Field

The present invention relates to the measurement of axial displacement and/or vibration of a rotating shaft.

Background

In several machines, for example jet engines, marine engines and wind turbines, it is desirable to monitor the movement of a rotating shaft, Such monitoring can warn, for example, of wear in a bearing which could ultimately result in machine failure.

The present applicants have previously proposed in EP 0608234 a measuring system that measures displacement of a rotating shaft in a direction transverse to the axis of the shaft. The present invention, on the other hand, is concerned with measurement of displacement of the rotating shaft in a direction parallel to the axis of rotation of the shaft .

There are available on the market various proximity sensors that can measure accurately the distance of the sensor from a target surface , such as presented by a

shoulder on the shaft or by a disk mounted on the shaft, Amongst the available sensors, there are sensors that rely on measuring inductance and others on measuring capacitance .

The present invention can use any sensor capable of

measuring small distances accurately and performing such measurement with a repetition frequency appropriate to the speed of rotation of the shaft, i.e. a repetition frequency that would enable several measurements to be taken within the time it takes for the shaft to rotate thro n °

The output signal of commercially available sensors is substantially linear over the distances of interest in the present invention. The output of the sensors is however prone to errors, for a variety of reasons, most notably changes in ambient temperature. As the response of the sensor is substantially linear, a graph of the sensor output versus distance is a straight line. One type of error, herein referred to as "drift", can result in a change in the position of the line on the graph, without any change in its slope. An error resulting from a change in the slope of the graph is herein referred to as a variation in "gain".

Obj ect

The present invention seeks to provide a way of

compensating for any drift that may occur in the sensor output that is not the result of axial movement of the shaft .

Summary

According to a first aspect of the invention, there is provided a method of measuring axial displacement and/or vibration of a rotating shaft, which method comprises

i) providing a stationary distance measuring sensor adjacent the shaft,

ii) providing on the shaft a disk having a first target surface in a plane normal to the axis of rotation of the shaft,

iii) providing a stationary second target surface adjacent the shaft within the measuring range of the sensor, the disk serving to interrupt the line of sight between the sensor and the second target surface only during

predetermined phases of rotation of the shaft,

iv) processing the output of the sensor to derive measurements of the distances from the sensor of the first and the second target surfaces, and v) utilising the measurements of the distance of the sensor from the second target surface to calibrate the sensor so as to correct the measured distance of the sensor from the first target surface for possible drift.

It should be made clear that the term "line of sight" should not be taken to mean that light travels between the sensor and the target surfaces, but merely that there is a straight path along which some form of energy (be it

electromagnetic, optical or acoustic) can flow between the sensor and the target surface.

In an embodiment of the invention, the disk has a third target surface in a plane normal to the axis of rotation of the shaft and lying at a predetermined distance from the first target surface, and, in the processing of the distance measurements made during rotation of the shaft to provide a calibrated output signal indicative of the distance of the first target surface from the sensor, the measurements from the third target surface are utilised to compensate for possible variation in gain.

According to a second aspect of the invention, there is provided a system for measuring axial displacement and/or vibration of a rotating shaft, which system comprises

i) a distance measuring sensor located in a

stationary position adjacent the shaft,

ii) a disk rotatable with the shaft having a first target surface in a plane normal to the axis of rotation of the shaft,

iii) a stationary second target surface located

adjacent the shaft within the measuring range of the sensor, the disk serving to interrupt the line of sight between the sensor and the second target surface only during

predetermined phases of rotation of the shaft, and

iv) a processor for processing the output of the sensor to derive measurements of the distances from the sensor of the first and the second target surfaces, the processor being operative to utilise the measurements of the distance of the sensor from the second target surface to calibrate the sensor, so as to correct the measured distance from the first target surface for possible drift.

In some embodiments, the disk has a third target surface in a plane normal to the axis of rotation of the shaft and lying at a predetermined distance from the first target surface, and, in the processing of the distance measurements made during rotation of the shaft to provide a calibrated output signal indicative of the distance of the first target surface from the sensor, the measurements from the third target surface are utilised to compensate for possible variation in gain.

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 shows schematically a system embodying the invention for measuring axial displacement of a rotating shaft,

Figure 2 is a section of the system in Figure 1 taken along the section line II-II,

Figure 3 is a graph to show how the output signal of the sensor may vary even when there is no displacement of any of the target surfaces,

Figure 4 shows three different possible outputs from the sensor as the shaft rotates despite no displacement of the target surfaces, and

Figure 5 shows a block diagram of the circuitry of the proximity sensor. Detailed description of the drawings

Figures 1 and 2 show a shaft 10 that is rotatable relative to a stationary housing 12, of which only some parts are shown diagrammatically in the drawing. A disk 14 is mounted for rotation with the shaft 10, the disk having a target surface 16 facing a stationary proximity sensor 18 that is secured by a bracket 20 to the housing 12. The sensor 18 may be of any known type capable of measuring small distances accurately at a rate appropriate to the speed or rotation of the shaft. In this context, it should be noted that several readings need to be taken during each revolution of the shaft if axial vibration is to be

detected. One suitable type of sensor is that which

measures the mutual inductance between two coils, the inductance varying with the distance of the sensor from the target surface. Other suitable sensors are those that measure a capacitance that varies with the distance from the target surface.

