Circuit for determining the capacity or the capacity change of a capacitive circuit or module

The invention relates to a circuit arrangement for detecting the capacitance or a change in capacitance of a capacitive circuit element or component, with a Clock generator, a controlled by the clock generator changeover, a Storage capacitor, a voltage source and an evaluation step, in which one electrode of the capacitive circuit element or component with the input of the Changeover switch is connected, the first output of the changeover to the first electrode of the storage capacitor, the first electrode of the storage capacitor one hand, via a resistor network to the voltage source and on the other hand with the evaluation stage and the second electrode of the storage capacitor are connected to a reference potential.

Within the scope of the invention is to "capacity", the capacitance value of a capacitive Circuit or device meant; a "capacity change" therefore means a Changing the capacitance value of a capacitive circuit element or component. By "capture" of the capacitance or capacitance change is within the Invention both only qualitative detection and quantitative detection, So a real measurement, meant. "Capacitive circuit element or component" means within the scope of the invention, each circuit element and each component which capacitive properties, is often referred to as a capacity, in which case not the capacitance value is meant. A "capacitive circuit element or component" is in particular a capacitor. As "capacitive circuit element or component" is in the scope of the invention as well as the electrode of a capacitive proximity switch, in cooperation with an influencing body, respectively. "Capacitive circuit element or component" within the scope of the invention, z. B. but also the capacity, representing each capacitive, lines. Subsequently, instead of a "capacitive circuit element or component" always spoken by a sensor capacitor, without this being a limitation on a capacitor is connected in the strict sense.

In the context of the invention, by "voltage source" includes both a voltage source total as meant a connection of such a voltage source.

Finally explanatory even mentioned that in the invention of "changeover" a switch, it is meant, which is often also referred to as change-over, Thus, having an input and two outputs, the input of either is connected to the first output or the second output.

The circuit configuration described initially is known from German Offenlegungsschrift 40 39 006. However, the circuit arrangement described therein intended for use in a capacitive proximity switch neither yet particularly suitable. Rather, it serves as a capacitance-to-frequency converter for Generating a substantially rectangular transducer output signal with a of the capacitance of a capacitor to be measured dependent frequency.

The invention is based on the object, a circuit of the in specify speech standing type, suitable for use in a capacitive proximity switch is particularly suitable, but also used for other beneficial can be, with the help of well interference, such as component tolerances or temperature drift can be compensated for in the measurement.

The inventive circuit arrangement is first and essentially characterized in that the evaluation stage is adapted for flow analysis and the changeover therefore virtually no voltage swing occurs. In the context of the invention means current evaluation that at the outputs of the changeover contact virtually no voltage difference occurs. The changeover switches between same Voltages such that the input of the changeover switch no voltage swing occurs.

In circuit arrangements of the type described in the introduction occur particularly in the range of the sensor capacitor, a plurality of parasitic capacitances on which the can distort the measurement result. Basically, such parasitic capacitances can only have an effect on the measurement result when a voltage rise occurs and thereby recorded or altered amount of charge on the measurement result impacts. If such a voltage swing prevented, then can the the circuitry inherent parasitic capacitances no longer affect. Thus, there is a first important advantage of the circuit arrangement in accordance with the teaching of the invention is that the measurement result for the negative influence of the parasitic Capacity is completely or largely avoided.

If, as stated above, at the entrance of the changeover no voltage swing occurs, this means that the sensor is not sensitive capacitor and thus, for example, a proximity switch does not detect a be left near object would. For this reason, in the circuit arrangement according to the invention an auxiliary voltage to the capacitive circuit element or component is applied, which is synchronized with the changeover frequency of the clock generator. As a result of that by the applied supply voltage at the input of the changeover and thus also on the sensor capacitor, a voltage rise occurs, the capacity or a Change in capacitance of the sensor capacitor are measured.

