DE102005024772A1 - Circuit arrangement for a capacitive proximity switch - Google Patents

Abstract

A Circuitry for a capacitive proximity switch for determining its operating state with a capacitive sensor element is described whose capacity (C3) itself dependent on of the operating state changes. she still has a common capacitor (C2), a control signal generator (SG) for generating a control signal (U3) at a control signal output (N1) and a controllable switching means (T1). This is with one first terminal (A1) connected to the capacitive sensor element (C3) and with a second connection (A2) to the collecting capacitor (C2) connected. Dependent on from the drive signal (U3), the controllable switching means connects the Sensor element (C3) with the collecting capacitor (C2) for the transfer of Charge from the sensor element (C3) to the accumulation capacitor (C2). Between the control signal output (N1) and the first terminal (A1) of the switching means (T1), a charging resistor (RL) is looped.

Description

The The invention relates to a circuit arrangement for a Capacitive proximity switch according to the preamble of claim 1, in particular according to the charge transfer principle.

Circuit arrangements of this type are known and have, for example, in the EP 0 859 468 A1 a capacitive sensor element whose capacitance changes depending on its operating state. This capacitance change is evaluated to determine the actuation state. For this purpose, the sensor element is charged with a charging voltage, whereby a certain electrical charge is transferred to the sensor element as a function of its capacity and the charging voltage. After a charging time, the sensor element is disconnected from the charging voltage and connected to a collecting capacitor, whereby a charge transfer from the sensor element takes place on the collecting capacitor. The process of charging and subsequent recharging is repeated for a predetermined number of cycles, whereby the charge of the collecting capacitor reaches a certain value, which is determined inter alia by the value of the capacitance of the sensor element. The charge or the resulting voltage of the collecting capacitor is therefore a measure of the capacitance of the sensor element to be measured. By evaluating the voltage of the collecting capacitor can be concluded that the operating state of the proximity switch. After voltage evaluation, the common capacitor is discharged in a defined manner and a new measuring cycle can follow.

The switching operations become conventional realized by analog switch, which are relatively expensive. Farther the sensor element can only up to the instantaneous voltage of the Unloading bulk capacitor, causing the transferable charge decreases with increasing charging of the collecting capacitor and consequently reduces the signal resolution becomes.

To solve this problem, the DE 103 03 480 A1 a circuit arrangement for a capacitive proximity switch with a control signal generator for generating a control signal, which serves to drive two connection means. The control signal causes the connection means to be alternately conductive, thereby causing charging of the sensor element followed by charge transfer to the collection capacitor.

Task and solution

Of the The invention has for its object the provision of a circuit arrangement based on the type mentioned, the safe determination of the operating state of the proximity switch guaranteed under all operating conditions, inexpensive to produce and insensitive to EMC and RF interference is.

The Invention solves This object is achieved by a circuit arrangement having the features of Claim 1. Advantageous and preferred embodiments of the invention are the subject of the further claims and will become hereafter explained in more detail. Of the Wording of the claims is by express reference to the content of the description.

The circuit arrangement according to the invention comprises a capacitive sensor element whose capacitance changes as a function of the actuation state, a collecting capacitor, a control signal generator for generating a control signal at a control signal output, a controllable switching means and a charging resistor. The controllable switching means is connected to a first terminal with the capacitive sensor element and connected to a second terminal to the collecting capacitor. It connects in dependence on the drive signal, the capacitive sensor element with the collecting capacitor for transferring the charge from the capacitive sensor element to the collecting capacitor. The charging resistor is connected between the control signal output and the first terminal of the switching means. The switching between a charging phase of the sensor element and the charge transfer phase takes place in the clock of the control signal, whereby an additional switching logic can be omitted. In this case, the control signal is applied to the sensor element via the charging resistor when the switching means is open, ie during the charging phase of the sensor element. The resistance value of the charging resistor is dimensioned such that the sensor element is charged during the charging phase substantially to a level of the control signal or the control voltage. During the charge transfer phase, ie with the switching means closed, the substantial part of the charge-reversal current flows via the switching means into the collecting capacitor, since the resistance value of the charging resistor is further dimensioned so that it is substantially higher-impedance than a resistance value of the switching means. Compared to a diode as a connection means between the control signal output and the first terminal of the switching means, the charging resistor has a lower temperature dependence and is less expensive. Such a circuit arrangement is therefore easy to set up low cost and insensitive to interference.

