VAMOSI Steven B., KHOMUTOV Kostyantyn, FESZTY Daniel, and NITZSCHE Fred for

TITLE: SMART ACTIVE VIBRATION SUPRESSION SYSTEMS

CROSS-REFERENCE TO RELATED APPLICATIONS This application claims the priority right of prior U.S. patent application Ser. No 61/680,639 filed on August 7, 2012 by applicants herein.

FEDERALLY SPONSORED RESEARCH Not Applicable

SEQUENCE LISTING OR PROGRAM Not Applicable

TECHNICAL FIELD The described embodiments relate to the field of vibration reduction technology and more particularly to a physical structure and mechanisms for adaptively changing its stiffness, damping and mass. The structure is typically attached to fixed and rotating blade systems, such as helicopter blades, aircraft and ship propeller blades, wind turbine blades, engines and vehicle suspensions, power tools and home appliances and similar vibration systems. BACKGROUND TO THE INVENTION ' Vibration impacts the performance and service life of mechanical devices, and the health of any present operators can also be adversely affected. Noise and vibration on rotary-wing vehicles (i.e., helicopters) occurs as a result of the unique aerodynamic environment over the rotating blades. The unique aerodynamic environment of the flow-field over a helicopter rotor featuring transonic flow on the advancing blade, dynamic stall on the retreating blade and a

characteristic helical tip vortex emanating from each blade called the Blade Vortex Interaction (BVI). First, the advancing blades operate at a low angle of attack but high airflow speed (close to the speed of sound), leading to shock-induced boundary layer separation and therefore increased drag as well as increased power demand. Second, dynamic stall is known to be a limiting factor of the forward flight speed of a helicopter due to excessive torsion and vibratory loads on the rotor blades and the pitch link (via which the pilot controls the overall rotor aerodynamics). Third, vortices impact the blades of the main rotor, the tail rotor or the fuselage and generate the characteristic "slapping" noise of a helicopter, as well as creating

aerodynamic perturbations on the blades that result in additional aerodynamic loading and vibration.

The periodic occurrence of these conditions over each revolution leads to further vibration and noise. Thus, excessive vibration and noise on helicopters arise due to blade aerodynamics. To control these phenomena, control systems have been proposed that are capable of affecting the aerodynamics of the blades. Conventional passive vibration control techniques are capable of reducing vibration, but only in narrow operational modes, without the ability to adapt to a different flight environment and such systems also generate significant weight constraints for helicopter design.

Conventional active vibration control systems are capable of reducing vibration in a wide range of frequencies with improved vibration reduction levels. Typical active vibration reduction strategies include: (i) higher-harmonic control, (ii) individual blade control via an actively controlled flap, (iii) active twist rotor, (iv) hydraulic/mechanical pitch link actuators and (v) active rotor of structural response. These systems can be effective, but some require high voltage to operate, others are heavy and complex, and all systems constantly overcome large ^• aerodynamic forces from one to multiple times per revolution (i.e., controlling the blade aerodynamics by a resultant pitch angle corresponding to about one degree change within one revolution). Such operations shorten service life, the constant power supply creates potential for instability of the system, and requires large force, and high actuator stroke. Additionally, the active rotor of structural response strategy effectively counteracts vibration transferred to the airframe but leaves the rotor of a helicopter exposed to high fatigue loads and reduced service life.

The newer approach of a structural component - a smart spring - having active, adaptive varying stiffness characteristics associated with dynamic systems to which the smart spring is attached, is capable of reducing vibration. As the smart spring changes its effective stiffness, it alters the boundary conditions of the system thus altering its effective stiffness, providing a parametric excitation of the elastic degrees of freedom of the system. As the effective stiffness of the system is changed, energy is redistributed in the spectrum of vibration and displaced from the lower, more distractive and less desirable transmissibility to higher harmonics and lower transmissibility, and less distractive frequencies. Studies of active cyclic helicopter blade root stiffness variations leading to 5-7% of a helicopter blade stiffness change have shown as much as 60% to 90% reductions in vibratory loads.

