GB2342988A

GB2342988A - Monitoring turbine Vibration

Info

Publication number: GB2342988A
Application number: GB9822805A
Authority: GB
Inventors: John Taylor
Original assignee: ROTADATA Ltd
Current assignee: ROTADATA Ltd
Priority date: 1998-10-19
Filing date: 1998-10-19
Publication date: 2000-04-26
Also published as: GB9822805D0

Abstract

Method and apparatus for monitoring turbine blade vibration comprises an emitter (E) and detector (D) arranged to emit and detect pulsed optical radiation which is focussed on a point (P) traversed by the turbine blades (B). The total duration of each burst of pulses reflected by a blade is repeatedly detected and an apparent blade width signal derived and analysed to determine amplitude and frequency of blade vibration in the circumferential direction. The optical radiation may be directed generally radially and may be directed and collected via fibre optics.

Description

APPARATUS AND METHOD FOR MONITORING MOTION OF ROTARY MEMBER The present invention relates to a method and apparatus for monitoring the motion of a rotary member. The invention is applicable particularly, but not exclusively to monitoring of vibration in a turbine assembly.

The monitoring of vibration in a rotating turbine assembly is an important practical problem and our granted UK patent no. 9609677.1 discloses a capacitive method for detecting bending and rotation of the axle of such an assembly during rotation, the method involving the detection of capacitance between the tip of a capacitive probe and the periphery of the turbine blades. Such a method is suitable for detecting any motion in the radial direction but is less suitable for detecting motion in the circumferential direction because any such motion will result in only a small change in capacitance and any measurement will be complicated by fringe effects.

Accordingly, in one aspect the invention provides a method of monitoring the motion of a rotary member, the method comprising the steps of directing modulated optical radiation at a projecting portion of the rotary member, the projecting portion being arranged to travel across the path of the optical radiation during rotation of the rotary member and thereby modulate further the optical radiation, detecting the further-modulated radiation and analysing the detected radiation to derive information about said motion.

By optical radiation is meant any electromagnetic radiation obeying the laws of optics, e. g.

IR and UV radiation as well as light.

Preferably the modulated optical radiation is focussed onto a surface region of the projecting portion and radiation reflected or scattered from said surface region is detected.

By concentrating the radiation onto a small region and detecting the scattered or reflected radiation, a relatively sharp transition in the detected signal is obtained as that region crosses the path of the optical radiation.

Preferably the optical radiation is directed generally radially onto a radially-extending projection. This feature facilitates the detection of peripheral vibration.

Preferably the optical radiation is modulated (e. g. chopped) with a periodic input signal.

This preferred feature facilitates the processing of the detected signal by, e. g. a digital processor.

In another aspect the invention provides apparatus for monitoring the motion of a rotary member, the apparatus comprising means for directing modulated optical radiation at a projecting portion of the rotary member, whereby in use the projecting portion is arranged to travel across the path of the optical radiation during rotation of the rotary member, thereby to modulate further the optical radiation, a detector arranged to detect the furthermodulated optical radiation, and processing means arranged to analyse the detected radiation to derive information about said motion.

Preferred features of the invention are defined in the dependent Claims.

Preferred embodiments of the invention are described below by way of example only, with reference to Figures 1 to 6 of the accompanying drawings, wherein: Figure 1 is a diagrammatic representation of apparatus in accordance with the invention for monitoring turbine blade vibration; Figure 2 is a plot against time of the output signal of a detector of Figure 1; Figure 3 is a set of plots derived from the plot of Figure 2 and showing the variation of arrival time of a given blade during revolutions 1, 2,... n ; Figure 4 is a similar set of plots showing the variation in apparent blade traverse time during revolutions, 1,2... n; Figure 5 is a schematic plan view of the probe and turbine blade of Figure 1 showing the detection of blade"flapping" ; and Figure 6 is a schematic plan view of two probes and a turbine blade in a variant of the apparatus of Figure 1 arranged to detect torsional blade vibration.

Referring to Figure 1, a turbine rotor assembly T is provided with a probe PR which is advancable and retractable through the turbine casing (not shown) in the radial direction.

The probe comprises a silicon photodetector D cooled by a Peltier cooler C and having a lens arrangement L1 arranged to focus optical radiation from the exit face of an optic fibre F1 (or other waveguide) which collects radiation reflected and/or scattered from point P on the periphery of a turbine blade B. Fibre F1 can have a graded index (GRIN) portion for this purpose, or altematively a further lens arrangement (not shown).

