Patent Publication Number: US-2013228003-A1

Title: Coriolis Mass Flowmeter and Method for Operating a Coriolis Mass Flowmeter

Description:
The present invention relates to a Coriolis mass flowmeter and to a method for operating a Coriolis mass flowmeter. 
     Coriolis mass flowmeters generally have a single measuring tube or a number of measuring tubes, for example a pair, through which there flows a medium (for example fluid), of which the mass flow is to be determined. Various arrangements and geometries of the measuring tubes are known for this. 
     There are, for example, Coriolis mass flowmeters with a single straight measuring tube and Coriolis mass flowmeters with two curved measuring tubes running parallel to one another. The latter measuring tubes, formed identically as a pair, are induced by an excitation system placed in the middle region to vibrate in such a way that they oscillate in opposition to one another, that is to say the vibrations of the two measuring tubes are phase-offset with respect to one another by 180°, to achieve a mass equalization. The position of the center of mass of the system formed by the two measuring tubes thereby remains substantially constant and forces occurring are largely compensated. As a positive consequence, this has the result that the vibrating system has scarcely any external effect as such. Provided upstream and downstream of the excitation system are vibration pickups, between the output signals of which a phase difference can be evaluated as a measuring signal when there is a flow. This is caused by the Coriolis forces prevailing when there is a flow, and consequently by the mass flow. The density of the medium influences the resonant frequency of the vibrating system. Consequently, apart from the mass flow, it is also possible to determine, inter alia, the density of the flowing medium. 
     Coriolis mass flowmeters are used in installations for measuring the flow of a wide variety of media. Deposits in the measuring tubes, for example due to limescale, the curing of polymers or the depositing of food residues, influence the measuring accuracy of these meters, both with respect to the measurement of the mass flow and with respect to the determination of the density of the medium. In particular in the case of Coriolis mass flowmeters with at least two measuring tubes, deposits are problematic whenever they are formed asymmetrically, with the result that the flow through the two measuring tubes becomes uneven. As a result, the overall pulse, which in the case of two tubes oscillating symmetrically in opposition to one another is altogether zero in the deposit-free state on account of the mass equalization, is different from zero. If there are asymmetric deposits, the mass flowmeter is consequently more susceptible to react to external vibrations or itself transfers vibrations to the flange-mounted process pipes. A further problem of asymmetric flow is the complete blockage of a measuring tube, for example due to solid constituents such as fruit pips in the medium. As a result, the pressure drop caused by the mass flowmeter increases considerably. Sensitive media, for example jam, may be rendered unusable by the high pressure that occurs in this case. 
     The invention is consequently based on the object of providing a Coriolis mass flowmeter and a method for operating such a meter that make a self-diagnosis of the mass flowmeter possible on the basis of asymmetric flow and/or other asymmetry errors occurring. 
     To achieve this object, the novel Coriolis mass flowmeter of the type mentioned at the beginning has the features specified in the characterizing part of claim  1 . Advantageous developments are described in the dependent claims and a method for operating a Coriolis mass flowmeter is described in claim  9 . 
     The invention has the advantage that various asymmetry errors of the mass flowmeter that may occur during operation can be detected by self-diagnosis. Examples of such errors are:
         deposits in one of the two tubes,   blocking of a tube in the flow divider and   errors based on uneven deposits in the two tubes or   asymmetrical changing of the ability of the tube to oscillate, for example due to a crack or fracture.       

     The self-diagnosis on the basis of asymmetric flow occurring can consequently provide an operator of the Coriolis mass flowmeter with valuable information about the safety of a process in which the meter is used. 
