GB2590426A - Monitoring of rotating shafts - Google Patents

Abstract

A system for monitoring an operating parameter of a rotating shaft comprises a sensor 30 and two disks 10, 12, mountable for rotation with the shaft at axially spaced locations. The discs have optical markers 22. The sensor comprises a light source 30a which emits a beam along a path that is intersected by the markers of both disks, and a detector 30b for detecting light from the source that has intersected the markers provided on the two disks. Preferably, the system comprises a set of sensors 130 associated with each disk and located at different points about the circumference of the disks 110,112 with a processor (not shown) receiving output signals from both sets of sensors, the processor evaluating the output signals to derive the centre of rotation of the shaft relative to the sensor of each set and thence determine the location and inclination of the axis of rotation of the shaft using the derived values for the axially spaced centres of rotation. The system may comprise at least one Incremental Motion Encoder or IME.

Description

MONITORING OF ROTATING SHAFTS

Field of the invention

The present invention relates to devices for monitoring the rotation of shafts.

Background

Many, if not most, machines incorporate a rotating shaft through which torque is transmitted. For the control and maintenance of such machines, systems have previously been proposed for monitoring the angular position of a shaft at any point in time and the speed of the shaft.

To determine the angular position of a shaft at any given moment, it is possible to use a shaft encoder. Shaft encoders are known that comprise a disk for mounting on the shaft that carries markers and a stationary sensor capable of detecting the passage of the markers as the shaft rotates. Such a shaft encoder, which can also be used to measure the speed of rotation of a shaft, can use electromagnetic or optical sensors to detect the markers, optical sensors only being used normally in clean environments.

It has also been proposed to use two shaft encoders mounted at a distance from each other on a common shaft to measure twisting of the shaft and thereby provide an indication of the torque transmitted through the shaft.

Hitherto, when a monitoring system included more than one shaft encoder, each disk carrying markers was associated with a respective sensor and the output signals of the different sensors were processed to determine their relative phase.

Object of the invention In its broadest aspect, the invention seeks to simplify the complexity of monitoring systems that employ two or more shaft encoders.

Summary of the invention

In accordance with a first aspect of the invention, there is provided a system for monitoring an operating parameter of a rotating shaft, comprising two disks mountable for -2 -rotation with the shaft at axially spaced locations and formed with optical markers, and a sensor common to both disks, the sensor comprising a light source for emitting a beam along a path that is intersected by the markers of both disks, and a detector for detecting light from the source that has intersected markers in both disks.

The invention allows the timing of markers on two disks to be detected using a common sensor, thereby simplifying both the construction of the monitoring system and the signal processing. The markers in the first disk, i.e. the one closer to the light source, are windows that allow light to pass through the disk in selected angular positions of the disk relative to the light source. The second disk, i.e. the one more remote from the light source, may also have optical markers constructed as windows to allow the light to reach a detector positioned further away from the light source than the second disk. As an alternative, the light beam may be reflected to follow the same path in reverse to be detected by a detector located near the light source and in such an embodiment, a mirror may be located beyond the second disk or the markers on the second disk may themselves be mirrors.

By suitably sizing and aligning the optical markers in the two disks, it is possible to ensure that at the time that the leading edge of a marker in the first disk intersects the light beam, light will be able to reach the detector after passing through, or being reflected by, a marker in the second disk and when the light beam intersects the trailing edge of the marker in the second disk, the window in the first disk will still be open. Consequently, the sensor will generate from its detector an output pulse of which the leading edge is determined by the marker on the first disk and the trailing edge is determined by the marker in the second disk. An indication of the transmitted torque may therefore be derived from the mark to space ratio of the output signals produced by the sensor as the shaft rotates.

In certain application, it is desirable to be able to monitor runout of a shaft, this being a lateral displacement of the axis of rotation of a shaft, as may result from worn support bearings. In EP 0608234, the present inventor has previously proposed comparing the output signals from multiple sensors located at different points about the circumference of the same disk to determine when a shaft is rotating off-axis. By comparing the phases of the multiple sensors, it is possible to calculate the position of the centre of rotation of the shaft. A system having a disk carrying markers, multiple sensors located at different points about the circumference of the disk and circuitry for -3 -determining runout by analysing the outputs of the sensor is hereinafter referred to as an Incremental Motion Encoder, or IME for short.

