Background, theory and practical limitations of metrology ......Metrologies at Radio Astronomy...

Metrologies at Radio Astronomy AntennasMetrology Systems for Radio Astronomy at MTM

© MT Mechatronics GmbH3rd RadioNet3 European Radio Astronomy Technical Forum

Technical Workshop1

Background, theory and practicallimitations of metrology systems at

Radio Astronomy Antennas

Technical Workshop

Chalmers University of Technology,Gothenburg, SwedenSeptember 1-2, 2014

Martin SüßMT Mechantronics GmbH,

Mainz, Germany

Metrologies at Radio Astronomy AntennasBackground, theory and practical solutions of metrology systems


Technical Workshop2

What is Metrology ?

Metrology is the science of measurement. Metrology includes all theoretical and practical aspects of measurement. …

In Ancient Greek the term μετρολογία (metrologia) meant "theory of ratios".

source: wikipedia (en)

+

measurements how to use the information ?

logosmetron +

+

http://www.wikipedia.org/wiki/Science

http://www.wikipedia.org/wiki/Measurement

http://www.wikipedia.org/wiki/Ancient_Greek



Technical Workshop3

What are we looking for?

Task 1: Pointing accuracy:

vertical axis (Azimuth)

horizontal axis

(Elevation)

line of sight (LOS)

Sometimes a 3. axis

(Tilt or XEl)



Technical Workshop4

What are we looking for?

Task 2: Surface accuracy

Deviations of theoretical vs. actual reflecting surface

Surface Error

But careful:Direction! (normal to surface / normal to ray ?)

Pathlength vs Surface

RMS – calculation: consideration of weighting



Technical Workshop5

Error Budgets:

Dependent on the project`s scientific goals or systematic requirements (link budgets), both errors are restricted to acceptable figures, also mainlyinfluenced by the frequency ranges to be observed and the antenna aperture.

Error Source Contribution

Axis Alignement 25 %

Servo System 15 %

Environment 60 %

Gravity 20 %

Wind 80 %

Thermal 20 %

Pointing error budget

Error Source Contribution

Panel Manufacturing 30 %

Panel Alignement 30 %

Environment 40 %

Gravity 20 %

Wind 40 %

Thermal 40 %

Surface error budget



Technical Workshop6

Side-Note on the surface vs. pointing accuracy

Surface accuracy is analytically optimized by „best fitting“ an ideal paraboloid into the actual (deformed) shape.

„Best fitting“ = tbd DOFs !!! (dependent on the active DOFs of thesystem.

Least square fit, the fitting parameters are derived according toweighting factors, influence functions, etc.

Solving this equation delivers a unique solution for the minimumsurface accuarcy. This solution may be disadvantageous for the usein pointing equations !!!



Technical Workshop7

What can we do about it?

Generally, there are two possible approaches:

Measuring the causes: (wind, gravity, temperatures)(typically easy to measure, but how to handle the results?)

or

Measuring the consequences (tilts, offsets, pointing errors)(typically very small, not easy to measure, but relevant for thefinal result)

Radio Telescopes are suffering from 2 facts:a.) 2-dimensional target acquisition (like on a optical CCD) to compensatefor pointing (end 2 end) b.) wavefront sensors to compensate for surface accuracy



Technical Workshop8

What can we do about it?

Gravity: mostly repeatable look up- tables

if not, accelerometers (not ground-based or other accelerations)

Thermal: predictable, but many varying load case scenarios !!!

Wind: partly, but hardly predicatable !!!

(separation into quasi-static and dynamical frequency range, very much site specific !!!)



Technical Workshop9

Gravity:

Easy to analyse and predict by use of FEM models.

Repeatable as a function of elevation

Standard compensation, but also some limited due to de-tuning of the optical system !



Technical Workshop10

Gravity:

Displaced BodyDOF Factors feed plane Y-axis (*)DOF Factors feed plane X-axis(*)Physical Unit

M1 tilt rotx 1,720 roty 1,720 mdeg/mdeg

M1 trans uy -19,022 ux 19,022 mdeg/mm

M2 tilt rotx -0,089 roty -0,089 mdeg/mdeg

M2 trans uy 8,995 ux -8,995 mdeg/mm

Feed uy 10,199 ux -10,199 mdeg/mm

(*) : Z-axis colinear to the pointing direction of the telescope

Pointing Sensitivity




Gravity:

Easy to analyse and predict by use of FEM models.

