Abstract:
A device for monitoring a predetermined level in a container contains a sensor formed by a mechanical vibratory system comprising two vibratory rods, of which at least one vibratory rod is tubular and surrounds the other vibratory rod coaxially. Each of the two vibratory rods is secured to a common support via a resilient holding member acting as a return spring so that each rod is able to execute vibrations transversely to its longitudinal direction. An excitation arrangement causes the two vibratory rods to vibrate transversely in opposite senses at the natural resonant frequency of the mechanical vibratory system. The sensor is fitted to the container so that the tubular outer vibratory rod comes into contact with the material in the container when the material attains the level to be monitored. An evaluation circuit serves to trigger display or switching actions as a function of the vibration amplitude of the mechanical vibratory system. To compensate changes in the resonant frequency of the vibratory assembly formed by the outer vibratory rod and its resilient holding member when a deposit materializes, the inner vibratory rod is provided with a compensating mass which is shiftable in the longitudinal direction of the vibratory rod. By shifting the compensating mass the resonant frequency of the vibratory assembly formed by the inner vibratory rod and its resilient holding member can be adapted to the resonant frequency of the outer vibratory assembly.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a device for monitoring a predetermined level in a container, including a sensor formed by a mechanical vibratory system comprising two vibratory rods, of which at least one vibratory rod is tubular and surrounding the other vibratory rod coaxially, each of the two vibratory rods being secured to a common support via a resilient holding member acting as a return spring so that each rod is able to execute vibrations transversely to its longitudinal direction, an excitation arrangement causing the two vibratory rods to vibrate in opposite senses at the natural resonant frequency of the mechanical vibratory system, and an evaluation circuit for triggering display or switching actions as a function of the vibration amplitude of the mechanical vibratory system of the sensor. 
     2. Description of the Prior Art 
     A device of this kind is known from U.S. Pat. No. 4,499,765. Each vibratory rod forms with the resilient holding member acting as a return spring a mechanical vibratory assembly, the natural resonant frequency of which is dictated by the mass moment of inertia of the vibratory rod and the spring constant of the resilient holding member. The two mechanical vibratory assemblies are configured so that they have the same natural resonant frequency which is simultaneously the natural resonant frequency of the mechanical vibratory system as a whole. For a given excitation power their opposed vibrations then have a maximum vibration amplitude when the outer vibratory rod vibrates in air, whereas when the outer vibratory rod is covered by the material, the level of which is to be monitored, the vibrations of the mechanical vibratory system are damped so that their amplitude becomes smaller or the vibration even collapses altogether. Due to the differing vibration amplitudes the evaluation circuit is thus able to detect whether the material has attained the level to be monitored or not. 
     In known devices of this kind there is the problem that the natural resonant frequency of the mechanical vibratory assembly formed by the outer vibratory rod is altered when a deposit of the material forms on the outer vibratory rod, since this results in the mass moment of inertia of the outer vibratory rod becoming larger, whereas the natural resonant frequency of the mechanical vibratory assembly formed by the inner vibratory rod remains unaltered. The two mechanical vibratory assemblies are then no longer tuned to each other, resulting in the vibration amplitude of the mechanical vibratory system becoming smaller. There is then the risk that the evaluation circuit is unable to recognize whether the reduction in the vibration amplitude is due to a deposit having been formed, although the outer vibratory rod is vibrating in air, or is due to the outer vibratory rod being covered by the material. This can result in false indications. 
     SUMMARY OF THE INVENTION 
     It is the object of the invention to define a device of the aforementioned kind which always vibrates in air with maximum amplitude, irrespective of any deposits having formed, and thus definitively indicates by a reduction in the vibration amplitude that the level being monitored has been attained. For achieving this object the invention provides a device for monitoring a predetermined level in a container, including a sensor formed by a mechanical vibratory system comprising two vibratory rods, of which at least one vibratory rod is tubular and surrounds the other vibratory rod coaxially, each of the two vibratory rods being secured to a common support via a resilient holding member acting as a return spring so that each rod is able to execute vibrations transversely to its longitudinal direction, an excitation arrangement causing the two vibratory rods to vibrate transversely in opposite senses at the natural resonant frequency of the mechanical vibratory system, and an evaluation circuit for triggering display or switching actions as a function of the vibration amplitude of the mechanical vibratory system of the sensor, wherein at least one of said vibratory rods is provided with a compensating mass which is shiftable in the longitudinal direction of said vibratory rod and wherein adjusting means for shifting said compensating mass are provided. 
