SHAFT COUPLING DISPLACEMENT MONITORING

Abstract: A shaft misalignment monitoring system comprising a coil disposable on one side of a coupling connecting two co-rotating shafts and a ferromagnetic rod disposable on the other side of the coupling, the rod being so positioned within the confines of the coil that misalignment of the shafts alters the relative disposition of the rod and coil so as to affect the inductance of the coil, the coil forming part of an alternating current circuit an operating parameter of which is variable in dependence upon the inductance of the coil and which variable operating parameter is monitored to give an indication of the position of the rod with respect to the coil and hence of the degree of misalignment of the shafts.

Description: SHAFT MISALIGNMENT MONITORING This invention relates to the monitoring of shaft misalignment and particularly to misalignment between two shafts at a coupling between the shafts, the coupling being of the sort which is intended to accommodate such misalignment between the shafts as may be found to exist. Although the coupling may be of the kind which accommodates misalignment it is nevertheless very much more preferable if the shafts are substantially in line during normal operation otherwise the coupling will be under stress and hence is more likely to fatigue sooner than it would if that stress was not present. Unfortunately, the accurate lining-up of two shafts while the system is cold and at rest is not satisfactory because amost invariably, once the system is at working speed and temperature and has reached equilibrium, several factors tend to disturb that alignment. For instance thermal expansion of the shafts will usually result in some axial misalignment between them and meshing of gears may resut in angular misalignment. Thus it has been proposed that a method of whot misalignment monitoring" should be employed where the misalignment between two shafts is monitored while the system is running at equilibrium and then making any necessary adjustments when the system is stopped. U.S. Patent No. 4148013 describes such an arrangement where a proximity probe or a strain gauge or a differential transformer is used to give an indication of shaft misalignment. That indication is transmitted from the sensing means on the shaft to receiving means off the shaft through a rotary transformer. The use of a differential transformer is unsatisfactory in a number of ways. Firstly it is necessary for there to be three coils, one primary and two secondary with a ferromagnetic rod providing the magnetic coupling between the secondary and primary coils. Thus in physical terms . the differential transformer is long and may present windage problems at high rotary speeds. Because the rod is coupling two coils its angular disposition within the coils is critical and since the rod is connected to one coupling flange and the coils to the other and since the coupling may be angularly misaligned, an arrangement must be provided to keep the rod square in the coils. This usually involves the use of a spring which introduces further problems of spring bounce at high rotary speeds if significant angular misalignment exists. The rod is usually a sliding fit within the coils. This may present frictional problems and increased wear of the parts. The use of a proximity probe is not entirely satisfactory either. These are either of the inductive or of the capactitive type and neither provides a linear output in response to misalignment. Furthermore, if it is of the capacitive type there can be dialectric problems caused by oil mist or the like if that should present. With either type there occurs calibration problems if no account is taken of the non-linearity of proximity probes or else linearising means must be provided. It is therefore an object of this invention to provide an improved method of shaft misalignment monitoring. In accordance with this invention there is provided a shaft misalignment monitoring system comprising a coil disposable on one side of a coupling connecting two co-rotating shafts and a ferromagnetic rod disposable on the other side of the coupling, the rod being so positioned within the confines of the coil that misalignment of the shafts alters the relative disposition of the rod and coil so as to affect the inductance of the coil, the coil forming part of an alternating current circuit an operating parameter of which is variable in dependence upon the inductance of the coil and which variable operating parameter is monitored to give an indication of the position of the rod with respect to the coil and hence of the degree of misalignment of the shafts. By employing this method the disadvantage of a long differential transformer is overcome as only a single coil of few turns, perhaps about 25, is used. Moreover such a coil can easily be buried in the flange of many type of coupling so that almost no windage problems are encountered at even the highest rotary speeds. Significantly the rod can be a loose fit in the coils and it can therefore be fixed rigidly to one flange of the coupling. This is because any angular position of the rod in the coil does not affect the inducatance of the coil unless the length of rod within the coil is also changed. Thus no spring is required with its attendent problems. There are no capacitive problems either, should an oil mist be present in vicinity of the coupling: and with the rod inside the coils the inductance of the coil is linearly dependent on flange displacement of the coupling. Thus no account need to taken of non-linearity associated problems not need any linearisation techniques be employed. Preferably, the variable frequency of said oscillating circuit is mixed with a reference frequency to