Department of Aerospace Engineering
Rotorcraft Center of Excellence
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TASK PS 2.3a
Passive, Semi-Active, Active Reduction of
Gearbox Vibration and Noise
Principal Investigators:
George A. Lesieutre, Professor
Edward C. Smith, Professor
tel: (814) 863-0103
tel: (814) 863-0966
email: g-lesieutre@psu.edu
email: ecs@rcoe.psu.edu
Graduate Students:
François LeHen, MS
Joseph T. Szefi, Research Associate
tel: (814) 865-1986
email: fxl132@psu.edu
email: szefi@psu.edu
PSU RCOE Program Review
May 3, 2005
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Background and Technical Barriers
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BACKGROUND
Problem: Irritating
High Frequency
Gearbox Noise
Transmitted to
Fuselage through
Rigid Links
• Helicopter gearboxes transmit irritating
high frequency noise and vibration to
the cabin (500 - 2000 Hz)
• Cabin suspended by struts, with quasistatic loads between -10 kN and +30 kN
• Many active control treatments
proposed, some for retrofit
– Issues: complexity, reliability, BW
• Layered isolators exhibit desirable low
and high frequency behavior
– Passive or semi-active
TECHNICAL BARRIERS TO SOLVE
10
10
10
• At least minimum level of axial stiffness
to ensure flight controls integrity
10
• Must have a low weight penalty
10
• Elastomers must stay in compression
• Must not exceed size constraints
• Must be nearly rigid at low frequencies,
while attenuating high frequency
vibrations (500 - 2000 Hz)
10
10
10
Transmissibility
3
Steel Strut
k = 273 MN/m
2
Vibrations
Vibrations
Transmitted
Transmitted
1
0
Frequency Range
of Interest
-1
Ideally,
Rigid at Low
Frequencies,
Soft at High
Frequencies
-2
-3
Frequency
Range of
Interest
-4
10
1
10
2
Frequency (Hz) 10
3
10
4
Background:
Helicopter Gearbox Design Constraints
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Quasi-static Stiffness Constraint
• From literature:
– GW = 12,000 lbs, axial stiffness of 14 MN/m at 4 foot
locations sufficient to minimize deflections of highspeed transmission shafts
GW = 12,000 lbs
Maximum Mass Constraint
• Sikorsky correspondence
– GW = 12,000 lbs, 50 lbs of added weight for high
frequency vibration control acceptable
• Total Isolator weight < 0.5 % GW (soundproof 2-3%)
Geometry Constraints
• Foot-type gearbox mounting
Typical Stiffness
at Four Feet
kaxial = 14 MN/m
S-76B Main Gearbox Supports
– No suspension struts
– Assumed d ~ 20 cm, h ~ 10 cm
• Strut-type gearbox connections
– Typically strut diameter, d ~ 8 cm
Fatigue Constraints
• Extensive fatigue tests performed to develop design
guidelines for layered elastomeric bridge bearings
• Design guidelines expressed in terms of shear modulus
and shape factor
• Included as fatigue constraint in layered isolator design
20-year-old Elastomer Bridge Bearing in UK
S < 2.00 GS < 12.00 MPa
D < 1.00 GS
S = Static Stress
D = Dynamic Stress
G = Shear Modulus
S = Shape Factor
Background:
Helicopter Gearbox Design Constraints
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• Brennan, Pinnington, Elliot (UK) report that dominant
strut vibration is axial, although lateral component is
significant (‘94)
• Sikorsky developed Active Noise Control system (‘98)
–
–
–
–
Gearbox connected with ‘feet’ type mounting
Inertial force actuators at gearbox / fuselage connection
Sensors inside cabin
In-flight noise reduced 10-20 dB at primary gear-mesh tone
• Gembler, Schweitzer (Eurocopter BK117) developed
smart strut concept (‘98)
STRUT
SHEAR
FORCES
PZT’s
– Piezoelectric ceramics bonded to struts
– Discrete frequency (1.9 kHz) reduced 11 dB
• Sutton, Elliot, Brennan, et al., (UK) used three axial
magnetostrictive actuators on strut (‘97)
STRUT
ACTUATORS
(three)
– 30-40 dB reduction in strut kinetic energy transfer
– Practical for frequency range of 250 - 1250 Hz
• Baz, Pines, (Univ. MD)
– Investigating “active periodic struts”
– Experimental results suggest average transmitted vibration
reduced by factor of 10 in high frequency range
Base
Structure
Piezoelectric
Insert
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Background:
Layered Isolator Behavior in Compression
•
•
A cell is an elastomer layer plus
metal layer
