DE10250523A1 - Method for measuring the capacity or capacity change of a capacitive circuit or component, especially for use with a liquid level measurement instrument, whereby measurement windows are set to compensate deposition effects - Google Patents

Abstract

Method for measuring the capacity or capacity change of a capacitive circuit or component in which a transmission pulse is generated an applied to the circuit or component. The transmission pulse generates a corresponding current pulse within a measurement time window that is displaced relative to the transmission pulse. The duration of the measurement window has a value that is independent of the frequency of the transmission pulse. An Independent claim is made for a method for measuring the liquid level in a container with a capacitive measurement instrument, where he duration of the measurement window for the current pulse generated by the sensor has a value that is independent of the frequency of the transmission pulse.

Description

The invention relates to a method for detecting the capacity or a Change in capacitance of a capacitive circuit or component, at which generates a transmission pulse and on the capacitive circuit or Component is given, the transmission pulse depending on the Capacitance of the capacitive circuit or component a current pulse generated and the current pulse within a compared to the transmit pulse shifted acquisition time window is detected. The invention also relates to also - as a preferred application - the method described above to determine the level of a medium in a container with a capacitive level measuring device, wherein the capacitive level measuring device has a fill level sensor with several sensor fields.

In the context of the invention, "capacity" is the capacity value of a capacitive circuit or component meant; a "change in capacity" consequently means a change in the capacitance value of a capacitive Circuit or component. With "recording" the capacity or one Capacity change in the context of the invention is both only a qualitative one Acquisition as well as a quantitative acquisition, meaning a real measurement. "Capacitive circuit or component" means within the scope of the invention every circuit element and every component that has capacitive properties has, is often referred to as capacity, but then not the Capacity value is meant. A "capacitive circuit or component" is especially a capacitor. As a "capacitive circuit or component" is in the context of the invention but also the electrode of a capacitive Proximity switch or the capacitive sensor field of a level sensor, in Interaction with an influencing body or influencing medium, designated.

The method described at the outset can be carried out using a large number of Circuit arrangements and in a variety of specific applications be used. A possible circuit arrangement is for example from the DE 197 44 152 A1 known. This circuit arrangement, at least one Clock generator, a changeover contact controlled by the clock generator, has a storage capacitor and an evaluation stage in particular from the fact that those adhering to the circuit arrangement parasitic capacities no longer affect, because the evaluation level for Current evaluation is suitable and there is virtually no voltage swing at the changeover contact occurs.

A preferred application of the method described in the introduction is included a capacitive level meter. Capacitive level gauges are used for the capacitive detection of the fill level of liquids and bulk goods and other media in a - closed or open - container and are widely known.

In the case of capacitive level measuring devices, the fact becomes exploited that the medium whose level is to be determined, the capacity the level sensor or the capacitive circuit or component, d. H. of a sensor field of the level sensor, because it affects the Capacity of the level sensor or the sensor field of the level sensor also the essential dielectric constant of the medium differs from that Dielectric constant differs from air. Depending on the design of the The level sensor becomes the change in capacitance between a sensor field and a reference electrode, between two adjacent sensor fields or the change in the basic capacity of a sensor field, which essentially on the geometry of the sensor field and the dielectric constant of the Depends on the sensor field surrounding medium, evaluated.

Especially with capacitive level gauges - but basically also with other capacitive sensors or proximity switches - One problem is that there is buildup or buildup on the capacitive circuit or component or at the level sensor can come, whereby the measurement result is falsified. With capacitive Level measuring devices, for example the level of the Monitoring the coolant of a machine is in the coolant due to contamination very often metallic, conductive particles that deposit on the level sensor. While the impact of this Deposits in the area of the level sensor that is in the medium, can be neglected, the deposits on the level sensor outside of the medium to falsify the measurement result. This can cause the capacitive level meter to be wrong - too high - Displays the fill level of the medium to be monitored. The previous one Example of monitoring the fill level of the Such a wrong measured value of the cooling lubricant of a machine Level measuring device - due to a too low level of the cooling lubricant is recognized in good time - lead to damage to the machine.

