GB2208548A - Method of analysis of vibrations by a measuring system with automatic testing of the complete measuring chain for gas turbines - Google Patents

Abstract

The method for supervising the electronic units of a gas turbine, more particularly a gas turbine on an aircraft consists in determining the amplitude and the phase of the unbalance of each module of the gas turbine. To this end, there are provided accelerometers A1-A4 detecting the amplitude of the unbalance and inductive speed sensors 7A, B and 8A, B which are used to measure shaft speed and torque, and hence power. Each inductive sensor has an extended tooth on its toothed disc D to provide an identifiable pulse for each shaft revolution, and thus an indication of shaft angle. The phase of the unbalance output from the accelerometers is determined with respect to this angle. This phase remains constant for a module working properly but becomes unstable if the unit or any sensor becomes defective. <IMAGE>

Description

METHOD OF ANALYSIS OF VIBRATIONS BY A MEASURING SYSTEM WITH AUTOMATIC TESTING OF THE COMPLETE MEASURING CHAIN FOR GAS TURBINES, MORE PARTICULARLY EMBARKED GAS TURBINES AND DEVICE FOR CARRYING OUT THE METHOD BACKGROUND OF THE INVENTION The present invention relates to a method of analysis of vibrations by a measuring system with automatic testing of the complete measuring chain for gas turbines, more particularly embarked gas turbines and to a device for carrying out the method. There exists actually in the domain of gas turbines a tendency to locate the electronic of the propulsion system directly on the propelling, which necessitates a simplification of this electronic. By simplification it is meant here a reduction of the number of electronic components. This leads to an increase of the reliability.Moreover, it is known that in the field of aircrafts each electronic unit which processes important informations has to be continuously supervised for knowing its state of working. This monitoring or "self test" is executed mostly by electronic circuits relatively sophisticated which, as the case may be, may comprise up to 1/4 or 1/3 of the total number of electronic components of the unit. Moreover, in the case of systems for the supervision of vibrations, these circuits are not capable to check the state of working of the sensors, cables or transmission lines and connectors.

It is known from the document Patent Abstracts of Japan, vol.9 (P-333)(1753), no.30, February 8, 1985 & BR< JP 59-173720 (A) (NIPPON KOKAN K.K.) October 1, 1984 a method and a device for the diagnostic of vibrations of low speed turning machines. A vibration sensor delivers a signal which after processing is delivered to a peak detector and to a microprocessor where it is converted to a digital signal. A histogramme is prepared by calculating the number of apparitions of the maximum value within a given interval of time and the period between the maximum values. This permits to diagnose an abnormal behaviour of the machine.

However, such a system does not permit an automatic testing of a complete measuring chain which comprises also the testing of the sensors, cables and connectors of the system.

SUMMARY OF THE INVENTION It is therefore the object of the present invention to simplify the system for the supervision of vibrations and more particularly the automatic test circuit (self test) of the electronic unit, permitting to reduce the number of components of this circuit and to include the supervision of the sensors, cables or transmission lines and connectors. This problem is solved by a method characterized by the following steps: providing at least one module of the gas turbine with sensors and determining from the signals delivered by these sensors the amplitude and the phase of the unbalance of each module, checking the behaviour of said phase for obtaining an indication on the correct or incorrect working of the electronic unit which supervises the vibrations and of the sensors, cables and connectors to which said electronic unit is connected.

The device according to the invention comprises sensors attached to the modules of the gas turbine, said sensors being connected to an electronic unit supervising the vibrations which determines the amplitude and the phase of the unbalance of each module, the behaviour of said phase giving an indication on the state of correct or incorrect working of said electronic unit which supervises the vibrations as well as of the sensors, cables, transmission lines and connectors to which said electronic unit is connected.

The phase of the unbalance has the propriety to remain constant at middle term when the measuring system comprising the electronic unit, the sensors, cables and connectors is in a correct condition of working. On the contrary, every defect in this system, in principle no matter at which level of the latter, renders the phase of the unbalance extremely unstable, leaping or hazardous. The observation of the phase of the unbalance gives therefore an indication easy to be interpreted upon the state of working of the unit to be supervised and of the elements associated to it like sensors, cables and connectors.

Taking into consideration that at least part of the speed sensors are already present for other purposes in the gas turbine, it is easy to understand for the one skilled in the art that the principle based on the measurement of the phase of the unbalance of the modules as mentioned above permits a simplification of the electronic, that is a reduction of the number of components of this electronic.

Moreover, the knowledge of the amplitude of the unbalance determined by the acceleration sensors and of the phase determined by correlation between the signals of the acceleration sensors and the signals of the speed sensors permits a correction of the unbalance when the latter reaches and exceeds a predetermined level. This correction of the unbalance renders the turbine more noiseless and contributes to protect and to increase the life of the bearings or ball bearings of the turbine as well as the various accessories like tanks, gear boxes, etc.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 shows schematically the arrangement of the speed and acceleration sensors in a gas turbine, Figure 2 shows a vibration monitoring unit according to the invention, Figure 3 is a diagram showing the signal corresponding to the unbalance of one module, and Figure 4 is a diagram showing the speed signal with a reference of phase.

