GB2169712A - Method and apparatus to generate angular velocity signals by magnetic recording and playback - Google Patents

Abstract

A method and apparatus for obtaining a repetitive signal from a rotating shaft, wherein the repetition rate of the signal is proportional to rotational speed of the shaft. In a preferred embodiment, an electromagnetic probe is spaced proximate the rotating shaft and energized for a predetermined interval at a repetitive rate equal to a multiple of shaft rotation speed in order to record discrete regions of magnetization, the number of which is independent of shaft rotation speed, on the surface of the shaft. The probe is then de-energized and used as a sensor to detect playback pulses generated from the magnetised portion of the rotating shaft. The repetition rate of playback pulses is indicative of instantaneous angular velocity of the shaft. Alternatively, a separate record and playback electromagnetic probe made be used, and recording and playback may be continuous. <IMAGE>

Description

SPECIFICATION Method and apparatus to generate angular velocity signals by magnetic recording and playback This invention pertains to the field of tachometry, and in particular to a method and apparatus for recording discrete regions of magnetism onto a rotating body to generate, in a playback mode, a repetitive signal indicative of the angular velocity of the body.

Background of the Invention Knowledge of rotational speed or angular velocity of a rotating element is often vital in the application, study, and diagnosis of operating machinery. Tachometers based on various operating principles have been developed to meet the need for obtaining angular velocity. For example, one well-known method is to paint axial stripes on a rotatable shaft and use an optical pickup to detect a change in reflected light as the stripes pass by the pickup while the shaft is rotating. In another method, a gear, or similar article having teethlike projections, is attached to a rotatable shaft and a magnetic pickup detects the passing teeth which are counted to determine the average shaft speed. Numerous other tachometric methods and devices have also been devised, all of which indicates the prevalent need to obtain a rather fundamental measurement.

Although numerous tachometric methods, which perform entirely satisfactorily under certain circumstances are available, inherent requirements preclude their use under other circumstances. For example, the methods described above require a physical installation of some added element to a rotatable body.

Even in those cases where such an installation is physically possible, the necessary expenditures may be prohibitive. Also, an optical method has the further disadvantage that the light path between the indicator (e.g. stripes) and detector must be clear and free of obstructions. This is not always possible, particularly where there may be light obscuration due to oil or other liquid mists or to eventual dirt or grime build-up on the indicator and/or detector.

One area in which tachometric signals are of particular importance, and in which conventional means have proven unsatisfactory, is in monitoring torsional vibrations which occur in large rotors, exemplified by rotors and shafts found in turbine-generator sets used for electrical power generation. Signals having a repetition rate proportional to rotor rotational speed serve as an important input to vibration monitors, such as were disclosed in U.S. Patents 3,885,420 and 4,148,222, both in the name of J.F. Wolfinger and assigned to the assignee of the present invention. If timely corrective action is not taken against undesirable torsional vibrations, permanent damage may occur to the rotor and/or other elements of the turbine-generator set.To provide utmost reliability in torsional vibration monitoring instruments, a highly reliable rotor speed indication, which is capable of denoting very small changes in the instantaneous angular velocity of the rotor, is necessary.

Accordingly, it is an object of the present invention to provide a method and apparatus for generating an electrical signal whose frequency, or repetition rate, is indicative of instantaneous angular velocity of a rotating body, without need of a special attachment or added indicia upon the rotating body.

In one embodiment of the invention to be described tachometric signals are generated by inducing a plurality of discrete magnetic regions into the surface of a rotating body along a circumferential path situated in a plane which is substantially perpendicular to the axis of rotation of the rotating body, thereby forming separate or discrete regions of permanent magnetism which can be sensed as the body rotates to provide the desired response signal, which is indicative of the instantaneous angular velocity of the body. In a preferred embodiment, a single probe in proximity to a rotatable shaft and coupled to an induction excitation source provides, in a first mode of operation, means to induce magnetic regions, and, in a second mode of operation, means to detect movement of the magnetic regions.A time varying signal whose frequency is controlled to be a multiple of shaft rotational speed or angular velocity may be used to synchronize the induction excitation source, such that induced magnetic regions are uniformly and continuously spaced around the rotatable shaft, and the number of regions induced along the entire circumferential path is independent of the rotational speed of the shaft.

