ARTICLE IN PRESS

Nuclear Instruments and Methods in Physics Research A 568 (2006) 686–691
www.elsevier.com/locate/nima

Robotic mounting of ATLAS barrel SCT modules
R.B. Nickersona, G. Viehhausera,, R. Wastiea, S. Teradab, Y. Unnob, T. Kohrikib,
Y. Ikegamib, K. Harac, H. Kobayashic, G. Barbierd, A.G. Clarkd, E. Perrind, A.A. Cartere,
J. Mistrye, J. MorriseaParticle and Nuclear Physics, Oxford University, Oxford OX1 3RH, UK
High Energy Accelerator Research Organisation (KEK), Oho 1-1, Tsukuba, Ibaraki 305, Japanc
Institute of Physics, University of Tsukuba, Tsukuba 305-8571, Japand
Nuclear Physics Department, University of Geneva, 24 Rue Ernest Ansermet, CH-1211 Genève 4, SwitzerlandeDepartment of Physics, Queen Mary University of London, Mile End Road, London E1 4NS, UKb

Received 7 August 2006; received in revised form 8 August 2006; accepted 1 September 2006
Available online 27 September 2006

Abstract
The 2112 silicon detector modules of the barrel part of the ATLAS SemiConductor Tracker (SCT) have been mounted on their carbon
ﬁbre support structure. Module insertion, placement and ﬁxing were performed by robotic assembly tooling. We report on our
experience with this assembly method. Part of the mounting sequence involves a partial survey of elements of the support structure which
is needed to align the modules properly during insertion. An analysis of these data is used to estimate the positional accuracy of therobots.
r 2006 Elsevier B.V. All rights reserved.
PACS: 29.40
Keywords: ATLAS; SCT; Barrel; Mounting; Assembly; Robotic; LHC

1. Introduction
The barrel section of the ATLAS SemiConductor
Tracker (SCT) consists of four concentric carbon ﬁbre
cylinders (labelled B3 to B6 from the innermost to theoutermost) with outer diameters of 568, 710, 854 and996 mm [1]. The outer surface of each of the cylinders ishermetically tiled with silicon detector modules [2,3]. The
tiling was designed so that modules have a small overlap
with all adjacent modules (Fig. 1). The gaps between
modules in the overlap regions are very small (approximately 1.6 mm), making insertion of individual modules
difﬁcult.

Corresponding author. Physics Department, Oxford University, Denys
Wilkinson Building, Keble Road, Oxford OX1 3RH, UK.
Tel.: +44 1865 273410.
E-mail address: g.viehhauser1@physics.ox.ac.uk (G. Viehhauser).

0168-9002/$ - see front matter r 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.nima.2006.09.006

Modules are laid out in rows of 12. The number of rows
for B3, B4, B5 and B6 are 32, 40, 48 and 56, respectively. In
total, 2112 modules were mounted to complete the barrel
SCT. Modules are mounted almost parallel to the local
surface of the support cylinders, at a small angle and withevery second module at a slightly larger radius toaccommodate the overlaps. The tilt angles accommodate
overlaps between adjacent rows and are 111 for the two
inner cylinders and 11.251 for the two outer cylinders.
Modules at smaller radius are referred to as lower modules,
and those at larger radius are called upper modules.
The precision mechanical connection between the support barrels and the silicon detector modules is achieved bycarbon ﬁbre brackets (Fig. 2). Each of the modules is
supported on two points located on one bracket and a third
point on the bracket in the next row. At the two ﬁrst points
there are precision bushes (inner diameter speciﬁed as
1.8 mm H7, i.e. 1.800–1.809 mm) where the module ispositioned by a precision washer (outer diameter 1.8 mm

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Fig. 1. Modules mounted on a barrel. The modules shown in the centre
are upper modules, and the modules to the left and right are lower
modules. Rows are horizontal across the picture. Note the overlaps at theends of the modules. Unprotected wirebonds are located along the hybridboards on both sides of the module

