november 2003 kc-135 spheres flight test results

November 2003 KC-135 SPHERES flight test results

Mark O. Hilstad, Simon Nolet, Dustin Berkovitz, Alvar Saenz-Otero, Dr. Edmund Kong, and Prof. David W. Miller

MIT Space Systems Laboratory2003-11-24

Nov. 2003 KC-135 results 2

Overview

• Attitude control and beacon tracking– Tracking of a hand-held beacon by a sphere.– Tracking of a sphere by another sphere.

• Simple search pattern– Open-loop three-axis rotation

• Coordinated search pattern– Elements of the “lost in space” maneuver

• Docking– Initial approach stage only

• Identification of inertia and center of mass• Lessons learned

A series of four KC-135 flights during the week of 3 Nov 2003 was sponsored by the Jet Propulsion Laboratory’s Terrestrial Planet Finder program. These flights were used to test algorithms designed by several members of the SPHERES team, and by NASA Ames. The results of these tests are presented herein.

This presentation referencesthe following video files:

• KC135_Nov03_flight2_para23.mpg• KC135_Nov03_flight4_para02.mpg• KC135_Nov03_flight4_para03.mpg• KC135_Nov03_flight4_para10.mpg• KC135_Nov03_flight4_para13.mpg• KC135_Nov03_flight4_para18.mpg• KC135_Nov03_flight4_para19.mpg• KC135_Nov03_flight4_para29.mpg


Beacon tracking experiments

• Objective– Demonstrate tracking of a beacon

and optimal rotation along the shortest path

– Validate the 3-D control law by demonstrating off-axis 3-D attitude control

• Experiment description– Tracking of a beacon randomly

located in the test volume

– Tracking of a free floating sphere’s on-board beacon

– Two spheres simultaneously tracking each others’ on-board beacons

Flight 2, parabola 23



Beacon tracking results

• Flight 4, parabola 18– Pointing error is reduced in each test (q1, q2 and q3 tend toward zero, while

q4 tends toward one)

– Body rates show angular acceleration and deceleration, as expected

Test 1 Test 2Test 1 Test 2 Test 1 Test 2


Simple searchF

light 4, parabola 2F

light 4, parabola 3

• Initial conditions– A single sphere stationary with respect

to the KC frame

• Open-loop spin– Try to point the onboard beacon in as

many directions as possible• Attempt to map 4 steradians

• Alternating thrusters – Two thrusters on at any given time

• Very limited time, so maximize actuation

– Used to change the plane of rotation– Thruster state changes once per

second.

• A propellant-efficient algorithm will be used for SPHERES-TPF.


Simple search results

• Day 4, parabola 2– Alternating z, y, x torque commanded– Some coverage due to thrusters, some due to disturbances such as bumping walls– Sparse coverage in the -x direction, but within the half-cone angle of the beacon– Quaternion integration issues add uncertainty to validity of results

• Quaternion behavior appears erratic, but rapid change is expected• Rate gyroscope saturation leads to inaccurate quaternion integration• Quaternion normalization errors also led to inaccurate integration

– Video shows multi-axis rotation, as intended.

0 2 4 6 8 10 12 14 16-1

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1P

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m]

Sphere 1 state telemetry.

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Time [s]

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dy

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xyz

xyz

q1

q2

q3

q

xyz

Coverage maps

-10

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Coordinated search

• Experiment description– Two spheres begin with the

beacon faces offset from their common line of sight by 135°.

– Both spheres initiate open-loop z-axis spins.

– Acquisition occurs when one sphere hears the other’s beacon.

– When a sphere hears a beacon, it sends a stop message to the other sphere.

– When a sphere receives a stop message, it initiates rate damping.

– Both spheres use the same algorithm.




Coordinated search results

• Flight 4, parabola 10– Local 0° are offset from common line by -135°– Perform +z spin at ~30°/s until acquisition/communication, followed by rate damping

• Beacon half angle is ~30-45°(equivalent to 1-1.5 seconds of spin time)– Telemetry from both spheres shows expected behavior

• Sphere 1 shows rate damping at z-quat≈0.6 → z-angle≈106°• Sphere 2 shows rate damping at z-quat≈0.45 → z-angle≈127°

– Primary maneuver (spin search, then decelerate upon acquisition) is circled.

Sphere 1

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Rate damping begins

q3 at start of rate damping

Sphere 2

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Rate damping begins

q3 at start of rate damping


Glide slope docking experimentsF

light 4, parabola 19F

light 4, parabola 29

• Objective– Demonstrate the first phase of the

docking approach computed by a glide slope algorithm

• Experiment description– The two spheres maintain their

orientation toward each other

– The chasing sphere initiates a translation along its x-body axis to move toward the target

– The algorithm is set such that docking should occur in about 8 seconds!


