JPH0830663B2 - DEVICE FOR DETECTING AND/OR MONITORING A DETERMINED FILL LEVEL IN A CONTAINER - Patent application - Google Patents

Description

The present invention relates to a device for detecting and/or monitoring a predetermined filling level in a container, the device comprising a sensor, an excitation device and an evaluation circuit, the sensor being arranged in the container at the height of the filling level to be monitored so that the sensor comes into contact with the filling material when the filling material reaches the predetermined filling level, the excitation device exciting the sensor to mechanically oscillate at its natural resonant frequency,
The evaluation circuit is designed to trigger a display process and/or a switching process depending on whether the mechanical vibration frequency of the sensor is above or below a switching frequency.

A device of this type is described, for example, in German Patent No.
A known device is disclosed in Published Japanese Patent Application No. 3336991. In this known device, the sensor has two vibrating rods. These rods are attached to a vibrating plate, spaced apart from one another, and are vibrated in opposite directions transverse to their longitudinal direction by an exciter. However, devices of the above type are also known, in which the sensor has only one vibrating rod or, in a complete departure from this, is configured without a vibrating rod. In all cases, the device operates based on the change in the natural resonant frequency of the sensor when it comes into contact with the filling. When the sensor is covered with the filling, its natural resonant frequency is lower than when it is not covered with the filling. The switching frequency is selected to lie between the lower natural resonant frequency when the sensor is covered and the higher natural resonant frequency when the sensor is not covered. If the evaluation circuit detects that the vibration frequency of the sensor is lower than the switching frequency, this is a measure or indicator that the sensor is covered with the filling, i.e., that the filling level in the container has exceeded the height to be monitored. On the other hand, if the evaluation circuit detects that the vibration frequency of the sensor is higher than the switching frequency, this is an indication that the sensor is not covered with a filling material, i.e., the filling level in the container is lower than the height to be monitored. This type of sensor is particularly suitable for monitoring the filling level of liquids, since the change in the natural resonant frequency when changing from an uncovered state to a covered state and vice versa can always be clearly detected in the case of liquids.

Due to manufacturing factors, the resonant frequencies of the sensors are different from each other. On the other hand, especially in the case of low-concentration liquids, the natural resonant frequencies of the covered and uncovered sensors may be quite close to each other, so the switching frequency must be set for each sensor. Therefore, the evaluation circuit is inevitably coupled to the sensor.
This means that if the evaluation circuit is to be used with another sensor, the evaluation circuit must be adjusted to the other sensor, and for this purpose the natural frequency of the other sensor must be known. Such adjustments usually cannot be made on-site, which makes repair and maintenance of the device difficult.

The object of the present invention is therefore to make it possible in a device of the type mentioned at the beginning to use any evaluation circuit with any desired sensor without the need to adjust the evaluation circuit separately for each individual sensor.

According to the present invention, this object is achieved by providing an electric circuit element associated with the sensor, the electric circuit element having a characteristic quantity whose value is uniquely related to the natural resonance frequency of the sensor, and the switching frequency being set and adjusted in the evaluation circuit depending on the value of the characteristic quantity of the electric circuit element.

When manufacturing the device according to the invention, all evaluation circuits are constructed identically: for each sensor, its natural resonance frequency is measured in its free state, and the values of the characteristic quantities of the electrical circuit elements are set and adjusted as a function of the measured natural resonance frequency.
The circuit elements are permanently attached to the sensors and are accommodated, for example, in threaded connections of the sensors. The circuit elements are connected to an evaluation circuit so that an electrical signal representing the value of the characteristic variable of the circuit element is supplied to the evaluation circuit. Each evaluation circuit has a device for setting and adjusting a switching frequency individually assigned to each sensor as a function of the value of the characteristic variable of the circuit element.

In this way, any evaluation circuit can be combined with any sensor without the need for special setting adjustments to adapt the evaluation circuit to the sensor.
One evaluation circuit can be used in conjunction with several sensors, which are scanned in turn by the evaluation circuit, so that after each scan, the switching frequency in the evaluation circuit is automatically adapted to the sensor currently being scanned.

The dependent claims set out advantageous embodiments of the invention.

Further features and advantages of the present invention will become apparent from the following detailed description of the preferred embodiments with reference to the accompanying drawings.

FIG. 1 is a block diagram of an apparatus according to the present invention.