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facing overlaps with the cooling block. The 100 mm-thickgap between the two is ﬁlled with thermally conductive
grease (Dow Corning 340). While the gap thickness isdeﬁned by the relative position of module and cooling loop
their contact is secured by two clips per module.
Before mounting the modules, all brackets and services,
LMTs, ﬁbres [5] and cooling pipes had been installed.1 As
the cooling pipes are suspended from the modules, dummy
modules are in place at this stage of the assembly.
All the services leave very small clearances (around
1 mm) for the insertion of modules (Fig. 2). In addition theinsertion path required to avoid clashes was not simple.
These were the main reasons for the choice of robotic
assembly. For the size and schedule of the barrel SCT
project distributed assembly on different sites was notnecessary and it was sufﬁcient to build and operate two
assembly stations with one robot each at Oxford University. Each robot was used to assemble two cylinders.
The ﬁrst cylinder to be assembled was B3, followed by B6,B5 and B4, where during assembly of the latter three both
assembly stations were used in parallel. B3 and B5 wereassembled using one robot, and the B4 and B6 with theother. The robots were used to assist in mounting as well as
in removal, in cases where there was a problem with thedetector module.
2. Robots

Fig. 2. Cross-section of insertion of module (dark) into barrel. Existingparts of the barrel (brackets, services and mounted modules) are in alighter grey. The module is shown after insertion into the spring clamp at
the left edge and before contacting the cooling block.

f6, i.e. 1.794–1.788 mm) and secured by an M1 screw withan M1.6 screw head, constraining three degrees of freedom
on one and two degrees of freedom on the other point. The
third mounting point is a simple sprung clamp constraining
out-of-plane movement.
The part of the module which makes mechanical contact
is the encapsulated thermal pyrolitic graphite (TPG)
baseboard [4] with beryllium oxide (BeO) facings, with
precision glued aluminium washers at the two screw
locations. The holes in the washers provide accurate
module location and the BeO facing is also used as themechanical handling point during the assembly. The
carbon ﬁbre cylinder also supports the services: electrical
power cables (referred to as low mass tapes, LMTs), opticalcontrol and data ﬁbres with the electro-optical converters,
and cooling pipes. All the services are segmented in rows or
multiples of rows on the barrel.
The electrical connections are made through one Samtec
SFMC-120-L3-S-D connector (2  18 pins, 1.27 mm pitch).
Thermal contact between the modules and the coolingpipes is made through cooling blocks soldered around thecupro-nickel (CuNi) pipe. A 60  8 mm2 section of the BeO

The robots used in the assembly were originally designed
and fabricated at KEK by the groups of KEK and
University of Tsukuba [6] and extensively adapted andcommissioned at Oxford University, where the mountingof SCT modules onto the barrel cylinders took place.
During the assembly the functions of the robot were to: (1)pick up a module from a custom module box; (2) transport
the module to the location on the cylinder along a safe
path; (3) align the module to the ﬁxing holes of the bracket
on the cylinder; (4) position the module on the ﬁxing
points; and (5) tighten the screws with torque-limited
motorized screw drivers. Similar procedures, in reverse
order, were performed when removal of a module was
required.
Another sequence was a recovery procedure. In the casethat the module insertion had to be stopped at any
arbitrary point during the process, the module could be
returned to its box by exactly reversing its approach path.
The modules were placed in the custom-built module
boxes [2] at the module assembly sites. The boxes were thenused to protect the modules during transportation, storage
and pre-assembly tests, and also worked as an interface forthe robot during the assembly. For this, each box was
positioned at a pre-deﬁned position in the robot from
where the module was picked up by the robot so thatmanual handling of the modules was never required.
1
Services were attached to the cylinders at Rutherford Appleton
Laboratory, UK.

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Fig. 3. Module held by robot jaws, ready for insertion into last positionon B3.