Repeatable as a function of elevation

Compensation requires active surface large number ofactuators (or on M2 !) + no closed loop compensation




Wind:

2 Scenarios:

b.) Wind gustStructural deformation

+ servo error

Can be large depending on closed loopperformance ( high bandwith high eigenfrequency of the structure

a.) Static Wind:

Structural deformationServo Error small




Thermal:

2 Scenarios:

a.) constant ΔT:

b.) Temperature Gradients:

Analytically easily to resolve (linear theory)

Practical difficulties: How to represent typical (actual) scenarios ???




Example: Axis Alignment procedure

-60,0

-40,0

-20,0

0,0

20,0

40,0

60,0

0 30 60 90 120 150 180 210 240 270 300 330 360

angl

e [

md

eg]

Azimuth angle [deg]

Example Measurement after EL/XEL correction

Inclinometer

Fitting

Offset

Amplitude

Phase

-80

-60

-40

-20

0

20

40

60

180 165 150 135 120 105 90 75 60 45 30 15 0 -15 -30 -45 -60 -75 -90 -105 -120 -135 -150 -165 -180

Messung 1

Messung 2

Messung 3

Messung 4

Test

Messung 3 + test

Messung 5




Example: SRT Mirror Alignment

Laser Tracker

M4

M5

M3





64m diam. ca. 33 μm accuracy

1 : 2.000.000 !!!





0

100

200

300

400

500

600

Sep11

Sep11

Sep11

May12

May12

May12

June12

June12

June12

final

Surf

ace

Acc

ura

cy [

µm

RM

S]

global RMS developement 2012

Global RMS

Spec.

Goal

Nr. 7 8 9 10 11 12 13 14

Messung

Messung04,

12.05.2010

Messung01,

13.05.2010

Messung02,

13.05.2010

Messung03,

13.05.2010

Messung03,

13.05.2010,

gefittet

Messung01,

15.05.2010,

gefittet

Messung02,

15.05.2010,

gefittet

Messung02,

16.05.2010,

gefittet

Mittelwert [mm] 0,010 0,004 -0,006 -0,018 -0,023 -0,02 -0,032 -0,024

RMS [mm] 0,259 0,312 0,138 0,187 0,111 0,066 0,080 0,057

0,000

0,050

0,100

0,150

0,200

0,250

0,300

0,350

7 8 9 10 11 12 13 14

RMS [mm]

RMS [mm]




(RA, Dec, TLE, …)Pointing Strategy:

(for closed loop corrections)

Source

Site(lat., long., time,

variations (abberation, precession, nutation, etc.

Telescope Coordinates(Az, El, Xel)

Refractioncorrection f(T,P,El)

Modified TelescopeCoordinates

Specific models torepresent thetelescope (incl. optics, BWG, etc.)

Pointing Model

Metrology Telescope Set-PointsServo systemcommands, encoderfeedback loops

Constant offsets, alignemnt corrections, etc

Actual, sensor based measurements

(Concept similar to „ The ALMA Pointing System“ by Jeff Mangum)

off

on




Practical Results

(observed under transient conditions, approx. 2h during sunset,200 stars, Metrology on / off every 10 stars, 5 sec integration time)

Metrology off: 2.79“ rms Metrology on: 1.01“ rms




Generally, there are two possible approaches:

Measuring the causes: (wind, gravity, temperatures)

or

Measuring the consequences (tilts, offsets, pointing errors)

Sometimes it is not distinguished between the two. Ifcompensated by an active, closed loop metrology systems, thiscan cause „overcompensation“, meaning that you compensatetwice for an error contribution that only occurs once!

This seems trivial, but to avoid, deep insight into all systems isrequired!

Conclusions:




Telescope performance can be improved by metrology systems undernon-ideal conditions

Best observations achieved are during perfect conditions withoutmetrology

Metrology systems are introducing addituional levels of complexity andsources of errors (calibration, noise in feedback loops)

Metrology is effort in terms of:

infrastructure (sensors, actuators, wiring, control)

Test and Comissioning

Calibration time (lost for observation)

EXPERIENCE!

Conclusions:

Therfore: First design a proper telescope, then add metrology systemsAND a button to switch them off in good nights !!!




Final Conclusion:

+

Metrology is not magic, for 2 reasons:

1.) It can be done and works!

2.) does not enchant a poor design into a perfect beauty!

Background, theory and practical limitations of metrology ......Metrologies at Radio Astronomy...

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