According to a preferred, particularly advantageous embodiment of the invention, the auxiliary voltage is temporarily powered down and out of the then remaining measurement a Kompensationsmeßwert generated. If the auxiliary voltage is switched off, then the sensor capacitor electrically isolated from the outside world, and it will affect only influences from arising within the circuit arrangement. The then remaining measurement includes all offset errors of the entire circuit arrangement, such as component tolerances, manufacturing tolerances or temperature influences. The results from all these disturbances remaining measured value can now be as new "zero" value is stored and taken as a reference for the further measurement will. For such a compensation cycle, in addition to switching off the auxiliary voltage different techniques possible, all based on the modulation the auxiliary voltage is based in order from the currently measured value with the actual Capacitance value and the actual change in capacitance of the sensor capacitor to determine which then disappears all unavoidable offset influences are. It is possible, for example, a phase or amplitude modulation of the auxiliary voltage as well as a phase inversion of the auxiliary voltage by 180 ° in synchronism with the Switching time of the changeover.

An additional improvement experienced by the inventive circuitry in that one related to the output stage reference branch for measuring a reference capacitance with a second changeover contact, a second storage capacitor is and a second resistor network are provided. there are the reference capacitor, the second changeover contact, the second storage capacitor and the second resistor network in the same way connected to each other, as the sensor capacitor, the first changeover contact, the first storage capacitor and the first resistor network are connected. Becomes without such a reference branch performed the circuit of the invention, so it is very simple and due to the small number of components also very cost-effective to implement, but it requires a relatively large payload because no difference is possible. Such circuitry may, however, in particular in connection with the possible use of a compensation cycle, used an inexpensive alternative one example, as a manual button his proximity switch.

By arranging a parallel reference branch connected to the same clock generator is connected, are two distinct and completely separate measuring channels available that do not affect each other. The outputs of the two Measurement channels can be supplied for the purposes of a differential signal of the evaluation stage will.

As with the use of a single measuring channel can also using a additional reference channel both switching cycles of each channel being actively used be such that in spite of low interference purest possible common mode signal is concluded that with appropriate amplifier technology by CMRR can be separated from the useful signal. Advantageous for good CMRR of a low-frequency interference signal is also when both Switching cycles in time are of equal length, ie, the clock generator a duty cycle yields of about 50%.

According to a preferred embodiment of the invention, the second output is the first changeover switch to the first output of the second changeover switch and the first output of the first changeover switch to the second output of the second changeover switch connected. Such interconnection of the measuring channel and the reference channel provides two active outputs, which for a good Niederfrequenzstörunterdrückung can be used at the same time two measuring inputs are present, as it is particularly advantageous for proximity switches. Of the Useful current in the one switching cycle is from the useful current in the second switching cycle withdrawn without prior amplification is necessary, which may could provide a distortion of the measurement signal.

The coupling of the auxiliary voltage to the capacitive circuit element or component is possible in different ways. For example, the auxiliary voltage via an external probe construction to the one electrode of the sensor capacitor docked. Then, if necessary, this electrode is provided as a capacitively insensitive Receiving electrode, given that it perform any voltage swings have to be. Alternatively, the coupling of the auxiliary voltage can also via an internal Coupling capacitor made to the one electrode of the sensor capacitor. The then present series circuit of the coupling capacitor and sensor capacitor represents a capacitive voltage divider so that the voltage swing at the one electrode of the sensor capacitor depends on its capacitance value. A Change the voltage swing then causes current to flow through one of the Electrode of the sensor capacitor downstream resistance and thus the required Current variation in the measurement channel.

According to a further embodiment of the invention, the evaluation step from a Differential current amplifier, where a microprocessor is connected downstream. Contains Inventive circuitry such evaluation stage, then the Compensation cycle described above are particularly effectively used. It is then possible, for example, such a compensation cycle in certain trigger time intervals or triggered by certain events and to store the then-resulting new "zero" value in the microprocessor and for the to use next, as a reference.

In particular, there are a variety of ways, the circuit arrangement designing and further accordance with the teachings of the invention. Reference is made on the one hand to the claims subordinate to claim 1, on the other hand to the description of a preferred embodiment in connection with the drawing. The only figure of the drawing, a preferred Embodiment of a circuit arrangement for measuring a capacity to the teaching of the invention.

