Active Vibration Control

1. Vibration Actuators and SensorsProfessor Mike BrennanInstitute of Sound and Vibration ResearchUniversity of Southampton, UK

2. Active Vibration Control Actuators Sensors Controlled Structure Controller WHY ? • Structures become lighter • Space and weight constraints

3. Vibration actuators and sensors • Actuators • Piezoelectric • Magnetostrictive • Electrodynamic • Hydraulic • Sensors • Piezoelectric • Controllability/Observability • Shaped actuators/sensors (spatial filtering) • Applications

4. Piezoelectric actuators and sensors Converse Piezoelectric effect (actuator) - + A change in dimensions of a material due to the Application of an electric field - + Piezoelectric effect (sensor) An electric field is generated due to a change in dimensions of a material (Curie brothers 1880)

5. Polarisation of a piezoelectric material • Subject a piezoelectric material to a large voltage near the Curie temperature • then the dipoles align dipole • Curie temperature is the temperature above which the material loses its piezoelectric property

6. Piezoelectric actuators and sensors • Piezoceramic (PZT) • Relatively stiff • Large piezoelectric constant • Piezopolymer (PVDF) • Relatively flexible • Large voltage capacity

7. Direct piezoelectric effect (sensor) (element in free-space) + -

8. Indirect piezoelectric effect (actuator) (element in free-space) - + - + Poling axis

9. Conventional use of piezoelectric material in transducers Modes of deformation + + + ceramic ceramic ceramic - - - shear elongation compression piezoelectric capacitance • Charge devices have a low capacitance • (high impedance) and hence require • pre-amp with a very high impedance q Equivalent electrical circuit • Used in accelerometers and force transducers. Generates an electrical • charge proportional to strain • Typical materials are polycystalline materials, e.g. barium titanate and • lead zirconate

10. Practical Accelerometer Designs Compression Type • Advantages • Few Parts / Easy to Fabricate • High Resonant Frequency • Disadvantages • Very high thermal transient • sensitivity • High base strain sensitivity

11. Practical Accelerometer Designs Bending Type • Advantages • Few Parts • Small Size and Low Profile • Low Base Strain Sensitivity • Low Thermal Transient Sensitivity • Disadvantages • Low Resonant Frequency

12. Practical Accelerometer Designs Shear Type • Advantages • Low Thermal Transient Sensitivity • Very Low Base Strain Sensitivity • Small Size • Disadvantages • ???

13. Force Transducer The stress is related to the applied force by and the stress is related to the strain by Therefore the strain is related to the force by As the electrical output is proportional to the strain, and the strain is proportional to the applied force, then the electrical output is proportional To the applied force Principle of Operation Force, F Material of cross-sectional area A and Young’s modulus E

14. Piezoelectric Force Transducer Preload stud Electrical output Piezoelectric element • Can be used in tension and compression • Fragile to moments

15. One-dimensional piezoelectric equations • The piezoelectric equations are electrical mechanical + Piezoelectric material of thickness t Conductive electrodes -

16. Piezoelectric elements as strain sensors Voltage generator C V Charge generator C q • The piezoelectric equations are 3 1

17. Flexural (bending) vibration sensor l w b x which evaluates to If a flexural wavelength is much greater than l, then

18. Longitudinal vibration sensor which evaluates to If a longitudinal wavelength is much greater than l, then l b u

19. 2-dimensional sensor 2 3 PVDF sensor 1 plate Recall for the one-dimensional case (for no applied field) For the two-dimensional case Thus the electrical output is proportional to both S1 and S2

20. Strain or Strain-rate measurement R - + V Piezoelectric sensor connected to a current amplifier measures “strain rate” C - + V Piezoelectric sensor connected to a charge amplifier measures “strain”

21. Piezoelectric actuators Stack Single element Connected electrically in parallel and mechanically in series

22. Coupling an Actuator to a structure The piezoelectric equation is Structure Actuator

23. Coupling an Actuator to a structure force Increasing voltage Max power transfer displacement Structure Actuator

24. Flat piezoelectric actuators High displacement – Low force actuators

25. Some piezoelectric actuator configurations Stacks Fans Curved actuators Bimorphs (benders)

26. Amplified piezoelectric actuators

27. PZT actuators for beam vibration actuators driven out-of-phase – bending vibration induced PZT element beam PZT element actuators driven in-phase – longitudinal vibration induced PZT element beam PZT element

28. PZT actuators for beam vibration Ratio of thicknesses Ratio of stiffnesses Free strain actuators driven out-of-phase – bending vibration induced PZT element beam PZT element M M

29. PZT actuators for beam vibration Ratio of stiffnesses Free strain actuators driven in-phase – longitudinal vibration induced PZT element F F beam PZT element

30. Piezoceramic Elements • The two piezoelectric elements can be excited: • in phase to generate longitudinal vibration • out-of-phase to generate flexural vibration F F

31. Piezoceramic Elements LONGITUDINAL VIBRATION FLEXURAL VIBRATION Works best at high frequencies when the length of the actuator is equal to half a wavelength

32. Excitation of a plate PZT patch plate

33. Controllability and Observability A Example - beam • A sensor positioned at point A will observe modes 1 and 2 but not mode 3 • An actuator positioned at point A can control modes 1 and 2 but not mode 3 Mode 2 Mode 3 Mode 1

34. Shaped piezoelectric film bonded to a beam structure Simply supported Cantilever Mode 1 Mode 2

35. Modal filters

36. Modal filters experimental results Point accelerance

37. Modal filters experimental results Mode 2 filter Mode 1 filter

38. Shunted Piezoelectric Absorber

39. The Smart Ski (ACX.com)

40. The Smart Ski (ACX.com) Piezo patches

41. Fundamental bending mode (215 Hz) Second bending mode (670 Hz)Third bending mode (1252 Hz) The Smart Bat (ACX.com)

42. The Smart Bat (ACX.com)

43. Electro / Magneto -Rheological Fluids What are they ? • micron sized, polarizable particles in oil What do they do? • Newtonian in absence of applied field • develop yield strength when field applied ER fluids respond to electric field MR fluids respond to magnetic field

44. Magneto-Rheological Fluids - Applications Ride Mode Switch MR Fluid Damper Sensor/Controller

45. Magneto-Rheological Fluids - Applications Single Degree of Freedom System - Heavy Duty Vehicle Suspended Seats • off-highway, construction and agricultural vehicles Acceptable motion transmitted • class 8 trucks ("eighteen wheelers") • buses Sensor Seat Controller Controllable shock absorber Spring Off-state Random pattern On-State Ordered pattern Road input

46. Change in Stiffness – shape memory alloys Memory metal is a nickel-titanium alloy This piece has been formed into the letters ICE, heat-treated, and cooled. When the memory metal is pulled apart, it deforms. When placed into hot water, the metal "remembers" its original shape, and again forms the letters ICE.

47. Change in Stiffness – shape memory alloys Soft Stiffness increases With temperature Stiff

48. Change in Stiffness – shape memory alloys • Material whose Young’s modulus changes with temperature Composite panel } Embedded SMA wires • Activating the fibres (by passing a current through them and hence • causing a temperature change) causes local stiffening and hence the • natural frequencies can be shifted to avoid troublesome • excitation frequencies.

49. Active Control of Helicopter Vibrations/Structure-Borne Sound • Active control of rotor • vibrations at about 18 Hz • Active control of gearbox • noise at about 500 Hz

50. Active Control of Structural Response (Westlands, 1989) Application of ACSR to the Westland/Agusta EH101 Helicopter.