Method for operating a magnetic-inductive flowmeter

Abstract

The invention relates to a method for operating a magnetic-inductive flowmeter (1), comprising the following method steps: - applying an operating signal (11), wherein the operating signal (11) has a first time segment (I) having a first partial time segment (i) and a second partial time segment (ii), wherein in the first partial time segment (i) a signal value S +,1 is applied in each case for a time duration t 1 , l <sb / >and in the second partial time segment (ii) a signal value S −,1 is applied for a time duration t 2,l <sb / >, wherein the time duration t 1 , l is longer than the time duration t 2 , l , wherein a magnetic field polarity during the first time duration t 1,l differs from the magnetic field polarity during the time duration t 2 , l , wherein the operating signal has a second time segment (II) with a third partial time segment (iii) and a fourth partial time segment (iv), wherein in the third partial time segment (iii) a signal value S +,ll is applied for a time duration t 1,ll and in the fourth partial time segment (iv) a signal value S -,ll is applied for a time duration t 2, ll , wherein the time duration t 2, ll is longer than the time duration t 1 , ll ; - measuring a first measurement voltage value U l of the induced measurement voltage during a measurement segment of the first time segment (I) and a second measurement voltage value U ll of the induced measurement voltage during a measurement segment of the second time segment (II); and – determining the flow-rate-dependent measurement variable on the basis of the first measurement voltage value U l and the second measurement voltage value U ll .

Description

[0001] Method for operating a magnetic-inductive flow measuring device

[0002] The invention relates to a method for operating a magnetic-inductive flow measuring device, a magnetic-inductive flow measuring device and a magnetic-inductive flow measuring probe.

[0003] Magnetic-inductive flow measuring devices are used to determine the flow velocity and volume flow of a flowing medium in a pipeline. In this case, a distinction is made between inline magnetic-inductive flow measuring devices and magnetic-inductive flow measuring probes, which are inserted into a lateral opening of a pipeline. A magnetic-inductive flow measuring device has a magnetic field-generating device for generating a magnetic field. A main axis of the magnetic field runs essentially perpendicular to the flow direction of the flowing medium. Saddle or cylindrical coils are typically used for this purpose. To create a predominantly homogeneous magnetic field, additional pole pieces are shaped and mounted relative to the flow direction so that the magnetic field lines run across the entire pipe cross-section essentially perpendicular to the transverse axis or parallel to the vertical axis of the measuring tube.In addition, a magnetic-inductive flowmeter has a measuring tube for conveying the medium, on the outer surface of which the magnetic field-generating device is arranged. A pair of measuring electrodes attached to the outer surface of the measuring tube taps an electrical measuring voltage or potential difference perpendicular to the flow direction and the magnetic field, which arises when a conductive medium flows in the direction of flow with a magnetic field applied. Since the tapped measuring voltage depends on the velocity of the flowing medium according to Faraday's law of induction, the flow velocity and / or—with the addition of a known pipe cross-section—the volume flow can be determined from the measured induced measuring voltage.

[0004] In contrast to a magnetic-inductive flowmeter, which comprises a measuring tube for conveying the medium with an attached device for generating a magnetic field penetrating the measuring tube and measuring electrodes, magnetic-inductive flow measuring probes with their usually circular-cylindrical housing are inserted into a lateral opening of a pipeline and fixed in a fluid-tight manner. A special measuring tube is no longer necessary. The measuring electrode arrangement and coil arrangement on the outer surface of the measuring tube mentioned above are omitted and replaced by a device for generating a magnetic field arranged inside the housing and in immediate proximity to the measuring electrodes. This device is designed such that an axis of symmetry of the magnetic field lines of the generated magnetic field intersects the front surface or the area between the measuring electrodes perpendicularly.There are already a variety of different magnetic-inductive flow measuring probes available in the state of the art.

[0005] Magnetic-inductive flowmeters are widely used in process and automation technology for fluids with an electrical conductivity of approximately 5 pS / cm and above. The applicant markets corresponding flowmeters in a wide variety of designs for various applications, for example, under the names PROMAG or MAGPHANT.

[0006] WO 2020 / 001876 A1 discloses a method for commissioning a magnetic inductive flowmeter, in which, before the start of the measuring phase - in which the operating voltage alternates with a first frequency - with a higher second frequency between +U shotThis leads to the magnetic field-generating device warming up more quickly, thus reducing the settling time required until the magnetic field stabilizes. However, WO 2020 / 001876 A1 does not address how the zero point can be stabilized during the measurement phase.

