Shaft resonance control

Abstract

A method of actively controlling torsional resonance of a rotating shaft of an engine is provided. The shaft has a rotational velocity characterised by a low frequency, rotational velocity term and a high frequency, oscillatory term superimposed on the low frequency term, the oscillatory term being caused by torsional resonance. The method including: measuring the rotational velocity of the shaft; extracting the oscillatory term from the measured rotational velocity; and on the basis of the extracted oscillatory term, applying a torque component to the shaft, the torque component being modulated at the same frequency as the torsional resonance to counteract the torsional resonance.

Claims

1. A method of actively controlling torsional resonance of a rotating shaft of an engine, the shaft having a rotational velocity characterised by (i) a low frequency, rotational velocity term and (ii) a high frequency, oscillatory term superimposed on the low frequency term, the oscillatory term being caused by torsional resonance, the method comprising: measuring the rotational velocity of the shaft; extracting the oscillatory term from the measured rotational velocity; and applying a torque component to the shaft based on the extracted oscillatory term, the torque component being modulated at a same frequency as the torsional resonance to counteract the torsional resonance, the torque component being applied by modulating a flow rate of fuel to the engine, which includes an engine fuel control system that generates a fuel flow demand signal in response to an acceleration demand signal and a steady state fuel flow requirement, the flow rate of fuel to the engine being modulated by frequency modulating the fuel flow demand signal to generate the torque component at the same frequency as the torsional resonance.

2. The method according to claim 1, wherein: the measurement of the rotational velocity is performed at a frequency that is higher than the torsional resonance frequency of the shaft.

3. The method according to claim 1, wherein the extraction of the oscillatory term from the measured rotational velocity includes demodulating the measured rotational velocity.

4. A system for reducing torsional resonance of a rotating shaft of an engine, the shaft having a rotational velocity characterised by (i) a low frequency, rotational velocity term and (ii) a high frequency, oscillatory term superimposed on the low frequency term, the oscillatory term being caused by torsional resonance, the system comprising: a device configured to measure the rotational velocity of the shaft; and a control unit configured to perform: extracting the oscillatory term from the measured rotational velocity; and generating and issuing a command to apply a torque component to the shaft based on the extracted oscillatory term, the torque component being modulated at a same frequency as the torsional resonance to counteract the torsional resonance, the torque component being applied by modulating a flow rate of fuel to the engine, which includes an engine fuel control system that generates a fuel flow demand signal in response to an acceleration demand signal and a steady state fuel flow requirement, the flow rate of fuel to the engine being modulated by frequency modulating the fuel flow demand signal to generate the torque component at the same frequency as the torsional resonance.

5. The system according to claim 4, wherein the device for measuring the rotational velocity of the shaft includes: a phonic wheel that is mounted coaxially to the shaft for rotation therewith, the phonic wheel having a circumferential row of detectable features; and a sensor configured to detect a passage of the row of detectable features by generating an alternating measurement signal having a frequency that is a multiple of the rotational frequency of the shaft.

6. The system according to claim 4, wherein: the measurement of the rotational velocity by the device is performed at a frequency which is higher than the torsional resonance frequency of the shaft; and the extraction of the oscillatory term from the measured rotational velocity by the control unit includes filtering the measured rotational velocity within a frequency range including the torsional resonance frequency.

7. A gas turbine engine having the system for reducing torsional resonance according to claim 4.

Description

(1) Embodiments will now be described by way of example only, with reference to the Figures, in which:

(2) FIG. 1 is a sectional side view of a gas turbine engine;

(3) FIG. 2 is a close up sectional side view of an upstream portion of a gas turbine engine;

(4) FIG. 3 is a partially cut-away view of a gearbox for a gas turbine engine;

(5) FIG. 4 shows schematically an engine fuel control system;

(6) FIG. 5 shows schematically a modified engine fuel control system for active shaft damping;

(7) FIG. 6 shows schematically at top a phonic wheel and sensor, and at bottom an alternating measurement signal generated by the sensor and software data averaging of the signal; and

(8) FIG. 7 shows a similar view to FIG. 2, but of an alternative gas turbine engine.

