Abstract:
The invention relates to a method and a device for determining and/or monitoring the fill level (F) of a medium in a container or, respectively, to a method and a device for establishing the density (ρ) of a medium in the container. The invention allows a highly accurate determination or monitoring of the fill level (F) or the density of a medium in a container. With reference to the method, the object is solved in that the influence of at least one disturbing parameter on the oscillation frequency of an oscillation-capable unit is established and correspondingly compensated in the determination of the fill level (F) of the medium in the container or, respectively, in the determination of the density (ρ) of the medium in the container.

Description:
FIELD OF THE INVENTION 
   The invention relates to a method and a device for determining and/or monitoring the level of a medium in a container or for establishing the density of a medium in a container. 
   BACKGROUND OF THE INVENTION 
   Devices with at least one oscillation element, so-called vibration detectors, have been known for detecting or monitoring the level of a medium in a container. The oscillation element is usually at least one oscillation bar, which is secured to a membrane. The membrane is caused to oscillate using an electro-mechanical transducer, e.g. a piezo-electric element. Due to the oscillations of the membrane, the oscillation element secured to the membrane also oscillates. 
   Vibration detectors embodied as level detectors utilize the effect that the oscillation frequency and the oscillation amplitude depend on the particular degree to which the oscillation element is covered: While the oscillation element can carry out its oscillations in air free and undamped, it experiences a damping, and, as a consequence thereof, a frequency and amplitude change, as soon as it is submersed partially or completely in the medium. On the basis of a precalibrated frequency change, an accurate conclusion can be drawn as to the particular level in the container. Level measuring devices are also applied especially for overfill protection or as protection against pumps running empty. 
   The oscillation frequency of the oscillation element is also influenced by the particular density of the medium. Consequently, at constant degree of covering, there is a functional relationship for the density of the medium, so that vibration detectors are best suited both for level determination and for density determination. In practice for monitoring and detecting the level or density of the medium in the container the oscillations of the membrane are picked up and changed into electrical received signals by means of at least one piezo-element. 
   The electrical received signals are then evaluated in an evaluation electronics. In the case of a level determination, the evaluation electronics monitors the oscillation frequency and/or the oscillation amplitude of the oscillation element and signals the condition ‘sensor covered’ or ‘sensor uncovered’, as soon as the measured values move under, or over, a pre-assigned reference value. A corresponding report to the operating personnel can then be given optically or acoustically. Alternatively or supplementally, a switching process is triggered; in this way perhaps a feed or drain valve on the container is opened or closed. 
   The devices mentioned above for measuring level or density are used in a multitude of industrial branches, for instance in chemistry, in the foods industry or for water treatment. The band width of monitored fill materials reaches from water through yogurt, paints and lacquers, to highly viscous fill materials, like honey, or to strongly foaming fill materials, like e.g. beer. 
   Vibration detectors are, however, only completely dependent on the above-mentioned parameters ‘level’ and ‘density’ to a first approximation. Besides these, other physical parameters also influence the oscillation behavior of the oscillation element, process parameters such as pressure and temperature or the viscosity of the medium. Thus, as soon as the requirement is made, that the sensor be applied for highly accurate measurements or that it be used as a universally applicable measuring device in the high and low temperature range or in the high or low pressure range, then the influence of these parameters on the oscillation behavior must be taken into account. In principle, the influence of temperature and pressure on the measurement results becomes that much more important, the greater these parameters deviate from normal conditions. Similar considerations are true also with respect to the viscosity of the medium: A measurement device must in the future be able to produce reliable measurements for media of greatly differing viscosities. 
   SUMMARY OF THE INVENTION 
   An object of the invention is to provide a method and a device which allow highly accurate determination or monitoring of the level or the density of a medium. 
   With reference to the method, the object is attained by detecting and correspondingly compensating the influence of at least one disturbing parameter on the oscillation frequency of the oscillations-capable unit when determining the level of the medium in the container or when determining density of the medium in the container. In this way, it is assured that, for the case of level measurement, the switching points of the measuring device, which signal the conditions ‘sensor covered’ and ‘sensor uncovered’, are exactly defined. A malfunction of the measuring device, which can happen if temperature or pressure deviations create a false showing of attainment of the pre-assigned switching points, is reliably prevented. In the case of density measurement, the error tolerance is significantly reduced by the compensation of the influence of the different disturbing parameters on the oscillation behavior of the oscillation element, so that the method of the invention and the corresponding device are suited for highly accurate density measurements. 
