Abstract:
A device and a method for monitoring the operational state of rotor blades of a turbine from a position facing the blade tip of a passing rotor blade: A wavesource generates an electromagnetic wave and a wave guide directs the wave towards the blade tip. The waveguide is included in a resonator defining at least one discrete resonance frequency of the wave on the waveguide. An aperture at its front end emits an energy fraction of the wave toward the blade tip. The frequency of the wave is adjusted to the resonance frequency of the resonator at the momentary operation conditions, and a measuring unit compares at least one measurement parameter of the wave directed towards the blade tip with the corresponding measurement parameter of the wave reflected from the blade tip.

Description:
FIELD OF THE INVENTION 
     The invention relates to a device for monitoring the operational state of rotor blades of a turbine from a position facing the blade tip of a passing rotor blade, the device comprising a wavesource for generating an electromagnetic wave and a waveguide for directing the wave towards the blade tip. The invention further relates to a turbine comprising such a monitoring device and to a monitoring method of a generic kind. 
     BACKGROUND OF THE INVENTION 
     Driven by ever-increasing requirements for improved fuel efficiency, reduced undesirable emissions and reduced noise, future aero-engines and land-based turbines will have to incorporate new systems to monitor the turbine conditions, analyse the incoming data and modify operating parameters to optimise operations and thus achieve improved performance. Sensing technology is the foundation upon which such systems are based. New sensors that are able to operate under the harsh environment present in a turbine must be developed to enable the measurement of previously unmeasurable parameters critical for monitoring the overall health of the turbine. The potential benefits from those systems are significant and may be categorised into two primary areas: turbine efficiency and turbine maintenance. For both purposes, the monitoring of the operational state of the turbine&#39;s rotor blades is an essential requirement, in particular for a detailed understanding of the functioning and health of a turbine. 
     In order to perform an accurate measurement of the operational state of the rotor blades, a microwave sensor is typically mounted through a hole or attached to the inside of the engine case to enable the microwave sensor to cast its beam onto the blades, which will be rotating and subsequently passing by the sensor during engine operation. Yet the environment encountered in turbine engines is harsh with gas path temperatures exceeding 1300 K in high-pressure turbines and temperatures around 900 K in the rear stages of a high-pressure compressor of aero-engines, most often with a high thermal gradient as well. It is a dirty environment with oil, combustion by-products and other contaminants. A sensor being able to operate reliably at those extreme temperatures and in such a harsh environment is therefore a key component. 
     A sensor to solve this problem has been addressed in patent application No. US 2010/0066387 A1 which discloses a device for determining the distance between a rotor blade and a wall of a gas turbine surrounding the rotor blade. The device comprises a waveguide that guides electromagnetic waves with at least two frequencies. Waves with one of the frequencies are emitted from the sensor and reflected back by the rotor blade and waves with the other frequency are reflected by a sealing element at the end of the waveguide. The distance of the device with respect to the rotor blade is then determined by comparing phases of the waves. 
     A disadvantage of this sensor is that a temperature dependent expansion of the turbine walls, in which the device is mounted, also causes a corresponding expansion of the waveguide, making the desired distance determination inaccurate. The problem may be circumvented by subtracting the phase comparison values of the waves of the two frequencies from each other and assigning this value to a previously measured value for the distance under those temperature conditions, e.g. on the basis of a value table. Nevertheless, a more direct way of accounting for the temperature changes would be highly desirable in order to make sure that the momentary operating conditions of the turbine are met, which generally do not solely dependent on the temperature gradient. Apart from the reliability of the measurement, it would also be desirable to improve the detection performance, in particular to increase the measurement resolution, and to extend a monitoring of the rotor blades beyond the detection of the single parameter of a distance measurement in between the blade tips and the sensor. 
     OBJECTS AND SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to avoid at least one of the above mentioned disadvantages and to provide an improved monitoring device and method, in which the harsh operation conditions within a turbine environment are accounted for. 
     Relating to the monitoring device, the invention suggests that the waveguide is included in a resonator defining at least one discrete resonance frequency of the wave on the waveguide and comprising an aperture at its front end through which an energy fraction of the wave can be emitted towards the blade tip. Thus, the resonator is provided in such a way that it supports at least one discrete resonance frequency of the wave on the waveguide. In this way, the resonator can be used to store inside electromagnetic energy of the wave at the resonance frequency. Due to the aperture at the front end, a fraction of the electromagnetic energy stored inside the resonator can be emanated from the resonator to interact with the rotor blades. Preferably, the back end of the resonator is short-circuited. 
     The resonance frequency of the wave can depend on the momentary operation conditions of the turbine and the resonator is preferably configured to support the resonance frequency shifting within a range of changing operation conditions. The device further comprises a frequency adjusting unit that is configured to set the frequency of the wave to the resonance frequency of the resonator at the momentary operation conditions. Thus, the monitoring device can be matched to momentary changes of operation conditions within the turbine environment by an adjustment of the wave frequency with respect to occurring changes of the resonance frequency of the resonator. Those operation conditions of the turbine environment affecting the resonance frequency of the resonator may comprise a temperature gradient of the propagation medium of the electromagnetic wave. 
