the RASNIK Alignment System

Particle PhysicsCERN, Geneva, Swiss

pp collisions

2) heavy collisions: A proton is a bag filled with quarks en gluonen

The ATLAS ExperimentCERN, Geneva, Switzerland

‘Tracking’ of charged particlesMeasurement of position of tracks

Track curvature: measure formomentum & energy

Momentum Measurement of chargedparticles in the L3 experiment:Chamber Position Monitoring

• Lorentz Force:• Track Curvature measurement

Detector

Muon Particle Track

Detector

Detector

Principle of CCD-RASNIK

Coded Mask

Light Source LensCCD

Title: The RASNIK opto-electronic alignment system: a high-precision, large range, fast andzero-drift monitor for displacements or deformations.

In RASNIK, the image of a (back-illuminated) coded mask is projected, by means of a lens,onto a (pixel) image sensor. A displacement of the mask, or lens, or sensor, relative to the othertwo components, results in a displacement of the mask’ image on the sensor. This can beregistered accurately by means of a processor connected to the (USB) sensor.Displacements in the two transversal directions cause an image shift, and a displacement inthe direction of the optical axis results in a change of the image scale. In addition, the relativerotation around the optical axis of mask and sensor can be recorded, making RASNIK a 4Dmeasurement device. With image frame rates up to 100 Hz, vibrations can be measured as well.

With RASNIK, the bending of a (roof) bar can be monitored accurately. When the lens andsensor are coupled on one (CAM) base plate, the displacement of the mask with respect tothis base plate is measured; this ‘proximity’ RASNIK is applied as displacement monitorfor adjacent tunnel sections. The deformation of a complete tunnel could be measured by mounting a series of identical plates, each carrying a mask, lens and sensor, forminga chain of coupled RASNIK systems.

dX, dY: Image Displacement

dXLED = -2 dXLEN = dXSEN

Rasnik 3-point alignment system

Alternatives:

- Taylor Hobson telescope- Stretched wire system: electronic version after 1985

Measurement & Precision

- Translation (X, Y): 50 nm per image- Scale: 1.0000 +/- 0.00001- Rotation around Z-axis: 0.1 mrad

Number of images: depending on pixel sensor:- webcam: 30 – 60 images per second: measurement of vibrations!- special graphic image sensor: 10.000 images/s

Practical limitation:Temperature gradient in air

dT

Image info ~ 1Mbis converted into only 4 parameters

ATLAS Muon Chambers

Image Sensors

Lenses

LightSources

RASNIK ‘In-Plane’ systems

Measures:• Chamber sag• Chamber torque• Temperature gradients

RASNIK systems in the ATLAS Muon Spectrometer

ProjectiveAxialProximity‘Praxial’

Applications

4 D, no-contact Dial Gauge

lens Image sensormask

laptop

Weena Rasniks

Measurement of relative displacement of adjacent sections

Measurement of (variation of) pillar height

Intergral measurement, in 3D, of deformation of large (long) object:- tunnel- bridge

- RasChain plate includes light source, lens and image sensor- mount RasChain plate at ~ 10 m pitch, over 1 km- readout chain at both ends- Deformation is measured with mm precision!

RasChain

Segment2 m Laser

Diffraction plate (hole)

Image sensor

RasChain

Laser

Diffraction plate (hole)

Image sensor

Microcontroller

Bus and Power

µC

Link

data power

IDaddress

Level 0

Cameras on a bus

link

Level 0

Level 1

Chain

ChainControl

Next chain~256m

~128m

Communicationlayer

RasChain measuring the integral deformation of along object, i.e. a tunnel

Fig. 1 The leap-frog Rasnik system. All plates are identical and each includean illuminated coded mask, a lens and an image sensor.

Position resolution with Gaussian noise on Rasnik dataThe noise per Rasnik system is rather arbitrary. With direct shadow images,50 nm has been achieved (image position on sensor: XR and YR).With RasDif, 20 nm has been reached (over 140 m!).If images of a static system are combined, even lower values are reached.The lower limit is hard to measure due to the presence of systematic image shiftsdue to non-homogeneity of the ambient medium, causing both a random and a systematical error.Assuming a random Gaussian error in XR of 50 nm in all of the 100 Rasnik systems,the resulting errors in the monitored plate positions is shown in fig.2.   

  As expected, the uncertainty is the largest in the middle of the RasChain.The value, however, is in the order of 10 µm and small enough to be relevant for the presenceof long-distance alignment systems such as (long) RasClic or the stretched wire system.

Fig. 2. The random error as a function of plate number,due to a Gaussian error of 50 nm on the Rasnik data(common for all 100 systems).

Laser zone lenshole dia. 50 mm

RasCam

100 m (vacuum tube!)

RasDif: replace lens by diffraction plate:just a round hole!

Laserexpanded beamjust monochromaticlight source

diffraction platehole dia. 50 mm

RasCam

RasDiflong baseline: lens becomes unpracticalReplace lens by ‘diffraction plate’: just a hole!

Image position on sensor. Response of earthquake in Mid-Atlantic,5 Richter Scale, on March 1, 2007

Rasnik as seismic sensor

RasNap

Air-refraction corrected telescope

Practical limitation:Temperature gradient in air

Rasnik: a new displacement monitor

- based on a wide and 27 years long experience- very precise:- high data rate: dynamical measurement- no drift in measurement: monitoring of slow motions- simple, digital, robust & low-cost- a new means of product parameter verification

But:- needs 220 V and Ethernet (compare old t, P-sensor: plot)- custom/case-specific application (use ‘standard’ components)- interpretation of data: skilled, educated personel