2, 3 and 4 show various embodiments of the circuit elements assigned to the sensor and the connection of the sensor to the evaluation circuit.

FIG. 5 is a diagram showing an embodiment of a delay circuit included in the evaluation circuit of FIG.

FIG. 6 is a diagram illustrating the operation of the delay circuit of FIG.

The drawing shows two parallel screws supported by a threaded joint 11.
The sensor 10 is shown with two vibrating rods 12 and 13. The threaded portion 11 has a threaded portion 14 and a hexagonal head 15. The threaded portion 14 is threaded into an opening in the internally threaded container wall so that the vibrating rods 12 and 13 extend into the container interior. The hexagonal head 15 is used to apply a wrench to tighten the threaded portion 11 against the container wall. The openings are located in the container wall at a height such that the vibrating rods 12 and 13 are covered by the contents of the container when the container fill level exceeds a predetermined height, but are not covered by the contents when the container fill level falls below the predetermined height. The device can be installed in any position; the vibrating rods can be positioned vertically if the openings are located above the container wall, or horizontally if the openings are located in the container sidewall.

The threaded portion 11 is hollow and contains a transducer that, when supplied with an AC voltage, vibrates the vibrating rods 12, 13 in opposite directions transverse to their longitudinal direction. The AC voltage is supplied by an excitation circuit 16 connected to the transducer via a line 17, which excites the mechanical vibration system consisting of the vibrating rods 12, 13 and the transducer to vibrate at its natural resonant frequency. A sensor constructed in this way is known, for example, from German Patent No. 3336991, in which the transducer is formed by a piezoelectric element. Therefore, the transducer will not be described in further detail here.

The output of the excitation circuit 16 is connected via a line 18 to the input of an evaluation circuit 20.
The sensor 10 emits a signal having a vibration frequency, which will be referred to hereinafter as the sensor frequency, between 200 and 500 Hz.
The evaluation circuit 20 checks whether the sensor frequency is above or below a predetermined switching frequency, and thereby can detect whether the vibrating rods 12, 13 of the sensor 10 are covered with a filling material or not. That is, the natural resonant frequency at which the vibrating rods 12 and 13 vibrate is relatively high if the vibrating rods are vibrating in air, and is relatively low if the vibrating rods are covered with a filling material.

The evaluation circuit 20 has a counter 21, which receives at its control input a signal having the sensor frequency supplied by the excitation circuit 16. At its clock input, the counter 21 receives clock pulses supplied by a clock generator 22. The frequency of these clock pulses is significantly higher than the sensor frequency, for example, it is of the order of 500 kHz. The counter 21 is controlled by a signal applied to its control input via line 18 so that it counts each clock pulse supplied by the clock generator 22 during one period of this signal. The counting state reached at the end of one period is transmitted to a memory 23, after which the counter 21 counts the number of clock pulses supplied by the clock generator 22.
21 is reset to zero and starts a new count of clock pulses during one period of the sensor signal. The digital value in memory 23 is therefore always proportional to the period of the sensor signal and is therefore a measure of the sensor frequency.

The digital value in the memory 23 is input to the input 24a of the comparator 24.
, which also receives at a second input 24b a digital value representative of the period of the switching frequency. Comparator 24 delivers at its output a binary signal which has one signal value (for example, a signal value "H") if the digital value supplied to input 24a is smaller than the digital value supplied to input 24b, and another signal value (in this example, a signal value "L") if the digital value supplied to input 24a is greater than the digital value supplied to input 24b.

Thus, in the selected embodiment, a signal value "H" at the output signal of comparator 24 indicates that the sensor frequency is higher than the switching frequency, and a signal value "L" indicates that the sensor frequency is lower than the switching frequency. Thus, if the switching frequency is properly set and adjusted for the connected sensor 10, a signal value "H" indicates that the vibrating rod of sensor 10 is vibrating in air, i.e., the fill level to be monitored has not been reached, and a signal value "L" indicates that the vibrating rod of the sensor is covered with the fill, i.e., the fill level to be monitored has been reached or exceeded.

If the switching frequency is fixed to a suitable value for a particular sensor, the evaluation circuit may no longer function correctly if the sensor is replaced with another sensor that has a different natural resonant frequency due to manufacturing factors. For example, if the natural resonant frequency of the vibrating rod when it is not covered is low enough to be below the switching frequency, the output signal of the comparator will always indicate the "covered" state. However, if the natural resonant frequency of the sensor is high enough to be above the switching frequency even when the vibrating rod is covered, the output signal of the comparator will always indicate the "uncovered" state.

