Appunti del Corso Advanced Measurement Techniques

Experimental set-up

Activators

Electrodynamic shaker - f ↓, bandwidth ↑
Hydraulic actuators - f ↑, bandwidth ↓
Vibrodrivers - f ↓, bandwidth ↑
Electrodynamic linear motor - f ↓↑, bandwidth ↑↓
Piezo actuators - to bend at crack - f ↓, bandwidth ↑
Low sound effect for benders
No rigid link with the structure for benders
Can be used as sensor
Amplifier needed to have ΔV
Nonworking for benders
Benders provide low energy
Wide band is needed to excite all the modes

Unloaded cables

To produce "map book"

Impact hammer

• Easy and fast
• No sound effect
• Input force not controllable

Natural excitation - you don't know the input force or see the excitation point, so you cannot measure the FRF. That means you don't know the force that is exciting the system, and for this reason you can't know how to make the model parameters - phantomous model analysis.

Various meanings

The input PS and CS - auto and _skip correlation. You cannot build a modal model, so you use this approach when you have to check your structure's model. Eigenvalues, eigenvectors, and fr are estimated. You said you don't know the model mass excited.

Experimental set-up

Activators
Electrodynamic shaker - f ↑, bandwidth ↑
Hydraulic actuators - f ↓, bandwidth ↓
Vibrodriver - f ↑, bandwidth ↑
Electrodynamic linear motor - f = mω² ↑↓, f ↑ @ ω ↓
Piezo-actuator - based on stack - f ↑, bandwidth ↑

1. Low sound effect for benders
2. No rigid link with the structure for benders
3. Can be used as sensor
4. Amplifier needed to have ΔV
5. Resonance for benders
6. Benders provide low energy
7. Wide band needed to excite the modes

Unmodulated cables

To produce "map, book"

Impact hammer

1. Easy and fair
2. No sound effect
3. Input force not continuous

Natural excitation -> you don't know the input force or see the excitation point, so you cannot measure the fit. That means you don't know the force that is exciting the system and for this reason, you can't know how to make the model parameters -> operational model analysis (the system is in its operational condition).

Sensors measuring the response -> PS and CS -> auto and _{________} correlation -> eigenvalues, eigenvectors, and etc. are estimated (you said you don't know the model mass assisted). You measure the structure response but you can't know the force input, you cannot find the relating force fit. You cannot build a model model, so you use this approach when you have to check your structure's model.

T_p = 1/f_p
f_s = 1/Δt
Δf = 1/T
To avoid aliasing f_s > 2f_p
To avoid leakage f_p = NA
f_o = D T = Nf_p

Which can be measured in terms of displacement, velocity, acceleration

Transducers to measure the temperature

Displacement transducers -> useful in rotative motion, monitoring only.
Velocity transducers -> Not used, however, the local angular velocity is very useful because torque, but the price is huge and each data theft-> the working principle is that the rotating gauge changes easily speed that is caught by the receiver.

Retentive displacement transducers ->
Mechanical accelerometers -> it is the most diffused, good sensitivity, low weight, biggest dynamic range, wide frequency range, simple design, high component resolution, low thermal minimality, simple mounting methods ->

Dynamic range = max measurable amplitude / min measurable amplitude. Higher free scale means higher dynamic range, that almost limits noise S/N 1-> The force is measured through strain gauge/load cell/piezoelectric (There are accelerometers) strain-gauge accelerometers -> measure the excitation --> mounted on the structure-> mode shape identification through modal analysis.

The aim of the dynamic mode analysis is characterizing the dynamic behavior of the system, so finding natural frequencies, mode shapes, and non-dimensional characteristics. Practically it is done in various steps:

1. Design of the experimental set-up, how to realize it?