POSITION MONITORING SYSTEM for the Endcap DetectorS

The Endcap systems (for track matching and momentum measurement ) define their positions relative to the Central Tracker and Barrel muon systems by connecting to the Link-Barrel system. In order to trigger on and define the correct position of passing particles (after software alignment correction), mechanical chamber positions and orientations need to be known with a reasonable tolerance, especially the R coordinates and f angle rotations. The typical total error budget on these measurements is around 150 mm (smaller than the baseline 200 mm chamber resolution) but smaller at the ME1 muon momentum defining ring (75 mm).

In the Endcap Muon Position Monitoring System (EMPMS) critical tracker F plane references will be transferred to each of the Endcap system detector layers at the CMS outer radial boundaries. Twelve interleaved LINK Straight Line Monitors (SLMs) through the Barrel muon system are defined by the two outer end MABs. These MABs are connected by the Link alignment system to the Central Tracker coordinate system. See Fig.1 and reference [1]. The Endcap Link lines transfer six (RF, R) references to each detector CSC station. There is also an Endcap Z coordinate transfer system to all chamber layers. It consists of a concatenation of laser-detector triangulation distance measurements between the Barrel end MABs and carbon fiber tubes/reference surfaces on the outer boundary of the YEn iron between all the layers of chambers. See Fig. 2. Thus diametrically opposing (RF,R,Z) link points around the endcap iron are defined. Two possible optical beam position sensors are under development/test. ALMY [2,3 ,4] and our DCOPS [5,6,7].

 

Fig. 1: Tracker F plane transfer to the Endcap Muon System by the Link system

 

Fig. 2: Z coordinate transfer to (MEn/m) layers from references on the MABs.

The Link line SLMs consist of laser+optics sources at each end of the CMS Endcaps (ME4) projecting across CMS through a string of eight (ten if ME4) transparent optical position sensors (ALMY or DCOPS). The two optical sensors on the end MABs of the Barrel system define the Link line in the tracker coordinate system. For ALMY, the bi-directional sensors respond to both Z+ and Z- source. A bi- directional version of DCOPS with an optical wedge has been succesfully tested. Also 20m laser sources have been prototyped. Estimated error on Link transfer at Endcap Link Sensors range 71- 108 m m.

Z transfer exists along all six Link SLM lines on each Endcap. The Z tubes are rigidly connected (referenced) to the link transfer fixtures and float (slide) in spherical bearing supports on the YEn iron. We have tested a commercial distance sensor (OMRON Z4MW40) that works on a 40 ± 10 mm stand-off with a long term resolution (3 week measurement periods) of 1-2 mm with a slow drift of less than 0.1 mm/day. This sensor has observed diurnal motions of 1 mm. It is insensitive to light backgrounds, fringe magnetic fields, and is thermally compensated. It will be used to transfer a Z reference surface on the MABs without touching them.

Between the outer boundaries of the Endcap iron discs there is a much smaller differential motion, so we have tested another low cost commercial distance sensor, the OMRON Z4DA01. It has a linear range of 2.5 mm at a 6 mm stand-off with a sensitivity of 2 mm and long term stability (2 weeks) of 10 mm at a 20% duty cycle.

Detector resolution is insensitive to the Z coordinate and we need to track only the large (several mm) magnetic iron motions across the local layers. Estimated local Z errors (148 mm) are dominated by transfer block torsion errors due the endcap iron bending (129 mm).

Across each of the local planes (MEn/m) of the endcap cathode strip chambers, there will be three pairs (six) laser diode beamlines between and defined by the radially opposite link points. These are the chamber layer RF plane and Z reference straight line monitors (SLMs). These lines are defined by a laser source and a two dimensional optical beam position detector at each link point (ALMY or DCOPS). These are referenced to the Rasnik detectors by linkplate construction, CCM measurement of the linkplate assemblies, and Q , F precision tiltmeters.Two transparent optical detectors (ALMY or DCOPS) mounted on each cathode strip chamber in each line will define these chamber positions (RF, Z) relative to the laser beams. The four intermediate cathode strip chambers within each ring will be monitored by local charged tracks in the overlapping cathode strips. In the ME1/3 ring where there is no overlap, the intermediate chambers in the ring will be measured in RF, R and Z by low cost but high resolution optical distance sensors (OMRON Z4DA01) between chambers (at each radial end) and at F plane boundaries. As costs allow, we will also apply these optical distance sensors to the ME1/2 and other overlapped chamber layers. To decouple R and F, there is a radial position measurement between the outer link references and the outer ring of cathode strip chambers, and between the outer and inner ring of cathode strip chambers. A schematic of the cathode strip chambers (inner and outer rings) with Z, boundaries straight line monitors, and R measurements is shown in Fig.3.

