Laboratory Test Bed Research Articles

Engineering Test Satellite VII Flight Experiments For Space Robot Dynamics and Control: Theories on Laboratory Test Beds Ten Years Ago, Now in Orbit

The Engineering Test Satellite VII (ETS-VII), an unmanned spacecraft equipped with a 2-m long, six-degree-of-freedom manipulator arm, was developed and launched by the National Space Development Agency of Japan (NASDA). ETS-VII has successfully carried out a variety of on-board experiments with its manipulator arm, and these key technologies are essential for an orbital free-flying robot. These results will provide a solid basis for future satellite servicing missions. This paper highlights manipulator control utilizing the concepts of the generalized Jacobian matrix and the reaction null-space. These concepts have been proposed and discussed for the past ten years using laboratory test beds, and their practical application has now been demonstrated in orbit.

USE OF ADAPTIVE CONTROL ALGORITHMS FOR AUTOMATIC CALIBRATION OF ELECTROHYDRAULIC ACTUATOR CONTROL

Load and flow dependent nonlinear characteristics of open–center hydraulic systems are some of the major difficulties in electrohydraulic (E/H) actuator control. Proportional–integral–derivative (PID) control algorithms, widely used in other applications, cannot effectively deal with such hydraulic nonlinearity. Fuzzy and neural network–based adaptive control algorithms were developed for improving the performance of E/H control systems, especially the compensation of hydraulic nonlinearity caused by the load and flow dependent characteristics. Those adaptive algorithms were also capable of calibrating the controllers automatically. This article presents the comparisons of the experimental results from a fuzzy adaptive controller and a neural network–based adaptive controller to those from a classical PID controller in terms of velocity tracking accuracy on an open–center E/H actuator. Evaluations on a laboratory test bed showed that both the adaptive algorithms were capable of compensating the system nonlinearity effectively, and calibrating the controller accurately and promptly. Both the adaptive controllers resulted in satisfactory velocity tracking accuracy and stable dynamic performance.

Modeling the imaging process in optical stellar interferometers

Optical interferometers on the ground, like ESO's Very Large Telescope Interferometer (VLTI) and the Keck Interferometer, and in space, like the InfraRed Space Interferometer (IRSI/Darwin) and the Space Interferometry Mission (SIM), will bring a major breakthrough in optical and near-infrared high angular resolution astronomy at the beginning of the next millennium. These instruments are complex systems with an exceptionally interdisciplinary character involving active/adaptive optics, structural mechanics, control engineering, electronics and various environmental disturbances (e.g. atmospheric turbulence and absorption, wind, seismic noise). For their design and development an approach from two sides is appropriate: laboratory testbeds are used for experimental investigations while numerical modeling tools perform an End-to-End instrument simulation. We have developed a set of numerical modeling tools to simulate the dynamic imaging process of an interferometer. The time-dependent point spread function (PSF) mainly characterizes the imaging performance of the instrument. It is computed by an optomechanical model. Based on the knowledge of the PSF the image of an incoherently radiating extended object is computed using a Fourier optical method. This article describes the modeling approach including an extension to more than two interferometric beams. Some results of simulations on the VLTI as a representative example are shown.

Vertical secondary flows in submersed plant‐like arrays

Obstructions that protruded from a laboratory test bed into the benthic boundary layer were exposed to gradients in longitudinal velocity (δu/δz) that produced vertical pressure gradients along the surface of the obstruction. These pressure gradients generated vertical secondary flows that may have ecological significance for benthic fauna and aquatic macrophytes. Laboratory experiments demonstrated that secondary flows of up to 15% of the local longitudinal velocity were produced behind individual obstructions within a submersed plant‐like or animal tube array and for conditions like those found in aquatic canopies or colonies. Our observations support theoretical predictions based on a reduced form of the Navier‐Stokes equations, confirming that the ascending flow is controlled by a local balance of vertical pressure gradient, proportional to δu2/δz, and viscous stress. The secondary flows were shown to transport dye from the bottom to a height dictated by the in‐canopy current speed and turbulence intensity. By extension, the ascending flows can potentially contribute to the advection of nutrients from sediments, where they have been regenerated by microbial processes, to areas higher in the canopy, where they can be used by epiphytes and macrophyte leaves. Pressure gradients generated near the stem base also produce pore‐water exfiltration. The coupled pressure‐driven exfiltration and vertical advection have the potential to control nutrient availability in the bed.

