Plaian, A

Ursu, I I

CERN Document Server

POSITION CONTROL SYSTEM FOR THE NOTTE EXPERIMENTAndrei Plaian, Ioan Ursu,“Elie Carafoli” National Institute for Aerospace Researches,B-dul Iuliu Maniu 220, 77538 Bucharest 6, Romania, E-mail: aplaian@aero.incas.roAbstractDuring the total solar Eclipse in august 1999 in Romania,the NOTTE experiment, concerning the elementaryparticles physics was developed. It was an airborneexperiment, to increase the duration of the measurement.An active control system was used to direct themeasurement system to the Eclipse and to ensure theproper insulation against the angular perturbationsinduced by the flying aircraft. The measurement systemand a digital TV camera are mounted on a large massmobile platform with two angular degrees of freedom,driven by DC torque servomotors. Due to its large inertia,the platform's angular movements are very slow, giving toa computer the necessary time (0.16 s) to analyze theimage from the TV camera and to find the angularposition errors. These errors are used by a controlalgorithm that gives commands to the torque motors inorder to correct the position of the platform. Theadmissible angular position errors are no larger than ±10angular minutes.1 INTRODUCTIONDuring the total Solar Eclipse in August 1999 inRomania, “Elie Carafoli” National Institute for AerospaceResearch and ISS, both from Romania and Universitadegli Studi di Bologna, Italy, carried out an experimentconcerning the study of the solar neutrinos. The technicaldifficulties of the NOTTE Experiment (NeutrinoOscillations with Telescope during the Total Eclipse)came from the fact that the measurement system wasmounted on a supersonic aircraft that pursued the EclipseShadow, to increase the duration of the experiment. Themain problem was to direct the measuring device towardsthe Eclipse and to maintain its orientation with very lowerrors (less than ±10 angular minutes), despite the factthat the flying aircraft vibrates and executes smallcorrections of the trajectory. To fulfil this task, an activeposition control system was used.2 DESCRIPTION OF THE POSITIONCONTROL SYSTEMProblem SettingThe main problem of this position control system was toget the position information. The two needed angularcoordinates of the center of the eclipse are available byanalyzing the image of the eclipse, given by a digital TVcamera. Due to the large image processing time, a set ofcoordinates could be available only at every 0.16 s (aPentium II/333 MHz was used). It is obvious that acontrol system, which uses only that information, wouldbe very slow and unable to ensure the requestedpositioning precision. Therefor, it was necessary to use asupplementary stabilization method.The adopted solution was to use a large mass mobileplatform with a good static and dynamic equilibrationsustained by a very low friction cardanic articulation.This solution offers a good mechanical insulation of themobile platform on those two rotation axes. Themechanical system was mounted on the plane so that thedirection versus the Eclipse would be approximatelyrectangular to the two axes of the cardanic articulation.This fact was possible because of the straight flight andthe almost constant position of the Eclipse during theapproximately 6 minutes of the experiment. The angularperturbations induced by the flying aircraft on this axisresults only in the rotation of the image, which does notdecrease the pointing precision. The perturbations on therest of 3 degrees of freedom (3 translations) have nosignificant effect on the pointing precision, because of thevery large distance to the Moon. However, due to the(low) friction torque of the bearings the orientation of themobile platform is slowly modified, which results inangular position errors. To maintain these errors at anacceptable level, it is necessary to use a position controlsystem, which, in these conditions, is able to ensure therequested precision.2.2 The Position Control SystemThe position control system was called “ControlledPositioning Mobile Platform” (CPMP).The angular position of the mobile platform MP iscontrolled on two rectangular axes, both approximatelyrectangular to the direction versus the Eclipse.The scheme of the CPMP is presented in fig.1. The twoangular coordinates of the center of the eclipse areavailable by analyzing the image of the eclipse, given bya digital TV camera. Based on this information, thealgorithm implemented in the computer C givescommands to the DC torque servomotors M1 and M2through the interface TMI, in order to correct the positionerrors.The first pointing to the Eclipse is made by bringing theplatform to a relative to the plane pre-determinedposition, with an initial positioning system IPS, driven bythe computer through an interface.The