December 3-4, Green BankGBT PTCS In Progress Review
Instrumentation
K. Constantikes
2GBT PTCS In Progress Review – December 3-4, 2003
Overview
• Conventional methods for improving pointing, focus are promising– JCMT inclinometry, LMT thermal model
• Instrumentation is simple, robust, relatively low cost• Requires crafty approaches and antenna characterization• Achievable on short time scales• Thermal corrections: 8 months for
– Sensor design, fabricate, install– Communications design, fabricate, install– Astrometric experiments– Correction algorithms, validation– Prototype real-time correction implementation
3GBT PTCS In Progress Review – December 3-4, 2003
Status
Instrument Purpose Status Initial Capability
Notes
Structural temp Correct thermal deformation
Commissioned Up to 10 additional Units
RS232 Concentrators
Sensor Communications
Commissioned 21 Ports Used,
48 Total
Quadrant Detector Dedicated Feed Arm Position
Installed December Calibrations
Inclinometers Alidade Thermals
Wind Effects
Structure Modal Tests
In Design,Material ordered
December Highest Priority
Star Tracker
(Guide Scope)
Differential Orientation of GBT Components
In Test Moderate Priority
Data Acquisition System (GDAQ)
High Resolution ADC and Discretes
Material On Hand
Device Testing
No Date Set Low Priority
Vertical air temp Group IndexQD Path BendingStructure Coupling
Prototypes Installed
No Date Set Up to 6 units
Accelerometers Structural health On hold No Date Set Low Priority
4GBT PTCS In Progress Review – December 3-4, 2003
Structural Temperature
• 19 locations, 0.2C interchangeable accuracy, 0.01C resolution, 1Hz, range –35 to 40C. (actual accuracy is ~0.1C, temp control of conversion elex)
• Design documentation:– PTCS Wiki (AntennaInstrumentation) – PTCS Project Note PTCS/PN12
• Accuracy tested in lab:– Solar/convective loading– Selected unit-to-unit accuracy, repeatability– Electronics temperature range
• RFI mitigated, ESD protected• Two thermistor failures, forensics with YSI• Integrated into M&C • First cut pointing, focus predictive algorithms tested
5GBT PTCS In Progress Review – December 3-4, 2003
Structural Temperature
6GBT PTCS In Progress Review – December 3-4, 2003
Temperature Sensor Locations
TSR
TF1
TF4
TF5
TF3
TF2
TH2
TH3 TB2
TB1 TB3
TB5
TB4
TE1
TE2
TA3
TA1
TA2
TA4
7GBT PTCS In Progress Review – December 3-4, 2003
Structural Temperature
8GBT PTCS In Progress Review – December 3-4, 2003
Structural Temperature
9GBT PTCS In Progress Review – December 3-4, 2003
Concentrators
• 8 port terminal servers, up to 115kb/sec.• Fiber interconnect to GB net• RFI mitigated, ESD protected, thermal tests• 5 currently on GBT: 21 ports used, 40 available
– 1 alidade– 1 actuator room– 2 vertex– 1 receiver room
• 6th unit to be installed at actuator room location• Units provide (will provide) RS232 for:
– Structure and air temperature sensors– Quadrant detector status and control– Alidade inclinometers– Data acquisition units (GDAC)– Etc…
10GBT PTCS In Progress Review – December 3-4, 2003
Air Temperature
11GBT PTCS In Progress Review – December 3-4, 2003
Air Temperature
• Modified structure temp sensor package– Forced convection cell– 1 sec time constant thermistor– Effective 5 sec time constant (swept volume)
• Applications:– More accurate LRF group index – Correct QD for path bending (gradient normal to ray)– Possible applications in model convective heat transfer
to/from structure, improved night-time thermal pointing corrections
12GBT PTCS In Progress Review – December 3-4, 2003
Quadrant Detector
• Used to measure feed arm motion WRT ~elevation axle, 10 Hz sample rate.