In a development of the circuit arrangement is the control signal generator for generating a control signal in the form of a non-constant voltage designed, in particular a square-wave voltage. This allows a Switching between the charging phase of the sensor element and the charge transfer phase in time with the level change of the control signal.

In a development of the circuit arrangement, the control signal generator a DC voltage source and a square-wave voltage source with common reference potential, wherein between the control signal output and the DC voltage source looped a clamping diode in the reverse direction is. Between the control signal output and the square-wave voltage source a capacitor and a resistor are connected in series. By Such an arrangement makes it possible to have a rectangular control voltage to generate at the control signal output, in the clock of the square wave voltage source between the potential of the DC voltage source and a sum potential from the potential of the DC voltage source and the potential of the Square-wave voltage source alternates. This allows an almost complete and discharging the sensor element independently of the control or charging voltage or the state of charge of the common capacitor, whereby a linear Voltage rise is effected on the collecting capacitor. The possible signal resolution is greatly improved.

In a development of the circuit arrangement is the switching means a bipolar transistor, in particular a pnp transistor. With help a bi-polar transistor, it is easy and inexpensive, a Switching function depending to realize from the control signal or the control voltage. expensive and sensitive analog switches can be omitted.

In a development of the circuit arrangement is the basis of the transistor connected to the control signal output. This wiring will reaches that transistor in response to the control signal is conductive, further control signals are not necessary.

In a development of the circuit arrangement is between the first Connection of the switching means and the sensor element is a resistor looped. The resistor makes the circuit less sensitive across from EMC and RF interference, the above the sensor element or its wiring are coupled.

In a development of the circuit arrangement is a switch in parallel switched to the common capacitor. This allows a discharge of the common capacitor before starting a new measurement. This leads to a sawtooth course a voltage at the collecting capacitor.

In a development of the circuit is the collective capacitor a resistor connected in parallel. The resistance is associated with the accumulation capacitor is an averaging agent, so that sets an approximately static voltage at the collecting capacitor.

In a development of the circuit arrangement, the circuit arrangement a plurality of capacitive sensor elements, each having a switching means and a charging resistor is assigned, and only a single one Collecting capacitor. This with the respective Schaltmit stuff over each a decoupling diode connected in the forward direction, wherein the Anode of the decoupling diode by a selection diode in the forward direction is connected to a respective selection signal. With the help of a Such circuitry, it is possible the operating state several proximity switches to evaluate in multiplex mode. The selection of the corresponding Proximity switch is done by the selection signal, through which the charge transfer from the selected one Sensor element is released to the single collecting capacitor. The charge of the non-selected sensor elements flows over the respective selection diode from. The charging voltage can be central to disposal be put.

In one development of the circuit arrangement, the capacitive sensor element is designed to be applied to a lower side of a surface or cover with dielectric properties, wherein it preferably has a smooth planar surface for abutment. In a further development of the circuit arrangement, the capacitive sensor element is a voluminous, elastic, preferably elongated body of electrically conductive material. Such a sensor element is for example in the EP 0 859 467 A1 The contents of this description are incorporated herein by express reference.

These and other features will become apparent from the claims and from the description and drawings, wherein the individual features each alone or more in the form of sub-combinations in one embodiment of the invention and in other fields be realized and advantageous and protectable Represent embodiments for which protection is claimed here. The subdivision of the application into individual sections as well as intermediate headings does not limit the statements made thereunder in their generality.

Summary the drawings

advantageous embodiments The invention are shown schematically in the drawings and are described below. Hereby shows:

1 a circuit diagram of a circuit arrangement for capacitive proximity switches for determining their operating state,

2 a diagram of the voltage waveform of a square wave voltage source U2 of 1 and a control signal U3 at a control signal output N1 of 1 .

3 a diagram of a first voltage waveform at a collecting capacitor C2 of 1 depending on the operating state of a proximity switch,

4 a circuit diagram of a circuit arrangement with a plurality of capacitive sensor elements and

5 a diagram of a second voltage waveform at the collecting capacitor C2 of 1 depending on the operating state of the proximity switch.

detailed Description of the drawings

1 shows a circuit diagram of a circuit arrangement for capacitive proximity switches for determining their operating state. The capacitive proximity switch is in 1 modeled by a capacitive sensor element or a capacitor C3. The capacitive sensor element may, for example, be applied to a lower side of a surface or cover with dielectric properties.