Originally, alignment of the modules with the mounting
brackets was achieved visually. The operator viewed boththe module ﬁxing holes and the bracket holes with video
cameras in the robots and aligned them with manual
steering. This was demanding on the visual abilities andconcentration of the operator in order to maintain therequired precision reliably. Given the large number ofmodules in the SCT, an automated method was developed
to survey the mounting positions on the barrels and each
module as picked up by the robot. The mounting position
was surveyed by inserting a precision pin in the ﬁxing holes
in the bracket and by measuring the location of this pin
with two crossing lasers attached to the robot. The module
holes were surveyed after pick up with another laser beam,
which detected the passage of light through the mounting
hole as the module was moved across. This method ofalignment did not demand excessive operator concentration and was almost fail-safe.
The safe path was deﬁned for each barrel before theassembly based on construction drawings, relying on therequirement that all objects stay within their nominalenvelopes. The operator watched the insertion to ensurethat there was no unexpected interference.
The robot held the modules in a set of air-driven jaws,
which gripped the BeO facing of the cooling tab of themodule from two sides next to the mounting screw holes(Fig. 3). These jaws had plain steel surfaces. The pistonsprovided about 17 N per jaw. On one robot the jaws wereopened by the pistons and closed by a spring exerting aforce of 4.4 N, thus maintaining grip even in the case of lossof air pressure. On the other robot the jaws were closed by
the air pressure and held open by the spring, yielding a grip
force of 12.6 N. In this case the fail-safe feature wasprovided by a back-up nitrogen bottle. The closed gap
thickness of the jaws was set to 830 mm, with thebaseboards being 930770 mm (mechanical tolerance).
The symmetry of the barrels invites assembly in rows.
The robot therefore had one long (1.5 m) stage in parallelFig. 4. Side view of robot. White arrows indicate travel of selected motion
stages.

Fig. 5. Robot in assembly position in front of B3. Note the long parallel
stage at the bottom of the picture (covered by accordion-type protective
cover).

with the barrel, which allowed access to all 12 positions
along one row as well as the module pick-up position, andseveral smaller stages, which controlled the position andorientation of the robot jaws (Fig. 4). These latter stages
were used for the insertion of the module. All stages were
open-loop controlled and driven by electrical stepper
motors with 500 steps/revolution. The step size along thestages was 10mm. Access to the other rows was possible
through rotation of the barrel.
In addition to the jaws the robot was equipped with a setof two air-driven screwdrivers, which inserted the mounting screws together with the precision location washers
with a force of 4.9 N and tightened them to a torque of
4 Ncm. It also had ﬁxtures for the different alignment andsurvey laser sensors.
The cylinders were mounted on a spindle which was
supported at each end on a pedestal (Fig. 5). The cylinders
could be fully rotated around the axis using a hand crank.
An indexing ring on the barrel support spindle allowed theposition at each row to be mechanically locked. The

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precision of this angle was moderate (1 mm) as the exact
location in this direction was calibrated during theassembly of each row by the survey.
The cylinders were enclosed by an aluminium proﬁleframe, which protected the cylinder during the assemblyand supported assembly infrastructure. This frame wasalso used as the skeleton for the innermost protective box
during transport of the cylinders to and from Oxford.
3. Safety measures
The power and weight of the robot far exceeded thestrength of any component in the assembly (modules,
carbon ﬁbre barrels, cooling loops, etc.), so special
precautions were taken to avoid catastrophic failures.
Electrical back-up power was supplied by an uninterruptible power supply (UPS) and the compressed air
supply was guaranteed by a back-up nitrogen bottle.
When the robot jaws were in a position close to thebarrel, where a sideways movement of the jaws or themodule would create signiﬁcant damage operation of
the long axial stage was disabled by hardware.
The screwdrivers had a cut-out switch, which limited theforce with which the screws with their location washers
were inserted to 6.9 N.
A generous number of emergency stop switches were
distributed throughout the assembly area and on the robot.
4. Robot commissioning
Most of the commissioning was done on a prototype
resembling a 1/8 sector in azimuthal angle of B3 with fulllength in the axial direction. The sector had a reasonably
realistic set of services attached. The ﬁnal insertion
trajectory was tuned for each barrel using a small accurate
model section of each cylinder.
Once the hardware and software was in a stable state theﬁnal parameter tuning to the as-built geometry of aparticular barrel was done using the actual cylinders.
Before mounting modules the correct behaviour was
veriﬁed with dummy modules with realistic envelopes on
known critical positions, in particular the positions at theends of the barrel.
5. Robot alignment
The ﬁrst step in the set-up of a robot was the parallelalignment of the robot with the barrel and the setting of theazimuthal angle of the barrel. For this the robot support
table had adjustable mounts on both ends, the position ofwhich could be ﬁne-tuned with the help of set screws andindicators. The correct parallel alignment was veriﬁedusing mechanical dummy modules mounted on thebrackets. A reﬂective laser sensor head on the robot
scanned the module surface, measuring the distance fromthe sensor to the surface. The robot support position
was then adjusted until parallel alignment was achieved.

