Oct 13-15, 2003 ADASS 2003
The GBT Precision Telescope Control System
Kim Constantikes
2ADASS 2003
Introduction
• Brief overview of the Green Bank Telescope (GBT) and scientific requirements on pointing, efficiency,etc.
• Experimental results• Brief overview of the Precision Telescope Control System (PTCS)• Overview our exploratory data processing environment, the Engineering
Measurement System (EMS)
The GBT is performing surprisingly wellWe are enhancing instrumentation and investigating GBT phenomenology at level of arcsecs, 100’s of microns wavefront errorThe EMS has been used to perform GBT laser rangefinder surveys at millimeter levels, will be used for other GBT investigations andprototype real-time compensations
3ADASS 2003
Telescope Structure and Optics
• Offset-Gregorian design• Operation to 115 GHz, 40 GHz winter 2003-2004• Optics: 110 m x 100 m of a 208 m parent paraboloid
• Effective diameter: 100 m• Off axis feedarm
• Elevation Limit: 5°• Slew Rates: Azimuth - 40°/min; Elevation - 20°/min• Main Reflector: 2209 actuated panels with 68 µm rms.
• Total surface: rms 400 µm• FWHM Beamwidth: 740"/f(Ghz)• Prime Focus: Retractable boom• Gregorian Focus:
• 8-m elliptic subreflector with 6-degrees of freedom• Rotating Turret with 8 receiver bays
4ADASS 2003
Telescope Structure and Optics
5ADASS 2003
Scientific Requirements for High-Frequency Observing
6ADASS 2003
Pointing Accuracy, 5° C Gradient, 5° ElLoral Tech Memo 52 Table 2-20 El=5, ∆T=5,Vw=0,Sun Az=180
Error Sources El Errors (arcsec) X-El Errors (arcsec)Repeatable Nonrepeatable Repeatable Nonrepeatable
Mechanical AlignmentsRF/El Axes Orthogonality 0.0 0.0El/Az Axes Orthogonality 0.0 5.2Az Axis Verticality 5.0 0.4Structural DeformationsReflectorWindThermal Gradient 0.3AlidadeWind Thermal Gradient 11.5Servo and Drive 0.9 0.3MiscellaneousEl Bearing Wobble 1.0 0.4 0.1 0.0Az Bearing Wobble 1.0 0.4 0.1 0.0Encoder Accuracy 1.2 1.2Encoder Coupling 1.8 2.1Encoder Referencing 10.0 10.0RSS Subtotals 11.3 12.1 11.3 2.4
7ADASS 2003
The Real vs. Ideal GBT: All-Sky Pointing
8ADASS 2003
The Real vs. Ideal GBT: Half-Power Tracking
9ADASS 2003
The Real vs. Ideal GBT: Efficiency and Beam Shape
10ADASS 2003
The Real vs. Ideal GBT: Wind Effects
11ADASS 2003
The Real vs. Ideal GBT: Servo Effects and Structure Vibration
12ADASS 2003
The Real vs. Ideal GBT: Temperature and Focus
The Real vs. Ideal GBT: Temperature Gradients
14ADASS 2003
PTCS: Paradigms, Models
• PTCS composed of four elements (April 2003 CODR):– High Frequency Observing System: The observer’s interface– Engineering Measurement System: Algorithm/analysis/control exploration tools– Precision Measurement System: Production measurements– Precision Control System: Control and models wrapped around existing Antenna
Manager (M&C) components• Alternative approaches for risk mitigation:
– Direct measurement of figure/position/orientation, additional closed loop control– Empirical corrections to pointing, focus– Model structure, measure perturbing influences (mainly wind and thermal gradients)
• Direct measurements not currently sufficient for 100 Ghz, – Empirical methods in development and test– Some models in use: FEM adjustment of primary, structure linearity/superposition– Considering new models to identify/predict perturbations (thermal-structural model),
plate scales and abberations (optical model)• Additional closed loop control not currently needed
– Structure vibration is typically small, not significantly excited by shaped trajectories or wind-pumping, lowest structure mode ~ 0.6 Hz
• Current emphasis is antenna characterization using multiple instruments and astronomical inferences
15ADASS 2003
PTCS: Instruments (Current)
• 12 laser rangefinders in ring around GBT• 19 precision structural temperature sensors
– Thermal corrections for pointing, primary shape (?)• 2 precision air temperature sensors
– Characterize group refractive index variation degradation to laser rangefinder accuracy, convective heat transfer model for GBT structure
• Quadrant Detector– Measure feed arm position (angle-angle) from elevation axle
• 3 weather stations• Servo monitoring for azimuth and elevation drives
– Torques, rates, etc.• 1 2-axis 10µG accelerometer set on FA tip
16ADASS 2003
Laser Rangefinder Geometry
-1.5 -1 -0.5 0 0.5 1 1.5
x 105
-1.5
-1
-0.5
0
0.5
1
1.5
x 105
0
5
10
x 104
Y
X
Node: ZAG731D Azimuth: 0 Elevation: 38
ZAG731D
ZY112
ZY111
ZY109
ZY110
Z
-1.5 -1 -0.5 0 0.5 1 1.5
x 105
-2
-1
0
1
2
x 105
0
5
10
15
x 104
YZY104
ZY103
ZY105
ZY102
ZEG41040R
X
Node: ZEG41040R Azimuth: 0 Elevation: 38
Z
17ADASS 2003