The device shown in the drawings automatically adapts the switching frequency to the respective connected sensor. For this purpose, an electrical circuit element 25 is associated with the sensor 10, which has a characteristic value that is uniquely related to the natural resonant frequency of the sensor 10. As symbolically shown in the drawings, the circuit element 25 is preferably accommodated in the screw connection 11 of the sensor 10 and is connected via a line 26 to a second input of the evaluation circuit 20. The evaluation circuit 20 receives an electrical signal representing the value of the characteristic value of the circuit element 25 via the line 26. If this electrical signal is analog, it is supplied to an analog-to-digital converter 27 in the evaluation circuit 20, which converts the analog signal to a digital value, for example, an 8-bit value. This digital value is supplied to an input 28a of an adder 28, which receives a fixed digital base value at its second input 28b. The adder 28 forms the sum of the values supplied to both of its inputs and delivers at its output a digital value representing this sum, which is supplied to a second input 24b of the comparator 24, which represents the switching frequency, and which is compared with the sensor frequency represented by the contents of the memory 23.

The digital base value supplied to input 28b
The distance between the switching frequency and the natural resonant frequency of the sensor is determined, and thus the response sensitivity of the device is determined. For example, for water and liquids of similar density, this value is set so that the switching frequency is 11% below the natural resonant frequency at which the sensor vibrates when uncovered. Closing switch 29 applies a signal to the control input of adder 28 that changes the value to be added in adder 28.
28c. This allows the device to match its response sensitivity to different density fills.
The interval between the natural resonant frequency of the sensor and the switching frequency is varied. If, as mentioned above, the digital base value supplied to input 28b is selected so that the switching frequency is 11% below the natural resonant frequency of the uncovered sensor when switch 29 is open, then when monitoring the fill level of a liquid with a lower density, the vibration frequency of the covered sensor will be 11% below the vibration frequency of the uncovered sensor.
% lower. In this case, the comparator
In this case, the switch 29 would not emit a signal indicating that the fill level to be monitored has been reached.
Closing the cover acts, for example, to ensure that the switching frequency is only 5% lower than the natural resonant frequency of the uncovered sensor.

Passive circuit elements that do not require an inherent current supply within the sensor are preferably used for the electrical circuit element 25. Some examples of suitable circuit elements are shown in Figures 2, 3 and 4. Each of these figures also shows a portion of the evaluation circuit 20, as well as the connecting lines 26 between the circuit element 25 and the evaluation circuit 20.

Typically, the evaluation circuit 20, together with the excitation circuit 16, is housed in an electronics housing, which can be mounted directly onto the threaded portion 11 of the sensor 10. When mounted, a connection is established between the excitation circuit 16 and the electromechanical transducer device located in the threaded portion 11, on the one hand, and between the evaluation circuit 20 and the circuit element 25, on the other hand. This is preferably achieved by means of a plug-in connector attached to the threaded portion 11 on the one hand and to the electronics housing on the other hand. However, depending on the installation or ambient conditions, it may be necessary to position the electronics significantly further away from the sensor 11. In this case, the lines 17 and 26 have a correspondingly long length, but plug-in connectors or similar connecting means are still provided, allowing for easy and rapid connection and disconnection. An advantage in both cases is that the electronics can be easily and quickly replaced, for example, in the event of a malfunction, without the need to disassemble the sensor for this purpose. A plug-in connector 30 is shown as the connecting means in FIGS. 2-4.

FIG. 2 shows a preferred embodiment in which the electrical circuit element 25 is a fixed resistor 31, the characteristic quantity which is uniquely related to the natural resonant frequency of the sensor being the fixed resistor 31.
31. In this case, line 26 is a double line, one of which is connected to ground and the other is connected to a reference voltage _UR via a pre-resistor 32. The electrical signal representing the value of the characteristic quantity is then an analog voltage U _K , i.e., the voltage drop across fixed resistor 31. This analog voltage U _K is supplied to an analog-to-digital converter 27 in evaluation circuit 20, where it is converted into a digital value as already described.