     In the device in accordance with the invention shifting the compensating mass automatically results in the natural resonant frequency of the two mechanical vibratory assemblies being tuned so that they always have the same natural resonant frequency even when the natural resonant frequency of one vibratory assembly has been altered due to deposits having been formed or due to any other effect. The optimum setting at which the two vibratory assemblies have the same natural resonant frequency can be detected by the vibration amplitude of the mechanical vibratory system having attained a maximum. 
     Advantageous modifications and improvements of the invention are characterized in the sub-claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     Further features and advantages of the invention read from the following description of an example embodiment as shown in the drawing in which: 
     FIG. 1 is a section view through a device for monitoring a predetermined level in a container, the mechanical vibratory system of which comprises two coaxially arranged vibratory rods, and 
     FIG. 2 is a block diagram of the excitation and evaluation circuitry of the device as shown in FIG.  1 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The device as shown in FIG. 1 of the drawing has a sensor  10  secured by means of a fastener part  12  in an opening of a wall  14  of a container housing the material, the level of which is to be monitored by the device. The sensor  10  comprises an outer tubular vibratory rod  16  and an inner vibratory rod  18  arranged coaxially in the interior of the former. The sensor  10  is secured to the container wall  14  so that the vibratory rods  16 ,  18  protrude into the interior of the container and the outer vibratory rod  16  is in contact with the material when the latter attains the level to be monitored. 
     The end of the outer vibratory rod  16  facing the container wall  14  is connected to the fastener part  12  by a resilient annular diaphragm  20  so that the outer vibratory rod  16  is able to execute vibrations transversely to its longitudinal direction, the diaphragm  20  acting as a return spring. The natural resonant frequency of the outer vibratory assembly formed by the outer vibratory rod  16  and the diaphragm  20  is dictated by the mass moment of inertia θ a  of the outer vibratory rod  16  and the spring constant C a  of the diaphragm  20 . The inner vibratory rod  18  is connected by an elastic constriction  22  to a holding block  24  so that the inner vibratory rod  18  is able to execute vibrations transversely to its longitudinal direction, the constriction  22  acting as a return spring. The natural resonant frequency of the inner vibratory assembly formed by the inner vibratory rod  18  and the constriction  22  is dictated by the mass moment of inertia θ i  of the inner vibratory rod  18  and the spring constant C i  of the constriction  22 . The outer vibratory assembly  16 ,  20  and the inner vibratory assembly  18 ,  22  form together the mechanical vibratory system of the sensor  10 . The two vibratory assemblies are coupled to each other by the holding block  24  being connected circumferentially by an annular connector part  26  to the diaphragm  20 . When one of these two vibratory assemblies is caused to vibrate at the natural resonant frequency, due to this coupling the other vibratory assembly is caused to vibrate opposite in phase when the two vibratory assemblies are correctly tuned to each other so that they have the same natural resonant frequency, i.e. when 
     C a =C i    
     θ a =θ i    
     In this case the reaction torque exerted by the outer vibratory assembly  16 ,  20  on the mounting of the fastener part  12  in the container wall  14  is oppositely equal to the reaction torque exerted by the inner vibratory assembly  18 ,  22  on this mounting, and in the same way the reaction forces exerted by the vibratory assemblies on the container wall  14  are oppositely equal. The reaction torques and the reaction forces cancel each other out so that no vibration energy is transmitted to the container wall. 
     The vibration of the mechanical vibratory system is excited electrically by means of electromechanical transducers. In the example embodiment shown, for this purpose a piezoelectric excitation transducer  28  and a piezoelectric receiving transducer  30  are applied to the diaphragm  20 . Each piezoelectric transducer consists in a manner known per se of a flat piezoelectric ceramic disk, provided with metallic coatings on both sides serving as electrodes. The one metallic coating of each piezoelectric transducer is electrically conductively connected to the diaphragm  20  serving as the ground connection. The opposite metallic coatings are electrically connected to an excitation and evaluation circuitry  32 . The excitation transducer  28  is configured and applied so that on application of an electric alternating voltage the diaphragm  20  is caused to vibrate mechanically, these vibrations being transmitted to the outer vibratory rod  16 , as a result of which this vibratory rod executes vibrations transversely to its longitudinal direction. The receiving transducer  30  converts mechanical vibrations of the diaphragm  20  into an electric alternating voltage which is transmitted to the excitation and evaluation circuitry  32 . 
