Status of the ITSAMS


Calculation of measurements from the centeral ITSAMS sub-system and their effects on the SSD ladder spacings { html, pdf, ps }.

Calculation of the stability of the Camera/mirror mounts to the SSD wings, Original 2D version{ html, pdf, ps }.

Notes/E-mail about ITSAMS Review at OSU Feb. 17 2005
Dear Flavio, Paolo, Beppe, and Pierluigi:

Here at OSU, we are going though a review of the alignment system. The outcome of the first discussions are outlined below. We would like your comments and/or suggestions.

Let me break the full 16 camera system up into the 4 sub-system, each using 4 camera-mirror sets. On the RB26 side, the Muon Absorber side, the Muon_Absorber-Beam_Pipe_monitoring sub-system. This sub-system is supposed to monitor for changes to the location of the beam-pipe with respect to the beam-pipe coming out of the absorber. We find that there are a large number of problems with this sub-system, most of which are out of our control. There appears to be no room to Mount on to the Muon Absorber especially given the apparent size and location of the V0, cable trays, and patch panels, not to mention all of the T0, V0, and FMD cabling in that extremely tight space. Additionally, the corresponding mirror is likely to need to be mounted on the SPD support cone and the not the Beam-Pipe support cone because of the expected radius of the sub-system. It would just miss the top of the beam pipe support cone. In either case, there has been no talk about how to mount to the "face" of either cone (and be out of the way of the cables). Because of the baffles on the beam pile and the mounting of the beam pipe support cone to the ITS, this sub-system would measure the movement of the Absorber with respect to the ITS. Without some other systems (capacitive sensors for example) no real knowledge of the beam pipe location would be expected. Lastly, from our perspective, this sub-system is completely unreachable and would require full remote adjustablity which we have not yet found a full solution for. Consequently we are strongly considering dropping this sub-system from our final design.

Also on the RB26 side, the TPC-ITS-RB26_side sub_system. Here I know that Pierluigi Barberis has gone to a lot of effort to "give" us 4 empty cable trays for us to use for this sub-system. These trays are not at the same 45 degree angle as the SSD cone wings which is OK since the optical path required to stay inside of the cable trays preclude using them. The optical path would require the mirror to be located (just above the Muon_Absorber-Beam_Pipe_monitoring sub-system mirrors) again on the SPD support cone with all of the same problems for the Muon_Absorber-Beam-Pipe sub-system mirrors. Again these mirrors would need at least 1 if not the full 2 degrees of adjustment freedom remotely. Additionally, because of the need to use the cable trays, a narrower laser beam separation would be necessary and the location of the mirrors could not be moved to accommodate cables or any thing else. This sub-system is well known to be redundant with respect to the last two. Consequently we are also strongly considering dropping this sub-system from our final design.

The Third, Central, sub-system would use the sets of wings on the SSD cone. If this sub-system would require the ability to adjust it after its installation, this sub-system would require the same remote adjustments as the two above. We are working on a solution for such remote adjustment but are have not found a suitable solution to date. If, on the other hand, once this sub-system would be installed there would be no need for adjustments (we could "glue" things into place) then this sub-system would basically have been completely solved. What remains is the question of what such a sub-system could tell us about any deformations on the inside (ladders). Talking to Flavio a while back, he suggested that Beppe had done a finite element analysis which might suggest that such a sub-system would not tell you about anything. I have a back of the envelope calculation, which assumes that there is no SSD support cylinder and that the ladders could freely pivot on their mounts, suggests that such a sub-system could measure how close the ladders get to each other as the cones either slide parallel to each other in the x and/or y direction and similarly for rotations about the z axis, but with rather poor resolution. Specifically I find that the fraction change in the spacing of the ladders is proportional to 0.5* (ratio of the measured cone displacement/the ladder length)**2 or the spacing between the ladders would get smaller by detla=2.297E-6[mm^-1] *(DeltaX)**2 where DeltaX is the displacement measured in the sub-system. At the same time, this sub-system would allow for the knowledge about the cone on the RB24 side when used in conjunction with the last sub-system. Consequently we are not sure about the usefulness of this sub-system.

Lastly, the forth, TPC-ITS-RB24_side sub-system, would measure the location of the ITS with respect to the TPC. Here we are in need of the design of the ITS cable trays and their support sub-system. On the other hand, all we would need is about 1cm of space (possibly less) between these cable trays and the TPC (at least at these 45 degree locations). Additionally we feel that we only need 1 degree of freedom for the mirrors attached to the SSD cone wings. This can easily be done with a hinge like design. With the other degree of freedom of the mirror moved to a translation of the camera sub-system along the TPC support structure. David has done a calculation, assuming a rigid ITS cylinder, and found that the ITS's position can be monitored to within the sub-system resolution (2-3microns, or better than 0.5microns if fringes are used) (translations in x,y and rotations). The Z resolution he finds is about a factor of 5 worse. This sub-system we feel would be the easiest to construct and would clearly tell us about the location of the ITS with respect to the TPC.

From the considerations presented above, we are converging towards a streamlined system which would include an adjustable TPC-ITS-RB24 sub-system and, if sufficient justifications is found, a fixed ITS, Central, sub-system, i.e. using a total of 4 or at most 8 camera-mirror sets. We believe that such a simplified system would satisfy all reasonable requirements for monitoring the ITS alignment and be of a manageable scope such that we could successfully deal with all of the engineering/mounting issues remaining in time.

Like I said above we would really like your comments on these discussions and it would be nice to get some additional information from you about the following.
1) Any information about from any existing Finite-element analysis that have been done.
2) Any ideas about what, if any, modes of failure we should be considering and should be monitoring for.
3) Any suggestions about how to have a compact way of having 2 rotational degrees of freedom in a more "rigid" way.
4) A time/construction schedule indicating when we need to be installed and if there would be any opportunity to "integration testing".
Tom will be at this next ALICE week and could discuss this with some of you then. Additionally we are also working on some more detailed analysis which we can share with your, but would like your comments sooner than that if possible.

Regards Bjorn, Tom, David, and Ivan