provide a signal including a resultant frequency representative of the difference between the variable and reference frequencies. The resultant frequency may be arranged to amplitude modulate a driven signal generated in a secondary winding of a rotary transformer by a primary winding of said transformer, said resultant frequency being detectable in a driving signal applied to the primary winding through reflected impendance changes from the secondary winding and being filtered from said driving signal to provide said indication of the position of the rod with respect to the coil and hence of the degree of misalignment of the shafts. This arrangment has the further advantage that only one pair of coils is employed to provide both the motive power for the circuit detecting misalignment and the means for communicating the deflected misalignment off the shaft. The invention is further described hereinafter with reference to the accompanying drawings in which: Fig. 1 is a schematic illustration of a shaft coupling including a rod and coil arrangement according to the invention; Fig. 2 is a circuit diagram of electronic components enabling the invention to be put into effect and being disposed within the coupling as shown for instance in Fig. 1; Fig. 3 is a block diagram of a further such components being those disposed off the shaft; Figs. 4 and 5 illustrate the mechanical structure of another embodiment of the invention; and Fig. 6 illustrates the circuitry of the embodiment of Figs. 4 and 5. In Fig. 1 a coupling 10 comprises two flanges one an input flange 12 and the other an output flange 14. Each is disposed on a respective shaft not shown, 'and each is connected to the other by some flexible means allowing rotary motion to be transmitted through the coupling but at the same time allowing some axial movement between the flanges 12, 14 and also some angular movement therebetween. On either flange 12, 14 (it is not important which, in this case it is the flange 14) there is disposed a coil Ll and within the confines of the coil L1 is arranged a ferrite rod Al which is mounted on the other flange 12. The rod Al is adjustable by means not shown in Fig. 1 such that its rest position is more or less half-way into the coil L1. Thus axial movement in either direction of the rod in the coil will change the amount of rod within the coil and hence the latter's inductance. Although ferrite or the like is the preferred material for the rod Al, since that has perhaps the greatest influence on the inductance of the coil L1, almost any remotely ferromagnetic material will do. The coil L1 forms a part of the electronic circuit illustrated in Fig. 2 which conveniently is mounted on a small printed circuit board carried inside the confines of the hub of one flange of the coupling 10. The board is powered by a rotary transformer 13 A & B the secondary winding L3 of which is shown in Fig. 2. That also is mounted on one hub and is driven by a primary winding L4 (see Fig. 3) mounted off the coupling. The AC output of the coil L3 is rectified by a full wave rectifier 16 whose output is smoothed by DC regulator 18 whose output in turn is fed to lines 20, 21. The coil L1 forms the inductor in a Colpitt oscillator 22, comprising capacitors C2 and C3, transistor T1, resistors R2 and R1 and capacitor C1. The inductor r;i is so designed as to ensure that the oscillator frequency varies linearly with the position of the rod or armature (Al). A transistor T2 provides an emitter follower buffer stage 24 which isolates the oscillator circuit 22 from a mixer stage 26. A transistor T5 and its associated components L2, C7 and C8, R13, C10 and Rll, form another Colpitts oscillator 28 of fixed frequency. The output of this oscillator 28 is fed through a similar buffer stage 30 using transistor T4 to the mixer stage 26. The mixer stage 26 comprises a transistor T3 which acts as a sum and difference mixer with the variable frequency from transistor T1 being fed to the base of transistor T3, and the fixed frequency signal from transistor T5 being fed to the emitter of transistor T3. The nominal output of transistor Tl1 the variable oscillator, may be arranged at about 150 KHz and this is mixed with the output of transistor T5 arranged to operate at 170 KHz. The resultant output of the transistor T3 in line 32 therefore includes a frequency of 20 KHz for zero displacement of the flanges 12, 14. The system is so scaled that, for maximum deflection of the flanges either plus or minus from the nomial datum, the output of transistor T1 changes between about 140 KHz and 160 KHz giving a resultant output of 30 KHz to 10 KHz in line 32. The function, therefore, of the rotating electronics of Fig. 2 is such that it provides a linearly changing frequency, proportional to changes in flange displacement. The resultant output in line 34 after having been filtered somewhat by capacitors C6 and C9 and resistor R12, which form a low-pass filter, is used to switch transistor T6, which acts as a shunt modulator 36 across the rotor power and signal coil L3. The circuit of Fig. 2 is powered by a 700 KHz power oscillator circuit 40 (see Fig. 3) connected to the primary winding or stator coil L4 of the rotary transformer 13. The effect of the transistor T6 in the shunt modulator 36, is effectively to shunt the rotor coil L3 by a low impedance at a repetition rate equal to the frequency appearing from transistor T3. The switching effect of the transistor T6 is detectable on the primary coil L4 due to the effects of reflected impedance changes. Thiv results in an amplitude modulation of the 700 KHz power signal proportional to the output of transistor T3. This amplitude modulated signal is fed through a pre-amplifier 42 into a frequency-to-voltage converter 