DE3304223A1 - Method and device for detecting the presence or absence of a substance at a detector or the distance between substance and detector - Google Patents

Abstract

The detector used in this method is a device (1,2) capable of oscillating, the natural oscillations of which are influenced by the substance. In a calibrating phase, after the device has been excited into oscillation for a short time, measurement values are detected during the period of the natural oscillations and permanently stored as reference values. The reference values in each case refer to a previously accurately determined reference state with respect to the distance between substance and detector or with respect to the presence or absence of the substance at the detector. During later excitations of the device (1,2) into oscillation, measurement values are compared in-phase with the reference values in order to detect the presence or absence of the substance at the detector or the distance between substance and detector via the correspondence or the differences. <IMAGE>

Description

Method and device for determining the

or absence of substance on a detector or the distance between substance and detector The invention relates to a method and a Device for determining the presence or absence of a substance on a detector or the distance between the substance and the detector, its output variable through the substance being affected.

After the installation of detectors used for monitoring substances usually a calibration of the detectors and the downstream ones have to be carried out Transmission and registration facilities existing arrangement are carried out. In the case of detectors for level measurement, for example, this calibration relates to the adjustment of the arrangement with respect to an initial value, which is an empty one Container and an end value of the measuring range that is displayed when the container is full shall be. The display is therefore to the the beginning, the end and if applicable Adapt intermediate values of the measuring range to the determinant measured variables.

For a rough comparison between the detectors and the downstream ones Transmission and registration arrangements, step switches can be used, while the fine adjustment can be made with potentiometers. A comparison however, using these means is cumbersome and time consuming. Leave adjustment errors do not always avoid each other.

There is already a method for largely automatic comparison one of a detector, a transmission link and a registration device existing arrangement known. With this method, setpoints are used for the start of the range and the end of the range is specified.

The detector output corresponding to the beginning of the range becomes a Input of a differential amplifier and a pulse train via a low-pass filter to the second Input of the differential amplifier fed. From the output signal of the low pass a system deviation signal is generated with the setpoint for the start of the range, with which the frequency or the pulse duration / pulse pause ratio of the pulse train in the sense of a reduction of the control deviation to the value zero or almost zero being affected. The values of the frequency control corresponding to the minimum control deviation or the pulse duration / pulse pause ratio are stored permanently. Thereafter is determined by the detector output signal that corresponds to the end of the measuring range, the pulse sequence corresponding to the start of the range is subtracted. The difference signal is converted into a further pulse train, which is converted into an average direct current or direct voltage level is converted which is compared with a nominal value, which corresponds to the end of the range. The output signal obtained from the comparison is used as a control deviation to influence the frequency or the pulse duration / pulse pause ratio the further pulse train is used in the sense of reducing the control deviation. If there is a control deviation of zero or almost zero, this is the pulse sequence corresponding value for the frequency or the pulse duration / pulse pause ratio also permanently saved. Through the contents of the respective permanent storage locations are the frequencies or pulse duration / pulse pause ratios during the subsequent measurements of the two pulse trains (EU-US 81 110 445).

The invention is based on the object of a method for determining the presence or absence of a substance on a detector or the distance between Develop fabric and detector in a simple, easily repeatable manner a calibration can be carried out automatically.

The object is achieved according to the invention in that as a detector a vibratory device is used whose natural vibrations by the Substance are influenced that the vibratory device in each case briefly is excited to vibrate that first in a calibration phase at predetermined Reference states with regard to the presence and absence or the distance of the substance compared to the vibratory device during the decay of the natural vibrations Measured values of the natural oscillation are saved as reference values, so that afterwards in a measuring phase in the case of brief excitations of the vibratory device during the measured values obtained after the natural oscillations decay in phase with the reference values are compared and that the result of the comparison is output or for actuation a switching element is used. The measures outlined above exist in principle, the decay characteristic of any impulse triggered To save the device in the given states in order to later adjust the decay characteristics to be used as a reference in the event of renewed impulsive excitations of the device. An advantage of the method is to be seen in the fact that specimen variations and their Influence on the evaluation circuits can be avoided. From the respective circumstances The detectors' installation-dependent influences no longer have to be taken into account or compensated, as the decay characteristic given after assembly is used as a reference.

The measured values are preferably taken during the decay time of the natural oscillation by periodic sampling of the output variable of the vibratory device generated.

