Space Systems LaboratoryMassachusetts Institute of Technology SPHERES Alvar Saenz-Otero Synchronized Position Hold Engage Reorient Experimental Satellites

Space Systems LaboratoryMassachusetts Institute of Technology Outline Objective & Requirements SPHERES Description Communications Lab Test Results KC-135 Flight Results Conclusions Future Work

Space Systems LaboratoryMassachusetts Institute of Technology Objective To develop a testbed that demonstrates formation flying algorithms between multiple autonomous satellites with six degrees of freedom, in a microgravity environment.

Space Systems LaboratoryMassachusetts Institute of Technology Requirements Develop a set of multiple distinct satellites that interact to maintain commanded position, orientation, and direction –Allow for the interchange of control algorithms, the acquisition and analysis of data, and a truth measure –Demonstrate key formation flying maneuvers –Demonstrate autonomy and status reporting –Ensure the adaptability of control algorithms to future formation flying missions –Ensure testbed operability on KC-135, Shuttle mid-deck and ISS

Space Systems LaboratoryMassachusetts Institute of Technology Terrestrial Planet FinderDS - 3TechSat 21 Motivation Space Telescopes Lower Cost Reduce Size Modularity

Space Systems LaboratoryMassachusetts Institute of Technology The Big Picture Commands STG communication Metrology STS communication

Space Systems LaboratoryMassachusetts Institute of Technology Today Commands STG communication STS communication

Space Systems LaboratoryMassachusetts Institute of Technology SPHERES Divided into 6 sub-systems –Propulsion –Structures –Power –Avionics –Communication –Metrology

Space Systems LaboratoryMassachusetts Institute of Technology Structures Internal aluminum structure –12 identical members –4 different ‘ends’ Lexan covers –fully enclosed –3 access doors

Space Systems LaboratoryMassachusetts Institute of Technology Propulsion Compressed CO 2 system 70-80psi 860psi) 3-5min of operation TankRegulator 5-way Valves Nozzles 5-way 3-way Reserve

Space Systems LaboratoryMassachusetts Institute of Technology Power Power Source 13 AA (1.5Vea, 19.5V total) ~1.5 hours Regulator 19.5V  5V Regulator 19.5V  12V Metrology Tattletale Rate Gyros & Accelerometers US Sensors Regulator 5V  3.3V Comm Propulsion 19.5V  24V C40 Xmit/ Rcv SolenoidsIR Sensors Bypass Capacitor

Space Systems LaboratoryMassachusetts Institute of Technology Metrology 6 dof IMU –3 axis accelerometer –3 gyroscopes GPS-like Ultrasound/Infrared System –IR sets ‘time-0’ –US delay from IR gives distance –Newton’s Method used for triangulation

Space Systems LaboratoryMassachusetts Institute of Technology Avionics Power C40 DSP Comm TT8 TT8 Comm STG Comm STS 916 MHz 868 MHz USIR Propulsion Solenoids IMU

Space Systems LaboratoryMassachusetts Institute of Technology Software Main –Initializes  P –Starts communication –“Background” All sends (STS, STG, Telemetry) Command Reception Error Detection ISR - Interrupt –Runs controller –Handles ‘critical’ communications Main interrupt controller background Controller output input calculations

Space Systems LaboratoryMassachusetts Institute of Technology Communications STS & STG –Wireless 19200bps –Half-duplex system  can only transmit or receive at one time –Data telemetry and commands TT8 –Wired bps –Full duplex –IMU updates at 50Hz –Global metrology updates at 10Hz

Space Systems LaboratoryMassachusetts Institute of Technology Communications Restrictions: One at a time 916 MHz ± 200KHz 868 MHz ± 200KHz “Master” “Slave”

Space Systems LaboratoryMassachusetts Institute of Technology Communications Need to create ‘packets’ of information –Adds overhead, but... –Handles ‘asynchronous’ data –Adds error-detection –Maximum 256 bytes of data per packet –Since ‘start’ is not a unique number, must ‘empty’ data each time to prevent packet header confusions. To FromSize DataChecksum Type Start

Space Systems LaboratoryMassachusetts Institute of Technology Communications Token Ring protocol –‘Token’ allows member to send data –It is passed on in pre-specified sequence –Currently token is only passed one-way to simplify operations A “Master” “Slave” 3