44, which in turn provides an output voltage proportional to the frequency of the modulated signal and therefore proportional to change in flange position. That change in flange position will occur through two sources, firstly angular misalignment and secondly axial misalignment. Thus a change in frequency which repeats at the rate of revolution of the shaft gives a precise indication of the degree of angular misalignnent: the greater the change then the greater the angular misalignment. The average frequency is of course an indication of the axial misalignment: the greater the average then the nearer together the two flanges approach one another. Thus the signal emanating from the frequency to voltage converter 44 has two components of information and these are extracted and separated in filter 46 before being applied to power amplifiers 48, 50 which in turn provide outputs representative of the angular and axial misalignment respectively. A power supply 52 drives the oscillator circuit 40 via line 53 and also components 42, 44, 46, 48 and 50 via line 54. A reference probe 56 provides a signal at X (see Fig. 1) which in Fig. 3 is applied to a buffer amplifier 58, the output of which gives an indication of the rotational position of the coupling 10 at any given moment. By appropriate phasing of the position signal at A with the angular misalignment signal at B, not only the degree of angular misalignment of the coupling can be determined but also the rotational position of such misalignment can be established. How such information is relayed to the end user is a matter of choice and various techniques could be used such as to display the information directly on to an oscilloscope or perhaps after manipulation thereof in a microprocessor. This forms no part of the present invention however. In applications of the invention where high temperatures are present, for example temperatures above 100 OC, it is difficult to mount electronics on rotating components of the system if a high degree of reliability is to be achieved. In more general applications it may also be desirable to provide an arrangement in which no rotating electronics are required. Such an arrangement is described below with reference to Figs. 4 to 6. With reference to Figs. 4 and 5, a ferromagnetic rod or armature 59 is mounted on one flange of a coupling .and extends into a coil 60 mounted on the other flange of the coupling. A further annular coil 61 is mounted on the same flange as coil 60, and an annular stator coil 62 is arranged around the coil 61. Referring to Fig. 6, coils 62 and 63, together with resistors 64 and 65 (resistors 64 and 65 could also be inductive elements in certain applications), form a full Wheatstone Bridge. The bridge is supplied with an A.C. voltage at a suitable amplitude and suitable frequency, the amplitude and frequency being largely dependent upon the mechanical configuration of the coupling and installation. This frequency will usually but not necessarily lie in the region of 100 KHz to 700 KHz. The coil 62 forms the stator coil of the shaft system with components 63, 64 and 65 mounted in a suitable housing adjacent to or with coil 62. The rotating coil 61 is mounted at a convenient point on the shaft, for example as illustrated on the shaft flange assembly. The coils 60 and 61 are connected in parallel. The armature 59 can move within the coil 60 due to any relative movements between the two flanges. In operation, the inductance change in coil 60 is arranged to be large for small movements of the armature 59, compared to any inductance changes due to small movements in the relative positions of coils 61 and 62 that may arise during normal running. With the flanges in a nominally aligned position, th-) system is arranged such that the bridge is in balance and the output at terminals 66 and 67 of the bridge is at a minimum value. Any relative movement of the flanges causes the position of the armature relative to coil 60 to change, thus changing the inductance of coil 60. This causes the bridge to become unbalanced due to the coupling between coils 61 and 62. This causes an output to appear at terminals 66 and 67, the magnitude of the voltage being in direct relationship to the misalignment of the flanges. The bridge is excited with an alternating voltage, and thus it is possible to compare the phase of the output voltage appearing at terminals 66 and 67 with the frequency of the A.C. supply using a phase detector circuit 68. The circuit will produce an output 69 which is a D.C. voltage. The magnitude of the D.C. voltage represents the magnitude of the misalignment and the sine of the D.C. voltage represents the movement, being either plus or minus from a datum point on the flanges. The system of Figs. 4 to 6 gives various advantages. Primarily, there are no rotating electronics, and therefore the system should be capable of operating at high temperatures, e.g. 150 or. The system should therefore be capable of continuous duty operation throughout the life of the plant with which it is associated. Further, the simple structure should offer significant financial savings in the final product cost. Finally the low power circuitry is inherently safe, making spark generation most unlikely so that the system can be used in hazardous areas. In normal operation, it is necessary with embodiments of the present invention to have two complete systems as described above on each shaft in order to monitor fully the misalignment at each end of the coupling, that is to monitor axial displacement, relative angular displacement about the rotation axis, and also relative angular displacement resulting in the flanges being non-parallel. A less complete picture of misalignment can however be obtained using just one system.

