PROCEEDINGS of the23rd International Congress on Acoustics
9 to 13 September 2019 in Aachen, GermanyHybrid Mass Damper Using Electromagnetic Resonator:
Application to an Helicopter
Simon CHESNE1; Guillaume INQUIETE2; Paul CRANGA3
1

Univ Lyon, CNRS INSA-Lyon, LaMCoS UMR5259, F-69621, Villeurbanne, France
2

Airbus Helicopters, Marignane airport, 13725 Marignane Cedex, France

3

Airbus Helicopters, Marignane airport, 13725 Marignane Cedex, FranceABSTRACTThis work aims to illustrate how to increase the efficiency of an initial passive damped electromagnetic
resonator by controlling the mass response actively. In aerospace and aeronautical engineering a fail-safe
design for active systems is usually required. Hybrid Mass Dampers (HMD) tend to realize these objectives.
Design rules and limitations of the so-called Hybrid Mass Damper (HMD) are discussed and analyzed. The
main idea is to modify a DVA (Dynamic Vibration Absorber) to use it as an AMD (Active Mass Damper).
Objectives are to take the best of the two technologies and increasing the performance of the passive device
and/or decreasing the control effort of the actuator in comparison with a fully active system. A specificelectromagnetic resonator is then designed and an appropriate control law is proposed. The methodology isillustrated and validated with an application to a helicopter. Theoretical and numerical results are completed
by experimental validation.
Keywords: Mass Damper, Active and passive control.

1. HYBRIDIZATION PRINCIPLE
Consider an initial structure simplified to a one degree of freedom system. This represents the mainstructure to control (Fig.1). We fix a Tuned Masse Damper (TMD) to this structure, optimally tuned
using Den Hartog’s law (1), then the active control force (fa) is introduced betw een the two masses.
Classically, it can be a voice-coil actuator, the mass of the TMD is the moving part of the actuator(usually the magnets). The concept of this dual loop controller, briefly presented in (2), is to combine
two control laws using two different inputs. The two loops are in parallel (Fig 1) and act on the sametransducer. These two loops are presented successively for a better understanding. The first one (inblue in Fig.1) is a proportional-derivative controller fed by the relative displacement between theinertial mass and the main structure. The second one is a proportional controller fed by the absolute
velocity of the main structure (in red in Fig.1).
Figure 2 illustrates the pole locations of this device. In Fig2a, the initial pole of the main structure
can be seen in grey. By adding an optimal passive TMD with equal peak design parameters, one cansee the two new poles and the increasing of damping. In the Fig 2b the two control laws are added
successively, starting with the optimal passive device.
It is known that direct velocity feedback associated with a TMD results in very poor stability
margins (3). The purpose of the first control loop (H1) is to increase these margins. It aims to reduce
the stiffness of the TMD and to add damping. It is shown by the blue root locus in fig 2b. One canobserve that the low frequency pole, linked to the actuator, is moved away from the imaginary axis.
The high frequency mode tends to return to the initial pole of the structure as the TMD is no more
tuned to absorb vibrations. At this point the device behaves like a classical AMD.
The second step consists of applying a direct velocity feedback (red root locus in fig 2b). One cansee that the two poles are moving to opposite directions. The gain of thi s controller appears as a
1

Simon.chesne@insa-lyon.fr
Paul.cranga@airbus.com
3Guillaume.inquiete@airbus.com
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tradeoff between the damping of the high frequency mode and the stability margins as explained if(4). These schemes show that this kind of dual loop controller, when adequately tuned, can in theory,
yield high damping on both resulting modes with adequate stability margins.

TMDδ

H1(s)
+fax2

ck+x1.
x1

H2(s)Main structure
c0k0x0Figure 1 – SDOF of the main structure with an hybrid vibration absorberWith TMD

Im(s)

Im(s)
ω2

ω2

Initial pole
Final polesω1a)ω1Re(s)b)Re(s)Figure 2 – a. Pole location of the passive devices (without and with TMD) b. Root locus of the Hybrid
device by successively adding the H1 control loop (in blue) and the H control loop (in red).

