CONTROL SYSTEM AND METHOD FOR RESONANT APPARATUS SUCH AS ADAPTIVE TUNABLE VIBRATION ABSORBERS

Abstract: A control system and method for resonant apparatus such as Adaptive Tuned Vibration Absorbers (ATVAs). In the control method aspect, which has applicability beyond ATVA control, two input signals in channels (39, 41) are maintained in a phase relationship of about 90 degrees via input from a reference sensor (36). The system includes a base sensor (40) providing a base vibration signal, a reference sensor (36) for providing a signal synchronized with the vibration source, a mass sensor (38) for providing a vibration signal of the tuning mass (43), and a preferably digital electronic controller (42) for demodulating the signals with quadrature waveforms generated from a waveform generator (47) to generate a control signal to the resonant apparatus, e.g. an ATVA (35). Optionally, detuning is accomplished via an offset associated with the method.

Description: CONTROL SYSTEM AND METHOD FOR RESONANT APPARATUS SUCH AS ADAPTIVE TUNABLE VIBRATION ABSORBERS Field of the Invention The invention relates to systems and methods for controlling mechanical vibration within a structure. More specifically, it relates to control of resonant apparatus, such as tuned absorbers. Background of the Invention Annoying acoustic noise and /or mechanical vibration may be created within an aircraft's cabin due to rotational unbalances of the aircraft's engine(s). For example, on fuselage-mounted engines, the rotational unbalance(s) may cause vibration transmission into the pylon structure and aircraft fuselage. If the fuselage vibration is well coupled to the acoustic space in the aircraft cabin, a predominantly tonal sound (generally characterized as a low frequency drone) may be generated within. In particular, this drone is related to the rotation of the various engines' components, for example, the rotation of the fan stage (at an Nl frequency) or of the compressor stage (at an N2 frequency), or both. In aircraft with aft-fuselage-mounted engines, such as the McDonnell Douglas DC -9, any rotational unbalance of the engines may result in such low-frequency cabin noise. This is particularly noticeable in the cabin's aft-most portion. Elimination or reduction of these Nl and /or N2 tones can dramatically reduce the annoyance experienced by passengers. Within the prior art, various means have been employed to counter such aircraft noise. These include passive blankets, Active Noise Control (ANC), Active Structural Control (ASC), Active Isolation Control (AIC), passive Tuned Vibration Absorbers (TV As), and adaptive TV As (ATVAs). Passive blankets are generally effective for attenuating high-frequency noise, but are generally ineffective at attenuating low-frequency noise. Where a higher level of noise attenuation is desired, Active Isolation Control (AIC) systems are utilized. AIC systems include active mountings which accommodate engine loads /motions and include an actively-driven element to provide active control forces. These active forces prevent vibration transmission from, for instance, the aircraft's engines into the pylon structure. The resultant effect is a reduction of cabin noise. AIC systems include the feedforward-type, in which reference signals, such as from reference accelerometers or tachometers, are used to provide reference signals indicative of the Nl and /or N2 vibrations of the engine(s). A plurality of distributed sensors, such as microphones, provide signals representative of the residual noise at various cabin locations. These reference and error signals are processed by a digital controller to generate drive signals to the active elements. These anti-vibration signals are of the appropriate amplitude, phase, and frequency to control vibration transmission from the engine to the pylon, thereby minimizing to the extent possible unwanted interior acoustic noise. US Pat. No. 5,551,650 entitled "Active Mounts for Aircraft Engines" describes such an AIC system. Disadvantages of AIC systems include space requirements for housing the active element and difficulty of accomplishing the appropriate actuation directions for vibration attenuation. Active Noise Control (ANC) systems are also well known. ANC systems may be used on turboprop aircraft, and include a plurality of acoustic output transducers, i.e., loudspeakers, strategically located within the aircraft's cabin. These loudspeakers are driven responsive to information derived from reference sensors and error sensors dispersed within the aircraft cabin. Reference signals may be derived from engine tachometers, from accelerometers on the engine(s), or from accelerometers placed on the fuselage wall in the area receiving propeller wash. The drive signals to the loudspeakers in ANC systems are generally adaptively controlled via a digital controller according to a known adaptive control algorithm, such as the Filtered-x Least Mean Square (LMS) algorithm. Co-pending US patent application Serial Number 08/553,227 to Billoud entitled "Active Noise Control System For Closed Spaces Such As Aircraft Cabins", describes one such ANC system. ANC systems have the disadvantage that they do not generally address any mechanical vibration problems and may be difficult to retrofit into existing aircraft due to potentially significant interior modifications. Furthermore, as the frequency of noise increases, large numbers of error sensors and speakers are required to achieve sufficient global noise attenuation. Certain Active Structural Control (ASC) systems, known in the prior art, may solve this problem of needing a large number of error sensors by attacking the fuselage's vibrational modes directly. For example, by attaching a vibrating device such as an actuator or shaker directly to the interior surface of the fuselage as described in US Patent No. 4,715,559 to Fuller, global attenuation may