M. Kubantsev

04/09/07

# Combining Photogrammetry with Coordinate Measuring Machine Survey of the CMS Forward Pixel Detector

##  

##  Introduction

Photogrammery technique is widely used in physics experiment for survey
experimental setup geometry especially on large scale (of meters to tens
of meters and more) to connect different parts of the detector installed
in the experimental hall. Less known application is possibility to
measure relative position of various parts of the detector on scale of
few centimeters to a meter, where accuracy of measurements is
approaching to one obtained with coordinate measuring machines, and of
order of few microns to 10 microns. In this case the advantage is to
make measurements from save distance of fraction of meter (down to 50
cm) from a very delicate object of complicated shape. The same surveyed
object has to be installed on the coordinate measuring machine with a
touch probe or optical lens and touched with force of few tens of grams
and/or approached to the distance of less then few centimeters (4-10 cm)
with bulky lens. This creates limitations for measurements of some deep
fiducials or targets inside the object and creates serious hazards
during measurements mechanically endangering the delicate and very
precious object (like a silicon detector) and the machine itself.

The CMS Forward Pixel Detector consisting of eight half disks of
complicated shape installed in four service half cylinders (Fig. 1). The
turbine geometry of the individual

![](media/image1.png){width="4.5in" height="2.908333333333333in"}

Fig.1 The half cylinder with two half disks installed for the 07-Pilot
Run

half disk carrying panels with the silicon sensors requires precision 3D
survey of the silicon sensor positions with limited access. The sensors
have fiducials with dimensions of 60 microns, which can be surveyed only
with a coordinate measuring machine with optical probe. These fiducials
cannot be seen on the photogrammetry pictures, because in this case only
special optical targets of size over 1 mm in diameter can be clearly
visible and processed through the geometry reconstruction.

**Survey work steps**

To make full survey of the detector we suggest the following chain of
measurements allowing us to align all elements of the detector with
global coordinate system of the CMS detector

First, we measure sensor positions (using fiducials) on individual
panels with the OGP CMM with typical precision of 1 micron in a sensor
plane and 2-3 microns in orthogonal direction.

Second, after installation of the panels on the half-disk we measure
again the sensor fiducial positions relative to each other together with
survey spheres mounted on the outer ring of the half-disk and several 1
mm photogrammetry optical targets installed on several panels. The
half-disk was installed on the support plate using three mounting legs.
The CMM with inter-calibrated optical lens and touch probe is used. The
maximum diameter of the targets is limited to 1 mm due to optical lens
field of view. Use of other lens is not permitted by the size and
visibility of silicon sensor fiducials. Note, that each side of the half
disk is measured separately. Additional survey spheres together with the
spheres mounted on the disk are used to connect these to sets of
measurements in one coordinate system.

Third, we make a set of photogrammetry pictures of the 1mm targets
together with other coded targets using the VSTAR system and make 3D
reconstruction of these targets. We compare the results with CMM
measurements. Typical agreement is about 10 microns. Additional 3 mm
targets installed on the half-disk inner and outer rings are used in
addition to 1 mm targets due to difficulty to reconstruct 1 mm targets
at CERN. Note, that the picture shooting is made from minimal distance
allowed by the camera of 50-70 cm, which allows recognize and
reconstruct 1 mm optical targets on the panels with nominal accuracy
better then 10 microns.

Fourth, after installation of the half-disks into the service cylinder
we survey their positions relative to the supporting structure (leg
balls) using combination of touch probe measurements of the survey
spheres on the disks, service half-cylinder and supporting structure and
photogrammetry targets. It allows inter-calibration of these two methods
and determining of sensor positions relative to the supporting
structure.

Fifth, we suggest after shipment to CERN, when the half-disks are
dismantled and delivered separately and the detector is re-assembled,
the CERN alignment group will re-survey the service cylinder using
photogrammery. At this step the half cylinder with the half-disks is
surveyed relative to the supporting rails using all optical targets
available including 1 mm ones. Additional removable optical targets
required on the end flange to make further connection with external
fiducial CMS system during final installation into the detector.

Sixth, we suggest the CERN alignment group will make final survey of the
end flange removable targets relative to the CMS targets using
photogrammetry after Forward Pixel installation into the CMS. Four CERN
photogrammetry targets are to be installed into holes in the end flange
using proposed fixtures described in the Appendix.

The results of the measurements described above are analyzed in
assumption that the half disk is a rigid body.

