GB2190198A - Vibrational analysis system for a machine - Google Patents

Abstract

A vibration analysis system is provided for analysing operation of a machine having a periodically repeating operating cycle. A cycle monitor (12) generates signals related to the machine cycle rate. The analysis system includes a time synchronous analyser (15) for receiving the vibration signals from vibration sensing means (10, 11) for each of a plurality of operating cycles of the machine. The time synchronous analyser (15) includes a sampler (16) to sample the vibration signals at a plurality of sampling points within each of the operating cycles; a time base controller (17) operative to carry out on the sample vibration values a compression or expansion operation to compensate for any variations between the durations of any of the operating cycles; and a sample analyser (18) operative to provide a vibration measure related to the machine vibration. A vibration output means (20) is coupled to the time synchronous analyser (15) for display and/or recording the vibration measure data for the machine. <IMAGE>

Description

SPECIFICATION Vibrational analysis system for a machine This invention relates to vibration analysing or monitoring, particularly although not exclusively in reciprocating engines for engine performance and condition diagnosis. However the invention is not limited to reciprocating engines but is applicable to other machinery having periodically repeating operating cycles including pseudo-reciprocating engines such as rotary and orbital internal combustion engines.

Diagnostic methods for rotating machines, including vibration monitoring, are well developed. However reciprocating machines have not been subject of the same attention, particularly in relation to diagnostics.

Nevertheless there have been studies of the behaviour of reciprocating engines in relation to the generation of vibration energy and hence the radiation of noise. The distinction between "mechanical noise" due to mechanical processes and other noise generated by such sources as inlet and exhaust gas flows has been made. The relevance of this work to fault diagnosis is that radiated engine noise is the direct result of engine surface vibration, which in turn is generated by mechanical sources within the engine. The measured airborne noise however is a space-averaged quantity and includes energy from all radiating surfaces. Consequently airborne noise measurement does not readily enable the distinguishing between areas of highly local vibration not discrimination between similar adjacent sources, such as cylinders in an engine.Furthermore, the physical surroundings of the test engine are of utmost importance to the measurement of noise and repeatability is poor except under laboratory conditions. Therefore, the past research into the characteristics and origins of noise has little direct bearing on the identification and trending of engine faults.

Presently known systems for reciprocating engine condition monitoring are based primarily on the measurement of pressures, temperatures and crank angle. These systems are primarily intended for application in large engines and test-bed environments and afford excellent data logging and analysis capabilities for some functions. However to achieve this power they rely on the processing of signals from many flow, pressure and temperature transducers. Apart from the high cost of pressure transducers, extensive labour is required for fitting including the extensive machining required in small to medium engines in order to provide ports into which the transducers need to be fitted. The information provided by these systems, relying as it does on measured cylinder pressure data, enable accurate balancing of the pressure per cylinder.However many important engine events which affect engine condition are not assessed, e.g. valving, timing gears, bearings, and "piston-slap" due to lateral movement of a piston within the cylinder arising from clearances between the piston and cylinder. Therefore, although these known systems provide extensive data on overall performance and even individual cylinder performance, they provide only limited diagnostic insight to many of the faults which may be present.

Vibration frequency spectrum analysis which has been successfully applied in rotating machines has in recent times been applied to reciprocating machines. This has been promoted as a means of detecting engine faults by the processing of engine surface vibration.

"Cascade" spectrum plots can be used to detect a change in machine vibration character, particularly at firing, however the method suffers from several limitations. In particular, the typical apparatus enables between 8 and 15 spectra to be computed per engine revolution at relatively low machine speeds. This results in an angular resolution of 24 to 45. The vibration pulse generated by valve operation can have a duration of less then 5 and so the pulse is included in the same angular spectrum as other sources. This smearing has the result that the effect of valve operation is masked to a point where such events cannot be detected. Also for slower speeds, for low numbers of spectra/rev, data between samples from which the spectra are computed can be completely lost.To maintain a significant frequency bandwidth sample durations need to be of a certain length and this can cause spectra to overlap, resulting in smearing of data and inability to distinguish all but the most gross combustion events, particularly at high machine speeds. Another limitation arises from the fact that in a diesel engine most vibration sources are impactive (e.g. a rocker arm impacting a valve), or quasi-impactive (e.g. the very sharp pressure rise on initiation of combustion). This excites parts of the engine structure at resonance frequencies and the spectral shape of the engine vibration response is then determined by the characteristic period of the impact. This means that the broad shape of each spectrum will be largely independent of which source caused the impact making it difficult to distinguish between different sources of noise.

It is an object of the present invention to provide a vibration analysis system suitable for diagnostic use in connection with one or more components of a machines having a periodic operating cycle and producing vibration in use.

