GB2248110A - Vibration analysis system for a multi-shaft turbine - Google Patents

Abstract

An aero-engine vibration analysis system for a multi-shaft turbine engine comprises at least one vibration transducer 14, 15 attached to the engine, means 17, 18 responsive to the rotational speed and phase of each of the engine shafts, means for sampling the output of the or each vibration transducer, means for storing the transducer output samples together with indications of the correlation of the rotations of the engine shafts with the order of collection of the output samples, and processing means arranged to perform a transform operation on said stored samples to transform the information contained therein from the time domain to the frequency domain. The system operates during aircraft flight but stored data in an EEPROM 40 is only down-loaded after a flight when an indication is given that overwritten memory values show vibration deterioration. <IMAGE>

Description

ENGINE VIBRATION ANALYSIS SYSTEM FOR A MULTI SHAFT TURBINE This invention relates to an engine vibration analysis system for a multi-shaft turbine, particularly a system for use with a gas turbine aeroengine.

Tt is desirable to be able to monitor the condition of an engine in use and one known way of doing this is to measure engine vibration. Monitoring engine vibration can allow engine wear to be calculated and predicted and can be used to detect and diagnose the failure of engine components.

Aircraft engine vibration monitoring systems are known which measure engine vibration in flight and produce a warning if the amplitude of engine vibration goes beyond some pre set limit, indicating that there is some fault in the engine.

Detailed analysis of engine vibration data is only possible using a powerful computer and takes too long to be carried out in real time in flight. So any such analysis must take place when the aircraft is run on the ground when being attached to a further ground based engine vibration monitoring system. As a result when a fault is suspected due to excessive engine vibration, or for any other reason, it is necessary to run the engine again on the ground in order to diagnose the problem.

It has been suggested that engine vibration data obtained in flight could be stored and later supplied to a ground based computer for diagnosis but this has the drawback that very large quantities of data will be produced by the vibration sensor during a flight and must be stored, for example on tapes, requiring a large and costly data storage system.

This invention was intended to produce an engine vibration analysis system for a multi-shaft turbine which stores the vibration data needed to allow later analysis in a more efficient manner.

This invention provides an engine vibration analysis system for a multi-shaft turbine engine comprising at least one vibration transducer attached to the engine, means responsive to the rotational speed and phase of each of the engine shafts, means for sampling the output of the or each vibration transducer, means for storing the transducer output samples together with indications of the correlation of the rotations of the engine shafts with the order of collection of the output samples, and processing means arranged to perform a transform operation on said stored samples to transform the information contained therein from the time domain to the frequency domain.

Systems employing the invention will now be described with reference to the accompanying diagrammatic figures in which, Figure 1 shows a gas turbine engine incorporating an engine vibration analysis system according to the invention; Figure 2 shows a cut away view of the engine of figure 1; and Figure 3 shows the internal structure of the engine vibration analysis system of figure 1; identical parts having the same reference numerals throughout.

Referring to the figures, a gas turbine engine 1 is formed by, in flow series, an air intake 2, a compressor 3, a fuel burner 4, a turbine 5 and an outlet jet pipe 6. The compressor 3 is formed by a multi-stage low pressure compressor 7 upstream of a multi-stage high pressure compressor 8, while the turbine 5 is formed by a high pressure turbine 9 upstream of a low pressure turbine 10.

The low pressure turbine 10 and the low pressure compressor 7 are both attached to a low pressure (LP) shaft 11 so that the low pressure compressor 7 can be driven by the low pressure turbine 10. Similarly the high pressure turbine 9 and the high pressure compressor 8 are both attached to a high pressure (HP) shaft 12 so that the high pressure turbine 9 can drive the high pressure compressor 8. The high pressure shaft 12 is a hollow tube surrounding the low pressure shaft 11 and coaxial with it. The entire engine 1 is enclosed by a casing 13.

Two accelerometers 14 and 15 are secured to the casing 13 to sense vibrations of the engine 1. The two accelerometers 14 and 15 are placed 900 apart around the circumference of the engine 1. The signals produced by these accelerometres are supplied to an engine vibration analysis system 16.