     Since the acceleration pickups are attached to the measuring tubes in addition to conventional vibration pickups, the novel approach in this context of functional separation is taken, with the result that optimum components can be used for the respective function. That is to say that the vibration pickups can continue as before to be optimized for the measurement of the phase differences, but the acceleration sensors can be adapted in the best possible way to their task, detecting an asymmetry in the measuring tubes. Tests with a Coriolis mass flowmeter that has only conventional vibration pickups on the basis of magnetic plunger coils have shown that the complete blocking of one of the two measuring tubes by a cork plug in the flow divider leads to relative measuring errors of the mass flow of 2% to 3% with water as the medium. This gives a magnitude of error that is well above the specified measuring error of, for example, 0.15%. However, no significant differences from the undisturbed case can be found in the vibration signals that are obtained in the evaluation device of the meter during measurement, for example, current flow, amplitude or differential signal of the vibration signals. A self-diagnosis of this error case on the basis of the vibration signals of conventional vibration pickups has consequently proven to be scarcely possible. On the other hand, with the novel use of additional acceleration sensors, a self-diagnosis with significantly improved reliability of the diagnosis finding is achieved. 
     Preferably, acceleration sensors that are made using MEMS technology (MEMS—Micro Electro Mechanical System), or with piezoelectric signal generation, may be used. These can be applied with particularly little effort. 
     In a particularly advantageous exemplary embodiment, the acceleration sensors are attached at the same location in the longitudinal direction of the at least one measuring tube at which the vibration pickup is also secured. As a result, additional mounting points can be avoided and it is possible to use the same securing means for both components. 
     If in the case of a symmetrical measuring tube arrangement the acceleration sensors are likewise arranged symmetrically in relation to one another, this has the advantage that the evaluation of the acceleration signals becomes particularly easy, since a logic operation performed on the signals can be reduced to a simple addition or subtraction. 
     In a further, particularly advantageous refinement of a mass flowmeter with two measuring tubes, two acceleration sensors upstream of the excitation system and two further acceleration sensors downstream of the excitation system are arranged symmetrically in relation to one another. This makes particularly good sensitivity of the arrangement with respect to asymmetry errors possible, and consequently particularly good reliability of the diagnosis finding reached in a self-diagnosis. 
     In the case of a symmetrical arrangement of acceleration sensors in pairs, the evaluation of the acceleration signals emitted by them can be carried out in a particularly easy way in that the signals are added to form an aggregate signal, the aggregate signal is compared with a predeterminable or predetermined first threshold value and an asymmetry error is indicated by a message signal if the first threshold value is exceeded by the aggregate signal. This evaluation allows an asymmetry to be established in a particularly reliable way. This is so because, in the case of a symmetrical oscillation, each of the acceleration sensors arranged symmetrically in relation to one another in pairs generates a precisely inverted signal of the other sensor respectively belonging to the same pair. If the two signals are added, the resultant aggregate signal therefore has an amplitude that is ideally equal to zero in the error-free case. If deposits that have a density deviating from the flowing medium or change the elastic bending properties of the measuring tube concerned occur in one of the two measuring tubes, this leads to changing of the amplitude of the respective acceleration signal, and consequently to an aggregate signal different from zero. Deposits in one measuring tube or uneven deposits in the two measuring tubes can consequently be established in an easy way by a comparison of the aggregate signal with a threshold value. 
     To allow for asymmetries specific to a particular meter, which are virtually unavoidable in a Coriolis mass flowmeter after its production, the evaluation device may be provided with a memory in which there is stored a correction value, for the first threshold value or the aggregate signal, determined meter-specifically during a calibration or initial operation of the Coriolis mass flowmeter. This advantageously allows avoidance of an error diagnosis on the basis of asymmetries of the tubes or tolerances of the acceleration sensors or of the evaluation device. 
     In addition or as an alternative to the addition of the acceleration signals in pairs described above, the respective phase difference of the acceleration signals of the two pairs of acceleration sensors arranged on the same measuring tube upstream and downstream of the excitation system may be determined by the evaluation device. The deviations of the two phase differences are compared with a predeterminable or predetermined second threshold value and an asymmetry error is indicated by a message signal if the second threshold value is exceeded by these deviations. This type of evaluation of the acceleration signals leads to a likewise very high sensitivity, and also makes it possible to establish blocking of a measuring tube in the region of the flow divider, since in this error case there is a great deviation of the two phase differences. If the flow through a pair of measuring tubes is asymmetric, this leads to Coriolis forces that deviate greatly from one another, and the phase difference in the direction of flow caused by the Coriolis force, which is comparatively great, is evaluated in this evaluation. 