It is a preferred object of the present invention to provide a vibration analysis system suitable for use in the diagnostic analysis of a reciprocating internal combustion engine and which overcomes or alleviates the aforementioned problems of the prior art.

According to the present invention there is provided a vibration analysis system for ana lysing operation of a machine having a periodically repeating operating cycle, the machine in use having a vibration sensing means operatively associated with the machine, the vibration sensing means in use providing vibration signals indicative of vibration of components of the machine, the machine in use further having operatively associated therewith a cycle monitor for generating signals related to the machine cycle rate; the analysis system including: (a) a time synchronous analyser for receiving the vibration signals from the vibration sensing means for each of a plurality of operating cycles or part-cycles of the machine and for analysing vibration levels over the plurality of operating cycles or part-cycles, the time synchronous analyser including: (i) a sampler operative to sample the vibration signals at a plurality of sampling points within each of the operating cycles or part-cycles so as to provide a plurality of sample vibration values for each operating cycle or part-cycle; (ii) a time base controller associated with or responsive to the cycle monitor and operative to carry out on the sample vibration values a compensation operation for any variations between the durations of any of the operating cycles or part-cycles whereby each of the compensated sample vibration values for each operating cycle or part-cycle after the compensation operation has been carried out by the time base controller substantially corresponds to the same point in the machine operating cycle or part-cycle as a respective compensated sample vibration value for each other operating cycle or corresponding part-cycle; and (iii) a sample analyser operative to provide a vibration measure related to the machine vibration for each of the plurality of sampling points within the machine operating cycle or partcycle over the plurality of such cycles or partcycles; (b) vibration output means coupled to the time synchronous analyser for display and/or recording the vibration measure data for the machine.

Preferably the sample analyser includes an event variability means for determining the variability of the compensated sample vibration values for each sampling point of the machine operating cycle or part-cycle over a plurality of such cycles or part-cycles. The event variability means comprises means for determining a statistical measure of variability of respective compensated sample vibration values within the plurality of operating cycles or part-cycles so as to thereby provide an indication of the variability of the machine vibration throughout the operating cycle or part-cycle whereby relatively high values of the statistical measure anywhere within the operating cycle or partcycle will reveal events that display significant vibration variability throughout the plurality of operating cycles or part-cycles.

In one possible embodiment the sample analyser includes a sample accumulator operative to provide a cumulative vibration measure indicative of or related to the total machine vibration for each of the plurality of sampling points over the plurality of machine operating cycles or part-cycles.

In an alternative embodiment the sample analyser includes a sample averager operative to provide an average vibration measure indicative of or related to the average machine vibration for each of the plurality of sampling points over the plurality of machine operating cycles or part-cycles. The sample averager is preferably operative to carry out a time synchronous averaging of the compensated sample vibration values for the plurality of operating cycles or part-cycles.Preferably the sample averager comprises a memory device in which are accumulated respective compensated vibration values for corresponding points in the machine operating cycles for a plurality of cycles or part-cycles, the averager being operative at the end of the predetermined number of operating cycles or part-cycles to divide the total vibration value in the memory by the number of sampled operating cycles or part-cycles to produce a linear arithmetic average vibration value for each sampling point.

Wherein the cycle monitor in one possible embodiment comprises an angular position encoder for continuously generating output signals relating to the instantaneous position of the machine in its operating cycle or part-cycle, the encoder constituting the time base controller and being operative to generate the output signals at the desired increments of cycle position corresponding to the desired vibration value sampling frequency whereby the encoder output signals serve to trigger the sampler directly.

In an alternative embodiment, the cycle monitor comprises a tachometer signal generator for generating a tachometer signal at the same point(s) in each the plurality of machine operating cycles or part-cycies, the time base controller being operative to process the first operating cycle or part-cycle of the machine as a datum for comparison with subsequent operating cycles or part-cycles being monitored whereby all operating cycles or partcycles to be averaged after the first operating cycle or part-cycle are normalised by the time base controller to the duration of or time base established from the first operating cycle.

The data output by the time synchronous analyser may be is passed through a filter stage operative to eliminate some vibration frequencies arising from indeterminate or nonsignificant causes or to selectively separate several coincident events having different characteristic frequency spectra. The filter stage is preferably operative to filter at predetermined frequencies solely for predetermined parts of the machine operating cycle or part-cycle whereby enhancing discrimination of events relating to the machine components without masking effects caused by non-significant vibration forces.

The system may further include vibration analysis means including an event identification means operative to automatically determine the occurrence of a particular event in the operating cycle or part-cycle as revealed by the vibration measures. Preferably the event identification means is operative to identify events in time within the operating cycle or part-cycle.