Embodiments of the present invention are particuarly well suited for generating signals indicative of rotor angular velocity that further enable measurement of torsional vibrations in rotors of large turbine-generator sets.

Brief Description of the Drawing While the specification concludes with claims particularly pointing out and distinctly claiming the subject matter regarded as the invention, the invention will be better understood from the following description taken in connection with the accompanying drawing in which: Fig. 1 illustrates a preferred circuit arrangement for generating a signal indicative of shaft rotational speed; Fig. 2 illustrates a preferred embodiment of a record/playback probe suitable for use with the circuitry of Figure 1; and Fig. 3 is a side elevational view of the record/playback probe of Figure 2 as it may be positioned to monitor the speed of and/or induce magnetic regions onto a rotating shaft.

Detailed Description of the Invention Fig. 1 schematically illustrates circuitry and apparatus for generating a signal having a repetition rate or frequency indicative of the instantaneous angular velocity, or rotational speed, of a rotatable shaft 10. A record/playback probe 12 is disposed in close proximity to shaft 10 and is operable in selectable modes-a first mode for recording, in which magnetic flux produced by probe 12 induces small, localized circumferential permanent magnetic regions into the surface of shaft 10, and a second mode for sensing or playing back, in which movement of previously induced magnetic regions is sensed and an electrical signal responsive to the movement is generated.

Shaft 10 may be a large shaft or rotor of the type typically found in electrical utility turbinegenerator sets. Record/playback probe 12 may be an electromagnetic device (more fully described hereinafter).

A switch 13 is operable to select either of the first or second mode of operation of probe 12, designated by switch positions R (Record) and P (Playback), respectively. In the record mode, common terminal C of switch 13 is connected to terminal R of switch 13 in order to connect circuitry for supplying an excitation signal to probe 12, so that a sufficiently intense electrical signal for inducing magnetic regions on the surface of shaft 10 is applied to probe 12. In the preferred embodiment illustrated in Fig. 1, magnetic regions are induced while shaft 10 is rotated. Synchronization means 14, may comprise a shaft rotation sensor or variable reluctance pickup for detecting a physical non-uniformity, such as a nub or notch, in shaft 10.Synchronization means 14 produces a synchronization or shaft rotation signal indicative of a complete revolution of shaft 10, which is coupled through an amplifier 16 to an induction excitation source 15 comprising a limiter 17, a phase-locked loop 18, a time base generator 20, an active filter 25, having an appropriate pass band for signal frequencies determined by frequency divider 20, and an amplifier 26. It is also possible to operate without active filter 25, whereby values of the divisor of frequency divider 20 may be changed without need to change the frequency characteristics of active filter 25.The synchronization signal available at the input of limiter 17 causes induction excitation source 15 to synchronize the excitation signal provided to probe 12 with the anguiar velocity of shaft 10, thereby generating uniformly spaced regions of magnetization on the surface of shaft 10 and ensuring that the number of induced magnetic regions along the entire circumferential path on the surface or shaft 10 is independent of the angular velocity of shaft 10. That is, excitation source 15 is not free running at a constant frequency, but generates an output or excitation signal, having a frequency which is a predetermined multiple of the angular velocity of shaft 10, to ensure that the circumferential magnetic pattern to be induced around shaft 10 is coincidently induced on each revolution of shaft 10.

The synchronization signal supplied to the input of amplifier 16 may be derived in any convenient manner. In the case of a utility turbine-generator set, it is most satisfactorily obtained from the generated electrical power signal, having a nominal frequency in the United States of 60 Hz. It is to be noted, however, that the exact frequency of the synchronization signal is not critical, only that it be proportional to shaft angular velocity. The synchronization signal is amplified in amplifier 16 and the output of amplifier 16 is coupled to limiter network 17, which limits the amplitude of the synchronization signal, so that a signal having a substantially constant amplitude and same frequency as the synchronization signal, is available at the output of limiter 17 . The output of limiter 17 is coupled to a first input of phased locked loop 18.Phaselocked loop 18 and time base generator 20 cooperate to form a frequency synthesis network for generating an output signal from phased-locked loop 18, having a frequency which is an integral multiple of the signal provided to the first input of phase-locked loop 18.