be achieved. However, the modifications necessary to retrofit Active Vibration Absorbers (AVAs) in this manner may be prohibitive, as the interior trim may have to be removed and structural modifications made to the frame members. Therefore, prior art ASC systems are necessarily difficult to retrofit. US Patent No. 5,310,137 to Yoerkie, Jr. et al., describes the use of AVAs and a feedback-type control system to cancel high-frequency vibrations of a helicopter transmission. Further descriptions of AVAs and active mounts can be found in WO 96/12121 entitled "Active Systems and Devices Including Active Vibration Absorbers (AVAs)". As should be apparent, active systems are attractive but somewhat complex and, thus, tend to be more expensive than simpler attenuation systems. Passive systems may offer these simpler, less-expensive alternatives. Passive Tuned Vibration Absorbers (TVAs) are effective at attenuating low-frequency vibration, but are generally limited in range and effectiveness. Passive TVAs include a flexibly-suspended tuning mass which is tuned by adjusting the stiffness of its flexible suspension or the mass of the suspended tuning mass, such that the device exhibits a stationary resonant frequency (fn). Movement of the tuning mass absorbs vibration of a vibrating member at its attachment point to the structure. However, TVAs may be ineffective if the engine speed changes, such that the TVA's resonant frequency no longer coincides with the disturbance frequency. US Patent No. 3,490,556 to Bennett, Jr. et al. entitled: "Aircraft Noise Reduction System With Tuned Vibration Absorbers" describes a passive vibration absorber device for use on the engine- mounting pylon of an aircraft for absorbing vibration at the Nl and N2 frequencies. When vibration cancellation over a wider range of frequencies is required, various Adaptive TVAs (referred to herein as ATVAs) may be employed. For example, US Patent No. 3,487,888 to Adams et al, entitled "Cabin Engine Sound Suppresser" teaches an ATVA whose resonant frequency (fn) can be adaptively adjusted by changing a bending length of a beam. The frequency range of attenuation may be greatly increased with ATVAs over passive TVAs. Various systems and methods for adaptive control have been disclosed in the prior art. US Patent No. 3,483,951 to Bonesho et al. compares the phase relationships between base and mass accelerometer signals and controls stiffness based upon the phase difference, generally trying to keep the phase difference at exactly 90degrees. PCT WO 92/15088, G10K 11/16 to Lotus Cars Ltd. describes control of Helmholtz resonators. The convergence and tracking properties of these systems are susceptible to noise present in the input signals. When the convergence and tracking properties of an ATVA control system are adversely affected, vibration, and thus noise, can reach unacceptable levels while the control system adjusts to changes in the speed (frequency) of the disturbance source (e.g. an aircraft engine frequency changing during accelerating or decelerating). Specifically, PCT WO 92/15088, G10K 11/16 to Lotus Cars Ltd. addresses the problem of noise in a Helmholtz resonator by implementing a tachometer signal which is used to control the center frequency of a bank of narrowband tracking filters. The need for this type of filtering complicates, and adds expense to the control system. In the area of fully active systems, the prior art teaches the use of an engine tachometer producing a reference signal to improve the tracking and convergence of the system, i.e., feedforward-type control systems. US Patent No. 3,172,630 to T. P. Goodman teaches the use of the tachometer signal in an active feedforward configuration in which the frequency, magnitude, and phase of the actuator command signals are manually adjusted. Finally, the US Patent to Adams, et al. listed above teaches the use of the tachometer signal to specify the pre-compression of an elastomeric mount in an "open-loop" fashion, thus altering the resonance frequency. Since there is no dynamic adjustment for actual vibration attenuation, the performance of open- loop control systems is known to degrade with changes in the vibration-absorber over time or with temperature changes. For example, if the system softens over time, vibration may, undesirably, be imparted to the structure, rather than be absorbed. Therefore, there is a recognized need for an adaptive control system which provides excellent tracking and convergence over a significant frequency range which can adjust for changes in the resonant device (ATVA or other resonant device) or system due to aging, temperature, or other parameter changes. Summary of the Invention Therefore, in light of the advantages and drawbacks of the prior art, the present invention, in one aspect thereof, is an ATVA system offering excellent convergence and tracking when in the presence of noisy input signals. The ATVA system comprises a base sensor for providing a signal indicative of residual vibration in the structure to be isolated. A reference sensor provides at least one reference signal indicative of the frequency of the source of the vibration (the disturbance). A mass sensor provides a signal indicative of the vibration of the ATVA's tuning mass. A controller processes the mass and base input signals with clean waveforms generated from the reference signal to form demodulated components. These demodulated components are further processed to generate a control signal. The control signal is used to tune the ATVA resonant frequency such that the phase angle between the base error signal and the motion signal representing the vibration of the ATVA device itself is maintained at or about 90 degrees phase difference. Optionally, the control system may include an offset to allow detuning of the ATVA. Maintaining the phase exactly at 90 degrees or slightly detuned therefrom (hereinafter referred to as being at substantially 90 degrees phase difference) provides excellent vibration absorption by the ATVA. Another aspect of the invention with applicability beyond control of ATVAs is a method for automatically controlling a resonant frequency of resonant apparatus, the method comprising generating a first input signal representing an oscillatory characteristic of a resonant component; generating a second input signal representing an oscillatory characteristic of an cancelable component; generating a reference signal that is a function of a oscillatory frequency of the oscillatory source (the disturbance); and processing said first input signal, said second input signal, and said reference signal and adjusting a parameter of said resonant component so as to maintain a substantial quadrature relationship between said first input signal and said second input signal based upon demodulated components. A sum of demodulated in-phase and quadrature components of said input signals is used as a control signal. Clean, i.e., spectrally pure, sine and cosine waveforms are generated from the reference signal and multiplied by each of the first and second input signals. These signals are then low pass filtered, thereby demodulating each input signal into the demodulated in-phase and quadrature components of the input signals. These various demodulated components are then multiplied and summed to generate a delta (control) signal. This delta signal can be used directly as a control signal to adjust a resonant apparatus, for example, to tune the position of an ATVA mass such that the vibration of the ATVA mass and the vibration of the structure to which the ATVA is attached are maintained at a phase differential of substantially 90 degrees. According to another aspect, the method is used for automatically controlling a natural frequency of an adaptive tunable vibration absorber, said adaptive tunable vibration absorber having a spring rate and a tuning mass, so as to minimize a vibration of a vibrating structure, said vibration being caused by a oscillating source, said method comprising the steps of: generating a first input signal representing a vibratory characteristic of said tuning mass; generating a second input signal representing a vibratory characteristic of the vibrating structure; generating a reference signal that is a function of a frequency of the oscillating source; and processing said first and second input signals and said reference signal and adjusting a resonant frequency of said adaptive tunable vibration absorber so as to maintain a substantial quadrature relationship between said first and second input signals. According to another aspect of the invention, a system is provided for controlling vibration and acoustic noise generated within a space that results from vibration generated by at least one vibrating source, said vibration being transmitted into said closed space through a connection structure between the at least one vibrating source and said closed space, thereby generating acoustic noise and vibration within the said closed space, the system comprising at least one adaptive tunable vibration absorber having a vibrating tuning mass, said adaptive tunable vibration absorber being fixedly attached to said connection structure, at least one mass sensor for generating at least one first input signal representing a vibratory characteristic of said tuning mass, at least one base sensor for generating at least one second input signal representing a vibratory characteristic of said connection structure proximate to an attachment point of said adaptive tunable vibration absorber to said connection structure, at least one reference sensor for generating at least one reference signal that is a function of a vibratory frequency of said vibrating source, and a controller for processing said at least one first input signal, said at least one second input signal, and said at least one reference signal and providing at least one control signal to said at least one adaptive tunable vibration absorber for adjusting said resonant frequency of said at least one adaptive tunable vibration absorber for maintaining a substantial quadrature relationship between said at least first input signal and said at least one second input signal. It is an object of the present invention to provide a control method that exhibits improved tracking and convergence properties over a wide range of vibration frequencies. It is another object of the present invention to provide an ATVA control system that can operate satisfactorily without the need for expensive narrowband tracking filters. It is yet another object of the present invention to provide a control system that can be implemented using a multi-sampling-rate algorithm. Brief Description of the Drawings The accompanying drawings which form a part of the specification, illustrate several embodiments of the present invention. The drawings and description together serve to fully explain the invention. In the drawings, like reference numbers are used to designate the same or similar items throughout the several figures wherein: Fig. 1 is a schematic frontal view of an ATVA control system as implemented in an aircraft showing the main components of the system; Fig. 2 is a simplistic representation of an ATVA control system showing the flow of input and control signals; Fig. 3a is a detailed block diagram of the generalized control system operation; and Fig. 3b is a detailed block diagram of a digital implementation of the control system operation. Detailed Description of the Preferred Embodiment of the Invention For a mechanical TVA or ATVA, resonance occurs when the base motion, that is, the motion of the base surface to which the ATVA device is attached, is in quadrature with the ATVA mass motion. Two objects in motion are said to be in quadrature with one another when they