measuring system consists in a photon detector FD,mounted on the mobile platform and a data recorder DR(a second computer). The measuring system isindependent to the position control system.International Conference on Accelerator and Large Experimental Physics Control Systems272International Conference  Accel rator and L rge Experim ntal Physics Control Systems, 1999, Trieste, ItalyThe power supply is ensured by a power source PSseparate from the aircraft (batteries and a 12 V DC / 220V AC inverter).2.3. The electro-mechanical part of the systemThe mobile platform is sustained by two DC torqueservomotors. The axis of a motor is rectangular to the axisof the other motor. To reduce the friction torque, no gearsare used. However, the torque developed by the motorsdirectly to their axes is large enough to fulfil the demandsof the control process.The static equilibration was made by adjusting theposition of a few counterweights attached to the platformand to the body of one of the motors. The staticequilibration eliminates the effects of the linearaccelerations (gravitational acceleration and the variablelinear accelerations of the aircraft on 3 directions).The angular movements of the plane on the directionwhich is rectangular to the axes of the motors giveseffects such as torques as a result of the centrifugal forcesand a dynamic coupling of the movements of the platformon its two axes. Both these effects were eliminated orstrongly reduced by experimentally adjusting the positionof some other counterweights. The operation wassomehow similar to a dynamic equilibration.Fig. 1. The control system used in the NOTTEExperimentThe initial positioning system is simply electro-magnetical, with no control involved. Its task is to bringthe platform to a pre-determined position (the position ofthe eclipse relative to the flying aircraft). The initialpositioning system is used until the Eclipse image entersthe TV camera field, than it is automaticallydisconnected. After that, the computer analyzes the imageand gives commands to the torque motors in order tobring the Eclipse to the center of the TV camera field andmaintain it there.The optical axes of the photon detector and of the TVcamera are set to be parallel, which ensures the pointingof the photon detector to the center of the Eclipse duringthe experiment.3. THE NOTTE EXPERIMENTThe experimental model of the CPMP was built byECNIAR and ISS from Romania (the position controlsystem) and Universita di Bologna, Italy (the measuringsystem).For the lab tests, an experimental device was especiallyconceived, which could simulate the angularperturbations, induced by the flying aircraft on two axes.After the successful passing of the lab tests (the obtainedprecision was approx. ± 7 angular minutes), all the partsof the CPMP were mounted on a MIG 29 chair, fig. 2.Fig. 2. The CPMP mounted on a MIG 29 chairThe chair equipped with the CPMP was mounted in thesecond cabin of a double command MIG 29.After the ground tests, which proved that the systemfunctioned well and did not electro-magneticallyinterfered with the systems of the plane, came the flightTVFD DRCM1 M2TMIPSIPS IPSIMP273tests. There were 11 flights, during which theperformances of the CPMP were analyzed and improved.During the tests, both the TV camera and the photondetector were covered with optical filters, to prevent theirdeterioration when pointing them directly to the Sun.4. CONCLUSIONSThe day before the total solar eclipse, a final test wasmade, simulating all the phases of the experiment. TheCPMP functioned well.Finally, the experiment during the total solar eclipsefailed due to the extremely bad weather conditions, whichmade the taking off of the plane impossible.Acknowledgments: The authors are very grateful toICALEPCS ‘99 Organizing Committee and ECNIAR forthe granted financial support and also to our colleaguesfrom ISS and Universita di Bologna for their cooperationin NOTTE Experiment.5. REFERENCES[1] I. Ursu, A. Plaian and F. Ursu, “PositioningAlgorithms in the NOTTE Experiment”, presented atICALEPCS’99.[2] P. Dorato and A. H. Levis, “Optimal LinearRegulators: The Discrete-Time Case”, IEEE TAC,AC-16, no. 6, December 1971, pp. 613-620.[3] T. Chen and B. A. Francis, “Sampled-Data OptimalDesign and Robust Stabilization”, Transactions of theASME, Journal of Dynamic Systems, Measurement,and Control, 114, December 1992, pp. 538-543.[4] M. M’Saad, “Commande LQG” Conceptionoptimisée des systémes: Commande optimale,Institute National de Grenoble et Université“Politehnica” Bucarest, 1995.[5] K. Zhou, J. C. Doyle and K. Glover, “Robust andOptimal Control”, Prentice Hall, 1996.274

Position Control System for the NOTTE Experiment

Abstract

Similar works

Full text

Available Versions

CERN Document Server