     Shown in FIG. 2 is a block diagram of the excitation and evaluation circuitry  32 , this figure also showing symbolically the excitation transducer  28  and the receiving transducer  30 . The receiving transducer  30  is connected to the input of an amplifier  34 , to the output of which the excitation transducer  30  is connected. The alternating voltage generated by the receiving transducer  30  having the frequency of the mechanical vibrations of the mechanical vibratory system is amplified by the amplifier  34  and applied to the excitation transducer  28 , thus amplifying the vibrations of the mechanical vibratory system. Accordingly, the two electromechanical transducers  28 ,  30 , which are coupled to each other via the mechanical vibratory system, are located in the feedback circuit of the amplifier  34 . When the gain of the amplifier  34  is so high that the self-excitation condition is satisfied the mechanical vibratory system is caused to vibrate at its natural resonant frequency. 
     Connected to the output of the amplifier  34  is an amplitude discriminator  36  which outputs a signal indicating whether the amplitude of the alternating voltage signal which is furnished at the output of the amplifier  34  and is proportional to the vibration amplitude of the mechanical vibratory system, is above or below a predetermined threshold value. The output signal of the amplitude discriminator  36  is applied to a display  38 . 
     The device described above operates in the following way: 
     When the outer vibratory rod  16  is not covered by the material in the container it is able to vibrate undamped and the mechanical vibratory system is caused to vibrate at its natural resonant frequency. These vibrations have a maximum amplitude when the two vibratory assemblies are tuned to each other in the way as described above so that they have the same resonant frequency. The amplitude discriminator  36  then detects that the amplitude of the output alternating voltage of the amplifier  34  is above the predetermined threshold value, and the display  38  connected to the output of the amplitude discriminator  36  indicates that the level to be monitored has not been attained in the container. 
     As soon as the outer vibratory rod  16  is covered by the material, however, its vibrations are damped to such a degree that the amplitude of the output alternating voltage of the amplifier  34  drops below the threshold value of the amplitude discriminator  36  or even becomes zero due to the vibration collapsing. The display  38  then indicates that the level to be monitored has been attained or exceeded. 
     As long as the condition that the inner vibratory assembly  18 ,  22  has the same resonant frequency as the outer vibratory assembly  16 ,  20  is satisfied, level monitoring is done by means of the sensor  10  with high sensitivity, since in the non-covered condition vibrations having a considerable amplitude can be maintained by relatively little energy and accordingly the vibration amplitude drops off sharply already when the outer vibratory rod is just slightly damped. 
     This ideal status no longer exists, however, when a material deposit has formed on the outer vibratory rod as may happen more particularly in the case of viscous, sticky or also damp powdered materials. Due to such a deposit forming, the tuning of the resonant frequencies of inner and outer vibratory assembly is disturbed. The resonant frequency of the outer vibratory assembly is reduced due to its mass moment of inertia θ a  becoming larger, whereas the resonant frequency of the inner vibratory assembly remains unchanged. This mis-tuning results in the vibration amplitude of the mechanical vibratory system and thus the amplitude of the alternating voltage output by the amplifier  34  becoming smaller, even when the sensor  10  vibrates in air. As a result of this the sensitivity of the sensor  10  is reduced and there is a risk of false indications due to the amplitude discriminator being unable to detect whether the reduction in the vibration amplitude is due to a deposit having formed or due to the sensor being covered by the material. 
     To get round this problem the sensor  10  as shown in FIG. 1 is configured so that the tuning of the outer vibratory assembly  16 ,  20  and of the inner vibratory assembly  18 ,  22  to the same natural resonant frequency is maintained. For this purpose a cavity  42  is formed at the front end of the inner vibratory rod  18 , and an axially shiftable compensating mass  44  is located in the cavity  42 . Shifting the compensating mass  44  is done by a positioning device  46  via a rod  48  extending axially through the inner vibratory rod  18 . The positioning device  46  is controlled by the excitation and evaluation circuitry  32 . 