    
    
     
       The invention and also refinements and advantages are explained in more detail below on the basis of the drawings, in which exemplary embodiments of the invention are represented and in which: 
         FIG. 1  shows a perspective view of a Coriolis mass flowmeter, 
         FIG. 2  shows a basic representation of the path followed by a measuring tube, 
         FIG. 3  shows a further basic representation of a measuring tube in another view, 
         FIG. 4  shows a block diagram to illustrate the signal evaluation in the case of two acceleration sensors, 
         FIG. 5  shows a block diagram to illustrate the signal evaluation in the case of four acceleration sensors and 
         FIG. 6  shows a block diagram to illustrate the signal evaluation in the case of four acceleration sensors and evaluation of the phase differences. 
     
    
    
     In the figures, the same parts are provided with the same designations. 
       FIG. 1  shows a Coriolis mass flowmeter  1  according to a preferred exemplary embodiment of the present invention. The mass flowmeter  1  measures the mass flow and the density of the medium on the basis of the Coriolis principle. A first measuring tube  2  and a second measuring tube  3  are arranged substantially parallel to one another. They are usually made from one piece by bending. The path followed by the measuring tubes is substantially U-shaped. A flowable medium flows according to an arrow  4  into the mass flowmeter  1 , and thereby into the two inlet portions of the measuring tubes  2  and  3  located downstream of an inlet splitter, which cannot be seen in the figure, and according to an arrow  5  out again from the outlet portions and the outlet splitter located downstream thereof, which likewise cannot be seen in the figure. Flanges  6 , which are respectively fixedly connected to the inlet splitter and the outlet splitter, serve for securing the mass flowmeter  1  in a pipeline not represented in  FIG. 1 . The geometry of the measuring tubes  2  and  3  is kept largely constant by a stiffening frame  7 , so that even changes of the pipeline system in which the mass flowmeter is fitted, for example caused by temperature fluctuations, lead at most to a minor shift of the zero point. An excitation system  8 , which is schematically represented in  FIG. 1  and may comprise, for example, a magnetic coil that is secured on the measuring tube  2  and a magnet that is attached to the measuring tube  3  and plunges into the magnetic coil, serves for generating mutually opposed vibrations of the two measuring tubes  2  and  3 , the frequency of which corresponds to the natural frequency of the substantially U-shaped middle portions of the measuring tubes  2  and  3 . 
     Vibration pickups  9   a  and  9   b  that are likewise schematically represented in  FIG. 1  serve for sensing the Coriolis forces and/or the vibrations of the measuring tubes  2  and  3  that are based on the Coriolis forces and are caused by the mass of the medium flowing through. They are likewise embodied as plunger coils. Vibration signals  10   a  and  10   b , which are generated by the vibration pickups  9   a  and  9   b , respectively, are evaluated by an evaluation device  11 . For the evaluation, the evaluation device  11  comprises a digital signal processor, which carries out the necessary calculation steps. Results of the evaluation, in particular measured values for the mass flow and density and also diagnosis messages, are output on a display  13  or transmitted via an output not represented in the figure, for example a fieldbus, to a higher-level control station. Apart from the evaluation of the vibration signals  10   a ,  10   b , the evaluation device  11  in the exemplary embodiment represented also undertakes the activation of the excitation system  8  as well as the carrying out of the evaluations for a self-diagnosis of the Coriolis mass flowmeter  1 . The self-diagnosis is performed on the basis of four acceleration signals  14   a ,  14   b ,  14   c  and  14   d , which are supplied by four acceleration sensors, of which only acceleration sensors  15   a  and  15   c  can be seen in  FIG. 1 . Two acceleration sensors  15   b  and  15   d  are located on the remote side of the measuring tube  3  and consequently cannot be seen in  FIG. 1 . 