The position and coplanarity of the ME1/2 chamber ring will be monitored at six F positions using sattelite radial Link laser beams (offset 50mm from the primary ) in order to have high precision RF reference lines. The position and orientation of 6 (out of the 36 chambers in each station) chambers will be monitored using three out-of-line measurements per chamber, as follows:

Internally within the ME1 station, the relative RF position of the chambers will be monitored with proximity measurements at the two edges of the chamber. Furthermore, ME1/3 layer RF, Z straight line monitors ending in reference sensors on the ME1/2 provide a measurement of their positions.

The outer radius Link transfer fixture (on outer ring CSCs) is a RF, R reference. The system will measure the radial position of the outer ring chambers near the SLM lines using simple cable extension linear potentiometers between the link fixture and the precision end frame of the cathode strip chamber. These potentiometers have been tested to have a 3 mm sensitivity and to yield a long term resolution of 25 mm. They have a very small temperature dependence dx/dT = 0.57 mm/degC. Near the same SLM lines, the inner ring cathode strip chamber MEn/1 (at its outer boundary) will be linked to the outer ring chamber by another linear potentiometer between the precision end frames. The potentiometers span the physical support gaps for the cathode strip chambers.

 

 

Fig. 3: Linking of the (MEn,m) layer Straight Line Monitors.

The relevant temperature distribution of the cathode strip chambers, electronics, hardware, fixtures, etc. will be measured by simple current source transducers AD592. We have been able to calibrate a large sample of these devices on a common heat sink to better than 0.2 degC. Even if we define a 0.5 degC temperature uncertainty of the outer cathode strip chamber, its use as an R connection only represents a 37 mm error. The accumulated local R measurement error is about 80 mm.

We have experimentally studied our baseline transparent amorphous silicon detector (ALMY) with crossed X, Y strip readout and demonstrated adequate resolution in a FNAL layer SLM testbench [8,9,10]. We have also studied a Link line testbench SLM with ALMY at FNAL and again demonstrated adequate resolution [11].

These are under continued development at Max Planck Institute for ATLAS [4]. The MPA (Multi-Point Alignment) sensors consist of a 64x64 grid of 300 mm wide (312 mm pitch) readout strip channels over a 20x20 mm2 active area of 1 mm thick amorphous Si photodiode detector sensing a laser optical beam.

We are also developing and testing an alternative digital CCD detector DCOPS for the SLMs which avoids refraction, reflection, and beam absorption/scattering effects [5,6,7]. There are a number of tests that need to be completed before a baseline change can be made. Preliminary tests and layout configurations indicate that DCOPS can work in both the Link and CSC layer SLMs. However it would require special radiation hard photodiode array detectors for use in ME1. At present this is unclear, so ALMY will remain as baseline here.

The accumulation of all estimated local errors in the local SLMs to the RF, R measurements is about 80 mm. Testbench sensor resolutionsare consistent withn this.

The process for precise sensor positioning on the cathode strip chambers is as follows: the chambers are mechanically supported by precision extrusion frames. The radial end frames are located on the precision alignment pins (each end of the centreline) used for the chamber assembly. These frames have CNC drilled pin locations for all the alignment sensors (SLM, radial, proximity). See Fig. 4. These pin/sensor locations are measured in the strip assembly survey. Photogrammetry targets will be located in premeasured CNC holes/pins in the frames. Survey measurements on cathode strip layer reference positions will be done at CSC assembly. Panel accuracy is establised from CNC machining.

 

Fig. 4 Cathode strip alignment references on precision end support frames

Table.1

Components required for the Endcap position monitoring system (ME4 descope).