Space Systems LaboratoryMassachusetts Institute of Technology Lab Results PD controller –KD = 0.25; KP = 0.50; deadband ~1º Slew –90º turn, 15º/s turn, via raised cosine: –calculates correct  for raised cosine Master always does the same slew –5 second initial synchronization period ang_ref_z = A × ( 1 - cos(  × t)) / 2 rate_ref_z = A ×  × sin (  × t) /2

Space Systems LaboratoryMassachusetts Institute of Technology Lab Results STS Test Matrix Independent slew –Slave passes Master data directly to ground STS Control

Space Systems LaboratoryMassachusetts Institute of Technology Lab Results Independent Test –Each SPHERES programmed to independently go to 90deg at 15deg/sec average –‘Slave’ is just a passthrough for ‘Master’ data

Space Systems LaboratoryMassachusetts Institute of Technology Lab Results Independent Test Video

Space Systems LaboratoryMassachusetts Institute of Technology Lab Results STS Full state, 10Hz –Master carries slew –Slave follows master with both angle and rate controls

Space Systems LaboratoryMassachusetts Institute of Technology Lab Results STS Full state, 10Hz

Space Systems LaboratoryMassachusetts Institute of Technology Lab Results STS Half State, 10Hz –Master carries slew –Slave follows master with only angle –Rate control reference set to zero

Space Systems LaboratoryMassachusetts Institute of Technology Lab Results STS Half state, 10Hz

Space Systems LaboratoryMassachusetts Institute of Technology Lab Results STS Full State, 1Hz –Master carries slew –Slave follows master with both angle and rate –Rate reference set to zero –Slave updates at 1Hz

Space Systems LaboratoryMassachusetts Institute of Technology Lab Results STS Full state, 1Hz

Space Systems LaboratoryMassachusetts Institute of Technology Lab Results STS Half State, 1Hz –Master carries slew –Slave follows master with only angle –Slave only updates reference commands at 1Hz

Space Systems LaboratoryMassachusetts Institute of Technology Lab Results STS Half state, 1Hz

Space Systems LaboratoryMassachusetts Institute of Technology KC-135 Results STS Full state, 10Hz –3D motion –Limited ~15sec test –‘Master’ measures KC rates –‘Slave’ follows master attached to KC

Space Systems LaboratoryMassachusetts Institute of Technology KC-135 Results KC-135 Airframe Slave

Space Systems LaboratoryMassachusetts Institute of Technology KC-135 Results STS Full state, 10Hz –3D motion –Limited ~15sec test –‘Master’ turned by hand, not in KC frame –‘Slave’ follows master turns –High body blockage

Space Systems LaboratoryMassachusetts Institute of Technology KC-135 Results KC-135 Manual Turn

Space Systems LaboratoryMassachusetts Institute of Technology Conclusions SPHERES is operational for formation flying control research –Simple controller is available, can be expanded –Possible to program and select different control modes –STS and STG communications functional –Reasonable bandwidth

Space Systems LaboratoryMassachusetts Institute of Technology Conclusions From lab results –Independent Formation flying may use ‘independent’ slews –For STS Higher rate is most important Full state is not essential Need to estimate Master state during communications breakup

Space Systems LaboratoryMassachusetts Institute of Technology Conclusions KC-135 –Higher control authority is needed –Global Metrology is essential for KC operations –Body blockage problem must be solved Communications and metrology –Formation Flying is minimal due to 20s parabola time, 10s operational time Basic FF maneuvers must be developed to prove 6dof operation

Space Systems LaboratoryMassachusetts Institute of Technology Future Work SPHERE improvements (HW) –Reduce mass –Complete access panels –Re-position thrusters –Improve communications –Improve IMU sensors –Implement Global Metrology

Space Systems LaboratoryMassachusetts Institute of Technology Future Work SPHERE Improvements (SW) –Shell for wireless programming –Fully implement communications protocol –Define operational modes for formation flying –Implement state estimators –Optimize code in general (especially communications and metrology algorithms) –oh, yeah, Thesis...

Space Systems LaboratoryMassachusetts Institute of Technology KC-135 is fun! Top KC Blooper