[0007] JP 2002-310751 A discloses a magnetic-inductive flowmeter which is operated with a two-frequency operating signal. This means that during the measuring phase the excitation current changes direction at a first frequency and simultaneously alternates between a maximum excitation current and no excitation current at a second frequency which is higher than the first frequency. The advantage of such an operating signal is the stabilization of the zero point. The disadvantage is that time periods are provided in which no magnetic field is generated or in which a remanent field is present which typically amounts to only a few percent of the nominal magnetic field. This means, on the one hand, that no induced voltage is generated at the measuring electrodes and therefore no flow velocity-dependent process variable can be measured during these time periods, and, on the other hand, the amplitude of the measuring signal is temporarily very low.This leads to increased measurement errors in applications with very high flow changes.

[0008] The invention is based on the object of remedying the problem.

[0009] The object is achieved by the method according to claim 1, the magnetic-inductive flowmeter according to claim 14 and the magnetic-inductive flowmeter probe according to claim 15.

[0010] The method according to the invention for operating a magnetic-inductive flow measuring device for determining a flow velocity-dependent measured variable of a flowable medium, which comprises a magnetic field generating device for generating a magnetic field and a device for detecting an induced measuring voltage in the medium, comprising the method steps:

[0011] - Impressing an operating signal on the magnetic field generating device for generating the magnetic field, wherein the operating signal has a first time period, wherein the first time period comprises at least a first time sub-period and a second time sub-period, wherein in the first time sub-period a signal value S +J each for a period of time is impressed, whereby in the second time segment a signal value S_ :I for a period of time t 2; / is impressed, whereby the duration is greater than the time period t 2J , whereby a magnetic field polarity changes during the period of the

[0012] Magnetic field polarity during the time period t2, / , wherein the operating signal has a second time period which, in particular directly, follows the first time period, wherein the second time period comprises at least a third time period and a fourth time period, wherein in the third time period a signal value S +JI for a period of time t i n is impressed, whereby in the fourth time segment a signal value S_ n for a period of time t 2 / / is impressed, where the time period t 2 / / is greater than the time period t i n , where the magnetic field polarity changes during the time period t i n of the

[0013] Magnetic field polarity during the time period t2,, differs;

[0014] - measuring a first measuring voltage value t / , of the induced measuring voltage during a measuring section of the first time period;

[0015] - Measuring a second measuring voltage value U n the induced measuring voltage during a measuring section of the second time period; and - determining a current zero point based on the first measuring voltage value / / , and the second measuring voltage value U n to compensate for the flow velocity-dependent measured variable.

[0016] The operating signal can be a voltage signal that is applied to the magnetic field generating device. In this case, the voltage signal comprises a time-varying voltage that is generated by an operating circuit and applied to the magnetic field generating device. The magnetic field generating device is designed such that, based on the impressed operating signal, it generates a time-varying magnetic field. Typically, the magnetic field generating device comprises at least one coil that is electrically connected to the operating circuit. Alternatively, the operating signal can also be a current signal that is set or predetermined on the magnetic field generating device. In this case, the operating circuit is configured to generate the current signal and make it available to the magnetic field generating device.

[0017] The operating signal is such that after the second time period, the first time period begins again, and after the first time period, the second time period follows. While the operating signal is applied, the measuring voltage induced in the medium or the electrical potentials present at the individual measuring electrodes are measured by means of a measuring circuit on the device for detecting the induced measuring voltage. The operating signal is therefore applied during the measuring phase. An evaluation circuit is set up to determine the flow velocity-dependent measured variable based on the measured measuring voltages or electrical potentials of different time intervals. The flow velocity-dependent measured variable can be the flow velocity, the volume flow, the mass flow, and / or variables derived therefrom.

[0018] The current zero point can be determined as follows. In a first process step, a difference or an average value of two measuring voltages from preferably consecutive time intervals of opposite magnetic field polarity of the first time interval is determined. The first measuring voltage value U, is derived from the difference. This is used to determine the flow velocity-dependent measured variable. In a second process step, the difference or the average value of two measuring voltages from preferably consecutive time intervals of the second time interval with opposite magnetic field polarity is also determined. The second measuring voltage value / / ,, is derived from the difference. This is also used to determine the flow velocity-dependent measured variable. The first measuring voltage value U Iand the second measured voltage value Un. For example, the difference between the respective absolute values ​​of the two determined measured voltage values ​​is determined, which itself has no flow velocity-dependent component and essentially depends exclusively on the zero-point effect. The currently determined measured value of the flow velocity-dependent measured quantity is then corrected by the current zero point.

