Week 13

1

Vibration Measurement

Week 13

2

Vibration Measurement Schemex, v, a, F

electrical signal

amplified signal

signal data

The type of vibration measuring instrument used will depend on:

- The expected range of frequencies and amplitudes

- The size of machine / structure involved

- Conditions of operation of the machine / structure

- Type of data processing used

Week 13

3

Dynamic Testing of Machine/Structure

� Dynamic testing is a valuable complement to dynamic analysis

� Modal testing:

� Natural frequencies

� Damping

� Mode shapes

� Operational testing:

� In-service loads

� Various cases

� Often random

Week 13

4

Dynamic Testing of Machine/Structure

Measurement Hardware

Week 13

5

Transducer

� A transducer translates changes in mechanical quantities (such as displacement, velocity, acceleration or force) into electrical signals (such as voltage or current).

� The size of the transducer is importatn

� Especially when measuring small structures

� Ideally should not influence the structure’s dynamics through added mass or stiffness

� Analytical models often include effects of transducer mass

Week 13

6

Variable Resistance Transducer

Mechanical motion produces a change in electrical resistance of the strain gage, which in turn causes a change in the output voltage or current.

R ∞ ε ∞ x(t)

Week 13

7

Variable Resistance Transducer

R ∞ ε ∞ x(t) Wheatstone bridge, sensitive to small changes in R

Week 13

8

Piezoelectric Transducer

E

Piezoelectric material, such as quartz and Rochelle salt, generate electrical charge when given deformation or mechanical stress.

crystal

Piezoelectric accelerometer

Week 13

9

Electrodynamic Transducer

The movement of the coil in a magnetic field produces a voltage Vout.

Voltage ~ Velocity

Current ~ Force

Week 13

10

LVDT

Linear Variable Differential Transformers:

- coils and magnetic core

- output voltage depends on the axial displacement of the core

- Displacement transducer

- Insensitivity to temperature

- Mass of core restricts application

- Best for low frequency, high displacement application

Week 13

11

Vibration Pick-ups

� Instruments that measure vibrations, e.g. vibrometer, accelerometer.

Most commonly used pick-ups are known as seismic instruments

- a mass-spring-damper system in a cage � inertial sensor

- a certain type of transducer is integrated in the instrument.

- fastened onto the vibrating body

Week 13

12

Vibration Pick-ups

Week 13

13

Vibrometer

- Measures the displacement of a vibrating body

- Works in the right (blue) range where r >= 3 � Z/Y = 1

if

Recorded z(t) lags from measured y(t)

Week 13

14

Accelerometer

Measures the acceleration of a vibrating body, from which the velocity and displacement are obtained by integration.

recorded z(t)ωn2 lags from the

acceleration of the base ytt(t)

Week 13

15

Accelerometer

Works in this range

Week 13

16

Vibration Exciter

Application:

- in the determination of the dynamic characteristics of machines and structures;

- In fatigue testing of material.

Types: mechanical, electromagnetic/electrodynamic, hydraulic.

Also known as ‘shakers’.

Week 13

17

Mechanical Exciter

Week 13

18

Mechanical Exciter

Week 13

19

Electrodynamic Shaker

Also known as electromagnetic exciter.

A reverse of the electrodynamic transducer.

Apply a force through a range of frequencies (harmonic or random input)

Week 13

20

Electrodynamic Shaker

- Current (A) passes through coil

- Generates force F (N) ~ I (A)

- Generates magnetic flux intensity B (Tesla)

F = BIl

l : length of coil

This accelerates the component on the shaker table.

AC: F harmonic

DC: F constant

Week 13

21

Signal Conditioning

Week 13

22

Signal Analyzer

Week 13

23

Signal Analyzer

Week 13

24

Impulse Testing

Week 13

25

Relationship between components and vibration spectrum