december 3-4, green bank gbt ptcs in progress review instrumentation k. constantikes

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


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


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


Temperature Sensor Locations

TSR

TF1

TF4

TF5

TF3

TF2

TH2

TH3 TB2

TB1 TB3

TB5

TB4

TE1

TE2

TA3

TA1

TA2

TA4


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…


Air Temperature


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


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


Quadrant Detector


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


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


Quadrant Detector


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


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


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


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…


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.


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?


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

december 3-4, green bank gbt ptcs in progress review instrumentation k. constantikes

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