Operation of phase-locked loop 18, a network well known to those skilled in the art, is designed to control the frequency of the signal available at the output of time base generator 20 and applied to a second or feedback input of phase-locked loop 18, such that the frequency of the signal supplied to the second input of phase-locked loop 18 is precisely the same as the frequency of the signal applied to the first input of phase-locked loop 18. Time base generator 20 comprises a frequency divider, or counter, whose signal output frequency is a pre-selected submultiple of the input frequency of the signal supplied to time base generator 20 from the output phaselocked loop 18. Therefore, since the frequency of the signal at the second input of phaselocked loop 18 is conditioned to be precisely equal to the frequency of the signal at the first input of phase-locked loop 18, the output signal of phase-locked loop 18 is forced to be a multiple of the frequency of the signal available at the first input of phase-locked loop 18, wherein the multiple is precisely equal to the divisor of time base generator 20. For example, time base generator 20 may be configured to cause its input signal to be divided in frequency by a factor of sixty.Phase-locked loop 18, acting to maintain the frequency of the signal provided to the second input thereof at, for example, one cycle per revolution, in order io be equal to an assumed sig nal frequency of one cycle per revolution at the first input of phase-locked loop 18, is forced to provide a signal having a frequency of 60 cycles/revolution at the output of phase-locked loop 18.

Active filter 25, having the input thereof coupled to the output of phase-locked loop 18, comprises a narrow bandpass and smoothing filter whose pass band is centered about the desired excitation frequency. The smoothing action of filter 25 is designed to restore the output signal of phase-locked loop 18, which may be substantially square-wave in character, to sine wave quality before application to a power amplifier 26. The output signal from power amplifier 26 is applied through series resonance capacitor 27 and switch 13 to probe 12. Capacitor 27 in combination with the inductance of probe 12 also provides filtering so that active filter 25 may be eliminated if desired.

Probe 12, illustrated in detail in Figures 2 and 3, includes a laminated iron core 51 having a narrow transverse slot 52 through one end and a pair of coils 53 and 54 wound about opposite arms of core 51. Slot 52 is preferably long enough in the axial direction d to encompass axial movement of shaft 10, which may occur, for example, due to thermal expansion or bearing wear, in order to maintain adequate magnetic flux communication between probe 12 and magnetic regions induced on the surface of shaft 10. A toroidal ferrite core may also be used for core 51. Coiis 53 and 54 may, for example, each be approximately 550 turns of AWG No. 26 wire. Coils 53 and 54 are preferably connected in parallel during the record mode and in series during the playback mode; the desired connection configuration may be made switch selectable.

In Fig. 1, probe 12 is shown including only a single coil for the purpose of explaining the preferred circuit arrangement and to facilitate an understanding thereof.

In operation, probe 12 is fixedly disposed, with respect to rotatable shaft 10, in close proximity to shaft 10 so that slot 52 is substantially parallel to the rotational axis of shaft 10 as illustrated in Fig. 3. Magnetic flux generated within laminated core 5 1 by excitation current from capacitor 27 (Fig. 1) through coils 53 and 54 of probe 12 is essentially confined within core 5 1 except across slot 52. When excitation current is supplied to coils 53 and 54 of probe 12, magnetic flux traverses slot 52 such that a portion of flux diverges outside the margins of core 51 defining slot 52 and passes into rotating shaft 10, thereby inducing localized regions of maone- tism into the surface of shaft 10.The magnetic pattern so induced conforms to the time varying shape of the excitation current supplied to coils 52 and 53 from amplifier 26 through capacitor 27 so that, with sine wave excitation current, the magnetic pattern around the circumference of shaft 10 is substantially sine wave in character. A sine wave is preferred as the excitation signal to avoid inductive effects, such as ringing, with signals having a more rapid rate of change. The positioning of probe 12 to induce magnetic regions onto the surface of shaft 10 ensures that probe 12 will be in magnetic flux communication with the induced magnetic regions during the playback mode.

