The present paper proposes a sliding-mode control method incorporating a strategy to avoid singularities during target capture for a space robot equipped with a manipulator. The position and attitude of the end-effector of the manipulator are controlled by a conventional sliding-mode control employing the transpose of the generalized Jacobian. In the proposed algorithm, the attitude/position of the main body of the space robot is controlled to increase the manipulability when the manipulator approaches a singularity. The direction of the main body translational motion is selected in order to effectively avoid the singularity, and the direction for acceleration of the end-effector is modified taking into account the acceleration of the main body. The gain scheduling technique is also incorporated in order to reduce the control input effort at the beginning of operation as well as to suppress the steady-state error. The effectiveness of the proposed method is verified through numerical simulations for a simple model of a space robot.
A new Reynolds-stress model developed by the author is assessed for the computations of turbulent Couette flow and turbulent rotating channel flow. The turbulent Couette flow has two distinctive properties. In the wall layer, the flow is wall turbulence, but the flow is also plane homogenous shear flow in the core region. The turbulent rotating channel flow is affected by Coriolis forces associated with the rotation. There is no direct effect of rotation on the turbulence energy budget, but in the budget for the Reynolds-stress tensor, the rotational term appears explicitly. Thus, the two flows are suitable for assessing the Reynolds-stress model. Computational results are compared with the available experimental data, proving that the performance of this Reynolds-stress model is effective.
This paper presents the development of a miniature multifunctional GPS receiver at NASDA. The design and implementation method for a spaceborne GPS receiver has been investigated, and a breadboard model of a parallel signal search system incorporating matched filtering, an essential technique for next-generation GPS receivers, has been manufactured. The time to acquisition (TTA) of a GPS signal was measured on the breadboard model using a GPS simulator. The test results of the trial product show that TTA is within 60 msec, and time to first fix (TTFF) of the navigation calculation in a low-altitude orbit is within 5.3 min in the worst-case scenario.
This paper applies robust control techniques to a blimp. The project “Mine Detection System Using Blimps” is in progress. The aim of the project is the development of a practical technique for the autonomous detection of landmines and their positions. In order for a blimp to perform observation and detection, precise flight path control is needed. The error resulting from modeling a real system cannot be avoided, however accurate the model is. In order to aim for realistic control, robust control can compensate model uncertainty. In the two-motion models presented (experimental model and analytical model), it was found that since the yawing motion mode is unstable, lateral-directional movement becomes unstable. This paper proposes a robust stability augmentation system for the yawing motion of the blimp developed for the project. The numerical simulation presents a comparison between a Kharitonov theory controller and a H∞ theory controller. The experimental results show that control of the blimp using a H∞ controller is more suitable for the project objective. An emphasis is placed on implementation of the controller.
A stepped-nose with various step lengths and heights, attached to a square cylinder, can significantly reduce the drag coefficient compared to that of the square cylinder. The underlying physics are that (1) the vortices trapped in the step regions produce the thrust forces acting on the step surfaces facing against the uniform stream which cancel the drag force acting on the front surface of the stepped-nose obstacle, and (2) the tangent reattachment of the flow separating from the front surface edges to the side surfaces of the main body decreases the suction pressure acting on the back surface of the main body. In the present study, these favorable effects of the stepped-nose are experimentally documented by presenting the measurement results of surface pressure coefficient, streamwise velocity, and turbulence intensity of side flow and flow visualization pictures. It is demonstrated that when step height takes a value of about one-tenth of the main body length, there is a rather wide range of step length, for which the net drag force acting on the stepped-nose almost vanishes and the side flow is stabilized by the stepped-nose.
On-orbit servicing, such as refueling, repairing, and orbit recovery, will be essential for space activities in the next generation for both manned and unmanned space systems. One of the most important and most difficult tasks in on-orbit servicing is capturing a “customer satellite” using a manipulator that can move dynamically in a wide range of space. A visual servoing technique that controls and guides the manipulator based on a camera image is required to perform this dynamic task. It is necessary to establish boundary conditions; in other words, to specify the task by assessing the environment and setting proper conditions for in order to execute it under the constraints of on-board computing power and the severe lighting conditions of space. This paper describes the design concept of a visual servoing system for a space robot and presents the results of an on-orbit experiment using Japanese Engineering Test Satellite VII (ETS-VII) that was designed based on this concept.
We detail the design, fabrication, and experimental and calculation results for three cavity solar thermal propulsion (STP) thrusters — medium, small, and very small — made of single-crystal molybdenum (SC-Mo), developed in the National Aerospace Laboratory of Japan (NAL). We obtained very high temperatures — 2,300 K for the propellant gas and 2,200 K on the outer surface of the thruster — at an appropriate propellant flow with the small thruster by solar-heating with a suitable concentrator of 1.05 m in diameter, which corresponds to an 800-second-class specific impulse thruster. Temperatures obtained in experiments were much lower than expected, however. To find points for improvement and evaluate thruster performance in space, we conducted a model calculation that confirmed the thruster could achieve an 800 to 900-second-class specific impulse with a 0.1–2 N class thrust magnitude in space. STP is thus a candidate for near-future propulsion in applications such as upper stages of orbit transfer vehicles.
The experimental airplanes of Supersonic Transportation, SST, are being developed in Japan. The wings are thin to augment cruise aerodynamic performance. If these wings are equipped with ailerons, the shock wave motions on the ailerons might cause aileron buzz not accompanied by heavy separation of the boundary layers. On the other hand, unsteady aerodynamics research regarding thin wings has not been extensive. In order to precisely investigate the shock wave motions on thin wings, two-dimensional CFD simulations were conducted in our study. In the simulations, Navier-Stokes equations are solved around the NACA0003 airfoil with an oscillating aileron. The results show that the imaginary component of the unsteady aileron hinge moment, abbreviated “AHMI,” has the maximum and positive value when the shock wave oscillates around the mid-chord on the aileron. The aileron length does not strongly affect the shock wave motions on the aileron if the scale of the shock wave motions is normalized with the aileron length. But a longer aileron shows larger AHMI than a shorter one. The experimental results on SST arrow wing are compared with simulated results, too. In order to compare the results acquired from the models having different space dimensions, the local Mach number component normal to the aileron hinge line is calculated as a common parameter. The comparison shows that, for both results, AHMI becomes positive when the shock wave oscillates around the aileron mid-chord. In addition, as the shock wave oscillates more rearward on the aileron, AHMI increases.
A blimp is introduced as a stable platform for remote-sensing instruments required for unmanned aerial observation and surveillance. In order to develop flight control systems for a blimp, two series of experiments were conducted to identify flight dynamics: constrained flight tests, and indoor free-flight tests. This paper addresses the blimp configuration, experimental set-up, method for identifying dynamics, and the results of identification in comparison with the analytical estimation for each experimental method. Both tests employed a full-scale blimp. The identification method for the constrained flight tests used the extended least-squares method involving the gradient algorithm, and the indoor free-flight tests, the eigen-system realization algorithm involving the autoregressive model fitting algorithm. The results suggest that analytical formulas for estimating the parameters, including added mass effects and stability derivatives, may yield values consistent with experimentally identified ones.