The event identification means may be operative to select a window within the operating cycle within which to conduct analyses for identifying the onset of a particular event, the window location within the operating cycle being carried out by an algorithm which commences with a nominal timing value for the particular event and then determining a range of times before and/or after that nominal timing value. The event identification means is preferably further operative to compare the determined vibration measures with a predetermined threshold vibration measure and to estimate the commencement time of the event by extrapolation to zero or by determining the last zero crossing for the vibration measure before the first threshold crossing vibration measure.The event identification means may be operative to estimate the commencement time of the event by extrapolation to zero by constructing mathematically an envelope or fitted curve for the vibration measure identifiable within the event and then projecting that envelope back to zero to yield the estimated starting time of the event.

The event identification means may further include event magnitude identification means operative to integrate the area under a plot of the vibration measures throughout the part of the operating cycle between the start and.end of an event as determined by the event identification means.

Possible and preferred features of the present invention will now be described with particular reference to the accompanying drawings. However it is to be understood that the features illustrated in and described with reference to the drawings are not to be construed as limiting on the scope of the invention. In the drawings: Fig. 1 is a block diagram of a vibration analysis system according to a possible preferred embodiment of the present invention, Fig. 2 is an example plot of average vibrational values throughout the operating cycle of a four stroke engine, Fig. 3 is a section of the plot of average vibration values versus angular position of Fig.

2 on an expanded rotational axis scale, and Fig. 4 is a section of a plot of the time standard deviation of the average vibration versus angular position on an expanded rotational axis scale.

The vibration analysis system in Fig. 1 is for use with vibration sensing means 10, 11 which in use is operatively associated with a machine (not shown) having a periodically repeating operating cycle The sensing means 10, 11 in use provide vibration signals indicative of vibration of components of the machine. A cycle monitor 12 generates signals related to the machine cycle rate. The cycle monitor 12 may comprise an angular position encoder for continuously generating signals relating to the instantaneous point or position of the machine in its operating cycle. Alternatively, the cycle monitor 12 may comprise a tachometer signal generator for generating a tachometer signal at the same point(s) in each of a plurality of machine operating cycles so as to enable determination of a limit or limits of a time base for each cycle or part-cycle.

The vibration analysis system includes a time synchronous analyser 15 for receiving vibration signals from the vibration sensing means 10, 11 for each of the operating cycles or part-cycles and for analysing vibration levels over the plurality of operating cycles or partcycles. The time synchronous analyser 15 includes: a sampler 16 operative to sample the vibration signals a plurality of times within each operating cycle or part-cycle so as to provide a plurality of sample vibration values for each operating cycle or part-cycle.A time base controller 17 is associated with or responsive to the cycle monitor 12 and is operative to carry out a compensation operating for any time base variations between operating cycles or part-cycles, whereby each of the sample vibration values for each operating cycie or part-cycle output by the time base controller 17 substantially corresponds to the same point of the machine operating cycle or part-cycle as a respective sample vibration value for each other operating cycle or corresponding part-cycle. A sample analyser 18 is operative to provide a vibration measure related to the machine vibration for each of the plurality of vibration sampling points within the machine operating cycle or part-cycle over the plurality of such cycles.The vibration analysis system also includes vibration output means 20 coupled to the time synchronous analyser 15 for display and/or recording the vibration measure data for the machine.

It will be convenient to hereinafter describe the invention in relation to analysis of vibration throughout complete machine operating cycles, although as indicated above the invention is not limited to complete cycle analysis but may be applicable to analysis of corresponding parts only of a plurality of operating cycles.

The sample analyser 18 includes an event variability means 22 for determining the variability of the sample vibration values for each point of the machine operating cycle over a plurality of such cycles. This possible embodiment will be further described later. In an alternative possible embodiment (not shown), the sample analyser 18 may comprise a sample accumulator operative to provide a cumulative vibration measure indicative of or related to the total machine vibration for each of the plurality of vibration sampling points over the plurality of machine operating cycles. In a further possible embodiment the sample analyser 18 comprises a sample averager 23 operative to provide an average vibration measure indicative of or related to the average machine vibration for each of the plurality of vibration sampling points over the plurality of machine pperating cycles.The "average vibration measure" may be a linear arithmetic average vibration level, a root mean square vibration level or other statistical vibration level. This third possible embodiment of sample averager 23 being provided will be described in further particular detail but the invention is not limited to this form of time synchronous analyser 15.

The preferred embodiment of the present invention illustrated utilises time synchronous averaging which is based on the principle that the vibration signal averaging process does not proceed randomly but is synchronised to the engine operating cycle or parts of it. This enables data averaging to be carried out so that the variability of individuai operating cycle events such as cylinder firings is expected to have minimal effect on the usefulness of the technique as a diagnostic tool. That is, unless the vibration signals can be averaged such that variation between averages is less that the change caused by a significant fault then the fault cannot be reliably detected.Without limiting on the invention, it is believed that between five and thirty averages may represent a good compromise between the two requirements that: (a) event variations be averaged to produce a valid statistical mean that will enable small scale events and faults to be detected, and (b) averaging should not proceed too long otherwise machine speed variations and torsional vibration will cause time base errors which will eventually average the high frequency content of the signal to zero.