• Analogous to multi-stage
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First 4 Axisymmetric Mode Shapes of a 3-Celled Isolator
Predicted high-frequency stopbands validated experimentally
for different numbers of cells
Stop Band
(Szefi, Smith, Lesieutre, SDM 2001)
•
•
Experiments suggest that
damping not essential
Mode 1
Mode 2
3 or 4 cells needed in practice
Experimental Transmissibilities
1.E+02
1.E+01
1.E+00
1 Cell
1.E-01
2 Cells
1.E-02
1.E-03
3 Cells
1.E-04
4 Cells
Noise Floor
1.E-05
10
100
1000
Frequency
Frequency (Hz)
(Hz)
10000
Mode 3
Mode 4
Beginning of
Stop Band
End of
Stop Band
4 - Celled Isolator Transmissibility
1.E+02
1.E+01
1.E+00
1.E-01
1.E-02
1.E-03
1.E-04
1.E-05
1.E-06
1.E-07
1.E-08
1.E-09
1.E-10
1.E-11
Experimental
Noise Floor
Analytical
Approx.
10
100
1000
Frequency (Hz)
Frequency
(Hz)
10000
PENNSTATE
Background:
Passive Performance Limits of Layered Isolators
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• Optimization routine was used to study performance limits of passive layered isolators
• Passive layered isolators cannot always adequately attenuate vibration at lower
frequencies (~ 500 Hz ) given stiffness and mass constraints
• Embedded fluidic amplifiers effectively amplify inertia between layers
• Fluid-filled mounts provide an efficient means of inertial amplification
Metal Layer
of Layered
Isolator
Embedded
Inertial
Amplifier
Transmissibility
1
2
10
1e2
2
3
Stop Band Begins
Too High (~ 700 Hz)
0
Rigid Link
Tuned
Mass on
Lever
Arm
Elastomer
Layer
Stiffness
10
1e0
-2
10
1e-2
-4
Minimum Desired
Attenuation Level
Achieved in Target
Frequency Range
( < 0.01)
10
1e-4
-6
10
1e-6
Vibration
Input
-8
10
1e-8
1
10
10
Tuned Absorber
Frequencies
10
2
100
3
10
1,000
Frequency (Hz)
4
10
10,000
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Background:
Layered Isolator with Fluidic Amplification
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• Need to reduce isolator mass
• Solution: Embedded Fluidic Motion Amplifiers
• Mass amplification due to the presence of inner cylinders
• Lighter than mechanical vibration absorbers
• Fluid acts as tuned mass of a Vibration Absorber
 Mass Reduced
 Frequencies of Interest
in the Stop Band
 More Compact Design
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Background:
Experimental Results for Layered Fluidic Isolator
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Experimental Comparison
Larger inner
diameters
Fluid
Tuned
Absorber
Frequencies
Config. 1, Fluid
Minimum
Desired
Attenuation
Level in Target
Frequency
Range (~0.01 )
Smaller
inner
diameters
Config. 2, Fluid
Specimen 2, No-Fluid
• Experimental results are compared for
• Specimen 2 with no fluid, and Configurations 1 & 2 with fluid
• Experimental stop band beginning frequency 450 Hz
• Tuned absorber frequencies around 600, 710, and 1050 Hz
• As inner diameters increase, beginning frequencies decrease, absorber frequencies decrease
• However, higher amplification ratios result in reduced attenuation within the stop band
Background:
High Force Tension-Compression Testing
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• In practice, layered isolators would be subject to high axial loads
• Compression and tension
• May be precompressed to accommodate tensile loading
• Experimental results suggest that
stop band characteristics are not
significantly affected by the
presence of high axial loads
Agusta A109E
• Installed layered isolators should be
stiff enough to avoid driveshaft
misalignment
• Quasi-static stiffness tests reveal
that shape factor stiffness
prediction method is accurate
Low Frequency Isolators Widely
Used in Machinery
McGuire, AHS 2003
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Objectives, Issues
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OBJECTIVES
• Investigate the use of periodically-layered mounts for high frequency gearbox isolation
(500 - 2000 Hz)
– Axisymmetric model to capture transmission of longitudinal waves through isolators
• Investigate passive, semi-active, and active enhancements to improve isolator performance
– Embedded fluidic amplifiers lower stop-band range
– Semi-active tunable fluid ports to track disturbance frequencies?