One of the types of the smart spring has been proposed - for example, in U.S. patent 5,973,440 (1999) to Nitzsche et al. Although the smart spring is capable of reducing vibration, it is unable to stay stiff in the incidence of loss of power to the piezo mechanism. Thus, there is no fail safe mode, making the smart spring a high-risk device and less attractive to end users. The direct piezo engaging mechanism is in a direct load path thereby limiting its friction force, increasing wear-and-tear and endangers breaking its piezo-ceramic components. A proposed Active Pitch Link (APL) in U.S. patent 8,210,469 B2 (2012) to Nitzsche et al., is capable of varying its stiffness, reducing vibration, but its large size limits the number of rotor hubs the APL can be installed in. Its asymmetric geometry generates significant pulling force and may jam the APL and not allow it to vary the stiffness. ^' There is therefore a need to improve active vibration reduction systems. Thus, several advantages of one or more aspects of active vibration reduction systems are to (a) be capable of changing stiffness at high frequency, (b) withstand high axial, transverse and centrifugal loading, (c) be equipped with a fail-safe option, and (d) have variant ability of achieving stiffness between two effective stiffness states of the device. These and other advantages of one or more aspects will become apparent from a consideration of the ensuing description and accompanying drawings.

SUMMARY OF THE INVENTION Certain exemplary embodiments may provide a physical structure characterized by having two effective stiffness states that can be actively switched in between to control vibration, comprising: a housing having an internal conical guiding sleeve; an axially expansible mechanism mounted inside the housing; a movable inner rod joined with an elastic element of different stiffness coefficient mounted to the housing; a clutch having a plurality of slots operable to engage with a movable inner rod, the clutch being mounted inside the conical guiding channel of the housing, the slots of the clutch supporting expansion and compression around the inner rod when moving within the clutch for; and inner biasing means for engaging the clutch with the inner rod as the expansible mechanism urging the clutch within the conical guiding sleeve whereby changing stiffness of the structure. BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described with reference to the drawings in which:

Figs. 1A and IB illustrate schematic representations of a physical structure and clutch mechanism having an outer spring placement for adaptively changing the stiffness of the system according to an embodiment; Fig. 2 illustrates a schematic representation of a physical structure and clutch mechanism having an inner spring placement for adaptively changing the stiffness of the system according to an embodiment;

Fig. 3 illustrates schematic representations of a helicopter with an integrated physical structure according to an embodiment;

Fig. 4 illustrates stiffness modulation of a physical structure;

Fig. 5 illustrates closed-loop feedback block diagram of a unity-gain system;

Fig. 6 illustrates schematic representations of a scaled helicopter blade with an integrated physical structure according to an embodiment; Fig. 7 illustrates a graphical representation of a test of a physical structure for vibration reduction according to an embodiment;

Fig. 8 illustrates schematic representation of a physical structure integrated into a wind turbine pitching mechanism according to an embodiment;

Fig. 9 illustrates a schematic representation of a home appliance with an integrated physical structure according to an embodiment;

Fig. 10 illustrates a schematic representation of a power tool with an integrated physical structure according to an embodiment; and

Fig. 11 illustrates a schematic representation of a vehicle with an integrated physical structure into engine mounts and suspension according to an embodiment. LIST OF REFERENCE NUMERALS OF THE DRAWINGS

10 elastic element 20 inner solid rod

30 physical structure 40 inner springs 50 conical guiding sleeve 60 clutch slots

70 housing 80 clutch

90 axially expansible mechanism 100 solid bottom floor

110 internal spring 120 hollow inner rod

210 electrical slip rings 220 rotor hub

230 helicopter 240 control module

310 scaled helicopter blade

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Figs. 1A and IB illustrate operating modes of active vibration suppression systems embodied as a physical structure 30. The structure 30 actively switches between stiffness of a solid state (Kl) and a soft state (K2), where K is a stiffness coefficient of solid or soft states of the structure. The solid state (Kl) is defined by solid pitch link stiffness and includes the stiffness of a housing 70 and an inner solid rod 20, which are connected by a grip, clasp, clamp or clutch 80. The inner solid rod 20 has a circular cross-section. However the inner solid rod can have different polygon based cross-sections such as triangle, square, rectangular, hexagon, octagon, etc. The soft state (K2) is defined by the stiffness of an elastic element 10. The elastic element can have different material composition and shape (e.g. rubber bushing, metal flexure, metal rod, spring, etc.) with a different stiffness coefficient than Kl. The K2 state is rotor-specific and can be selected to target various harmonics of vibration.