     By further effectuating a comparison of at least one measurement parameter in between the outgoing and reflected wave, reliable informations about the operational state of a rotor blade can be deduced irrespective of the momentary operation conditions within the turbine environment. Accordingly, the device also comprises a measuring unit that is configured to compare at least one measurement parameter of the wave emitted towards the blade tip with the corresponding measurement parameter of the wave reflected from the blade tip. In this way, accurate measurements of the operational state of a rotor blade can be obtained even if the resonance frequency of the resonator shifts with temperature or due to other disturbing effects. Preferably, the measurement parameter comprises at least one of the phase and the amplitude of the emitted and reflected wave. More preferred, the emitted and reflected wave are compared with each other with respect to both of these measurement parameters. 
     According to the invention, these advantages can also be achieved in a method in which the wave is generated inside a resonator defining at least one discrete resonance frequency of the wave. The method comprises the step of setting the frequency of the wave to a resonance frequency of the resonator at the momentary operation conditions and of emitting an energy fraction of the wave from the resonator towards the blade tip. The method further comprises the step of comparing at least one measurement parameter of the wave emitted towards the blade tip with the corresponding measurement parameter of the wave reflected from the blade tip. Thus, the comparison of the measurement parameter can be advantageously related to the operational state of a rotor blade irrespective of the momentary operation conditions within the turbine environment, such as a temperature gradient. 
     Preferably, the comparison of a measurement parameter of the emitted and reflected wave is used to obtain information with respect to the relative position and/or relative motion of one or more rotor blades. In a first preferred implementation of the method, the radial gap in between the resonator and the blade tip is deduced from this comparison. Accordingly, the measuring unit of the monitoring device is preferably configured with logic to relate the comparison of the measurement parameter to the radial gap in between the resonator and the blade tip. In this way, clearance measurements between the tips of the rotor blades and the stationary case of the turbine may be conducted. Preferably, the phases of the emitted and reflected wave are used as a measurement parameter to determine the radial gap in between the resonator and the blade tip. 
     In particular, the clearance between the tips of rotor blades and the stationary engine case can be used as a critical parameter with respect to the efficiency of the turbine. Thus, tip clearance measurements may be primarily focused on improving turbine efficiency through leakage reduction that may be achieved by a reduction in clearance. More specifically, for a given operating state of the turbine the tip clearance should be preferably reduced as much as possible whilst avoiding blade rubbing. 
     Accurate tip clearance measurement may offer three major advantages:
         it may be used to enable closed loop active clearance control on a turbine, which allows reducing the clearance between the blade tips and the case as much as possible. As a consequence the turbine efficiency can be improved, the specific fuel consumption reduced and undesirable emissions such as NOx and CO 2  limited to a minimal value;   it may be used to help to prolong a turbine lifecycle by preventing a blade from rubbing which may result from a case distortion, rotor dynamics/shaft bending or environmental factors, by preventing blade cycling damage and by increasing hot gas path component life through reduced EGT (exhaust gas temperature); and   it may be used to provide dimensional measurement from each blade, which may allow prognostics and optimised condition-based maintenance in the turbine hot section.       

     Furthermore, in axial compressors, tip clearance measurements can be used as a key to compressor stability and stall margin. 
     Turbine locations on which blade tip clearance measurements are preferably applied include a high-pressure turbine and/or a high-pressure compressor, which are expected to bring the most benefits. 
     In a second preferred implementation of the method, the comparison of a measurement parameter of the emitted and reflected wave is used to determine a time difference during which a subsequent passage of one rotor blade or of two different rotor blades occurs. Accordingly, the measuring unit of the monitoring device is preferably configured with logic to relate the comparison of the measurement parameter to a time difference during which a subsequent passage of one rotor blade, in particular after one or more revolutions, or of two different rotor blades, in particular of two neighbouring or two diametrically opposing rotor blades, occurs. In this way, time of arrival measurements on a rotor blade may be conducted. Preferably, the amplitudes of the emitted and reflected wave are used as a measurement parameter to determine the time difference during which a subsequent passage of one rotor blade or of two different rotor blades occurs. 
     Such a time-of-arrival (also called tip timing) measurement can be primarily focused on the mechanical integrity of rotating blades by providing a measure of blade vibration. The measurement preferably consists of detecting the time at which a point on a rotating blade tip passes a stationary point (hence the term “time-of-arrival”). In the absence of blade structural vibration, the time-of-arrival normally depends only on the rotational speed of the blade. However, when a blade structural vibration occurs, the time-of-arrival also depends on the amplitude, frequency and phase of vibration. 
     Accurate time-of-arrival measurement may offer the following advantages:
         by measuring the blade time-of-arrival, any anomalous signal predicting advanced blade or disk failures can be detected and damages can be avoided through prompt reaction. Time-of-arrival can therefore be used as a key measurement for predicting component life and enables thus condition-based maintenance;   it can be used to enable safety monitoring during early testing of new turbine engine designs and can help to validate these new designs during development engine testing; and   it can be used to enable the measurement of asynchronous vibrations for blade flutter, rotating stall and compressor surge detection.       

     Turbine locations on which time of arrival measurements are preferably applied include the early stages of a low-pressure compressor up to the rear stages of a high-pressure compressor, which are expected to bring the most benefits. 
     In general, land-based turbines, aero-derivatives, aero-ground tests and aero-engines (non-exhaustive list) can greatly benefit from accurate tip clearance and time-of-arrival measurements. Therefore, according to a third preferred implementation of the method, the radial gap in between the resonator and the blade tip and the time difference in between a subsequent passage of rotor blades are deduced from the comparison of the measurement parameter. Accordingly, the measuring unit of the monitoring device is preferably configured with logic to relate the comparison of at least one measurement parameter of the emitted and reflected wave to the radial gap in between the resonator and the blade tip and the time difference in between a subsequent passage of rotor blades. Preferably, at least two measurement parameters, in particular the phases and the amplitudes of the emitted and reflected wave, are used as a measurement parameter to determine both tip clearance and time-of-arrival of the monitored rotor blade independently from each other. 
