Vibration Actuators and Sensors Professor Mike Brennan Institute of Sound and Vibration Research University of Southampton, UK
Dec 22, 2015
Vibration Actuators and Sensors
Professor Mike Brennan
Institute of Sound and Vibration ResearchUniversity of Southampton, UK
Active Vibration ControlWHY ?
• Structures become lighter
• Space and weight constraints
Actuators Sensors
ControlledStructure
Controller
Vibration actuators and sensors
• Actuators Piezoelectric Magnetostrictive Electrodynamic Hydraulic
• Sensors Piezoelectric
• Controllability/Observability
• Shaped actuators/sensors (spatial filtering)
• Applications
Piezoelectric actuators and sensors
Piezoelectric effect(sensor)
An electric field is generated due to a change in dimensions of a material(Curie brothers 1880)
+
-
-
+
Converse Piezoelectric effect (actuator)
A change in dimensions ofa material due to theApplication of an electric field
Polarisation of a piezoelectric material
• Subject a piezoelectric material to a large voltage near the Curie temperature then the dipoles align
• Curie temperature is the temperature above which the material loses its piezoelectric property
dipole
Piezoelectric actuators and sensors
Property PZT (PC5H) type VI PVDF
Curie temperature (°C)
Longitudinal Young’s modulus (Nm-2)
Piezoelectric constant – d31 (mV-1)
Max E-field (Vm-1)
212
93 10959.5 10
100
12212 10 1223 10
60.4 10 640 10
• Piezoceramic (PZT) • Relatively stiff• Large piezoelectric constant
• Piezopolymer (PVDF) • Relatively flexible• Large voltage capacity
Direct piezoelectric effect (sensor) (element in free-space)
+
-
FA
Stress ( )T F A
33Generated field ( )E g T
33Charge ( )q d F
Indirect piezoelectric effect (actuator) (element in free-space)
L
H
W
+
-
L L
H HW W
+
-
H H
L LW WP
olin
g ax
is
31
Ld E
L
32
Wd E
W
33strain ( ) V
S dH
Electric field ( ) V
EH
Conventional use of piezoelectric material in transducers
• 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
Modes of deformation
ceramic
+
-
compression
+
-
shear
ceramic
+
-
elongation
ceramic
qpiezoelectric capacitance
Equivalent electrical circuit
• Charge devices have a low capacitance (high impedance) and hence require pre-amp with a very high impedance
Practical Accelerometer Designs
Advantages– Few Parts / Easy to Fabricate– High Resonant Frequency
Compression Type
Disadvantages– Very high thermal transient
sensitivity– High base strain sensitivity
Practical Accelerometer Designs
Advantages– Few Parts– Small Size and Low Profile– Low Base Strain Sensitivity– Low Thermal Transient Sensitivity
Bending Type
Disadvantages– Low Resonant Frequency
Practical Accelerometer Designs
Advantages– Low Thermal Transient Sensitivity– Very Low Base Strain Sensitivity– Small Size
Disadvantages– ???