The structure 30 operates via an enclosed electric axially expansible mechanism 90 located on the inside of the housing 70 and is fixed to a solid bottom floor 100. The axially expansible mechanism 90 is an annular stack of multiple layers of ceramic material capable of producing piezoelectric effect, which is defined as a change in the mechanical state of an actuator as a result of an electrical change of state. However, the axially expansible mechanism can have a different shape of the stack of multiple layers of ceramic material capable of producing piezoelectric effect such as plurality of stacks of squares, rectangles, circles, etc. When electricity is applied (source not shown) to the actuator 90, it elongates to push onto the clutch 80 to disengage it from the inner solid rod 20 by moving the clutch 80 out of a conical guiding sleeve 50 of the housing 70 of the structure 30 (Fig IB). Motion of the inner solid rod 20 is guided by the disengaged clutch 80 and the stroke of the inner solid rod is limited by the outer elastic element 10.

When the supply of electricity to the actuator 90 stops, inner springs 40 are open to its equilibrium state and apply force on to the clutch 80 to engage it with the inner solid rod 20 by pushing the clutch into the conical guiding sleeve 50. The clutch 80 engages with the inner rod 20 by contracting plural clutch slots 60, whereby friction is created between the clutch 80 and the inner solid rod 20 to stop the motion of the inner rod 20 and engage it with the housing 70 of the structure 30. However, the inner springs 40 can have different material composition and shape (e.g. rubber bushing, metal flexure, metal spring bushings, etc.). A transition phase between free motion and engagement with the housing 70 of the inner rod 20, allows for the structure 30 to achieve stiffness between maxima (Kl) and minima (K2) of the effective stiffness state limits of the structure 30. During the transition phase, the structure 30 performs stiffness modulation due to a dynamic (complex-valued) equivalent spring constant that is created due to the Coulomb friction between the moving surfaces, K=Re(K)+/lm(K). This effect significantly increases vibration damping and reduces settling time of the system to which the structure is attached. It increases stability of the system as part of the mechanical energy is converted into heat.

Fig 2 shows another embodiment where an internal elastic element 110 is imbedded inside of a hollow inner rod 120. The other elements operate in the same manner as described in relation to Figs. 1A and IB.

The structure 30 can, according to another embodiment, be used as a replacement for a conventional pitch link on a helicopter 230 as shown in Fig 3. The structure 30 is replacing an original pitch link located in a rotor hub 220. The rotor hub 220 must also be equipped with electrical slip rings 210 and a control module 240. Additionally, the helicopter is equipped with a power supply (source not shown) capable of actuating the structure 30.

As the structure 30 actuates, the stiffness of a solid state is switched from its maximum level, Kl to K(a), which is composed by the time-averaged stiffness constant, K0, modulated with the amplitude of the solid state stiffness, Kl, as shown in Fig. 4. Hence, K(a) = K0 + Kl/(ct), where the periodic actuation signal function, /(a), depends on the control frequency, ω, and the control phase angle, φ, and has zero time average. The control frequency is set at certain harmonics of the rotor fundamental frequency (rotor spinning rate), ω = ρΩ, where p is an integer, phase angle φ is determined by the rotor azimuth angle from the predetermined base line, and α = ωί + φ.

The harmonics of /(a) are responsible for reshaping the vibration spectrum, spreading the vibration energy at multiples of the fundamental frequency. In the case of the square wave

, , . . 2 , . sin3 sin5a

control signal shown in Fig. 4 /(or) =— (since + +— -— + ...)

π 3 5 The following equitation is the first order dynamic system parametric excitation, which is generally described by Hill's equation, and can be seen as the dynamic equivalent of the structure 30, valid at any instant of time, with neglected mass variation due to much higher effective stiffness of the structure compared to external vibration force, thus m is the only mass of the structure. mx + k(t)x = F{t)

This equitation is known as Mathieu's equation, where the stiffness modulation is purely sinusoidal and provides a form of parametric excitation of the system. In the present case, the external force F(t) = F _n sin(nQt) is the n-th harmonic of the fundamental frequency, Ω whose control objective is to reduce the forced response (uncontrolled) amplitude, x(t) = x _n sin(nQt), thus: _r K0

" ^— Κλ

l + (— /(α) - η ² Ω ² )

m

This expression shows the natural harmonic control character of the structure as represented by closed-loop feedback block diagram of a unity-gain system in Fig. 5. The block diagram in Fig. 5 is shown as the feedback from the uncontrolled response signal from the n-th harmonic, Fn/K0, to its controlled harmonic response signal, x _n , through the structure 30.

The structure 30 is designed to act as a filter for the higher-harmonics of the load vibration spectrum. The structure 30 feature of stiffness control changes the effective stiffness (flexural torsional characteristics) of the blade that allows control of an aeroelastic response of the blade. As a result of the structure 30 actuation, energy is redistributed in the spectrum of vibration removing vibration transmissibility from high and less desirable to lower

transmissibility frequencies in the rotor hub 220. Small changes in the effective stiffness of the blade allow the achievement of reductions in the vibration spectrum.

