Vibration sensor with self-test function

Abstract

The invention relates to a method for operating a vibration sensor (10), wherein the vibration sensor (10) has a mechanical oscillator (18) and a drive unit (16), and the drive unit (16) causes the mechanical oscillator (18) to oscillate, wherein the vibration sensor (10) has a control and evaluation unit (14), and the control and evaluation unit (14) regulates the drive unit (16) such that the mechanical oscillator (18) is driven in resonance, wherein a main signal path (I) is formed which extends from the control and drive unit (14) via a transmission chain (24) to the drive unit (16) and from the drive unit (16) via a reception chain (26) to the control and evaluation unit (14), wherein the transmission chain (24) and / or reception chain (26) is tested. The invention further relates to a vibration sensor (10).

Description

The invention relates to a method for operating a vibration sensor according to claim 1 and to a vibration sensor according to claim 6.Vibration sensors are generally known from the prior art. As standard, such a vibration sensor has a membrane which can be excited to produce a vibration via a drive and by means of which a mechanical vibrator arranged on the membrane can be excited to produce a vibration. A piezoelectric drive or an electromagnetic drive can be used as the drive, for example. Depending on a degree of coverage of the mechanical oscillator with a filling material and depending on the viscosity of the filling material, the mechanical oscillator oscillates with a characteristic resonant frequency, which can be detected by the vibration sensor and converted into a measurement signal.A typical application for a vibration sensor is the detection of a limit level, i.e. a predefined fill level, in a process container, such as a process tank, a storage tank, a silo or a pipeline in the process industry. Limit level sensors are often used as so-called limit switches, i.e. for determining whether a filling medium exceeds or falls below a certain filling level, the so-called limit level. Usually, when a defined limit level is reached, a switching command is generated. A switching command of the limit level sensor can, for example, start or stop filling devices or emptying devices in order to correspondingly avoid an overflow or idling of the respective process container. Limit level sensors can be used for different liquids, as well as granulated and powdery bulk materials.Proper functioning of a vibration sensor and in particular of a limit level sensor is particularly important in particular with regard to these switching commands. If the vibration sensor does not function properly or emits an incorrect signal, a process container may run dry or overflow, and thus considerable damage, even to other installations. Therefore, vibration sensors used as limit level sensors are often subject to stringent safety requirements.The underlying object of the invention is to provide a method for operating a vibration sensor and a vibration sensor, by means of which the operating reliability of a vibration sensor is increased.The invention relates to a method for operating a vibration sensor. The vibration sensor has a mechanical vibrator and a drive unit, wherein the drive unit vibrates the mechanical vibrator. The mechanical oscillator is in particular a single-rod oscillator connected to a membrane or an oscillating fork connected to a membrane, in particular having two prongs. A piezoelectric drive or an electromagnetic drive can be provided as the drive unit in a known manner.The vibration sensor has a control and evaluation unit, wherein the control and evaluation unit controls the drive unit such that the mechanical oscillator is driven in resonance. The vibration sensor has a control loop such that the mechanical vibrator is driven at a resonance frequency and the resulting vibration characteristics are detected.The vibration sensor has a main signal path which extends from the control and drive unit via a transmission chain to the drive unit and from the drive unit via a reception chain to the control and evaluation unit.The transmission chain serves for conditioning and, if appropriate, amplifying the excitation signal for the drive unit generated by the control and evaluation unit. The transmission battery can have, in particular, one or more of the following electronic components: a signal generator, a comparator, an integrator, a driver stage (in particular in the case of electromagnetic drive), a microcontroller and / or an analog resonant circuit. For example, a square-wave signal can be generated with the signal generator, which can then be converted by means of the integrator into a sinusoidal signal.The receiving chain serves for conditioning and amplifying the vibration signal of the mechanical vibrator detected by the drive unit. The reception chain has in particular at least one of the following electronic components: a filter stage for filtering any harmonics of the measurement signal, an amplifier stage, a comparator for converting a received sine signal into a square-wave signal, a DDS module, a VC oscillator and / or digital potentiometer which is set to the parameters of the corresponding mechanical oscillator.The transmission chain and / or reception chain are tested, i.e. it is checked whether the transmission chain and / or the reception chain have functioning properly. According to the method, in particular a functional test of only the electronics is carried out. In particular, the electronics are checked independently of the mechanical components, since only the electronic part of the resonant circuit or control circuit is tested with the transmission chain and the reception chain. In particular, the transmission chain can be checked individually and / or the reception chain can be checked individually. Alternatively or additionally, there is the possibility of checking the transmission and reception chain together. In particular, the test is intended to exclude the presence of a short circuit or a line break. Furthermore, integrated circuits can be present in the transmission chain and / or the reception chain, which can thus be checked for correct functioning. Possible fault scenarios here are, for example, that the output of a component of an integrated circuit remains in saturation, both in the upper and in the lower region, that the output begins to oscillate, that the gain changes in amplifiers or that the input resistances change.The possibility of testing the transmission and / or reception chain, i.e. in particular the electronic components, provides a self-test for the vibration sensor, so that the functionality of the vibration sensor can be checked regularly and in particular without removing the vibration sensor from ongoing operation or removing it and disassembling it.In order to check the transmission chain and the reception chain, a transmission and reception signal path is in particular formed in such a way that the drive unit is located outside the transmission-reception signal path. In other words, the drive unit is bridged. The signal generated by the control and evaluation unit is conducted via the transmit-receive signal path without passing through the drive unit. This offers the advantage that the influence of the drive unit on the excitation or measurement signal is excluded.In a practical embodiment, a transmit chain signal path is formed for testing the transmit chain, wherein the drive unit and the receive chain are arranged outside the transmit chain signal path. The transmission chain signal path extends in particular from the control and evaluation unit via the transmission chain directly back to the control and evaluation unit. This makes it possible to test only the transmission chain. This is then relevant in particular in order to further limit a detected error.In a further practical embodiment, a receive chain signal path is formed for testing only the receive chain, wherein the drive unit and the transmit chain are arranged outside the receive chain signal path. The reception chain signal path extends in particular directly from the control and evaluation unit via the reception chain to the control and evaluation unit. Analogous to the separate checking of the transmission chain, the separate checking of the reception chain makes it possible to locate a possible error more precisely.In particular, in order to check the transmission chain and / or the reception chain, a test signal can be transmitted via the main signal path, wherein the test signal is received and analyzed by the control and evaluation unit. For this purpose, the evaluation electronics are in particular in a test state during the self-test. The test signal is in particular only for the self-test as a signal. The test signal is particularly parameterizable and can be adjusted in its frequency and / or amplitude in order to signal, for example, a covered or uncovered state. In particular, the test signal passes through the main signal path and the control and evaluation unit checks whether the corresponding test signal exhibits a corresponding anticipated behavior. For example, the test signal changes when passing through the reception chain if filter and amplifier circuits are installed there. The transfer function of the receive chain is known and can also be calibrated. Thus, the transmitted signal can be counted back. The defined amplitude and frequency of the transmitted test signal and the known transfer function of the reception chain therefore result in a predictable signal which results after passing through the reception chain. If these predictable and actually measured signals match, a functioning circuit can be deduced. If it does not match, an error can be assumed.The test signal is modulated onto the excitation signal in particular by the control and evaluation unit. The modulated signal differs from the excitation signal in frequency and amplitude. In particular, the amplitude of the test signal is small in relation to the excitation signal in order to influence the measurement as little as possible. The frequency of the test signal is higher than the frequency of the excitation signal by a factor of two to 3. An advantage of a modulated test signal which runs through the main signal path is that the checking of the electronic components can be carried out while the vibration sensor is operating.The test signal is alternatively an invalid signal, i.e. a signal which lies outside the defined parameters. Such a test signal can have, for example, a frequency outside the permitted measurement range. Likewise, the test signal can have strong frequency jumps, which are then detected as invalid. An invalid test signal