DETAILED DESCRIPTION OF THE INVENTION Figure 1 shows schematically the modules 1 to 4 of an airplane reactor, respectively the low pressure compressor 1, the high pressure compressor 2, the high pressure turbine 3 and the low pressure turbine 4. It is visible that the reactor comprises two coaxial shafts 5 and 6, the shaft 5 connecting the modules 1 and 4 together and the shaft 6 the modules 2 and 3 together. As an example, inductive speed sensors 7A, 7B, 8A and 8B are disposed by pairs in the vicinity of the extremities of the shafts 5 and 6. These sensors are located opposite to disks D bearing an indentation and mounted at the extremities of the shafts 5 and 6.Each tooth of the disks passing in front of the associated sensor produces a variation of the magnetic field in this sensor and the output signal of the sensor 7A is illustrated in Figure 4, each sensor delivering a signal similar to the one of Figure 4. These output signals of the sensors are designated in Figure 1 by N1A, N1B for the shaft 5 and by N2A, N2B for the shaft 6.

The pairs of sensors 7A, 7B; 8A, 8B are connected to an information processing unit 9. This unit determines the torque Cn1, Cn2 on each shaft from the phase difference of the signals N1A, N1B et N2A,N2B of each pair of sensors, this phase difference being proportional to the torsion of the shaft. The unit 9 determines also the power Pn1, Pn2 transmitted by each shaft by calculating the product of the torque by the number of turns per unit of time: Pn1 = Cn1 x N1; Pn2 = Cn2 x N2. Figure 1 shows that the unit 9 delivers signals which are representative of the speeds N1, N2, of the torques Cnl, Cn2 and of the powers Pn1, Pn2 as well as of a signal of reference formed of one pulse per turn of each shaft 1/Rev N1, 1/Rev N2 and signals Ln1, Ln2 which are the signals of supervision of the speed sensors. This supervision takes place by comparison of the signals of the pair of sensors from one shaft, these signals having identical amplitude and phase if the sensors are in good working condition.

The invention is not limited to gas turbines with two shafts but it applies also to turbines or reactors; having more than two shafts. The total power is obtained from the partial powers on the shafts. The knowledge of the power on each shaft and the correlation with other parameters like pressure and temperature permits, in case of a drop of the total power or of the propulsive force, to detect easily the shaft which does not deliver the assigned power. From this information and by knowledge of certain parameters like temperature and pressure already normally measured in the turbine, it is possible to locate the defective module and to repair it.It is visible that the principle of supervision according to the present invention permits to prevent a disassembling of the complete reactor as was the case until now, which leads to an important gain of time of the operation of revision and to an important reduction of the time during which the turbine is not working. The invention permits also to greatly improve the efficasiousness of the supervision and of the maintenance of gas turbines, more particularly of the embarked gas turbines. Moreover, the total power calculated by the sum of the partial powers may be set, with other parameters,in relation with the fuel consumption which permits to know the efficiency of the gas turbine. If necessary, the total power may also serve to determine the propulsive force of the reactor of an airplane.

In the present invention, the power is determined by a small number of parameters: the torque and the speed on each shaft of the turbine which are multiplied one by the other. The determination of the power is thus relatively simple in comparison to the usual method utilized up to now which is based on the measurement of the pressure and the temperature. The method according to the present invention requires only a small number of components of the electronic unit which supervises the vibrations. This increases the reliability of this electronic unit.

Figure 2 shows a device for supervising of the vibrations in a gas turbine, more particularly in an embarked gas turbine or a reactor of an airplane. It is to be seen that the device 10 schematically illustrated is coupled in this exemple to four accelerometers Al to A4 arranged respectively in the vicinity of the modules of the turbine. In principle, two accelerometers are necessary. The device 10 receives from the electronic unit 9 the speed signals N1 and N2 et the signals of reference formed of one pulse per turn 1/Rev N1 and 1/Rev N2.

Figure 3 shows the amplitude A of the unbalance of the module 1 as a function of the time (rotation of the shaft 5) as detected by the accelerometer Al. Each accelerometer detects the unbalance of the module to which it is associated and it delivers a signal similar to the one of Figure 3. Figure 4 shows the signal N1A delivered by the speed sensor 7A. Each of the other sensors 7B, 8A and 8B delivers a signal similar to the one of Figure 4. One sees that this signal comprises periods of greater amplitude than the normal periods, these periods of greater amplitude being spaced from each other by an interval of time T corresponding to the duration of one revolution of the shaft. The periods of greater amplitude serve as a reference 1/Rev with respect to which the phase of the unbalance is measured.They are produced in the example described by a tooth longer than the other teeth of the indentation of the disks D associated to the speed sensors 7A, 7B, 8A and 8B. In order to obtain the angular position of the unbalance, it is sufficient to adjust previously the angular position of the disk D such that the longer tooth be in alignment with a particular, well determined position of the turbine, for example in alignment with the vane 1.