In an expedient embodiment, by scanning the output of the vibratory device obtained measured values in digital Values converted into a non-volatile when measuring the reference states Memory can be entered. The reference values then decrease with each measurement Natural vibrations for comparison to determine the presence or absence of a Substance at the detector or the distance between the substance and the detector.

In a preferred embodiment, the obtained reference and samples are subtracted from each other, the absolute values of the differences summed up and to indicate the presence or absence of a substance on the detector or the distance between the fabric and the detector. The sums of the differences are for the presence or absence of the respective substance at the detector or the distance characteristic between substance and detector. Leave in a relatively easy way values are thus obtained from which the respective conditions prevailing at the detector can be recognized. For example, the sum of zero equals absence of the substance at the detector. Both the reference values and those obtained later The measured values are the same, which means that the natural vibrations are not additional be dampened by the fabric. So if the sum is zero it means that there is no additionally dampened decaying oscillation.

The sum of the absolute values of the differences is expediently compared with an evaluation factor, the the absence of the Substance is assigned to the detector.

With the weighting factor, the influence of measurement inaccuracies of the devices used for the measurement are taken into account. Small deviations in the total the differences from the value zero can be caused by such measurement inaccuracies will.

In a further preferred embodiment it is provided that successive measured values converted into absolute values and from each other Subtract that a positive difference to the factor one and a negative one Difference should be rounded to the factor zero and that the sum of the factors with the number of measured values is compared. With this measure it is additionally checked, whether the vibratory device is decaying compared to the reference vibration Performs vibrations.

With this further test, damped natural vibrations recognize despite the influence of interference.

A decaying natural oscillation .8. Is present when this condition does not apply.

The sum of the rounded differences of the absolute values is preferably used neighboring samples of the natural oscillation with the product of the number of samples and a correction factor. The weighting factor becomes a threshold introduced through which the influences of errors are eliminated. Come as error influences Noise effects or quantization errors in question.

With the measures explained above, a new calibration can be carried out at any time reach. This can be desirable, for example, for the range limits other reference states are desired after a measurement. Another reference state for the beginning of a measuring range results, for example, if after some work cycles the vibratory device with a layer adhering to it Material is covered, although it is not immersed in the fabric. This case occurs when a container after being filled with a material that exhibits adhesion, has been emptied again.

An apparatus for performing the methods described above consists according to the invention that a vibratory device is provided which emits an electrical output variable at least during the oscillation or causes a transducer to emit an electrical output variable that the vibratory device can each be excited to vibrate by a pulse is that is synchronized with clock pulses of a clock that the electrical Output variable of a sample and hold circuit synchronized with the clock pulses can be supplied, which is followed by an analog-to-digital converter, which is in the calibration phase connected to a non-volatile memory and in the measuring phase to an arithmetic unit whose other inputs are connected to the outputs of the memory in the measuring phase are.

This arrangement is simple in structure. You can connect can be used with different types of vibratory devices. Cumbersome setting and adjustment measures are not required for the calibration to set reference states.

In an expedient embodiment it is provided that the clock generator, the arithmetic unit, the sample-and-hold circuit and the analog-to-digital converter are connected to a control unit, which is connected to the oscillatable via a frequency divider Facility and via an address circuit to the memory is connected and the input-side controls for switching from the Has calibration phase to the measurement phase and sensitivity setting.

The changeover to the calibration or measurement phase requires this Arrangement only the actuation of a switch.

After the natural vibrations have subsided, the measurement result is available Disposal. The properties of the vibratory device in relation to the Damping of the natural vibrations therefore determine the duration of the measurement. For level monitoring are the time constants of the natural oscillations in the majority of cases very small compared to the time periods for a noticeable change in the fill level. Therefore, the arrangement enables a die even during the level change Measurement that accurately reflects the height at the time of actuation.

A further memory is preferably connected downstream of the first gate circuit, in which the signals emitted by the sample and hold circuit during the measurement phase can be entered, the content of the further memory successively with different Contents of the first memory corresponding to reference states can be fed to the arithmetic unit is. With this arrangement, the decay characteristics can be stored in the first memory numerous reference states can be stored. These decay characteristics correspond e.g. the different fill levels. It can, for example, the level differences of a few centimeters each be assigned a decay characteristic. With the arrangement described above is stored in the measurement phase from the Reference characteristics that determines that of the measured decay characteristic comes closest. This selected characteristic determines the actuation of the the arithmetic unit downstream devices, e.g. a relay and / or a display device.