If one could rely on the response curve of the sensor remaining constant under all operating conditions, one could take the output of the sensor when facing the target surface 16 as being a precise measurement of the axial position of the shaft. In practice, however, one cannot make any such assumption. Figure 3 shows what may happen in practice to the response curve of the sensor. When operating under reference conditions, the response curve may be that

designated A in Figure 3. A change in an external factor, can result in this response curve drifting to the curve designated B in Figure 3. This curve is also a straight line and has the same slope as the curve A, but it is offset from it so that the error, i.e. the difference between the two output signals of the sensor, is constant over the

measurement range. Such an error is referred to herein as drift. Other external factors may modify the slope of the response curve, as represented by the curve C in Figure 3. With this type of error, herein termed a variation in gain, the error varies with the measured distance. In order to permit the sensor output to be corrected for both types of error, the embodiment of the invention shown in Figures 1 and 2 enables the same sensor to measure its distance from additional target surfaces.

Referring again to Figures 1 and 2, the system includes a second target surface 22, that is mounted on the housing 12 at a preset distance from the sensor 18. The disk 14 includes a cut-out 24 so that this second target surface 22 is in the line of sight of the sensor 18 in between certain angular positions of the disk 14. Furthermore, the disk 14 has a projecting land 26 that presents a third target surface 28 to the sensor 18 between certain other angular positions of the disk 14.

On account of these additional target surfaces of the measurement system, the sensor output varies with time as the shaft rotates in the manner shown in Figure 4. The waveforms A, B and C would be generated under different operating conditions, corresponding to the three sensor response curves shown in Figure 3. In each of these

waveforms, the regions 30 where the distance measurement is lowest occur when the sensor 18 is measuring its distance from the third target surface 28 of the projecting land 26 on the rotating disk 14. The highest regions 32 correspond to the measured distance from the stationary second target surface 22 and the intermediate regions 34 correspond to the measured distance from the first target surface 16 that is the surface of the disk 14 itself.

The sensor 18 produces an analogue output and this is converted by an analogue to digital converter 40 into a digital signal. The digitised signal is applied to a digital processor 42 that performs a mathematical calculation, as explained further below, on the measured distances from the three target surfaces and reference values of the distance to the second and third target surfaces made under reference operating conditions and stored in memory in the processor 42.

The stored distance to the second target surface 22 in the reference position (curve A) as measured under reference operating conditions, is compared with the measured distance under the prevailing condition (curve B or curve C as the case may be) , and the error is noted. Since in the case of a drift error, the error is constant over the entire

measurement range, it remains only to combine this error with the measured distance from the first target surface 16 to correct the latter value for drift.

If the same correction is now made to the measured position of the third target surface 28, and the corrected measurement is compared with the distance stored for the third target surface 28 under the reference measurement conditions, a match between the two values will indicate that there has been no variation in gain, that is to say the slope of the response curve has not changed.

However, as can be seen by comparing the waveforms A and C in Figure 4, when there has been a variation in gain, adding to the regions 30 of the waveform C the offset needed to make regions 32 of the waveforms A and C coincide, will not make those regions 30 coincide with the corresponding region 30 of waveform A. The difference between the two is a measure of the variation in gain. As the distance between the first and third target surfaces is fixed by them both being part of the rotating disk, the correction that needs to be applied to the measurement of the first target surface to correct for gain will always be a predetermined fraction (C) of the correction required to return the measurement of the third target surface, after correction for drift, to the value stored for the distance from sensor to the third target surface under the reference operating conditions.

Thus, assuming that under reference operating conditions the measured distances from the sensor 18 to the second 22 and third 28 target surface are Y and Z respectively and that under the prevailing measurement conditions the

obtained readings for the first, second and third target surfaces are X, Y' and Z', respectively.

Then the processor will first compute the drift error Y'-Y and apply a correction to X to give a drift corrected value X-(Y'-Y) for the position of the first target surface 16.

The drift error (Y'-Y) is then applied to the third target surface Z'- (Y'-Y) and its value is compared with the stored value Z. If the two are not identical then the variation in gain error will have a value of Z- ( Z ' - (Y ' -Y) ) at the position of the third target surface and a

predetermined proportion C of this error will need to be applied to the measured distance to the first target

surface, which will now have a value

X - (U' -Y) -C (Z- (Z ' - (U' -Y) )

where C is a constant factor dependent on the distance between the first and third target surfaces. The above equation yields a calibrated value of the distance from the sensor 18 to the first target surface 16 that is compensated for both drift and variation in gain.

It will be noted that the processor 42 is required only to perform a simple arithmetic computation that can readily be performed several times during each rotation of the shaft 10, even for a shaft rotating at high speed.

It is an advantage of the described embodiment that, by virtue of the land 26, it also provides a reference signal at a predetermined angular position of the shaft. This signal can be used by other equipment in the machine. For example, if the shaft is the crankshaft of an internal combustion engine, the system can provide an indication of top dead centre. Systems employed to detect lack of balance during rotation of the shaft also require an angular

reference signal which can be provided by the axial

displacement measurement system of the invention.