Therefore, when manufacturing any sensor, it is necessary to measure its natural resonant frequency in a free state and select a fixed resistor 31 whose resistance is such that, after its resistance is converted into an analog signal transmitted via line 26, this analog signal is converted into a digital value, and this digital value is added to a fixedly set base value, a digital value is obtained that represents the switching frequency, just as the value stored in memory 23 represents the sensor frequency after acquisition by counter 21. Since all this is precisely determined and known, the selection of a fixed resistor with an appropriate resistance value is not difficult for a person skilled in the art. Fixed resistors with sufficiently finely graduated resistances are commercially available. Advantageously, metal-coated film resistors are used, which are distinguished by their high precision and low temperature dependence.

2 shows that an adjustable resistor 33 can be used instead of the fixed resistor 31. This adjustable resistor 33 can be a trimmer resistor whose resistance value can be adjusted by turning a tap, or it can be a film resistor whose resistance value can be adjusted by processing the resistive layer.
These possible configurations are well known to those skilled in the art.

3 shows the use of a Zener diode 34 as another embodiment of the electrical circuit element 25. In this case, the characteristic quantity that is uniquely related to the natural resonant frequency of the sensor is the Zener voltage of the Zener diode. Zener diodes are also commercially available with Zener voltages that are graduated to a sufficiently fine range. When a Zener diode is used, the line 26 is also a double line, one of which is connected to ground and the other is connected to a pre-resistor 35.
The reference voltage U _R is connected via a Zener diode.
A voltage divider consisting of resistors 36 and 37 is provided in parallel with 34. The electrical signal representing the characteristic quantity is again an analog voltage U _K , which is converted into a signal by an analog-to-digital converter.
27. This analog voltage _UK corresponds to the Zener voltage of the Zener diode 34 reduced by the voltage divider ratio of the voltage divider 36-37. The advantage of using a Zener diode instead of a fixed resistor is that the analog voltage _UK is independent of fluctuations in the reference voltage _UR .

While in the previous embodiments the electrical signal representing the characteristic quantity was an analog signal, Fig. 4 shows an embodiment in which the electrical signal representing the natural resonant frequency transmitted via line 26 to evaluation circuit 20 is a digital signal. In this embodiment, circuit element 25 is a multiplex switch 38 with a number of switch contacts corresponding to the number of bits of the digital value to be set. In Fig. 4, switch 38 has eight switch contacts, which allows a setting of eight binary digits. In miniaturized designs, such switches are so-called dual-switches.
In-line switches are commercially available. In this case, line 26 has one dedicated conductor for each switch contact of switch 38, i.e., eight conductors in this embodiment. Each conductor is connected to reference voltage _UR via a resistor 39. For simplicity, only the resistors 39 of the first and last conductors are shown in FIG. 4. When the switch contact associated with a conductor is open, this conductor supplies the potential of reference voltage _UR . When the switch contact is closed, this conductor supplies ground potential. In this embodiment, the analog-to-digital converter 27 in evaluation circuit 20 is omitted, and the conductors of line 26 are connected directly to input 28a of adder 28. In the embodiments of FIGS. 2 and 3, this input was connected to the output of analog-to-digital converter 27. The switch contacts of switch 38 are adjusted so that the natural resonant frequency of the sensor is represented by a digital value equal to the digital value delivered as a result of conversion of analog voltage _UK by analog-to-digital converter 27. In this case, the characteristic quantity of the circuit element 25 that has a unique relationship with the natural resonance frequency of the sensor is the result of adjusting the setting of the switch contact of the switch 38.

As mentioned above, any sensor 10, each with one circuit element 25, can be combined with the same evaluation circuit 20, with the sensor-specific switching frequency being automatically set and adjusted, eliminating the need for factory tuning of the electronics to match a specific sensor.

The output signal of comparator 24 represents the undelayed sensor status signal, which indicates whether the sensor is "uncovered" or "covered." This sensor status signal is supplied to input 40a of delay circuit 40, whose output 40b is connected to final stage control circuit 50. Final stage control circuit 50 can control different final stages using the delayed sensor status signal. For example, output 50a of final stage control circuit 50 can control an AC voltage final stage providing an AC voltage signal dependent on the sensor status signal, output 50b can control a DC voltage final stage, output 50c can control a pulse frequency modulation final stage, and output 50d can control a relay final stage operating a relay dependent on the sensor status signal. An operating mode control signal supplied to control input 50e of final stage control circuit 50 determines which of these final stages is controlled.