In another advantageous embodiment it is provided that a clock generator via a frequency divider with an oscillatory device is connected, the output of which is connected to a rectifier and a low-pass filter Comparator is connected, which is in the calibration phase at increased threshold voltage connected to a memory and in the measuring phase via the blocking input of a gate circuit, whose second input is connected to the memory, with a retriggerable one Monoflop is connected, the output of which is connected to the second input via a resistor of the comparator is fed back. This arrangement is characterized by a very simple construction.

The vibratory device is preferably an electromagnetic one Rod that can be excited to vibrate and whose natural vibrations can be determined by means of a coil are.

At least part of this rod can consist of a permanent magnet or consist of a section that can be magnetized by direct current.

The vibratory device is expediently an electromagnetic one Rod that can be excited to vibrate and whose natural vibrations can be determined by a microphone or a piezoelectrically excitable rod whose natural oscillations are piezoelectrically are detectable.

In another advantageous embodiment, there is the vibratory Device consisting of a container that vibrates electromagnetically or piezoelectrically can be excited and its natural vibrations can be determined by a microphone. With this arrangement, the decaying sonic or ultrasonic vibrations a container containing the filling material is used to determine the filling level.

Further details, features and advantages emerge from the following Description of exemplary embodiments shown in the drawing.

1 shows a time diagram of an exponentially damped harmonic Natural oscillation of an oscillatable one containing two independent energy stores Facility; Fig. 2 is a block diagram of an arrangement for determining the or absence of a substance on a detector or the distance between substances and detector; Fig. 3 is a circuit diagram of a further arrangement for determining the Presence or absence of a substance on a detector of the distance between substances and detector; Fig. 4 is a block diagram of a circuit with which the circuit according to FIG. 1 is expanded.

The time course of the natural oscillation shown in FIG. 1 results for example in the case of a vibratory device that consists of a mass and a spring, at the end of which the mass is attached. The movement of the Mass is also affected by damping, which is friction and other non-reversible energy sales. One on a spring link suspended mass is suitable as a detec- gate to determine the Presence or absence of a substance at the detector, as the attenuation of the movement of the Mass is different with and without the presence of the substance. The vibration of the Mass is monitored with a device that provides an electrical output issues.

Such a device can be a microphone. The course according to FIG. 1 occurs with weak damping when the mass for example oscillates in air.

This case is used as a reference state. After the crowd for example via a piezoelectric or magnetorestrictive excitation of the spring is set in oscillation, are subsequently during the natural oscillations of the mass in air at periodic time intervals t1, t2, t3, t4 and t5 A1, A2, A3> A4 and A5 measured and used as reference values for the absence of the Substance saved from the crowd. Substance should be any liquid or Substance consisting of free-flowing particles can be understood, their level is determined by the mass. With the mass there is a limit level monitoring carried out. The reference values A1 to A5 characterize a state in which the mass is not surrounded by the substance to be monitored.

A second reference state exists when the mass in the Fabric is immersed. In this reference state, the mass is also briefly set in vibration. After the force that stimulates the vibrations has ceased the natural vibrations are at the same periodic time intervals tl, t2, t3, t4 and t5 sampled. The sampling period does not generally match the period coincide with the exponentially damped harmonic natural oscillation. The period duration must be selected in such a way that the number of measured values increases the course the decaying oscillation is determined with sufficient clarity. The at second reference state of the mass obtained measured values due to the vibrations the mass in the fabric will be smaller than the deflections A1 to A5 saved as reference values. With the saving of the presence and absence The calibration phase is the reference values of the natural oscillation obtained from the material completed.

The reference values are then used in the measurement phases. A measurement phase requires, like a calibration phase, the brief excitation of the mass. In the measuring phase, the reference values should be used to determine whether the mass is or is not surrounded by fabric. During the duration of the natural vibrations of the mass are just as in the calibration phase with the same sampling period and in phase Measured values of the deflections of the vibrations recorded. These measured values are saved with the Reference values compared. The comparison shows whether the measured values match the Reference values of the first or second reference state match. It is also possible, the measured values only with the reference values of one reference state to compare. For example, the first reference state can be used for this whose deflections A1 to A5 are shown in FIG. If they agree of the measured values with the reference deflections A1 to A5, the mass is not of substance surround. So the material does not yet have the level determined by the position of the mass reached. If there is no match, then the mass is at least partial immersed in the fabric. The substance has the same level as the crowd reached or exceeded.