The engine vibration analysis system 16 receives engine vibration signals from the accelerometers 14 and 15. In order to allow the state of the engine to be derived from these signals the engine is run across its full operating range by the engine being slowly accelerated from minimum LP shaft revolutions, idle speed, to maximum LP shaft speed, maximum dry power, and then slowly decelerated back to idle speed. During the acceleration phase vibration readings are taken at each of 100 evenly spaced LP shaft speed bands, the width of each speed band being; Max NL Speed - Min NL Speed 101 Each reading is taken over 10 full revolutions of the LP shaft. Vibration readings are taken at the same 100 LP shaft speed bands during the deceleration phase.Each vibration reading is transformed from the time domain into the frequency domain and compared to readings held in memory for that speed band and acceleration sense and first and second groups of preset vibration values in permanent memory.

The first group of vibration values define vibration levels above which maintenance action should be taken after the aircraft has landed. The second group of vibration values define vibration levels above which action should be taken immediately to reduce the strain on the engine; the values in the second group are higher than the values in the first group.

If the new vibration readings have lower values than the readings in memory and both of the groups of pre-set values, no action is taken and the new vibration readings are not recorded. If the new vibration readings have higher values than the readings in memory the vibration reading values in memory are overwritten with the new values. If the new vibration readings have values above those of the first group a light is lit indicating that the aircraft will require maintenance on landing. If the new vibration readings have values above those of the second group a light is lit indicating to the pilot that he must take action to reduce the power demanded from that engine or risk engine failure.

In order to do this the vibration analysis system 16 is supplied with tachometer signals from LP and HP tachometer units 17 and 18 respectively giving the speed of rotation of the LP and HP shafts 11 and 12 respectively. The LP tachometer signal is produced by the 56 tooth phonic wheel (not shown) mounted on the LP shaft 11 and producing a signal at 56 times the frequency of rotation of the LP shaft 11. The HP tachometer signal is produced by a 28 toothed phonic wheel (not shown) mounted on a power take off shaft (not shown) geared to the HP shaft 12 to rotate at the same speed as the HP shaft 12, to produce a signal at 28 times the frequency of rotation of the HP shaft 12.

The tachometer signals from the tachometer units 17 and 18 are passed through filters 19 and 20 respectively to remove noise. The filtered LP tachometer signals are supplied to a divider circuit 21 which divides their frequency by 56 to produce a signal at a frequency of once per revolution of the LP shaft 11. Similarly the filtered HP tachometer signals are supplied to a divider circuit 22 which divides their frequency by 28 to produce a signal with a frequency of once per revolution of the HP shaft 12.

The once per revolution HP shaft signals from the divider 22 are supplied to an interrupt generator 23.

The once per revolution signal from the divider 21 is supplied to a divide-by-ten circuit 24 to produce a signal with a frequency of once-per-ten revolutions of the LP shaft 11, this divided-by-ten signal is then supplied to the interrupt generator 23.

The once per revolution LP shaft signal is also supplied to a counter 25 and a counter 39, the counter 25 is also supplied with a 500 KHz signal by a local oscillator 26.

The counter 25 is clocked by the 500 KHz signal from the local oscillator 26, incrementing by one of each clock cycle. When the counter 25 receives an NL shaft once per revolution signal from the divider 21 it sends the number it holds to a processor 37 and then resets to zero. The number sent by the counter 25 describes the speed of the NL shaft 11, the larger the number the slower the NL shaft 11 is rotating. The counter 39 is clocked by the NL shaft once per revolution signal from the divider 21 and so counts the number of revolutions of the LP shaft 11.

In normal flight only the LP tachograph unit 17, its associated filter 19 and divider 21, local oscillator 26 and counter 25 and the processor 37 are operational.

The processor 37 monitors the LP shaft speed number produced by the counter 25 and when this number indicates that the LP shaft is at idle speed and then changes to indicate that the LP shaft speed has increased into the first speed band above idle speed the processor 37 activates the rest of the vibration analysis system 16.

The signals produced by the accelerometers 14 and 15 are passed through amplifiers 28 and 29 and filters 30 and 31 respectively, and the amplified and filtered signals are then passed through integrators 32 and 33 respectively to produce vibrational velocity signals.

The vibrational velocity signals from the integrators 32 and 33 are then provided to analogue to digital (A to D) converters 34 and 35 respectively.