[0019] The advantage of the inventive solution is that it avoids time segments in which no current flows through the magnetic field-generating device or no voltage is applied to the magnetic field-generating device, thus preventing the generation of a magnetic field. This ensures that an induced voltage proportional to the flow velocity of the medium can always be detected at the device for detecting the induced measuring voltage. Especially in applications where the flow velocity varies rapidly—such as in filling processes—it is advantageous if the induced measuring voltage can be detected continuously over the entire measurement period.

[0020] Another advantage is that by imprinting the signal values ​​S + / , S_ :I , S +JI and - all of which deviate from zero - a significantly higher signal amplitude can be achieved than in the method according to JP 2002-310751 A, thus making it less sensitive to 1 / f noise. Furthermore, the operating signal according to the invention results in a significant simplification of the control process, since no sections are provided in which no current flows.

[0021] Advantageous embodiments of the invention are the subject of the subclaims.

[0022] One embodiment provides that during the first time period, the first time sub-interval, in particular the signal value S +J and the second time sub-interval, in particular the signal value S_ :I at least temporarily with a first frequency alternate.

[0023] One embodiment provides that during the second time period, the third time sub-interval, in particular the second signal value S +iIIand the fourth time interval, in particular the second signal value at least temporarily with a second frequency f 2 II alternate.

[0024] This does not only mean that the operating signal exceeds the signal value S more than once +J and the signal value S_ , but also periodically. The same applies to the repetitions of the signal value S +iII and the signal values ​​S_ w The advantage of this is that a more stable flow velocity-dependent measured variable can be determined from the measured voltages during the repeating time intervals. One embodiment provides for the first time interval and the second time interval to alternate at a third frequency f3.

[0025] One embodiment provides that the third frequency f3 is smaller than the first frequency u and the second frequency f 2 II .

[0026] One embodiment provides that a ratio of the third frequency f3 and the first frequency f1 and / or the second frequency f 2 n between 2 and 1000, in particular 2 and 10 and preferably 3 and 5.

[0027] One design provides that a relationship between the duration and the time period t2, / is between 1 and 10, in particular 1,5 and 5 and preferably 2 and 3.

[0028] One embodiment provides that a ratio between the time period t2,n and the time period t ul between 1 and 10, in particular 1, 5 and 5 and preferably 2 and 3.

[0029] One embodiment provides that the time period t w equal to the time period t 2 n and / or the time period t 2; / equal to the time period t 1; / / is.

[0030] Measured values ​​of the coil current and coil voltage can also be used to control the operating signal. This allows the time profile and / or measured values ​​of the coil current and coil voltage during the longer time intervals to be determined and used to control the duration of the short time intervals and to determine the inductance of the magnetic field-generating device. This allows the shorter time intervals to be selected to be shorter than in operation with constant time intervals. This is particularly advantageous for suppressing 1 / f interference signals. The determined inductance can be used for diagnostics, compensation, and / or flux density control to ensure that the flow velocity-dependent measured values ​​are correct and reliable even in the event of changes to the magnetic field-generating device or under the influence of external interference.

[0031] One embodiment provides that the first time period has a time period t 31 continues.

[0032] One embodiment provides that a relationship between the time period t 31 and the duration between 2 and 1000, in particular 3 and 10 and preferably 5 and 7. One embodiment provides that a ratio between the time period t 3J and the time period t 2; / between 4 and 2000, in particular 6 and 20 and preferably 10 and 14.

[0033] One embodiment provides that during the first time segment and / or the third time segment, a shot signal value S + , shot for a period of time t +iSftot is impressed, and / or wherein during the second time segment and / or the fourth time segment a shot signal value S_, shot for a period of time t_, shot is imprinted.

[0034] The advantage of this design is that the additional imprinting of shot signal values ​​in the individual time intervals results in a faster oscillation and thus stabilization of the generated magnetic field.

[0035] The magnetic-inductive flowmeter according to the invention for determining a flow velocity-dependent measured variable of a flowable medium comprises:

[0036] - a measuring tube for guiding the medium;

[0037] - a device for detecting an induced measuring voltage in the medium

[0038] - a magnetic field generating device arranged on the measuring tube for generating a magnetic field penetrating the measuring tube; and

[0039] - a measuring, operating and / or evaluation circuit which is designed to carry out the method according to the invention.