The vibratory device is expediently connected to the respective Adapted to the measurement conditions.

If, for example, fill levels are to be measured continuously, becomes a resilient rod instead of a mass suspended from a spring used, which, depending on the fill level, covers the material for different lengths of time is. The rod is firmly clamped at one end or near one end and is excited to vibrate piezoelectrically, magnetostrictively or electromagnetically. The vibrations can take place longitudinally or transversely to the rod axis.

2 is a block diagram of a first arrangement for detection the presence or absence of a substance on a detector or the distance between Substance and detector shown. A vibratory device 1 contains a resilient elastic rod 2, which is immersed in the substance, not shown. By means of the rod 2, the fill level is determined over the immersion depth. The rod 2 consists, for example, of magnetostrictive material. He's from one Surrounding coil, not shown, via which it is stimulated to vibrate with pulses will. It also has a receiver coil, not shown, in which the Vibrations of the rod 2 electrical signals are induced. The coil for excitation the vibrations of the device 1 is connected to the output of a frequency divider 3, which is connected to a control unit 4. The control unit 4 is controlled by a clock 5, which generates a constant high-frequency clock oscillation.

The output of the receiver coil of the vibratory device 1 is connected to a sampling and holding device 6, which is controlled by the control unit 4 is controlled. The sample and hold circuit 6 includes an analog-to-digital converter 7 connected, its control inputs as well to the control unit 4 are laid. The outputs of the analog-to-digital converter 7 are connected to inputs of a first gate circuit 8 and a second gate circuit 9 placed. Both gates 8 and 9 are actuated by the control unit 4.

The gate circuit 8 is followed by a non-volatile memory 10, whose address inputs are connected to an addressing circuit 11, whose The content is influenced by the control unit 4. The outputs of the gate circuit 9 and of the memory 10 are each connected to the inputs of an arithmetic unit 12, the output of which is connected to a relay 14 via an exclusive-OR circuit 13.

The second input of the circuit 13 containing two inputs is placed on a switch 15, the switching of the relay 14 from the operating current mode permitted in closed-circuit operation. The control unit 4 has inputs to three more Setting switches 16, 17, 18 connected. With the switches 16 and 17 achieve different response sensitivities. The switch 18 is used for instruction the storage of the measured values in the memory 10 and is thus only during the Reference phase actuated. The frequency divider 3, the control unit 4, the addressing circuit 11, the gate circuits 8, 9, 13 and the arithmetic unit 12 are preferably through a Microcomputer realized.

If at a reference state the decay characteristics of the natural vibrations the vibratory device is to be detected, the switch 18 is actuated. The control unit 4 forwards the pulses from the clock generator 5 to the frequency divider 3, which, according to its divider ratio, generates a pulse to excite the vibratory Device 1 generated. Via this, the rod 2 is exemplified way set in transverse vibrations. The decaying natural vibrations of the rod 2 are input to the sample and hold circuit 6 at the clock frequency and digitized via the analog-to-digital converter 7. Locks during the calibration phase the control unit 4, the arithmetic unit 12 and the gate circuit 9, while the gate circuit 8 is opened. At the same time, the addressing circuit 11 is activated by the control unit 4 continuously switched to the recording of the output from the analog-to-digital converter Control reference values in successive storage locations of the memory 10. The reference values contained in the memory 10 are available for later measurements.

The storage of reference values can be for different reference states be repeated.

After the switch 18 has been opened, the one shown in FIG. 2 is available Arrangement for the measurement of states related to the presence or absence of a Fabric available. The gate circuit 8 is blocked during the measuring phase. That Control unit 4 actuates the addressing circuit 11, opens the gate circuit 9 and controls the arithmetic unit 12. The rod 2 is in the same way as in the calibration phase to Vibrating excited. The decaying natural vibrations are also as in the Calibration phase measured and digitized. Get in contrast to the calibration phase the digital measured values not to the memory 10, but to the arithmetic unit 12. The im Reference values contained in memory 10 are in phase with respect to the natural oscillation of the rod 2 with the measured values passed via the gate circuit 9 to the arithmetic unit 12 supplied. The arithmetic unit 12 compares the measurement fed to its inputs and reference values in the following ways.

