Measuring circuit for determining a measuring capacity

p0001The invention relates to a circuit arrangement for determining a measuring capacitance.

State of the art

p0002To measure very small capacities or capacity changes are for example capacitive proximity switches are used, which have been known since the late sixties. They operate essentially on the principle of influencing an electric alternating field by a dielectric of an object or medium that is significantly greater than the dielectric of air or vacuum. The introduction of such a dielectric in the near field of one or several measuring electrodes where it leads to a corresponding concentration of the electric field lines and thus the amplification of the electric field. This increases the capacity, for example, increasing a capacitor, such as a plate capacitor, between the plates of the dielectric is introduced, by a small amount. This capacitance change is evaluated in an evaluation circuit. For example, a switching signal is output by the evaluation circuit if the capacity exceeds a predetermined value. Such capacitive proximity switches come to the most diverse technical fields of application. They can be applied on the monitoring of Grenzfüllständen media of all kinds, for example, aqueous media, granules, powders, oils and the like. They are used both in a medium dipping for detecting for example the filling level as well as non-contact, for example by arranging outside of a non-metallic container wall, for example to detect the filling level in the container. Another application is the detection and recognition of objects over a certain distance, for example, the detection of a paper stack, the detection of a metal object, a glass and the like.

p0003The capacitive sensors are operated by means of electronic circuits, the operation of which can be divided in terms of their operating principle into two main groups.

p0004On the one hand there are oscillatory processes in which typically the oscillation condition of the oscillator is controlled as a feedback amplifier shortly before the onset of vibration on an electrode system. The electrode system forms in this case usually a capacitive voltage divider with the measurement capacitance as an integral part. The height of the measuring capacitance affects the overall gain, the phase position and thus the vibration behavior. Such oscillator circuits for the detection of small capacities and capacity differences for use in capacitive sensors go for example from<patcit id="pcit0001" dnum="DE10156580A1"><text>DE 101 56 580 A1</text></patcit> or from <patcit id="pcit0002" dnum="DE1673841A1"><text>DE 1673841 A1</text></patcit> forth. With the help of such circuits and methods are low circuit complexity high sensitivities or switching distances of capacitive sensors and outstanding "sensory qualities", such as the compensation of buildup of a medium distance. A disadvantage of this oscillatory method, however, is their high susceptibility to electrical alternating fields in a more or less broad band around the oscillation frequency. Therefore, such sensors meet in many cases not the EMC Immunity requirement, part IEC61000-4-6.

p0005In addition to the above-described oscillatory method of operating the sensors in a sense exist "externally controlled" process, in which a generator is a measurement circuit according to different principles controls, for example in the form of capacitive bridges, oscillating amplifiers, charge balance method, phase comparators and the like.

p0006Capacitive sensors, which operate according to this principle, for example, go from the <patcit id="pcit0003" dnum="DE19949985A1"><text>DE 199 49 985 A1</text></patcit>, from the <patcit id="pcit0004" dnum="DE19701899C2"><text>DE 197 01 899 C2</text></patcit>, from the <patcit id="pcit0005" dnum="EP1093225B1"><text>EP 093 225 B1 1</text></patcit> or from <patcit id="pcit0006" dnum="DE102005057558A1"><text>DE 10 2005 057 558 A1</text></patcit> forth. Although capacitive sensors, which are operated in this way, in many cases significantly less sensitive to interference voltages in a wide frequency range. A disadvantage, however, is that such sensors can be realized only with a much higher circuit complexity at most lower sensory quality. As "sensory quality" should be understood in this application the following parameters or characteristics:<ul><li>a large adjustable and usable sensitivity range, ie a wide range between the minimum and maximum adjustable operating distance;</li><li>low temperature dependence of the switching distance;</li><li>the ability of the compensation damp, conductive buildup in level applications;</li><li>the series stability and independence of component tolerances.</li></ul>

p0007So far there are no capacitive sensors that exhibit these properties at the same time a high EMC immunity. For example, in the from of<patcit id="pcit0007" dnum="DE102005057558A1"><text>DE 10 2005 057 558 A1</text></patcit> stemming sensor for the contactless detection of the level of a liquid and adhering medium high conductivity, for example, blood, although obtained by a non-metallic container wall of a container and the method described there an excellent compensation of buildup in level applications with high noise immunity. For this sensor, and the method for the contactless detection of the level achieved by liquid media no high switching distances, as required in the distance sensor.

p0008In the out of <patcit id="pcit0008" dnum="DE19945330A1"><text>DE 199 45 330 A1</text></patcit> Although stemming method for detecting and evaluating a change in capacitance and the sensor described there are, for example, higher operating distances achievable and it is a compensation excessive noise possible, but this sensor allows no compensation of buildup ionizing media.

p0009The invention is therefore based on the object to provide a circuit arrangement for determining a measurement capacity, in particular for use in capacitive proximity switches, which in simple circuitry and high immunity meets all sensory qualities mentioned above in an excellent manner.