[0040] The magnetic-inductive flow measuring probe according to the invention for determining a flow velocity-dependent measured variable of a flowable medium, comprising:

[0041] - a housing with a front section to be supplied with the medium,

[0042] - a device for detecting an induced measuring voltage in the medium;

[0043] - a magnetic field generating device arranged in the housing for generating a magnetic field penetrating the housing, in particular the front section; and

[0044] - a measuring, operating, and / or evaluation circuit configured to carry out the method according to the invention. The invention is explained in more detail with reference to the following figures. It shows:

[0045] Fig. 1 : an embodiment of a magnetic-inductive flow meter;

[0046] Fig. 2: a perspective view of a partially sectioned embodiment of a magnetic-inductive flow measuring probe;

[0047] Fig. 3: a schematic representation of a first embodiment of the operating signal;

[0048] Fig. 4: a schematic representation of the measurement signal which would result from the operating signal of Fig. 3 at a flow velocity of 0 m / s.

[0049] Fig. 5: a schematic representation of a second embodiment of the operating signal; and

[0050] Fig. 6: an embodiment of the method according to the invention.

[0051] Fig. 1 shows a cross-section through a magnetic-inductive flowmeter 1. The structure and measuring principle of a magnetic-inductive flowmeter 1 are generally known. A flowable medium which is electrically conductive is passed through a measuring tube 2. The measuring tube 2 comprises a support tube 3 which comes into contact with the medium and is usually made of steel, ceramic, plastic, or glass, or at least comprises these. A magnetic field generating device 5 for generating a magnetic field is arranged on the support tube 3 such that the magnetic field lines are oriented substantially perpendicular to a longitudinal direction defined by a measuring tube axis. The magnetic field generating device 5 usually comprises a saddle coil or at least one (cylindrical) coil 6i. A coil core 14i usually extends through a receptacle 15 of the coil 6i.The receptacle 15 is understood to be the volume delimited by the coil wire forming the coil 6i. The receptacle 15 of the coil 6i can thus be formed by a coil holder or by the imaginary enclosed volume. The latter occurs when the coil wire of the coil 6i is wound directly around the coil core 14i. The coil core 14i is formed from a magnetically conductive, in particular soft-magnetic, material. The device 5 for generating the magnetic field comprises a pole piece 21i arranged at one end of the coil core 14i. The pole piece 21i can be a separate component or monolithically connected to the coil core 14i. In the embodiment shown in Fig. 1, two diametrically arranged coils 6a, 6b each have a coil core 14a, 14b and a pole piece 21a, 21b. The two coil cores 14a, 14b are connected to each other via a field return 22.The field return 22 connects the opposite sides of the coil cores 14a, 14b. However, they are also magnetically inductive.

[0052] Flowmeters with exactly one coil 6 with exactly one coil core 14 and without field feedback are known. The coil 6 is connected to an operating circuit 7, which drives the coil 6 with an operating signal. The operating signal can be a voltage with a time-varying profile and is characterized by operating signal parameters, wherein at least one of the operating signal parameters is controllable. The magnetic field generated by the magnetic field-generating device 5 is generated by a direct current of alternating polarity clocked by an operating circuit 7. This ensures a stable zero point and makes the measurement insensitive to influences from electrochemical interference. The two coils 6a, 6b can be connected separately to the operating circuit 7 or connected in series or parallel to one another.

[0053] When a magnetic field is applied, a flow-dependent potential distribution is created in the measuring tube 2, which can be detected, for example, in the form of an induced measuring voltage. A device 8 for tapping the induced measuring voltage is arranged on the measuring tube 2. In the embodiment shown, the device 8 for tapping the induced measuring voltage is formed by two oppositely arranged measuring electrodes 17, 18 for forming a galvanic contact with the medium. However, magnetic-inductive flowmeters are known which have measuring electrodes arranged on the outer wall of the support tube 3 that are not in contact with the medium. As a rule, the measuring electrodes 17, 18 are arranged diametrically and form an electrode axis or are intersected by a transverse axis that runs perpendicular to the magnetic field lines and the longitudinal axis of the measuring tube 2.However, devices 8 for tapping the induced measuring voltage are also known, which have more than two measuring electrodes. The flow velocity-dependent measured variable can be determined based on the measured voltage. The flow velocity-dependent measured variable includes the flow velocity, the volume flow rate, and / or the mass flow rate of the medium. A measuring circuit 23 is configured to detect the induced measuring voltage applied to the measuring electrodes 17, 18, and an evaluation circuit 24 is designed to determine the flow velocity-dependent measured variable. The evaluation circuit 24 can be part of the measuring transducer.