First of all, the respectively scanned measured value is assigned by the stored reference value is subtracted. the Difference will be thereafter converted to an absolute value.

The absolute values of the differences are added up. The sum is compared with a reference value. The relationship applies to the sum of the absolute values of the differences between measured values and reference sample values URk I sample value of the reference signal UM - sample value dou measurement signal where URk denotes the reference sample values and UMk denotes the sample values of the measurement signal. If the arithmetic unit 12 determines the value 0, then reference values and measured values agree. The rod 2 accordingly has the same natural oscillation characteristics as was the case with the reference signal storage. This means that the rod 2 is not immersed in the fabric if, as explained above, the reference values were measured with the rod uncovered.

Due to measurement errors in the output signal of the vibratory device and the electrode connected to it, the reference values and the measured values can differ slightly from one another, although the natural vibrations are the same. In order not to obtain an incorrect result in this case, the sum of the reference samples is multiplied by a weighting factor L1 which lies between the values zero and one. The following comparison is made to determine the correspondence between the measured and reference samples: L1 to 0 1 There is agreement between the reference samples and the measurement samples if the sum is less than the product of the weighting factor and the sum of the maturity samples. The response sensitivity can be set in a simple manner with L1.

The rod 2 is immersed in a container that is exposed to vibrations is, then the condition explained above can be met even though the rod 2 performs more strongly damped natural vibrations by dipping into the material.

Such vibrations can be generated, for example, by vibrating sieves will. In order to prevent a wrong result in this case, the Arithmetic unit 12 carried out a further test. The test is that two consecutive measurement samples, namely the Kte and the K + 1st sample for all K from 1 to n are subtracted from one another as absolute values.

The difference in the absolute values of successive measurement samples is rounded up, namely to the value 1 if the difference is greater than zero and to the value 0 if the difference is less than zero. These difference values are added up and compared with a reference value. If the sum corresponds to the number of samples, then there is a damped oscillation. The following relationship applies: whereby a Ixl p1 feeds X »o 10 fW xO is. The measured samples are denoted by UMk and the difference between the absolute values is denoted by -1.

In order to eliminate errors caused by noise effects such as quantization errors, a correction factor L2 is introduced. It must then be the condition be fulfilled if there is no natural oscillation.

If the latter condition is also not met, then the output signal is of the arithmetic unit 12 of the exclusive-OR circuit 13 is supplied. If there is a match the output signal is zero or approximately between measurement and reference sample values zero. The relay 14 is therefore not actuated.

If the output signal exceeds the threshold then the rod 2 dips into the fabric. A corresponding output signal is passed on via the exclusive-OR circuit 13 to the relay 14 and stimulates it. An optical or acoustic message can be generated via the relay 14.

Analogous to this, the arrangement shown in FIG. 2 for the determination the presence or absence of a substance on rod 2 can be used, the decay characteristic the natural oscillation for the rod 2 covered with fabric is stored and with the measured values is compared.

By means of an addition, as shown in FIG. 4, the arrangement can 2 can also be used for the continuous measurement of the level. Identical elements in FIGS. 2 and 4 are provided with the same reference symbols. The gate circuit 9 is not direct according to FIG. 4, but via a further, non-volatile one Memory 19 is connected to the inputs of the arithmetic unit 12.

Both memories 10 and 19 are controlled via the addressing circuit 11.

In the calibration phase, the memory 10 is used for numerous reference states the respective decay characteristics of the natural vibrations of the rod 2 are entered.

For example, fill levels in a container are used as reference states entered, which differ from each other by a certain amount. Depending on the desired accuracy, for example, fill level heights are recorded, which differ from each other by 1 cm, 10 cm, 20 cm, etc. In the measuring phase the decay characteristic of the natural oscillations is initially stored in the memory 19 entered, its capacity in contrast to memory 10 only for the samples a decay characteristic must be measured. The decay characteristics contained in memory 10 are compared with the decay characteristic of the memory 19 up to the above criteria that have been explained. The determined reference state The corresponding fill level is shown on a digital or analog display.