The circuit arrangement now comprises a clock generator 1, one of the Clock generator 1 controlled changeover 2, a memory 3, a Voltage source 4 and an output stage 5. An electrode 6 of this capacity is connected to the input 7 of the changeover second The second electrode the sensor capacitor represents a not shown here external object. The required capacity is then between the electrode 6 and the external by the Object formed mass. The first output of the changeover 8 2 is the first electrode of the storage capacitor 3 is connected. This electrode is a hand via a resistor network 9 to the voltage source 4 and the other hand with the evaluation stage 5. The second electrode of the storage capacitor 3 is connected to reference potential 10. In the embodiment shown here, a Circuitry corresponds to the Bezrngspotential 10 mass.

Next, the circuit configuration shown in the drawing, an auxiliary voltage 11, which is synchronized with the changeover frequency of the clock generator the first The auxiliary voltage 11 may additionally an unrepresented frequency or amplitude modulator be followed. In the present case the auxiliary voltage is 11 coupled via an external probe construction 21 to the electrode 6 of the capacitive circuit element or component.

The circuit arrangement shown in the drawing further includes a second Changeover contact 12, a second storage capacitor 13 and a second resistor network 14. Thus, the circuit arrangement comprises a measuring arm and a reference branch. For measuring arm includes the first changeover 2 to the input 7, the first output 8 and a second output 15 and the Sensor capacitor whose one electrode is shown 6, further, the storage capacitor 3 and the resistor network 9, and an additional resistor 16 to the reference branch includes a first reference capacitor whose one electrode 17 is shown, the second change-over contact 12 to a first output 18 and a second output 19, the second storage capacitor 13 and the second Resistor network 14 and a further resistor 20. The second electrode of the Reference capacitor provides, as the sensor capacitor, which is not shown here However, external object. The reference capacitor as a whole can also are formed by a fixed capacitor.

The two resistor networks 9 and 14, each comprising a variable resistor included, the optimum operating point can be adjusted. Both resistor networks 9 and 14 should advantageously be approximately the same size.

In the embodiment of the invention shown in the figure Circuitry, the second output 15 of the first changeover switch 2 to the first output 18 of the second changeover switch 12 and the first output 8 of the first changeover switch 2 to the second output 19 of the second Changeover switch 12.

As evaluation stage 5 can in principle each amplifier circuit are used, has a low input impedance and operates in the sense of a current evaluation. Such an amplifier circuit, for example, a current-voltage converter be.

The evaluation stage 5 shown in the figure consists of a differential current amplifier circuit with two operational amplifiers 22 and 23 and a downstream, as subtractor working third operational amplifier 24. The internal Interconnection of the evaluation stage 5 is ensured that the voltage U₁ am not inverted input of the operational amplifier 22 of the voltage U₂ the non-inverted Input of operational amplifier 23 corresponds. Incidentally, the operational amplifier 24, which is not shown here, a microprocessor connected downstream will. A sol-cher microprocessor can be used, the compensation cycle, ie switching off the auxiliary voltage and modulating the auxiliary voltage, control. With such a microprocessor, it is possible to Compensation cycle at certain time intervals or by certain events triggered trigger.

Both the measuring channel and the reference channel may in each case before the switch-over 2 or 12 for suppressing high-frequency interference components - here not shown - RC low-pass filters are switched. Again, it has a positive effect out that no voltage swings occur at the change-over contacts 2 and 12, so that without significant effect on the measured value smaller capacities in the Measuring and reference channel can be switched.

Further up has been carried out, that the auxiliary voltage to the capacitive Circuit element or component is applied to the switching frequency of the clock generator synchronized. Here, the auxiliary voltage relative to the voltage of the Clock generator also be delayed or phase-shifted, preferably phase 90 °, because the particularly simple and reliable from the double can be obtained switching frequency of the clock generator. This measure can the temperature-related influences on the gate delays the changeover be minimized, if it is ensured that the switching time the evaluated charge transport is substantially complete. That means, that a slight change in gate delay any change in the measured Current integral between the switching times causes more.