Today

1. Today • Accelerometers • Reference model and siblings • Calibration • Types and models • Common usages • Fixing and precautions

2. m kel c Reference scheme and siblings Vibration to be measured Vibration of the inertial mass Vibration of the inertial massrelative to the shell Imposing the dynamic equilibrium: Second order model z(t) is the output signal, which can be sensed by a suitable relative transducer.

3. Reference scheme and siblings Substituting complex expressions in (1)…

4. Reference scheme and siblings • Different quantities could be the input signal: • Displacement • Velocity • Acceleration Seismometer Velocimeter Accelerometer

5. Reference scheme and siblings Dividing both numerator and denominator by kel … and Thus: Seismometer Velocimeter Accelerometer

6. x Seismometer The measurand is the displacement of the vibrating element.

7. Accelerometer The measured displacement of the inertial mass is the output signal → deflection accelerometer. K=m/ks – static sensitivity

8. Strain gauge Vibration to be measured Inertial force on the mass m Deformation of the beam Deformation measurement (R) Reference scheme and siblings • Deflection accelerometers can be realized by way of different transducing principles, adopted to sense the inertial mass displacement: • relative vibrometer accelerometers: capacitive, inductive, potentiometric • strain gauge accelerometers • piezoelectric accelerometers • MEMS accelerometers – capacitive or piezoresistive • For example let us take into account the strain gauge accelerometer.

9. Reference scheme and siblings Deflection accelerometer properties:

10. Reference scheme and siblings • Accelerometers LOAD EFFECT: • on mass-load relationship • on the resonant frequency of the measurand system m Thereforeaccelerometer mass shouldbesmallcomparedto the vibrating mass beingmeasured M F M F K C K C

11. Calibration of accelerometers Accelerometers are calibrated at 1000 rad/s = 159.2 Hz The applied acceleration is 10 m/s2.

12. Calibration of accelerometers • The aforementioned interferometric technique is used for high level calibration (primary standards). Other techniques are commonly used for lower levels instruments: • Back to back Vu amp. Accelerometer to be calibrated meter Vr amp Reference accelerometer Sensitivity Aosint Vibrating table

13. Calibration of accelerometers

14. Accelerometers types and models • Capacitive accelerometers sense a change in electrical capacitance, with respect to acceleration.The accelerometer senses the capacitance change between a static condition and the dynamic state. • Piezoelectric accelerometers use materials such as crystals, which generate electric potential from an applied stress. This is known as the piezoelectric effect. As stress is applied, such as acceleration, an electrical charge is created. • Piezoresistive accelerometers (strain gauge accelerometers) work by measuring the electrical resistance of a material when mechanical stress is applied • Hall Effect accelerometers measure voltage variations stemming from a change in the magnetic field around the accelerometer

15. Accelerometers types and models • Magnetoresistive accelerometers work by measuring changes in resistance due to a magnetic field. The structure and function is similar to a Hall Effect accelerometer except that instead of measuring voltage, the magnetoresistive accelerometer measures resistance. • Heat transfer accelerometers measure internal changes in heat transfer due to acceleration. A single heat source is centered in a substrate and suspended across a cavity. Thermoresistors are spaced equally on all four sides of the suspended heat source. Under zero acceleration the heat gradient will be symmetrical. Acceleration in any direction causes the heat gradient to become asymmetrical due to convection heat transfer. • Micro-Electro Mechanical System technology is based on a number of tools and methodologies, which are used to form small structures with dimensions in the micrometer scale (one millionth of a meter). This technology is now being utilized to manufacture state of the art MEMS-Based Accelerometers.

16. Null mode accelerometers The acceleration is compensated by a retroaction that keeps the mass into position: null mode • high sensitivity (> 1000 mV/g) • low frequencies (0500 Hz)

17. Piezoelectric accelerometer • The spring-damper parallel is given by a piezoelectric prism, having a small mass and a high stiffness. • Normally the crystal is pre-loaded. • No damping devices are inserted: the damping ratio is very low. High frequency accelerometer High sensitivity accelerometer Mass 43 g Sens.10 pC/(ms-2) Freq. 1-4800 Hz Mass 0.63 g Sens. 0.15 pC/(ms-2) Freq. 1-26000 Hz

18. Piezoelectric accelerometer Once an acceleration is applied and a charge distribution is realized on the crystal, this has to be measured. The crystal has high impedance and so the measurement should be undertaken by a high input impedance voltmeter; in this way a discharge of the crystal would be prevented. Since this is often impossible, a pre-amplifier is connected to the transducer before passing the signal to a common meter. Pre-amplification can be either charge or voltage amplification. Typically charge pre-amplifiers are used or built-in tension amplifiers (located within the transducer or immediately at its output connection). Tension amplification far from the transducer is to be avoided since it is sensitive to the length of the cables connecting the transducer to the pre-amplifier.

19. Piezoelectric accelerometer These are available on the market with different names: ICP (trade mark of PCB piezotronics), IEPE (Bruel & Kjaer) and many others. The commonly used acronym is ICP – Integrated Circuit Piezoelectric.

20. Piezoelectric accelerometer ICP/IEPE amplification needs a power supply.

21. Piezoelectric accelerometer Three axial ICP They are very useful systems, but they can have problems due to temperature sensitivity, related to the presence of the microelectronic pre-amplifier. Mono axial ICP

22. Accelerometers types and models Please notice: piezoelectric transducer are insensible to static and quasi-static excitation due to the crystal self adjustment in time

23. Common usages • Vibration measurements • human comfort evaluations • industrial diagnostics on machinery • civildiagnostics on buildings • Accelerationmeasurement • inertialnavigationsystems • impact detection and measurement • dynamicforce/displacementcharacterization

24. Fixing and precautions

25. Fixing and precautions

26. Fixing and precautions

27. Fixing and precautions