[0054] The support tube 3 is often made of an electrically conductive material, such as steel. To prevent the measuring voltage applied to the first and second measuring electrodes 2, 3 from being dissipated via the support tube 3, the inner wall is lined with an insulating material, for example, a (plastic) liner 4.

[0055] Commercially available magnetic-inductive flowmeters have two additional electrodes 19, 20 in addition to the measuring electrodes 17, 18. Firstly, a level monitoring electrode 19, ideally located at the highest point in the measuring tube 2, serves to detect partial filling of the measuring tube 1 and is configured to transmit this information to the user and / or to take the level into account when determining the volume flow. Furthermore, a reference electrode 20, which is typically located diametrically opposite the level monitoring electrode 19 or at the lowest point of the measuring tube cross-section, serves to establish a controlled electrical potential in the medium. The reference electrode 20 is typically used to connect the flowing medium to a ground potential.

[0056] The operating circuit 7, controller circuit 10, measuring circuit 23, and evaluation circuit 24 can be part of a single electronic circuit or form separate circuits. The measuring, operating, and / or evaluation circuit 7, 23, 24 is configured to carry out the method according to the invention. For this purpose, it can comprise a microprocessor and / or electrical components (electrical lines, electromechanical components, passive and / or active components). For this purpose, the operating circuit is configured to generate the operating signal and provide it to the magnetic field-generating device. Furthermore, the measuring circuit is configured to determine the measured voltage values ​​and forward them to the evaluation circuit. The evaluation circuit is configured to determine the current zero point and to take it into account for determining the flow velocity-dependent measured variable.

[0057] The measuring principle underlying the invention is first explained using the perspective and partially sectioned illustration in Fig. 2. A flow measuring probe 101 comprises a generally circular-cylindrical housing 102 with a predetermined outer diameter. This housing is adapted to the diameter of a bore located in a wall of a pipeline (not shown in Fig. 1) and into which the flow measuring probe 101 is inserted in a fluid-tight manner. A medium to be measured flows through the pipeline, into which the flow measuring probe 101 is immersed practically perpendicular to the flow direction of the medium, which is indicated by the wavy arrows 118. A front end 116 of the housing 102 projecting into the medium is fluid-tightly sealed by a front body 115 made of insulating material. By means of a coil arrangement 106 arranged in the housing 102, a magnetic field 109 can be generated that extends through the end section into the medium.A coil core 111, which is at least partially made of a soft magnetic material and arranged in the housing 102, ends at or near the end section 116. A field return body 114, which encloses the coil arrangement 106 and the coil core 111, is configured to return the magnetic field 109 extending from the end section into the housing 102. The coil core 111, the pole piece 112, and the field return body 114 are each field guide bodies 110, which together form a field guide arrangement 105. A first and a second measuring electrode 104i, which forms a galvanic contact with the medium to be guided, form the device 103 for detecting a measuring voltage induced in the medium and are arranged in the front body 115 and, like the outer walls of the housing, contact the medium.An electrical voltage induced at the measuring electrodes 104i based on Faraday's law of induction can be tapped by a measuring and / or evaluation circuit. This voltage is maximum when the flow measuring probe 101 is installed in the pipeline such that a plane spanned by a straight line intersecting the two measuring electrodes 104i and a longitudinal axis of the flow measuring probe runs perpendicular to the flow direction 118 or the longitudinal axis of the pipeline. An operating circuit 107 is electrically connected to the coil arrangement 106, in particular to the coil 113, and is configured to impress a clocked operating signal onto the coil 113 in order to thus generate a clocked magnetic field 109.

[0058] The measuring, operating, and / or evaluation circuit 107, 120, 121 is suitable and configured to carry out the method according to the invention. For this purpose, it may comprise a microprocessor and / or electrical components (electrical lines, electromechanical components, passive and / or active components). For this purpose, the operating circuit is configured to generate the operating signal and provide it to the magnetic field-generating device. Furthermore, the measuring circuit is configured to determine the measured voltage values ​​and forward them to the evaluation circuit. The evaluation circuit is configured to determine the current zero point and to consider it for determining the flow velocity-dependent measured variable.