The vibration sensing means 10, 11 associated with the machine being monitored may be of any convenient construction and operation. For example the vibration sensing means 10, 11 may comprise an accelerometer or other vibration sensor mounted to the machine. It will be convenient generally to describe the present invention in relation to a diesel engine and in this case the accelerometer may be mounted to the cylinder head adjacent the cylinder under study. However it will be appreciated that the invention is not limited to diesel engine analysis or monitoring nor to such mounting vibration sensing means 10, 11. For example the invention may be applicable to other reciprocating engines such as petrol engines and to pseudo-reciprocating engines such as rotary and orbital internal combustion engines.

The upper frequency limit of sensitivity of the vibration sensing means 10, 11 may be for example about 10 to 30 kHz. This frequency may be a function of the accelerometer. In order to attenuate lower frequency signals, say under 500 Hz (e.g. arising from sources such as the pistons being of different weights, misalignment of output drives etc.) the output vibration signals from the vibration sensor 10, 11 may be passed through filter 30, which may be a high-pass analogue or digital filter in order to attenuate the lower frequency signals and improve the signal-tonoise ratio of small scale events such as valve opening. Band pass filtering may also be desirable to reduce the effect of vibration transmission from adjacent cylinders. "Enveloping" of the sensor signal prior to averaging may also be included.

If the engine is a two-stroke engine then a once-per-revolution pulse is required for the same events to be included one in each time base. In the case of analysis or monitoring of a four stroke engine, the tachometer signal generator 12 is preferably operative to generate the tachometer signal one-per-two-revolutions of the engine. A marker (not shown) mounted for rotation with the engine cam shaft may be provided and as the marker passes a monitor 12 the tachometer pulse is generated. Alternatively, a vibration sensing means may be attached to an injector pump or line and used to arm an electronic circuit which is then triggered by a once-per-revolution pulse derived from a marker on the camshaft thereby resulting in a once-per-revolution tachometer pulse. Other electronic or electromechanical means may aiternatively be devised for generating the required tachometer pulses.The tachometer pulse may conveniently be used to determine the commencement of a time base for each engine cycle although if desired it is possible to use the tachometer pulse to terminate a time base.

The vibration signal from the vibration sensing means 10, 11 may pass via an amplifier 31 to a filter 30 or envelope stage 34 and thence to an analogue to digital converter 32.

The operating frequency ofthe analogue to digital converter 32 may be equal to or greater than the rate of sampling the vibration signals by the sampler 16 of the time synchronous analyser 15. It is possible for the sampler 16 of the time synchronous analyser 15 to comprise the analogue to digital converter 32. It is also possible for the filter 30 to be a digital filter incorporated after the analogue to digital converter 32, or for the sampler 16 to comprise the filter 30.

The sampler 16 which in use is operative to sample the vibration signals is preferably operative to take a number of samples large enough to enable discrimination of events within the operating cycle and small enough so that random variations in component vibrations between cycles do not tend to cancel to zero over a significant number of cycles. For example in the case of a diesel engine operating at about 25Hz, 3600 samplings per engine cycle (720 of crankshaft rotation) per cylinder may be taken. With this resolution variations of engine rotational speed within one cycle of operation of the engine are not normally significant in causing data "smearing" or loss of significant event data in the average vibration measure. A number or all engine cylinders may be sampled simultaneously.In this case, the vibration analysis system may use multiplexer 33 techniques to process the input data from a plurality of vibration sensing means 10, 11.

Also it will be appreciated that it may take more than the duration of one machine operating cycle to process the data from that cycle or a preceding cycle and therefore subsequent cycle analysis may be carried out by loading input data into a buffer or storage means for subsequent retrieval and processing. Alternatively the sampling of the vibration signals may be delayed for one or more machines operating cycles so that, say, only every second or third etc. cycle is sampled and subject to analysis.

The analogue to digital converter 32 may be operative to sample the vibration signals at a rate determined by a clock signal. For example, where there are say 36,000 analogue to digital conversions per cycle, the sampler 16 may be operative to average each consecutive set of ten values represented by the analogue to digital converter output to thereby obtain 3,600 vibration value "samples". The sampler may be responsive to the cycle monitor 12 to produce the vibration value samples or may itself be clock driven. In the case where the cycle monitor 12 comprises an encoder indicating the instantaneous point or position of the machine in its operating cycle, the sampler 16 may be responsive to the encoder output to provide a vibration value sample the required number of times per cycle.In the case of 3,000 samples per cycle being required, the sampler 16 may be operative to generate its vibration value sample output every 0.2 of crankshaft rotation (for a four-stroke engine) as indicated by the encoder 12.

