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Alignment/Position

It is planned to align all wafers with respect to each other and with respect to the SVT support structure before the placement in STAR. Details of this plan were presented in the conceptual design report. The relevant part of the CDR is attached to this plan.

The present provisions in the mechanical design of the SVT are described in the following:

There are in total 36 carrier assemblies, which are designed to support detectors and FEE's. These beryllium carriers are tiled to make three concentric super layers around the beam pipe. Two pairs of precision end rings (outer and inner end rings) and standoffs assemblies are provided on both ends of the carrier assemblies to support these carrier assemblies. In-between the end rings, there are four .02 inch thick beryllium sheets (which are quadrants of a 1 inch diameter beryllium pipe) to maintain the parallelism of these two pairs of end rings. The whole assembly then will be assembled to and supported by two water manifolds through connecting elements, such as brackets, rods, and screws. The whole assembly (up to the manifolds) is a clamp shell design, which is held together by a lock mechanism. The water manifold is then mounted onto a cone support structure (made of composite material) with four sets of screws on each side. The SVT assembly will be secured onto the TPC support wheel, with one end fixed to the TPC support wheel and the other end simply supported (to eliminate the stress due to thermal expansions). The cone support structure is not separable and should be installed before the installation of beam pipe. The ends of the cone will be covered by light barriers and air duct support structures.

Requirement:

All requirements can be described by five parameters: displacements in x and z and the rotational displacements around axis perpendicular to the wafer.

Justification:

It is assumed that there are no contributions by `bows' and `twists' due to the stiffness of the carrier. The wafer thickness was measured to be within 5% of the avarage value (305 m). Therefore no displacement in y has to be considered.

Status:

A coordinate system based on the BABAR coordinate system was adapted.

Requirement:

The relative uncertainty between wafers should be less than 20 m in x and z and less than 1 mrad in all three rotational coordinates. The uncertainty is defined as the offset of all wafers with respect to the origin (middle) of the detector carrier.

Justification:

An uncertainty comparable to the intrinsic resolution of the detector leads to sufficiently small degradation in the impact parameter resolution. The assembly accuracy adds in quadrature to the position resolution.

Status:

Fiducial marks at each corner of each wafer and each end of each carrier are contained in the present design. Subsequent use of existing coordinate measuring machines (CMM) during the assembly meet the requirements. Details can be found in the mechanical design documents.

Requirement:

The gravitational sag (`bow') of each detector carrier is estimated to be less than 5 m.

Justification:

This sag is in accordance with the maximum degradation factor of the impact parameter resolution.

Status:

Finite element analysis shows that the present setup meets the requirement. Test have to be conducted on prototypes.

Requirement:

fiducial marks have to be located at the following locations for local alignment:

Justification:

These fiducial marks are needed to meet the 20m accuracy requirement

Status:

Fiducial marks are being provided in the wafer design and the mechanical design

Requirement:

The SVT has to be fully assembled (detector plus cone support) outside of the main detector. The accuracies specified above have to be met after the cone supports are installed.

Justification:

A unistrut is used during the assembly to lower the cost

Status:

The design and an assembly plan were drafted accordingly

Requirement:

The position of a wafer with respect to its carrier has to meet the same requirements than the stability of wafer positions with respect to each other.

Justification:

The stability of absolute wafer position enters into the position resolution in the same way than the relative wafer position.

Status:

The assembly plan based on the accuracy of the coordinate measuring machine will meet all requirements.

Requirement:

The internal survey of the assembled part has to meet the same requirements than the wafer positioning accuracy.

Justification:

The survey accuracy enters into the position resolution in the same way than the relative wafer position.

Status:

The accuracy of the coordinate measuring machine will meet the survey requirements.





next up previous
Next: Global Alignment Hardware Up: Mechanical Previous: Installation



Claude Andre Pruneau
Thu Oct 12 17:29:54 EDT 1995