• Excellent for vibration measurements, wind effects. Need to resolve calibrations for 1 hour absolute accuracy (unmodeled elevation dependence)
• Prototype unit removed Fall 2002 for improvements• Upgrades:
– Reduced electronic thermal drifts– Reduced electronics noise– Monitoring and control functions
• Reinstalled Nov 2003, completed functional tests• Engineering interfaces only
13GBT PTCS In Progress Review – December 3-4, 2003
Quadrant Detector
14GBT PTCS In Progress Review – December 3-4, 2003
Quadrant Detector
• Stability in lab ~ path refractive effects (~ 200 m in lab) (Estler, et al., equivalent to 1C/m vertical air temp gradient)
• Resolution ~ 12 m (GBT equiv)• Noise in lab ~100 m position (GBT equiv)• Residual nonlinearity in lab cal ~ 1.3 millimeters RMS (GBT
equiv)• Works in foggy/rainy conditions• Noise on GBT < 150 m • Dynamic range from 10 to 85 deg elevation • SR translation plate scale <4”/mm
15GBT PTCS In Progress Review – December 3-4, 2003
Quadrant Detector
Before upgrade Notes Current
Quantize 12m 15 bits 12m
Path and Elex Noise
<230m Measured in lab <100m
PSD Nonlinearity 4000m Measured in lab < 1300m
Electronic Thermal 400m 10°C No noticable trend on 2C
Tipping calibration,
½ hour
~< ± 2000m Residual repeatable cross-el error
Mechanical/Thermal Drift, Path
± 3500m Guess. Need to characterize
Quantities in GBT configuration with 87M path
16GBT PTCS In Progress Review – December 3-4, 2003
Quadrant Detector
17GBT PTCS In Progress Review – December 3-4, 2003
Quadrant Detector
18GBT PTCS In Progress Review – December 3-4, 2003
Inclinometers
• Plan to install two-axis inclinometers on each el bearing…– Dynamic range of angle motion is small, can use conventional
inclinometers
– Tilts of el bearings directly cause pointing errors
– Might be able to use alidade as spring-balance (along with observations) to infer primary shift/rotation, FA bending as fcn of wind speed and relative azimuth- generate lift/drag relations semi-empirically
– Can also distinguish thermal from other distortions via NCP experiments with range of wind speeds…
– Can be used as accelerometers for structure mode shape and frequency analysis
– Will use existing concentrators for communications
19GBT PTCS In Progress Review – December 3-4, 2003
Inclinometers
• ± 1° Gas-damped capacitively coupled pendulum• Design goal 0.2 arcsec over one hour
– 24 Hour limits of error (20C)= 0.6” + 0.07% RO– Uncompensated temp coef = 1.4”/C +0.2% RO– Resolution 0.46” @ 10 Hz, 0.14”@ 1 Hz, 0.07” @ 0.1 Hz
• Will thermally couple to el bearing casting, T/ t < 5C over 24 hours, max rate ~< 1C/hour– Factory temp compensation curves will improve this…
• Natural frequency of inclinometer?– During real-time correction, average out. – During modal testing?
• Correction for az rate centripetal effects (a=2r)~ 14e-9 G at 15”/sec, => < 0.001” error~ 90e-6 G at 20°/min, => 18” error a = (da/d) = 2r , so ~0.2” error => < 7”/sec
20GBT PTCS In Progress Review – December 3-4, 2003
Star Tracker
• Need to measure/infer rotations of components on tipping structure– Dynamic range (5 to 95° elevation) prevents fixed
conventional inclinometer– Primary rotations wrt el axle – SR rotations wrt el axle
• Guide cameras have been used to stabilize tracking to ~ 1”
• Propose use of star tracker to measure structure rotations (differentially)
• Additional use as optical guide scope
21GBT PTCS In Progress Review – December 3-4, 2003
Star Tracker
• Acquired SBIG ST7-XE with 100mm F5 optic– CCD Kodak KAF-0401E + TI TC-211 Pixel Array – 765 x 510 pixels,6.9 x 4.6 mm – Pitch 9 x 9 microns – Full Well Capacity ~100,000 e- – Dark Current 1e¯/pixel/sec at 0° C– Electromechanical shutter, min exposure 0.1sec
• IFOV ~20”• Have started testing w/ 100mm and 300mm optics• Need to prove that sub-pixel interpolation yields ~ 1” tracking• Interface to M&C though USB/fiber• Small form factor, will be easy to mount and move…
22GBT PTCS In Progress Review – December 3-4, 2003
Accelerometers
• Silicon Designs 1221 MEMS accelerometers, 2G/root-Hz.
• 3-axis design• Signal conditioning PCBs completed• Will test with existing QD Laser data acquisition
system• Will be used to
– replace existing FA accelerometer– Additional accelerometers for structure modal analysis– Potential for less-accurate inclinometry, but thermal stability
is poor.
23GBT PTCS In Progress Review – December 3-4, 2003
Data Acquisition
• Need for 24-bit, up to 1kHz modules to service– Accelerometers– Replace existing QD VXWorks ADC
• Identified Analog devices ADUC845 – 24 bit, sigma-delta, 10 single-ended or 5 differential– Embedded 8051 core– Have samples in hand– Have prototyping system in hand– Will evaluate (soon?)– Communications via RS232
• Remaining technical issue is time-distribution protocol– Will evaluate round-robin latency measurement and 10-5sec/sec
local clock: Estimation window of 100 secs for 1ms accuracy?
24GBT PTCS In Progress Review – December 3-4, 2003
PTCS Instrumentation Team
• Jason Ray: Temp sensor lead, GDAQ, electronics• JD Nelson: QD lead, air temp sensor lead,
electronics• Randy McCullough: Electronics design• Jeff Cromer: Mechanical• Paul Marganian: Software• John Shelton: Metrology