would be processed in particular by the reception chain, so that a malfunction of the reception chain could be detected therewith.The testing of the electronic components and in particular of the transmission and / or reception chain takes place in particular regularly after respectively defined time intervals in order to ensure proper operation of the vibration sensor. It can be provided that in the case of certain irregularities, such as measurement deviations, a check is carried out on the occasion.The invention also relates to a vibration sensor having a mechanical vibrator and a drive unit for vibrating the mechanical vibrator. The vibration sensor further comprises a control and evaluation unit, wherein the control and evaluation unit controls the drive unit such that the mechanical oscillator is driven in resonance. For further details on the vibration sensor, reference is made to the above description.The vibration sensor has a main signal path which extends from the control and evaluation unit via a transmission chain to the drive unit and from the drive unit via a reception chain to the control and evaluation unit, wherein the vibration sensor has means for checking the transmission chain and / or reception chain.As already described above, these means for checking the transmission chain and / or the reception chain enable a simple self-test of the vibration sensor, which thus increases the operational reliability of the vibration sensor.In a practical embodiment, the vibration sensor has at least one switch, wherein the switch can be switched back and forth between a first position and at least one second position. The switch is in particular an electrical switch. The at least one switch and, connected thereto, the signal paths made possible therewith can be regarded as a means for testing the transmission chain and / or reception chain.The at least one switch connects the transmission chain to the drive unit in the main signal path, in particular in a first position. In a second position, the switch connects the transmission chain to a transmission chain signal path, wherein the drive unit and the reception chain are arranged outside the transmission chain signal path. In other words, the transmission chain signal path leads from the control and evaluation unit to the transmission chain and directly back to the transmission and evaluation unit. A simple switch can be used to check the transmission chain independently of the drive unit and the reception chain.At least one switch connects the receiving chain to the drive unit in particular in a first position. In a second position, the switch connects the receive chain to a receive chain signal path, wherein the transmit chain and the drive unit are arranged outside the receive chain signal path. The receive chain signal path leads in particular from the control and evaluation unit to the receive chain and directly back to the transmit and evaluation unit. A simple switch can be used to check the reception chain independently of the drive unit and the reception chain.Preferably, two switches are provided, one for the transmission chain and one for the reception chain, wherein the two switches can be switched independently of one another.In a practical embodiment, two switches are provided, wherein in a first position the transmission chain with the drive unit and the reception chain with the drive unit are connected to one another via the main path (=configured in the standard operation of the vibration sensor). In each case in a second position of the switch, the transmission chain and the reception chain are connected to one another via a transmission-reception signal path, wherein the drive unit is arranged outside the transmission-reception signal path. The drive unit is then bridged in order to be able to test the transmission chain and reception chain separately.The described two switches each have in particular at least three positions, wherein in the third position of the first switch the transmission chain is arranged in the transmission chain signal path and in the third position of the second switch the reception chain is arranged in the reception chain signal path. This third position allows the transmission chain to be incorporated either in the main signal path, in the transmit-receive signal path or in the transmission chain signal path. The same applies to the reception chain. Depending on the position of the respective switch, different units can be tested.In a further practical embodiment of the vibration sensor, a substitute component for simulating the drive unit is arranged in the transmission-reception signal path. This is advantageous in particular when the drive unit is bridged, but the optimum behavior of the drive unit is to be simulated in the transmit-receive signal path. A capacitor is particularly suitable as a replacement component. In order to be able to represent additional parasitic effects as well, in particular further resistors and capacitances can be used. In order to vary the capacitance and to achieve a greater test depth, a capacitance diode can in particular additionally be used.Further practical embodiments and advantages are described in connection with the figures. The following are shown: FIG. 1 shows a vibration sensor in a schematic illustration, FIG. 2 shows a vibration sensor according to a first embodiment