The angular position B of the unbalance is then determined with respect to this particular position taken as a reference. The device can thus deliver an indication on the amplitude of the unbalance and of its angular position, this position being the one of the center of gravity as is well known. These informations permit to correct the unbalance if the latter reaches or exceeds a predetermined value. Generally speaking, the correction of the unbalance is executed only on the modules located at the extremities LPT (Low Pressure Turbine) and LPC (Low Pressure Compressor) by means of screws fastened to or removed from a disk of these modules.From the signals received from the accelerometers Al to A4, from the signals of speed N1 and N2 and from the references 1/Rev N1 and 1/Rev N2, the device 10 determines the value of the unbalance of the modules LPC, HPC (High Pressure Compressor), HPT (High Pressure Turbine) and LPT as well as the corresponding phases B LPC, 8 HPC, a HPT and a LPT. The device delivers also a signal Vib. Syst. OK which controls a display indicating that the level of vibration of the system is within acceptable limits. The possibility, in the present invention, to correct the unbalance permits a more noiseless working of the reactor and contributes to reduce the wear e.g. in the bearings of the shafts 5 and 6 of the reactor.Moreover, the visualization of the phases of the four modules permits to know if the system of autosupervision of the information processing electronic units including these units themselves, the speed and acceleration sensors, the transmision line to a display device (not represented) and the connectors along this transmission line is properly working or not Effectively, in the case of a normal working, the phase of the unbalance remains constant at middle term.On the contrary, in the case of an abnormal working, e.g. in presence of poor speed signal due e.g. to an intermittent contact or to a defect of the sensibility of the sensor, in presence of a reference signal of too small amplitude or intermittent or even variable in the time, or in presence of high harmonics of great amplitude masking the unbalance signal, the one or the many phases detected become unstable and behave irregularly, leaping or haardous which indicates a defect of the system. It is to be seen that there exists practi cally no defect of the autosupervision which could not be detected by the visualization of the displayed phase of the unbalance.

The preceding shows that the utilization of the phase of the unbalance as described above for providing an autosupervision of the electronic units for the treatment of the information including the sensors, the transmission line and the connectors is particularly simple and efficatious and that it necessitates only a relatively small number of electronic components which increases its reliability.

Claims (8)

1. A method of analysis of vibrations by a measuring system with automatic testing of the complete measuring chain for gas turbines, more particularly embarked gas turbines comprising shafts and low and high pressure compressor and turbine modules, characterized by the following steps: providing said modules of the gas turbine with sensors for detecting the speed and the acceleration of said modules, said sensors being arranged at the extremity of said shafts, in the vicinity of said modules, utilizing the signals delivered by said sensors for determining by means of an electronic unit for supervising the vibrations, the amplitude and the phase of the unbalance of each module, displaying and checking the behaviour of said phase for obtaining an indication on the proper or improper working of said electronic unit for supervising the vibrations as well as of said sensors, cables and connectors to which said electronic unit is connected.

2. A method according to claim 1, characterized in that the sensors are acceleration and speed sensors detecting respectively for each module the amplitude of the unbalance and the phase of the unbalance with respect to a reference signal of the speed sensor.

3. A device for analysing of vibrations by a measuring system with automatic testing of the complete measuring chain for gas turbines, more particularly embarked gas turbines, said turbines comprising shafts and low and high pressure compressor and turbines modules, characterized in that it comprises sensors for detecting the speed and the acceleration of said modules, said sensors being mounted at the extremity of said shafts, resp. in the vicinity of said modules, said sensors being connected to an electronic unit for supervising the vibrations, said electronic unit determining the amplitude and the phase of the unbalance of each module and displaying said phase, the behaviour of said phase providing an indication of the proper or improper working of said electronic unit for supervising the vibrations as well as of the sensors, cables and connectors to which said electronic is connected.

4. A device according to claim 3, characterized in that said sensors are acceleration and speed sensors detecting respectively for each module the amplitude of the unbalance and the phase of the unbalance with respect to a reference signal delivered by one speed sensor per shaft, said reference signal being formed of one pulse per revolution of said shaft.

5. A device according to claim 3 or 4, characterized in that in the case of a proper working of the electronic unit for supervising the vibrations, the phase of the unbalance remains constant at middle term while any breakdown or defect of said electronic unit- including the sensors, cables or connectors gives rise to a phase which is unstable or leaping or hazardous.

6. A device according to claim 3 provided for determining and correcting the unbalance of rotating modules of gas turbines, more particularly embarked gas turbines, characterized by the determination of the value and the phase of the unbalance of said modules, said phase being determined with respect to a signal of reference corresponding to a well determined angular position relatively to the module to be corrected, said signal of reference being delivered by the corresponding speed sensor.

7. A method of analysis of vibrations by a measuring system with automatic testing of thecompletemeasuring chain for gas turbines, substantially as herein described.

8. A device for analysing of vibrations by a measuring system with autanatic testing of the complete measuring chain for gas turbines, substantially as herein described with reference to the accompanying drawings.