MIT 16 83X - Metrology System Design (17 pages)

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Metrology System Design



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EMFFORCE OPS MANUAL 1 Space Systems Product Development Spring 2003 G Metrology System Design AA G 1 Subsystem Outline The purpose of the metrology subsystem is to determine the separation distance and relative orientation of the craft The data will be used to control the system and stored for analysis of the system performance The primary purpose of the metrology system is to provide an input for the control algorithm The data will be used to maintain control of the cluster This purpose is the main source of the system requirements Massachusetts Institute of Technology 1 Dept of Aeronautics and Astronautics EMFFORCE OPS MANUAL 2 Space Systems Product Development Spring 2003 G 2 Requirements The metrology system is required to determine the position and orientation of the vehicles The metrology system is also required to determine the angular rate of each vehicle This data will be used to control the system and recorded for analysis Specifically the metrology system must Determine the separation distance of the vehicles to within 1cm Determine the bearing angle between vehicles to within 5 degrees Work at a maximum separation distance of 4m Return data at a rate of 1 Hz Massachusetts Institute of Technology 2 Dept of Aeronautics and Astronautics EMFFORCE OPS MANUAL 3 Space Systems Product Development Spring 2003 G 3 Current System Design The current metrology system is legacy technology modified to suit our needs The current design for the metrology system is a hybrid of the system used for project SPHERES and the system utilized by the Electronic Ink Corp in their Mimio Electronic whiteboard The design of the receivers is taken directly from SPHERES since it had proven effective on that project and met our current system requirements The transmitter is based on the design of the Mimio pen transmitter The Mimio transmitter proved to be more effective for location in 2D a plane where as the SPHERES system was designed to work in 3D space The original designs have been modified to suit the specific needs of project EMFFORCE however it can be assumed that any part of the design which is not explicitly described below is based solely on legacy technology The metrology system uses triangulation of emitted ultrasonic signals to determine the location of the vehicles Infrared emitters and receivers facilitate timing of the ultrasonic pulses Each vehicle will be equipped with three omni directional ultrasonic receivers 1 omni directional ultrasonic transmitter and an array of infrared transmitters and receivers to cover 360 degree line of sight In this document omni directional is taken to mean that the transmitters emit signals of the same amplitude and phase in all directions in the horizontal plane 360 degrees and the receivers can sense a signal from any direction in a plane This is not strictly omni directional but since the test bed operates in 2D the term is adequate During one cycle of the sensing system each vehicle will take turns emitting and receiving pulses and then calculate the position of the other vehicles by triangulation The first vehicle will first emit an infrared pulse followed by an ultrasonic pulse at a known time later The other two vehicles will first receive the infrared pulse followed by the ultrasonic pulse at each of the three receivers The time difference between the reception time of the IR and Ultrasonic pulses will give time of flight for the ultrasonic pulse This will be used to determine the distance of the transmitter from each of the receivers A simple calculation will give the position of the transmitter After the vehicles have the position data for the first vehicle the second will take its turn to emit followed by the third There is an inherent delay in this system The ultrasonic pulse must be allowed to leave the test area before another can be emitted otherwise the first pulse might interfere with measurement of the subsequent pulses Given the speed of sound at standard conditions and the current geometry of the system the metrology team has calculated that it will require 23 milliseconds for a pulse to leave the test area before the next can be sent As a result a three vehicle system will require 69 milliseconds to determine the orientation of all three vehicles This is a delay inherent to the physics of the system and cannot be significantly changed From this approximation and added margin for known and unknown factors it is currently approximated that the metrology system will operate at 1 Hz Massachusetts Institute of Technology 3 Dept of Aeronautics and Astronautics EMFFORCE OPS MANUAL 4 Space Systems Product Development Spring 2003 G 4 Hardware Component Design G 4 1 Structural Support The Structural support is designed to keep the other components in the proper orientation It is necessary to keep the relative positions of the receivers fixed in order to calculate the position and orientation of the vehicle Since the positioning of the receivers is so important the metrology designed the structural support independently of the structure of the rest of the vehicle The metrology system works by using differences in reception time at each of the receivers thus it is important to keep them as far apart as possible If the receivers are close together the difference in reception times will be very small If the receivers are farther apart the average difference will be greater Since errors in reading the receivers should be independent of receiver location the greater difference between the reception times will make the errors less significant The extent of this effect has not been calculated directly The general assumption during design was that the receivers should be as far from the center of the vehicle without overhanging the edge of the vehicle The structural support is attached to the vehicle using Velcro For accurate position and angle measurement the system needs to be aligned with the axis of rotation and center of mass of the vehicle it was assumed that they would be in the same place directly below the center of the support It was assumed that axis of rotation might be hard to find or might change due to design changes so adjustments might be necessary Velcro was used to make small adjustments easy The support structure of the metrology system is a large Y cut from a single sheet of 1 8inch thick aluminum stock a diagram is shown in Figure G 4 A The structure was cut using the water jet The legs each extend 0 38m from the center and


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