17-03-2001 The barrel reference system 1

THE BARREL REFERENCE SYSTEM

C.Guyot (Saclay)

Goal: Provide a geometrical survey of the whole barrel muon spectrometer giving the absolute chamber positions with a precision < 0.5 mm on translation and < 0.5 mrad on rotations

Why: • Correction of the projective system sagitta prediction• Pattern recognition requirements (relative chamber positions are needed with a precision < 1mm )• Magnetic field reconstruction (aims at a reconstruction of the toroid coil positions w.r.t. chambers with a precision of about 1mm)

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d

Rotation axis

Chamber rotated by an angle

Distance d between the plane of the projective sensors plane and the wire plane

Projective sensors

Wire plane

Correction to the projective system sagitta prediction

The projective system predicts zero sagitta correction for this muon whereas a false sagitta of the of order d is measured with the wire plane. With d = 50mm, the angle has to be known with a precision < 0.5 mrad to keep this error on the sagitta below 25 m.

BIL BML BOL

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Principle of the reference system (only one sector shown)

(camera - LED sources)

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Reference system implementation: plate-chamber connection

Closed wheel of plate-plate connections

Foot plate

BIL BIR

BML

Standard sector plate

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Plates on the toroid rib

Supported via a 3 balls system

Calibration:• Ball triplets location on the plate measured with a 3D CMM (+- 10 m)• Camera (Cf BCAMs calibration procedure in the end cap system)• LED holes location w.r.t. balls known to < +- 100 m by construction

CMOS camera

Platform on MDT tubes

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Reference system implementation: BL-BS chambers connection

BIL-BIS connection

BML-BMS connection

BOL-BOS connection

Projective system

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Implementation on MDT chambers

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SIMULATION/RECONSTRUCTION (1):

Principle of the simulation/reconstruction program:

Mechanical elements (MDT chambers, sensor platform, reference plates) are simulated by a 3D nodes in space, connected by either rigid mechanical bars (fixed distance), by proximity sensors (distance measurement), by RASNIKs (sagitta measurement over 3 nodes) or camera/LED system (angular and distance measurements):

A MDT chamber is described by 4 nodes at the corners + one central node, connected by rigid bars and RASNIKs (inplane). The 4 corner nodes also carry the praxial sensors, connected to other praxial nodes via axial RASNIKs and proximity measurements. The projective nodes and the reference system nodes are rigidly connected to the 4 MDT corner nodes.

The reference frame is defined by 3 reference plate nodes (one is fixed, the second has 2 dof, the third only one dof). In the simulation, all nodes are moved in space with ranges [+- 5mm] in all 3 directions. The reconstruction algorithm is based on a global fit which accounts for all the nodes connections.

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Nodes layout in the simulation/reconstruction program (one sector)

RASNIK

Camera(BCAM)

Plates on toroid ribs

BIL

BML

BOL

In-plane RASNIKs

BOL

BML

BIL

z

y

x

y

z

x

cm

cm

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SIMULATION/RECONSTRUCTION (2):

AXIAL RASNIK resolutions: 5m in x/y, 1.10-4 in magnification 100 rad on mask/pixel lines rotation

CAMERA resolutions: 100 rad on absolute pointing direction 20 rad on relative spot angles (=> 5.10-4 on distance measurement)

PROXIMITY sensor resolutions: 20 m in x/y/z

Precision on chamber NODE positioning:Praxial and projective nodes: 20 m in z/y , 150 m in x (tube direction)Reference system nodes : 200 m in x/y/z

Precision on distance measurement between 2 plates on the same toroid coil: 100 m

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Reconstruction accuracy of the nodes corresponding to the toroid rib plates

x= 306 m

y= 216 m

z= 393 m

x= 117 rad

y= 99 rad

z= 76 rad

cm

cm

cm

rad

rad

rad

Translations Rotations

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Reconstruction accuracy of the nodes corresponding to the MDT corners

x= 321 m

y= 252 m

z= 482 m

cm

cm

cm

Translations

Projective track sagitta reconstruction accuracy

Generated

z= 4.5 mm

z = 20 m

Reconstructed

cm

cm

The measured position of a straight track hit in a chamber is calculated by interpolation w.r.t. the assumed positions of the 4 corner nodes. The generated sagitta results from the initial random nodes movements (supposed to be unknown).

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Conclusion:

The proposed system provides the required accuracy on the absolute chamber positioning within a sector. The simulation/reconstruction study of the whole barrel system is yet to be performed.

Status and planning:

• 05/2001: Final drawings of the MDT platforms (similar to the platform for the magnetic field sensors) and of the positioning tools (if required)• 10/2001: Delivery to the MDT construction sites of the first platforms and of their positioning tools.• 12/2001: first barrel MDT chambers build with reference platforms• 2003: Assembly and calibration of the cameras • 2003: Assembly and measurement of the reference plates.• 2003: Construction of the LED supports• 2003/2004: Assembly and calibration of the cameras for the BL-BS connections • 2004: Installation on the toroid ribs before the chamber installation.• 2004-2005: Installation in the ATLAS cavern of the BL-BS extension plates with their cameras