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The typical orientation difference between the robot andthe barrel coordinate systems was within 20 mm perpendicular to the barrel axis over the length of the barrel(1.6 m). The j angle of the barrel was adjusted to bewithin 0.051 with respect to the robot coordinate system.
6. Mounting procedures
The assembly of modules on the barrels was usually donein groups of six, either upper or lower modules of one row.
The ﬁrst step was a survey of the mounting points on thebarrel. For this precision pins were inserted in the fully
constraining mounting bushing at each module location.
The protruding ends of the precision pins were located witha crossed beam laser sensor and their position recorded in a
database. After this, the survey pins were removed and agrease layer was applied on each cooling block by theoperator. The modules were then attached to the surveyed
locations.
Modules were delivered and stored individually in
precision boxes. During assembly, the lid of the box andmodule holding clamps were removed. The box was moved
to the pick-up position, where the module was picked up
with the robot.
The exact location of the module after pick-up withrespect to the robot coordinate system was obtained from alaser beam, which scanned the position of the module
mounting holes.
Once the coordinates were known the insertion took
place and the mounting screws, together with thepositioning washers, were inserted in the mounting holes
and tightened by the robot. The robot retracted and theprocedure was repeated until the group of six modules was
mounted. To complete mounting of a group of modules theoperator manually attached the cooling loop clips andmated the module connector with the service connector on
the barrel.
Up to 24 modules per assembly station were mounted
this way in an 8-hour shift.
7. Dismounting modules
In a small number of cases (40 modules) it was necessary
to remove a module after initial functionality checks on thebarrel. After removal of cooling loop clips and de-matingof the electrical connector a survey of the module on thebarrel was performed, as during insertion except that the
precision pins were adapted to work with modules in place.
The module was gripped in the mounted position by the
robot jaws and the mounting screws removed. Because of
its strong adhesion it was necessary to break the grease
joint by inserting a small wedge by hand. After removal themodule holes were surveyed to ascertain the exact gripping
location, and the module was then returned into its box.
Before mounting a new module in this position theremaining grease was removed by hand. For the removal

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of lower modules it was necessary to remove ﬁrst the twoneighbouring upper modules.
8. Yield and observed problems
The robotic assembly of the ATLAS barrel SCT was
highly successful. Including replacements, over 2180
modules were mounted on the barrels. Forty modules
had to be removed and replaced for various reasons, mostof which were not associated with the assembly process.
Only four modules were damaged during assembly. Two
were operator errors; one was due to a design ﬂaw which
was subsequently corrected; and the baseboard of the
fourth was damaged when the grease joint was split during
removal made necessary by an unrelated cause.
The most prominent problem during the assembly waswith the bush tolerances. These tolerances are tight toensure accurate location of modules, but on 33 modules therobot screwdrivers cut out due to torque limits and thescrews had to be tightened by hand. Examination of
the washers and bushes involved showed that the overly
tight ﬁt was caused by burrs on the bushings.
At an advanced point in the assembly one stage in one
robot started to seize causing a loss of steps in its motor.
This was traced to improper selection of materials in thesliding joint. The problem, which was the only one
associated with the robots during assembly, was rectiﬁed
with lubricant to avoid schedule slippage.
The small clearances made the setting of the parametersdeﬁning the insertion path critical and on a few occasions
these had to be adapted slightly after problems during theinsertion. Mostly these involved the jaws touching coolingpipes, which were not well constrained in space, and where
the jaws failed to clear the cooling block due to insufﬁcient
clearance programmed for the approach. These re-evaluations of the parameters were done with dummy modules.
9. Repeatability and absolute precision
The accuracy required on the module insertion path toguarantee clearance was about 100 mm. This was also theprecision required for alignment of module and bracket
holes, where the clearances were smaller but a chamfer on
the washer provided guidance. These were not absolute
precisions, but the relative accuracy with which theposition of the brackets and the modules were measured
and the module was moved along its insertion path. This
precision only had to be maintained during the time
between the survey and the insertion, which typically wasless than an hour.
During commissioning the positioning repeatability of
the robot was measured to be better than 10 mm byrepeatedly driving the robot from its home position to thesame programmed location.
The positioning accuracy was also estimated using thesurvey data recorded during the assembly, although it wasnecessary to account for large systematic shifts between