**Half-disk photogrammetry survey.**

Photogrammetry survey of half-disk is done just after CMM measurements
on the same support plate as shown in Fig.2. The disk is placed
vertically and surrounded by coded targets. AutoBar is placed in the
center of the array.

![](media/image2.png){width="5.804166666666666in"
height="6.166666666666667in"}

Fig.2 Photogrammetry session for --Z2 half disk.

![](media/image3.png){width="5.375in" height="3.8375in"}

Fig.4a Optical targets for --Z1_3-side half disk

![](media/image4.png){width="5.25in" height="3.4319444444444445in"}

Fig.4b Optical targets for --Z1_4-side half disk

![](media/image5.png){width="5.25in" height="4.034722222222222in"}Fig.4c
Optical targets for --Z2_3-side half disk

Fig.4d Optical targets for --Z2_4-side half disk

At the first step the 1mm optical targets are photographed and
reconstructed simultaneously on both sides of the half disk (12-16
targets on each side, the picture of the targets with numbers is shown
in Fig.4a-d). The results are presented in the files attached:

***-Z1 Half Disk_20mar07***

***-Z2 Half Disk_29mar07***

Here all photogrammetry target coordinates (X,Y,Z) and estimated
reconstruction errors (Sigma = R.M.S./SQRT(Nmeasurements) are presented
in the working coordinate system of the STAR package (usually connected
with the AutoBar).

**Comparison of two survey methods.**

Four files (txt) with optical target coordinates are extracted from the
CMM measurement data:

***3-side\_-z1_targets***

***4-side\_-z1_targets***

***3-side\_-z2_targets***

***4-side\_-z2_targets***.

Photogrammetry measured coordinates of the optical targets on the panels
are fitted and translated to the coordinates of the corresponding
targets on two half-disk sides measured by CMM. The results are shown in
following attached txt.files for two sides of the half-disk in the
corresponding CMM coordinate systems separately:

***3-side\_-z1_targetsTrans***

***4-side\_-z1_targetsTrans***

***3-side\_-z2_targetsTrans***

***4-side\_-z2_targetsTrans***

Some unmeasured 1mm targets has suffix "u". Coordinates of other
auxiliary photogrammetry targets are presented. The differences between
reconstructed target coordinates by two methods are shown in:

***3-side\_-z1_targetsTrans_R***

***4-side\_-z1_targetsTrans_R***

***3-side\_-z2_targetsTrans_R***

***4-side\_-z2_targetsTrans_R***

The following observation is to be made: as the repeatability of the CMM
measurement is of order 1-3 microns and photogrammetry error estimation
is about 4-10 microns the agreement between two data sets is
significantly worse and is about 10-20 microns. Close inspection shows
that about half of the points have significantly higher discrepancy
between two measurements (20-40 microns) then average indicating
possible systematics involved. As the CMM measurements are done using
automatic pattern recognition of the targets with measuring effective
"center of gravity" of the target image, there is possibility that some
irregularities in the target shape can cause such discrepancy. Further
investigation is under way.

As the targets on both sides of the half-disk are reconstructed together
with photogrammetry it provides independent check of the CMM based
procedure of connection of coordinates of two sides of the disk with
survey spheres. Such comparison is pending.

**APPENDIX**

M. Kubantsev 04/09/07

**An Adapter for CERN Survey Targets on the End Flange of the Half
Cylinder.**

The message below describes requirements for a photogrammetry target
adapter we may use for final survey of the half cylinder in the CMS
detector. This adapter can be attached to the end flange using
designated holes. The sketch of the possible adapter is attached
together with CERN drawing. I propose to make 4 such adapters for each
half cylinder, total 16.

\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_

From Antje Behrens \<Antje.Behrens@cern.ch\>

Date Thu, 21 Sep 2006 16:30:56 +0200

To Paolo Petagna \<Paolo.Petagna@cern.ch\>, swalk@fnal.gov

Cc Raphael Goudard \<Raphael.Goudard@cern.ch\>

Subject Survey reference hole

Dear Simon,

Please find enclosed a drawing of the survey reference hole. Only the

parameter A3 (8mm H7 hole), B3 and C3 are valid, we do not need the

thread.

If you manage to integrate these holes with the necessary space to

insert a target, we will be free later to choose the measurement system

(theodolite / photogrammetry).

Please let us know if you succeeded, or if we have to switch to a

different hole diameter (and a different target type).

Best regards

Antje