     As a departure from the exemplary embodiment represented, it goes without saying that the measuring tubes may have different geometries, for example a V-shaped or a-shaped middle portion, or a different number and arrangement of excitation systems, vibration pickups and/or acceleration sensors may be chosen. The Coriolis mass flowmeter may alternatively have a different number of meaning tubes, for example one measuring tube or more than two measuring tubes. 
     In a memory  12  of the evaluation device  11 , parameters determined during the calibration of the Coriolis mass flowmeter  1  are stored, for example a correction value that has been determined meter-specifically and serves for the adaptation of a first and a second threshold value, which are used in the self-diagnosis for deriving a diagnosis finding. 
     The way in which the acceleration sensors are applied in principle to the measuring tubes is explained once again on the basis of  FIGS. 2 and 3 . In this respect, it is unimportant whether the acceleration sensors  15   a  . . .  15   d  are applied on the outer side of the measuring tubes, as shown in  FIG. 2 , or on the sides facing one another of the measuring tubes  2  and  3 , as represented in  FIG. 3 . By contrast, it is of importance for the logic operation that is used in the evaluation of the acceleration signals that the acceleration sensors  15   a  . . .  15   d  are sensitive in the same direction. This is achieved by suitable selection of the type of acceleration sensors and by a suitable application to the measuring tubes  2  and  3 . In a particularly advantageous refinement, the acceleration sensors respectively have a preferential direction of their sensitivity, which is aligned parallel to the oscillating direction of the measuring tubes  2  and  3 . In  FIG. 2 , the oscillating direction of the measuring tubes is represented by an arrow  20 . Suitable preferential directions of the sensitivities of the acceleration sensors  15   a  . . .  15   d  are marked correspondingly by arrows  21   a ,  21   b ,  21   c  and  21   d , respectively. The acceleration respectively measured by the acceleration sensors  15   a  . . .  15   d  may be regarded as a directional vector. If the acceleration sensors are applied with a sensitivity direction deviating from the exemplary embodiment represented, it goes without saying that the logic operation in the evaluation must be correspondingly adapted. If, for example, the acceleration sensor  15   b  has its direction of sensitivity opposite to that of acceleration sensor  15   a , the addition of the acceleration signals  14   a  and  14   b  carried out in the evaluation ( FIG. 1 ) is replaced by a subtraction. The vibration pickups  9   a  and  9   b  are embodied like the excitation system  8  as plunger coils. The measuring tubes  2  and  3  are induced by the excitation system  8  to vibrate with oscillations in phase opposition. The two vibration pickups  9   a  and  9   b  are arranged symmetrically in relation to the middle of the measuring tubes  2  and  3 , consequently at the same distance from the excitation system  8 . The sampling frequency with which vibration signals  10   a  and  10   b  supplied by the vibration pickups  9   a  and  9   b  are sampled ( FIG. 1 ) in this case usually lies approximately at a frequency 15 times higher than the excitation frequency of the excitation system  8 . Plunger coils, which as vibration pickups  9   a  and  9   b  make a very exact detection of the phase difference possible, are however scarcely suitable for detecting an asymmetric flow, since they pick up the relative movement of the two measuring tubes  2  and  3  in relation to one another. For better detection of an asymmetry, and consequently to improve the reliability of diagnosis findings in this respect, acceleration sensors  15   c  and  15   d  and also  15   a  and  15   b  are respectively applied in addition to the vibration pickups  9   a  and  9   b  at the same level in the longitudinal direction of the measuring tubes  2  and  3 . Therefore, the same securing means as are also used for the attachment of the vibration pickups  9   a  and  9   b  may serve for the application of the acceleration sensors  15   a  . . .  15   d . The acceleration sensors may, for example, be secured directly to the plunger coils. As a result, additional mounting points on the measuring tubes  2  and  3  can be avoided. In principle, although a pair of acceleration sensors, for example the acceleration sensors  15   a  and  15   b , would be sufficient for the detection, the sensitivity can be improved significantly by the use of two pairs. 