2. INNOVATIVE TRANSDUCER
For this project, a specific dual transducer has been designed and patented. Basically, it consistsof a one mechanical degree of freedom system designed as a T MD.
The moving mass is guided by two set of flexible membranes. Two magnets are fixed at theextremities of the moving mass, contributing to the total mass of the moving part. Each magnet issurrounded by a coil. One magnet and coil is dedicated to the pas sive behavior of the TMD the otherone to the active one. The principle is illustrated in Fig3.a (For better readability, the metal structures
surrounding the magnet-coil systems for guiding the magnetic field lines are not shown on thisscheme). The final prototype can be seen in fig 3.b.
Two parts can be clearly identified. The passive part (represented at the bottom of fig 3a) consistsof a load (tunable resistance) and a system to measure voltage. The load is used to tune the dampingof the TMD. The mechanical damping of the system is very low. To reach the optimal damping value,
one uses the possibility of adapting the load to optimize damping of the coupled system. As theresulting voltage on the load is directly proportional to the relative velocity, this value is used to feed
the first loop of the controller (H1). The active part (in red in fig 3a) uses a current amplifier in orderto obtain a control force proportional to the command.

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iBN

CommandSmTMD

Current amplifier(Passive part)

NV

RS

Voltage measurement:
Relative velocity ߜሶa)Tunableresistance

Dual absorber(Active part)uX2kcb)Figure 3 – a) Schematic diagram of the dual transducer, b) the final prototype

3. FULL SCALE VALIDATION
The final aim of the project is to control a specific vibration mode on an helicopter. This section
aims to illustrate the efficiency of the proposed device on a real structure for which the system has
been designed. For confidentiality reasons, not all the data can be shown. This abstract only gives aninsight of the potential applications. The helicopter used for the tests is shown in Fig 4.

Figure 4 – AH225 used for the tests
The transfer functions between a force sensor linked to the shaker located on the main rotor, andan accelerometer glued near the HMD are plotted in Fig 5. The responses of the passive systems arerepresented in black (helicopter alone), and in red (with the passive TMD). The effective modal massof the mode to control is around 3 tons. In this case, the mass ratio is very low. The passive reduction
is very small (around 2dB). No effects are observable at other frequencies.
Fig 5 shows two different tunings (in blue, continuous and dotted line). The continuous blue line
uses a classical detuning (half of the passive stiffness) and high gain for the “H2 loop”. Two effects
can be observed. The first one is a massive reduction of the vibration of the mode of interest, almost
8 dB in comparison with the passive system. The spectacular results are tarnished by the increase of
the vibration of a lower frequency mode corresponding to the new frequency resonance of the HMD.
A simple way to avoid this phenomena is to slightly change the detuning created by the “H1 loop”.
The second tuning (light blue dotted line) shows that with a bigger detuning, this phenomena isavoided. The amplitude reduction on the mode of interest is around 5dB in comparison with thepassive TMD, but a gain of 7.5dB is also observed on the previous low frequency mode. After themode of interest, a global reduction of 2-3dB is observed. No high frequency spillover happens.
The tuning procedure of the various loops appears as a tradeoff between the modal repartition of
the main structure, the stroke of the actuator and the stability margins. This tradeoff depends on thedesign of the transducer and the modal distribution of the main structure.

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0-45
Without TMD
With TMD
With HMD (tuning 1)
With HMD (tuning 2)

Magnitude (dB)

-50

-55
-10-60

-65
-20

-70

Mode of interest-75-30-80-8551015Frequency (Hz) BBQ

2025Frequency

Figure 5 – Normalized transfer function without TMD, with TMD and with HMD using two
different tunings. (For confidentiality reasons, the frequencies and the shapes of the mode are notgiven).

4. CONCLUSIONSThe study validates an innovative transducer dedicated to a dual loop controller. This transducer
is a Hybrid Mass Damper, combining passive Tuned Mass Damper behavior and actuator capabilities
for active vibration control. Sensing and actuating systems are linked to a single degree of freedom
device constituted of two voice-coil systems, one dedicated to the passive damping and the other one
to the active damping. The proposed control loops provide stable and efficient control. The resulting
damping device is also fail-safe. In comparison to a classical optimal TMD, the damping performanceis increased without adding any mass on the TMD. These aspects are very important for embedded
applications. The proposed hybrid control device has been built at full -scale and successfully tested
on both an academic bench and on an helicopter. Future studies will show that the power consumptionof this kind of device is much lower than a full active system. More details can be found in (4).

REFERENCES
1. DenHartog J, Mechanical Vibrations, 4th ed., Mc Graw-Hill, New York, 1956.
2. Preumont A. and Seto K, Active control of Structures, John Wiley and sons, West Sussex, United
Kingdom, 2008.
3. Chesne S. and Collette C, Experimental validation of fail-safe hybrid mass damper, Journal of Vibrationand Control Vol 24 (19) 2018.
4. Chesné C. et al. Innovative Hybrid Mass Damper for Dual-Loop Controller, Mechanical Systems andSignal Processing, Elsevier, 2019, 115, pp.514 - 523.

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