Vibration Control of Flexible Robots--Theoretical and Experimental Results

Using robots for heavy loads and huge operating ranges causes that elastic deformations of the links lead to vibration problems. A new, basically model independent approach to active damping control is presented in this paper where the actuators are controlled to act like virtual spring-damper-systems. The paper illustrates this technique with the help of a simple and easy to model one link flexible robot and shows experimental results from two laboratory testbeds.

Modelling and Model Fitting of Flexible Robots – a Multibody System Toolkit Approach

During the last decade robots with flexible links became a popular research object for control engineers. This is because of their sophisticated properties referring to feedback control, e.g., non-minimum phase behaviour in end-effector control. Massive problems already occur trying to obtain an accurate analytic model for multilink flexible robots. This paper presents an effective way for numerical modelling of multilink flexible robots using the multibody system toolkit M{\mobilelogo}BILE. The experimental model fitting to a laboratory test bed of a two-link flexible robot is documented.

New Wavelength-Routed Architecture for Wavelength-Division Multiplexed WANs: Concept and Demonstration

After a brief presentation of the context of the French transport network, a new cell-based photonic network architecture is presented. This architecture, in which wavelength routing is carried out, fulfills major identified requirements for high-capacity wavelength-division multiplexed wide area networks. This architecture relies on the concatenation of geographically distributed cells, each of them being arranged in a broadcast and select topology. Simplicity, high flexibility, multiple bit rate, upgradability, and scalability are the main features of this cellular architecture, which also exhibits broadcasting and intrinsic equipment protection. Implementation issues and key components are reviewed. The laboratory testbed of this network is then described. It consists of four network nodes separated by 60 km, implementing one complete cell which comprises 16 wavelengths with a mean frequency spacing of 200 GHz. Four of the 16 channels are carrying 2.488 Gbit/s data streams. System validations and measurements performed are presented. From the measured cascadability results, this architecture proves to be implementable with commercially available components. Upcoming technologies that could further improve the architecture performances are addressed.

Rule-based control for a flexible-link robot

This paper presents a design and implementation case study that focuses on endpoint position control of a two degree-of-freedom robot with very flexible links. Linear and nonlinear conventional control techniques have been shown to be somewhat successful in achieving various control objectives for the laboratory test beds of this and many other studies; however, their reliance on an accurate mathematical model of the process often limits their chances of achieving good endpoint position control. Here the authors investigate an alternative to conventional approaches where they employ rule-based controllers to represent and implement two general forms of knowledge that they have about how to best control the mechanism: i) experience gained from the use of a mathematical model and conventional control; and ii) an intuitive understanding of the dynamics of the two-link flexible robot. The authors begin the case study by assuming that the controls for the two links of the robotic mechanism can be designed and implemented independently and investigate the performance of rule-based fuzzy controllers which only use simple intuitive knowledge about how to control two independent links. Next, the authors show that if the rule-base is augmented with knowledge about the coupling effects between the two links, the controller can achieve improved performance over the uncoupled case. The final portion of the case study, which represents the authors' primary contribution, investigates the use of a two-level hierarchical rule-based controller with a simple upper-level "expert controller" that captures knowledge about how to supervise the application of low-level fuzzy controllers during movements in the robot workspace. Overall, the rule-based supervisory control results have proven to be extremely effective for vibration suppression in the laboratory test bed of this study, comparing favorably (in terms of performance, design complexity, and implementation issues) to a variety of conventional techniques attempted to date (including linear robust designs, feedback linearization, input command shaping, and adaptive approaches).< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

A Test Bed for Experiments with Intelligent Vehicles

This paper describes the design considerations of a laboratory test bed for experiments with intelligent vehicles. A detailed description of the hard-and software elements is provided. Furthermore a specific operational mode is developed.