The time base controller 17 is provided because, even at constant running speed (which is preferred in use of the invention), there will be significant random fluctuations in actual running speed between engine operating cycles. The time base controller 17 is associated with or is responsive to the cycle monitor 12. In the case of the cycle monitor 12 comprising an encoder, the time base controller 17 may be also constituted by the encoder if the encoder output is generated at the desired increments of cycle position corresponding to the desired vibration value sampling frequency since in that case the encoder output will serve to trigger the sampler 16 directly.

In the preferred and simpler case of the cycle monitor 12 comprising a tachometer signal generator (as illustrated in Fig.

1), the first operating cycle of the machine conveniently may be taken as a datum for comparison with subsequent operating cycles being averaged. That is, all subsequent operating cycles to be averaged during the analysis operation would be normalised by the time base controller 17 to the duration of or time base established from the first operating cycle. The time base controller 17 is operative to measure the duration of the first and each subsequent operating cycle and to adjust the timing of the samples in the respective cycle to the standard time base (first cycle duration) so that each sequential value of the sample vibration values for each of the second and subsequent cycles corresponds to a respective sample vibration value in the first operating cycle.To achieve this compensation for time base variations the sampler 16 is operative to sample at known time intervals, the series of samplings for each cycle after the first being stored by the sampler 16 while the time base is being measured and the time base compensation factor being determined by the controller 17, after which the series of stored samplings are compressed or expanded into the same number of samples as were taken during the first operating cycle.

In the time synchronous analyser 15 which includes a sample averager 23, the averager 23 may comprise a register or other electronic memory device in which are accumulated respective vibration values for corresponding points in the engine operating cycle for a plurality of cycles. At the end of the predetermined number of operating cycles being monitored (e.g. between five and thirty cycles) the total vibration value in each of the number of registers (corresponding to the number of samplings of the vibration signals in the first engine operating cycle) may be divided by the number of operating cycles to give a linear arithmetic average vibration value for each sampling point. In the preferred embodiment this average vibration value may be in units of positive or negative acceleration.A root mean square vibration value or other statistical moments may be calculated in possible alternative embodiments.

At the end of the predetermined number of operating cycles over which the analysis operation is carried out, the series of average vibration values, one value for each of the sampling points within the machine operating cycle, will represent the average vibration experienced at the location of the vibration sensing means 10,11 for the engine operating cycle i.e. in the time domain.

The vibration output means 20 coupled to the time synchronous analyser 15, although not limited thereto, may comprise a screen or plotter for printing of the average vibration values throughout the engine operating cycle.

In the preferred embodiment, before the data output by the time synchronous analyser 15 is displayed or stored in archive 24, it may be passed through a filter or enveloping stage 40. This filter stage 40 in an embodiment not illustrated may be provided between the sampler 16 and the sample averager 23 so that the average vibration value in the sample averager 23 at the end of the analysis operation will represent a filtered average vibration value. However in order to use less processing time, the filtering as illustrated in Fig. 1 may be carried out on the average vibration values at the end of an analysis operation.

The post-averaging filter stage 40 is a digital filter operative to eliminate some vibration frequencies arising from indeterminate or nonsignificant causes, such as vibrations arising from combustion in a cylinder adjacent to the one being monitored. This post-averaging filtering may serve to selectively separate several coincident events having differing characteristic frequency spectra. The post-averaging digital filter 40 may be uniform in its frequency response for the entire operating cycle. Alternatively, or in addition, digital filtering at certain predetermined frequencies may be carried out solely for predetermined parts of the entire operating cycle.This may enable enhanced discrimination of events related to the cylinder being analysed without masking effects caused by non-significant vibration sources and also without filtering out significant data for the cylinder being analysed. That is, different digital filtering operations may be carried out for increments of the entire operating-cycle according to predetermined criteria.

The filtering criteria may also be changed depending on the nature and location of the vibration sensor. For example, if an accelerometer is mounted on the crank case instead of on the cylinder head, different frequencies may need to be filtered out because of the different resonant responses of the crank case to sources of vibration and different transmitted vibration from other resonant surfaces.

The vibration analysis system of the illustrated embodiment includes vibration analysis means 45 for analysing the vibration measures output by the time synchronous analyser 15.

The vibration analysis means 45 includes an event identification means 46, 47 operative to automatically determine the occurrence of an event in the operating cycle as revealed by the vibration measures, in the preferred em bodiment being the average vibrational values.

Events that have proved to be possible to be automatically determined include events relating to operation of valves and related drive train, injection and combustion and piston slap. Perhaps bearing faults and other events may be possible to identify.