Disclosure of the Invention

p0010This object is achieved by a circuit arrangement for determining a measuring capacitance comprising a reference circuit portion for defined periodic charging and discharging a predetermined reference capacitance and a measurement circuit portion for defined periodic charging and discharging to be determined measuring capacity and further comprising at least one circuit element to form at least one the timing of the charging of the reference capacitance characterizing size and to form at least one and the timing of the charge of the measuring capacitance characterizing size and a circuit section for comparing the timing of the charging of the reference capacitance variable characterizing the temporal course of the charging of the measuring capacitance variable characterizing Close includes the value of the measured capacitance due to this comparison.

p0011The basic idea of ​​the invention is that circuitry into two parts, a reference circuit portion and a measurement circuit part, divide, the circuit parts are symmetrical, so formed each corresponding equal and both circuit parts periodically charging a reference or measuring capacity each and each time variation of the detect charging variable characterizing both the reference and the measuring capacitance. In another circuit component, the measuring capacity is then determined based on the comparison. Such symmetrical configuration of the reference circuit portion and the measuring circuit section allows very advantageous high noise immunity of the circuit, because all disorders, for example, temperature variations and the like affect both circuit parts the same, it being understood that these circuit parts arranged in the capacitive sensor in the immediate vicinity are such that they are exposed to substantially identical ambient conditions and disorders in both circuit parts have the same effect.

p0012The evaluation of the temporal course of the charging parameter characterizing both the reference capacitance and the measuring capacitance is preferably carried out by a comparison of these quantities in the corresponding circuit part. Whenever the time course of the charging of the measuring capacitance characterizing size corresponds to the temporal course of the charging of the reference capacitance variable characterizing a circuit signal is output. In this case the value of the measuring capacitance is the value of the reference capacitance.

p0013Purely in principle, it would be conceivable to capture both the temporal course of the charging of the measuring capacitance characterizing size and the time profile of the charging of the reference capacitance characteristic value in a single circuit element. A particularly simple implementation of circuitry provides, however, assign both the reference circuit portion and the measurement circuit part each have their own circuit portion for detecting the time course of the charging of the measuring capacitance and the reference capacitance characterizing size.

p0014With regard to the timing of the charge of the measuring capacitance and the reference capacitance characterizing size are purely in principle the most varied sizes, which permit a statement about the timing of the charging of the measurement or reference capacitor, conceivable. According to a particularly advantageous embodiment this size is for example the temporal arithmetic mean or the DC voltage component of the charging voltage of the measurement or reference capacitor.

p0015According to a particularly advantageous embodiment, the charging and discharging of the reference capacitance and the measuring capacitance clocked periodically carried out such that in a first time interval of a period, a simultaneous charging of the reference capacitance and the measuring capacitance and in a second time interval of the period, a simultaneous discharge both the measurement and reference capacitance effected.

p0016It is preferably provided that the second time interval is made substantially shorter than the first time interval, so that the charging takes place much longer than the sudden discharge. This ensures that the value of the temporal arithmetic average of the charging voltage of the measurement or reference capacitor and the common mode component of the alternating signal of the charging and discharging of the measurement and reference capacitance have larger values ​​with the associated advantages of detecting these values. In the case of the detection of the time course of the average value of the charging voltage of the reference or measuring capacity is advantageously arranged both in the reference circuit portion and in the measurement circuit part in each case at least one RC element. Such RC network provides a simple approach in the case of a charging curve, ie an exponential function, a temporal arithmetic averaging.

p0017The part of circuit for comparing the mean values ​​and for closing on the measurement capacitance is preferably a comparator, at whose input the output signals of the circuit parts for averaging.

p0018In order to achieve a simultaneous charging and discharging of both the measurement and the reference capacity with high precision, it is provided according to an embodiment that the periodic charging and discharging is performed by a clock generator which controls a switch arrangement that simultaneously the reference capacity and the measuring capacitance to ground connects. It is - as mentioned above - the time interval in which the reference and measuring capacitance are connected to ground, is substantially shorter than the time in which the switch assembly is opened in each case, ie charging of the reference or measuring capacitance occurs.