   According to a preferred further development of the method of the invention, there is provided that a change in the oscillation frequency of the oscillation-capable unit caused by a change in the viscosity of the medium is compensated in the manner that the exciter frequency exhibits a phase-shift other than 90-degrees relative to the oscillation frequency of the oscillation-capable unit. 
   In particular, the phase-shift between the exciter frequency and the oscillation frequency of the oscillation-capable unit is so measured that a change occurring in the oscillation behavior is essentially independent of the viscosity of the medium and thus essentially only dependent on the immersion depth of the oscillation-capable unit in the medium, or, in other words, on the density of the medium. In practice, it has been found that a phase-shift of about 70-degrees in liquid media is best suited for eliminating the influence of viscosity on the measurement results. If the medium, however, is strongly foaming, then a phase-shift of about 120-degrees between exciter and oscillation frequency permits a sufficiently good compensation of the influence of the viscosity of the foam. Naturally, the phase-shift for compensating the influence of viscosity also depends decisively on the particular embodiment of the oscillation-capable unit. 
   According to an advantageous further development of the method of the invention, it is provided that at least one disturbing parameter is directly measured or indirectly detected. Preferably, characteristic curves are produced and stored on the basis of empirically established data to show the dependence of frequency change on at least one disturbing parameter. Naturally, it is also possible to calculate the characteristic curves on the basis of a mathematical model and to store that, in which case the mathematical model possibly again is based on empirically established data. 
   In one embodiment of the method of the invention, further parameters are taken into consideration in the choice of the correct characteristic curves. Especially of interest in the case of these parameters are the geometry and/or the dimensioning of the oscillation-capable unit, the material from which the oscillation-capable unit is made and/or, in the case of level determination, the installation position of the oscillation-capable unit in the container. The characteristic curves are thus also provided to be sensor-specific and/or system-specific. 
   A preferred form of the method of the invention provides that the at least one disturbing parameter is measured or determined and that the corresponding frequency change is taken into consideration in the case of level measurement in the determination of the switching point or in the case of density measurement in the determination of the density of the medium. In this way, it is possible to react immediately to fluctuations in the disturbing parameter and, therefore, to use the measuring device universally, that is to say independently of the conditions present at the location of measurement. 
   With reference to the device, the object of the invention is solved in that the control/evaluation unit determines the influence of at least one disturbing parameter on the oscillation frequency of the oscillation-capable unit and that the control/evaluation unit so corrects the frequency change that is registered on reaching the predetermined level that the influence of the disturbing parameter is eliminated, or that the control/evaluation unit takes into consideration the measurement error caused by the disturbing parameter in density determination. 
   As already indicated above, reference to the at least one disturbing parameter is a reference to temperature or pressure, or a reference to the viscosity of the medium. Naturally, the invention enables compensation of every other empirically available disturbing parameter that has an influence on the oscillation behavior of the oscillation element. 
   In order to always have the actual values of the temperature and/or pressure available, a temperature sensor and/or a pressure sensor are/is provided, which determine/determines temperature or pressure in the environment of the oscillation-capable unit. According to one embodiment of the device of the invention, the temperature sensor, e.g. a PT 100, and/or the pressure sensor are/is integrated into the device for determining level or density. Naturally, it is also possible to provide the temperature and/or pressure sensor as separate units and to position them in the container. Furthermore, it is also possible to determine, for example, the pressure or the temperature by way of the oscillation-capable unit itself. Especially in this case the stiffness between the drive/receiver unit and the membrane is measured, on which membrane for example oscillation bars in the form of a tuning fork are attached. Then, the reaction of the oscillation-capable unit to the exciter frequency is evaluated for the purpose of determining temperature or pressure. 
   According to a preferred further development of the device of the invention, a data transmission line or data bus is provided. The sensors and the separate units of the device of the invention send their data on this connection to the control/evaluation unit, or the sensors and the separate units of the device of the invention communicate on this connection with the control/evaluation unit. Preferably current industry standards are utilized for the communication, examples being PROFIBUS PA, FIELDBUS FOUNDATION, or HART. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is explained in greater detail on the basis of the following drawings, which show as follows: 
     FIG.  1 : a schematic representation of a preferred embodiment of the device of the invention, 
     FIG.  2 : a flowchart for operation of the control/evaluation unit in the case of level determination, 
     FIG.  3 : a flowchart for operation of the control/evaluation unit in the case of density determination, 
     FIG.  4 : a graphical representation of the characteristic curves E(Δf) for different viscosities and 
     FIG.  5 : a schematic representation of a circuit for compensating frequency changes resulting from viscosity of the medium. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1  shows a schematic representation of the device of the invention for determining and/or monitoring the fill level F of a medium  2  in container  3 . In a few words, of concern here is a limit detector. The device  1  shown in  FIG. 1  is, of course, also, as already explained above, suitable for determining the density of the medium  2  in container  3 . While in the case of level determination the oscillation-capable unit is immersed in the medium, or not immersed in the medium, only upon reaching the detected limit level, for monitoring or determination of density ρ it must be continuously in contact with the medium down to a predetermined immersion depth E. For container  3 , naturally a pipe can be used, with the medium  2  flowing through the pipe. 