Operation - Fig 3

Referring to Fig 3, electricity is applied to the structure 30 from a non-rotating frame of the helicopter 230 to the rotating rotor hub 220 through the electrical slip rings 210 or similar electric transmission. Frequency of the applied electricity and the actuation pattern of the structure 30 are controlled via the control module 240. The control module 240 includes characteristics of the structure 30 and a control algorithm.

Plurality of vibration detection sensors installed in various most vulnerable vibration locations on the helicopter, collecting vibration data and transferring it to the control module 240.

Vibration data is being analyzed by the control module 240 to determine load strength as well as vibration frequency and amplitude via Fourier Transform. Furthermore, sensors area collecting and transferring contracting and expanding motion, and time periods of the structure operation to the control module 240. A preferred closed-loop control law for the structure 30 in the present embodiment is to decrease the dynamic response at the critical harmonics of the fundamental frequency, associated with the integer multiples of the number of blades and their immediate neighbors, neither affecting the rotor mean thrust (collective control at 0 per one revolution of the blade frequency - 0/rev) nor the blade azimuth pitch control system (cyclic control at 1 per revolution of the blade frequency - 1/rev). Based on a control transfer function determined from experimental results, the control module 240, through its control algorithm, sends an actuation signal to the power supply installed on the helicopter 230. The power supply generates a charge for structure 30 actuation from the engine of a helicopter, electricity outlet, battery or similar and provides a charge to the structure 30 for disengagement, partial disengagement or engagement of the clutch 80 to switch between stiffness of the solid state (Kl) and the soft state (K2) of the structure 30.

It has been shown that the structure 30 should be actuated at a frequency of 2 to 3 times per revolution for 1-2 bladed rotary systems, or N+/-1 per revolution where N is the number of blades of the helicopter. For a full-sized, 4 bladed helicopter, the blade rotating at 250 RPM (4.2 Hz) would correspond to 5 x 4.2 Hz = 21 Hz the structure 30 actuation frequency, while for a scaled rotor rotating at 1500 RPM (25 Hz), it would correspond to 5 x 25 Hz = 125 Hz the structure actuation frequency. Furthermore, a duty cycle of the structure 30 or a time period of the switch between one stiffness state to the other and back should have variable frequency of actuation (as an example, the axially expansible mechanism is at rest for a longer period of time, then actuated per one actuation cycle) to address the periodic vibratory loading of the system, effective stiffness change of blades due to the rotor fundamental frequency (rotor spinning rate), and to reset initial condition and to adjust for wear-and-tear of the structure.

It has been shown, by the maximum energy extraction method, that the structure can reduce vibration when the structure is activated to switch to the soft stiffness state K2 when top and bottom of the structure are moving one against the other and when top and bottom move one away from the other no actuation is done and the structure switch to the solid stiffness state Kl. Additional Embodiments -

Another application of the structure 30, according to another embodiment, is in a scaled blade 310 of a scaled rotor application as shown in Fig 6. Full-size helicopter rotors typically have a 4- 6 m radius and rotate at a frequency of 200-250 RPM (corresponding to 3.3 to 4.2 Hz).

However, before installing and testing a new technology on a full-size helicopter rotor, it must be whirl tower and wind tunnel tested. Whirl tower and wind tunnel tests are usually done on scaled rotors since operating large wind tunnels capable of housing a full-size helicopter are extremely rare and costly. As a result, the structure 30 can be used with the scaled helicopter blade 310 for a proof of concept testing.

Typical scaling factors are in the range of 1:6 to 1:2, while the same tip speed (around Mach 0.6 = 204 m/s) must be maintained between the full-size scaled blades. For example, a 1:6 scaling with a Mach 0.6 tip speed would correspond to about 6 x 4.2 =25.2 Hz rotational frequency for the rotor (i.e., 1500 RPM). Small scaled blades are typically 1 m in radius, 80mm in chord and 10mm in thickness. The structure 30 can be scaled to provide a lightweight, capable of handling high centrifugal loading, and sufficiently compact device to fit the scaled helicopter rotor hub and the scaled blade in such a manner as to not compromise the aerodynamic and weight properties of the blade and the rotor hub.