     The following preferred aspects of the invention may be advantageously implemented in the monitoring method and/or the monitoring device. 
     Preferably, the wave emitted through the aperture is characterized by at least two field components: First, a reactive near field can be provided by the emitted wave, in which electromagnetic energy is stored in the absence of a receiving object and which transfers energy in the presence of a receiving object. Preferably, the near field component is measured within a distance from the aperture smaller than the emission wavelength, more preferred within a distance of at most three quarters of the emission wavelength, and most preferred within a distance of at most one half of the emission wavelength. Secondly, a propagating field can be provided by the emitted wave, in particular at significantly larger distances from the aperture as compared to the emission wavelength, which is radiating energy away from the resonator regardless of the presence of a receiving object. 
     Preferably, the resonator is only resonant for a number of discrete resonance frequencies. Thus, no continuum of resonance frequencies is supported by the resonator. Preferably, the design and geometry of the resonator is chosen to obtain the resonance at a desired operation frequency of the wave, preferably within a frequency range from 1 to 100 GHz. Moreover, the resonator is preferably configured to obtain a resonance for a dominant mode of the wave, such as the TE11-mode or the TEM-mode. 
     Preferably, the measurement parameter comprises the phase of the emitted and reflected wave. Thus, a phase comparison which is obtained by the measuring unit when the resonator is at resonance may be used to determine the operational state of a respective rotor blade. More preferred, the phase comparison is carried out by determining the phase of the reflection coefficient of the resonator, i.e. the phase component of the ratio between the reflected wave and the transmitted wave. Thus, a measured modification of the reflection coefficient can indicate the presence of a rotor blade and can further be used to determine its operational state. In particular, the quality factor and/or the input match of the emanated and reflected waves may be extracted from the phase comparison. 
     Alternatively or supplementary, the measurement parameter preferably comprises the amplitude of the outgoing and reflected wave. Thus, a comparison of the amplitudes which is obtained by the measuring unit when the resonator is at resonance may be used to determine the operational state of a respective rotor blade. For instance, the amplitude of the wave may be measured as a power distribution of the wave, in particular a gaussian distribution, wherein the maximum peak and/or integral and/or width of the power distribution may be used to compare the power distributions of the emitted and reflected wave. 
     According to a first preferred implementation, the comparison of the measurement parameter of the emitted and reflected wave relies at least on the reactive near field of the resonator that is interacting with the rotor blades. In this way, a large transverse resolution of the monitored object can be obtained. According to a second preferred implementation, the comparison of the measurement parameter of the emitted and reflected wave relies at least on the propagating field of the resonator interacting with the rotor blades, providing a large longitudinal range for distance sensing. According to a most preferred implementation, the comparison of the measurement parameter of the emitted and reflected wave relies on the interaction of both field components with the rotor blades in order to combine both advantages. 
     Preferably, the frequency adjusting unit is configured to vary the wave frequency within at least one predetermined frequency range and to determine the resonance frequency on the basis of the frequency variation. Preferably, the frequency variation is performed to determine and/or adjust at least one of the following parameters according to the momentary operation conditions of the turbine:
         the resonance frequency of the resonator, in particular its center frequency;   the phase length on a feeding transmission line connected to the resonator, in particular a coaxial cable connecting the resonator to a wavesource.       

     In this way, the frequency can be readjusted to allow an accurate blade measurement, e.g. to account for a phase shift of the center frequency of the resonator due to a change of the dielectric constant under temperature. Most preferred, both parameters are determined within one frequency variation. The frequency setting carried out by the frequency adjusting unit may be regarded as a calibration procedure which allows to establish accurate measurement conditions for the blade within a subsequent comparison of the measurement parameter of the measuring unit. 
     According to a preferred implementation, the change of the resonance frequency of the resonator and/or of the phase length of the feeding transmission line determined during the frequency variation is employed as a reference phase. Preferably, the reference phase is subsequently used by the measuring unit as a measurement parameter of the emitted wave to be compared with the wave reflected from the blade tip. 
     According to a preferred configuration, the frequency adjusting unit is configured to set the resonance frequency of the resonator each time before and/or after an actual measurement of the operational state of a rotor blade is carried out. For instance, at least one frequency variation can be performed before and/or after a measurement of the measurement parameter. In this way, a highly reliable blade monitoring can be ensured. To further increase the measurement accuracy, repeated frequency variations may be applied before the measurement of the measurement parameter. This may further contribute to a reduction of the measurement noise, e.g. by means of averaging the frequency values that have been determined after each frequency variation. 
     According to another preferred configuration, the frequency adjusting unit is configured to set the resonance frequency of the resonator each time after a predetermined number of measurements of the rotor blade&#39;s operational state or after a predetermined time interval. The latter configurations may be in particular applicable in turbine environments in which only small or exceptional temperature gradients are expected. 
     Preferably, the resonator is a microwave resonator. Correspondingly, the frequency range in which the resonance frequency of the resonator is set by the frequency adjusting unit lies preferably within the microwave frequency range. The wavesource is preferably constituted by a microwave source. 