[0059] Fig. 3 shows a simplified schematic representation of a first embodiment of the operating signal (solid line) and an operating signal according to the prior art (dashed line). The operating signal can be a voltage signal or a current signal. In the case of a voltage signal, it comprises time-varying voltage values ​​that are applied to the magnetic field-generating device. In the case of a current signal, it comprises time-varying current values ​​that are set on the magnetic field-generating device. The graph shown represents the current or voltage as a function of time. According to the prior art, the operating signal alternates periodically between two time intervals. In the first time interval, the operating signal alternates between the signal value S +iIand a signal value of zero. The first time interval has four time intervals in which the operating signal assumes the signal value S+ and three time intervals in which the operating signal is zero. In the second time interval, the generated magnetic field changes the magnetic field polarity. In the second time interval, the operating signal alternates between the signal value " and a signal value of zero. The second time interval has four time intervals in which the operating signal assumes the signal value S_ n and three time intervals in which the operating signal is zero. The respective time period in which the signal value S +J or S_ n The time period during which the operating signal assumes a signal value of zero is identical. The operating signal depicted is referred to as a dual-frequency operating signal.

[0060] The operating signal according to the invention differs from the prior art essentially in that there are no time intervals in which the operating signal assumes a signal value of zero. Furthermore, the time durations and t2, / or t i n and t 2 / / the time intervals i, ii, iii, iv in which the signal values ​​S +J and S_ :I or S +iII and S_ :II In the illustrated embodiment, the time periods t w and t 2; / a ratio of 1 :2 and the time periods t i n and t 2 / / a ratio of 2:1. The ratio changes when changing from the first time interval to the second time interval or from the second time interval to the first time interval. The first time interval lasts a time period t 3JIn the illustrated embodiment, the duration of the first time interval is identical to the duration of the second time interval. The signal values ​​S are also identical. +J and S +iII and the signal values ​​S_ and S_ :II or the absolute values ​​of the four signal values ​​S +J , S +JI , S_ :I and S_ w . The time period t w is identical to the time period t 2 / / and the time period t 2; / is identical to the time period t il Alternatively, a relationship between the time duration and the time period t 2J between 1 and 10, in particular 1,5 and 5 and preferably 2 and 3. Alternatively, a ratio between the time period t 2 / / and the time duration between 1 and 10, in particular 1.5 and 5 and preferably 2 and 3. For the optimal design of the operating signal, a ratio between the time duration t 3 1 and the duration between 2 and 1000, in particular 3 and 10 and preferably 5 and 7 and a ratio between the time period t 3 1 and the time period t 2J between 4 and 2000, in particular 6 and 20 and preferably 10 and 14.

[0061] Fig. 4 shows schematic representations of the measurement signal that would result from the operating signal according to the invention shown in Fig. 3 and the operating signal according to the prior art at a flow velocity of 0 m / s. The measurement signal comprises the measurement voltage induced by the flowing medium (in this case, the measurement voltage is zero) with a simplified interference signal that provides an exponentially decaying contribution. The interference signal arises from crosstalk from the magnetic field generating device to the device for detecting the induced measurement voltage. Typically, the measurement voltage is measured in the later part of a time interval of constant magnetic field polarity, since the influence of the interference is minimal there. Operating the magnetic field generating device at a higher frequency leads to a larger zero point (see / / , and U n), because a disturbance induced when switching the coil current direction has less time to decay before the next switching occurs. This disturbance cannot be reliably distinguished from an actual flow. For example, the mean value of two measurement voltages with two consecutive time intervals with opposite magnetic field polarity (red double arrows) and without the respective flow-dependent signal component is not dependent on the zero-point effect.

[0062] In light of the measurement signal resulting from the operating signal 11 according to the prior art (dashed line), the advantage of the operating signal according to the invention is clearly evident. The measurement signal resulting from the operating signal according to the invention exhibits a significantly higher amplitude of the measurement signal for all time sub-intervals. Especially at the beginning of the switching process, the amplitude of the conventional solution is very low and close to 0 V. Only after several time sub-intervals does the amplitude increase for the time sub-intervals in which no coil current flows through the magnetic field-generating device, until it essentially assumes half of the maximum amplitude.