Shear Type
Force Transducer
Principle of Operation
Force, F
Material of cross-sectional area A and Young’s modulus E
The stress is related to the applied force by
F
A
and the stress is related to the strain by
ES
Therefore the strain is related to the force by F
SEA
As the electrical output is proportional to the strain, and the strain is proportional to the applied force, then the electrical output is proportionalTo the applied force
Piezoelectric Force Transducer
Preload stud
Piezoelectric element
• Can be used in tension and compression
• Fragile to moments
Electrical output
One-dimensional piezoelectric equations
Conductiveelectrodes
Piezoelectricmaterial of thickness t
• The piezoelectric equations are
3 31 3 33 3D d T E
113 3 33 3ES S T d E
mechanical electrical
strainS stressT
electric fieldE V t piezoelectric constantd
1 1 modulus of elasticityES Y
permittivity
dielectric displacment (charge area)D
+
-
Piezoelectric elements as strain sensors
• The piezoelectric equations are
3 31 1 33 3D d T E
111 3 31 3ES S T d E
1
3
q cV
is the capacitance of the sensorc
is the voltage generatedV
is the charge generatedq
V
CVoltage generator
C
q
Charge generator
Flexural (bending) vibration sensor
l
31 10
Aq d T dA
b
which evaluates to
31 ( ) (0)p pq d w Y b w l w
pwbw
If a flexural wavelength is much greater than l, then
31p
pb
wqd Y bl
S w
x
w
Longitudinal vibration sensor
l
31 10
Aq d T dA
b
which evaluates to
31 ( ) (0)pq d Y b u l u
If a longitudinal wavelength is much greater than l, then
31 p
qd Y bl
S
u
2-dimensional sensor
plate
PVDF sensor
1
3 2
Recall for the one-dimensional case (for no applied field)
3 31 1 31 1 1D d T d Y S
For the two-dimensional case
31 2 32 1 13 32 1 31
2 22
1
1
1D dY S d Yd d S
1 2where and are Poisson's ratios
Thus the electrical output is proportional to both S1 and S2
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”
Piezoelectric actuators
h
hV
Single element
33Free strain, h V
dh h
Connected electrically in parallel and mechanically in series
33For a stack of disks, h V
n n dh h
Stack
h
h
Coupling an Actuator to a structure
Actuator
Structure
ak
sk
sx
bF
sF
The piezoelectric equation is
33a
aa a
T VS d
Y l
is the actuator straina a aS x l is the actuator stressaT is the actuator Young's modulusaY is the actuator lengthal is the applied voltageV
Set 0 to get the blocked forceaS
Now and b a aa a
a a
F Y AT k
A l
where actuator cross-sectional area
and actuator longitudinal stiffnessa
a
A
k
33So b aF d k V
Coupling an Actuator to a structure
Actuator
Structure
ak
sk
sx
bF
sF
1Now
1s b
a
s
F Fkk
33So 1
as
a
s
d kF V
kk
33and 1
ss
a
dx V
kk
displacement
forc
e
Increasing voltagebF
( free)sx
Max power transfer
a sk k
If then will be smalls a sk k x
Flat piezoelectric actuators
High displacement – Low force actuators
Some piezoelectric actuator configurations
Fans
Bimorphs (benders)
Stacks
Curved actuators
Amplified piezoelectric actuators
PZT actuators for beam vibration
beam
PZT element
PZT element
actuators driven out-of-phase – bending vibration induced
beam
PZT element
PZT element
actuators driven in-phase – longitudinal vibration induced
PZT actuators for beam vibration
beam
PZT element
PZT element
actuators driven out-of-phase – bending vibration induced
M M
2
2
1
6 12 8b b bt Y b T
MT T
p
b
tT
t b b b
p p p
Y b t
Y b t 31
p
Vd
t
Ratio of thicknesses
Ratio of stiffnesses
Free strain
PZT actuators for beam vibration
2
6b b bt Y b
F
b b b
p p p
Y b t
Y b t 31
p
Vd
t
Ratio of stiffnesses
Free strain
beam
PZT element
PZT element
actuators driven in-phase – longitudinal vibration induced
FF
Piezoceramic Elements
The two piezoelectric elements can be excited:
• in phase to generate longitudinal vibration• out-of-phase to generate flexural vibration FF
LONGITUDINAL VIBRATION
Piezoceramic Elements
FLEXURAL VIBRATION
Works best at high frequencies when the length of the actuator is equal to half a wavelength
Excitation of a plate
plate
PZT patch
Controllability and Observability
Example - beam
Mode 1 Mode 2 Mode 3
A
• 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
Shaped piezoelectric film bonded to a beam structure
Simply supportedCantilever
Mode 1
Mode 2
Modal filters
Modal filters experimental results
Point accelerance
Modal filters experimental results
Mode 1 filter Mode 2 filter
Shunted Piezoelectric Absorber
1n LC
The Smart Ski (ACX.com)
The Smart Ski (ACX.com)
Piezo patches
The Smart Bat (ACX.com)
Second bending mode (670 Hz)
Third bending mode (1252 Hz)
Fundamental bending mode (215 Hz)
The Smart Bat (ACX.com)
Electro / Magneto -Rheological Fluids
• micron sized, polarizable particles in oil
What do they do?
