Alignment Bars

RequirementsStrategyLayoutLength PredictionShape Prediction

Readout Scheme

Model to predict bar shape in the experiment

CalibrationVibrationsBar MountSummary

C.Amelung, W.Andreazza, J.Bensinger, F.Cerutti, C.Fabjan, K.Handrich,K.Hashemi, S.Palestini, J.Rothberg, I.Trigger, A.Schricker

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Requirements

A. Schricker

•Have to provide a bar that carries sensor components.

•Provide an absolute length scale over the full length of the bar (~9.6m).

•Know each platform position on the bar within 30µm RMS.

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Strategy

A. Schricker

Strategy to meet the specifications:

•Measure the position of the sensor platforms and calibrate the Rasniks with the CMM in Freiburg.

•Use temperature sensors to follow the thermal expansion of the bar.

•Use optical in-bar instrumentation (3 Rasniks) to measure three points on the bar (middle and the two ends) and to follow the bar deformation.

•Use Rasnik data and analytical model to predict the bar shape in the experiment.

CMM: coordinate measuring machine

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The bar consists of an aluminum tube:(l=9600mm, D=80, d=72, intrinsic straightness: 1 mm/m)

A skeleton inside carries thein-bar instrumentation:3 overlapping RASNIK 9x3 T-sensors

Skeleton

Temperature sensor

Optical path

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A. Schricker

Layout

Skeleton

PT100 sensor

Rasnik mask

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Station 9

CCD Mask

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Length Prediction

A. Schricker

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•Measure thermal expansion coefficient of each bar.•Using PT100 sensors to predict the thermal expansion of the alignment bar.

•For ambient temperature the error in prediciton is 4 µm RMS.

•Length predictable with 10µm RMS if T-gradient is 0.5 K/m.

Shape Prediction

A. Schricker

From the 3 overlapping Rasniks:Position of 3 points on the bar (a,b,c) relative to 2 points on the bar (d,e)

Readout Scheme

Model to predict bar shape in the experiment

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Shape Prediction

A. Schricker

The actual bar shape is approximated with a model using the in-bar Rasnik data and three additional virtual forces.

Disentangle shape and sag with CMM measurement.

Refine model with the calculated load distribution for inclined bar positions.

Readout Scheme

Model to predict bar shape in the experiment

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Calibration

A. Schricker

•Transformation of the 3 independent Rasnik coordinate systems into the bar coordinate system.

•Measure positions of sensor – platforms relative to BCS with the CMM.

•Calibrated short Rasnik systems to 11µm accuracy.•Found a method to calibrate 9.6m long Rasnik system (CMM range 6.5m).

CMM range

x

y

z

BCS: bar coordinate system

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CMM in Freiburg

Coordinate measuring machine in Freiburg.

Precision:In one axis 5µm absolute, 2µm/m relativeIn space 10µm absolute, 5µm/m relative

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Proposed Modification

A. Schricker

Present in-bar instrumentation

•Measurement of each Rasnik system without moving the bar.•Larger dynamic range of the in-bar Rasnik.•Maintenance of components inside the bar.

Proposed in-bar instrumentation

Dynamic range

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Vibrations

A. Schricker

Hit bar at one end and measured amplitudes.

(1kHz sampling rate, 8192 samples)

Spectrum of uninstrumented bar

(supported at its Bessel points)

Vibration studiesplanned for H8 !

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Bar Mount

A. Schricker

Gimbal mount:2 degrees of freedom permittedRotation around two intersectingaxes normal to the bar axis.

6 degrees of freedom of the bar have to be constrained without indroducing stress.

Lower mount:4 degrees of freedom permitted.Rotation around all three axes andlinear motion along the bar axis.

Kinematic Mount:

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Shape distortion due to a 2 mm shift of the support point.

Consequences of Non Perfect Bar Mounts

A. Schricker

Shape distortion due to a 0.16 Nm torque. (Corresponds to a 20 mrad twist of the lower mount.)

Error in prediction

Error in prediction

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Summary•We understand the behavior of the alignment bar.

•We have a model to predict the bar shape.

•We meet actual requirements for length prediction.

•We have demonstrated the calibration of the in-bar instrumentation for the short Rasnik systems.

•Demonstrate the calibration of the long Rasnik system.

•Implement online quality control for the CMM measurements in Freiburg.

•Test our ideas about damping vibrations (H8).

•Demonstrate model of describing the bar shape for inclined bars (H8).

•Investigate maintenance of the bar instrumentation in the experiment.

A. Schricker

To Do

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