Referring again to Fig. 1, the value of capacitor 27 is selected such that with switch 13 in record position R, probe 12 and capacitor 27 provide a series tuned circuit, resonant at the frequency of the excitation signal. Characteristic of series tuned circuits, the reactance of probe 12 (mostly inductive) and of capacitor 27 cancel, with the result that the current flowing through probe 12 during the record is much higher than generally otherwise attainable without a series tuned circuit, thereby enhancing magnetic recording. The value of capacitor 28 is selected such that in the playback position P, switch 13 connects probe 1 2 in parallel with capacitor 28 to provide a parallel tuned circuit, resonant at the frequency of the excitation signal used to record the magnetic regions into the surface of shaft 10.In those instances where the shaft of a turbinegenerator is being monitored, the resonant frequency may for example be 3600 Hz. The parallel tuned circuit in the playback mode provides a higher amplitude induced signal, than would an untuned circuit, at and near the resonant frequency. The signal induced in the parallel tuned circuit is supplied to the input of an amplifier 30. The playback signal, having a frequency proportional to the instantaneous velocity of shaft 10 encoded therein, is amplified by amplifier 30 and the amplified signal, available at the output of amplifier 30, may, for example, be used as the input to a torsional vibration monitoring instrument, such as for electric utility turbine-generators. Of course, it is not necessary that the circuits including capacitor 27 and capacitor 28, respectively, be exactly tuned to the respective predetermined resonant frequency.However, it is to be understood that for circuits not tuned to the respective predetermined resonant frequency, the voltages and current will be less than what is achievable in a resonant tuned circuit, and, it may be possible to provide values of capacitor 27 and capacitor 28 such that the respective circuits are detuned so much that the voltages and currents in the respective circuits are inadequate to produce the desired effects in accordance with the present invention.

Operation To monitor instantaneous angular velocity of shaft 10, operation of the circuitry of Fig. 1 is as follows. Switch 13 is initially placed in record mode position R to enable magnetization of small circumferentially disposed localized areas onto the surface of shaft 10 by probe 12, which operates as an electromagnet to induce a magnetic pattern in response to excitation signal current, preferably sinusoidal, provided thereto from amplifier 26 thru capacitor 27. The induced magnetic regions are preferably disposed along a circumferential path situated in a shaft section which is substantially perpendicular to the axis of rotation. The inductance of probe 12, along with the predetermined capacitance of capacitor 27, preferably forms a series tuned circuit resonant at the desired frequency of the excitation signal.

The excitation signal supplied to probe 12 is synchronized to rotation of shaft 10 by the synchronization signal, such as a one per revolution signal as hereinbefore described, available from shaft rotation sensor 14. The synchronization signal is multiplied in frequency by phase-locked loop 18 and time base generator 20, and a signal having a frequency which is an integral multiple of the synchronization signal is supplied to power amplifier 26 through active filter 25. The permanence of the magnetic pattern recorded onto shaft 10 depends on the permeability of the material of shaft 10 and on physical treatment of shaft 10, e.g. exposure to extraneous magnetic fields.However, for typical shafts comprising magnetic material, such as iron, especially when such shafts are disposed in an operating environment, the pattern is relatively permanent so that once the magnetized regions are induced onto the surface of shaft 10, switch 13 may be placed in the playback position P for monitoring the rotational speed or playback signal induced into probe 12 by the magnetized regions as shaft 10 is rotated.

The playback signal is developed in a parallel tuned circuit including the predetermined capacitance of capacitor 28 and inductance of probe 12, and the parallel tuned circuit is preferably resonant at the frequency of the excitation signal used to induce the magnetic regions onto the surface of shaft 10. The playback signal has encoded therein a frequency proportional to the instantaneous angular velocity of shaft 10, thus permitting very small changes in shaft speed to be detected, say changes on the order of 2X10 radians per second.

While there has been shown and described what is considered a preferred embodiment of the invention, and there has been set forth the best mode contemplated of carrying out the invention, it is to be understood that various modifications and other uses may be made thereof. For example, although one electromagnetic probe 12 having dual modes of recording and playback is described, it will be recognized that separate probes 12 and 40, probe 12 for recording and probe 40 for piaX- back, may be employed in the practice of the invention as illustrated by the circuitry shown in Fig. 1, in which case switch 13 permanently selects the R position, or probe 12 is directly coupled to capacitor 27 without need of switch 13. This alternate configuration permits continuous recording and continuous playback while shaft 10 is rotating.Probe 40, fixed with respect to shaft 10 and circumferentially displaced from probe 12, is in magnetic flux communication with the magnetic pattern induced by probe 12 onto the surface of shaft 10.