In the arrangement shown in Fig. 3, the vibratory Device 1, 2 on the input side with a frequency divider 22 to a clock generator 23 connected, which generates clock pulses with a constant frequency. The clock generator 23 is connected to an address counter 24 which controls a memory 25. To the Output of the vibration capable device 1 is a rectifier 26 connected, which is connected to a low-pass filter 41, which has an input a comparator 27 is in communication.

The output of the comparator 27 is connected to an input of an AND gate 28 and applied to an input of a gate circuit 29. The AND gate 28 is the input of the memory 25 connected downstream. The gate circuit 29 feeds the blocking input of a Gate circuit 30, the second input of which is connected to the output of memory 25 is. The output of the gate circuit 30 is connected to a retriggerable monoflop 31, on the one hand with the input of an Ex-Or member 32 and on the other hand via a Resistor 33 is connected to the second input of comparator 27. The second The input of the Ex-Or element 32 is connected to a switch 34. The exit of the Ex-Or link is in communication with a relay 35.

A switch 36, which is closed in the calibration phase, is on with one input of the AND gate 28, the blocking input of the gate circuit 29 and the Center tap of a voltage divider consisting of two resistors 37,38 in Link. The resistor 37 is still connected to the second input of the comparator 27 connected. The resistor 38 is connected to the second connection to ground. Of the The second input of the comparator 27 is connected to ground via a further resistor 39 and applied via a further resistor 40 to positive potential.

The clock generator 23 controls the excitation via the frequency divider 22 of rod 2. When switch 36 is closed, each pulse becomes different Memory cell of the memory 25 addressed. The decaying natural oscillation of the rod is rectified, screened and is available at the first input of the comparator 27. When switch 36 is activated, the second input receives a higher threshold lens tension than supplied in the measuring phase. When the signal of the rectified oscillation falls below the threshold voltage prevailing at the second input of the comparator, the comparator 27 changes its output signal.

In the calibration phase, the AND gate 28 is permeable, i.e. there are In the cycle of the pulses of the clock generator 23 binary ones in the memory 25 for as long entered until the output signal of the comparator 27 changes. Then be the remaining memory cells are filled with binary zeros.

In the measuring phase, the AND gate 28 is blocked when the switch 36 is open and the gate circuit 29 permeable. The threshold voltage at the second input of the Comparator 27 is less. The difference between the threshold voltages in the calibration and the measurement phase is decisive for the response sensitivity. Via the gate circuit 30, the signals output by the comparator 27 are compared with the output signals of the Memory 25 compared. When the output of the comparator 27 is ahead of the output of the memory 25 becomes zero, then the monoflop 31 receives a trigger pulse. That Monoflop actuates the relay 35 via the Ex-Or element 32 and increases the resistance 33 the comparator threshold to introduce a hysteresis.

The natural oscillation characteristics can also be determined with the measures described above for those cases where the substance is at a certain distance from the Detector is arranged. The prerequisite for this, however, is that the damping at this distance is different from the other distances. Is particularly suitable for this purpose a container which is different at different fill levels Shows decay characteristics. With only small dimensions of the vibratory This facility is more suitable for establishing presence or absence of the substance.

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Claims (19)