A somewhat similar arrangement comprising an optic fibre F2 optically coupled to a photodiode E (or other emitter) by a lens L2 focusses optical radiation onto point P. This radiation is chopped by means of a square wave signal generated by a square wave oscillator 2 and amplified by an amplifier 1 before being fed to photodiode E.

The frequency of the square wave signal is suitably 10 MHz or greater and can be generated by a timing signal t fed to oscillator 2.

Peltier cooler C is energised from a power supply 3. This arrangement reduces electrical noise and drift in detector D.

The output of detector D is a 10 MHz square wave signal when a portion of the tip of a blade B lies at point P and is otherwise zero-in other words the signal, initially modulated by the 10 MHz square wave signal, is modulated further by the rotating assembly of turbine blades. This is illustrated in Figure 2, which shows three bursts B1, B2 and B3 of the 10 MHz square wave signal output by the detector and associated with successive rotations of a given blade B past point P. In this embodiment there will be similar bursts associated with the traverse of each other blade across point P, which, for the sake of simplicity, are omitted from Figure 2.

Tuming again to Figure 1, the output signal from detector D is first amplified by amplifier 6, then fed directly to digital processor arrangement 4. The amplifier 6 as well as the Peltier cooled C are preferably inegral with detector D. as shown in Figure 4. Processor 7 is suitably arranged to limit the ampliture of the output signal by cutting off the peaks (thereby sharpening the leading and trailing edges and eliminating some noise) and to count the pulses in each burst. Processor 7 also preferably measures the pulse width of the leading and trailing pulses PL and PT, to generate an output signal representative of the aggregate of the pulse durations. This signal is shown in Figures 3 and 4 and comprises a single pulse associated with the passage of a single blade part the detector at each revolution. Each plot a), b), c) represents the signal associated with parolution 1, 2,... n respectively and a given blade generates pluses 1,2,... n in the respective revolutions.

The above signal is mathematically processed in the digital domain by a digital signal processor 15 having a stored program in ROM 16, and finally processed in a microprocessor 17 (provided with RAM 18) and fed via an interface 19 to be displayed on the screen of a computer 20.

The mathematical processing of the digital signal (representation of blade velocity V and apparent blade thickness in the direction of travel) will now be described.

Typically the speed of a blade tip will be from Mach 0.7 to Mach 1.5 and the thickness in the direction of travel will be 1 mm or more. Hence the time between the leading edge and trailing edge of a blade B crossing point P will typically be about 3 microseconds, or more, corresponding to about 30 full wave reflectors per blade passing.

This assumes no vibration.

However, given that the blade will vibrate in the circumferential direction, the number of pulses in successive bursts B1, B2 and B3 (Figure 2) will vary, such that the amplitude of vibration will be proportional to the difference in the number of pulses (or, if fractions of a full pulse are taken into account, the aggregate pulse width of each burst). The extreme positions of the blade in the rotating frame of the turbine rotor are indicated at B and B'in Figure 5.

The signal of Figure 2 is processed firstly, to determine the amplitude of vibration of a given blade and secondly, to determine the speed of the blade tip relative to its root.

These data enable the frequency of vibration to be detrmined, on the reasonable assumption that the blade exhibits simple harmonic motion.

Figure 4 illustrates the derivation of the speed of vibration. t,, t2,... tn is the period between the leading and trailing edges of a given blade crossing the point of focus P (Figure 1) of the beam during revolutions 2,2,... n respectively. By taking measurements over a large number of revolutions (say 1000 or more) the maximum and minimum period t max and t min can be found. t min occurs when the blade tip is moving in the same direction as the direction of rotation of the root and t max occurs when the blade tip is moving in the opposite direction. The peak speed V max of the blade tip relative to the blade root can be found from the expression:

Where x is the length of the blade periphery (as shown in Figure 1).

Figure 3 illustrates the derivation of the amplitude of vibration from the arrival times of the (mid-point) of each blade T1, 12,... ln. The maximum and minimum arrival times c max and X min can be found from a large number of samples and the amplitude A can be found from the expression: A = Vp (i max-T min) where Vp is the peripheral speed of the blade root. The frequency f of vibration (assuming simple harmonic motion) can be found from the expression: f = V max/2nA.