A capacitor 44 and probe 40 form a parallel tuned circuit and the value of the capacitance of capacitor 44 is selected such that the parallel tuned circuit is preferably resonant at the predetermined frequency of the excitation signal. The output playback signal from the parallel tuned circuit comprising probe 40 and capacitor 44 is provided to an amplifier 42 whose output signal may be used as the input signal to a torsional vibration monitoring instrument, such as for electric utility turbinegenerators as hereinbefore described.

Further, it is not necessary that shaft 10 have magnetic regions induced while it is disposed in its desired operational environment.

The magnetic regions may be induced onto the surface of shaft 10 at a location remote from its ultimate place of use, due in part to the permanence of the induced magnetic regions, and the piayback circuitry, as described herein, may be used to detect the previously induced magnetic regions at the ultimate place of use of shaft 10. Knowing the frequency of the excitation signal used to induce the magnetic regions permits the value of capacitor 28 or 44 to be selected to provide a parallel tuned resonant circuit.

Thus has been illustrated and described a method and apparatus for generating an electric signal whose frequency, or repetition rate, is indicative of instantaneous angular velocity of a rotating body, without need of a special attachment or added indicia upon the rotating body.

While only certain preferred features of the invention have beep shown by way of illustration, many modifications and changes will occur to those skilled in the art. It is to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit and scope of the invention.

Claims (26)

1. Velocity measuring instrumentation for measuring angular velocity of a rotatable shaft, comprising: an electromagnetic probe fixed with respect to the shaft and disposed proximity the surface of the shaft, said probe operable in a first mode for inducing a circumferential magnetic pattern on the surface of the shaft and operable in a second mode for generating an output signal in response to movement of said magnetic pattern while the shaft is rotating, said output signal having a frequency indicative of the instantaneous angular velocity of the shaft; switching means coupled to said probe for switching between said first and second mode; and excitation means having an output coupled to said probe through said switching means in said first mode, said excitation means for generating an excitation signal to be supplied to said probe during said first mode, thereby causing said probe to induce said magnetic pattern, said excitation means including synchonization means for synchronizing the frequency of said excitation signal with the rotational speed of the shaft so that said magnetic pattern is independent of the rotational speed of the shaft.

2. The instrumentation as in claim 1 further including: first capacitor means coupled between the output of said excitation means and said switching means for forming, in the first mode, a series resonant circuit with said probe; and second capacitor means coupled to said probe for forming, in the second mode, a parallel resonant circuit with said probe.

3. The instrumentation as in claim 2 wherein said series resonant circuit and said parallel resonant circuit are each respectively resonant at the frequency of the excitation signal.

4. Velocity monitoring instrumentation for measuring angular velocity of a rotatable shaft, comprising: a first electromagnetic probe fixed with respect to the shaft and disposed proximate the surface of the shaft, said probe operable for inducing a circumferential magnetic pattern on the surface of the shaft; a second electromagnetic probe fixed with respect to the shaft, and disposed in magnetic flux communication with the magnetic pattern, said second probe for generating an output signal in response to movement of the magnetic pattern while the shaft is rotating, said output signal having a frequency indicative of the instantaneous angular velocity of the shaft; and excitation means coupled to said first probe for generating an excitation signal, thereby causing said first probe to induce the magnetic pattern, said excitation means including synchronization means for synchronizing the frequency of the excitation signal with the rotational speed of the shaft so that the magnetic pattern is independent of the rotational speed of the shaft.

5. The instrumentation as in claim 4 further including: first capacitor means coupled between said first probe and said excitation means for forming a series resonant circuit with said first probe; and second capacitor means coupled to said second probe for forming a parallel resonant circuit with said second probe.

6. The instrumentation as in claim 4 wherein said excitation means includes a phase-locked loop for generating said excitation signal and said synchronization means includes a shaft rotation sensor for providing a synchronizing signal to said phase-locked loop.

7. The instrumentation as in claim 5 wherein said excitation means includes a phase-locked loop for generating the excitation signal and said synchronization means includes a shaft rotation sensor for providing a synchronizing signal to said phase-locked loop.

8. The instrumentation as in claim 5 wherein said series resonant circuit and said parallel resonant circuit are each respectively resonant at the frequency of the excitation signal.