with a transmission chain and a reception chain in a schematic illustration, and FIG. 3 shows a vibration sensor according to a second embodiment with a transmission chain and a reception chain in a schematic illustration.A vibration sensor 10 is shown in FIG. 1. The vibration sensor 10 has a housing 12 with a control and evaluation unit 14 arranged therein and a drive unit 16 (not recognizable in FIG. 1, cf. FIG. 2 ).The vibration sensor 10 further comprises a mechanical vibrator 18, which is designed here as a vibration fork with a first tine 20 and a second tine 22. The mechanical oscillator 18 is excited to oscillate by the drive unit 16, so that it oscillates at a resonant frequency.When the degree of coverage of the mechanical vibrator 18 changes, the resonant frequency at which it vibrates changes, whereby the change in coverage can be detected.Such a vibration sensor 10 is frequently used as a limit switch, which is subject to particularly high safety requirements.FIG. 2 shows a first embodiment of a vibration sensor 10 having a control and evaluation unit 14 and a drive unit 16.By means of the control and evaluation unit 14, an excitation signal is generated, which is conducted via a transmission chain 24 to the drive unit 16. The transmission chain 24 has a plurality of electronic components, which are represented here in simplified form by a box.The measurement signal generated by the drive unit 16 is conducted via a receiving chain 26 (likewise represented in simplified form as a box) to the control and evaluation unit 14 and analyzed there.The signal path from the control and evaluation unit 14 via the transmission chain 24, the drive unit 16 and the reception chain 26 up to the control and evaluation unit 14 is referred to here as the main signal path I. The excitation signal and measurement signal relevant for the measurement are transmitted via the main signal path I.The vibration sensor 10 also has a first switch 28, which is arranged here downstream of the transmission chain 24 in the signal direction. The switch 28 is arranged between the transmission chain 24 and the drive unit 16. The switch 28 has two switching positions. In the first switching position, which is not shown in FIG. 2, the switch 28 connects the transmission chain 24 to the excitation unit 16 according to the main path I.In the second switching position shown, the switch 28 connects the transmission chain 24 to a transmission chain signal path II. The transmission chain signal path II extends from the control and evaluation unit 14 to the transmission chain 24 and back again to the control and evaluation unit 14. In this second switching position of the switch 28, it is possible to test the transmission chain 28 separately from the drive unit 16 and the reception chain 26.The vibration sensor 10 also has a second switch 30, which is arranged here in front of the receiving chain 26 in the signal direction of the main signal path. The switch 30 is disposed between the drive unit 16 and the receiving chain 26. The switch 30 has two switching positions. In the first switching position, which is not shown in FIG. 2, the switch 30 connects the receiving chain 26 to the excitation unit 16 according to the main path I.In the second switching position shown, the switch 30 connects the receiving chain 26 to a receiving chain signal path III. The receiving chain signal path III extends from the control and evaluation unit 14 to the receiving chain 26 and back again to the control and evaluation unit 14. In this second switching position of the switch 30, it is possible to test the receiving chain 26 separately from the drive unit 16 and the transmitting chain 24.FIG. 3 shows a second embodiment of a vibration sensor 10. In the following, the same reference numerals are used for identical or at least functionally identical components as for the description of the first embodiment in FIG. 2.The second embodiment differs substantially from the first embodiment in that the vibration sensor has a transmit-receive signal path IV in addition to the main signal path I, the transmit chain signal path II and the receive chain signal path II.The vibration sensor 10 has a first switch 28 and a second switch 30, wherein the two switches 28 and 30 each have three switching positions.In a first switching position (not shown here) of the switches 28 and 30, the transmission chain 24 and the reception chain 26 are connected to the drive unit 16 according to the main signal path IIn a second switching position (likewise not shown) of the switches 28 and 30, the transmission chain 24 (analogously to FIG. 2 ) is incorporated into the transmission chain signal path II and is not connected to the drive unit 16 and the reception chain 26. The reception chain 26 is incorporated (analogously to FIG. 2 ) in the reception chain signal path III and is not connected to the drive unit 16 and the transmission chain 24.FIG. 3 shows the third switching position, wherein the transmission chain 24 is connected to the reception chain 16 via the transmission-reception signal path IV. The transmission-reception signal path IV leads from the control and evaluation unit 14 via the transmission chain 24, via the reception chain 16, to the control and evaluation unit 14. In this third switching position of the first switch 28 and of the second switch 30, the transmission chain 24 and the reception chain 26 can be tested separately without a drive unit 16.List of reference characters10 Vibration sensor 12 Housing 14 Control and evaluation unit 16 Drive unit 18 Mechanical oscillator / oscillator fork 20 First prongs 22 Second prongs 24 Transmission chain 26 Reception chain 28 First switch 30 Second switch I Main signal path II Transmission chain signal path III Reception chain signal path IV Transmission-reception signal path