Claims: CLAIMS: 1. A shaft misalignment monitoring system comprising a coil disposable on one side of a coupling connecting two co-rotating shafts and a ferromagnetic rod disposable on the other side of the coupling, the rod being so positioned within the confines of the coil that misalignment of the shafts alters the relative disposition of the rod and coil so as to affect the inductance of the coil, the coil forming part of an alternating current circuit an operating parameter of which is variable in dependence upon the inductance of the coil and which variable operating parameter is monitored to give an indication of the position of the rod with respect to the coil and hence of the degree of misalignment of the shafts. 2. A system as claimed in claim 1, wherein the alternating current circuit is an oscillating circuit and the variable operating parameter is the frequency of oscillation of the oscillating circuit. 3. A system as claimed in claim 2, in which said oscillating circuit is a first Colpitts oscillator. 4. A system as claimed in claim 2 or 3, in which the variable frequency of said oscillating circuit is mixed with a reference frequency to provide a signal including a resultant frequency representative of the difference between the variable and reference frequencies. 5. A system as claimed in claim 4, in which the resultant frequency is arranged to amplitude modulate a driven signal generated in a secondary winding of a rotary transformer by a primary winding of said transformer, said resultant frequency being detectable in a driving signal applied to the primary winding through reflected impedance changes from the secondary winding and being filtered from said driving signal to provide said indication of the position of the rod with respect to the coil and hence of the degree of misalignment of the shafts. 6. A system as claimed in any one of claims 2 to 5, in which said variable frequency is employed to give an indication of the angular misalignment between the shafts by noting the change in said variable frequency per revolution and of the axial misalignment between the shafts by noting the average level of said variable frequency per revolution. 7. A system as claimed in claim 6, in which a positional reference probe is arranged to provide a position signal from which the rotational position of the shafts at a given moment can be determined and from which the angular direction of said angular misalignment can be established. 8. A system as claimed in claim 5 and 6, in which said driving signal is applied to a frequency to voltage converter the output of which is filtered to provide a first signal representative of the degree of angular misalignment and a second signal representative of the degree of axial misalignment. 9. A system as claimed in claim 7 and 8 in which the position signal is phased with said first signal to determine the angular position of any angular misalignment between the shafts. 10. A system as claimed in claim 4 and or any of claims 5 to 9 when dependent on claim 4 in which the reference signal is provided by a second Colpitts oscillator. 11. A system as claimed in claim 4 or any of claims 5 to 10 when dependent on claim 4, in which said variable frequency is in the range 130 to 170 KHz, and said reference frequency is about 10 KHz larger than the maximum value for said variable frequency. 12. A system as claimed in claims 5 and 11 in which said driving signal has a frequency in the range 400 to 1000 KHz. 13. A system according to claim 1, wherein the said operating parameter of the alternating current circuit is a voltage the magnitude of which varies in dependence upon the inductance of the coil. 14. A system according to claim 13, wherein the alternating current circuit comprises a further coil mounted on the said one side of the coupling and connected to the said coil, and a stator coil magnetically coupled to the further coil. 15. A system according to claim 14, wherein the further coil is annular and arranged around the periphery of the said one side of the coupling, and the stator coil is annular and arranged around but spaced from the further coil. 16. A system according to claim 15, wherein the stator coil is connected in one arm on a Wheatstone bridge supplied with an alternating current signal, the bridge output being the said variable operating parameter. 17. A system according to claim 16, wherein a phase detector is provided to monitor the phase difference between the bridge output and the alternating current signal. 18. A shaft misalignment monitoring system substantially as hereinbefore described with reference to the accompanying drawings.

Inventors: BRITTON, JOHN D.
Applicants: FLEXIBOX LIMITED
Application year: 1985
Main classification: G01B7
IPC codes: G01B7, F16D1