The delay of the sensor status signal in the delay circuit 40 causes the final stage control circuit 50 to respond not immediately to a change in state of the sensor status signal, but only after the sensor status signal has maintained its state for a predetermined period of time, thereby preventing sporadic faults from triggering a switching process or indication.

A control signal supplied to the control input of delay circuit 40 via switch 41 allows the function of the device to be switched from monitoring the maximum filling level to monitoring the minimum filling level. When monitoring the maximum filling level, an indication or switching process must be triggered when a previously uncovered sensor is covered by the filling material, i.e., when the sensor frequency falls below the switching frequency. By contrast, when monitoring the minimum filling level, an indication or switching process must be triggered when a previously covered sensor is no longer covered by the filling material, i.e., when the sensor frequency exceeds the switching frequency.

5 shows an embodiment of the delay circuit 40, in which the delay time is determined by an up/down counter 42. This configuration allows the delay circuit to be constructed entirely with digital circuits that are compatible with the other digital circuits of the evaluation circuit 20. Furthermore, this embodiment makes it easy to realize asymmetric delay times, for example, when the sensor status signal is
The delay time is 1 second when the sensor status signal assumes a value corresponding to the "uncovered" state, and 0.4 seconds when the sensor status signal assumes a value corresponding to the "covered" state. Such asymmetric delay times are preferable when the fill is a liquid that may generate waves on its surface. If such waves have a period consistent with the symmetric delay times, unstable device operating characteristics may result. This type of instability is avoided by the asymmetric delay times.

The undelayed sensor status signal coming from the output of the comparator 24 is fed to an input 43a of a counter control circuit 43. The counter control circuit 43 has two further inputs
At 43b and 43c, clock pulses having two different clock frequencies are received from clock generator 44. For example, the frequency _F1 of the clock pulses supplied to input 43b is 400 Hz and the frequency _F2 of the clock pulses supplied to input 43c is 1 kHz, so that both clock frequencies differ from each other by a factor of 2.5.
The counter control circuit 43 transmits clock pulses of frequency _F1 or clock pulses of frequency _F2 from its output connected to the clock input 42a of the up/down counter 42, depending on the value of the sensor status signal applied to its input 43a.
An undelayed sensor status signal is generated at a second output connected to 42b. If this sensor status signal has a high signal value "H", which corresponds to an "uncovered" sensor state, this signal switches up/down counter 42 to count up, so that clock pulses with a low clock frequency _F1 can be transmitted to clock input 42a of up/down counter 42. If, on the other hand, the sensor status signal has a low signal value "L", this signal switches up/down counter 42 to count up.
is switched to count down, which allows clock pulses with a high clock frequency _F2 to be transmitted to the clock input 42a of the up/down counter 42.

The up/down counter 42 has two outputs 42c and 42d, which are connected to two inputs of a memory 45 and to two control inputs 43d and 43e of a counter control circuit 43. The up/down counter 42 emits a signal from the output 42c when it reaches a lower count state _Z1 during counting down, and a signal from the output 42d when it reaches an upper count state _Z2 during counting up. Advantageously, the lower count state _Z1 is the count state 0, and the upper count state _Z2 is the maximum count state, i.e., the count state 4095 in the case of a 12-bit counter, for example.

The memory 45 can be in one or the other of two states: the output 42 of the up/down counter 42
A signal supplied from output 42c of up/down counter 42 places memory 45 in one state, for example, state "0", and a signal supplied from output 42d of up/down counter 42 places memory 45 in the other state, which in this embodiment is state "1".
outputs at output 45a a signal whose value corresponds to the memory state and at output 45b a complementary signal, so that memory 45 can be implemented, for example, by a simple flip-flop.

Both outputs 45a and 45b of the memory 45 are connected to a multiplexer 46
The control input of the multiplexer 46 is connected to the switch 41.
Depending on the position of the switch 41, the multiplexer 46 transmits the signal supplied from the output 45a or the signal supplied from the output 45b to the final stage control circuit 50. In other words, the multiplexer functions as a simple change-over switch.
It is connected to the control input side 43 f of the counter control circuit 43 .