(9) FIG. 1 illustrates a gas turbine engine 10 having a principal rotational axis 9. The engine 10 comprises an air intake 12 and a propulsive fan 23 that generates two airflows: a core airflow A and a bypass airflow B. The gas turbine engine 10 comprises a core 11 that receives the core airflow A. The engine core 11 comprises, in axial flow series, a low pressure compressor 14, a high-pressure compressor 15, combustion equipment 16, a high-pressure turbine 17, a low pressure turbine 19 and a core exhaust nozzle 20. A nacelle 21 surrounds the gas turbine engine 10 and defines a bypass duct 22 and a bypass exhaust nozzle 18. The bypass airflow B flows through the bypass duct 22. The fan 23 is attached to and driven by the low pressure turbine 19 via a shaft 26 and an epicyclic gearbox 30.

(10) In use, the core airflow A is accelerated and compressed by the low pressure compressor 14 and directed into the high pressure compressor 15 where further compression takes place. The compressed air exhausted from the high pressure compressor 15 is directed into the combustion equipment 16 where it is mixed with fuel and the mixture is combusted. The resultant hot combustion products then expand through, and thereby drive, the high pressure and low pressure turbines 17, 19 before being exhausted through the nozzle 20 to provide some propulsive thrust. The high pressure turbine 17 drives the high pressure compressor 15 by a suitable interconnecting shaft 27. The fan 23 generally provides the majority of the propulsive thrust. The epicyclic gearbox 30 is a reduction gearbox.

(11) An exemplary arrangement for a geared fan gas turbine engine 10 is shown in FIG. 2. The low pressure turbine 19 (see FIG. 1) drives the shaft 26, which is coupled to a sun wheel, or sun gear, 28 of the epicyclic gear arrangement 30. Radially outwardly of the sun gear 28 and intermeshing therewith is a plurality of planet gears 32 that are coupled together by a planet carrier 34. The planet carrier 34 constrains the planet gears 32 to precess around the sun gear 28 in synchronicity whilst enabling each planet gear 32 to rotate about its own axis. The planet carrier 34 is coupled via linkages 36 to the fan 23 in order to drive its rotation about the engine axis 9. Radially outwardly of the planet gears 32 and intermeshing therewith is an annulus or ring gear 38 that is coupled, via linkages 40, to a stationary supporting structure 24.

(12) Note that the terms “low pressure turbine” and “low pressure compressor” as used herein may be taken to mean the lowest pressure turbine stages and lowest pressure compressor stages (i.e. not including the fan 23) respectively and/or the turbine and compressor stages that are connected together by the interconnecting shaft 26 with the lowest rotational speed in the engine (i.e. not including the gearbox output shaft that drives the fan 23). In some literature, the “low pressure turbine” and “low pressure compressor” referred to herein may alternatively be known as the “intermediate pressure turbine” and “intermediate pressure compressor”. Where such alternative nomenclature is used, the fan 23 may be referred to as a first, or lowest pressure, compression stage.

(13) The epicyclic gearbox 30 is shown by way of example in greater detail in FIG. 3. Each of the sun gear 28, planet gears 32 and ring gear 38 comprise teeth about their periphery to intermesh with the other gears. However, for clarity only exemplary portions of the teeth are illustrated in FIG. 3. There are four planet gears 32 illustrated, although it will be apparent to the skilled reader that more or fewer planet gears 32 may be provided within the scope of the claimed invention. Practical applications of a planetary epicyclic gearbox 30 generally comprise at least three planet gears 32.

(14) The epicyclic gearbox 30 illustrated by way of example in FIGS. 2 and 3 is of the planetary type, in that the planet carrier 34 is coupled to an output shaft via linkages 36, with the ring gear 38 fixed. However, any other suitable type of epicyclic gearbox 30 may be used. By way of further example, the epicyclic gearbox 30 may be a star arrangement, in which the planet carrier 34 is held fixed, with the ring (or annulus) gear 38 allowed to rotate. In such an arrangement the fan 23 is driven by the ring gear 38. By way of further alternative example, the gearbox 30 may be a differential gearbox in which the ring gear 38 and the planet carrier 34 are both allowed to rotate.