Various methods are known to rule out such errors, through either the buildup or deposit of dirt particles prevented on the level sensor or its effect on the measurement result should be suppressed. An oil level sensor is known from DE 100 11 860 A1 known in which the deposition of dirt on the sensor should be prevented that the oil during the operation of the machine The measuring section of the sensor flows continuously. To ensure the constant flow of oil necessary measures - especially the Using a required pump - will result in one disproportionately high additional effort in the implementation of the oil level sensor or to a very limited area of application of the oil level sensor.

In a capacitive level meter sold by the applicant The "LK" series uses the method described at the beginning, whereby the current pulse is out of phase, namely out of phase by 90 ° is evaluated. However, this cannot guarantee that Buildup is reliably differentiated from level.

The present invention is therefore based on the object described method for recording the capacity or a Specify change in capacitance of a capacitive circuit or component with its help, especially in the case of a capacitive level measuring device, Interferences, in particular buildup or deposits on the capacitive Circuit or component, are recognized or an impact of Interference on the measurement result is prevented. In addition, that should Procedures can be carried out easily, so that it is particularly in a capacitive level sensor applied with little effort can be.

In the method described above for detecting the capacitance or a change in capacitance of a capacitive circuit or component, this object is first and essentially achieved in that the duration T _{S of} the acquisition time window has a predetermined value regardless of the frequency of the transmission pulse. This frequency-independent duration of the acquisition time window compared to the transmission pulse is based on the knowledge that buildup or deposition on the capacitive circuit or component leads to a broadening of the current pulse to be evaluated in comparison to the width of the current pulse without buildup. This is due to the primarily ohmic nature of the attachments. By an appropriate choice of the duration T _{S of} the acquisition time window, it is thus possible to choose the acquisition time window so large or small that the wider, ie longer-extended current pulse does not completely "fit" into the acquisition time window during buildup, while the narrower, ie Shorter current pulse without build-up is completely within the acquisition time window.

According to a preferred embodiment of the method according to the invention the current pulse is detected with the aid of a controlled Rectifier. Both cycles of the rectifier are preferably active used. The evaluation of the current pulse using a rectifier is particularly easy to implement, using both cycles of the rectifier due to component tolerances or Temperature drift can be compensated.

According to a further preferred embodiment of the method according to the invention, as pure a rectangular pulse as possible is used as the transmission pulse. Such a rectangular pulse, which has a steep rising edge and a steep falling edge, as the transmission pulse leads - if there is no buildup - to a very narrow - ideally dirac-shock-shaped - current pulse. The narrower the current pulse, the shorter the duration T _{S of} the acquisition time window can be selected, at which the current pulse nevertheless decays completely within the acquisition time window.

It is further advantageous if the method according to the invention is designed in such a way that the duration T _{S of} the acquisition time window can be set, controlled or regulated. This makes it possible to optimally adapt the acquisition time window to the respective operating conditions, in particular to the respective medium. In addition, an accurate analysis of the respective medium can be carried out by several measurements with different time periods T _{S of} the acquisition time window.

In the method according to the invention for determining the fill level of a medium in a container with a capacitive fill level measuring device, with a fill level sensor having several sensor fields, in which, according to the invention, the duration T _{S of} the acquisition time window also has a predetermined value regardless of the frequency of the transmit pulse, that the level measuring device has a microprocessor and the microprocessor determines the frequency of the transmission pulse as a function of the medium to be measured. The frequency of the transmission pulse runs through a frequency range of, for example, 50 kHz to 5 MHz, for example when the capacitive level measuring device is started up, the frequency of the transmission pulse at which the largest possible output signal is available then being selected depending on the medium to be measured. The selection can be made simply using a threshold value.

