atlas pixel detector discussion of tolerances november 12, 1998 pixel mechanics d. bintinger, lbnl...

ATLAS Pixel Detector

Discussion of Tolerances

November 12, 1998Pixel Mechanics

D. Bintinger, LBNLE. Anderssen, LBNL/CERN

November 98Mechanics Meeting

D. Bintinger LBNL


Tolerance Scheme• Mechanics should not significantly add to inherent Pixel resolution

– Goal: 15 to 18 in Azimuth

• Two possible Approaches– A.) Fabricate with loose tolerances and rely on track alignment (particles)– B.) Fabricate with very tight tolerances to minimize track alignment effort

• Desire to fall closer to option B than A, but certainly in between– Desire to use Stave as fundamental alignment unit to minimize track fitting effort

for 1500 modules with 6 DOF

• This discussion only applies to the specific geometry of the stave

Tolerances presented are with view in mind that Stave is a well know unit


D. Bintinger LBNL


Relation of Assembly to Tolerances

• Modules Placed on Local Support– Minimum accuracy required for module to module registration – All modules are to E3 within 1 pixel width of desired position

• Modules Surveyed on Local Support– Modules’ Positions are determined relative to stave mounts and each other– CMM Accuracy limits fundamental accuracy of this measurement to E5

onefor CMM)

• Local Support placed in Shell/Disk– Last time to physically measure module location– CMM Accuracy limits fundamental accuracy here as well.

• Powered on in operating environment (Flow, CME, CTE, etc)– Changes location of modules from surveyed position

• X-Ray survey in powered on condition (arbitrary accuracy)• Stability/Repeatability

– Gradient of stability motions should be less than accuracy/calibration-time-constant

Add in Q

uadra

ture

Change of StateNot Statistical

AffectsFundamentalPerformance

Consider that this rationale requires a thorough X-ray Survey

Only fundamental requirements are Module placement, and Stability

Fundamental toModule but

Unrelated to rest


D. Bintinger LBNL


Comments on “Change of STATE”• Should not be treated as stochastic variation, however if small,

affects can be estimated by adding in quadrature– The change from nominal is not statistically based, but highly correlated with

temperature/% moisture and does not average to zero– This could be viewed as “Systematic” error and “removed” if well understood

• Operation in Powered Up Configuration only– Measurements in Power-Off configuration only– Movement from last measured positions occurs in definite “repeatable” fashion,

but stability affects accuracy of X-Ray survey– X-Ray survey could “remove” the offset caused by powering up, but would require

changing from Stave-based to Module-based alignment (13 times the number of parameters)--this sets limit of stave deflection from nominal

• If X-Ray survey is not done, and errors are near limit– Static and Power-up deflections become very important for convergence of

alignment software at start-up of ATLAS

Repeatability and Stability should be on the same order


D. Bintinger LBNL


Relating Survey to Position

• Last measurement of Stave is in Half Shell• Going to full shell, center ring sags 2

– Negligibly affects staves

• Supporting from ends, Barrel sags 50– This is static, and affects barrel uniformly,

equivalent to moving detector axis

• All static deflections and assembly tolerances “disappear” after X-ray survey and/or track alignment

*Figures may change based on detail design


D. Bintinger LBNL


Map Global Tolerance to Stave Dimension

5

azimuth

radia

l

Stave

• Global Tolerances based on 3 effects– Tilt angle– Module does not change dimension as it moves (R maps to

– Low momentum tracks have bend radius (negligible)

• Global Tolerances are mapped to stave coordinates

– Lateral Tolerance is approximately equal to Azimuth tolerances (projectively: cos(tilt) y 1)

– Out of plane (normal) motion of stave maps to azimuth via tilt angle--azimuthal tolerance sets limit on out of plane motion, not Radial Tolerance

lateral

norm

al

Limits Normalexcursion

Tolerance Box

Nominal Dimension

E Normal

(requires touch)Try to define tolerances in terms easy tomeasure on CMM


D. Bintinger LBNL


Construction Tolerances• Modules within E3 will be within 1 pixel width of nominal• Global--only tight enough to allow unambiguous alignment of modules

dimensions are 1 values except as noted– Lateral: 15 – Normal: 20 B-Layer

25 Layers 1,2– Z: 50 – Planarity: E10 (limits not 1)– Rotation about normal axis: .34 mrad– Rotation about longitudinal axis: 1.8 mrad B-Layer