The event identification means 46, 47 may be operative to identify events in time within the operating cycle. The event timing determination may firstly include selecting a "window" within the operating cycle within which to carry out analyses for the onset of a particular event. The determination of the location of the window may be predetermined by an operator using for example manufacturers' data providing nominal timing values for events within the operating cycle of the engine.The determination of the window location within the operating cycle may be carried out by an algorithm which commences with the manufacturers' nominal timing value for a particular event (input by an operator or preprogrammed) and then determines a window on either side of that nominal timing value, e.g. plus or minus 10 of crankshaft rotational position from the nominal position.

The second step in the event timing identification operation may cause comparison of the determined vibration measures with a predetermined threshold value. The threshold value for example may be a percentage of the highest peak vibration measure within the window.

Thirdly, the commencement time of the event may be estimated or identified by extrapolation to zero or determining the last zero crossing for the vibration measure before the first threshold crossing peak. That is, the second and third steps of the procedure carried out by the event identification means 46, 47 may comprise finding the highest peak within the window, then the first peak within the window that reaches a predetermined percentage of the magnitude of the highest peak (i.e.

reaches the threshold) and the extrapolating back to zero or counting back to the zero crossing before that first peak that reaches the threshold Alternatively, the determination of the time of commencement of an event within the engine operating cycle in the embodiment utilising extrapoiating to zero may comprise constructing mathematically an envelope or fitted curve for the vibration measures identifiable with the event and then projecting that envelope back to zero to give the estimated starting point of the event. This extrapolating technique may be used for (but is not limited to) the embodiment where the sample analyser 18 comprises event variability means 22.

The determination of the timing of events within the operating cycle can be recorded and displayed for analysis or monitoring of the operation and condition of the engine, e.g.

ffor diagnostic purposes. For example, after setting the correct valve opening timing, if valve closure is in error then this can imply a cam profile error. The drift in timing of valve operations over a period of time provides a measure of wear, e.g. of the valves, valve seats, rockers, cam. Likewise the injector timing as determined directly or indirectly by examination of the timing of the combustion pulse can be used to adjust injection to produce a uniform and optimum power balance between cylinders on multi-cylinder engines. If desired the injector timing for each cylinder may be automatically adjusted to its optimum in response to comparison of the determined time of combustion onset with manufacturers' nominal data.

The vibration analysis means of the illustrated embodiment further includes event magnitude identification means 48, 49. The magnitude identification means 48, 49 is operative to integrate the area under a plot of the time synchronous average vibration measure throughout the operating cycle between the start and end of an event as determined by the associated event identification means 46, 47. In the case where the vibration measure comprises average vibration values, the magnitude identification means 48, 49 is operative to determine the root of the sum of the squares of the vibration value at each sampling point identified with the event to thereby give a measure of the magnitude of that event. The end of each detected event may be determined in any convenient way. For example an allowed end of an event may be determined by presenting allowed durations for events.These allowed durations may be stored in editable tables to which the magnitude identification means 48, 49 refers in use.

The values in the tables may be preset by an engineer commissioning the vibration analysis system and based on manufacturers' data or on sample analysis operations carried out on a machine to be analysed or monitored over a period of time.

The magnitudes of events within the operating cycle provide valuable diagnostic data. For example, the integrated magnitude of valve events at constant timing is a measure of valve integrity (cracking etc.). The integrated magnitude of the combustion pulse is a measure of power per cylinder and together with injector timing can be used to optimise engine operation. In the case of piston slap, large magnitudes of piston slap cause internal scuffing of the bore and external erosion of the liner from cavitation of the coolant. The amplitude of piston slap is highly dependent on piston clearance. Therefore the integrated magnitude of piston slap events relates approximately to the actual diametral clearance which exists at the time of the test and the magnitude can be trended with time to establish deterioration.

The output data from the event timing identification means 46, 47 and event magnitude identification means 48, 49 may be angular timing of each event and integrated power of each event respectively. These two parameters can form the basis of comparisons between cylinders, between engines and for the same cylinder with time.

As mentioned previously, the sample analyser 15 may comprise event variability means 22 for determining the variability of events within the operating cycle. This may be an alternative to or in addition to the sample averager 23 or sample accumulator (not shown).

The event variability means 22 may comprise means for determining the standard deviation of corresponding respective sample vibration values within the plurality of operating cycles so as to thereby provide an indication of the variability of the vibration within each sample throughout the operating cycle. A plot of these values over the duration of the operating cycle will reveal any events that display significant variation in the resulting vibration in the plurality of operating cycles. This may enable identification of some events more readily than examination of synchronously averaged vibrationmagnitudes alone. Therefore the vibration analysis means 45 includes event identification means 46 associated with the event variability means 22 operative in a manner generally analogous to the event identification means 47 operating on the output of the sample averager 23.