– Active piezoelectric stack for improved tonal attenuation in stop band?
• Validate proposed isolator designs with experiment
– Pre-compression, quasi-static axial stiffness
– Provide rotorcraft industry with experimentally validated design tools
ISSUES
• Can active enhancement to passive layered fluidic mount improve performance
with minimal weight penalty?
– Electroactive materials for increased stop band attenuation
• Implementation in realistic application
– Maintain compression
– Lateral shear and moments
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Approach, Accomplishments
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YEAR 1
•
•
Develop axisymmetric model to predict layered isolator behavior
Determine isolator design constraints
–
•
Determine performance limits of passive layered isolators
–
•
Quasi-static stiffness, mass, geometry, fatigue
Use design optimization with approximate axisymmetric model
Evaluate use of passive or semi-active enhancements to improve performance
YEAR 2
•
Develop enhanced model of layered isolators to include additional components
–
Embedded fluid elements
•
Validate model via fab & testing of preliminary fluidic layered specimen
•
•
YEAR 3
Complete testing of preliminary fluidic layered specimen
Refine layered isolator design, construct and test new compact device
–
–
•
Investigate issues associated with high-force environment
–
•
Accommodate realistic constraints for gearbox isolation
Reduce weight and height of preliminary specimen by half
Effects of precompression; validate quasi-static axial stiffness
Continue to investigate semi-active or active enhancements
PENNSTATE
Approach, Accomplishments (continued)
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•
YEAR 4
Experimentally evaluate compact layered, fluidic passive isolator
–
–
–
•
Met performance goals for isolation in specified frequency range
Met realistic constraints of application
Half the weight and height of preliminary specimen
Develop concept for active enhancement
•
Use piezoelectric stack to focus on residual tonal disturbances
FEEDBACK FROM YEAR 4
•
“The task made good progress and the interaction with others is good. However, the
payoff and real applications are somehow questionable. Work with Lord and using
piezoelectric stack are commendable.”