   The device  1  exhibits an essentially cylindrical housing  12 . A thread  10  is provided on the jacket surface of housing  12 . Thread  10  serves for securing device  1  at the height of the predetermined fill level F in container  3  and is, in the illustrated case, arranged in a corresponding opening in lid  11  of container  3 . Other types of securement, for example using a flange, can easily be substituted for the arrangement of the device  1  of the invention on the container  3  illustrated in FIG.  1 . 
   Housing  12  is closed by a membrane, or diaphragm,  5  on its end region extending into container  3 . For this purpose, membrane  5  is clamped at its edge region into housing  12 . The oscillation-capable unit  4  extending into container  3  is secured to membrane  5 . In the illustrated case, the oscillation-capable unit  4  is embodied as a tuning fork, thus including two oscillation bars separated from one another, attached to membrane  5 , and extending into container  3 . 
   Membrane  5  is caused to oscillate by a drive/receiver element  6 , with the drive element  6   a  exciting the membrane  5  to oscillate with a predetermined exciter frequency. Drive element  6   a  is, for example, a stack drive or a bimorph drive. Both types of piezoelectric drives are sufficiently known from the prior art, that a description of them can be dispensed with here. Because of the oscillations of the membrane  5 , the oscillation-capable unit  4  also executes oscillations. The oscillation frequency of unit  4  differs, depending on whether it is in contact with the medium, in which case the mass of the medium must move too, or, instead, it is oscillating freely and without contact with the medium  2 . 
   As in the case of the drive unit  6   a , the receiver unit  6   b  can likewise be a single piezoelectric element. The drive/receiver unit  6  excites the membrane  5  to oscillate as a function of a transmitted signal present at the piezoelectric element, and it serves to receive and convert the oscillations of the membrane  5  into an electrical received signal. 
   Piezo-electric elements change their dimensions (thickness, diameter, etc.) as a function of a voltage difference applied in the polarization direction. If an alternating voltage is applied, then the thickness oscillates: When the thickness increases, the diameter of the piezo-electric element decreases; conversely, when the thickness decreases, then the diameter of the piezo-electric element increases. 
   Because of this oscillating behavior of the piezoelectric element, the voltage difference causes the membrane  5  clamped into the housing to bend through. The oscillating bars of the oscillation-capable unit  4  arranged on the membrane  5  execute oppositely directed oscillations about their longitudinal axis because of the oscillations of the membrane  5 . These oppositely directed oscillations have the advantage that the alternating forces exerted by each oscillating bar on the diaphragm  5  cancel one another. This minimizes the mechanical stress of the clamping, in that essentially no oscillation energy is transferred to the housing  12 . 
   Also provided in the container are a temperature sensor  13  and a pressure sensor  14 . Both sensors  13 , 14  and the vibration sensor deliver their measurements to the control/evaluation unit  7  for evaluation. 
     FIG. 2  shows a flowchart for operation of the control/evaluation unit  7  for the case of fill level determination. The associated frequency change established under standard conditions is input as setpoint for characterizing the switching point. Following program start at point  20 , the actual temperature value T and the actual pressure value p are made available at program points  21 , 22 . The corresponding frequency change Δf(p,T) is calculated at program point  23  for the measured values T,p. The calculation can, for example, be done using an empirically established characteristic curve. This curve can be described by the following formula:
 Δ f ( p,T )= p °( a°T+b°T   2   +c )+ d°T+e.   
   In this formula, a,b,c,d,e are real numbers reflecting sensor- and system-dependent parameters. Establishment of these parameters proceeds, for example, using empirically established characteristic curves. For different sensors or installation types of a level- or density-measuring instrument in the container, a preferred variant of the device of the invention provides different sets of characteristic curves. In the simplest case, operating personnel call up these sets of characteristic curves for the correct establishment of switching point or density by pressing a button, so that they are available subsequently for the control/evaluation unit. 