[0063] Fig. 5 shows a schematic representation of a second embodiment of the operating signal 11. The second embodiment of the operating signal 11 differs from the first embodiment essentially in that during the first time segment i and the third time segment iii, a shot signal value S + , shot (e.g. a shot voltage) for a time period t + , shot is impressed and that during the second time segment ii and the fourth time segment iv, a shot signal value S_, shot (e.g. a shot voltage) for a time period t_, sho t is applied. The absolute values ​​of the individual shot signal values ​​and the time durations are identical in the illustrated configuration. The absolute values ​​of the individual shot signal values ​​and the time durations during which the corresponding shot signal value is applied can differ from each other.

[0064] Fig. 6 shows an embodiment of the method according to the invention. An operating signal is impressed on the magnetic field generating device for generating the magnetic field (method step I). The operating signal has a first time period with at least one first time sub-period and a second time sub-period. In the first time sub-period, a signal value S +iI each for a period of time t w impressed and in the second time segment a signal value S_ :I for a period of time t 2; / The magnetic field polarity of the generated magnetic field differs during the period of the magnetic field polarity during the time period t2. According to the invention, the time period is greater than the time period t 2J . Thus, in the first time period, the time portion in which the signal value S +iIis greater than the time portion in which the signal value S_ :I is imprinted.

[0065] The operating signal further comprises a second time period, which directly follows the first time period. However, a further time period may be provided between the first time period and the second time period, in which the signal value is zero. The second time period comprises at least a third time sub-period in which a signal value S +iII for a period of time t 1; / / and a fourth time segment in which a signal value S_ :II for a period of time t 2Ji The magnetic field polarity differs during the time period t i n of the magnetic field polarity during the time period t 2iII According to the invention, the time period t 2JI greater than the time period t ul. Thus, in the second time period, the time portion in which the signal value S_ n is greater than the time portion in which the signal value S +ill is imprinted.

[0066] It is essential to the invention that during the first time period the signal value S +J and the signal value S_ , at least temporarily with a first frequency alternate and that during the second time period the second signal value S +iII and the second signal value S_ :II at least temporarily with a second frequency f 2 II At the same time, the first time period and the second time period alternate with a third frequency f3, which is smaller than the first frequency and the second frequency f 211 . Thus, the operating signal according to the invention is also a two-frequency operating signal.

[0067] While the operating signal is impressed, a first measuring voltage value t / , of the induced measuring voltage is measured during a measuring section of the first time period (I) (method step II).

[0068] Offset in time from the first time period (I) during a measuring section of the second time period (II), a second measuring voltage value U n the induced measuring voltage is measured (process step III).

[0069] From the measured first measuring voltage value t / , and the second measuring voltage value U„, a current zero point is determined to compensate for the flow velocity-dependent measured variable (process step IV). This can be achieved by forming a difference between the two measuring voltage values ​​U Iand Un. The flow velocity-dependent measured variable is determined using the measured voltage values ​​of different time segments of a time interval and compensated for the current zero point. Process steps I to IV are not to be considered as consecutive

[0070] To interpret process steps. For example, the induced measuring voltage is measured during the application of the operating signal in process step I.