ACTION TAKEN
•
RITA project with Bell & Lord initiated, with initial focus on passive mount
•
Active enhancement using piezoelectric stack developed further
•
EXPECTED RESULTS AND PRODUCTS
Development of a passive layered, fluidic broadband isolator with active tonal
enhancement to greatly reduce transmission of high frequency gearbox
vibrations to helicopter cabins
Background:
Gearbox Vibration Disturbance is Highly Tonal
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•
High-frequency noise with multiple tones
(500-2000 Hz)
Active Isolator Control:
Embedded Piezoelectric Stack
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• Passive Limitations
Sometimes impossible to satisfy: - Mass & Size Constraints
- Frequency Range of Interest
- Attenuation Level
 Solution: Active Enhancement
- Replace the last cylinder link
with a piezo stack actuator
- No static load
- Vibrations already attenuated
- Focus on the main
tonal disturbance frequencies
- Develop a suitable control scheme
- Validate with experiments
- Consider power and mass penalties
Piezoelectric actuator
150 V max, 55 x 20 mm
Active Control Options
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•
Features of this problem
• High frequency range (500-2000 Hz)
• High amplitude, periodic components
•
•
Tone frequencies may vary with RPM
Feedforward control
• Measurable disturbance reference input with sufficient
propagation delay to output
• Models of input-output and control-output
•
•
Slowly-changing
Feedback control
• Non-measurable excitation or rapid propagation
• Model of control-output sensor
• Computational delay & model accuracy limit bandwidth
•
Adaptive feedforward control
of high-amplitude periodic components
Determining Piezo Control Signal
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•
Store measured model of the isolator
(2 FRFs)
FRF1 = Syst_output / Syst_input
FRF2 = Syst_output / Piezo_input
•
During each control cycle (100 ms)
•
•
FFT of Syst_Input to select the main disturbance freqs
For each disturbance (frequency)
•
Measure amplitude & phase
•
Determine ideal Piezo_Input control
Apiezo_out 
AFRF1 (freq)
 Asyst_input
AFRF2 (freq)
piezo_out  FRF1 (freq)- FRF2 (freq)  syst_input  180
Amplitude and Phase Adaptation
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

FFT of Syst_Output at each control cycle
For each disturbance (frequency)
–
–
Amplitude & Phase
Modify model (stored FRFs) to improve control
 Adapt FRF2 (Syst_output / Piezo_input)
A FRF2_new (freq) 
(A sys_input  A FRF1 (freq) - A sys_output )
A piezo_out
 FRF2_new (freq)   FRF2 (freq)   adapted
adapted is found via successive increments / decrements that minimize output
Active Control Scheme
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•
Features
– Fast (one cycle every 100ms)
– Simultaneous control of multiple
disturbance tones
– Adaptive
•
•
Based on experimental models, measured
performance
Tracks changing freqs, models
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Experimental Layout
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•
Controller provides:
– Continuous multi-channel data acquisition at 12500 Hz
– Continuous data output at 12500 Hz
– One control cycle every 100 ms


Data acquisition during 80 ms
Processing during 20 ms
Fluidic Layered Isolator
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–
–
–
–
2 layers for simplicity
1 fluidic layer and actuator
13 cm overall length (but design not optimized)
2.5 kg (including actuator & fluid)
Passive Validation
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Experimental Data
vs.
Theoretical 3-D Model
Active Validation – With Fluid
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•
Very good coherence
• Optimum stored models
•
For each tone (600 – 2000 Hz)
• 40 dB additional reduction
• Fast convergence to steady state
(a few seconds)
•
Low voltage needed for realistic
input levels
• About 2.5 V for 10 g acceleration
• Fluid amplifies by a factor of 35
Active Validation – Effective Transmissibility
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•
3 Tonal disturbances (10 g each)
+ white noise input
•
Control of 3 disturbances
•
Average 36 dB
additional active reduction
Comparison to Other Solutions
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•
Active Layered Isolator
Stop Band over 600-2000 Hz
Passive Reduction of 40 dB
Active + Passive Reduction up to 80 dB
•
Active Periodic Strut
(Baz, Asiri, Pines)
Stop Band over 600-2000 Hz
Passive Reduction of 28 dB
Active + Passive Reduction of 40 dB
- Passive Layered Isolator
performs as well as
Active Periodic Strut
- Active Layered Isolator
achieves further reductions
at main disturbance tones
Note: This isolator has only 2 layers; 3 layers would provide even better results.