(15) It will be appreciated that the arrangement shown in FIGS. 2 and 3 is by way of example only, and various alternatives are within the scope of the present disclosure. Purely by way of example, any suitable arrangement may be used for locating the gearbox 30 in the engine 10 and/or for connecting the gearbox 30 to the engine 10. By way of further example, the connections (such as the linkages 36, 40 in the FIG. 2 example) between the gearbox 30 and other parts of the engine 10 (such as the input shaft 26, the output shaft and the fixed structure 24) may have any desired degree of stiffness or flexibility. By way of further example, any suitable arrangement of the bearings between rotating and stationary parts of the engine (for example between the input and output shafts from the gearbox and the fixed structures, such as the gearbox casing) may be used, and the disclosure is not limited to the exemplary arrangement of FIG. 2. For example, where the gearbox 30 has a star arrangement (described above), the skilled person would readily understand that the arrangement of output and support linkages and bearing locations would typically be different to that shown by way of example in FIG. 2.

(16) Accordingly, the present disclosure extends to a gas turbine engine having any arrangement of gearbox styles (for example star or planetary), support structures, input and output shaft arrangement, and bearing locations.

(17) Optionally, the gearbox may drive additional and/or alternative components (e.g. the intermediate pressure compressor and/or a booster compressor).

(18) Other gas turbine engines to which the present disclosure may be applied may have alternative configurations. For example, such engines may have an alternative number of compressors and/or turbines and/or an alternative number of interconnecting shafts. By way of further example, the gas turbine engine shown in FIG. 1 has a split flow nozzle 20, 22 meaning that the flow through the bypass duct 22 has its own nozzle that is separate to and radially outside the core engine nozzle 20. However, this is not limiting, and any aspect of the present disclosure may also apply to engines in which the flow through the bypass duct 22 and the flow through the core 11 are mixed, or combined, before (or upstream of) a single nozzle, which may be referred to as a mixed flow nozzle. One or both nozzles (whether mixed or split flow) may have a fixed or variable area. Whilst the described example relates to a turbofan engine, the disclosure may apply, for example, to any type of gas turbine engine, such as an open rotor (in which the fan stage is not surrounded by a nacelle) or turboprop engine, for example. In some arrangements, the gas turbine engine 10 may not comprise a gearbox 30.

(19) The geometry of the gas turbine engine 10, and components thereof, is defined by a conventional axis system, comprising an axial direction (which is aligned with the rotational axis 9), a radial direction (in the bottom-to-top direction in FIG. 1), and a circumferential direction (perpendicular to the page in the FIG. 1 view). The axial, radial and circumferential directions are mutually perpendicular.

(20) It is known to control the thrust of a gas turbine engine using a control system implemented by an Electronic Engine Controller (EEC), the thrust of the engine being indirectly measured using shaft speed, Engine Pressure Ratio (EPR) or Turbine Power Ratio (TPR). The EEC also controls (i) the shaft speeds within safe operational limits, and (ii) the temperature and pressure at different parts of the engine to avoid undesirable conditions such as surge or stall, and to ensure the integrity of the engine. Environmental considerations as well as growing power demands of modern aircraft require control systems that are robust and optimised to the operating conditions of the aircraft. In particular, electronic closed-loop fuel control systems have an integrating action which helps to ensure accurate control of the engine while meeting the pilot's demands for thrust and complying with safety limits.