   Clearly an interesting aspect of the invention is also that these different sets of characteristic curves, which were empirically established or calculated using a mathematical model and are sensor- and/or system-specific, can be applied also completely independently of the previously described temperature-, pressure- and/or viscosity-compensation. 
   The frequency change Δf(p,T) occurring under the influence of the disturbing parameters (pressure p, temperature T) is subsequently taken into consideration at program point  25  in the frequency change Δf(actual) reflecting the fill level F or the density ρ. Only when the corrected actual value Δf(actual)Corr agrees with the input setpoint Δf(set) of the frequency change is a report “sensor covered” issued at point  26 . So long as the aforesaid is not fulfilled, program points  21  to  25  are run through in a loop. When the report provided in  26  is obtained, the program ends at  27 . 
     FIG. 3  shows a flowchart for operation of control/evaluation unit  7  for the case of density determination. A characteristic curve established under corresponding standard conditions is input as the setpoint for the density ρ(Δf). This characteristic curve gives the density ρ as a function of the frequency change Δf. Knowing the values of temperature T and pressure p, which are measured at program points  29 , 30 , the associated frequency change Δf(p,T) is calculated or otherwise established at  31 . This frequency change Δf(p,T) is taken into account in determining the actual frequency change Δf′ uninfluenced by these disturbing parameters p,T (point  32 ), so that the corrected frequency change Δf′ reliably reflects the actual density ρ(Δf′) of the medium  2  (program point  33 ). 
     FIG. 4  shows graphically the immersion depth E as a function of the frequency change Δf for different viscosities V. The two extreme cases of viscosities of 1 mPasec and 60,000 mPasec are correspondingly marked in FIG.  4 . As can be clearly seen, the frequency change Δf does not depend only on the immersion depth E of the oscillation-capable unit  4  in the medium  2  but also significantly on the viscosity V of the measurement medium  2 . It is to be recalled at this point that the invention is to relate to a device which is universally applicable for fill level or density measurements in the most different of media  2 . If the different viscosities V of the media  2  were not taken into account in the invention, a switching procedure could, for example, be triggered, even though the input fill level had not yet really been reached. Likewise, the measurement errors in density measurement would be unacceptably large. 
     FIG. 5  shows a schematic representation of a circuit for compensating a frequency change Δf, which (as is clearly seen in  FIG. 4 ) arises due to the influence of viscosity V of the measured or monitored medium  2 . Stated in a few words, the illustrated circuit compensates for the disturbing parameter ‘Viscosity V’ automatically. As already described above, the phase shift Δ φ  between the exciter frequency f E  and the oscillation frequency f S  of the oscillation-capable unit  4  is sized for this purpose such that an occurring frequency change Δf is essentially independent of the viscosity V of the medium  2  and thus essentially only dependent on the immersion depth E of the oscillation-capable unit  4  in the medium  2 , that is to say, on the density ρ of the medium  2 . 
   In particular, the received signal representing the oscillation of the oscillation-capable unit is filtered in filter  17 ; then the filtered signal undergoes a phase-shift Δ φ  sized such that the frequency change Δf(V) caused by the viscosity V has no longer any influence on the frequency change Δf of the oscillation frequency of the oscillation-capable unit  4 . If the temperature- and/or pressure-values lie within a range in which they have no measurable effect on the frequency change Δf of the oscillation-capable unit  4 , then the influence of viscosity V is compensated without problem. On the other hand, if the temperature-and/or pressure-values lie in a range in which they influence the frequency change Δf so strongly that measurement errors and malfunctioning of the sensor occur, then the above-described compensation of the influence of temperature and/or pressure becomes additionally required. 
   It has been observed that, for a large number of liquids of different viscosity V, the influence of viscosity can be compensated with sufficient quality using a phase shift of 70-degrees. For dense foams (density&gt;0.6 g/cm 3 ), a phase shift in the range 120- to 140-degrees is best suited for the compensation. 
   When all of the frequency changes Δf(p,T,ρ) caused by temperature T, pressure p and/or density ρ are under control, then a determination of viscosity V is conversely possible. 
   List of Reference Numerals 
   
       
         1  device of the invention 
         2  medium 
         3  container 
         4  oscillation-capable unit, especially a tuning fork 
         5  membrane 
         6  drive/receiver unit 
         7  control/evaluation unit 
         8  data line 
         9  data line 
         10  thread 
         11  lid 
         12  housing 
         13  temperature sensor 
         14  pressure sensor 
         15  data line 
         16  data line 
         17  filter 
         18  phase shifter 
         19  amplifier