RAM 18 (Figure 1) can be arranged to store in separate locations data derived from respective blades, so that the vibration frequencies of the blades can be determined individually.

The torsional vibration of the blades can be determined from the outputs of two sensor arrangements directed at axially displaced points P1 and P2 as shown in Figure 6. The extreme positions of the blades are indicated at B and B'.

Claims

CLAIMS 1. A method of monitoring the motion a rotary member, the method comprising the steps of directing modulated optical radiation at a projecting portion of the rotary member, the projecting portion being arranged to travel across the path of the optical radiation during rotation of the rotary member and thereby modulate further the optical radiation, detecting the further-modulated radiation and analysing the detected radiation to derive information about said motion.
2. A method as claimed in Claim 1, wherein the modulated optical radiation is focussed onto a surface region of the projecting portion and radiation reflected or scattered from said surface region is detected.
3. A method as claimed in Claim 2, wherein the optical radiation is directed generally radially onto a radially-extending projection.
4. A method as claimed in any preceding Claim, wherein the optical radiation directed onto the projecting portion is modulated with a periodic input signal.
5. A method as claimed in Claim 4, wherein the optical radiation is chopped by said inputsignal.
6. A method as claimed in any preceding Claim, wherein the optical radiation is directed onto the projecting portion from the end of an optic fibre.
7. A method as claimed in any preceding Claim, wherein the optical radiation is reflected or scattered by the projecting portion and is then collecte into an end of an optic fibre leading to a detector.
8. A method is claimed in any preceding Claim, wherein the rotary member is a turbine assembly, and the modulated radiation is directed onto turbine blades of the assembly.
9. A method as claimed in Claim 8, wherein the optical radiation directed onto the turbine blades is modulated with a period shorter than the time taken for a turbine blade to travel across the path of the optical radiation.
10. A method as claimed in Claim 9, wherein the duration of modulation in the detected optical radiation associated with each traverse of a turbine blade is repeatedly determined and periodic differences therein are analysed to derive information about blade vibration.
11. A method as claimed in Claim 10, wherein the optical radiation is directed onto axially spaced apart regions of a turbine blade, the duration of modulation of the detected optical radiation associated with the traverse of the respective regions of the turbine blade is repeatedly determined and the results compared, and information about torsional blade vibration is derived from the comparison.
12. A method as claimed in Claim 10 or Claim 11, wherein said determination involves counting cycles of modulation.
13. A method as claimed in any of Claims 10 to 12, wherein said determination is phase-sensitive.
14. A method as claimed in any of Claims 10 to 13, wherein said information is derived by digitally processing one or more output signals from an optical detector arrangement.
15. A method as claimed in any preceding Claim, wherein the speed of vibration is obtained from said signal representative of apparent blade width and/or the amplitude of vibration is determined from variations in blade arrival time.
16. A method as claimed in any preceding Claim, wherein the further-modulated optical radiation is detected by a cooled photodetector.
17. A method of monitoring the motion of a turbine assembly, the method being substantially as described hereinabove with reference to Figures 1 to 7 of the accompanying drawings.
18. Apparatus for monitoring the motion of a rotary member, the apparatus comprising means for directing modulated optical radiation at a projecting portion of the rotary member, whereby in use the projecting portion is arranged to travel across the path of the optical radiation during rotation of the rotary member, thereby to modulate further the optical radiation, a detector arranged to detect the further modulated optical radiation, and processing means arranged to analyse the detected radiation to derive information about said motion.
19. Apparatus according to Claim 18, comprising beam-forming means arranged to direct a beam of optical radiation onto a surface region of the projecting portion, wherein the detector is arranged to detect radiation reflected or scattered from said surface region.
20. Apparatus according to Claim 18 or Claim 19, which is arranged to monitor the motion of a turbine assembly, the apparatus comprising a probe arranged to direct the optical radiation radially inwardly onto a surface region of a blade tip.
21. Apparatus according to Claim 20, arranged to generate a digital signal representation of apparent blade width.
22. Apparatus according to claim 21, comprising means arranged to acquire and store a succession of such digital signals as a given turbine blade repeatedly crosses the path of the optical radiation and processing means arranged to process the stored signals to derive information about the vibration of that blade.
23. Apparatus according to any of Claims 18 to 22, comprising means for cooling the detector.
24. Apparatus for monitoring the motion of a turbine assembly, the apparatus being substantially as described hereinabove with reference to Figures 1 to 7 to the accompanying drawings.