Transcript LHDC High-Performance Fine-Steering Mirror Family

Pointing and Stabilization of
Lightweight Balloon Borne Telescopes
presented at the
SwRI LCANS 09 Balloon Workshop on
Bridging the Gap To Space
Lightweight Science Payloads on
High-Altitude Long-Duration Balloons and Airships
26 October 2009
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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Precision Pointing Systems
Cover Large Ranges of
Precision and Field-of-Regard
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Fields-of-Regard from 100 microradian 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
100
10
1
0.001
Mass-Stabilized
Telescope
Satellite,
like HST
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
Single Full- or Reduced-Aperture
Flexure-Mounted Steering Mirror
1
10
100
System Precision (micro-radians)
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Line-Of-Sight Stabilization,
Stability Correction Ratio
Pointing System Cost is Related to the Correction Ratio Spectrum
60
Pointing
40
Correction Ratio
Amplitude
(dB)
20
0
10
Readily Available
Currently Achievable
In Development
Not Possible
20
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
•
LEO Spacecraft
– Thermal Shock from Transitions into & from Umbra
– Attitude Control System (ACS) exciting vehicle bending modes
– Solar Array Drives
•
High-Altitude Lighter-Than-Air
– ACS exciting pendulum & suspension cable bending modes
– Payload Mechanisms
– Station-Keeping Propulsion, if applicable
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High-Altitude Heavier-Than-Air
– Air Turbulence exciting vehicle bending modes
– Propulsion
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Typical Precision 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
Payload motion sensor suite: inertially or optically referenced
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 a 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,
116 x 87mm for a 75mm beam shown
-3dB closed-loop servo control
bandwidth up to 5,000 Hz
Range of travel up to +-175mrad
(+-10degrees) is available
A variety of mirror substrate materials
are proven
– Aluminum
– Beryllium (shown here)
– Silicon Carbide
– Silicon Carbide Foam
– Zerodur
– BK-7
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CE50-35-CV-RC2 FSM
Is Simple, Robust and Mature
•The CE75-35-BK SN140
•BK-7 mirror
•76.2mm diameter aperture
•+-35mRad travel
•120 Rad/Sec2/rootW efficiency
•2,300 Rad/Sec2 acceleration
•wave PV @633nm surface figure error
•450 Hz -3dB closed-loop servo control
bandwidth
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CE75-35-ZD Represents LHDC’s
line of Cost-Effective FSM
•CE75-35-ZD SN147, Zerodur mirror
•76.2mm diameter aperture
•+-26mRad travel
•A custom abbreviated frame
•9,000 Rad/Sec2 acceleration
•120 Rad/Sec2/rootW efficiency
•0.165 wave PV @633nm surface figure error
•250 Hz -3dB closed-loop servo control
bandwidth
•Coating is highly reflective at 1.5um
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FO50-175-AL
Has Space-Flight Experience
•FO50-175-AL SN106
•Aluminum mirror
•80.7 x 60mm polished aperture
•+-175mrad travel
•380 Hz -3dB closed-loop servo control
bandwidth
•7,000 Rad/Sec2 acceleration
•Proven in low-earth orbit
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FO50-35-SC-RT7 Achieves Record
Servo Control Bandwidth
•FO50-35-SC-RT7 SN133
•Silicon carbide mirror
•80.7 x 60mm polished aperture
•+-5mrad travel with the reduced-travel
option
•5,000 Hz -3dB closed-loop servo control
bandwidth when base-referenced
•6,000 Hz -3dB closed-loop servo control
bandwidth when optically referenced
•3,300 Rad/Sec2 acceleration
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The Fine-Steering Mechanism Can
Be An Active Isolation System
Non-Contacting 6-DOF Active Isolation System
• 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
• IS2-10 Isolation System
– Occupies a 25mm thick disk
– ±2mm travel in 3 axes
• IS5-40 Isolation System used here as a
base-motion simulator
– ±5mm travel in 3 axes
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Servo Functional Block Diagram
Pointing/Tracking
Error (2)
Tra cking Position
Command (2)
Tra cking Position
Error (2)
1st Base Referenced
Position Command (2)
2nd Base Referenced
Position Command (2)
Feed-Forward
Acceleration
Command (2)
+
+ 
+
+ 
-
Tra ck
Point
Position
Servo
Compensation
+
+

Current Driver
Electronics
Fine-Steering
Mirror
Mechanism
Thermal Sensor (2)
Base Referenced
Position Sensor
Output (2)
Tra cking
Optical Sum
Tra ck
Enable
Position
Sensor
Demodulation
Point/Track
Switching
Logic
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Flight-Format Servo Control
Electronics is Available
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SC03-BD
2 Channels Servo Control
– Position-Referenced Loops
– Current-Referenced Drivers
– Optical Tracking Reference
– Position Sensor Reference
Light Weight
– 150 Grams
Full Military Temperature
Up to +-45V, 10A Driver Capability
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Servo Control Electronics
Available in a VME-6U
Single-Card Format
SC02-BD
Single-Card VME-6U Format
Contains All Servo Functions
- Pointing and Tracking Modes
- Current-Referenced Driver
- High-Temperature Driver Shutdown
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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 and position reporting resolution
– Line-of-sight jitter and position reporting noise
– Short-term positioning drift and position reporting drift
– Long-term positioning drift and position reporting drift
– Positioning thermal sensitivity and position reporting thermal sensitivity
– Positioning linearity and 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
100
10
DiffractionLimited
1
Resolution
(m on ground at
30km altitude) 0.1
50
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
Fast beam steering is
defined as the ability to
follow a small-amplitude
sine wave at various
frequencies
10000
Servo Control Bandwidth
(Hz)
•
Servo Control Bandwidth vs. Aperture and Travel
for Fine-Steering Mirrors
1000
100
224
153
Major Axis Aperture (mm)
+-10 mR
+-35mR
+-175mR
116
80
56
Travel
10
Alternately defined as the
0dB open-loop frequency
+-10 mR Extended
+-35mR Extended
+-175mR Extended
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Generally defined as the
frequency at which the
closed-loop servo
response falls by 3dB
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•
21
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)
•
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 Positioning Accuracy
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Friction-induced pointing error
– Typically associated with ball or sleeve bearings
– Peaks at turn-around condition (stick-slip)
– Friction-induced error amplitude can be readily estimated
• Peak Pointing Error ~ 2 * Friction Torque / Inertia / Bandwidth2
•
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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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 5,000 Hz
– Correct disturbances up to 1,000 Hz
Stable Line-of-Sight
– Correct for platform vibrations
– Correct for aero turbulence
Agile Beam-Steering for scanning, chopping, dither, etc.
– Up to 15,000 rad/sec2 acceleration
– Up to 30 rad/sec rate
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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
0.1m resolution over a 20km circle on ground from 100km altitude
0.03m resolution over a 6km circle on ground from 30km altitude
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