In detail, there are now a multitude of possibilities Method according to the invention for detecting the capacity or a change in capacity a capacitive circuit or component or the inventive Method for determining the level of a medium in a container to design and develop with a capacitive level measuring device. For this purpose, reference is made, on the one hand, to claims 1 and 10 subordinate claims, on the other hand in connection with the description with the drawing. Show in the drawing

Fig. 1 is a schematic diagram of a capacitive level meter in a container as well as a technical equivalent circuit diagram, once without and once with adhesions,

Fig. 2 is a timing diagram of the current pulse, without and with buildup, according to a method known from the prior art, and

Fig. 3 is a timing diagram of the current pulse, with and without attachments, according to a first embodiment of the method according to the invention,

Fig. 4 is a diagram of a circuit for the first embodiment of the method according to the invention,

Fig. 5 is a timing diagram of the current pulse, without and with buildup, according to a second embodiment of the method and

Fig. 6 is a sketch of a circuit for the second embodiment of the method according to the invention.

Fig. 1 shows both in Fig. 1a as well as in Fig. 1b, respectively on the left side schematically illustrates a capacitive level measuring device 1, comprising a plurality of sensor fields Level Sensor 2, wherein the level sensor 2 projects into the interior of a container 3. Inside the container 3 there is a medium 4 , for example a lubricant for a machine tool, the fill level of which is to be monitored with the capacitive fill level measuring device 1 . The capacitive level measuring device 1 can be designed, for example, in accordance with DE 100 08 093 A1. In principle, however, the method according to the invention is independent of the specific implementation of the capacitive level measuring device 1 .

The difference between FIGS. 1a and 1b is, on the one hand, that in the example according to FIG. 1a, the medium 4 has a higher fill level than the medium 4 in the example according to FIG. 1b. In addition, in the example shown in FIG. 1b, the fill level sensor 2 is covered with buildup 5 in an area above the medium 4 , the amount of buildup 5 being shown in a greatly exaggerated manner for clarification. In practice, the buildup 5 is, for example, a thin layer of metal chips that are present as contamination in the medium 4 and are deposited on the fill level sensor 2 .

The respective right side of FIGS. 1a and 1b shows a - greatly simplified - technical equivalent circuit diagram of the respective fill level situation. The equivalent circuit diagram has a rectifier 6 for detecting the current pulse I, the amplitude of the current pulse I depending on the one hand on the amplitude of the transmit pulse, but otherwise essentially on the height and the dielectric constant of the medium 4 . The influence of the medium 4 on the current pulse I is shown in the equivalent circuit diagram in connection with a capacitor 7 . The presence of the buildup 5 in the example shown in FIG. 1b is taken into account in the corresponding equivalent circuit diagram by a resistor 8 which is connected in series with the capacitor 7 .

FIG. 2 shows a time diagram of the current pulse 9 "without adhesions" ( FIG. 2a) corresponding to the example in FIG. 1a or of the current pulse 9 '"with adhesions" ( FIG. 2b) corresponding to the example in FIG. 1b. In addition, in FIG. 2 — just as in FIG. 3 — corresponding to the time profile of the current pulse 9 or 9 ′, the time profile of the transmit pulse 10 and the switchover pulse 11 of the rectifier 6 are shown. Fig. 2 shows a method according to the prior art in which the current pulse 9, 9 'phase shifted by 90 ° is evaluated after transmit 10th As can be seen from FIG. 2, this results in four time windows 12 , 13 , 14 and 15 , each of which has the same duration. The time windows 12 and 14 form the acquisition time window within which the current pulse 9 is to be measured.

FIG. 2a shows the course of the current pulse 9 for the situation shown in FIG. 1a, in which the sensor field of the level sensor 2 that has just been evaluated is in the medium 4 . In this situation, the current pulse 9 has a relatively large amplitude and a relatively small width.

FIG. 2b shows the profile of the current pulse 9 'in Fig. 1b situation shown, the straight solved sensor field of the level sensor 2 is located at the outside of the medium 4 and is covered with adhesions. 5 The ohmic property of the adhesions 5 results in a current pulse 9 'with a smaller amplitude and a larger width compared to the current pulse 9 according to FIG. 2a. Since the integral of the current pulse 9 or 9 'within the detection window 12 or 14 is evaluated by measurement and the integral of the current pulse 9 is equal to the integral of the current pulse 9 ', a distinction must be made between the two situations shown in FIGS. 1a and 1b - inside the medium 4 or outside the medium and covered with adhesions 5 - not possible.

