Instrumentation

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Instrumentation

• K. Constantikes

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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

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Status
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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 MC
• First cut pointing, focus predictive algorithms
  tested

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Structural Temperature
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Temperature Sensor Locations
TF1
TF5
TSR
TF3
TF4
TF2
TH3
TB2
TH2
TE2
TB1
TB3
TB4
TB5
TE1
TA4
TA2
TA3
TA1
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Structural Temperature
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Structural Temperature
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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

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Air Temperature
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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

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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

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Quadrant Detector
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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 lt 150 ?m
• Dynamic range from 10 to 85 deg elevation
• SR translation plate scale lt4/mm

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Quadrant Detector
Quantities in GBT configuration with 87M path
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Quadrant Detector
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Quadrant Detector
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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

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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 _at_ 10 Hz, 0.14_at_ 1 Hz, 0.07 _at_
  0.1 Hz
• Will thermally couple to el bearing casting, ?T/
  ?t lt 5C over 24 hours, max rate lt 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, gt lt 0.001 error
  90e-6 G at 20/min, gt 18 error ?a (da/d?)??
  2?r ??, so 0.2 error gt ?? lt 7/sec

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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

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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 MC though USB/fiber
• Small form factor, will be easy to mount and
  move

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Accelerometers

• Silicon Designs 1221 MEMS accelerometers,
  2?G/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.

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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?

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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