     Preferably, at least one detector that is adapted for a signal detection of the wave in the waveguide and/or emitted from the waveguide and/or reflected from the rotor blade is operatively connected with the frequency adjusting unit and/or the measuring unit. Preferably, the detector comprises at least two detection units, in particular mixers, that are offset in phase in order to allow a phase measurement for the calibration procedure of the frequency adjusting unit and/or for the comparing procedure of at least one measurement parameter of the measuring unit. Preferably, the detector is configured to detect the signal of the wave that is emitted towards the blade tip and the signal of the reflected wave. Suitable algorithms for evaluating the detected signal within the calibration process of the frequency adjusting unit and/or within the comparison of at least one measurement parameter of the measuring unit are known in the art. 
     The monitoring device according to the invention based on a resonator can provide the advantage of much smaller dimensions as compared to conventional devices. Thus, the applicability of the monitoring device also in smaller spaced turbine environments can be greatly improved. The small dimension of the present device based on a resonator has the further advantage of yielding a smaller spot-size and consequently a larger measurement resolution. Moreover, the exploitation of the resonator&#39;s reactive near field rather than the propagating field of a patch antenna allows to further increase the measurement precision. 
     The frequency adjusting unit and the measuring unit may be realized as a single logical component, program code or the like, or as separate units. 
     According to a first preferred configuration, the side walls of the resonator are formed by a hollow tube constituting said waveguide. The hollow tube may have a circular, rectangular or other section allowing the guiding of waves. Such a resonator is subsequently referred to as a cavity resonator. It offers the advantage of smaller dimensions and of being less sensitive to the axial position relative to the rotor blade as compared to current measurement devices. Dielectric filling materials can be used to further reduce the size of the cavity resonator. Thus, a particularly preferred application area of the cavity resonator lies in the tip clearance measurement of small turbines, in particular to accommodate a small mounting hole and smaller/thinner blade dimensions. 
     According to a second preferred configuration, the waveguide is constituted by a central conductor extending coaxially through the resonator. Such a resonator is subsequently referred to as a coaxial resonator. It offers the advantage that a further miniturisation with respect to the cavity resonator is possible, such that the detection spot size can be even further reduced. Furthermore, an excellent waveform with a sharp rise time can be obtained. In particular, the sharper waveform provides the benefit of an improved time resolution. Dielectric filling materials can be used to further reduce the size of the coaxial resonator. Thus, a particularly preferred application area of the coaxial resonator lies in time of arrival measurements requiring a very small spot size and a sharp rise time of the waveform in order to achieve a high spatial resolution. 
     Preferably, in both configurations of a cavity resonator and a coaxial resonator the monitoring of the rotor blades is based upon a combination of reactive near field and propagating field interaction with the rotor blades. In particular, the contribution of the reactive near field can be advantageously exploited to achieve a high spatial resolution of the measurement as compared the current monitoring devices. 
     Preferably, the aperture at the front end of the resonator is covered by a protective cap. The protective cap is preferably used to prevent a contamination of the cavity of the resonator. More preferred, the protective cap consists of a dielectric material for high temperature applications. Preferably, the electromagnetic wave is generated in the resonator at the opposite side of the protective cap. 
     Preferably, the cavity of the resonator is filled with a dielectric material. In this way, the size of the resonator can be further reduced. Preferably, the length of the resonator is chosen in such way that the resonator is resonant for at most three quarter of the wavelength corresponding to the excitation frequency, more preferred for at most half of this wavelength, and most preferred for at most a quarter of this wavelength. 
     In order to ensure the functionality of the monitoring device in a turbine exhibiting a certain temperature gradient, the resonator is preferably configured to be resonant for at least one frequency of the varied frequency range for all temperatures within an intended operation temperature range. Preferably, the operation temperature range comprises a temperature value of at least 600 K, more preferred at least 900 K, and most preferred at least 1300 K. Preferably, the operation temperature range also comprises low temperature values of below 230 K. In this way, the requirements of specific turbine environments, such as the rear stages of a high-pressure compressor of aero-engines or high-pressure turbines, can be accounted for. 
     To further enable the monitoring device to operate within the harsh conditions of specific turbine environments, the resonator preferably consists of a material that is resistant to a temperature of at least 900 K, more preferred at least 1300 K. Preferably, the material of the resonator comprises at least one high temperature resistant metal or metal alloy. 
     In order to generate the wave directly in the waveguide, the wavesource is preferably connected to an excitation probe extending into the cavity of the resonator. The excitation probe can be shaped as a pin, an open loop or a closed loop, wherein also other shapes are conceivable which allow an excitation of the resonator for the desired frequency range. In many applications, loops may be advantageous in that they offer superior excitation properties. 
     A suitable transmission line for connecting the excitation probe with the wavesource is preferably constituted by the inner conductor of a coaxial cable. More preferred, the excitation probe is constituted by an end portion of the inner conductor that is protruding from the coaxial cable. Preferably, a high temperature coaxial cable is used. More preferred, the coaxial cable is bond to the outer surface of the resonator and the protruding end portion extends via a through hole inside the cavity. The joint in between the coaxial cable and the resonator is preferably realized using a braze, TIG welding, laser welding, or any other metal-to-metal joining technique. 
     According to a first preferred configuration, the excitation probe is connected with the wavesource through a hole in the back end of the resonator. According to a second preferred configuration, the excitation probe is connected with the wavesource through a hole in a lateral side wall of the resonator, more preferred at an end portion of the side wall that is close to the back end. Both configurations may be advantageous with respect to the mounting of the resonator in a turbine depending on the respective application area. 