A distinction between the situation "medium" or "buildup" shown in FIGS. 1a and 2a and FIGS. 1b and 2b is only possible if the choice of a correspondingly high frequency of the transmission pulse 10 and thus also of the switchover pulse 11, the time window will be shortened as far as 12 to 15 that the current pulse 9 'no longer completely "fits" into the detection window 12 and 14 respectively. Such an increase in the transmission pulse 10 is associated with various disadvantages. Initially, the interference radiation of the fill level sensor 2 also increases with the frequency, so that, on the one hand, additional measures must be taken to prevent the interference radiation in the environment, and on the other hand the fill level sensor 2 may only have a relatively limited rod length. However, both measures severely restrict the desired universal usability of the capacitive fill level measuring device 1 . In addition, the capacitive level measuring device 1 can no longer be used equally for any media at a correspondingly high - required - frequency.

A first variant of the method according to the invention will now be explained in more detail with reference to FIG. 3, in which the duration T _{S of} the acquisition time window 12 or 14 has a predetermined value regardless of the frequency of the transmission pulse 10 . As in Fig. 2a so 3a is also shown in Figure, the course of the current pulse 9 wherein in Fig situation shown 1a -.. The currently solved sensor field of the level sensor 2 is of medium 4, however, not covered by adhesions 5 - shown, while Fig 3b. the time course of the current pulse 9 'in the situation shown in Fig. 1b - the currently evaluated sensor field of the level sensor 2 is from buildup 5 . but not covered by medium 4 - shows. Figs. 3c and 3d show the corresponding Fig. 2c and 2d the time course of the transmitted pulse 10 and the switching pulse. 11

A comparison of FIGS. 2 and 3 first shows that in the method according to the invention according to FIG. 3, the time windows 12 to 15 have different time periods T _S and T _A , the acquisition time windows 12 and 14 having a shorter duration T _S and the supplementary time windows 13 and 15 have a longer duration T _A. Since the transmission pulse 10 and the switching pulse 11 have the same frequency, as desired, the duration T _{S of} the acquisition time window 12 and 14 and the duration T _{A of} the supplementary time window 13 and 15 together correspond to half the period length of the transmission pulse 10 and the switching desk 11 .

By choosing a suitable - short - duration T _{S of} the acquisition time window 12 or 14 it is now achieved that on the one hand the current pulse 9 completely "fits" into the acquisition time window 12 or 14 , on the other hand the current pulse 9 '- due to its build-up 5 conditioned larger width - does not completely "fit" into the acquisition time window 12 or 14 . This now leads to the fact that when the current pulse 9 or 9 'is integrated during the acquisition time window 12 or 14, the integral of the current pulse 9 ' is smaller than the integral of the current pulse 9 . This makes it possible to distinguish the state "medium present" (corresponding to FIG. 1a) from the state "buildup present" (corresponding to FIG. 1b).

According to a first preferred embodiment of the method according to the invention, the current pulse 9 or 9 'is also detected within the supplementary time window 13 or 15 by integration. In a according to Fig. 3 suitable choice of the period T _S of the detection time window 12 or 14 is within the supplement time window 13 or 15 of the current pulse 9 equal to zero, that is, the current pulse 9 is so short that it is completely within the detection time window 12 and 14 respectively subsides. In contrast, in the situation "build-up present" shown in FIGS . 1b and 3b, the current pulse 9 'is so wide that the current pulse 9 ' has a value other than zero within the supplementary time window 13 or 15 .