are moving out of phase with one another by exactly 90 degrees. Notably, the control system and method in one aspect of the present invention, has much broader applicability than just for ATVAs or for use in aircraft. This control method aspect of the present invention may be used for regulating the phase between two appropraitely chosen input signals within a resonant apparatus where the phase difference between the signals needs to be n x 90 degrees where n = {0, +1, +2, ... }. Within the generalized example of an ATVA, the motion of the base surface and the motion of the ATVA mass are measured by sensors which produce appropriate vibration signals. Generally, one vibration signal is generated for each motion, base and mass. The signals are then used to generate a control signal which can be used to adjust the relative phase between the two signals. For convenience, and without loss of generality, we restrict the following description to the case of a mechanically resonant apparatus. However, one skilled in the art will appreciate that equivalents of the present preferred embodiment can find application outside of the field of mechanical resonance, such as in Helmholtz resonators. By way of example only, and not by limitation, the invention is described with reference to an aircraft application, whereby the ATVA control system and method are implemented to minimize vibration transferred into the cabin of an aircraft from one or more fuselage-mounted engines. However, it will be apparent to a person having ordinary skill in the art that any closed structure, such as an automobile passenger compartment, truck cab, railroad car interior, etc. would benefit from the present invention control system and method. Further, the present invention is useful with any vibration utilization device, such as a vibrating tamping machine, or other resonant vibration device, where it is desired to maintain a mass at or near resonance. Referring now to Fig. 1, an aircraft engine 30 vibrates due to rotational unbalances generated by one or more unbalanced components found therein. This vibration occurs predominantly at the two frequencies Nl and N2 as discussed above. Vibration is transferred into the aircraft fuselage 31 through a C-shaped yoke 32 and pylon 33 or other like connection structure which interconnects the engine 30 to the fuselage 31. The resulting vibration of the fuselage 31 creates the annoying tonal noise which is heard by passengers seated within the aircraft's cabin 34. In order to prevent this vibration from reaching the aircraft cabin 34, at least one, and preferably, a plurality of resonant apparatus, such as ATVAs 35 are secured to the yoke 32 or pylon 33 structure between the engine 30 and the aircraft fuselage 31 in order to absorb vibration. Fig. 1 shows one ATVA 35 attached to the yoke 32 at the point of attachment to the pylon 33. The ATVA apparatus 35 vibrates in an orbital fashion within the tangential T/radial R plane of the engine 30, thereby absorbing both radial and tangential vibrations. Alternate embodiments would have the ATVAs 34 attached to the pylon 33, or in some other position, preferably between the engine 30 and the fuselage 31. The ATVAs optionally may also be attached to the fuselage 31 directly. In order to absorb vibrations at both the Nl and N2 frequencies, some ATVAs would be tuned and controlled to the Nl frequency and others to the N2 frequency. Effective vibration absorption is accomplished via appropriate tuning and orientation of the ATVAs. The ATVA's 35 should be tuned to the appropriate resonance frequency and then adaptively adjusted within its tunable range to maintain the appropriate relationship between vibrational frequency of the engine 30 (the disturbance frequency) and the resonant frequency of the ATVA 35. In this way, the ATVA apparatus 35 may absorb vibrations as the frequency of the engine 30 changes during acceleration and deceleration. In general, the ATVA 35 should be oriented such that it resonates in response to vibration of the member to which it is attached. Generally, the ATVA 35 should be attached such that radial (designated R) and /or tangential vibrations within the yoke 32 or pylon 33 are absorbed. With reference to Fig. 1 and Fig. 2, a reference sensor 36, such as a tachometer, or other suitable reference sensor associated with engine 30 generates a reference signal on a communication channel 37. This reference signal represents a function of the speed (vibrational frequency) of the engine 30 and, thus, is synchronized with the frequency of the vibrations produced by the vibration source (the engine 30). A first input sensor, such as an ATVA mass sensor 38 (mounted to mass) generates a first input signal on a communication channel 39 preferably representing the radial vibration of the mass 43 (Fig. 2) of the ATVA 35. A second sensor, such as base sensor 40 (interconnected to base of yoke 32) generates a second input signal (hereinafter referred to as an error signal) on a communication channel 41 representing the vibration of the base (the yoke 32, pylon 33 or other structure). Preferably, the base sensor 40 is substantially collocated with the ATVA 35 and measures acceleration or other vibration signal in the same direction as the mass sensor 38. Each of these signals on the communication channels 37, 39, 41 is delivered into a controller 42 for processing. The controller is preferably a digital processor. A control or drive signal is derived by the controller 42 through communication channel 44 and delivered to ATVA 35. The signal command to a driver, or other like imparting device controls the resonant frequency of the ATVA 35. In the Fig. 2 embodiment, the resonance is controlled via a driver which adjusts the exposed length of a beam 53. Throughout this description, the vibration signal representing the vibration of the mass 43 (Fig. 2) of ATVA 35 is referred