PTCS: Instruments (in Development)
• 2 2-axis clinometers (0.2 arcsec) on elevation bearings– Map azimuth track variations
• Additional air temperature sensors– Characterize vertical refractive index, convective heat transfer
• Additional 3-axis accelerometer sets– Compensation for vibration (need high-bandwidth tertiary element)– Structural health monitoring via modal analysis
• Hot wire (self-heating thermistor) wind speed– Measure 3-D wind with low time constants, compensate with
tertiary element • Wide-field star tracker
– Measure differential orientation of locations on structure, e.g., subreflector actuator mount points
18ADASS 2003
Engineering Measurement System
• Algorithm development and numerical analysis• Initially for real-time laser rangefinder multilaterations• Continues as exploratory environment for production measurement and control• Consists of:
– Top level signal flow-graph representation, data-driven process: Wit– Data transport to/from monitor and control: TCP and SOAP– Database connectivity for calibrations, configurations, etc.– Scripting/numerical/visualization environment: Matlab– API to C, C++, etc
• Wit provides easy probing of graph, drag and drop dataflow design• Matlab has rich algorithm and visualization environment• Databases organize large datasets, e.g., retro glass offsets, instrument
calibrations• SOAP transport provides direct access to GBT M&C data• Wit and Matlab provide migration to standalones, libraries
19ADASS 2003
Example: Multilateration with laser rangefinders
– Initialize coordinate transforms, rangefinder calibrations, retro excess path lengths, etc. from database
– Calculate and smooth path group refractive index, rangefinder zero points and “leakage” signals
– Correct measured path phases– Predict range from rangefinder to target using structural FEM– Calculate measured range from corrected phases, predicted ranges– Select range data by target– Non-linear least-squares estimate of retro position from ranges and
rangefinder positions– Save intermediate,final results to database; real-time probe and
plot quantities of interest
20ADASS 2003
Example: Multilateration with laser rangefinders
Graph edges carry primitive data types, structures, generic objects
Graph nodes execute when input data are available
Graph state defined by edge data, Matlab engine retains state in global structures
Can probe or break on edges, pause execution, and use full MATLAB IDE in paused state
Hierarchical graphs modularize operators
Node parameters can be changed on the fly, parameters can be promoted to inputs, etc.
Database operations via ODBC and SQL
21ADASS 2003
Example: Multilateration with laser rangefinders
22ADASS 2003
Other Applications
• Prototype real-time visualization and correction of low-rate effects:– Thermal pointing errors:
• Structural and air temp monitoring • Predictive algorithm, e.g. linear regression• SOAP/TCP to/from GBT Monitor and Control System
– Alidade tilts and azimuth track flatness: • Elevation bearing clinometers• SOAP/TCP .
– Wind-induced feed-arm motion:• Quadrant Detector• Pointing coefficients• SOAP/TCP
23ADASS 2003
Additional Information on PTCS
Project documentation can be found at:http://wiki.gb.nrao.edu/bin/view/PTCS/WebHome
• CODR• Project and System Notes• Various design and status information• Information for Astronomers (under construction)• Experiments and results
See our poster The GBT Engineering Measurement System (P9.9)
for more details…
24ADASS 2003
Additional Slides
25ADASS 2003
Telescope Structure and Optics: Active Surface
26ADASS 2003
Scientific requirements for high-frequency observing
27ADASS 2003
Scientific requirements for high-frequency observing
28ADASS 2003
Pointing Accuracy, 6 m/s wind
Error Sources El Errors (arcsec) X-El Errors (arcsec)Repeatable Nonrepeatable Repeatable Nonrepeatable
Mechanical AlignmentsRF/El Axes Orthogonality 0.0 0.0El/Az Axes Orthogonality 0.0 35.3Az Axis Verticality 5.0 2.9Structural DeformationsReflectorWind 10.5Thermal Gradient AlidadeWind 1.5Thermal Gradient Servo and Drive 0.9 0.3MiscellaneousEl Bearing Wobble 1.0 0.4 0.6 0.2Az Bearing Wobble 1.0 0.4 0.6 0.2Encoder Accuracy 1.2 1.2Encoder Coupling 1.8 1.7Encoder Referencing 10.0 8.1RSS Subtotals 11.3 12.3 36.4 2.1
29ADASS 2003
The Real vs. Ideal GBT: Gravity
Elevation
Del
ta-X
(mm
)D
elta
-Y (m
m)
Del
ta-Z
(mm
)
30ADASS 2003
The Real vs. Ideal GBT: Temperature and Azimuth
31ADASS 2003
The Real vs. Ideal GBT: Temperature and Elevation
32ADASS 2003
Architecture of current GBT Observing System
33ADASS 2003
Architecture of HFOS
34ADASS 2003
Architecture of PCS