The 500 KHz signal from the local oscillator 26 is passed through a divide by 200 circuit 36 to produce a 2.5 KHz signal which is then supplied as a clock signal to the A to D converters 34 and 35. On each pulse of the 2.5 KHz signal the A to D converters 34 and 35 each produce a digital word giving the levels of their respective vibrational velocity sigrals at that time and send this digital word to the RAM memory 27. The 2.5 KHz signal from the divider circuit 36 is also supplied to a counter 38 which counts the number of pulses of the 2.5 KHz signal which occur.

Each vibration information sampling cycle is started when the previous sampling cycle has been completed and the number supplied to the processor 37 by the counter 25 indicates that the LP shaft speed has entered a new speed band. When this occurs the processor 37 signals the interrupt generator 23, resets the divide-by-ten circuit 24 and the counters 38 and 39 to zero, and begins storing the digital words produced by the A to D conve,tters 34 and 35 in sequence in the RAM memory 27.

When 512 of these words have been stored from each of the A to D converters 34 and 35 a full vibration sample has been taken. The interrupt generator 23 sends a first interrupt signal to the processor 37 when it receives the first HP shaft once per revolution signal from the divider 22, a second when it receives the signal from the divide-by-ten circuit 24 and a third when it receives the next HP shaft once per revolution signal from the divider 22 after it has received the signal from the divide-by-ten circuit 24. Thus interrupt signals are generated on the first HP shaft rotation after the start of the sampling cycle, the tenth LP shaft rotation after the start of the cycle and the next HP shaft rotation after the tenth LP shaft rotation.

When each of these three interrupt signals are received by the processor 37 it stores the number contained in the counter 38 in the RAM memory 27 as a part of the vibration data sample. Also, when the first and third interrupt signals are received the processor stores the number held in the counter 39 in the RAM memory 27 as a part of the vibration data sample.

The LP shaft speed is then checked by the processor 37 reading the number sent to it by the counter 25, if this shows that the LP shaft speed has changed by three or more speed bands during the data sampling, the data is assumed to be unacceptable and the processor 37 initiates a new data sampling cycle and the vibration data sample stored in the RAM memory 27 is no processed further. This data will be overwritten by the next vibration data sample taken.

If the LP shaft speed has changes by two or less speed bands the processor 37 then validates the stored vibration data sample by checking that the three interrupt signals have been correctly responded to, if they have not been correctly responded to the vibration data sample is not processed further and a new data sampling cycle is started, again the data sample will be overwritten by the new vibration data sample.If the interrupt signals have been correctly responded to the processor 37 transforms the vibration data from the time domain into the frequency domain using the algorithm;

Where f is the number of NH or NL rotations during the sampling interval N t; t is the sample period; N is the number of data samples occurring during the 10 NL rotations or between the recorded NH interrupts; H(f) is the required vibration component corresponding to the shaft rotation speed; g(n) is the data samples requiring analysis.

This provides a vibration spectrum of vibrational components against frequency. This new spectrum is then compared by the processor 37 with an existing spectrum already held in an EEPROM non-volatile memory 40. The EEPROM non-volatile memory 40 holds two such spectrums for each speed band, one for an accelerating engine and one for a decelerating engine. The new spectrum is compared with the existing spectrum corresponding to the same speed band and acceleration sense, and if the peak values of the new vibration spectrum are higher than the peak values of the stored vibration spectrum the stored vibration spectrum is overwritten in the EEPROM memory 40 by the new vibration spectrum.

The peaks compared in the frequency domain will be the dominant vibrational frequencies of the engine and will be related to the frequency of rotation of the low and high pressure shafts 11 and 12 respectively. The relationship between shaft rotational frequencies and dominant vibrational frequencies will vary between engine designs, but will be fixed for each engine design.

The peak values of the new vibration spectrum are also compared with two sets of preset values in an EEPROM memory 41. A first set of values describe vibration levels above which action or investigation should be carried out by maintenance staff after the aircraft has landed, while a second set of values describe vibration levels higher that those of the first set and'above which the pilot should take immediate action to reduce the strain on the engine.

If the new vibration spectrum is above the first set of values the processor 37 lights a maintenance warning light 42 on a maintenance check panel. If the new vibration spectrum is above the second set of values the processor 37 lights a cockpit warning light 43 warning the pilot that the strain on the engine must be reduced or minimised to reduce the risk of engine failure.