There are many situations where the output shaft of a prime mover, such as an engine or a motor, is coupled to a driven shaft, an example being the coupling of an engine drive shaft to a ship's propeller. In such a situation, if the drive and driven shafts are not correctly aligned, the coupling is subjected to strain that may result in failure For correct alignment, it is necessary to ensure that there should be no lateral offset between the axes of rotation of the two shafts and that the two axes should not be mutually inclined In accordance with a second aspect of the invention, there is provided a system for monitoring the rotation of a shaft to determine both the position and the inclination of the axis of rotation of the shaft, the system comprising two disks each carrying markers and mountable at axially spaced locations on the shaft, a set of sensors associated with each disk and located at different points about the circumference of the disk, and a processor connected to receive output signals of both sets of sensors and operative to evaluate from the output signals of each set of sensors the location of the centre of rotation of the shaft relative to the sensor of the set and to determine from the two axially spaced evaluated centres of rotation the location and inclination of the axis of rotation of the shaft.

In such a system, the centre of rotation at each IME is determined relative to the fixed position of the set of sensors belonging to that IME. Misalignment of the sensors of the two [ME' s can therefore result in systematic errors in the measurement of the position and inclination of the axis of rotation of the shaft.

To mitigate such errors, it is desirable for the disks of the two IME's to be formed with optical markers, and for there to be only one set of sensors associated with both disks, each sensor comprising a light source for emitting a beam along a path intersected by the markers of both disks, and a detector for detecting light from the source that has been intersected by markers in both disks.

From a knowledge of the position of the centre of rotation at two axially spaced points, one can determined the inclination of the rotational axis of the shaft, thus enabling where the axis intersects a coupling plane between the two shafts and its angle of inclination relative to that plane. If the position and inclination of the driven shaft are fixed, then it suffices to determine the location and inclination of only the drive shaft to be able to correct for misalignment but if the driven shaft is not fixed then it is possible to use -4 -two further IME's on the driven shaft to determine the position and inclination of its axis of rotation.

If two EVIE's are mounted on each of the drive and driven shafts, it is desirable that a single set of optical sensors be provided common to all four EVIE's, each sensor comprising a light source emitting a beam intersected by markers of all four disks. In this case, the disks need to be formed with additional windows, or mirrors, that are not used for carrying out any time measurements but merely allow the light to reach markers of the other disks.

Brief description of the drawings

The invention will now be described further, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a diagram of a system for providing an indication of the torque transmitted through a shaft, Figure 2 is a diagram showing an idealized output signal of the sensor in Figure 1, Figure 3 is a diagram like that of Figure 1 of a system for determining the position and inclination of the axis of rotation of a shaft, Figure 4 is a diagram showing how two shafts coupled for rotation with one another can be misaligned, to risk causing damage to the coupling between them, Figure 5 is a diagram like that of Figures 1 and 3 of a system for determining misalignment between the axes of two shafts, Figure 6 is a diagram like that of Figure 2 showing the output signal of each of the three sensors in Figure 5, and Figure 7 is a diagram of a sensor comprising a light source and detector located within a common housing.

Detailed description of the drawings

Figure 1 shows two shaft encoders 10 and 12 mounted on a common shaft 16. Systems using two shaft encoders on a common shaft have previously been proposed to measure twisting of a shaft as a result of the torque transmitted it the shaft. Hitherto, the two shaft encoders would each comprise a disk carrying markers and a respective electromagnetic or optical sensor to detect the passage of the markers as the disk rotates -5 -In the system shown in Figure 1, each shaft encoder comprises a disk 20a, 20b having markers 22a, 22b around its circumference. The markers in the described embodiment are all "windows", this term being used to designate regions that allow the passage of a light beam through the disk only at selected angular positions of the shaft but need not necessarily be bounded on all sides. Instead of there being a respective sensor associated with each disk, in the system of Figure 1, the angular positions of the two disks are measured using a single sensor. The sensor is an optical sensor that comprises a light source 30a that transmits a narrow beam 24, usually a laser beam, aimed to pass through windows 22a and 22b in both disks 20a and 20b before reaching a light detector 30b. The light source 30a and the detector 30b will together be referred to below as sensor 30.