     The present invention also relates to a turbine, in which the above described monitoring device is mounted. Preferably, the monitoring device is mounted in such a way, that measurements based on the reactive near field interaction with the rotor blades can be accomplished. Thus, the resonator is preferably disposed in such a way that the radial gap in between the front end and the passing blade tip is at most three quarters, more preferred at most one half, of the wavelength which corresponds to the wave frequency within the turbine propagation medium for all temperatures within the intended operation temperature range. Preferably, the temperature range comprises a temperature value of at least 600 K, more preferred at least 900 K, and most preferred at least 1300 K. Preferably, the temperature range also comprises low temperature values of below 230 K. Preferably, the monitoring device is mounted in the housing of the turbine. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be described in more detail in the following description of preferred exemplary embodiments with reference to the accompanying drawings. In the drawings: 
         FIG. 1  is a schematic view of a turbine comprising a device for monitoring the operational state of the rotor blades; 
         FIG. 2  is a more detailed schematic view of the monitoring device shown in  FIG. 1 ; 
         FIG. 3  is an exploded view of an end launch cavity resonator that can be applied in the monitoring device shown in  FIG. 2 ; 
         FIG. 4  is a sectional view of the resonator shown in  FIG. 3 ; 
         FIG. 5  is an exploded view of a side launch cavity resonator that can be applied in the monitoring device shown in  FIG. 2 ; 
         FIG. 6  is a sectional view of the resonator shown in  FIG. 5 ; 
         FIG. 7  is an exploded view of a coaxial resonator that can be applied in the monitoring device shown in  FIG. 2 ; 
         FIG. 8   a, b  are sectional views of the resonator shown in  FIG. 7 ; 
         FIG. 9   a - d  are schematic illustrations of the resonators shown in  FIG. 3 to 8  with various embodiments of the excitation probes; and 
         FIG. 10   a - f  are schematic illustrations of the back end of the resonators shown in  FIG. 3 to 8  with various embodiments of the excitation probes. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The turbine  1  depicted in  FIG. 1  comprises a rotor shaft  2  on which rotor blades  3  are fixed in a flow channel  4 . The flow channel  4  is circumferentially bordered by a housing  5  surrounding the respective tip  6  of each rotor blade  3 . A resonator of a device  7  for monitoring the operational state of the rotor blades  3  is mounted in the housing  5 . In order to improve the reliability of the measurement in certain turbine environments, the resonator is preferably set back within the housing  5  in such a way, that its front end  8  has a larger spacing R from the respective tip  6  of a passing rotor blade  3  as compared to the adjoining inner wall of the housing  5 . In other turbine environments, the resonator may also have the same spacing R from the tip  6  of a passing rotor blade  3  as compared to the inner wall of housing  5  or extend into the housing. 
     The device  7  is employed to measure the distance R in between the tip  6  of a passing rotor blade  3  during the operation of turbine  1 , i.e. to determine the clearance of a passing tip  6 . A deviation from a standard value indicates that the turbine efficiency is suboptimal and that a leakage flow may occur. The device  7  is further employed to measure the time difference T in between a subsequent passage of two rotor blades  3  and/or the time difference T in between a subsequent passage of one particular rotor blade  3  after one or several revolutions. A deviation from a standard value, in particular with respect to the rotational speed of rotor shaft  2 , indicates that vibrations on a rotor blade  3  do occur. If the detected vibrations are larger than a predicted value, a damage of this rotor blade  3  must be suspected. 
       FIG. 2  schematically illustrates the assembly and functionality of the monitoring device  7 . The device  7  comprises a resonator  10  with an open circuited front end  8  and a short-circuited back end wall  9 . The lateral sides of resonator  10  are constituted by a hollow tube  11  forming a waveguide. The length of waveguide  11  is chosen to be optimized for a desired resonance frequency of the resonator  10 . The aperture of the open front end  8  is covered by a protective cap  12 . 
     Thus, the resonator  10  is based on a piece of waveguide  11 , circular, rectangular or other, that is operating in its fundamental propagating mode (TE11 for a circular waveguide for example). It is terminated on one end  9  by a short circuit and on the other end  8  by an open circuit, being thus resonant for a length that is for instance roughly equal to a quarter wavelength at the operating frequency. 
     An excitation probe  14  extends from the back end wall  9  of resonator  10  into its cavity  13 . A resonator  10  according to this specific embodiment is subsequently referred to as an end-launch cavity resonator. The excitation probe  14  is connected with a wavesource  15  via a feeding transmission line  29  which is constituted by a coaxial cable. The back end wall  9  is provided with a joint  21  comprising a through hole through which the central conductor  16  of the coaxial cable  29  passes. In this way, the cavity  13  can be excited by the excitation probe  14 . Thus, an electromagnetic wave can be generated in the waveguide  11  at the resonance frequency of resonator  10 . The wavesource  15  is typically working at a microwave frequency and the resonator  10  and excitation probe  14  are optimized for this frequency range. 
     At resonance, the electromagnetic signal coming from the transmission line  16  is coupled into the cavity  13 . The microwave resonator  10  then emits a fraction of the electromagnetic energy at its open end  8  and produces both propagating (radiating) and reactive fields, which are different behaviour characteristics of the electromagnetic field emanating from the source  10 . Propagating fields radiate energy away from the source regardless of whether there is a receiving circuit. On the other hand, reactive fields store energy in the absence of a receiving circuit whereas in the presence of a receiving circuit, reactive fields transfer energy. At the open-ended side  8  of the cavity  13 , reactive near field and propagating field components thus exist, and interact with any target passing in front of the sensor  10 . The reactive near field component provides transverse resolution, and the propagating field component provides longitudinal range for the distance sensing. 