The Development of an Integrated Experiment to Study the Controls/Structures Interaction Problem in Large Optical Systems

The next generation of ground and space-based optical systems is characterized by their large size and high performance requirements. They rely heavily on active control to compensate for inadequate mechanical accuracy caused by manufacturing processes, thermal distortions, and gravitational or dynamic loads. Because of the prohibitive weight penalty incurred by fabricating a monolithic reflector with sufficient stiffness, as well as the problem of packaging a large, fragile optical element for transport to orbit, the use of segmented large optical components is the only practical approach for large space systems. This paper describes an experimental apparatus developed at the Lockheed Palo Alto Research Laboratory called the Advanced Structures/Controls Integrated Experiment (ASCIE). The ASCIE is a laboratory test bed designed to support a number of interdisciplinary studies, especially experiments pertaining to the interaction between structural dynamics and control systems in large segmented optical systems. The test article emulates a Cassegrain telescope with a segmented primary mirror. The segments and the secondary mirror are actively controlled to simultaneously provide the correct alignment as well as image stabilization. The segment alignment control system utilizes a self-referenced ensemble of edge sensors, and an independent optical measurement system provides for initial calibration and scoring. A light-weight truss structure supports both the secondary and primary mirror elements. The whole assembly is tuned to replicate the dynamic behavior of a large system. Details of the segment alignment control system, as well as preliminary experimental results are presented.

Compact homopolar generator developed at CEM-UT

For electromagnetic launchers (EMLs) to become practical devices, they must evolve from laboratory test beds to field-portable systems. Such systems require the development of compact, lightweight, high-energy, high-current power supplies. Investigation of the candidate systems -- flux compressors, capacitors, inductors, batteries, and rotating machines -- showed the homopolar generator (HPG) to be a device with immediate potential for development. HPGs were selected because of their demonstrated ability to produce the high-energy, high-current electrical pulse required of an EML power supply from a relatively compact light-weight machine. By taking state-of-the-art HPG technology and integrating it with a machine designed specifically for high energy density, a field-portable HPG-powered EML system can be realized.

The DES raligun facility at CEM-UT

The Center for Electromechanics at The University of Texas at Austin (CEM-UT) has constructed a facility for the operation of electromagnetic (EM) launcher experiments. The facility was specifically designed to investigate distributed-energy-store (DES) railguns. Experiments conducted in the facility have demonstrated the DES railgun concept using a 1-m long, four-stage DES railgun. Investigations have begun on a 4-m, ten-stage DES railgun to demonstrate operation of such a system at higher projectile velocities. The capabilities and design of the major components of the facility are described. Also presented is a review of the experimental development of the railgun system. The DES railgun facility is a versatile laboratory test bed facility for EM acceleration experiments.

Modeling of Soil Densification by Vibroflotation

A model probe was constructed and a series of densification tests run in a laboratory test bed. The model probe was 2.6 in (66 mm) in diameter and 0.305 m long, powered by a 1/2-hp motor. The primary soil used in the tests was a poorly graded beach sand, maximum density=115.7 pcf. A gravelly sand, maximum density=113.8 pcf was used mainly as a backfill soil. Five separate tests were performed using these two soils in various combinations and at various initial densities. Details of the tests are outlined and the results and conclusions are presented. The final relative density at a given distance from the probe is a function of the initial relative density. The gravelly sand is a better backfill than that achieved at several probe radii from the center. The amount of backfill accepted was seen to be a function of depth, decreasing with increasing depth. It was observed that while the probe is working to densify the soil maximum input power is required and once maximum density is finally achieved at a given depth, the power input required goes to a minimum. At shallow depths, however, this phenomenon was not observed as large input powers are required and the soil appears to fail via heaving at the surface.