodimensional microgravity emulator, called air-floating test bed, verifies that the proposed open loop control law is capable of realizing a desired coefficient of restitution with fairly small errors.

Experimental study on autonomous burrowing screw robot for subsurface exploration on the Moon

Subsurface exploration on the Moon is a significant mission for future space developments. The authors have studied an autonomous robotic explorer which can burrow into the soils. This paper focuses especially on the excavation mechanism of a burrowing robot for near-future lunar subsurface exploration. The main objective of the proposed robot is to bury a scientific observation instrument, such as a long-term seismometer under the lunar surface, which is covered with very compacted lunar regolith. Therefore, an efficient drilling mechanism is required. In this paper, the authors propose a non-reaction screw drilling mechanism with double rotation, which is called the contra-rotor screw. This paper also evaluates the feasibility and effectiveness of the proposal of a burrowing robotic system through some experimental analyses.

Impedance-based contact control of a free-flying space robot with respect to coefficient of restitution

This paper presents an impedance-based contact control of a free-flying space robot with respect to the coefficient of restitution. Since no object is constrained in space, the contact between two objects in orbit must be treated carefully so as to prevent the objects from inducing undesired contact force and post-contact relative motion. As a solution to this issue, the authors propose a contact control method for a free-flying space robot based on the impedance control with respect to the coefficient of restitution. The coefficient of restitution is used as a reference value to tune the impedance parameters and is formulated through dynamic consideration to include both the impedance parameters and contact parameters in the coefficient of restitution. The fundamental experiment is conducted to verify the proposed control method, using a ground-based manipulator to realize the impedance control, and a fixed wall as a target surface. The experimental results showed that the proposed control method successfully controlled the coefficient of restitution between two bodies.

Earth-worm typed Drilling robot for subsurface planetary exploration

This paper presents a mobile robotic system designed to perform deep soil sampling for lunar or planetary subsurface exploration in the near future. Drilling robots have to carry the excavated fine sand, regolith backward because of the high density. Therefore a new scheme is proposed, to move forward under the soil by making use of reactive force caused by pushing the discharged regolith. The first simple experiments demonstrate the effectiveness of the proposed method.

Modeling, Analysis, and Control of an Actively Reconfigurable Planetary Rover for Traversing Slopes Covered with Loose Soil

Future planetary rovers are expected to probe across steep sandy slopes such as crater rims where wheel slippage can be a critical problem. One possible solution is to equip locomotion mechanisms with redundant actuators so that the rovers are able to actively reconfigure themselves to adapt to the target terrain. This study modeled a reconfigurable rover to analyze the effects of posture change on rover slippage over sandy slopes. The study also investigated control strategies for a reconfigurable rover to reduce slippage. The proposed mechanical model consists of two models: a complete rover model representing the relationship between the attitude of the rover and the forces acting on each wheel, and a wheel-soil contact force model expressed as a function of slip parameters. By combining these two models, the proposed joint model relates the configuration of the rover to its slippage. The reliability of the proposed model is discussed based on a comparison of slope-traversing experiments and numerical simulations. The results of the simulations show trends similar to those of the experiments and thus the validity of the proposed model. Following the results, a configuration control strategy for a reconfigurable rover was introduced accompanied by orientation control. These controls were implemented on a four-wheeled rover, and their effectiveness was tested on a natural sand dune. The results of the field experiments show the usefulness of the proposed control strategies.

Vibration suppression control of a space robot with flexible appendage based on simple dynamic model

This paper discusses a vibration suppression control method for a space robot with a rigid manipulator and flexible appendage. A suitable dynamic model that considers the coupling between the manipulator and flexible appendage was developed for the controller to accomplish the vibration suppression control of the flexible appendage. The flexible appendage was modeled using a virtual joint model, and the control method was developed on the basis of this model. Although this type of control requires feedback of the flexible appendage state, its direct measurement is generally difficult. Thus, an estimator of the flexible appendage state was constructed using a force/torque sensor attached between the base and flexible appendage. The control method was experimentally verified using an air-floating system.

Simultaneous control for end-point motion and vibration suppression of a space robot based on simple dynamic model

This paper addresses a dynamic model and a control method of a space robot with a rigid manipulator and a flexible appendage. The control method has been developed for performing multiple tasks: end-point motion control and vibration suppression control of a flexible appendage. A simple dynamic model that considers coupling between the manipulator and the flexible appendage is proposed for the control method. The tasks are performed simultaneously on the basis of their order of priorities using a redundant manipulator. Additionally, because vibration suppression requires feedback of the state of the flexible appendage, a state estimator of the appendage using a force/torque sensor is developed. Finally, the proposed model, control method, and state estimator were verified experimentally using an air-floating system.

Design of underactuated hand for caging-based grasping of free-flying object

This paper discusses an underactuated robotic hand for grasping a free-flying object. We introduce a concept of creating a closure called caging before locking the target in an immobilizing grasp. The caging is a closure that geometrically constrains the target such that the target can not escape from its closure created by robotic fingers. The design of the underactuated hand that performs caging-based grasping of the target is presented. The proposed underactuated hand was evaluated through the static analysis and under a condition called object closure. The prototype of the proposed hand was developed, and its performance was verified experimentally using an air-floating system.

Evaluation of influence of surface shape of locomotion mechanism on traveling performance of planetary rovers

The surfaces of both the Moon and Mars are covered with loose soil, with numerous steep slopes along their crater rims. Therefore, one of the most important requirements imposed on planetary rovers is their ability to minimize slippage while climbing steep slopes, i.e., the ability to generate a drawbar pull with only a small amount of slippage. To this end, the wheels/tracks of planetary rovers typically have parallel fins called lugs (i.e., grousers) on their surface. Recent studies have reported that these lugs can substantially improve the traveling performances of planetary rovers. Therefore, in this study, we conducted experiments using lightweight two-wheeled and mono-tracked rovers to provide a quantitative confirmation regarding the influence of lugs on the traveling performances of planetary rovers. Based on our experimental results, we confirmed that, although an increase in the number of lugs contributes to the high traveling performance of wheeled rovers, it does not contribute much to that of tracked rovers. Furthermore, an increase in lug height improves the traveling performances of both types of rovers.

Evaluation of the reconfiguration effects of planetary rovers on their lateral traversing of sandy slopes

Rovers that are used to explore craters on the Moon or Mars require the mobility to negotiate sandy slopes, on which slippage can easily occur. Such slippage can be reduced by actively readjusting the attitude of the rovers. By changing attitude, rovers can modify the position of their center of gravity and the wheel-soil contact angle. In this study, we discuss the effects of attitude changes on downhill sideslip based on the slope failure mechanism and experiments on reconfiguring the rover attitude and wheel angles. We conducted slope-traversing experiments using a wheeled rover under various roll angles and wheel angles. The experimental results show that the contact angle between wheels and slopes has a dominant influence on sideslip when compared with that of readjusting the rovers center of gravity.