The goal is to experimentally extract a *Spatial Model* (mass, damping, stiffness) of the structure (shown on figure [[fig:nass_picture]]) in order to tune the Multi-Body model.
#+name: fig:nass_picture
#+caption: Picture of the ID31 Micro-Station. (1) Granite (2) Translation Stage (3) Tilt Stage (4) Hexapod (5) Dummy Mass
#+attr_html: :width 500px
[[file:img/nass_picture.png]]
The procedure is represented on figure [[fig:vibration_analysis_procedure]] where we go from left to right.
#+name: fig:vibration_analysis_procedure
#+caption: Vibration Analysis Procedure
#+attr_html: :width 400px
[[file:img/vibration_analysis_procedure.png]]
First, we obtain a *Response Model* (Frequency Response Functions) from measurements.
This is further converted into a *Modal Model* (Natural Frequencies and Mode Shapes).
Finally, this is converted into a *Spatial Model* with the Mass/Damping/Stiffness matrices.
Theses matrices will be used to tune the Simscape (multi-body) model.
The modes we want to identify are those in the frequency range between 0Hz and 150Hz.
* Type of Model
The model that we want to obtain is a *multi-body model*.
It is composed of several *solid bodies connected with springs and dampers*.
The solid bodies are represented with different colors on figure [[fig:nass_solidworks]].
In the simscape model, the solid bodies are:
- the granite (1 or 2 solids)
- the translation stage
- the tilt stage
- the spindle and slip-ring
- the hexapod
#+name: fig:nass_solidworks
#+caption: CAD view of the ID31 Micro-Station
#+attr_html: :width 800px
[[file:img/nass_solidworks.png]]
However, each of the DOF of the system may not be relevant for the modes present in the frequency band of interest.
For instance, the translation stage may not vibrate in the Z direction for all the modes identified. Then, we can block this DOF and this simplifies the model.
The modal identification done here will thus permit us to determine *which DOF can be neglected*.
* Instrumentation Used
In order to perform to Modal Analysis and to obtain first a Response Model, the following devices are used:
- An *acquisition system* (OROS) with 24bits ADCs (figure [[fig:oros]])
- 3 tri-axis *Accelerometers* (figure [[fig:accelero_M393B05]]) with parameters shown on table [[tab:accelero_M393B05]]
- An *Instrumented Hammer* with various Tips (figure [[fig:instrumented_hammer]]) (figure [[fig:hammer_tips]])
#+name: fig:oros
#+caption: Acquisition system: OROS
#+attr_html: :width 500px
[[file:img/instrumentation/oros.png]]
The acquisition system permits to auto-range the inputs (probably using variable gain amplifiers) the obtain the maximum dynamic range.
This is done before each measurement.
Anti-aliasing filters are also included in the system.
#+name: fig:accelero_M393B05
#+caption: Accelerometer used: M393B05
#+attr_html: :width 500px
[[file:img/instrumentation/accelero_M393B05.png]]
#+name: tab:accelero_M393B05
#+caption: 393B05 Accelerometer Data Sheet
| Sensitivity | 10V/g |
| Measurement Range | 0.5 g pk |
| Broadband Resolution | 0.000004 g rms |
| Frequency Range | 0.7 to 450Hz |
| Resonance Frequency | > 2.5kHz |
Tests have been conducted to determine the most suitable Hammer tip.
This has been found that the softer tip gives the best results.
It excites more the low frequency range where the coherence is low, the overall coherence was improved.
- Be closer to the real dynamic of the station in used
- If the control system of stages are turned OFF, this would results in very low frequency modes un-identifiable with the current setup, and this will also decouple the dynamics which would not be the case in practice
This is critical for the translation stage and the spindle as their is no stiffness in the free DOF (air-bearing for the spindle for instance).
The alternative would have been to mechanically block the stages with screws, but this may result in changing the modes.
The stages turned ON are:
- Translation Stage
- Tilt Stage
- Spindle and Slip-Ring
- Hexapod
The top part representing the NASS and the sample platform have been removed in order to reduce the complexity of the dynamics and also because this will be further added in the model inside Simscape.
All the stages are moved to their zero position (Ty, Ry, Rz, Slip-Ring, Hexapod).
All other elements have been remove from the granite such as another heavy positioning system.
** Test Planing
The goal is to identify the full $N \times N$ FRF matrix (where $N$ is the number of degree of freedom of the system).
However, the principle of reciprocity states that:
The impact points are shown on figures [[fig:hammer_x]], [[fig:hammer_y]] and [[fig:hammer_z]].
#+name: fig:hammer_x
#+caption: Hammer Blow in the X direction
#+attr_html: :width 300px
[[file:img/impacts/hammer_x.gif]]
#+name: fig:hammer_y
#+caption: Hammer Blow in the Y direction
#+attr_html: :width 300px
[[file:img/impacts/hammer_y.gif]]
#+name: fig:hammer_z
#+caption: Hammer Blow in the Z direction
#+attr_html: :width 300px
[[file:img/impacts/hammer_z.gif]]
* Signal Processing
The measurements are averaged 10 times (figure [[fig:general_parameters]]) corresponding to 10 hammer impacts.
#+name: fig:general_parameters
#+caption: General Acquisition Settings
#+attr_html: :width 500px
[[file:img/parameters/general_parameters.jpg]]
Windowing is used on the force response signals.
A boxcar window (figure [[fig:window_force]]) is used for the force signal as once the impact on the structure is done, the measured signal is meaningless.
An exponential window (figure [[fig:window_response]]) is used for the response signal as we are measuring transient signals and most of the information is located at the beginning of the signal.