Endcap Alignment component

Number

CSC layer SLMs

  

ALMY / DCOPS sensor

180

Optical distance sensor (RF proximity measurement) ME1

  

Laser diode module

36

Temperature sensor AD592

480

Linear Potentiometer (Radial position monitor)

72

Optical distance sensor (RF proximity measurement) ME23

72
   

F and Z Linking

  

Link SLM (ALMY or DCOPS) sensors

48

Optical distance sensor (long range Z measurement) Z4M

12

Optical distance sensor (short range Z measurement) Z4D

48

Linkplates

36

Dual angle Precision Inclinometers

36

Link Laser-SMF-COLL-Optics

12

ME1/2 chambers

 

CCD camera view

12

LED sourceplate

12

ALMY ME1

24
   
   

Our prototype ALMY sensors are digital readout through a custom VME interface

which controls a string of sixteen detectors. The presaent prototypes require a six conductor power service ribbon cable : AGND, DGND,-12v, +12v, +3v rev bias, +5v

and a 50 pin flat control/databus cable chained through a string. ALMY II is similar.

Our prototype DCOPS sensors are serviced by a phone cable connection consisting of a bi-dectional differential data line (RS485), +12v, Ground. Local sensor board DC-DC conversion produces +9v CCD, +5v Analog, and +3.3.v Digital supplies.

Sample Linkplate layout design. Similar layouts have been done for layer SLMs.

We are in the process of optimizing geometrical parameters to satisfy CMS Integration requirements and to minimize the number of different linkplate and CSC

sensor mountings. Layout work is being done consistent with both ALMY II and DCOPSII

(separate sensor and electronics boards) so that our solutions are consistent with the use of

either optical beam position detector.

 

 

 

 

 

 

 

References:

 

 

[1] CMS Muon System Technical Design Report, Chapter 7; Muon Alignment Group,

CERN/LHCC 97-32.

[2] Transparent Silicon Strip Sensors for the Optical Alignment of Particle Detector Systems, W. Blum, H. Kroha, P. Widmann, Max Planck Institute for Physics/MPI-PhE/95-13

[3] A Novel Laser Alignment System for Tracking Detectors using Transparent Silicon

Strip Sensors, W. Blum, H. Kroha, P. Widmann, Max Planck Institute for Physics/MPI-PhE/95-05

[4] International Alignment Workshop at MPI, Munich, April 98; see

http://pcatlas4.mppmu.mpg.de/mdt/library/alignment_workshop.htm

[5] COPS Position Monitoring System for the CMS Endcap Muon System; D. P. Eartly,

T. F. Droege, E. Hahn, R. H. Lee, D. O. Prokofiev, J. Moromisato, E. von Goeler,

CMS Note 1996/021.

[6] A Two Dimensional Digital CCD Optical Position Sensor for Relative Alignment

Monitoring; D. P. Eartly, S. Hansen, T. F. Droege, D. O. Prokofiev, J. Moromisato,

E. von Goeler, Fifth Intl Workshop on Accelerator Alignment, Oct 97, see

http://eartly.fnal.gov/empms/ or

http://www.aps.anl.gov/conferences/iwaa97/iwaa97.html

[7] Study of Position resolution with COPS; J. Moromisato, E. von Goler, Sreucroft,

D. Eartly, T. Droege, D. Prokofiev (to be published)

[8] CMS Endcap Muon System Evaluation of MPI Transparent Amorphous Silicon - X, Y

Strip Readout Optical Beam Position Sensors; D.P. Eartly, R. H. Lee, A. Bujak, D. O.

Prokofiev, CMS IN 1997/005.

[9] CMS Endcap Muon System Tests of MPI Transparent Amorphous Silicon - X, Y

Strip Readout Optical Beam Position Sensors in two way split Laser Diode Module

Beams; D.P. Eartly, R. H. Lee, CMS IN 1997/020.

[10] CMS Endcap Muon System Long Term Resolution Tests of MPI Transparent

Amorphous Silicon Optical Beam Position Sensors in a Multi Sensor Straight Line

Monitor; D. P. Eartly, R. H. Lee, CMS Note 1998/004.

[11] CMS Endcap Muon System Tests of a Link Straight Line Monitor R, Rphi, Z using

MPI Transparent Amorphous Silicon Optical Beam Position Sensors, Laser/LED Z

distance sensors, and R wire displacement potentiometers; D. P. Eartly, R. H. Lee,

(CMS IN 1998/076 Oct 98); also see http://tycho.physics.purdue.edu/~leer