3.6 mrad Layers 1,2– Rotation about horizontal axis 0.5 mrad B-Layer

1.0 mrad Layers 1,2

• Values are Relative to Stave fixtured for assembly (vac-chuck?)– Radial and Azimuth map to normal and lateral directions, z is along stave--tilt affects can

be ignored– Need to understand assembly loading and spring back as related to tolerances


D. Bintinger LBNL


Stave Survey Tolerances• Knowledge of module positions w.r.t. stave coordinate system• Designed so that Stave can act as alignment unit• Global Tolerance R, Azimuth, Z

• These tolerances in quadrature add 6.6 to the pixel resolution I.e.(152 + 6.62)1/2

– Lateral: 5 – Normal: 10 B-Layer

15 Layers 1,2– Z: 25 – Planarity: E10 (limits not 1) – Rotation about normal axis: .17 mrad– Rotation about longitudinal axis: 1.8 mrad B-Layer

3.6 mrad Layers 1,2– Rotation about horizontal axis 0.5 mrad B-Layer

1.0 mrad Layers 1,2

• Measured with Stave in simulated mounting configuration– Need to consider the influences of measuring forces for contact measurements– Errors in z-position for a module in a shingled configuration map to r

• Tolerances at these levels exceed confidence limits of available CMM’s


D. Bintinger LBNL


Movement Tolerances• Movement due to coolant flow, power-up, cool-down, drying, w.r.t. Stave

Coordinate system• Global Tolerances as tied to Pixel Size

• These add another 6.6 in quadrature to pixel resolution: ((152 + 6.62) + 6.62)1/2 = 17.7

– Lateral: 5 – Normal: 10 B-Layer 15

Layers 1,2– Z: 25 – Planarity : E10 (limits not 1) – Rotation about normal axis: .17 mrad– Rotation about longitudinal axis: 1.8 mrad B-Layer 3.6

mrad Layers 1,2– Rotation about horizontal axis 0.5 mrad B-Layer 1.0

mrad Layers 1,2

• Tolerance on Stability can be directly tied to these numbers– Stability motions not to exceed half the value of the above numbers absolute– OR ---– Require that motions are not to exceed above numbers between software alignments (order 1-

day)--This requires module-based alignment.

• Motion of Stave in excess of specified tolerances precludes its use as a functional alignment element


D. Bintinger LBNL


• Approximation – Possible to estimate response

through analysis of a single degree of freedom oscillator

– Input acceleration PSD assumed to be a constant 1g2/Hz

– A fundamental mode at 100 Hz would have a response of ~25m rms, 1 sigma

– Q of stave materials has not been measured

30 40 50 60 70 80 90 1000

0.05

0.1

0.15

0.2

Q of structure = 40

Input acceleration1x10-6 g2/Hz--constant level (equivalent to 7 10-3grms)

Hz

RMS Motionmm

Estimate based 1DOF Oscillator

Comment on Vibration Tolerance Rationale

• Simple analysis indicates that vibration on the order of 100Hz yields 1sigma errors on the order of our tolerances

– This is not well qualified for stave structures, but is of the correct order– These numbers actually exceed tolerance limit– further work needs to be done to quantify this better


D. Bintinger LBNL


Conclusions

• Rational basis for tolerancing of assembly and deformations has been employed to set limits on errors tied to the physical precision of the detector

• Tolerances on motion are more stringent than originally thought

• Survey tolerances are at limit of CMM’s available• Stability, Vibration, and Hygrothermal strains are, at present

understanding, each in excess of allowances

atlas pixel detector discussion of tolerances november 12, 1998 pixel mechanics d. bintinger, lbnl...

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