to as x(t) and the error signal representing the base motion of, for example, the yoke 32 is referred to as e(t). For an appropriately sized ATVA 35, minimization will occur when the phase differential is at or slightly detuned from 90 degrees phase difference. Referring again to Fig. 1 and Fig. 2, a tuning mass 43 of the ATVA 35 is flexibly connected to the vibrating structure (ex. yoke 32 or pylon 33) via flexible beam 53, as shown schematically in Fig. 2, so that the mass 43 responds to the vibration of the vibrating structure (yoke 32 or pylon 33) at its own resonant frequency. The objective of the controller 42 is to generate a command signal on a communication channel 44 to adjust the relationship between the mass 43 and its flexible connection 53 to the vibrating structure (yoke 32 or pylon 33), such that the radial motion of mass 43 is in quadrature with the radial motion of the vibrating structure. The controller 42 processes the first and second input signals from the mass sensor 38 on the communication channel 39 and the base sensor 40 on the communication channel 41, and the reference signal from the reference sensor 36 on the communication channel 37 in order to generate the command signal on the communication channel 44 to adjust the resonant frequency of the mass 43. In this embodiment, an imparting device 64, such as the rotary motor shown, adjusts the position of the mass 43 by exposing more or less of a beam member 53. Without loss of generality, any known means for adjusting the spring stiffness, mass, or both may be employed for adjusting the resonant frequency of the ATVA 35 or other resonant apparatus. The command signal must have a monotonic causal relationship with the variable resonant frequency of the ATVA 35. That is, a command signal to imparter 64 to move the mass 43 in one direction must always move the resonant frequency in one direction while moving the mass 43 in the other direction would always have the opposite effect on resonant frequency. As described above, when signals x(t) and e(t) are in quadrature (at 90 degrees phase difference), the ATVA 35 resonance frequency substantially coincides with the excitation frequency of the vibration generated by the engine 30 and absorption is obtained. This minimizes the base response, for example, the vibration of the yoke 32 and, resultantly, minimizes the vibration of the fuselage 31 which generally would cause noise therein. As discussed above in the Background of the Invention, two related problems experienced in the prior art are: 1) noise in the signals x(t) and e(t) that is outside of the Nl or N2 frequency band, and 2) resultant slow tracking of the control system. The present invention uses a reference signal from the communication channel 37 in a novel way to increase the signal-to-noise ratio (SNR) of signals x(t) and e(t) with two main advantages over the closest related prior art. The first advantage in one aspect of the present invention is that this implementation will be less computationally expensive than the related art due to the reduced filtering requirements. The present invention can tolerate more noise in the input signals; although, as with all of the related art techniques, higher signal-to-noise ratio (SNR) input signals will always lead to better tracking of the disturbance frequency by the ATVA 35. The second advantage of another aspect of the present invention is that the control system may be implemented as a multi-rate algorithm, i.e., the several parts of the control system can sample signals at different rates, as dictated by the specific function being performed. This will be described more fully below. Referring now to Fig. 3a, which shows a schematic representation of the various functions of the controller 42, the block diagram functions may be implemented with analog components, a programmed digital computer, or hybrid combinations thereof. The preferred embodiment is to implement all functions on a programmed digital microprocessor as will be described with reference to Fig. 3b. In the generalized case illustrated in Fig. 3a, the input signal, x(t) on the communication channel 39 is generated by mass sensor 38 and base error signal e(t) on the communication channel 41 is generated by base sensor 40. A reference signal on communication channel 37 is generated by reference sensor 36 and is input into waveform generator 47. The waveform generator 47 produces spectrally pure sine and cosine signals which are synchronized with the disturbance vibration. Notably, any of the known waveform generators which derive spectrally pure sine and cosine signals from a reference signal 37 may also be used, such as the waveform generator taught in commonly assigned US 5,487,027. When the input signals e(t) and x(t) are in quadrature, the inventors herein determined that a unique relationship exists between the demodulated components of these signals. Demodulation may be understood at a combined multiplication and filtering step which provides the simplified DC components XC, XS, ES, and EC. The inventors herein determined that when the phase difference is 90 degrees, the following condition is required. Phase angle between e(t) and x(t) Required condition ± 90 degrees (XC * EC) + (XS * ES) = 0 In short, in order for the phase between x(t) and e(t) to be maintained at 90 degrees, the mathematical equation above must hold true. Therefore, the control process according to one aspect of the present invention, at all times, strives to maintain this relationship. This is physically accomplished by de-modulating the inputs x(t) and e(t) with cosine and sine waveforms generated by the waveform synthesizer 47 (Fig. 3a). First the input x(t) and error e(t) are multiplied by the cosine waveform at multipliers 49. Simultaneously, the input x(t) and error e(t) are also multiplied by the sine waveform at multipliers 49'. The multiplied signals