Claims

Method for operating a vibration sensor (10), wherein the vibration sensor (10) has a mechanical vibrator (18) and a drive unit (16), and the drive unit (16) oscillates the mechanical vibrator (18), wherein the vibration sensor (10) has a control and evaluation unit (14), and the control and evaluation unit (14) controls the drive unit (16) in such a way that the mechanical vibrator (18) is driven in resonance, wherein a main signal path (I) is formed, which extends from the control and drive unit (14) via a transmission chain (24) to the drive unit (16) and from the drive unit (16) via a reception chain (26) to the control and evaluation unit (14), characterized in that the transmission chain (24) and / or reception chain (26) is tested.Method according to the preceding claim, characterized in that, for testing the transmission chain (24) and the reception chain (26), a transmission-reception signal path (IV) is formed in such a way that the drive unit (16) is located outside the transmission-reception signal path (IV).Method according to one of the preceding claims, characterized in that a transmission chain signal path (II) is formed for testing the transmission chain (24), wherein the drive unit (16) and the reception chain (26) are arranged outside the transmission chain signal path (II).Method according to one of the preceding claims, characterized in that a reception chain signal path (III) is formed for testing the reception chain (26), wherein the drive unit (16) and the transmission chain (24) are arranged outside the reception chain signal path (III).Method according to one of the preceding claims, characterized in that, for testing the transmission chain (24) and / or the reception chain (26), a test signal is transmitted via the main signal path (I) and the received test signal is analyzed by the control and evaluation unit (14).Vibration sensor having a mechanical oscillator (18) and a drive unit (14) for setting the mechanical oscillator (18) into oscillations, having a control and evaluation unit (14), the control and evaluation unit (14) regulating the drive unit (16) in such a way that the mechanical oscillator (18) is driven in resonance, and having a main signal path (I) which extends from the control and evaluation unit (14) via a transmission chain (24) to the drive unit (16) and from the drive unit (16) via a reception chain (26) to the control and evaluation unit (14), characterized in that the vibration sensor (10) has means for testing the transmission chain (24) and / or the reception chain (26).Vibration sensor according to the preceding claim, characterized in that the vibration sensor (10) has at least one switch (28, 30), wherein the switch (28, 30) can be switched back and forth between a first position and at least one second position, a. wherein the at least one switch (28), in a first position, connects the transmission chain (24) to the drive unit (16) according to the main signal path (I) and wherein, in a second position, the switch (28) connects the transmission chain (24) to a transmission chain signal path (II), wherein the reception chain (26) and the drive unit (16) are arranged outside the transmission chain signal path (II); b. wherein the at least one switch (30) connects the receive chain (26) to the drive unit (16) according to the main signal path (I) in a first position and wherein the switch (30) connects the receive chain (26) to a receive chain signal path (III) in a second position, wherein the transmit chain (24) and the drive unit (16) are arranged outside the receive chain signal path (III).Vibration sensor according to one of the preceding claims, characterized in that two switches (28, 30) are provided, wherein in each case, in a first switching position of the switches (28, 30), the transmission chain (24) is connected to the drive unit (16) and the reception chain (26) is connected to the drive unit (16) via the main signal path (I), and wherein in each case, in a second position of the switches (28, 30), the transmission chain (24) and the reception chain (26) are connected to one another via a transmission-reception signal path (IV), wherein the drive unit (16) is arranged outside the transmission-reception signal path (IV).Vibration sensor according to the preceding claim, characterized in that the two switches (28, 30) each have at least three switching positions, wherein in the third switching position of the first switch (28) the transmission chain (24) is arranged in the transmission chain signal path (II) and in the third switching position of the second switch (30) the reception chain (26) is arranged in the reception chain signal path (III).Vibration sensor according to one of the two preceding claims, characterized in that a replacement component for simulating the drive unit (16) is arranged in the transmission-reception signal path (IV).

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