[0071] LIST OF REFERENCE SYMBOLS magnetic-inductive flowmeter 1

[0072] Measuring tube 2

[0073] Support tube 3

[0074] Liner 4 magnetic field generating device 5

[0075] Operating circuit 7

[0076] Device 8 for detecting the induced measuring voltage

[0077] Regulator circuit 10

[0078] Operating signal 11

[0079] Coil 13i

[0080] Coil core 14i

[0081] Measuring electrode 17i

[0082] Field feedback body 19

[0083] Pole piece 21 i

[0084] Measuring circuit 23

[0085] Evaluation circuit 24 magnetic-inductive flow measuring probe 101

[0086] Housing 102

[0087] Device 103 for detecting an induced measuring voltage

[0088] Measuring electrode 104i magnetic field generating device 105

[0089] Coil arrangement 106

[0090] Operating circuit 107

[0091] Magnetic field 109

[0092] Field guide body 110

[0093] Coil core 111

[0094] Pole piece 112

[0095] Coil 113

[0096] Field feedback body 114

[0097] Front body 115

[0098] End section 116

[0099] Flow direction of the medium 118

[0100] Measuring circuit 120

[0101] Evaluation circuit 121

Claims

PATENT CLAIMS 1 . A method for operating a magnetic-inductive flow measuring device for determining a flow velocity-dependent measured variable of a flowable medium, which comprises a magnetic field generating device (5, 105) for generating a magnetic field and a device (8, 103) for detecting an induced measuring voltage in the medium, comprising the method steps: - impressing an operating signal (11) on the magnetic field generating device (5) for generating the magnetic field, wherein the operating signal (11) has a first time period (I), wherein the first time period (I) comprises at least a first time sub-period (i) and a second time sub-period (ii), wherein in the first time sub-period (i) a signal value S +J each for a period of time t w is impressed, whereby in the second time segment (ii) a signal value S_j for a time period t 2; / is impressed, whereby the duration is greater than the time period t 2J , whereby a magnetic field polarity changes during the period from the magnetic field polarity during the time period t2, / , wherein the operating signal has a second time period (II), which follows, in particular directly, the first time period (I), wherein the second time period (II) comprises at least a third time period (iii) and a fourth time period (iv), wherein in the third time period (iii) a signal value S +JI for a period of time t i n is impressed, whereby in the fourth time segment (iv) a signal value S_ n for a period of time t 2 n is impressed, where the time period t 2 / / is greater than the time period t i n , where the magnetic field polarity changes during the time period t i n differs from the magnetic field polarity during the time period t2,, - measuring a first measuring voltage value / / , of the induced measuring voltage during a measuring section of the first time period (I); - Measuring a second measuring voltage value U n the induced measuring voltage during a measuring section of the second time period (II); and - Determining a current zero point based on the first measured voltage value U I and the second measuring voltage value Un to compensate for the flow velocity-dependent measured variable.

2. Method according to claim 1, wherein during the first time period (I) the signal value S +iI and the signal value S_ , at least temporarily with a first frequency alternate.

3. Method according to claim 1 or 2, wherein during the second time period (II) the second signal value S +JI and the second signal value at least temporarily alternate with a second frequency.

4. Method according to one of the preceding claims, wherein the first time period (I) and the second time period (II) alternate at a third frequency f3.

5. Method according to one of claims 2 to 4, wherein the third frequency f3 is smaller than the first frequency and the second frequency 2 / / .

6. Method according to one of claims 2 to 5, wherein a ratio of the third frequency f3 and the first frequency and / or the second frequency f 211 between 2 and 1000, in particular 2 and 10 and preferably 3 and 5.

7. Method according to one of the preceding claims, wherein a ratio between the time duration and the time period t 2iI between 1 and 10, in particular 1, 5 and 5 and preferably 2 and 3.

8. Method according to one of the preceding claims, wherein a ratio between the time period t 2JIand the time period t l w between 1 and 10, in particular 1, 5 and 5 and preferably 2 and 3.

9. Method according to one of the preceding claims, wherein the time period equal to the time period t2,w and / or the time period t 2; / equal to the time period t ul is.

10. Method according to one of the preceding claims, wherein the first time period (I) has a time duration t 3; / continues.

11. The method according to claim 10, wherein a ratio between the time period t 3J and the duration between 2 and 1000, in particular 3 and 10 and preferably 5 and 7.

12. The method according to claim 10 or 11, wherein a ratio between the time period t 3 I and the time period t 2J between 4 and 2000, in particular 6 and 20 and preferably 10 and 14.

13. Method according to one of the preceding claims, wherein during the first time segment (i) and / or the third time segment (iii) a shot signal value S +iShot for a period of time t +iShot is imprinted, and / or wherein during the second time segment (ii) and / or the fourth time segment (iv) a shot signal value S_, shot for a period of time t_ shot is imprinted.

14. Magnetic-inductive flowmeter (1) for determining a flow velocity-dependent measured variable of a flowable medium, comprising: - a measuring tube (2) for guiding the medium; - a device (8) for detecting an induced measuring voltage in the medium - a magnetic field generating device (5) arranged on the measuring tube (2) for generating a magnetic field penetrating the measuring tube (2); and - a measuring, operating and / or evaluation circuit (7, 23, 24) which is designed to carry out the method according to one of the preceding claims.

15. Magnetic-inductive flow measuring probe (101) for determining a flow velocity-dependent measured variable of a flowable medium, comprising: - a housing (102) with a front section (116) to be supplied with the medium, - a device (103) for detecting an induced measuring voltage in the medium; - a magnetic field generating device (105) arranged in the housing (102) for generating a magnetic field penetrating the housing (102), in particular the front section (116); and - a measuring, operating and / or evaluation circuit (107, 120, 121) which is designed to carry out the method according to one of claims 1 to 13.

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