Conclusions
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• Periodically-layered isolators offer a unique solution to the problem of highfrequency gearbox vibration
• Passively provide attenuation (300-1000) over broad frequency range
• Add less than 0.5 % GW in mass
• Accommodates axial precompression
• Fluidic amplification lowers the stop-band start frequency and improves control
• Provide broadband passive attenuation over frequency range (500–2000 Hz)
• Does not compromise axial stiffness
• Active enhancement improves performance at main disturbance tones
• Piezo-stack cancels passively-attenuated residual vibration
• Adaptive feed-forward control tracks frequency and model changes
•
Experiments validated adaptive feedforward control approach
• 40 dB passive reduction in range 600–2000 Hz
• 40 dB additional active reduction at each of multiple tones
•
Power required: a few W per controlled tone (realistic input levels)
•
Isolator Size: 2.5 kg and 11 cm length per isolator
• Lightweight controller & amplifier available
• About 4 kg and 18 cm for similar performance with pure passive approach
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Accomplishments
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Publications
• 2001 AIAA SDM paper - Developed axisymmetric Ritz-based model of layered periodicallylayered isolators in compression
• 2002 Journal of Sound and Vibration paper - Layered Isolators
• 2003 AIAA SDM paper, AHS Forum paper - Preliminary fluidic layered specimens
• 2004 AIAA SDM paper - Design and testing of compact, lightweight fluidic layered specimen
• 2005 AIAA SDM paper - Design & testing of fluidic layered specimen with active augmentation
Planned 2005 Accomplishments
RCOE Tasks
• Investigate best options for active fluidic layered isolator
• Lightweight powering and control
• Best combination of passive and active approaches (number of layers, fluidic layers)
• Validate isolator performance under realistic dynamic and quasi-static loading conditions
• Axial and lateral forces, and moments
Invercon Tasks (RITA project with Bell, Lord)
• Coordinate with Lord and Bell to pursue design of commercially-viable mount
• Determine detailed design constraints for Bell Model 427 helicopter
• Design and fabricate isolators in coordination with Lord
• Full scale testing at Bell or Lord facility, flight testing
• Possible future feature on Bell Model 429
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Plans for 2001 - 2005
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Completed
Tasks
STAGE ONE
Axisymmetric Isolator Model
Experimentally Validate
STAGE TWO
Characterize Strut Noise
Transmission Problem
Investigate Semi-Active or
Active Configurations
Recognized Use of Embedded
Fluid Elements
STAGE THREE
Detailed Isolator Model With
Embedded Fluid Elements
Experimental Validation
Refined Isolator Design
Experimental Validation
Prestrain, Stiffness Invest.
Semi-active, Active Mount
Control Architecture Design
Specimen Fabrication
Experimental Validation
INVERCON
Design and Fab. Of Specimen
Full Scale Testing
Short Term
2001
2002
Long Term
2003
2004
2005
Technology Transfer
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Technology Transfer and Leveraging
•
•
Worked with UTRC for 3 years developing layered isolator concept
Working with Lord on manufacturing of layered fluidic devices
2004 RITA Project (Invercon, Lord, Bell)
•
•
•
•
•
•
Invercon is RCOE spinoff company
•
Fosters technology transfer between RCOE and rotorcraft industry
2004 RITA grant to implement layered isolators in Bell Model 427 helicopter
Invercon heading design of preliminary specimens
Bell providing
•
•
Transmission mounting constraints to aid design
Interior noise spectrum of Model 427
Lord to fabricate and test preliminary specimens
Bell to provide full-scale transmission for experimental validation of layered mount
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Simulation of Active Enhancement
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•
1-D Model
– Consider measurement accuracy
•
If control can achieve:
Between 0.25 and 1 degree
error in phase
Between 0.1% and 1%
error in amplitude
•
Simulation predicts
33 – 47 dB reduction
at each controlled tone
• Consistent with experiments
Potential Active
Enhancement
by the control
algorithm
Comparison to Other Solutions
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
Active Layered Isolator


Stop Band over 600-2000 Hz
Passive Reduction of 40 dB
Active + Passive Reduction up to 80 dB

Eurocopter & EADS Active Strut

No Stop Band
No Passive Reduction
Active 11 dB reduction at 1900 Hz
(gearmeshing frequency for BK117)



Active Layered Isolator Advantages
- Passive and active reduction
- Multiple tones controlled
- Higher reduction
- No static loads carried by actuator
- Low power required as the force is already passively reduced
Recommendations – Active Enhancement
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•
•
•
Address additional tones
Improve adaptation to increase convergence for
imperfect models
Additional features
• Pre-select tones for each helicopter
• Automatic locking to known disturbance frequencies