Glide slope docking results

• Flight 4, parabola 19– The first 3.5 seconds of the docking maneuver were successfully achieved

– The spheres maintain relative pointing, as shown by the constant quaternions

– Similar data were acquired by both spheres using their own sensors

Docking Approach

Range to target

Approach velocity

Chasing Sphere

Docking Approach

Range to target

Approach velocity

Chasing Sphere Range

to target

Approach velocity

Target Sphere

Docking Approach


Inertial property identification

• Online gyro-based mass property identification– Use gyro data and calibrated thruster information to identify the error from nominal

values of center of mass offset and moment of inertia– Algorithms by SPHERES team and NASA Ames

Dry inertia

(kg m2)

Ixx0.022901

Iyy0.020546

Izz0.018165

Ixy0.000196

Ixz-0.000066

Iyz-0.000212

CM offset

(mm)

Full tank

CMx-0.054495248

CMy-0.810686405

CMz0.244119942

Empty tank

CMx-0.015665375

CMy-0.821387572

CMz3.081773236

Pre-flight estimated inertial properties• KC-135 flight goal: perform one

long test over many parabolas– Allows more time for estimates

to converge– Provides the most amount of

raw data for download

• Problem: disturbances corrupt identification

– Update stops by itself when gyro saturates, no thrusters are firing, or SNR is too low

– Pause/resume command used when sphere is handled


Inertia ID sample data• Estimated angular acceleration (using rate sensors)

– Data are taken from parabolas 32-40, flight #4– Green lines indicate when test was paused– Delays in pausing and unpredictable motion caused issues

• Parabola 35: attached proof mass (a spare battery pack) on the -x face of the sphere

• Parabola 37: replaced the full tank with one containing only 28g of gaseous CO2 (no liquid) to remove effects of propellant slosh

• Parabola 38: removed the proof mass 0 10 20 30 40 50 60 70 80

-1

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Filtered angular accelerations, clean data only

time [sec]

alp

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ax

ay

az

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abol

a 35

Par

abol

a 37

Par

abol

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ay and az dropWhen proof massIs attached


Inertia ID results

• Deviations from the previous best-estimate inertia – These are “dry” values; propellant is subtracted out

• The inertia in the y (green) and z (red) axes jumps when the proof mass is added– The change is about 80% of what was expected; this is under investigation

• Propellant slosh is not an issue

0 10 20 30 40 50 60 70 80

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x 10-3 ID'ed Inertia

time [seconds]

Inert

ia [

kg

-m2 ]

IxxIyyIzzIxyIxzIyzbetween parabolasproof mass added28g tank replacedproof mass removed

~ 3.2e-3 change(expected 3.73e-3)


Center of mass ID results

• Deviations from the nominal CM offset. – These are “dry” values; propellant is subtracted out

• Since it was impossible to filter out all handling of the sphere using pause and resume commands in real-time, the online estimate was corrupted

• Saved high-frequency IMU data from the flight were used successfully in an identical off-line algorithm (accurate to floating-point precision) to produce promising results for both inertial and CM estimation.

0 10 20 30 40 50 60 70 80

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ID'ed deltaC

time [seconds]

CM

off

set

[mm

]

deltaCx

deltaCy

deltaCz

Pa

rab

ola

35

Pa

rab

ola

37

Pa

rab

ola

38~ 5 mm change

(expected 6.3)


Lessons learned

• Care must be taken when integrating separate tests into one program– Tests being integrated in one program must have compatible initialization settings– Special attention is required when the tests are written by different individuals

• Unexpected infrared noise causes confusing behavior– We have identified a series of fixes for use in any future KC flights

• IR-opaque, visible-transparent sheeting on lights (will also improve video quality)• Form-fitted, pre-cut curtains to block IR from other experiments• Point laptop screen away from test area• We expect these fixes to fully alleviate all infrared problems

– Recent test results on the ISS suggest that infrared noise will not be a problem; however, experiences like this continue to help us better understand noise sources and to identify new ways to address the issue if it arises.

• SPHERES core code changes– Static variables must be explicitly initialized at the start of each test, not just at the

start of the program– Order of process initialization problems manifested on flights as corrupt IMU data

• Identified problem and fixed by changing initialization procedure– Will add data to the telemetry stream to notify us of suspected IR noise.


Conclusion

• Objectives accomplished– Showed that the attitude control algorithm works as

expected in 3D– Validated critical maneuvers and demonstrated key

components of the “lost in space” sequence• Open-loop search• Beacon acquisition• Intercommunication• Stop and hold response• Beacon tracking

• Saved precious experiment time on ISS– Gained confidence in current algorithms, and identified

areas for improvement prior to flight.– Improved the inertia and center of mass estimates of the

spheres.

november 2003 kc-135 spheres flight test results

Documents

handheld beacon

beacon trackingtracking

beacon tracking resultsflight

onboard beacon

spheres team

test volumetracking

test q1

simple search patternopenloop