measurements separated by long time intervals (and insome circumstances even between measurements taken
closer in time). Such shifts may make upper and lower
modules look displaced with respect to each other in thedata. These differences are larger in some directions,particularly azimuthally where positioning was given by
the indexing ring which had modest precision.
After removing known systematic problems, the z
position information was compared to the nominal
positions in the construction drawings. From the measured
position of individual brackets the distance between
neighbouring brackets was derived and compared to thenominal distance. As an example the difference between
nominal and measured distances in z for B4, is shown in
Fig. 6. The spread in the resulting distributions are causedby the build precision of the support structure and therobot positioning accuracy (although not all the movement
stages in the robot contribute in this measurement). The
precision of the barrel support structure was speciﬁed to
720 mm for the radial position of the pads, to which thebrackets were attached, and the holes used for this junction
were required to be within a cylinder of 40 mm diameter on
the barrel surface. With such lightweight and relatively
ﬂexible structures, these tolerances were considered to be at
the limit of what could be achieved and the actual
tolerances were most likely slightly larger. The brackets
were not systematically measured, but the forms tolaminate and to machine them were made on CNC
machines with a precision of about 50 mm.
There is a systematic effect along the length of the barrel,
which is likely to be a result of the positioning accuracy of
the robot (limited by the open-loop controlled stages and/or the linearity of the long stage), rather than of the barrel,
as similar behaviour can be found in the other cylinderassembled using the same robot (Fig. 7). Using mean
positions for each bracket for a correction for each robot a
standard deviation of 87 mm is obtained for the distributionof the difference for all module positions (Fig. 8).
Assuming no correlation this translates to a 62 mm spread

Fig. 6. Difference between measured axial (z) distance between neighbouring modules within a row and nominal distance for B4. The different
data points and lines correspond to the 40 rows of modules on this barrel.

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mechanically well-deﬁned handling points. This was wellachieved for the insertion and ﬁxing of the modules on the
SCT barrels. These criteria were met less well with thedesign of the cooling interface and the electrical connection, where attempts to automate assembly were abandoned as the size of the project or access and precisionconsiderations did not mandate it. For a larger or more
distributed assembly project issues of simplicity of assembly should be considered at the system design stage to allow
for extensive use of robots in the assembly.
11. SummaryFig. 7. Distribution of difference between measured axial (z) distance
between neighbouring modules within a row and nominal distanceaveraged for each barrel. B3 and B5 were assembled using the samerobot, whereas for B4 and B6 the other robot was used.

A total of 2112 SCT silicon modules have been mounted
on four support cylinders using robots. The excellent
success rate in attaching modules demonstrates thatrobotic assembly is a good choice for a tight geometry
like the ATLAS barrel SCT.
An automated survey method was selected to reduce thescope for operator error. No emphasis was placed on
absolute precision measurement. However, an analysis of
the survey data taken during assembly shows that excellent
precision can be obtained.
Acknowledgements

Fig. 8. Distribution of difference between measured axial (z) distance
between neighbouring modules within a row and nominal distance after
correction for robot-related systematic effects.

in the difference between the measured and nominal
location of a bracket position along the row. As discussed
above the actual build precision is not well known, but
assuming a value of 50 mm yields a robot measurement
precision of about 40 mm.
If it had been needed the described systematic effects inabsolute position measurement could easily have been
overcome by the use of closed-loop controlled stages or the
addition of a precision measurement system for absolute
distances (e.g. interferometer).
10. Other observations
Generally, robotic assembly is greatly facilitated bysimple, uniform design of the objects to be handled, with

We would like to thank J. Carter, M. Tyndel, and T.
Kondo for useful discussions during the setup andoperation of the robots. We are greatly indebted to all
the technical staff who worked on the project.
This work was supported by the UK Particle Physics andAstronomy Research Council; by the Fonds National
Suisse de la Recherche, Division of Science and Engineering, Switzerland; and by the Ministry of Education,
Culture, Sports, Science and Technology of Japan, andthe Japan Society for Promotion of Science.
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