     Preferably, the resonator  10  is arranged in the housing  5  of turbine  1  in such a way, that the radial gap R in between the front end  8  and the blade tips  6  lies within a preferred measurement range RND for the reactive near field of resonator  10 . The distance RND is preferably defined as: RND&lt;λ/2, wherein λ is the emission wavelength of the resonator  10 . This advantageously allows to exploit the combined field ranges of the reactive near field and the propagating field for the monitoring of the rotor blades  3 . In principle, an operation within the reactive near field range up to one half of the emission wavelength is preferred, in order to avoid ambiguities of measuring multiple wavelengths. However, also an operation in the far field range is conceivable. 
     Preferably, the monitoring at the position of the blade tips  6  mostly relies on the reactive near field component yielding the advantage of an increased transverse resolution of the measurement. The relation of reactive near field to propagating field at the open-ended side  8  of the resonator  10  depends on the relation of the transverse dimension of the resonator  10  to the wavelength: the smaller the electrical dimension, the higher the proportion of reactive near field is. This fact provides guidance during the design phase based upon application requirements. 
     The transverse dimension of the cavity resonator  10  is selected in such a way that for the operating frequency (for example 24 GHz), the dominant mode of the corresponding waveguide  11  can be propagated but not the first higher order mode. This means that the section of the cavity resonator  10  can not be smaller than a certain dimension, linked to the cutoff frequency of the dominant mode for the given dimension. Filling the cavity resonator  10  with a dielectric material allows further miniaturisation of its transverse dimension. The longitudinal dimension of the cavity resonator  10  is also given by the operating frequency. It is chosen in such a way to obtain a resonance at the operating frequency, and could for instance correspond to a quarter guide wavelength. 
     The resonance frequency of the resonator  10  is therefore defined by its length, inner diameter and the dielectric permittivity of the protective cap and, if applicable, of the filling dielectric material of the resonator. The resonance frequency thus depends on the temperature, due to the permittivity variations and dimension variations induced by a change in temperature. The position, dimension and shape of the excitation probe  14  define the coupling of the microwave signal into the resonator  10 . These features should be adjusted in order to obtain a critical coupling of the resonator  10  to the central conductor  16  of coaxial cable  29 . 
     The monitoring device  7  further comprises a signal processing unit  17  that is functional to prepare and conduct the signal processing for the monitoring process of the rotor blades  3 . For this purpose, the signal processing unit  17  comprises a frequency adjusting unit  18  and a measuring unit  19 . Both, the frequency adjusting unit  18  and the measuring unit  19  are operatively connected with at least detector  20  disposed at the end of transmission line  20  that is opposed to resonator  10 . Furthermore, the frequency adjusting unit  18  is operatively connected with the wavesource  15 . 
     The frequency adjusting unit  18  is functional to set the excitation frequency of excitation probe  14  to the resonance frequency of resonator  10  according to its momentary operation conditions, which are in particular influenced by its momentary operation temperature, as described above. For this purpose, the frequency adjusting unit  18  is configured to sweep the frequency of wavesource  15  within at least one predetermined frequency band. By varying the frequency of the signal transmitted from wavesource  15 , the frequency adjusting unit  18  changes the total amount of phase length between the wavesource  15  and the resonator  10  in order to accurately locate the resonance (i.e. the center frequency of the resonance of the resonator  10 ) and to measure changes in the phase length of the feeding transmission line  16 . 
     Both parameters can change under temperature due to the change in dielectric constant when the materials in the transmission line  16  or resonator  10  heat up. Therefore, the resonator  10  is prone to change its center frequency with temperature. According to one embodiment of the invention, the phase measurement conducted in the frequency sweep can be employed as a reference phase used by the measuring unit  19  to compare with the phase of the wave reflected from the blade tip  6 . 
     In the case of a negligible temperature change in the environment, this reference phase does not change. If the transmission line  16  and resonator  10  are not changing temperature or not changing very fast, then the frequency sweep can be done on a periodic basis (e.g. minutes to hours). Preferably, frequency sweeps for measurements in a harsh environment are repeated at a much faster rate, for instance in the range of once per second (for tip clearance measurements) or continuously (for time of arrival measurements). 
     The measuring unit  19  is functional to compare at least one measurement parameter of the wave emitted towards the blade tip with the corresponding measurement parameter of the wave reflected from the blade tip. The measurement parameter comprises the phase and/or the amplitude of the wave in order to perform tip-clearance and/or time-of-arrival measurements. In this way, the difference of the measurement parameters of the microwave signal reflected by the resonator at resonance and out of resonance can be used to determine the distance and/or nature, for example the shape, of the rotor blade  3  passing in front of the resonator  10 . 
     In particular, the phase of the reflection coefficient of the resonator  10  and/or the difference of the amplitude between the outgoing and reflected wave is determined by measuring unit  19 . The measurement unit  19  is enabled to perform this measurement in a reliable manner since the resonance frequency has been obtained and adjusted beforehand by the frequency adjusting unit  18 . This allows to take into account the changes in dimension and dielectric permittivity due to large thermal gradients present in the harsh environment of turbines, and allows performing accurate measurements even if the resonance frequency of the resonator  10  shifts with temperature. Thus, the modification of the phase of the reflection coefficient of the resonator  10 , in particular due to reactive near field interaction and/or propagating field interaction with the target  3 , and/or the difference of the amplitude between the outgoing and reflected wave provides a significant monitoring parameter of the operational state of the rotor blades  3 . 