from multipliers 49, 49' are then each low pass filtered in low pass filters 50 to remove unwanted high frequency components. This leaves only the low frequency components. Signals XC and XS are, respectively, the demodulated in-phase and quadrature components of the mass vibration signal x(t), and signals EC and ES are, respectively, the demodulated in-phase and quadrature components of the error signal e(t). Signals XC, XS, EC, and ES are all relatively low bandwidth; and, under stationary conditions (e.g. the engine at constant speed), they are all constant or DC. Because of the low bandwidth of these signals, all signals to the right of line A-A in Fig. 3a may be sampled at a much lower rate than that required for the signals to the left of line A-A. This is the multi-rate advantage mentioned above. To further illustrate the demodulation occurring in demodulator steps 51, 51', the following derivation for the demodulation of the x(t) and cos signals at 51 is supplied. This is exemplary, and other demodulations may be easily derived using similar steps. The mass vibration signal x(t) is preferably given by the equation: x(t) = X cos (ωt +φ) (1) multiplying x(t) and cos(ωt) at multiplier 49 we get: x(t) cos (ωt) = X cos (ωt) cos (ωt +φ) (2) using the trigonometric identity: cos A cos B = (112) cos (A-B) + (1 /2) cos (A+B) (3) equation (2) then expands to: x(t) cos (ωt) = (1/2) cos (-φ) + (1/2) cos (ωt +(ωt +φ)) (4) which then simplifies to: x(t) cos (ωt) = (112) cos (φ) + (1 /2) cos (2ωt +φ) (5) It should be recognized that the first term of equation (5) 1/2 cos (φ) represents a constant or DC component and the second term 1/2 cos (2ωt +φ) represents an oscillatory term that is correlated to twice the disturbance frequency (a 2X component). Notably, by low pass filtering the product of multiplication step at filter 50 , the 2X component is removed, thus leaving only the DC component. This demodulated component is XC. Preferably, the cutoff frequency of filter 50 will be below the 2X frequency, and more preferably less than IX. This allows the use of an inexpensive filter with a gradual roll off. In a best mode, the filter's cutoff frequency would be set to about 10 Hz. The other demodulated components XS, ES, and EC are derived in a similar fashion. The two in-phase demodulated components, XC and EC, and the two quadrature components, XS and ES, are multiplied at multipliers 49". These two multiplication results are then combined by an adder 51 thereby generating the JE signal referred to above. The JE signal is then passed on to a compensator 68 which may perform an If-Then logical operation if a stepper motor is used as imparter 64, or optionally may be a simple integrator outputting a set point to an open or closed loop control system for controlling the imparter 64. In short, the compensator 68 provides the appropriate signal to the imparter 64. The output logic used for the If-Then logical operation is described with reference to Fig. 3b. Now referring to Fig. 3b which describes a digital implementation, the mass and base signals 39, 41 are passed through analog-to-digital converters 45 which sample the analog signals in a way well known in the art of digital computers. Those signals to the left of line A-A should be sampled at a rate in accordance with the well-known Nyquist sampling criterion. Preferably, the sample rate is at least four times the highest frequency in the tunable range of the ATVA device 35. Before entering the analog-to-digital converters 45, the signals x(t) and e(t) preferably are passed through conventional analog anti-aliasing filters (not shown). Anti-aliasing filters are any type of filter, such as low-pass filters or band-pass filters, which remove spectral component signals above a certain frequency point. This is desired whenever a signal is to be sampled by a digital computer. Otherwise, high-frequency signals may appear in the digitally- sampled signal as "alias" signals at a lower frequency, thus adding unwanted noise. The sampled input signals may then optionally be passed through bandpass filters 46 to remove excess noise and improve their signal-to-noise ratio (SNR). As stated above, the present invention works well without such filtering and band-pass filters tend to add unwanted expense. However, tracking and convergence of the system are improved if the input signals have a higher SNR. Such filtering can also be accomplished in the preferred embodiment through digital filtering on a stored-program microprocessor. The function of the waveform synthesizer 47 may be implemented with a lookup table using a digital input function 48 to produce a synchronous oversampled signal from the tachometer (reference) signal on the communication channel 37 as input to a counter. The sine and cosine signals are then extracted from the table using values of the counter. The use of a lookup table to generate waveforms is described in copending US Patent application serial number 08/693,742. The remaining core steps of the control method are the same as are described in the Fig. 3a embodiment. The A signal from the control process is provided to the output logic function 52. The output logic function 52 performs the function of the compensator 68 in Fig. 3a in that it determines the direction and amount for which to command the imparter 64 to adjust resonance. In particular, the Δ signal is received from the adder 51. The output logic function 52 then converts the Δ signal into a appropriate signal to adjust the resonant frequency of the ATVA 35. For example, the output logic may operate under the following simple rules: If the Δ signal is negative, Then enable imparter 64 and move in a positive direction. If the Δ signal is positive, Then enable imparter 64 and move in a negative direction. If the Δ signal is within dead zone, Then don't enable imparter 64, where the dead zone is a range of values