p0019The switch arrangement is preferably realized by a diode network or Junction FETs, which are particularly susceptible to interference at the same time very precise switching characteristics. In particular, this arise compared to integrated analog switches very beneficial only small parasitic capacitances, for example, when from the<patcit id="pcit0009" dnum="DE19701899C2"><text>DE 197 01 899 C2</text></patcit> arising circuit that operates with switches, so two switches and thus double parasitic load on the measurement capacity, can occur.

p0020It is particularly preferred that the charging of the reference capacitance is carried out with an adjustable voltage source, whereas the charge of the measuring capacitance occurs at a fixed low voltage source. In this manner, can be set in a capacitive sensor, the operating distance, as will be explained in greater detail below.

p0021The reference circuit part and the measurement circuit part are preferably within the same housing in close proximity to each other, especially on the same printed circuit board arranged in order to compensate for the compensation of linear interference, for example, temperature influences or influences due to component drifts very beneficial can because this interference effect on you both symmetrical circuit portions of the same.

drawings

p0022Further advantages and features of the invention are the subject of the following description and the drawings illustrating embodiments.

p0023In the drawings:<dl id="dl0001" compact="compact"><dt>Fig. 1</dt><dd>a first embodiment of an inventive circuit arrangement;</dd><dt>FIG. 2</dt><dd>schematically the charging voltages of the reference and measurement capacity and their evaluation;</dd><dt>Fig. 3</dt><dd>a second embodiment of an inventive circuitry;</dd><dt>Fig. 4</dt><dd>a third embodiment of an inventive circuitry and</dd><dt>Fig. 5</dt><dd>another embodiment of an inventive circuit arrangement.</dd></dl>

Description of embodiments

p0024A first embodiment of an inventive circuit arrangement is in <figref idrefs="f0001">Fig. 1</figref> shown. Electronic switches 2, 2a are periodically turned on by a square wave generator 10 with a control frequency 1 / (Ton + Toff) for a time Ton and off for a time Toff. During the period Ton are a measuring capacitance or measuring electrode 1 and a fixed reference capacitance 1a through the switch 2 or 2a to a fixed potential, here, for example, connected to ground. This increases the measuring capacitance 1 and the reference capacitance 1a suddenly discharged. After this time, open sound, the switches 2 and 2a. The capacitors 1 and 1 a are charged via the respective associated resistors 3 or 3a. Here, charging currents are a fixed DC voltage source U_S 6, which can be implemented by means of a stabilized internal sensor supply, and an adjustable DC voltage source U_V 8, respectively. The charging voltages at the capacitors 1 and 1 a rise in accordance with an exponential function, which represents the Charge capacity at. The voltage increase is directly dependent on the size of the capacity, the size of the charge resistors and charge currents and of the supply voltages. With a suitable dimensioning of the maximum charging voltage U_Lend not reached after the time interval Toff before capacity could again be suddenly discharged by closing the switch 2 and 2a during the time interval Ton. The charging and discharging continues in clock generator 10th

p0025The waveforms are schematically illustrated in <figref idrefs="f0002">FIG. 2</figref> shown. From the periodic waveforms that appeal to the capacity in the form of voltages U_L1, The charging voltage of the measuring capacitance, and U_L2, The charging voltage of the reference capacitance, adjust is closed manner described below to the value of the measuring capacitance. 1 For this purpose, an averaging of the charging voltage U_L1 or U_L2 the measuring capacitance 1 and the reference capacitance 1a provided. This averaging is 7 and 7a realized by RC low-pass filters of the first order, formed by resistors 4 and 4a and capacities, which out seven the arithmetic mean or the DC voltage component of the alternating signal to a certain extent. The resistors 4 and 4a should preferably be at least five times greater than the resistances 3 and 3a for good decoupling from charging and discharging.

p0026The basic idea of ​​the invention is this averaging of up and discharge at the two capacities: measuring capacitance and reference capacitance. The averages and DC components of the charging voltages are each reversible clearly assigned to the reached Ladeendspannungen as in<figref idrefs="f0002">FIG. 2</figref> can tell where the charging voltage U_L1, A mean value M₁ and the charging voltage <b>U_L2</b> an average value M₂ assigned. These signals M₁ and M₂ a voltage comparator 9 are supplied. The voltage on the "-" - input is influenced directly by the measure. It decreases with increase in capacity 1, which is a decrease of charging relaxation U_Lend1 and thus the mean value M₁ leads to. Falls below the signal at the "-" - input of the comparator 9, the signal M₂ the average value of the charging voltage of the reference capacitance, which is formed in the reference circuit portion and the "+" - rests input of the comparator 9, tilts the comparator 9 by the low- to the high state and are, for example, a signal S, which, for example, the detection of a medium displays. The closer the value of the mean M₂ on the value of the average value M₁ is, the smaller the limit value of the measuring capacitance 1, from which the comparator 9 is tilted. The sensitivity setting can in a wide range by the adjustable DC voltage U_V by means of the adjustable voltage source. 8

p0027The reference circuit part which is, so to speak, against the same constructed symmetrically to the measurement circuitry for measuring circuit part, is in <figref idrefs="f0001">figure 1</figref> each marked with the numeral-suffix "a".