• Newtonian in absence of applied field
• develop yield strength when field applied
What are they ?
ER fluids respond to electric field
MR fluids respond to magnetic field
Magneto-Rheological Fluids - Applications
Ride Mode Switch
MR Fluid Damper
Sensor/Controller
Magneto-Rheological Fluids - Applications
Seat
Sensor
Controller
SpringControllable shock absorber
Road input
Acceptable motion transmitted
Off-stateRandompattern
On-State Ordered pattern
Single Degree of Freedom System -
Heavy Duty Vehicle Suspended Seats
• off-highway, construction and agricultural vehicles • class 8 trucks ("eighteen wheelers")• buses
Change in Stiffness – shape memory alloys
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.
Memory metal is a nickel-titanium alloy
This piece has been formed into the letters ICE, heat-treated, and cooled.
Change in Stiffness – shape memory alloys
Soft
Stiff
Stiffness increasesWith temperature
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.
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
Application of ACSR to the Westland/Agusta EH101 Helicopter.
Active Control of Structural Response (Westlands, 1989)
Active Control of Rotor Vibration
rotor
fuselage
Hydraulic actuators
• Active control at rotor blade passing frequency at about 18 Hz + harmonics
• Feedforward control
ACSRACSR - Actuator Installation for Production EH101
•sa
Steel downtube
CompositeCompliantElement
TitaniumLug End
ACSR Actuator
Hydraulic Supply
Main GearboxInstallation
Fwd
Support Strut/ACSR ActuatorAssembly
Magnetostrictive actuators
polepiece
polepiece
magnet
Solenoidcoil
Terfenol-D rod
Terfenol-D
Ter – Terbium
Fe – Iron
Nol – Naval Ordinance Lab
D – Dysprosium
• Needs to be pre-stressed for good operation• Low voltages required• Similar performance to PZT
Active Control of Gearbox Noise
rotor
fuselage
• Active control at gear meshing frequency at about 500 Hz + harmonics
• Feedforward control
magnetostrictive actuators
Active Control of Gearbox Noise
Kinetic energy of receiving blockmeasured using 6 accelerometers
Without control
With controlReal-time control
Active Control of Aircraft Noise
Original Equipment- 4 Tuned Vibration Absorbers per engine
Actuators 2 per engine
Active Control of Aircraft Noise (Lord Corporation)
Controller 1 in cargo bay 12”x8”x3.5” 5 lbs
Amplifier 1 in cargo bay 11”x18”x3.5” 17 lbs
Actuators 4 2 on each yoke 4.5”diax5” 15 lbs
Microphones 8 behind trim incorporated into actuator harness
Wire Harness from cockpit, overhead through cabin, to pylon 22 lbs
Active Control of Aircraft Noise
NVX™OFF
NVX™ON
Attenuation is up to 8dBC
SPL (dBC)
Active Control of Aircraft Noise
NVX Systems reduce noise by reducing vibration
Controller On Controller Off
0 50 100 150 200 250 -80
-60
-40
-20
0
Frequency (Hz)
Typical Vibration Reduction with NVX DC9 Ground Test @ 75% Power
~6.3 dB
~25.5 dB
N1 N2
Data measured on pylon
Active magnetic bearings (SKF)
bearing sensors
controller
Active Vibration isolation demonstration
Active Vibration isolation demonstration
Concluding Remarks
• Actuators and Sensors are required for all Active Control Systems:
• Actuators used•Hydraulic•Piezoelectric (PZT)•Electrodynamic•Magnetostrictive
• Sensors used•Accelerometers•Force gauges•PVDF•PZT
References
• C.R. FULLER, S.J. ELLIOTT and P.A. NELSON 1996. Active Control of Vibration. Academic Press
• P.A. NELSON and S.J. ELLIOTT 1992. Active Control of Sound. Academic Press
• C.H. HANSEN and S.D. SNYDER 1997 Active Control of Noise and Vibration. E & F.N. Spon
• R.L. CLARK, W.R. SAUNDERS and G.P. GIBBS 1998. Adaptive Structures. Wiley Interscience
• A.V. SRINIVASAN and D. MICHAEL McFARLAND 2001. Smart Structures. Cambridge University Press