     When mounting the acceleration sensors  15   a  . . .  15   d , allowance must be made for them having a preferred measuring direction if they are for example made piezoelectrically or using MEMS technology. If the measuring directions of the acceleration sensors  15   a  . . .  15   d  are chosen as represented in  FIGS. 2 and 3 , when there is symmetrical deflection of the two measuring tubes  2  and  3  the acceleration sensor  15   a  supplies an acceleration signal that corresponds approximately to the inverted acceleration signal of the acceleration sensor  15   b . This applies correspondingly to the acceleration signals that are output by the acceleration sensors  15   c  and  15   d . However, this no longer applies if, due to an error state, for example due to deposits in one of the two measuring tubes  2  or  3 , an asymmetry has occurred. If, for example, there occur in the measuring tube  2  deposits that are larger than the deposits occurring in the measuring tube  3 , the measuring tube  2  oscillates with a small amplitude and the acceleration sensors  15   a  and  15   c  supply acceleration signals of amplitudes that are consequently likewise smaller than the amplitudes of the acceleration signals supplied by the acceleration sensors  15   b  and  15   d.    
     This is used in the evaluation explained below on the basis of  FIG. 4 . The two acceleration sensors  15   a  and  15   b  supply acceleration signals  14   a  and  14   b , respectively, to the evaluation device ( 11  in  FIG. 1 ). There, the two acceleration signals  14   a  and  14   b  are initially subjected to a bandpass filtering, in which the signal components  40   a  and  40   b  of the fundamental oscillation of the measuring tubes are allowed to pass through. This takes place by means of two bandpass filters  41   a  and  41   b . The bandpass filtering has the effect of removing disturbing frequency components from the acceleration signals  14   a  and  14   b . This is optional and in an alternative embodiment may possibly be omitted. The signal components  40   a  and  40   b  are fed to an adder  42 , which calculates from them an aggregate signal  43 . In the case of an ideal symmetry of the measuring tubes, the aggregate signal  43  would be equal to zero, as already explained above. On the basis of the size of the aggregate signal  43 , an asymmetry error of the measuring tubes can therefore be detected in an easy way. In the exemplary embodiment represented, the aggregate signal  43  is passed for evaluation to an amplitude detector  44 , the output signal  45  of which is subjected to a subsequent normalizing operation in a functional block  46 . A normalized signal  47  obtained in this way is assessed in a functional block  48  by comparison with a first threshold value  49 . If it is greater than the threshold value  49 , the error state of an asymmetry that is present is output by means of a display signal  50 . For signal conditioning, as an alternative to the amplitude detector  44  and the normalizing operation in the functional block  46 , a rectification of the aggregate signal  43  with subsequent lowpass filtering is similarly possible. The determination of an assessment factor for the amplitude or the energy of the aggregate signal  43  can consequently be carried out in various ways. According to a functional block  51 , a correction of the first threshold value  49  or of the assessment factor may be additionally performed. Consequently, an adaptation to the respective meter- or application-specific conditions is possible during the calibration of the mass flowmeter or during its initial operation. This has the advantage that allowance can be made in the determination of the diagnosis finding for possible production-related asymmetries of the measuring tubes or tolerances of the acceleration sensors or variations in the measured value processing of the evaluation device. In the case of a perfectly symmetrical flow through the two measuring tubes, the aggregate signal  43  has the value zero. If the first threshold value  49  is exceeded by the normalized amplitude  47 , there is an asymmetry of the measuring tubes. This is reliably diagnosed and indicated by the method or by the novel Coriolis mass flowmeter. 