p0028The symmetrical, ie identical design structure of the two circuit parts occur component-related drifts, eg temperature drifts or drifts due to aging of components in both branches is equal to. The same drift in both branches gives a theoretical extent differential voltage zero. This drift are largely eliminated. The circuit arrangement has for this reason a very low temperature dependence and thus a very large achievable sensitivity. The sensory quality of the above-described circuitry is essentially determined by the properties of the electronic switches 2 and especially by their parasitic capacitances. The switches may for example be realized inexpensively with Junction FETs or diode networks. The high noise immunity due to the fact that overlap undisturbed in a linear system many signals. With an open switch 2 during the time interval Toff are only resistors, so-linear components of the electrode 1 at. The average value of the coupled-noise voltage after the RC low-pass filters 4, 7, 4a, 7a is always zero. The only prerequisite is that the electronic switch arrangement 2, 2a large when opened Störspannungshübe the electrode terminal not limited. This is achieved by adequate blocking voltages in a diode switch network or the use of junction FETs, which have a negative reverse bias voltage. During the time interval Ton, the measuring electrode 1 is connected to a fixed potential, and thus in this time period not be interfered with.

p0029A further embodiment of the circuit arrangement will be described in connection with <figref idrefs="f0003">Fig. 3</figref> described, in which like elements in <figref idrefs="f0003">Fig. 3</figref> with the same reference numerals as in <figref idrefs="f0001">Fig. 1</figref> are designated, so with regard to their description reference is made to the above incorporated by reference. The circuitry in<figref idrefs="f0003">Fig. 3</figref> differs from that in <figref idrefs="f0002">FIG. 2</figref> characterized in that the measuring electrode 1 surrounded by an annular compensation electrode 11th This compensation electrode 11 has the task of interference that are caused by capacitive loads, for example, a plastic housing in which the circuit arrangement and the sensor are installed to minimize the measuring electrode. 1 The compensation electrode 11 acts so far interference, for example, outer buildup of a medium on the sensor housing by dipping and pulling at a level query by a compensating alternating field meet. This circuitry is particularly in free zone application is being used.

p0030This in <figref idrefs="f0004">Fig. 4</figref> Illustrated embodiment differs from that in <figref idrefs="f0001">Fig. 1</figref> illustrated by the fact that the measuring electrode 1 surrounded by a shield electrode. 13 Again, in turn, the same elements with the same reference numerals as in<figref idrefs="f0001">Fig. 1</figref>So that with regard to their description reference is made to the above reference. The shield electrode 13 is the rear shield of the measuring electrode 1 and is regarded as directly back to the measuring electrode arranged or surrounds the measuring electrode back. The shield electrode 13 is buffered by a non-inverting amplifier 14 with the voltage gain 1 and hochimpedantem possible input and low-impedance output.

p0031Another embodiment of a circuit according to the invention is in <figref idrefs="f0005">Fig. 5</figref> shown, wherein again the same elements by the same reference numerals as in <figref idrefs="f0001">Fig. 1</figref> and <figref idrefs="f0002">FIG. 2</figref> are provided, so that with respect to the description thereof reference is made to the foregoing terms. The circuitry in<figref idrefs="f0005">Fig. 5</figref> differs from the in <figref idrefs="f0001 f0002 f0003 f0004">FIG. 1 to FIG. 4</figref> illustrated by the fact that a fixed DC power source 15 and, instead of the voltage source 8 and the resistor 3a is an adjustable DC current source 16 are provided instead of the voltage source 6 and resistance 3rd In this circuit, the charging of the measuring capacitance 1 and the reference capacitance 1a of these constant current sources 15 and 16 is not in accordance with an exponential function, but linearly with time.

p0032In addition, it should be emphasized that above-described circuit arrangements can also be designed such that all the polarities may be reversed, that is, during the time Toff could capacity 1 and 1a also, for example about negative voltage sources 6 and 8 or current sink in place of the constant current sources unloaded 15 and 16 and sound are suddenly charged to a positive value for a time. Also this allows to the above-described principle of the invention realized.