The operation of the delay circuit 40 of Fig. 5 will now be explained with reference to the diagram of Fig. 6. Initially, the switch 41 is open and the multiplexer 46 then transmits the signal from the output 45a to the final stage control circuit 50. Diagram A of Fig. 6 represents the sensor status signal provided by the comparator 24, diagram B represents the change in counting state of the up/down counter 42 between the two limit counting states _Z1 and _Z2 , and diagram C represents the output signal of the multiplexer 46 provided to the final stage control circuit 50.

In diagram A, a high signal value "H" corresponds to an "uncovered" sensor state, and a low signal value "L" corresponds to a "covered" sensor state. In diagram B, the up/down counter 42 has a count state exactly midway between the limit count states _Z1 and _Z2 at time _t0 . Because the sensor state signal has a signal "H" at time _t0 , the up/down counter 42 has switched to counting up, and therefore the count state increases at the low clock frequency _F1 .

At time _t1 , the sensor status signal changes from a signal value "H" to a signal value "L," causing the counter control circuit 43 to switch the up/down counter 42 to count down, and simultaneously supplying clock pulses having a high clock frequency _F2 to the clock input 42a. The count state of the up/down counter 42 therefore decreases at a faster rate than when counting up. Before the count state reaches the lower limit _Z1 , the sensor status signal again assumes a signal value "H" at time _t2 , so that between time _t2 and _t3 , counting up at the lower frequency _F1 resumes. At time _t3 , counting down begins again at the clock frequency _F2 , during which time the lower limit count state _Z1 is reached at time _t4 .

When the lower count state _Z1 is reached at time _t4 , the up/
Down counter 42 outputs a signal at output 42c, which is fed to memory 45. As shown by the solid lines in diagram C of FIG. 6, memory 45 already had the state "0" before time _t4 , and so this state is maintained. In contrast, as shown by the dashed lines in diagram C, memory 45 already had the state "1" before time _t4 .
, the memory 45 changes from state "1" to state "0".
and the signal at output 45a transmitted from multiplexer 46 to final control circuit 50 also changes accordingly, indicating that the sensor is now covered with filling material.

The signal delivered from the output 42c of the up/down counter 42 is also supplied to the control input 43d of the counter control circuit 43. This prevents the counter control circuit 43 from delivering counting pulses to the up/down counter 42, which would otherwise cause it to continue counting down. The up/down counter 42 therefore remains stopped until it starts counting up again at time _t5 . After this counting up and the subsequent counting down, if the lower count limit is reached again at time _t6 , the up/down counter 42 delivers a new signal from the output 42c, but this signal has no effect on the state of the memory 45, since the memory was already in state "0" before. This signal, again via the counter control circuit 43, prevents the up/down counter 42 from continuing to count down until it starts counting up again at time _t7 .

6, diagram B shows that the counting up, initiated at time _t7 , with a short interruption by the counting down, finally reaches the upper count state _Z2 at time _t8 . Therefore, at time _t8 , up/down counter 42 outputs memory 45 at output 42d to state "1".
In response to this, the output 45a
The output signal at also changes, and this signal is transmitted from the multiplexer 56 to the final control circuit 50 to indicate that the sensor is again vibrating freely, i.e., no longer covered by the filling. This signal is also supplied to the control input 43e of the counter control circuit 43 to prevent further counting up in the up/down counter 42 until it begins counting down again.

When switch 41 is closed, a signal supplied to control input 43f of the counter control circuit reverses the effect of the sensor status signal on the counting direction of up/down counter 42. That is, if the sensor status signal has a signal value "L", up/down counter 42 counts up, and if the sensor status signal has a signal value "H", up/down counter 42 counts down. Closing switch 41 causes multiplexer 46 to transmit to final stage control circuit 50 the complementary signal appearing at output 45b of memory 45 instead of the signal appearing at output 45a.

It can be seen from the diagram of FIG. 6 that the change in the sensor status signal is not transmitted immediately to the final control circuit 50, but with a delay time at least equal to the duration of the counting range between the _two limit counting states _Z1 and Z2 at the clock frequency valid for the corresponding counting direction, possibly extended by a temporary change in counting direction.
In this way, sporadic changes in the sensor condition signal are suppressed and only a sustained change over a predetermined period of time is indicated.