Example

An example of the physical structure 30 attached to a scaled rotorcraft blade, herein referred to as "Active Pitch Link (APL)", was simulated in a rotational condition in a whirl tower test environment. The test set-up parameters were as follows:

' Embedded Piezo-ring actuator Piezomechanik HPTs 150/14- 10/50

Voltage (Max) 150 V

Voltage (Min) O V

Test Section

Scaled Rotor Blade Carbon Fiber / Epoxy [12 mm x

80 mm x 986 mm]

Attachment Steel through bolts / Steel Hub

Power Supply Piezomechanik Amplifier [-10 V

to 150V & 1 Amp]

Drive 60HP, 575V, 3-phase motor

This example of the APL was tested for 2 minutes under 120+/-40 [|\|] vibratory loads, with the blade angle of attack (AOA) set to 2+7 degrees of periodic variation at 1 per revolution (1/rev) frequency. The data was analyzed using fast Fourier transform. Note that the AOA change was enabled through a fan located asymmetrically under the rotor disk, generating about 14 m/s up wash for about 20 deg range of the azimuth of the rotor disk. The example APL was actuated once per revolution or at 10 Hz. The rotational frequency for this test was 600 revolutions per minute (RPM) with 1-meter radius 1-bladed rotor, with the primary goal to demonstrate that "blade stiffness control" leads to vibration reduction.

The test objective was to keep the 1/rev vibrations at minimal change - since this simulates the cyclic control in the current whirl tower test setup, and to reduce the 2/rev vibrations. The test results in Fig 7 show that while the 1/rev vibratory load remains at 18% changed, the 2/rev vibration is reduced by as much as 73%. In one example, the stroke of the APL is measured to be 20 microns and exhibits no more than a 5% reduction in blade torsional stiffness.

Additional Embodiments - Figs. 8-11 - Other Implementations The structure 30 can be used in many different blade environments including wind turbines, home appliances (e.g., washer, dryer, air conditioning units), power tools and vehicles as show in Figs. 8 to 11.

Fig 8 illustrates an implementation of a structure 30 into a wind turbine pitching mechanism to reduce vibration and therefore stresses onto the wind turbine system to reduce failure rate due to fatigue loading on the wind turbine.

Fig 9 illustrates a schematic drawing of a home appliance and implementation of a structure 30 to reduce vibration to eliminate fatigue loading and home comfort.

Fig 10 illustrates another embodiment of a structure 30 implementation into power tool to reduce vibration passage to operator's body and reduce fatigue load of the tool.

Fig 11 illustrates an implementation of a structure 30 into the vehicle engine mounts and suspension to increase comfort of operators and decrease stress on their bodies.

Advantages

From the aforementioned description, a number of advantages of some embodiments of this application become evident: a) The actuator 90 is capable of providing the expansibility at high frequencies, and allow the structure 30 to vary its stiffness at much higher frequencies than traditional stiffness variation systems (i.e., hydraulic, magnetorheological, etc.), and is capable of accommodating actuation at high rotational speeds of the scaled helicopter blades. b) The structure 30 returns to the base profile in the event of power outage or other failure, thereby having the structure 30 change the stiffness to the solid state (Kl), to a conventional pitch link stiffness state and rendering it fail-safe. c) The low gap between the clutch 80 and the conical guiding sleeve 50 accommodates the required, much lower stroke from the actuator 90 to vary stiffness state of the structure 30, in comparison to the conventional active vibration reduction systems (e.g. actively controlled flap, active twist rotor, hydraulic/mechanical pitch link actuators). The actuation stroke is usually the main limitation in implementing any active vibration control technology. d) The structure 30 is capable of handling high axial, transverse, and centrifugal loading without jamming the actuation mechanism allowing the physical structure to vary its stiffness state. c) Partial expansibility of the actuator 90 allows the structure to achieve stiffness between maxima (Kl) and minima (K2) states increasing stability of the system. Conclusion, Ramifications, and Scope

Accordingly, the reader will see that at the various embodiments of the structure can provide faster actuation, lightweight, smaller in size, more robust, fail-safe, and smaller actuation stroke device that can be used on almost any vibration intensive machinery.

Although the aforementioned description provides many specificities, these should not be construed as limiting the scope of the embodiments but as merely providing illustrations of some of the presently preferred embodiments. For example, the inner solid rod can have a different polygon based cross-sections; the axially expansible mechanism can have a different shape of the stack of multiple layers of ceramic material capable of producing piezoelectric effect; and the spring and the inner springs can have different material composition and shape. Thus, the scope of the embodiments should be determined by the appended claims and their legal equivalents, rather than by the examples given.