Space Systems LaboratoryMassachusetts Institute of Technology SPHERES Alvar Saenz-Otero Synchronized...

Space Systems Laboratory Massachusetts Institute of Technology

SPHERESAlvar Saenz-Otero

Synchronized Position Hold Engage Reorient Experimental Satellites


Outline

• Objective & Requirements• SPHERES Description• Communications• Lab Test Results• KC-135 Flight Results• Conclusions• Future Work


Objective

To develop a testbed that demonstrates formation flying algorithms between

multiple autonomous satellites with six degrees of freedom, in a microgravity

environment.


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


Terrestrial Planet Finder

DS - 3 TechSat 21

Motivation

• Space Telescopes• Lower Cost• Reduce Size• Modularity


The Big Picture

Commands

STG communication

Metrology

STS communication


Today

Commands

STG communication

STS communication


SPHERES

• Divided into 6 sub-systems– Propulsion– Structures– Power– Avionics– Communication– Metrology


Structures

• Internal aluminum structure– 12 identical members– 4 different ‘ends’

• Lexan covers– fully enclosed– 3 access doors


Propulsion

• Compressed CO2 system• 70-80psi (tank @ 860psi)• 3-5min of operation

Tank Regulator

5-way

Valves

Nozzles

5-way

3-way 3-way 3-way 3-way 3-way 3-way

3-way 3-wayReserve Reserve


PowerPower Source

13 AA (1.5Vea, 19.5V total) ~1.5 hours

Regulator19.5V 5V

Regulator19.5V 12V

Metrology

Tattletale

Rate Gyros &Accelerometers

US Sensors

Regulator5V 3.3VComm

Propulsion19.5V 24V

C40

Xmit/Rcv

Solenoids IR Sensors

BypassCapacitor


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


Avionics

Power

C40 DSP

Comm TT8TT8

Comm STG

Comm STS

916 MHz

868 MHz

US IR

Propulsion

Solenoids

IMU


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


Communications

• STS & STG– Wireless 19200bps– Half-duplex system can only transmit or

receive at one time– Data telemetry and commands

• TT8– Wired 125000bps– Full duplex– IMU updates at 50Hz– Global metrology updates at 10Hz


Communications

Restrictions:

One at a time

One at a time916 MHz ± 200KHz

868 MHz ± 200KHz

“Master”

“Slave”


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.

ToFrom Size

Data ChecksumTypeStart


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

1

21

A

“Master”

“Slave”

3


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)) / 2rate_ref_z = A × × sin ( × t) /2


Lab Results

• STS Test Matrix• Independent slew

– Slave passes Master data directly to ground

• STS ControlFull State(angle and rate)

Half State(angle only)

10 Hz X X

1 Hz X X


Lab Results

• Independent Test– Each

SPHERES programmed to independently go to 90deg at 15deg/sec average

– ‘Slave’ is just a passthrough for ‘Master’ data


Lab Results• Independent Test Video


Lab Results

• STS Full state, 10Hz– Master carries

slew– Slave follows

master with both angle and rate controls


Lab Results• STS Full state, 10Hz


Lab Results

• STS Half State, 10Hz– Master carries


master with only angle

– Rate control reference set to zero


Lab Results• STS Half state, 10Hz


Lab Results

• STS Full State, 1Hz– Master carries


master with both angle and rate

– Rate reference set to zero

– Slave updates at 1Hz


Lab Results• STS Full state, 1Hz


Lab Results

• STS Half State, 1Hz– Master carries


master with only angle

– Slave only updates reference commands at 1Hz


Lab Results• STS Half state, 1Hz


KC-135 Results

• STS Full state, 10Hz– 3D motion– Limited ~15sec

test– ‘Master’

measures KC rates

– ‘Slave’ follows master attached to KC


KC-135 Results• KC-135 Airframe Slave


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


KC-135 Results• KC-135 Manual Turn


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


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


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


Future Work

• SPHERE improvements (HW)– Reduce mass– Complete access panels– Re-position thrusters– Improve communications– Improve IMU sensors– Implement Global Metrology


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...


KC-135 is fun!• Top KC Blooper

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Space Systems LaboratoryMassachusetts Institute of Technology SPHERES Alvar Saenz-Otero Synchronized...

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