around zero. This keeps the system from hunting which may wear out the imparter 64 prematurely. The block diagram of Figs. 3a and 3b use the following expression to represent the input into the compensator 68 or output logic 52: Δ = (XC EC + XS ES) + OFFSET Disregarding the OFFSET term, which will be described more fully below, when e(t) and x(t) are in quadrature, the input to the compensator 68 or output logic function 52 is Δ = 0, and, therefore, no adjustment is made to the resonant frequency of the ATVA 35. The OFFSET term is nominally zero but can optionally have a non-zero setting in order to "de-tune" the ATVA 35. Such de-tuning is done because maximum vibration absorption may, in some applications, occur when the phase difference between x(t) and e(t) is not exactly 90 degrees. See Adaptive Tuned Vibration Absorbers: Tuning Laws, Tracking Agility, Sizing, and Physical Implementations, by A. von Flowtow, A Beard, and D. Bailey, Noise Con 1994 which describes the desirability of detuning by mechanically tuning the resonant device. This substantially, but not-quite-quadrature condition can be maintained by setting the OFFSET to a small phase angle deviation from 90degrees. The appropriate setting for the OFFSET term is determined empirically. Notably, in a novel aspect, the detuning is accomplished via an adjustment within the algorithm. For the example of a mechanical ATVA, the phase angle between e(t) and x(t) is physically bounded between 0 degrees and -180 degrees. It was recognized by the inventors herein that the expression (XC EC + XS ES) is always negative whenever the phase angle between e(t) and x(t) is less than -90 degrees, and is positive over the remainder of the bounded phase region. This change in polarity around the point of 90 degrees phase difference provides a convenient way to determine which way the resonant frequency of the ATVA 35 must change in order to achieve vibration absorption. If the output signal from the compensator 68 is directly related to the desired resonant frequency, itself, rather than a desired change in the resonant frequency, the compensator 68 also nominally includes a gain on the adder input signal Δ in order to provide a means for setting the convergence rate of the control function. That is, the gain is empirically adjusted in order to determine and set the appropriate convergence rate for the control system. Any ATVA will have a time constant associated with it which represents how long the electromechanical (or other type) process takes to adjust its resonance frequency in response to a control signal on the communication channel 44. Therefore, as a general rule, the time constant of the control system should be designed to be at least as long as the time constant of the ATVA 35. Doing this will minimize mechanical overshoot and unwanted dither. The time constant of the control system may also be set as desired by careful selection of the low-pass filters 50. Wider-bandwidth low-pass filters 50 allow higher frequency signals to pass. Higher-frequency signals generally change more quickly than low-frequency signals, which generally results in a shorter control system time constant. Lower-bandwidth low-pass filters 50 generally result in a longer control-system time constant. In an embodiment of the control system in which the gain on the input signal from the adder 51 to the compensator 68 is used to adjust the control system time constant, a smaller gain would result in a longer time constant and vice versa. It should be recognized that the amplitude of the mass motion x(t) of the spring/mass system within the ATVA 35 can affect the amplitude of the Δ signal provided to the compensator 68 and, thus, the convergence rate. However, it is desirable that the convergence rate depend only on the magnitude of the error e(t). This is easily accomplished by using a signal processing technique known as "signal normalization". Using this technique, the amplitude of the mass motion x(t) is the signal to be normalized and the term "normalization" simply refers to the mathematical process of normalizing x(t) such that the magnitude of x(t) doesn't effect convergence. Implementing such a normalization technique generates the input signal to the compensator 68 which is: Δ = (XC * EC + XS * ES) / (^ XC * XC + XS * XS ) + OFFSET Due to the square-root function in the above equation, this normalized algorithm is preferably implemented on a digital computer. In the preferred embodiment of the present invention, which entails implementation of the functions shown in Fig. 3b on a programmed digital microprocessor, the output logic function 52 converts the input into two control signals to send to the ATVA to adjust its resonant frequency. One control signal from the output logic function 52 enables the imparter 64, while the other indicates the direction to move. Once the control system converges, it will not enable movement. Preferably, the resonant frequency is changed only if the absolute value of the Δ signal is greater than some specified positive minimum value (providing a dead zone). That is, the resonant frequency will only be changed if the difference between the current and desired resonant frequency is more than an arbitrary amount. The signal indicating the direction to change of the resonant frequency of the ATVA depends on the mathematical sign of the output of adder 51, i.e., the Δ signal. Notably, any other suitable means of taking the output of adder 51 and controlling the resonance of the ATVA 35 may be used. While the form of the apparatus, the methods, and the systems herein described constitutes a preferred embodiment of this invention, it is to be understood that the invention is not limited to this precise form of apparatus, methods, and systems and that changes may be made therein without departing from the scope of the invention which is defined in the appended claims.