     The diagnosis described on the basis of  FIG. 4  is based on the evaluation of the signals of two acceleration sensors arranged symmetrically in relation to one another. However, the sensitivity can be increased if instead four acceleration sensors are used. 
       FIG. 5  shows a diagnosis method in which the acceleration signals  14   a  and  14   c  of the acceleration sensors  15   a  and  15   c , respectively, are combined by a logic operation with the aid of a subtractor  52  to form a differential signal  53 . A second differential signal  54  is calculated with the aid of a second subtractor  55  on the basis of the acceleration signals  14   b  and  14   d  of the acceleration sensors  15   b  and  15   d , respectively. Obtained as a result are differential signals  53  and  54 , which have been freed of an acceleration component that would be obtained in a mass flow measurement without any flow. The two differential signals  53  and  54  are respectively conditioned by amplifiers  56  and  57  and a renewed calculation is subsequently carried out with the aid of a further subtractor  58  to give a differential signal  59 , which in a functional block  60  is subjected to an evaluation that corresponds in principle to that already described further above on the basis of the functional blocks  44  . . .  51  in  FIG. 4 . Consequently, an asymmetric flow in the measuring tubes  2  and  3  can be detected with good sensitivity. This is so because a strong asymmetry of the flow through the two measuring tubes leads to a corresponding increase in the energy of the differential signal  59 . 
     In the case of the exemplary embodiments of a diagnosis method shown in  FIG. 6 , the phase difference  62  between the vibration signals  14   a  and  14   c  for the vibration sensors  15   a  and  15   c , respectively, is calculated in a functional block  61  and the phase difference  64  between the acceleration signals  14   b  and  14   d  of the acceleration signals  15   b  and  15   d , respectively, is calculated in a functional block  63 . The phase differences  62  and  64  are measures of the respective mass flow through the measuring tubes  2  and  3 . In a downstream functional block  65 , the difference  66  of the phase differences  62  and  64  is calculated. The difference  66  in turn represents an assessment factor for the obtainment of a diagnosis finding as to the presence of an asymmetry and is evaluated in a functional block  67  in a way that has already been described in principle on the basis of the functional blocks  44  . . .  51  in  FIG. 4 . That is to say that the difference  66  may be subjected to a normalizing operation and allowance can be made for production-related asymmetries by means of a correction factor. This is followed by a comparison with a threshold value. If the threshold value is exceeded, there is an inadmissible asymmetry of the measuring tubes and it is indicated. The evaluation of the phase differences described on the basis of  FIG. 6  when using four acceleration sensors has the advantage that it is distinguished by particularly good sensitivity. Both measuring tubes are driven at their resonant frequency by the excitation system, with the result that the fundamental oscillation for both measuring tubes is in phase opposition with the same amplitude. The Coriolis force is generated in both measuring tubes by the flow respectively prevailing. If the flow is in this case asymmetric, the Coriolis force in the two measuring tubes also differs in its intensity. If only two acceleration sensors are used, as explained on the basis of  FIG. 4 , the difference of the acceleration signals can already be evaluated. Since, however, the Coriolis force occurring is comparatively small, the difference of the two acceleration signals is likewise small. If, as described on the basis of  FIG. 6 , acceleration signals of four acceleration sensors are used, the phase difference in the direction of flow that is caused by the Coriolis force and is greater by a multiple can be evaluated. 
     In  FIGS. 4 to 6 , the respectively different diagnosis methods are illustrated. Input signals are always acceleration signals, which in an actual implementation of the mass flowmeter are obtained by sampling the analog output signals of acceleration sensors. The various steps of the evaluation are actually implemented in the firmware of a signal processing processor or of a microcontroller. It goes without saying that, as an alternative to this, parts of the signal processing, for example the bandpass filtering or the addition, may be actually implemented before the sampling by an analog hardware circuit. This type of implementation has the advantage over purely digital measured-value processing that the required sampling rate and the required computing power are lower.