The windows 22a and 22b are sized and positioned on the disks 20a and 20b in such a manner that, for all possible values of the transmitted torque, at the time that the leading edge of the window 22a of the first disk 20a intersects the laser beam 24, the window 22b of the second disk 20b will already lie in the path of the beam and the beam will start reaching the detector 30b. As both disks continue to rotate, the trailing edge of the window 22b of the disk 20b will intersect the beam 24 while the window 22a of the first disk 20a still lies in the path of the beam, so that light will then cease to reach the detector 30b.

The resulting output of the sensor 30 is shown in idealized form in Figure 2. In this drawing, the leading edge of each pulse is designated LED1, to indicate that its timing is dictated by the Leading Edge of a marker in Disk 1, and the trailing edge of each pulse is designated FED2, to indicate that its timing is dictated by the Trailing Edge of a marker in Disk 2. Thus, the pulse width will change when torsion causes the two disks to be rotated slightly relative to one another, thereby providing an indication of the transmitted torque.

Fig shows the light source 30a and the light detector 30b as being spaced from each other. This however is not essential as the two components may be mounted adjacent one another and the laser beam may be reflected to pass twice through the windows of the first disk 20a. Reflection may either be achieved by a mirror located where the detector 30b is positioned in Figure 1, or by forming the optical markers of the more remote second disk 20b as reflectors.

The signal in Figure 2 is idealized in that it is represented by a perfectly square wave. In practice, the beam has a finite width and the light value does not change instantly -6 -between its minimum and maximum values. This however is taken into account in the processing of the detector output signal.

The system of Figure 3 differs from that of Figure 1 in that instead of being formed of two shaft encoders, it comprises two IME's 110, 112 mounted on a common shaft. In the same way as a shaft encoder, each IME comprises a disk carrying markers. In an IME, each disk is associated with multiple circumferentially spaced sensors 130. The system of Figure 3 has three sensors 130 arranged at intervals of 120°, the third of the sensors not being visible in the drawing. By comparing the output signal of the three sensors 130, it is possible to determine nmout, i.e. one can ascertain the exact position of the centre of rotation of the shaft relative to a stationary frame of reference defined by the position of the sensor 130. The operation of an IME is described in EP 0608234 and need not therefore be described herein in detail.

By placing two 1ME's on a common shaft, one can determine the centre of rotation of the shaft at each of two spaced locations and thereby determine the position the location and the inclination of the axis of rotation of the shaft. Figure 4 shows schematically why it is important in certain situations to be able to measure these parameters.

In Figure 4, the shaft 410 on the left is assumed to be a firmly mounted driven shaft. The shaft 420 on the right is the output shaft of prime mover coupled in some way to drive the shaft 410, the coupling plane being designated 430. For example, the shaft 410 might be the propeller shaft of a ship and the shaft 420 the output shaft of an engine. While the propeller shaft may be supported in pillow blocks that are firmly secured to the hull of the ship, the engine may be mounted on rubber bushings to avoid its vibrations being felt throughout the ship. As rubber bushings are prone to wear, the axes of the shaft 410 and 420 may with time become offset from one another and if the bushings wear unevenly the axis of the shaft 420 may tilt relative to the shaft 410. Both of these forms of misalignment are shown in Figure 4. Any misalignment places strain on whatever coupling is used to couple the two shafts to one another and can result in failure of the coupling.