Claims: Claims What is claimed is: 1. A method for controlling a resonant frequency of a resonant apparatus, comprising the steps of: a) generating a first input signal representing an oscillatory characteristic of a resonant component of the resonant apparatus, b) generating a second input signal representing an oscillatory characteristic of a cancellable component, c) generating a reference signal that is a function of a frequency of an oscillatory source, and d) processing said first and second input signals and said reference signal and adjusting a parameter of said resonant apparatus so as to maintain a substantial quadrature relationship between said first and second input signals by controlling based on demodulated components. 2. A method of claim 1 wherein said demodulated components are in- phase and quadrature components of said first and second input signals. 3. A method of claim 1 wherein a sum of said demodulated components is minimized. 4. A method of claim 3 wherein said sum is further comprised of a sum of: a) a product of an in-phase demodulated component of said first input signal and an in-phase demodulated component of said second input signal; and b) a product of a quadrature demodulated component of said first input signal and a quadrature demodulated component of said second input signal. 5. A method of claim 4 wherein: a) said in-phase demodulated component of each of said first and second input signals is formed by multiplying each said input signal by a cosine waveform and then low pass filtering; and b) said quadrature demodulated component of each of said first and second input signals is formed by multiplying each said input signal by a sine waveform and then low pass filtering. 6. A method of claim 1 wherein an in-phase demodulated component of each of said first and second input signals is formed by multiplying each said input signal by a cosine waveform and then low pass filtering. 7. A method of claim 1 wherein a quadrature demodulated component of each of said first and second input signals is formed by multiplying each said input signal by a sine waveform and then low pass filtering. 8. A method of claim 1 further including means for providing an offset to maintain said substantial quadrature relationship in a detuned state. 9. A method of claim 1 wherein said resonant apparatus is an adaptive tuned vibration absorber including a moveable tuning mass flexibly suspended from a vibrating structure. 10. A method of claim 9 further comprising the steps of: a) determining a direction to move said tuning mass based upon a sum of said demodulated components, and b) adjusting a position of said tuning mass based upon said direction so that a resonant frequency of said adaptive tuned vibration absorber is adjusted to effectuate vibration absorption. 11. A method of claim 10 further including an additional step of: a) preventing movement of said tuning mass if said sum of said demodulated components is less than a predetermined amount in order to avoid constant minute adjustments to said position. 12. A method of claim 1 wherein said resonant apparatus is an ATVA attached to a vibrating structure in an aircraft, said oscillatory source is at least one aircraft engine, and said reference signal is selected from a group comprising: a) a first reference signal indicative of an Nl vibration frequency produced by a rotation of said at least one aircraft engine's fan stage, and b) a second reference signal indicative of an N2 vibration frequency produced by a rotation of said at least one engine's compressor stage. 13. A method of claim 1 wherein sine and cosine waveforms used, in part, to form said demodulated components are generated by a waveform generator. 14. A method of claim 1 wherein said processing further includes the further steps of: a) generating a sine waveform from said reference signal; b) generating a cosine waveform from said reference signal; c) demodulating said first input signal into a in-phase component; d) demodulating said first input signal into a quadrature component; e) demodulating said second input signal into its in-phase component; f) demodulating said second input signal into its quadrature component; g) multiplying said in-phase component of said first input signal by said in-phase component of said second input signal to form an in-phase product; h) multiplying said quadrature component of said first input signal by said quadrature component of said second input signal to form a quadrature product; i) adding said in-phase product and said quadrature product together to form a component sum; and j) generating a control signal responsive said component sum for adjusting a resonant frequency of said resonant device. 15. A system for controlling vibration and acoustic noise generated within a space that results from vibration generated by at least one vibrating source, said vibration being transmitted into said closed space through a connection structure between the at least one vibrating source and said closed space, thereby generating acoustic noise and vibration within the said closed space, the system comprising: a) at least one adaptive tunable vibration absorber having a vibrating tuning mass, said adaptive tunable vibration absorber being fixedly attached to said connection structure; b) at least one mass sensor for generating at least one first input signal representing a vibratory characteristic of said tuning mass; c) at least one base sensor for generating at least one second input signal representing a vibratory characteristic of said connection structure proximate to an attachment point of said adaptive tunable vibration absorber to said connection structure; d) at least one reference sensor for generating at least one reference signal that is a function of a vibratory frequency of said vibrating source; and e) a controller for processing said at least one first input signal, said at least one second input signal, and said at least one reference signal and providing at least one control signal to said at least one adaptive tunable vibration absorber for adjusting said resonant frequency of said at least one adaptive tunable vibration absorber for maintaining a substantial quadrature relationship between said at least first input signal and said at least one second input signal.

Inventors: HIGHFILL JOHN H III, SOUTHWARD STEVE C, HIGHFILL, JOHN, H., III, SOUTHWARD, STEVE, C.
Applicants: LORD CORP, LORD CORPORATION
Application year: 1998
Main classification: G10K11
IPC codes: F16F7, F16F15, G10K11