If the sensor 10 is partially covered with filling, the natural resonant frequency of the sensor may be very close to the switching frequency, resulting in frequent changes above and below the switching frequency even though the filling level remains substantially the same. In order to achieve a clear filling level indication in such cases, the delayed sensor state signal provided by the delay circuit 40 is fed to the control input 28d of the summer 28.
As a result, an additional digital value is added or subtracted depending on the signal value, so that the digital value representing the switching frequency is changed in such a way that a switching hysteresis is obtained.

The evaluation circuit 20 further comprises a sensor parameter monitoring circuit 51, which receives, on the one hand, a digital value representing the sensor frequency and supplied by the memory 23, and, on the other hand, a digital value supplied by the analog-to-digital converter 27. Based on these values, the sensor parameter monitoring circuit 51 monitors the sensor vibration range and characteristic variable range as well as the line between the sensor and the evaluation circuit. If one of these digital values is outside a predetermined range or if one of these values is missing, indicating a line break,
The sensor parameter monitoring circuitry sends an alarm signal to the final stage control circuitry, which forwards the signal to the selected final stage, where it indicates that an error has occurred.

The main advantage of the evaluation circuit 20 is that it is made up entirely of digital circuitry and can therefore be manufactured as an integrated component.

──────────────────────────────────────────────────── Continued from the front page (72) Inventor: Pfendler, Martin Federal Republic of Germany, 79689 Maurburg Felix-Platterweg 6 (56) References: JP 63-58715 (JP, U) JP 52-15259 (JP, U) JP 55-26431 (JP, B1) JP 2-60245 (JP, B2)

Claims (11)

[Claims] [Claim 1] A device for detecting and/or monitoring a predetermined filling level in a container, comprising a sensor, an excitation device and an evaluation circuit, the sensor being arranged in the container at a height of the filling level to be monitored so that the sensor comes into contact with the filling material when the filling level reaches the predetermined filling level, the excitation device exciting the sensor to mechanically oscillate at its natural resonant frequency, and the evaluation circuit being configured to trigger an indication process and/or a switching process depending on whether the mechanical oscillation frequency of the sensor is above or below a switching frequency, characterized in that the sensor is assigned to an electric circuit element, the circuit element having a characteristic quantity whose value has a unique relationship to the natural resonant frequency of the sensor, and the switching frequency is set and adjusted in the evaluation circuit depending on the value of the characteristic quantity of the electric circuit element. 2. The apparatus according to claim 1, wherein said circuit element is a resistor and said characteristic quantity is a resistance value of said resistor. 3. The device according to claim 1, wherein said circuit element is a Zener diode and said characteristic quantity is a Zener voltage of said Zener diode. 4. The apparatus according to claim 1, wherein said circuit element is a switch having a number of changeover contacts for setting digital values, and said characteristic quantity is the setting state of the changeover contacts of said switch. 5. The circuit element is disposed on a sensor,
5. The method according to claim 1, wherein the sensor transmits an electrical signal representing the value of the characteristic quantity of the circuit element to an evaluation circuit.
The device described in paragraph . 6. The evaluation circuit according to claim 1, further comprising a device for converting the mechanical vibration frequency of the sensor into a first digital value, a device for converting the value of the characteristic quantity of the circuit element into a second digital value, and an adder circuit which adds a fixed base value to the second digital value to form a sum representing the switching frequency.
10. The device according to claim 9, [Claim 7] The device as described in claim 6, wherein the evaluation circuit has a comparator which receives the first digital number at one input side and the second digital number at a second input side and supplies an undelayed sensor status signal at its output side, the sensor status signal taking one or the other of two signal values depending on the comparison result of both digital numbers. 8. The comparator is followed by a delay circuit which applies a predetermined delay to the non-delayed sensor status signal provided by the comparator and provides a delayed sensor status signal at the output side, which is used to trigger an indication process and/or a switching process.
The apparatus described. 9. The delay provided by the delay circuit is of a different magnitude depending on whether the undelayed sensor status signal has one signal value or the other signal value.
9. The apparatus of claim 8. 10. The device of claim 9, wherein the delay circuit is constituted by an up/down counter, the counting direction of which is controlled by the undelayed sensor signal, and the counters are driven at different clock frequencies depending on the counting direction. 11. The evaluation circuitry includes a sensor parameter monitoring circuitry, the sensor parameter monitoring circuitry comprising: a first
and a second digital value, and provides an alarm signal if one of the digital values is outside a predetermined range.