Transcript LHDC High-Performance Fine

Pointing and Stabilization of
Lightweight Balloon Borne Telescopes
SwRI Balloon Workshop on
Low Cost Access to Near Space
27 April 2007
Larry Germann
Left Hand Design Corporation
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The Purpose of a Precision
Pointing System
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Perform line-of-sight stabilization
– Correct atmospheric turbulence
– Correct vehicle base motion
– Correct vibration of optical elements
– Correct force or torque disturbances
– Correct friction-induced pointing errors
Perform scanning function to extend the Field of Regard
beyond the telescope’s Field of View
Perform chopping function
Perform dither function
Quickly slew and stare among a field of targets
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When a Precision Pointing
System is Needed
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When the required pointing stability cannot be achieved by the
platform attitude control system
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When the field-of-regard requirement is larger than the
instrument’s achievable field-of-view
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When chopping is required to calibrate the optical sensor
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Line-Of-Sight Stabilization,
Stability Correction Ratio
Pointing System Cost is Related to the Correction Ratio Spectrum
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Pointing
40
Correction Ratio
Amplitude
(dB)
20
0
10
Readily Available
Currently Achievable
In Development
Not Possible
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50
100
200
500 1000
Frequency (Hz)
Correction Ratio Amplitude (f) = Base Motion (f) / Residual LOS Jitter Requirement (f)
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Dominant Sources of
Vehicle Base Motion
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LEO Spacecraft
– Thermal Shock from Transitions into & from Umbra
– Attitude Control System
– Solar Array Drives
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High-Altitude Lighter-Than-Air
– Attitude Control System
– Payload Mechanisms
– Station-Keeping Propulsion, if applicable
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High-Altitude Heavier-Than-Air
– Air Turbulence
– Propulsion
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Typical Pointing System
Components
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The components of a typical precision pointing system include:
– Beam-expander telescope
– Fine-steering mechanism or fast-steering mechanism: two-axis reducedaperture, full-aperture steering mirror or isolation system
– Coarse-pointing mechanism: vehicle attitude control system, two-axis
gimbaled telescope or full-aperture steering mirror
In general, both fine-and course-pointing mechanisms are required when
system dynamic range >10^5 @1kHz or >10^6 @10Hz is required, exceptions
include a mass-stabilized satellite ACS for the single pointing stage
Flexure-mounted fine-steering mechanism is required when system following
accuracy requirement exceeds friction- or hysteresis-induced limits
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Fine- and CoarsePointing Mechanisms
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Coarse-Pointing Mechanism
– Performs large-angle motions
– Can be vehicle ACS or bearing-mounted mechanism
– Keeps FPM near the center of its travel range
Fine-Pointing Mechanism
– Performs high-frequency portions of pointing motions
– Performs high-acceleration motions
– Accurately follows commands
– Corrects or rejects base motion and force and torque disturbances
– Can be reaction-compensated (a.k.a. momentum compensated)
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2-Axis Fast-Steering
Mechanism Technology
is Mature
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Apertures for beam sizes from 15mm
to 300mm are available
Servo control bandwidths to 5000 Hz
Range of travel up to +-175mrad
(+-10degrees) are available
A variety of mirror substrate materials
are proven
– Aluminum
– Beryllium
– Silicon Carbide
– Zerodur
– BK-7
– LEBG
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The FSM Can Be An Active
Isolation System
Non-Contacting 6-DOF Active Isolation
Systems are Available
• Non-Contacting electromagnetic actuators
• Non-Contacting sensors
• Highly flexible umbilical transfers signals
with <0.1 Hz suspension resonant frequency
– minimal transfer of base motion forces
• Accelerometer- and position-referenced
stabilization servos
• Shown here is an IS2-10 Isolation System
– ±2mm travel in all axes
• Shown here is an IS5-40 Isolation System
used as a base-motion-simulator
– ±5mm travel in all axes
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Precision Pointing Systems
Offer Many Benefits
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Extended Dynamic Range,
– up to 9 orders of magnitude
– up to +-180 degree Field of Regard
– as low as nanoradian line-of-sight stability
High servo control bandwidth, up to 3,000 Hz
Stable Line-of-Sight
– correct for platform vibrations
– correct for aero turbulence
Agile Beam-Steering
– up to 15,000 rad/sec2 acceleration
– up to 30 rad/sec rate
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Precision Pointing Systems
Cover Large Ranges of
Precision and Field-of-Regard
Fields-of-Regard from 1 milliradian to continuous rotation are considered
Precision is defined as positioning resolution, stability and following accuracy
Friction Limit
Fine-Steering Mechanism (FSM)
with a Coarse Steering Mechanism
1000
Mass-Stabilized
Telescope
Satellite
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10
1
0.001
FSM Sensor Noise Limit
with 10x Optical Gain
Field of Regard (+- milliradians)
10000
0.01
Coarse-Steering
Mechanism
Single Full-Aperture
Flexure-Mounted Steering Mirror
Full-Aperture FSM