The EEPROM memory 41 also stores the operating programme for the processor 37, the processor 37 refers to this as required.

The processor 37 also oversees the functioning of the vibration analysis system 16 as a whole and if the analysis system 16 fails the processor 37 lights a system failure light 44 in the cockpit to warn the pilot that the engine vibration analysis system 16 has failed.

When the aircraft has landed after a flight, ground maintenance personnel will check whether the maintenance warning light 42 is lit, if it is the stored vibration spectrums in the EEPROM memory 40 will be extracted and analysed.

This is done by attaching a ground based vibration data analysis system (not shown) to an input output terminal 45 linked to the processor 37. The ground based system signals the processor 37 via the terminal 45 to dump the accumulated vibration data, in response the processor 37 reads out the whole set of vibration frequency spectrums for all speed bands and both acceleration senses through the terminal 45 to the ground based analysis system.

The ground based analysis system can then be used to deduce what repairs or replacements of parts are needed or may be needed in the engine 1.

In a multi-engined aircraft a separate set of vibration transducers 14 and 15 and vibration analysis system 16 could be used for each engine.

The system described is intended for use in a two shaft turbine, but could be used in a three or more shaft turbine by adding appropriate tachometers, dividers and counters and interrupt signals.

The number of accelerometers is not related to the number of turbine shafts, although more than two nccelerometers could be used if desired, to increase redundancy for example, only one accelerometer is necessary regardless of the number of shafts.

Although accelerometers are used in the system described above any form of vibrational transducer could be used.

It is preferred to use two accelerometers spaced apart around the engine by 900, however accelerometers with other orientations, or one accelerometer only could be used.

Claims (17)

1 An engine vibration analysis system for a multi-shaft turbine engine comprising at least one vibration transducer attached to the engine, means responsive to the rotational speed and phase of each of the engine shafts, means for sampling the output of the or each vibration transducer, means for storing the transducer output samples together with indications of the correlation of the rotations of the engine shafts with the order of collection of the output samples, and processing means arranged to perform a transform operation on said stored samples to transform the information contained therein from the time domain to the frequency domain.

2 A system as claimed in claim 1 wherein the sampling means is arranged to collect a predetermined number of samples in a sampling cycle.

3 A system as claimed in claim 1 or claim 2 wherein collection of a first sample is synchronised with rotation of a first of the engine shafts.

4 A system as claimed in claim 2 or claim 3 wherein the rotational phase of the or each other shaft is correlated with the order of collection of the samples.

5 A system as claimed in any of claim 2 to 4 wherein an integer number of revolutions of each shaft is correlated with the order of the samples.

6 A system as claimed in any preceding claim wherein the sampling frequency is fixed.

7 A system as claimed in any preceding claim wherein a sampling cycle is performed at a multiplicity of predetermined engine speeds.

8 A system as claimed in claim 7 wherein the said predetermined engine speeds are spaced apart at regular intervals throughout an engine speed range.

9 A system as claimed in claim 8 wherein there are

101 sampling speeds spaced apart by 100 equal speed intervals

10 A system as claimed in claim 7 wherein sampling cycles are performed during engine acceleration and deceleration.

11 A system as claimed in any of claims 7 to 10 wherein a sampling cycle is triggered automatically by the engine attaining a predetermined speed.

12 A system as claimed in claim 1 wherein two vibrational transducers are attached to the engine.

13 A system as claimed in claim 12 wherein the two vibrational transducers are attached to the engine 0 in positions 90 apart around the engine.

14 A system as claimed in claim 1 wherein each stored sample, after transformation into the frequency domain, is compared to a previously stored sample and if it represents higher levels of vibration that the previously stored sample is used to replace this previously stored sample.

15 A system as claimed in claim 7, 10 and 14 where each stored sample is compared to a previously stored sample from a sampling cycle performed at the same engine speed and during engine acceleration in the same sense.

16 A system as claimed in claim 1 in which the each stored sample, after transformation into the frequency domain is compared to a set of predetermined values and a warning is issued if the stored sample exceeds the predetermined values.

17 A system substantially as shown in or as described with reference to the accompanying figures.