9. For use in measuring angular velocity of a rotatable shaft of a turbine-generator set, apparatus comprising: an electromagnetic probe in fixed proximity to the surface of the shaft, said probe operable in a first mode for inducing a circumferential magnetic pattern onto the surface of the shaft, and operable in a second mode for generating an output signal in response to movement of said magnetic pattern while the shaft is rotating, said output signal having a frequency indicative of the instantaneous angular velocity of the shaft; switching means coupled to said probe for selectively switching between the first and second mode;; excitation means having an output coupled through said switching means to said probe in the first mode, said excitation means for supplying an excitation signal to said probe during the first mode, thereby causing said probe to induce said magnetic pattern; first capacitor means coupled between the output of said excitation means and said switching means for forming, in the first mode, a series resonant circuit with said probe; and second capacitor means coupled to said switching means for forming, in the second mode, a parallel resonant circuit with said probe.

10. The apparatus as in claim 9 wherein said series resonant circuit and said parallel resonant circuit are each respectively resonant at the frequency of the excitation signal.

11. The apparatus as in claim 9 wherein said excitation means includes a phase-locked loop for generating said excitation signal in response to a synchronization signal supplied by a shaft rotation sensor to said phaselocked loop.

12. For use in measuring angular velocity of a rotatable shaft of a turbine-generator set, apparatus comprising: a first electromagnetic probe disposed in fixed proximity to the surface of the shaft for inducing a circumferential magnetic pattern onto the surface of the shaft; a second electromagnetic probe disposed in magnetic flux communication with said magnetic pattern; excitation means having an output coupled to said first probe for supplying an excitation signal to said first probe, thereby causing said first probe to induce said magnetic pattern; first capacitor means coupled between the output of said excitation means and said first probe for forming a series resonant circuit with said first probe; and second capacitor means coupled to said second probe for forming a parallel resonant circuit with said probe.

13. The apparatus as in claim 12 wherein said series resonant circuit and said parallel resonant circuit are each respectively resonant at the frequency of the excitation signal.

14. The apparatus as in claim 12 wherein said excitation means includes a phase-locked loop for generating said excitation signal in response to a synchronization signal supplied to said phased-locked loop by a shaft rotation sensor.

15. A method for generating a repetitive signal whose repetition rate is proportional to the instantaneous angular velocity of a rotatable shaft, comprising: inducing a predetermined number of discrete magnetic regions into the surface of the shaft along a circumferential path; detecting movement of said magnetic regions past a stationary means for sensing magnetic fields, while the shaft is rotating, to produce the repetitive signal, said means for sensing disposed in magnetic flux communication with said magnetic regions.

16. The method as in claim 15 further including inducing said magnetic regions while the shaft is rotating.

17. The method as in claim 16 wherein said magnetic regions are continuously induced into the surface of the shaft and movement of said magnetic regions is continuously detected.

18. The method as in claim 16 wherein the predetermined number of discrete magnetic regions is independent of rotational speed of the shaft.

19. The method as in claim 17 wherein the predetermined number of discrete magnetic regions is independent of rotational speed of the shaft.

20. The method as in claim 15 wherein said magnetic regions are induced in response to an excitation signal from a series tuned circuit resonant at the frequency of the excitation signal.

21. The method as in claim 16 wherein said magnetic regions are induced in response to an excitation signal from a series tuned circuit resonant at the frequency of the excitation signal.

22. The method as in claim 17 wherein said magnetic regions are induced in response to an excitation signal from a series tuned circuit resonant at the frequency of the excitation signal.

23. The method as in claim 15 wherein movement of said magnetic regions is detected by inducing a current in a parallel tuned circuit, said parallel tuned circuit including said means for sensing and resonant at the frequency of the excitation signal.

24. The method as in claim 16 wherein movement of said magnetic regions is detected by inducing a current in a parallel tuned circuit, said parallel tuned circuit including said means for sensing and resonant at the frequency of the excitation signal.

25. The method is in claim 17 wherein movement of said magnetic regions is dctected by inducing a current in a parallel tuned circuit, said parallel tuned circuit including said means for sensing and resonant at the frequency of the excitation signal.

26. Velocity measuring instrumentation as claimed in claim 1 substantially as described herein with reference to the accompanying drawing.