     For setting the compensating mass  44  use can be made of the fact that every change in the resonant frequency of the outer vibratory assembly  16 ,  20  changes the natural resonant frequency of the mechanical vibratory system as a whole. The mechanical vibratory system is thus caused to vibrate at the changed natural resonant frequency via the amplifier  34 , these natural resonance vibrations, however, having a smaller amplitude due to the lack of tuning between the natural resonant frequencies of the outer and inner vibratory assemblies. As soon as the excitation and evaluation circuitry detects this change in the natural resonant frequency it causes the positioning device  46  to shift the compensating mass  44  in the direction in which the resonant frequency of the inner vibratory assembly  18 ,  22  is caused to approximate the resonant frequency of the outer vibratory assembly  16 ,  20 . Thus, wher the resonant frequency of the outer vibratory assembly  16 ,  20  is reduced due to a deposit having formed, the positioning device  46  is controlled by the excitation and evaluation circuitry  32  such that it shifts the compensating mass  44  outwardly toward the free end of the vibratory rod  18 . As a result of this the mass moment of inertia θ i  of the inner vibratory assembly  18 ,  22  is increased and correspondingly the resonant frequency of the inner vibratory assembly  18 ,  22  is reduced until, in the end, equality of the resonant frequencies of the two vibratory assemblies is reestablished. When, however, the resonant frequency of the outer vibratory assembly  16 ,  20  again decreases due to a reduction in the deposit the compensating mass  44  is shifted inwardly. Attaining the tuned condition can always be detected by the the vibrations of the mechanical vibratory system having attained a maximum amplitude. 
     In FIG. 1 the position of the compensating mass  44  drawn in solid lines corresponds to the highest natural resonant frequency of the inner vibratory assembly  18 ,  22 ; the position depicted in broken lines at  44 ′ is the position of the compensating mass  44  which corresponds to the lowest natural resonant frequency of the inner vibratory assembly  18 ,  22 . 
     Shifting the compensating mass  44  by the positioning device  46  can be done in various ways as will be readily appreciated by the person skilled in the art. For instance, the rod  48  may be provided as a kind of threaded spindle having a screw thread running in a fixed threaded bore, and the positioning device  46  may be a motor causing the threaded spindle to rotate so that the threaded spindle together with the compensating mass is moved axially. Or the threaded bore may be provided in the compensating mass  44 , in which case only the part of the rod  48  located within the cavity  42  is configured as a threaded spindle so that the compensating mass  44  wanders along the rod  48  when the latter is caused to rotate by the positioning device  46 . Furthermore, the rod  48  may be located longitudinally shiftable in the inner vibratory rod  18  and shifted longitudinally by the positioning device  48  in some other suitable way. 
     In the block diagram as shown in FIG. 2 an example embodiment of a closed loop control circuit  50  is illustrated which permits tuning the mechanical vibratory system to the maximum amplitude of the natural resonant vibration by shifting the compensating mass  44  when there is a change in the natural resonant frequency. 
     The closed loop control circuit  50  contains a frequency measuring circuit  52  and an amplitude measuring circuit  54 , both of which receive the output signal of the amplifier  34 . The frequency measuring circuit  52  continually measures the frequency of the output signal of the amplifier  34 , which frequency is equal to the momentary natural resonant frequency of the sensor  10 . The frequency value measured in this way is applied to a frequency comparator  56  which also receives a reference frequency value stored in a reference frequency memory  58 . The frequency comparator  56  compares the frequency value measured by the frequency measuring circuit  52  to the reference frequency value and furnishes a signal as a function of this comparison to a positioning device control circuit  60 . 
     The amplitude measuring circuit  54  continually measures the amplitude of the output signal of the amplifier  34  which depends on the amplitude of the natural resonance vibration of the mechanical vibratory system and furnishes the measured amplitude value to a maximum value detector  62  and to an amplitude comparator  64 . The outputs of the maximum value detector  62  and of the amplitude comparator  64  are likewise connected to the positioning device control circuit  60 . 
     The functioning of the closed loop control circuit  50  is as follows: 
     The reference frequency stored in the reference frequency memory  58  corresponds to the natural resonant frequency last measured by the frequency measuring circuit  52 , this natural resonant frequency being that of the mechanical vibratory system of the sensor  10  when vibrating in air. In the frequency comparator  56  the reference frequency stored in the reference frequency memory  58  is continually compared to the frequency measured by the frequency measuring circuit  52 . As long as the natural resonant frequency of the mechanical vibratory system does not change, the frequency values compared to each other are equal and the frequency comparator  56  furnishes the positioning device control circuit  60  either with no signal or a signal indicating equality. 