     The comparison of the measurement parameter carried out by measuring unit  19  is based on the signal detected by detector  20 . For the purpose of a phase detection, detector  20  comprises at least two detection units, in particular mixers, that are offset in phase. The phase offset preferably corresponds to 90 degrees. In this way, an in-phase signal and a quadrature signal can be obtained. The phase component of the vector corresponding to these two signals can be derived according to algorithms known in the art. Note that a varying DC component, causing an unwanted shift of this vector by a constant value, can be effectively avoided by the frequency adjustment performed by the frequency adjustment unit  18 . Accordingly, the detector  20  further allows to determine the amplitude of the wave. 
       FIGS. 3 and 4  show a resonator  25  according to a first alternative embodiment that can be used in the place of resonator  10  in the device shown in  FIG. 2 . This resonator is also referred to as an end-launch cavity resonator  25 . Identical constituent parts with respect to the previously described resonator  10  are labelled with the same reference numerals. Further depicted in  FIGS. 3 and 4  is the coaxial cable  29 , in which the feeding transmission line  16  is implemented, surrounded by a dielectrics  22 . An excitation probe  28  is constituted by an end portion of this transmission line  16  projecting from the coaxial cable  29  and its dielectric filling  22  into the cavity  13 . The excitation probe  28  exhibits a curved shape. 
     The cavity  13  of resonator  25  is filled by a dielectric material  27  which allows to reduce its transverse dimension by maintaining a desired value or desired value range of its resonance frequency. The back end of resonator  25  is provided with an annular flange  26  which allows a simplified mounting of the resonator  25  in the housing  5  of turbine  1 . 
     Thus, the cavity resonator sensor according to this embodiment is composed of a circular waveguide-based cavity resonator body  25 , a feeding coaxial cable  29 , with its inner conductor  16  acting as the excitation probe  28 , a cavity filling dielectric material  27  and a protective cap  12 . The feeding coaxial cable  29 , including the dielectric material  22  and inner conductor  16 , has the capability to transmit a microwave signal at very high temperature. The length of the resonator body  25  is equivalent to a portion of the guided wavelength at the operating frequency. The section of the resonator body  25  is circular, but it could also be rectangular or exhibit any other adequate shape. It is designed in such a way that the dominant mode of the waveguide  11  propagates at the operating frequency but not the first higher order mode. Preferably, dielectrics  27  other than air are used to further reduce the size of the cavity. 
     The cavity resonator body  25  is made out of a high-temperature material. Preferably, the resonator comprises a high temperature resistant metal or metal alloy. Alternatively, any material that withstands the installation and environmental requirements could be used. The protective cap  12  is made of a dielectric material for high-temperature applications and is mounted at the front of the cavity  13  of resonator body  25 . Moreover, the protective cap  12  is hermetically sealed to the cavity resonator body  25  by using vacuum brazing or diffusion bounding. The feeding coaxial cable  29  and the protective cap  12  are hermetically sealed to the cavity resonator body  25  to prevent contamination and oxidation of the feeding coaxial cable  29 . High-temperature joining techniques, such as brazing or diffusion bonding, are used to join components of the cavity resonator sensor  25 . Joint  21  is typically a laser weld or a TIG weld. Other suitable materials and/or welding techniques are known in the art. 
     Thus, the cavity  13  is based on a circular waveguide  11  short-circuited at one end  9  and open-circuited at the other end  8 . The cavity filling dielectric material  27  is used to further reduce the size of the cavity. The inner conductor  16  of the microwave cable  29  forms an open loop and acts as the excitation probe. It excites the appropriate mode in the cavity, in this case a TE11n mode. As it is required that only the dominant mode can propagate in the waveguide  11 , all the possible cavity modes of this resonator  25  are of the TE11n type. 
       FIGS. 5 and 6  show a resonator  30  according to a second alternative embodiment which can be used in the place of resonator  10  in the device shown in  FIG. 2 . This embodiment is referred to as side-launch cavity resonator  30 . Thereby, identical constituent parts with respect to the previously described embodiments  10 ,  25  are labelled with the same reference numerals. The side-launch cavity resonator  30  essentially corresponds to the end-launch cavity resonator  25  shown in  FIGS. 3 and 4 , with the exception, that its excitation probe  31  extends into cavity  13  from the side walls  32 . Thus, joint  21  is located at an end portion of side walls  32 . Apart from this difference, the side walls  32  substantially correspond to the waveguide  11  of resonator  25 . Correspondingly, the back end  33  of resonator  30  substantially corresponds to the short-circuited back end  9  of resonator  25 , with the exception, that no joint for a coaxial cable is provided. 
     Thus, the side-launch cavity resonator sensor is composed by a circular waveguide-based cavity resonator body  30 , a feeding coaxial cable  29 , with its inner conductor  16  acting as the excitation probe, a cavity filling dielectric material  27  and a protective cap  12 . The feeding coaxial cable  29 , including dielectric material  22  and inner conductor  16 , has the capability to transmit microwave signal at very high temperature. The main difference with the end-launch cavity resonator sensor  10 ,  25  is that the fundamental mode in the cavity is excited on its side  32  rather than at its end  33 . 
     The cavity filling dielectric material  27  is used to further reduce the size of the cavity  13 . The protective cap  12  is hermetically sealed to the cavity resonator body  32  by using vacuum brazing or diffusion bounding. The high-temperature microwave cable  29  is hermetically sealed to the cavity resonator body  30 . The inner conductor  16  of the microwave cable  29  forms a pin  31  projecting into cavity  13  which acts as the excitation probe. The feeding coaxial cable  29 , including its dielectric material  22  and inner conductor  16 , has the capability to transmit a microwave signal at very high temperature. 
