Measurent of axial displacement of a rotating shaft

Abstract

A system for measuring axial displacement of a shaft 14 includes at least one first marker 10 mountable for rotation with the shaft 14 and a first stationary sensor 16 mounted adjacent the shaft 14 fo

Description

Field of the invention The present invention relates to measurement of axial displacement of a rotating shaft. Background of the invention The forces acting on the rotor of a wind turbine are considerable and place heavy demands on the support bearings. Maintenance of the bearings is therefore of importance, and it is desirable to be able to measure any axial and radial displacements of the rotor as it rotates, as these measurements are indicative of wear of the bearings. The fact that these bearings have a very large diameter (several meters) adds to the difficulty of measuring these displacements with a high degree of precision. In order to simplify maintenance and repair of bearings of large diameter shafts, it has been proposed to employ modular journal, or friction, bearings in place of anti-friction bearings, i.e. bearings with rolling bearing elements. Such bearings comprise individual oil lubricated bearing shells distributed about the circumference of the shaft that are replaceable without the need to remove the shaft. Accurate measurement of radial and axial displacement of the shaft as it rotates is of particular importance in the monitoring and control of such modular bearings. This is especially the case since conventional vibration monitoring does not function in these circumstances. It is known to measure the angular position of the shaft using a series of circumferentially spaced markers and a stationary sensor. EP 0 608 234 describes a measuring system for measuring run out of a rotating shaft, this term referring to the radial displacements, or wobble, of the axis of rotation of the shaft. The system comprises a series of circumferentially spaced markers and a plurality of sensors circumferentially spaced around the shaft. By comparing the phases of the signals of the different sensors, run out of the shaft can be determined accurately and precisely. Object of the invention The aim of the present invention is to provide a system that can measure axial movements of a rotating shaft, even one of large diameter, such as that of a wind turbine. Summary of the invention According to the present invention, there is provided a system for measuring axial displacement of a shaft, comprising at least one first marker mountable for rotation with the shaft, a first stationary sensor mounted adjacent the shaft for detecting as each first marker passes the first sensor, at least one second marker mounted for rotation with the shaft and a second stationary sensor mounted adjacent the shaft for detecting as each second marker passes the second sensor, wherein each first marker is configured to align with the first sensor at the same angular position of the shaft regardless of the axial position of the shaft relative to the first sensor, and each second marker is configured such as to align with the second sensor at an angular position of the shaft that varies with the axial position of the shaft relative to the second sensor, the system further comprising a processor for determining axial displacement of the shaft by comparing the phases of output signals of the first and second sensors. In some embodiments, the system comprises a plurality of first markers distributed about the circumference of the shaft. The first markers may be lines disposed on a cylindrical surface and extending parallel to the axis of rotation of the shaft. In some embodiments, a plurality of first sensors, circumferentially distributed around the shaft, is associated with the first markers and a processor is provided to determine the relative phases of the output signals of the plurality of first sensors to provide a measurement of run out of the shaft. The at least one second marking may suitably comprise a line disposed on a cylindrical surface concentric with the shaft and lying at a helical angle to the axis of cylindrical surface. It is advantageous to provide a plurality of such second markings equal in number and spacing to the first markings and extending parallel to one another. In different embodiments, the markers and sensors interact with one another either optically or magnetically. 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 lisa schematic representation of an embodiment of the invention, Figures 2A and 2B are projections onto a flat plane of two sets of markings shown in Figure 1, and Figure 3 shows idealised output signals of different sensors in Figure 1. Detailed description of the drawings In the embodiment of the invention shown in Figure 1, two sets of markings 10 and 12 are arranged on rings rotatable with a shaft of which the axis of rotation is represented by the line 14. The reference numeral 14 will be used herein to refer both to the shaft and its axis of rotation. The first set of markings 10 is associated with three sensors 16 the outputs of which are fed to a processor 18 that calculates the run out of the axis of rotation of the shaft. The second set of markings 12 is associated with a sensor 20 which, in common with one of the sensors 16, is connected to a processor 22 for measuring axial displacement, which may optionally also receive signals directly from the processor 18. The markings 10 are lines on a cylindrical surface concentric with the shaft that run parallel to the axis of the shaft, each line lying within a plane that contains the axis 14. As a result, if a line 10 is aligned with one of the sensors 16 axial movement of the shaft would not disturb the alignment. This will be clear from Fig. 2A which shows the projection of the markings 10 onto a flat surface that moves past the sensor 16; axial movement of the markings (i.e. in a direction parallel to the markings themselves) will not affect the image seen by the sensor 16. The markings can have any physical property that distinguishes them from the space between them, so long as the associated sensor can detect the difference between the markings and the background. Thus, for example, the markings and the sensors may interact optically or magnetically. An optical sensor can be a camera, while a magnetic sensor could for example be a Hall effect device. If the markings are magnetic, they could be formed by the teeth of a gear wheel. The sensors 16 are shown as being circular but they may be linear, in which case they should extend parallel to the markings 10. A linear sensor will produce pulses which, prior to processing, will have steeper leading and trailing edges and may provide an improved signal to noise ratio. If the shaft is rotated with a constant speed, each of the three sensors 16 will produce a signal which, after suitable processing, will be as shown at A in Figure 3. The signal processing is to convert the analogue output signals into square pulses. The signals from the sensors 16 are amplified, filtered to remove noise, and converted into a digital signal having only one of two states, depending on whether the magnitude of the analogue signal lies above or below a threshold. A signal as shown at A in Fig. 3 is produced by each of the three sensors in Fig. 1. A comparison of the phases of the three signals can provide a measure of the run out of the shaft. The processing by the processor 18 of the output signals of the sensors 16 to determine run out is well known in the art and described in EP 0 608 234, which is imported herein by reference to avoid the need to describe the signal processing in further detail in the present context. As with the markings 10, the markings 12 of the second set are evenly spaced and extend parallel to one another on a cylindrical surface concentric with the shaft but, as more clearly seen from projection onto a flat plane in Fig. 2B, they lie at a helical angle to the axis 14. The result of this is that the phase of the signal picked up by the sensor 20 will depend on the axial position of the shaft 14 relative to the sensor 20. If, in Fig. 2B, the markings are moved toward the top of the page, while the sensor 20 remains stationary, the resulting waveform will be as shown at B in Fig. 3, whereas if they are moved toward the bottom of the page, the resulting waveform will be as shown at C in Fig, 3. The line 24 is drawn in Figure 3 to emphasize the phase difference between the three signals A, B and C. Therefore, by comparing the phase of the output signal of the sensor 20 with the phase of the output signal of one of the sensors 16, it possible to determine the axial position of the shaft relative to the sensor 20. As with the markings 10, the markings 12 may be optical or magnetic and the sensor 20 may be circular or linear. If the markings are magnetic, they may in this case be the teeth of helically cut gear. If axial displacement of the shaft remains constant as it rotates, a single marking 12 would suffice to enable that displacement to be measured. However, the provision of multiple markings 12 allows variations in axial displacement to be determined. By having as many second markings as there are first markings 10, it is possible to use the same phase comparison method as used in the run out processor 18 to measure axial displacement in the processor 22. Depending on the positions relative to the shaft of the sensors 16 and 20 of which the phases are compared to determine axial displacement, it is possible that the measured phases differences may be affected by run out, rather than axial displacement. It is preferable to position the two sensors 16 and 20 connected to the processor 22 such that run out does not affect the relative phase of their output signals, but, if that should not prove possible, then compensation may be applied by the optional connection of the run out processor 18 to the axial displacement processor 22. The sensitivity of the measurement of axial displacement will depend on the inclination of the markings 12 to the axis 14. The greater the angle between them, the greater will be the measurement precision. As illustrated, because the markings 12 are straight lines, the dependence of axial displacement on phase will be linear. This, however, need not be the case. If desired, the marking may S-shaped, with greater inclination at the centre that at the ends, allowing smaller displacements to be measured with greater precision than larger ones. The markings 12 need to be provided on a cylindrical surface but this is not the case for the markings 10, so long as the outputs of the sensors 16 do not vary with the axial position of the shaft. Thus, the markings 10 may be radial lines on an axial end surface of the shaft. The two sets of markings must be fast in rotation with one another, but they need not necessarily be formed or mounted on the same shaft. For example, if two shafts are splined to slide linearly relative to one another, the two sets of markings can be mounted or formed on the respective shafts.