(21) For example, the engine 10 may have an engine fuel control system based on the Rolls-Royce Inverse Model, or RIMM, discussed in U.S. Pat. No. 8,321,104 (incorporated herein by reference). The control system, shown schematically in FIG. 4, relates the rotational speed of the high-pressure shaft NH to the fuel flow requirement W.sub.f. In more detail, it subtracts at summer junction 41 an estimate of the steady state fuel flow requirement W.sub.f.sup.ss from a feedback of the trimmed aggregate fuel flow demand signal W.sub.fd. The difference is multiplied at 42 by an estimated rate of change of engine speed with fuel increment NHdot/ΔW.sub.f to provide an estimate of engine acceleration NHdot, and this is then integrated at block 43 to provide an estimate for NH. The dynamics of the RRIM are tuned to the requirements of the engine via data within nonlinear modules of first 44 and second 45 engine model blocks or modules embedded within the feedback loop to respectively produce W.sub.f.sup.ss and ΔW.sub.f/NHdot values from the estimate for NH, W.sub.f.sup.ss being a steady state fuel flow requirement, and ΔW.sub.f being an overfuelling requirement. The system accounts for, e.g. the pilot's engine speed demand lever and an acceleration limiter loop comparator, to generate an acceleration demand signal NH.sub.ddot which is combined at multiplier 46 with the ΔW.sub.f/NHdot value to provide an overfuelling requirement ΔW.sub.f sent to a summer junction 47 for combining with the W.sub.f.sup.ss value to generate a minimum fuel flow requirement W.sub.f. This is then supplied to a logic block 48 for comparison with fuel schedules/limiting loops (providing e.g. a maximum fuel flow signal) to produce the trimmed aggregate fuel flow demand signal W.sub.fd for controlling the operation of a fuel system which in turn regulates the flow of fuel to the engine.

(22) Advantageously, such a system can be modified to provide active damping of either of the shafts 26, 27 of the engine. In particular, the feedback loop of the engine fuel control system typically runs at about 25 ms and caters for first order dynamics governed by turbine torque driving a total spool inertia. However, a representative model of higher order dynamics that includes shaft resonance can be represented by a second order transfer function such as:

(23) $H\left(s\right)=\frac{\theta }{\tau }=\frac{A{\omega }_0^2}{s^2+2\zeta {\omega }_{0}s+{\omega }_0^2}$

(24) or in differential form as:

(25) $\frac{d^2\theta }{{\mathrm{dt}}^2}+2\zeta {\omega }_0\frac{d\theta }{\mathrm{dt}}+{\omega }_0^2\theta =A{\omega }_0^2\tau$

(26) or:

(27) $\frac{d^2\theta }{{\mathrm{dt}}^2}+2\zeta {\omega }_0\omega +{\omega }_0^2\theta =A{\omega }_0^2\tau$

(28) where τ is the torque on the shaft, θ is the shaft wind up or twist, ω=dθ/dt is the rate of change of twist in the rotating frame of reference of the shaft (i.e. ω is the torsional oscillation of the shaft), A is a DC gain, ζ is a damping factor and ω.sub.0 is the undamped resonance frequency.

(29) The trimmed aggregate fuel flow demand signal W.sub.fd resulting from the control system of FIG. 4 results in a nominal torque on the shaft, τ.sub.nominal. To actively counteract or cancel a torsional resonance, τ.sub.nominal can be supplemented by a torque component applied to the shaft which is modulated at the same frequency as, but in opposition to, the torsional oscillation ω. Thus the overall torque on the shaft τ=τ.sub.nominal−αω, where α is a constant of proportionality.

(30) A modified engine fuel control system that can provide such active damping is shown in FIG. 5. In the modified system, a further summer junction 49 adjusts the aggregate fuel flow demand signal W.sub.fd from the logic block 48 by an amount proportional to—αω. Notably, although the fuel control system uses a measurement of the high-pressure shaft NH to determine W.sub.fd, the adjusted flow demand signal can be used to actively damp either the high-pressure shaft 27 or the low-pressure shaft 26. Indeed both shafts can be simultaneously actively damped by providing summer junction 49 with a respective−αω for each shaft. That the low-pressure shaft 26 can be damped in this way, despite the intervention of the high-pressure turbine 17 between the combustion equipment 16 and the low pressure turbine 19, is possible because the gas dynamics in the turbine end of the engine operate over substantially shorter time scales than the shaft dynamics. Moreover, there is little chance that a fuel flow demand signal modulation use to actively damp one shaft will cause resonance of another shaft because the shaft resonance frequencies are generally well separated.

