Engineering topics

Wen, J.

Publications and source records attributed to Wen, J..

7 records

DOE OSTI2024

Electron cyclotron emission quasi-optical transmission system on the HL-3 tokamak

A new quasi-optical (QO) Electron Cyclotron Emission (ECE) transmission system has been established on the HL-3 tokamak, which includes a focusing QO mirror combination and a long-distance transmission line. This system was developed to meet the requirements for poloidal spatial resolution and the high signal-to-noise ratio needed for magnetohydrodynamic (MHD) instability studies using ECE on the HL-3. The QO mirror combination was installed inside the vacuum chamber for focusing. Laboratory test results, theoretical calculations, and synthetic ECE simulation results indicate that the Gaussian beam can meet the spatial resolution requirements for the accurate measurement of the MHD instability on the q = 1/2/3 surfaces, corresponding to the poloidal mode numbers m = 3/6/9. This includes good diagnostic poloidal spatial resolution for the important 2/1 and 3/2 modes. At the front end of the transmission line, a high-efficiency mode converter was designed to transition the TE 10 mode to the HE 11 mode for input into the transmission line, with an insertion loss of less than 1.5 dB. A 30 m long-distance corrugated oversized waveguide was constructed, with transmission losses ranging from 6 to 10 dB in the 60–120 GHz range. Polarization adjustment results show that the polarization offset and geometric spatial polarization angle change consistently, which can provide a reference for polarization adjustment in other complex structured transmission lines. As a result, the newly established ECE QO transmission system will provide strong support for future physics research involving ECE on the HL-3.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗

NASA NTRS1997

Carbon dioxide in the atmosphere: isotopic exchange with ozone and its use as a tracer in the middle atmosphere

Atmospheric heavy ozone is enriched in the isotopes 18O and 17O. The magnitude of this enhancement, of the order of 100%, is very large compared with that commonly known in atmospheric chemistry and geochemistry. The heavy oxygen atom in heavy ozone is therefore useful as a tracer of chemical species and pathways that involve ozone or its derived products. As a test of the isotopic exchange reactions, we successfully carry out a series of numerical experiments to simulate the results of the laboratory experiments performed by Wen and Thiemens [1993] on ozone and CO2. A small discrepancy between the experimental and the model values for 17O exchange is also revealed. The results are used to compute the magnitude of isotopic exchange between ozone and carbon dioxide via the excited atom O(1D) in the middle atmosphere. The model for 18O is in good agreement with the observed values.

NASA Discipline Exobiology↗

NASA NTRS1993

A global path planning approach for redundant manipulators

A new approach for global path planning of redundant manipulators is proposed. It poses the path planning problem as a finite time nonlinear control problem. The solution is found by a Newton-Raphson type algorithm. This technique is capable of handling various goal task descriptions as well as incorporating both joint and task space constraints. The algorithm has shown promising preliminary results in planning joint path sequences for 3R and 4R planar robots to meet Cartesian tip tracking and goal endpoint planning. It is robust with respect to local path planning problems such as singularity considerations and local minimum problems. Repetitive joint path solutions for cyclic end-effector tasks are also generated. Eventual goals of this work include implementation on full spatial robots, as well as provision of an interface for supervisory input to aid in path planning for more complex problems.

Seereeram, Sanjeev↗

NASA NTRS1987

An algorithm for obtaining bang-bang control laws

A new computational method for solving control problems, when it is desired that the control action be bang-bang is presented. A gradient-based algorithm is developed where the switching times are updated to minimize the final state missed distance. The algorithm explains how a change in a switching time propagates through the other switching times and affects the final state. The algorithm is applicable to linear and nonlinear problems, including multiinput systems.

Wen, J.↗

NASA NTRS1986

Existence of the time optimal control for robotic manipulators

Using Filipov's Theorem, it is shown that the conditions oif nonfinite escape of trajectories, reachability, and convexity of the dynamics over all admissible controls are needed for the existence of a time optimal solution for the robotic equation. With a lower bound for the finite-escape time established using a Liapunov approach, and an upper bound for the time to reach the target established using the exact linearization idea, a single inequality is found which is closely related to the coriolis and the centrifugal terms, the absence of which implies that the domain of existence of the optimal solution can be made arbitrarily large with a large torque constraint. As the work space is finite, this is essentially a global result in practical situations.

Wen, J.↗

NASA NTRS1985

Stable Direct Adaptive Control of Linear Infinite-dimensional Systems Using a Command Generator Tracker Approach

A command generator tracker approach to model following contol of linear distributed parameter systems (DPS) whose dynamics are described on infinite dimensional Hilbert spaces is presented. This method generates finite dimensional controllers capable of exponentially stable tracking of the reference trajectories when certain ideal trajectories are known to exist for the open loop DPS; we present conditions for the existence of these ideal trajectories. An adaptive version of this type of controller is also presented and shown to achieve (in some cases, asymptotically) stable finite dimensional control of the infinite dimensional DPS.

Balas, M. J.↗

NASA NTRS1985

A minimum time control algorithm for linear and nonlinear systems

The minimum time control problem with bounded control has long been of interest to control engineers. Much of the theoretical study of this problem has been limited to linear systems and the results are usually problem-specific. This paper presents a new computational method for solving the minimum-time control problem when the control action is assumed to be bang-bang. A gradient-based algorithm is developed where the switching times are updated to minimize the final state missed distance. The algorithm is applicable to linear and nonlinear problems, including multi-input systems.

Wen, J.↗