       FIGS. 7 and 8   a, b  show a resonator  40  according to a third alternative embodiment that can be used in the place of resonator  10  in the device shown in  FIG. 2 . The resonator according to this embodiment is referred to as coaxial resonator  40 . Identical constituent parts with respect to the previously described embodiments  10 ,  25 ,  30  are labelled with the same reference numerals. The resonator  40  essentially corresponds to the resonator  25  shown in  FIGS. 3 and 4 , with the following exceptions: first, a central conductor  41  is arranged inside the cavity  13  extending coaxially through its total length and constituting the waveguide of resonator  40 . Secondly, the general shape of the side walls  42  of resonator  40  correspond to the hollow tube  11  of resonator  25 , but their transverse dimension can be advantageously reduced. Furthermore, excitation probe  43  is shaped differently. 
     In detail, the coaxial resonator  40  comprises an outer conductor  42  and an inner conductor  41 , having the same axis. The coaxial resonator  40  is terminated by a short circuit at its back end  9  and an open circuit at its front end  8 . The coaxial resonator  40  is further provided with the feeding coaxial cable  29 , which feeds the excitation probe  43 . The excitation probe  43  in this example is a closed loop and excites the coaxial resonator  40 . The dimension, shape and position of the excitation probe  43  are optimised in order to obtain a good impedance matching at the desired operating frequency. The front end of the coaxial resonator  40  is protected by a protective cap  12 . The coaxial resonator  40  can be filled with air or any other adequate dielectrics. 
     The length of the coaxial resonator  40  is chosen in order to obtain a longitudinal resonance of the coaxial resonator  40 . The transverse dimensions of the coaxial resonator  40  are chosen in such a way that only the dominant TEM mode propagates into the corresponding transmission line. 
     For low-temperature applications, i.e. up to typically 500 K, the protective cap  12  can be glued hermetically to the outer conductor  42 . For medium-temperature applications up to typically 900 K, the thermal expansion of different materials and thermal gradient should be considered for the design. High-temperature joining techniques, such as brazing or diffusion bonding, are typically used to join the protective cap  12  to the outer conductor  42 . The reliability of the sensor  40  is further increased by choosing an optimum alloy or metal with a low coefficient of thermal expansion. The high-temperature join  21  shown in  FIG. 8   a, b  can be recessed to further improve its life cycle. 
     Thus, the coaxial resonator  40  is based on the same principle as the cavity resonator  10 ,  25 ,  35 . The main difference between the coaxial resonator  40  and the cavity resonator  10 ,  25 ,  35  is that the waveguide is based on a section of rigid coaxial transmission line  41  instead of a hollow waveguide  11 ,  32 . The main advantage of this is that further miniaturisation is made possible by the fact that the dominant mode of a coaxial transmission line  41  is the TEM (Transverse Electro Magnetic) mode, which has a zero cutoff frequency. This means that there is no theoretical minimum size for the section of the coaxial resonator  40 , the minimum section size being thus dictated by manufacturing issues only. The section, however, has to be selected in such a way that the first higher order mode is below cutoff for the operating frequency. The length of the coaxial resonator  40  is selected in a way that the latter is resonant at the operating frequency. This can for instance correspond to a length of a quarter of the guided wavelength, but other solutions are also possible. 
       FIG. 9   a - d  schematically depict cross-sectional views of the cavity resonator  25 ,  30  based on a circular waveguide, wherein various different embodiments of excitation probes  36 - 39  are illustrated. L is the length of the resonator  25 ,  30 ,  40 , equivalent for example to a quarter of the guided wavelength at the operating frequency. D is the outer diameter and d the inner diameter. The parameters L, D, d are chosen in such a way that the dominant mode of the waveguide propagates at the operating frequency but not the first higher order mode. Furthermore, the feeding transmission line  16  (in this case a coaxial cable) and the protective cap  12  are shown. The excitation probe can be for example a coupling pin  38  or a coupling loop  36 ,  37 ,  39  in the case of the end launch resonator  25  and the side launch resonator  30 . More precisely, the coupling loop can be an open loop  36  or a closed loop  37 ,  39 . Note that these excitation probes  36 - 39  and their respective arrangement at the back end or side wall are also applicable in the coaxial resonator  40 . 
       FIG. 10   a - f  schematically illustrate further embodiments of excitation probes  46 - 41  on a cavity back wall  9 , which are also applicable on a cavity side wall. In the place of the coupling pin  49  also open loops  47 ,  50 ,  51  or closed loops  46 ,  48  are conceivable. Furthermore, circular loops  46 ,  47  or rectangular loops  48 ,  50 ,  51  can be applied. 
     The position, dimension and shape of the excitation probes  36 - 39 ,  46 - 51  define the coupling of the microwave signal into the resonator. These features should be adjusted in order to obtain a critical coupling of the resonator to the feeding transmission line. In many cases, the coupling loops  36 ,  37 ,  39 ,  46 - 48 ,  50 - 51 , in particular the closed loops  37 ,  39 ,  46 ,  48 , may offer the advantage of superior excitation properties. 
     The above described preferred embodiments are intended to illustrate the principles of the invention, but not to limit the scope of the invention. Various other embodiments and modifications to those preferred embodiments may be made by those skilled in the art without departing from the scope of the present invention.