It will be appreciated that instead of standard deviation determination, the event variability means 22 may be operative to determine some other measure of variability such as variance or kurtosis.

The vibration output means 20 as previously mentioned may include display or recording means. Display means for example may comprise plotting means. Preferably the apparatus is operative to enable display of a section only of the operating cycle on a relatively large or expanded base line. For example in the case of a vibration trace from 0 to 720" in the case of a four stroke engine, preferably it is possible to display on an expanded scale along the rotational angle axis, selected ranges of angular positions to thereby enable closer visual analysis of vibration data for analysis purposes. Preferably it is also possible to display on an expanded vertical (average vibration value) scale selected sections of the trace for more ready visual analysis.

Referring now to Fig. 2, events in the operating cycle of one cylinder of a four stroke diesel engine (represented by machine angular position in degrees on the horizontal axis) are revealed by clear increases in the average vibration values (vertical axis). For example, the expected or nominal exhaust opening position is shown at A, whereas actual exhaust opening (or possibly piston slap) is shown at B. At C is the vibration resulting from exhaust closing. Inlet close is shown at D. Injection is shown at E followed by combustion at F. The timing and magnitude of these events can be manually or automatically determined for analysis purposes. The expansion of the exhaust valve event shown in Fig. 3 (40004700) more readily enables manual analysis of this event.For example, actual exhaust closing can be readily estimated at point G, while manufacturers' nominal exhaust closure may be at point H (420 ).

Fig. 4 shows an alternative possible vibration measure plot for the same event as shown in Fig. 3, namely time standard deviation enabling manual or automatic event analysis and in particular event timing determination.

It will be seen that the vibration analysis system according to the preferred embodiment of the present invention as herein described and as illustrated in the accompanying drawings enables the identification of events in time within each operating cycle. The timing of events can be compared to manufacturers' data to enable identification of faults. The timing of events can be recorded so that the changing of event timing over a period of time within which the machine is in operation (e.g.

over a period of months or years) also enables fault identification. Simiiarly the magnitudes of events within the engine operating cycle can be determined and recorded for operating condition monitoring, fault identifications and for machine maintenance and replacement programmes to be efficiently carried out. Furthermore it is believed that there is a correlation between cylinder pressure and engine surface vibration and therefore it may be possible to reconstruct internal pressure traces from the vibration signals leading to pressure balancing of the engine cylinders without the need for pressure transducers.

Each one of a number of engines can be analysed or monitored and the condition at different times and changes in condition over a period of time can be determined. For example all data on each cylinder of an engine versus the time or analysis survey number can be recorded and displayed in tabular or graphical form. A particular event or its error on each cylinder versus time/analysis survey number can be displayed. A particular event or its error on any particular cylinder versus other engines in a fleet can be recorded and displayed.

The vibration analysis system of the preferred embodiment of the present invention utilises non-intrusive techniques. That is, tappings to the cylinders for example do not need to be provided for mounting of transducers.

The apparatus gives good definition of small scale faults and errors. Also the technique can result in summarising engine condition with quite small data files, thus lending itself to record keeping on a large number of engines.

It is to be understood that various alterations, modifications and/or additions may be made to the features of the possible and preferred embodiment(s) of the invention as herein described without departing from the spirit and scope of the invention as defined in the appended claims.

Claims (18)

1. A vibration analysis system for analysing operation of a machine having a periodically repeating operating cycle, the machine in use having a vibration sensing means operatively associated with the machine, the vibration sensing means in use providing vibration signals indicative of vibration of components of the machine, the machine in use further having operatively associated therewith a cycle monitor for generating signals related to the machine cycle rate; the analysis system including: (a) a time synchronous analyser for receiving the vibration signals from the vibration sensing means for each of a plurality of operating cycles or part-cycles of the machine and for analysing vibration levels over the plurality of operating cycles or part-cycles, the time synchronous analyser including:: (i) a sampler operative to sample the vibration signals at a plurality of sampling points within each of the operating cycles or partcycles so as to provide a plurality of sample vibration values for each operating cycle or part-cycle; (ii) a time base controller associated with or responsive to the cycle monitor and operative to carry out on the sample vibration values a compensation operation for any variations between the durations of any of the operating cycles or part-cycles whereby each of the compensated sample vibration values for each operating cycle or part-cycle after the compensation operation has been carried out by the time base controller substantially corresponds to the same point in the machine operating cycle or part-cycle as a respective compensated sample vibration value for each other operating cycle or corresponding partcycle; and (iii) a sample analyser operative to provide a vibration measure related to the machine vibration for each of the plurality of sampling points within the machine operating cycle or part-cycle over the plurality of such cycles or part-cycles; (b) vibration output means coupled to the time synchronous analyser for display and/or recording the vibration measure data for the machine.