The monitoring system shown in Figure 5 is aimed at mitigating this problem. In Figure 5, two IME 510,512 are mounted on one shaft 420 and two further IME's 610, 612 are mounted on the other shaft 410. As with the previously described embodiment, a single set of sensors 530 is common to all four 'ME's. As illustrated, the sensors 530 comprise light sources and separate light detectors, but once again the light beams may be -7 -reflected to detectors located adjacent the light sources. In this embodiment, it is necessary to ensure that the light beams used by one pair of IME's 610, 612 are not obstructed by the disks of the other pair of IME's 510, 512. Therefore, each disk has additional windows that are not used as markers but merely allow unobstructed passage of the light beams to the disks of the other IME The idealized output signal produced by each of the sensors 130 in Figure 5 is shown in Figure 6. In this case, each sensor will have repeating pairs of pulses. The leading and trailing edges of the first pulse in each consecutive pair are dictated by the angular positions of the disks of the IME's on one of the shafts, while the leading and trailing edges of the second pulse in each pair are dictated by the angular positions of the disks of the IME's on the other shaft. A timing marker, not shown, is additionally provided to indicate a fixed angular position from which the pulses should be counted.

For convenience, assuming that pulse edge LED] is indicative of the timing of the disk of IME 510, then by comparing the LED1 edges of all three sensors 530, a processor can determine the centre of location of the shaft at the location of the IME 510. By similar processing of the remaining three pulse the locations in space of all four of the locations marked with an X in Figure 4 can be calculated. The processor can thus determine both relative inclination and offset between the axes of rotation of the two shafts 410, 420.

Such data can in practice be used, for example, to determine the size and location of shims to be placed under a ship's engine to ensure correct shaft alignment and thereby avoid stress being placed on the coupling. The nature of the coupling is not material in the present context and it therefore only represented in Figure 4 by a reference plane. In practice, the coupling may be a rubber bush, a metal element bolted to both shafts or a universal joint.

As shown in Figures Ito 6, each sensor is shown as a light source and a separate detector located at the opposite end of the projected beam. However, as earlier described, the light beam may be reflected to a detector located near the light source. Figure 7 shows such a light sensor where both the light source and the detector are located at the same end of the detection beam.

In Figure 7, a laser light source 80 is arranged in a housing 82 within which there are also mounted a light detector 84 and a beam splitter 86. The use of a beam splitter ensures that the reflected light beam follows the same path as the light incident on the -8 -reflector, thereby avoiding possible parallax errors. The housing 82 is shown as fitted with baffles 88 which serves to minimize fouling of the light source and the detector when operating in a dirty environment. Any dirt or oil particles that are present in the ambient air but moving in a direction different from the laser beam will not be able to reach any of the optical elements 80, 84, 86 within the housing 82.

As an alternative to a beam splitter, it is possible to fire the laser through a bundle of optical fibres, and to surround that bundle with another set of fibres which are led back to a sensor. In practice, it has been found that, on account of beam spreading, it suffices to place the laser emitter and the sensor very close together.

Claims (1)

1. CLAIMS1. A system for monitoring an operating parameter of a rotating shaft, comprising two disks mountable for rotation with the shaft at axially spaced locations and formed with optical markers, and a sensor common to both disks, the sensor comprising a light source for emitting a beam along a path that is intersected by the markers of both disks, and a detector for detecting light from the source that has intersected markers in both disks.fo 2. A system as claimed in claim 1, wherein the markers are formed as windows.3. A system as claimed in claim 1 or 2, wherein a reflector is provided to reflect the light beam to pass through at least one disk twice before reaching the detector.4. A system for monitoring the rotation of a shaft to determine both the position and the inclination of the axis of rotation of the shaft, the system comprising two disks each carrying markers and mountable at axially spaced locations on the shaft, a set of sensors associated with each disk and located at different points about the circumference of the disk, and a processor connected to receive output signals of both sets of sensors and operative to evaluate from the output signals of each set of sensors the location of the centre of rotation of the shaft relative to the sensor of the set and to determine from the two axially spaced evaluated centres of rotation the location and inclination of the axis of rotation of the shaft.A system as claimed in claim 4, wherein a single set of optical sensors is provided common to all four EV1E' s, each sensor comprising a light source emitting a beam disks and a detector for detecting the light beam after it has been intersected by markers on all four disks, 6. A system as claimed in claim 5, wherein the markers are formed as windows 7. A system as claimed in claim 5 or 6, wherein a reflector is provided to reflect the light beam to pass through at least one disk twice before reaching the detector.