Claims

1. A system for measuring axial displacement of a shaft, comprising at least one first marker mountable for rotation with the shaft, a first stationary sensor mounted adjacent the shaft for detecting as each first marker passes the first sensor, at least one second marker mounted for rotation with the shaft and a second stationary sensor mounted adjacent the shaft for detecting as each second marker passes the second sensor, wherein each first marker is configured to align with the first sensor at the same angular position of the shaft regardless of the axial position of the shaft relative to the first sensor, and each second marker is configured such as to align with the second sensor at an angular position of the shaft that varies with the axial position of the shaft relative to the second sensor, the system further comprising a processor for determining axial displacement of the shaft by comparing the phases of output signals of the first and second sensors.

2. A system as claimed in claim 1, comprising a plurality of first markers distributed about the circumference of the shaft.

3. A system as claimed in claim 2, wherein the first markers are lines disposed on a cylindrical surface and extending parallel to the axis of rotation of the shaft.

4. A system as claimed in claim 2 or 3, wherein a plurality of first sensors, circumferentially distributed around the shaft, is associated with the first markers and a processor is provided to determine the relative phases of the output signals of the plurality of first sensors to provide a measurement of run out of the shaft.

5. A system as claimed in any preceding claim, wherein the at least one second marking comprises a line disposed on a cylindrical surface concentric with the shaft and lying at a helical angle to the axis of cylindrical surface.

6. A system as claimed in claim 5 when appended to Claim 2, 3 or 4, comprising a plurality of second markings equal in number and spacing to the first markings and extending parallel to one another.

7. A system as claimed in preceding claim, wherein at least one of first and second sensors is elongate and extends parallel to the associated markings.

8. A system as claimed in any preceding claim wherein the markings and sensors interact optically.

9. A system as claimed in any one of claims 1 to 7, wherein the markings and 5 sensors interact magnetically.

10. A system as claimed in claim 9, wherein the markings are formed by the teeth of a gear wheel.

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