     When there is a change in the resonant frequency of the outer vibratory assembly  16 ,  20 , however, for instance due to a deposit having formed, the frequency value measured by the frequency measuring circuit  52  no longer equals the reference frequency value stored in the reference frequency memory  58 . As soon as the difference detected by the frequency comparator  56  exceeds a predetermined minimum value, the frequency comparator  56  furnishes the positioning device control circuit  60  with a signal which indicates lack of equality and also shows in which direction the resonant frequency of the outer vibratory assembly has changed. The positioning device control circuit  60  controls the positioning device  46  so that the compensating mass  44  is shifted in the direction in which the resonant frequency of the inner vibratory assembly  18 ,  22  is caused to approximate the new resonant frequency of the outer vibratory assembly  16 ,  20 . 
     Due to the change in the resonant frequency of the outer vibratory assembly and the resulting mis-tuning, the vibration amplitude has also been reduced. During shifting of the compensating mass the vibration amplitude measured by the amplitude measuring circuit  54  reincreases. As soon as the maximum value detector  62  detects that the measured vibration amplitude has attained a maximum value it furnishes the positioning device control circuit  60  with a signal which discontinues further shifting of the compensating mass  44  by the positioning device  46 . 
     The mechanical vibratory system is now tuned to the new natural resonant frequency so that the outer vibratory assembly  16 ,  20  and the inner vibratory assembly  18 ,  22  vibrate opposite in phase with the same resonant frequency and maximum amplitude, as a result of which optimum sensitivity of the sensor  10  is reestablished despite a deposit having formed. 
     The maximum value detector  62  is able to detect the maximum value being achieved as a rule only by the vibration amplitude beginning to drop again after the maximum value has been exceeded. This is why the arrangement is preferably to be configured so that the signal furnished by the maximum value detector  62  to the positioning device control circuit  60  results in a reversal of the direction of shift every time the maximum value is exceeded. Since the system constitutes a closed loop control circuit it is automatically regulated to a position corresponding to the maximum vibration amplitude. Due to it always being the case that the deposit forms slowly in practice, very fast control is not needed. 
     The process as described is repeated in the same direction when the deposit on the outer vibratory rod  16  increases further, and it proceeds in the reverse direction when the deposit is reduced by it being removed or dropping off, for instance. 
     Once the system has regulated itself to the maximum value of the vibration amplitude, the natural resonant frequency measured in this condition is entered from the frequency measuring circuit  52  into the reference frequency memory  58  where it is stored as a new reference frequency. Subsequently, the vibration frequency measured by the frequency measuring circuit  52  is compared to the new reference frequency and shifting the compensating mass  44  as described above is re initiated when the difference sensed by the frequency comparator  56  again exceeds the predetermined minimum value. 
     When the outer vibratory rod  16  starts to dip into the material in the container, no frequency tuning is allowed, of course, by the compensating mass  44  being shifted. For this purpose use is made of the fact that the action of the sensor  10  dipping into the material is accompanied by a sudden, sharp reduction in the vibration amplitude which is exploited by the amplitude discriminator  36  to detect attainment of the monitored level. This reduction in amplitude may be so considerable, as already mentioned, that the vibration collapses and is not reinstated until the sensor is no longer covered by material. 
     This is why the amplitude value measured by the amplitude measuring circuit  54  is supplied to an amplitude comparator  64  which compares the measured amplitude value to a predetermined threshold value. When the measured amplitude value is less than the threshold value the amplitude comparator  64  furnishes the positioning device control circuit  60  with a signal which disables shifting of the compensating mass  44 . Shifting of the compensating mass  44  is not reenabled until the measured amplitude value is again above the threshold value. 
     It would also be possible to omit the amplitude comparator  64  and instead to apply the output signal of the amplitude discriminator  36  to the positioning device control circuit  60  to enable or disable frequency tuning, as is indicated by the broken line connection  66  in FIG.  2 . Frequency tuning is then disabled when the output signal of the amplitude discriminator  36  indicates that the sensor  10  is covered by material, and it is enabled when the output signal of the amplitude discriminator  36  indicates that the sensor  10  is vibrating in air. Using a separate amplitude comparator  64  permits, however, a more sensitive influencing of deposit compensation by the shifting of the compensating mass being disabled already when a reduction in amplitude occurs at which the amplitude discriminator  36  does not yet respond. It is then in particular possible to adjust the threshold value of the amplitude comparator  64  as a function of the maximum value last sensed in each case. 
     It will readily be appreciated by the person skilled in the art from his knowledge of the described basic principle in compensating deposits that various modifications of the embodiment as described above are possible. Thus, it would be basically possible to apply the shiftable compensating mass to the outer vibratory rod instead of to the inner vibratory rod or to provide both vibratory rods with shiftable compensating masses. However, such embodiments would prove to be more difficult to achieve technically.