The current pulse 9 'within the supplementary time window 13 or 15 , which will be referred to below as the residual current pulse 16 , is based on the presence of buildup 5 on the level sensor 2 . By switching the rectifier 6 from the acquisition time window 12 to the supplementary time window 13 or from the acquisition time window 14 to the supplementary time window 15 , the residual current pulse 16 can be subtracted from the current pulse 9 ', whereby the difference between the integral of the current pulse 9 and the integral of the current pulse 9 ' within the acquisition time window 12 or 14 is increased further, so that a distinction can be made more clearly between the situation "medium present" and the situation "buildup present".

Fig. 4 shows a partial schematically shows a circuit for performing the method described above, wherein detection is made of the current pulse I with the aid of a rectifier. For this purpose, the current pulse I is connected to the two connections 18 , 19 of the current evaluation 20 via a two-pole switch 17 . In the first cycle of the switchover signal 11 , in which the switch 17 is connected to the terminal 18 , the current pulse .9 or 9 'in the acquisition time window 14 and the residual current pulse 16 in the acquisition time window 13 are measured, while in the second cycle of the switchover pulse 11 , in which the switch 17 is connected to the connection 19 , the current pulse 9 or 9 'in the detection time window 12 and the residual current pulse 16 in the detection time window 15 are detected. In the example shown in Figs. 3 and 4 of the supplementary process time window is 13 and 15 of the residual current pulse 16 detects thus during the duration and the amount of the residual current pulse 16 are each of the amount of the current pulse 9 and 9 'subtracted.

In the in Fig. 3b shown time course of the current pulse 9 'there may be a situation in which the integral of the residual current pulse 16 is greater than the integral of the current pulse 9 depending on the type and amount of build-5', so that the above-described Subtraction of the residual current pulse 16 from the current pulse 9 'would lead to a "negative" sensor signal.

In order to prevent this condition or to avoid a more complex evaluation circuit required as a result, it is provided according to an alternative embodiment of the method according to the invention that the amount of the residual current pulse 16 is derived over the duration T _{A of} the supplementary time window 13 or 15 to ground 21 . FIG. 5 shows a corresponding time diagram of the current pulse 9 or 9 'and in FIG. 6 a corresponding sketch of a circuit. As in FIGS . 3a to 3d, the course of the current pulse 9 and 9 'as well as the time course of the transmission pulse 10 and the time course of a clock signal 22 are shown in FIGS . 5a to 5d. The time profile of the clock signal 22 corresponds to the time profile of the switching pulse 11 according to FIG. 3d. In addition, the time course of a switching signal 23 is shown in FIG. 5e.

In the circuit shown in FIG. 6, the switch 17 is now switched not only between the two connections 18 and 19 but additionally also between a third connection 24 , the third connection 24 being connected to ground 21 . According to the circuit according to FIG. 4, the two cycles in which the switch 17 is connected to the connections 18 and 19 are actively used. In these two cycles, either the current pulse 9 or 9 'in the acquisition time window 12 or the current pulse 9 or 9 ' in the acquisition time window 14 is measured. In contrast to the circuit according to FIG. 4, the switch 17 in the circuit according to FIG. 6 is connected to the third connection 24 over the duration of the supplementary time window 13 or 15 , so that the residual current pulse 16 is derived to ground 21 . In contrast to the circuit according to FIG. 4, in which the rectification of the current pulse I takes place with a two-pole switch 17 , in this embodiment the rectification takes place with a three-pole switch 17 . Of course, a multi-pole switch, for example a four-pole switch, can also be used.

As a result, the output signal is always positive, with a small signal corresponding to a small capacitance. Because the residual current pulse 16 is always derived to ground 21 , the duration T _{S of} the acquisition time window can be shortened even further, which has a positive effect on the immunity to interference of a corresponding sensor.

A value of the duration T _{S of} the acquisition time window 12 or 14 which is suitable in practice is between 10 ns and 500 ns, in particular between 100 ns and 500 ns. Because the duration T _{S of} the acquisition time window 12 or 14 is selected independently of the frequency of the transmission pulse 10 , the frequency of the transmission pulse 10 of the capacitive level measuring device 1 can be selected as a function of the medium 4 to be measured. Thus, a capacitive level measuring device 1 can be realized, which can be used universally with different media 4 .