(31) To perform the frequency modulation of W.sub.fd in proportion to—αω requires measurements of the torsional oscillation ω to be available to the engine fuel control system at a significantly higher rate than the 25 ms run time of the control system feedback loop. A suitable measurement rate of about 5 ms can be achieved using a phonic wheel on the shaft of interest. Any such phonic wheel would typically be mounted towards the front end of a shaft, i.e. adjacent its compressor, and at a distance from the midpoint of the shaft about which torsional oscillations are usually centred. This places the phonic wheel at a location where the torsional oscillations are of relatively high amplitude, and hence increases the sensitivity of measurements made using the wheel.

(32) Conventionally, phonic wheels and associated sensors are used to measure shaft speed. For example, FIG. 6 shows schematically at top a phonic wheel 51 which is mounted coaxially to the shaft for rotation therewith. The phonic wheel has a circumferential row of teeth, and a variable reluctance sensor 52 which detects the passage of the row of teeth by generating an alternating measurement signal. As each tooth of the phonic wheels passes close to the front face of a pole piece of the sensor there is a change in the magnetic flux experienced by a conductive wire wrapped around the pole piece, owing to the change in the reluctance of the magnetic circuit consisting of the pole piece, the phonic wheel and the air gap between the two.

(33) For shaft speed measurement, software determines rolling averages (based on e.g. 10 to 20 samples) of the timing pulses (tooth passing events) of the measurement signal and these averages are used to continuously calculate and update the rotational speed. In particular, the software typically includes a zero crossing detector that samples using a clock rate of a few MHz to determine the timing between zero crossings. This is then used to calculate the rotational speed of the phonic wheel 51. However, the software can also sample the sinusoidal waveform produced by the phonic wheel directly using a fast ND converter, at a suitable frequency (e.g. 20 kHz to 20 MHz).

(34) Using any of various frequency demodulation techniques known to the persons skilled in the art (e.g. using a Hilbert transform), it is then possible to recover the harmonic content associated with the phonic wheel vibration due to the torsional oscillation ω of the shaft.

(35) The true shaft speed signal generated by the phonic wheel sensor 52 when the shaft is oscillating is given by:
N=A sin(Ωt+B sin(ω.sub.0t))

(36) where Ω is the shaft rotational velocity. This is a frequency modulated signal and can therefore be de-modulated using phase-locked loops, quadrature detection and techniques known to people skilled in the art. This can be implemented in the EEC by deploying field-programmable gate arrays or digital signal processors, to recover the signal ω=B sin(ω.sub.0t)) in order to use in the engine fuel control system.

(37) Instead of a phonic wheel, the torsional oscillation ω can be measured using e.g. an optical encoder. Such a device can provide an improved signal to noise ratio relative to a phonic wheel.

(38) The active damping approach discussed above does not rely on designing out all possible interactions that may give rise to shaft resonance. It is thus more adaptable. It is also enabling of reduced shaft responses, thereby increasing shaft life.

(39) Although described above in respect of an aero gas turbine engine, the approach can be used e.g. for active damping of a shaft of a marine engine coupled to a propeller, or for active damping of a power-offtake shaft of an engine being used for electrical power generation.

(40) The active damping can be applied to the shaft by means other than modulating the fuel supply. For example, the engine may have an electric motor 53, such as a starter motor or generator such as that shown in FIG. 7, which can be used to apply the torque component—αω directly to the shaft.

(41) It will be understood that the invention is not limited to the embodiments above-described and various modifications and improvements can be made without departing from the concepts described herein. Except where mutually exclusive, any of the features may be employed separately or in combination with any other features and the disclosure extends to and includes all combinations and sub-combinations of one or more features described herein.