Sensor Noise Limit
0.1
1
Single Full- or Reduced-Aperture
Flexure-Mounted Steering Mirror
10
100
Precision (micro-radians)
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Near Space Relative to
Lower-Altitude Aircraft
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The primary advantages of near-space deployment relative to operating on
lower-altitude (up to 18km) aircraft platforms include:
– Additional FOV and FOR for earth observation and surveillance missions
– Potentially quieter platforms
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The primary difficulties are:
– Additional LOS stability is required for the same resolution on the ground
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Near Space Relative to
LEO Platforms
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The primary advantages of near-space deployment relative to operating on
LEO platforms include
– increased resolution for earth observation and surveillance missions
– relaxed environmental requirements
– the ability to loiter over an area of interest
– less hardware is required to cool actuators and servo control electronics
The primary difficulties associated with near-space deployment relative to
LEO platforms include
– aircraft and UAV encounter atmospheric turbulence and the resulting lineof-sight and platform disturbances
– reduced FOV and FOR
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Components of
Pointing Accuracy
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Fine- and course-steering mechanism pointing accuracy
is defined in several ways:
– positioning resolution
– position reporting resolution
– line-of-sight jitter
– position reporting noise
– short-term positioning drift
– long-term positioning drift
– positioning thermal sensitivity
– position reporting thermal sensitivity
– positioning linearity
– position reporting linearity
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Imaging Resolution Limit is
Related to Altitude and Aperture
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Imaging resolution
is constrained by
the optical
diffraction limit,
which is a function
of altitude and
telescope aperture
Image resolution is
defined as a
distance on the
ground from 30km
altitude
Diffraction-Limited Resolution
as a Function of
Aperture and Wavelength
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DiffractionLimited
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Resolution
(m on ground at
30km altitude) 0.1
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0.01
100
100
200
30
10
3
Wavelength (micron)
400
1
0.3
0.1
Aperture
(mm)
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Positioning and Reporting
Linearity
• Positioning linearity is defined as the difference between
commanded and achieved position over the operating ranges of
travel and temperature
– Dominated by friction, disturbances and position sensor error
– Position sensor error is dominated by thermal sensitivity
– Typically not much better than 0.04% of travel
• Reporting linearity is the difference between reported and
achieved position over the operating ranges of travel and
temperature
– Dominated by position sensor error
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Fast Beam Steering is
Defined as Servo Control Bandwidth
1000
100
+-10 mR Extended
+-35mR Extended
+-175mR Extended
224
153
Major Axis Aperture (mm)
+-10 mR
+-35mR
+-175mR
116
10
80
Travel
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Alternately defined as the
0dB open-loop frequency
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40
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Generally defined as the
frequency at which the
closed-loop servo response
falls by 3dB
Servo Control Bandwidth vs. Aperture and Travel
for Fine-Steering Mirrors
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Fast beam steering is
defined as the ability to
follow a small-amplitude
sine wave at various
frequencies
Servo Control Bandwidth
(Hz)
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Fast Beam Steering is also
Defined as Acceleration Capability
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Fast Beam Steering is
sometimes defined as the
highest frequency at
which the mechanism
can perform a full travel
sine wave
This is limited by the
mechanism’s
acceleration capability
Acceleration is shown
here in terms of peak and
continuous capability
100000
Acceleration (Rad/Sec2)
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Peak and Continuous Acceleration Capability
vs. Aperture and Substrate Material
for Fine-Steering Mirrors
10000
SiC Peak
Be Peak
Al & Zd Peak
Substrate Material
1000
24 40
56 80
116 153
Major Axis Aperture (mm)
224
SiC Continuous
Be Continuous
Al & Zd Continuous
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Non-Linear Characteristics
Limit Pointing Accuracy
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Friction-induced pointing error
– Typically associated with ball or sleeve bearings
– Peaks at turn-around condition (stick-slip)
– Error amplitude can be readily estimated
• pointing error ~ 2 * friction torque / inertia / bandwidth2
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Hysteresis-induced pointing error
– typically associated with ceramic actuators
– typically quantified in terms of % of travel range
– Effect are similar to friction effects
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Many Precision Pointing Instruments
are Suitable for
Near-Space Platforms
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LIDAR measurements of forest canopy
LIDAR measurements of foliage, carbon stock under canopy
LIDAR measurements of targets under foliage or camouflage
LIDAR topology measurements under foliage
5-10m resolution over a 60km circle on ground from 100km altitude
1-3m resolution over a 20km circle on ground from 30km altitude
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