The circuit arrangement comprises a control signal generator SG with a DC voltage source U1 and a square-wave voltage source U2 with common reference potential, ground for example, between a control signal output N1 of the control signal generator SG, at which a control signal or a control voltage U3 is output, and the DC voltage source U1 a Clamping diode D1 is looped in the reverse direction and between the control signal output N1 and the square-wave voltage source U1, a capacitor C1 and a resistor R1 are connected in series. The clamp diode D1, in conjunction with the capacitor C1, causes an increase in the voltage output by the square-wave voltage source U2 at the node N1 by the amount of the voltage of the DC voltage source U1. 2 shows this relationship in a diagram of the voltage waveform of the AC voltage source U2 and the control voltage U3 at the control signal output N1 over time.

Of others are a charging resistor RL and a switching means in shape a pnp transistor T1 provided. The base of the transistor T1 is connected to the control signal output N1. Between the emitter as the first terminal A1 of the transistor T1 and the capacitive sensor element C3 is an optional resistor R2 looped. The resistance R2 is used for damping of disturbances, the above the capacitive sensor element C3 are coupled. The collector as a second terminal A2 of the transistor T1 is connected to a common capacitor C2, whose other terminal is connected to the reference potential is. The charging resistor RL is between the control signal output N1 and the first terminal A1 and the emitter of the transistor T1 looped in.

One optional capacitor C4 connected in parallel to the sensor element C3 is, serves as a basic capacity, around when not actuated Sensor element C3 a reference value or a quiescent voltage signal to generate the collecting capacitor C2. The collecting capacitor C2 is a switch S1 connected in parallel, which closes before the start of a measurement and thus completely discharges the collecting capacitor C2. Will the voltage curve evaluated at the collecting capacitor C2 by a microcontroller, this can the collection capacitor C2 before the start of a measurement discharge when the corresponding input is short-term to reference potential is switched. The switch S1 is omitted in this case. dashed can be represented to the collecting capacitor C2 instead of the switch S1 also be a resistor R3 connected in parallel.

Of the Charge resistance RL and the base of the transistor T1 are connected to the Control voltage U3 applied. When the control voltage U3 their higher value has, flows a charging current from the control signal output N1 via the charging resistor RL in the sensor element C3 and the capacitor C4, whereby the capacitance of the sensor element C3 and the capacitor C4 approximately to the voltage level of the control voltage U3 charge. The transistor T1 locks in this case because its base-emitter voltage is positive is.

If the control voltage or supply voltage U3 drops to its lower value, the base-emitter path of the transistor T1 becomes conductive, ie the transistor T1 turns on. The resistance value of the charging resistor RL is dimensioned such that a resistance value of the through-connected transistor T1 is significantly lower than the resistance value of the charging resistor RL. Thus, the charge of the sensor element C3 and the capacitor C4 is substantially transferred or transferred to the collecting capacitor C2 and only a small proportion of charge flows back into the control signal generator SG.

The The amount of charge transferred is determined by the capacity C3 of the Sensor element and the known capacitance of the capacitor C4 determined. When pressed of the proximity switch takes the capacity C3 to, which increases the voltage at the collecting capacitor faster.

3 shows a diagram of the voltage waveform at the collecting capacitor C2 depending on the operating state of the proximity switch over time. When the proximity switch is not actuated, the voltage runs in a sawtooth fashion between the reference voltage and a first ramp voltage UR1. In a time interval between the times t1 and t2, when the proximity switch is actuated, the slope of the ramp increases sharply at time t1 and the voltage at the collecting capacitor C2 increases up to a ramp voltage UR3. The subsequent measuring cycles take place up to the time t2 with a high ramp gradient, wherein in each case a ramp voltage UR2 is achieved. The achieved ramp voltage thus indicates the operating state of the proximity switch and can be evaluated by a unit, not shown, for example, a microcontroller.

