LG 02/06/2004


Mechanical Testing of Pixel Detector Ladder Assembly Mockup


Vibration and Displacement


The existing pixel detector carbon fiber beam/detector mechanical prototype has the following construction.


The ladder assembly is 25.4 cm long and 2.13 cm wide. There is a ~1.5 cm wide mounting block glued to the carbon fiber beam at one end.

            To measure the displacement as a function of time, we are using a model C1-A capacitive probe from the Lion Precision Co. (More information can be found here http://www.lionprecision.com/modular/index.html)

The probe is calibrated to produce 40 mV / micron output signal and has a 1 KHz Bandwidth. This calibration was tested using the ladder mechanical assembly which has a kapton dielectric between the probe and the sensed Si and is not flat but has a slightly curved surface. The calibration was still found to be valid when checked against a micrometer driven mechanical stage.


All tests were done with the probe in the calibrated range from the Si (250-750 microns) measured at the far end of the ladder assembly away from the support.


Fundamental Resonance Frequency:


By slightly displacing the end of the ladder assembly with a fingernail and releasing we can find the Fundamental resonance frequency of the assembly.



On the upper trace we can see the initial displacement and the oscillations after the release. The lower trace is a FFT and gives the fundamental resonance frequency as 139.2 Hz.


Vibration and Displacement Produced by Air Flow:


We tested the airflow induced vibration and displacement for axial and off axis airflow at different speeds. The test setup is shown below.


The 10 degree data is considered the worst case and is shown below.


(The original scope data can be seen here)


This is at rest with the air off. The SD for the static case, which shows our probe resolution, is 0.157 microns.The “at rest” mean position of the beam is 28.016 microns.


(The original scope data can be seen here)



(The original scope data can be seen here)


(The original scope data can be seen here)


(The original scope data can be seen here)


We can see that as the airflow increases, the free end of the ladder assembly bends away from the capacitive probe and the magnitude of the vibrations induced by the airflow increases. This is shown below.