For example, the same capacitive level measuring device 1 may then, if it is to monitor the level of water (ε _r ≍ 81), working with a transmitted pulse having a frequency of 100 kHz, while the same capacitive level measuring device 1, when it is the level of oil (ε _r ≍ 2) to monitor, uses a transmission pulse 10 with a frequency of 2 MHz. In both cases, the duration T _{S of} the acquisition time window 12 or 14 can be, for example, 130 ns, the acquisition time window 12 or 14 then being sufficiently short so that the situation "medium present" can be distinguished from the situation "buildup present" can. In the case of a - according to the prior art - phase-shifted evaluation of the current pulse 9 with respect to the transmission pulse 10 , a transmission pulse 10 with a frequency of 1.92 would also have to be reached in order to achieve a detection time window 12 or 14 with a duration T _S of likewise 130 ns MHz are used, so that the capacitive level measuring device 1 would then no longer be suitable for monitoring the level of water.

Claims (17)

1. A method for detecting the capacitance or a change in capacitance of a capacitive circuit or component, in which a transmit pulse is generated and applied to the capacitive circuit or component, the transmit pulse depending on the capacitance of the capacitive circuit or component, a current pulse generated and the current pulse is detected within a detection time window shifted relative to the transmission pulse, characterized in that the duration T _{S of} the detection time window has a predetermined value regardless of the frequency of the transmission pulse. 2. The method according to claim 1, characterized in that the detection of the current pulse with the help of a controlled rectifier. 3. The method according to claim 2, characterized in that both cycles of the rectifier can be used actively. 4. The method according to claim 3, characterized in that each cycle has a collection time window and a supplementary time window and Any residual current pulse present within the supplementary time window is detected. 5. The method according to claim 4, characterized in that the Current pulse over the duration of the acquisition time window or the residual current pulse over the duration of the supplementary time window is integrated. 6. The method according to claim 5, characterized in that the amount of Residual current pulse is subtracted from the amount of the current pulse. 7. The method according to any one of claims 1 to 3, characterized in that the residual current pulse over the duration of the supplementary time window after Mass is derived. 8. The method according to any one of claims 1 to 7, characterized in that the transmission pulse is substantially rectangular. 9. The method according to any one of claims 1 to 9, characterized in that the duration T _{S of} the acquisition time window is adjustable or controllable or regulatable. 10. The method according to any one of claims 1 to 9, characterized in that the duration T _{S of} the acquisition time window is between 10 ns and 500 ns, in particular between 100 ns and 500 ns 11. Method for determining the fill level of a medium in a container with a capacitive fill level measuring device, with a fill level sensor having several sensor fields, whereby a transmit pulse is generated and applied to a sensor field, the transmit pulse generates a current pulse depending on the capacitance of the sensor field and the current pulse is detected within a detection time window shifted with respect to the transmission pulse, characterized in that the duration T _{S of} the detection time window has a predetermined value regardless of the frequency of the transmission pulse. 12. The method according to claim 11, characterized in that the detection the current pulse with the help of a controlled rectifier that both cycles of the rectifier are actively used and that each cycle is one Has acquisition time window and a supplementary time window. 13. The method according to claim 12, characterized in that within the Supplementary time window, any residual current pulse detected becomes. 14. The method according to claim 13, characterized in that the Current pulse over the duration of the acquisition time window or the residual current pulse over the duration of the supplementary time window is integrated and that the amount of the Residual current pulse is subtracted from the amount of the current pulse. 15. The method according to claim 13, characterized in that the Residual current pulse derived over the duration of the supplementary time window to ground becomes. 16. The method according to any one of claims 11 to 15, characterized in that the level meter has a microprocessor and the Microprocessor, depending on the medium to be measured, the frequency of the Transmit pulse determined. 17. The method according to any one of claims 11 to 16, characterized in that the frequency of the transmit pulse is between 50 kHz and 5 MHz, is in particular between 100 kHz and 2.5 MHz.