Method and device for determining the presence or absence of a substance on a detector or the distance between the substance and the detector Ver Method of determining the presence or absence of a substance on a detector or the distance between the fabric and the detector. its output size through the substance is influenced, characterized in that the detector is an oscillatable Device (1,2) is used, whose natural vibrations influenced by the substance that the vibratory device (1,2) each momentarily vibrate it is suggested that first in a calibration phase with predetermined reference states regarding the presence or absence or the distance of the substance from the vibratory Device (1,2) during the decay of the natural vibrations measured values (A1, A2, A3, A4, A5) A of the natural vibrations are stored as reference values that afterwards in a measuring phase with brief excitations of the vibratory device (1,2) Measured values obtained during the decay of the natural oscillations in phase are compared with the reference values and that the result of the comparison is output or is used to operate a switching element. 2. The method according to claim 1, characterized in that the measured values during the decay time of the natural oscillations through periodic sampling of the output variable the vibratory device (1,2) are generated. 3. The method according to claim 1 or 2, characterized in that the obtained by sampling the output of the vibratory device (1,2) Measured values are converted into digital values when measuring the reference states can be entered into a non-volatile memory (10). 4. The method according to claim 1 or one of the following claims, characterized characterized in that the decaying natural vibrations with the same phase positions obtained reference and sample values are each subtracted from one another, where the absolute values of the differences are summed up and used to detect the presence or absence of a substance on the detector or the distance between the substance and the detector. 5. The method according to claim 1 or one of the following claims, characterized characterized in that the sum of the absolute values of the differences with the product of the sum of the reference samples and a weighting factor is compared, which is assigned to the absence of the substance on the detector (1,2). 6. The method according to claim 1 or one of the following claims, d a d u r c h g e k e n n n z e i c h n e t that in each case successive measured values converted into absolute values and subtracted from each other that a positive one Difference or difference 0 equals one and a negative difference equals the value receives zero and that the sum of these values is compared with the number of readings will. 7. The method according to claim 6, characterized in that the after Claim 6 obtained sum with the product of the number of samples and a correction factor is compared. 8. Apparatus for performing the method according to claim 1 or one of the following claims, characterized in that an oscillatable Device (1,2) is provided which at least during the oscillation an electrical Output variable or a transducer for outputting an electrical output variable causes the vibratory device (1,2) each by a pulse can be excited to vibrate, which synchronizes with clock pulses of a clock generator (5) is that the electrical output is synchronized with the clock pulses Sample and hold circuit (6) can be supplied to which an analog-digital converter (7) downstream is the one in the calibration phase with a non-volatile memory (10) and is connected in the measuring phase with an arithmetic unit (12), the other Inputs in the measuring phase are connected to the outputs of the memory (10). 9. Apparatus according to claim 8, characterized in that the clock generator (5), the arithmetic unit (12), the sample and hold circuit (6) and the analog-digital converter (7) are connected to a control unit (4), which via a frequency divider (3) the vibratory device (1,2) and via an addressing circuit (11) the memory (10) is connected and the input side is an operating element (18) for triggering the calibration phase and operating elements (16, 17) for setting the sensitivity having. 10. Apparatus according to claim 8 or 9, characterized in that two gate circuits between the analog-digital converter (7) and the arithmetic unit (12) (8,9) are arranged, each via jointly addressable memories (10,19) to the Inputs of the arithmetic unit (12) are connected. 11. Apparatus for performing the method according to claim 1, characterized characterized in that a clock generator (23) via a frequency divider (22) with a vibratory device (1,2) is connected, the output of which via a rectifier (26) and a low-pass filter (41) is connected to a comparator (27) which is in the Calibration phase connected to a memory (25) when the threshold voltage is increased and in the measuring phase via the blocking input of a gate circuit (30), the second of which The input is connected to the output of the memory (25), with a retriggerable Monoflop (31) is connected, the output of which is connected to the second via a resistor (33) Input of the comparator (27) is fed back. 12. The device according to claim 11, characterized in that the Clock generator (23) is connected to the memory (25) via an address counter (24). 13. Apparatus according to claim 11 or 12, characterized in that that the memory (25) via an AND element (28) to the output of the comparator (27) and is connected to a switch (36) closed in the calibration phase, which is applied to the blocking input of a gate circuit (29), the second input of which is connected to the output of the comparator (27) and its output to the blocking input the gate circuit (3Q) is connected, which is connected upstream of the monoflop (31). 14. The device according to claim 11 or one of the following claims, characterized in that the monoflop (31) via an Ex-Or element (32) whose second input is applied to a switch (34), connected to a relay (35) is. 15. Device according to claim 7 or one of the following claims, characterized in that the vibratory device (1) is an electromagnetic is excitable to vibrate rod, whose natural vibrations can be determined via a coil are. 16. The device according to claim 7 or one of claims 8 to 14, characterized in that the vibratory device (1) is an electrical magnetic is excitable to vibrate rod, whose natural vibrations can be determined by a microphone are. 17. The device according to claim 7 or one of claims 8 to 14, characterized in that the vibratory device consists of a container exists, which can be excited electromagnetically or piezoelectrically to vibrate and whose natural vibrations can be determined by a microphone. 18. The device according to claim 7 or one of claims 8 to 14, characterized in that the vibratory device (1) consists of a rod exists, which can be excited magnetostrictively to vibrate. 19. The device according to claim 7 or one of claims 8 to 14, characterized in that the vibratory device (1) consists of a rod exists, which can be excited piezoelectrically to vibrate.