2. A system as claimed in Claim 1 wherein the sample analyser includes an event variability means for determining the variability of the compensated sample vibration values for each sampling point of the machine operating cycle or part-cycle over a plurality of such cycles or part-cycles.

3. A system as claimed in Claim 2 wherein the event variability means comprises means for determining a statistical measure of variability of respective compensated sample vibration values within the plurality of operating cycles or part-cycles so as to thereby provide an indication of the variability of the machine vibration throughout the operating cycle or part-cycle whereby relatively high values of the statistical measure anywhere within the operating cycle or part-cycle will reveal events that display significant vibration variability throughout the plurality of operating cycles or part-cycles.

4. A system as claimed in any one of Claims 1 to 3 wherein the sample analyser includes a sample accumulator operative to provide a cumulative vibration measure indicative of or related to the total machine vibration for each of the plurality of sampling points over the plurality of machine operating cycles or part-cycies.

5. The system as claimed in any one of Claims 1 to 3 wherein the sample analyser includes a sample averager operative to provide an average vibration measure indicative of or related to the average machine vibration for each of the plurality of sampling points over the plurality of machine operating cycles or part-cycles.

6. A system as claimed in Claim 5 wherein the sample averager is operative to carry out a time synchronous averaging of the compensated sample vibration values for the plurality of operating cycles or part-cycles.

7. A system as claimed in Claim 5 or 6 wherein the sample averager comprises a memory device in which are accumulated respective compensated vibration values for corresponding points in the machine operating cycles for a plurality of cycles or part-cycles, the averager being operative at the end of the predetermined number of operating cycles or part-cycles to divide the total vibration value in the memory by the number of sampled operating cycles or part-cycles to produce a linear arithmetic average vibration value for each sampling point.

8. A system as claimed in any one Claims 1 to 7 wherein the cycle monitor comprises an angular position encoder for continuously generating output signals relating to the instantaneous position of the machine in its operating cycle or part-cycle, the encoder constituting the time base controller and being operative to generate the output signals at the desired increments of cycle position corresponding to the desired vibration value sampling frequency whereby the encoder output signals serve to trigger the sampler directly.

9. A system as claimed in any one of Claims 1 to 7 wherein the cycle monitor comprises a tachometer signal generator for generating a tachometer signal at the same point(s) in each of the plurality of machine operating cycles or part-cycles, the time base controller being operative to process the first operating cycle or part-cycle of the machine as a datum for comparison with subsequent operating cycles or part-cycles being monitored whereby all operating cycles or partcycles to be averaged after the first operating cycle or part-cycle are normalised by the time base controller to the duration of or time base established from the first operating cycle.

10. A system as claimed in any one of Claims 1 to 9 wherein the data output by the time synchronous analyser is passed through a filter stage operative to eliminate some vibration frequencies arising from indeterminate or non-significant causes or to selectively separate several coincident events having different characteristic frequency spectra.

11. A system as claimed in Claim 10 wherein the filter stage is operative to filter at predetermined frequencies solely for predetermined parts of the machine operating cycle or part-cycle whereby enhancing discrimination of events relating to the machine components without masking effects caused by non-significant vibration forces.

12. A system as claimed in any one of Claims 1 to 11 and further including vibration analysis means including an event identification means operative to automatically determine the occurrence of a particular event in the operating cycle or part-cycle as revealed by the vibration measures.

13. A system as claimed in claim 12 wherein the event identification means is operative to identify events in time within the operating cycle or part-cycle.

14. A system as claimed in Claim 13 wherein the event identification means is operative to select a window within the operating cycle within which to conduct analyses for identifying the onset of a particular event, the window location within the operating cycle being carried out by an algorithm which commences with a nominal timing value for the particular event and then determining a range of times before and/or after that nominal timing value.

15. A system as claimed in Claim 14 wherein the event identification means is further operative to compare the determined vibration measures with a predetermined threshold vibration measure and to estimate the commencement time of the event by extrapolation to zero or by determining the last zero crossing for the vibration measure before the first threshold crossing vibration measure.

16. A system as claimed in Claim 14 wherein the event identification means is operative to estimate the commencement time of the event by extrapolation to zero by constructing mathematically an envelope or fitted curve for the vibration measure identifiable within the event and then projecting that envelope back to zero to yield the estimated starting time of the event.

17. A system as claimed in any one of Claims 13 to 16 wherein the event identification means further includes event magnitude identification means operative to integrate the area under a plot of the vibration measures throughout the part of the operating cycle between the start and end of an event as determined by the event identification means.

18. A system substantially as herein before described with particular reference to the accompanying drawings.