If, instead of the switch S1, the resistor R3 is connected in parallel with the collecting capacitor C2, the result instead of the one in FIG 3 shown sawtooth-shaped voltage curve at a key press the in 5 shown voltage curve. The collecting capacitor C2 together with the resistor R3 form an averaging device, so that a substantially constant voltage is established at the collecting capacitor C2.

4 shows a circuit diagram of a circuit arrangement with three capacitive sensor elements C3, each associated with a load resistor RL and a transistor T1. The control signal generator SG, consisting of the voltage sources U1 and U2, the clamping diode D1, the capacitor C1 and the resistor R1, is present only once and acts on the respective charging resistors RL and the transistors T1 with the control voltage U3. The collecting capacitor C2 is just if only once available. The diodes D3 and D4, which are connected to the collector of the transistor T1, serve for mutual decoupling. The selection of a proximity switch to be measured takes place with the aid of the corresponding selection signal SL1, SL2 or SL3. The selection signal SL of the selected or selected proximity switch carries a voltage which is greater than the maximum occurring ramp voltage. The selection signal of the non-selected proximity switches carries the reference voltage. The charge of the non-selected sensor elements flows off via the respective diode D3, while the charge of the selected sensor element is transferred or transferred via the corresponding diode D4 into the collecting capacitor C2.

The Circuit arrangements shown allow safe determination of the operating state the proximity switch (s) Under all operating conditions, are inexpensive to produce and insensitive across from EMC and RF interference.

Claims (11)

Circuit arrangement for a capacitive proximity switch for determining its operating state with - a capacitive sensor element whose capacitance (C3) changes depending on the actuation state, - a common capacitor (C2), - a control signal generator (SG) for generating a control signal (U3) at a control signal output (N1) and - a controllable switching means (T1), which is connected to a first terminal (A1) with the capacitive sensor element (C3), which is connected to a second terminal (A2) to the collecting capacitor (C2) and in dependence from the drive signal (U3) connecting the capacitive sensor element (C3) to the accumulation capacitor (C2) for transferring the charge from the capacitive sensor element (C3) to the accumulation capacitor (C2), characterized in that - between the control signal output (N1) and the first terminal (A1) of the switching means (T1) a charging resistor (RL) is looped. Circuit arrangement according to Claim 1, characterized in that the control signal generator (SG) generates a control signal is formed in the form of a non-constant voltage, in particular a square-wave voltage (U3). Circuit arrangement according to claim 1 or 2, characterized in that the control signal generator has a DC voltage source (U1) and a square-wave voltage source (U2) with common reference potential, wherein between the Steuersig nalausgang (N1) and the DC voltage source (U1) a clamping diode (D1) in the reverse direction is looped in and between the control signal output (N1) and the square-wave voltage source (U2), a capacitor (C1) and a resistor (R1) are connected in series. Circuit arrangement according to one of the preceding Claims, characterized in that the switching means is a bipolar transistor, in particular a pnp transistor (T1). Circuit arrangement according to Claim 4, characterized that the base of the transistor (T1) with the control signal output (N1) is connected. Circuit arrangement according to one of the preceding Claims, characterized in that between the first port (A1) of the switching means (T1) and the sensor element (C3) a resistor (R2) is looped. Circuit arrangement according to one of the preceding Claims, characterized in that the collector capacitor (C2) is a switch (S1) is connected in parallel. Circuit arrangement according to one of the preceding Claims, characterized in that the collecting capacitor (C2) is a resistor (R3) is connected in parallel. Circuit arrangement according to one of the preceding Claims, characterized in that it comprises a plurality of capacitive sensor elements (C3), each having a switching means (T1) and a charging resistor (RL), and only a single accumulation capacitor (C2) connected to the respective switching means (T1) via respectively a decoupling diode (D4) is connected in the forward direction, wherein the anode of the decoupling diode (D4) by a selection diode (D3) in the forward direction with a respective selection signal (SL1, SL2, SL3) is connected. Circuit arrangement according to one of the preceding Claims, characterized in that the capacitive sensor element (C3) thereto is formed on a bottom of a surface or cover with dielectric Properties to be created, it preferably a smooth flat surface to the plant has. Circuit arrangement according to one of the preceding Claims, characterized in that the capacitive sensor element (C3) a voluminous, elastic, preferably elongated Body out electrically conductive Material is.