Capacitive sensor device with associated evaluation circuit

Abstract

A capacitive sensor device includes a sensor electrode coupled to a first switch and coupleable either to a sensor operating voltage or to an evaluation circuit, configured as a power source circuit. A first current path is coupleable to the sensor electrode at an input end and to ground via a collector and emitter of a first transistor by an auxiliary resistor. A second current path is coupled to a reference potential at one end. A capacitor is coupled between the reference potential and a second transistor. A second auxiliary resistor is arranged in the second current path. The first transistor's base and collector are coupled to the base of the second transistor. A compensation capacitor has a first terminal coupled to the first current path and a second terminal couplable to a compensation voltage, to ground, or in a floating manner via a second switch.

Claims

1. A method for detecting a capacitance of a sensor electrode, comprising the steps: charging a sensor electrode by coupling the sensor electrode to an operating voltage, decoupling the sensor electrode from the operating voltage and coupling the sensor electrode to a compensation capacitor, in order to bring about a charge compensation between the compensation capacitor and the sensor electrode, wherein the compensation capacitor is formed by one or more capacitors, and is adapted to be selectively coupled, via a switch device, to a compensation voltage, a ground, or in a floating manner, in relation to the sensor electrode, discharging a remaining charge of the sensor electrode via a first current path of a current mirror, wherein a corresponding current is generated in a second current path of the current mirror, and wherein a support capacitor is coupled to the second current path and is recharged by means of the generated current, wherein the preceding steps are repeated and a charge state of the support capacitor is evaluated as a measure of the capacitance of the sensor electrode wherein the compensation voltage is variable and is defined, dependent on the determined charge state of the support capacitor; and wherein the coupling of the individual capacitors to the sensor electrode is carried out depending on the determined charge state of the support capacitor.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention shall now be explained in greater detail, based on the exemplary embodiments.

(2) FIG. 1 shows a schematic view of the capacitors present on the vehicle;

(3) FIG. 2 shows a first exemplary embodiment of the invention in the form of a reduced block diagram;

(4) FIG. 3 shows a second exemplary embodiment of the invention in the form of a reduced block diagram.

DETAILED DESCRIPTION

(5) A vehicle is depicted in FIG. 1, which has a capacitive sensor 2 in the rear. The sensor 2 is designed as a sensor electrode, disposed in the rear bumper of the vehicle. When a user wants to actuate a trunk lid, said user can execute a movement gesture, in particular move a foot under the vehicle rear, thus changing the capacitance of the sensor electrode, and thus trigger an opening of the trunk.

(6) The electrode extends in the width of the vehicle over the entire bumper, or a specific subsection thereof, in which the user can execute the gesture.

(7) The capacitors of the vehicle that are to be taken into account are shown schematically in the illustration. The vehicle chassis has a capacitance with respect to the actual ground potential, which is normally greater by orders of magnitude than the other capacitances. The capacitance C.sub.Chassis is normally multiple 100 pF, or even greater capacitances. For the following consideration, this capacitance of the vehicle chassis is normally negligible with respect to the ground potential, because the vehicle itself forms a vehicle ground, which is labeled C.sub.Mass in the schematic depiction, with respect to the sensor electrode.

(8) Furthermore, the sensor electrode 2 has a capacitance with respect to the ground potential, which amounts to a few pF, e.g. approx. 5 pF. Accordingly, C.sub.GND equals 5 pF in this schematic view.

(9) Moreover, a variable capacitance C.sub.S acts in parallel to C.sub.GND. This variable capacitance is dependent on environmental conditions or the movement of a body in the surrounding region of the sensor electrode 2. There are different fundamental explanations for this effect; on one hand, the dielectric surrounding the sensor electrode 2 changes, and on the other hand, a coupling of a further capacitor (the body of the operator) in parallel to C.sub.GND can be drawn from as an explanation. Independently of the explanation, however, the capacitance C.sub.S is that capacitance that is to be detected for the recognition of an actuation. The capacitance C.sub.S, thus the change in capacitance through external factors or actuation, is significantly lower than the capacitance C.sub.GND thereby. Normally, the capacitance C.sub.S is less than one pF, e.g. less than 0.5 pF.

(10) The capacitance C.sub.GND is altered by environmental effects, e.g. drying salt water, grease layers, or even lacquer applications on the bumper. When the capacitance C.sub.GND is increased, the actual detection capacitance C.sub.S is more difficult to detect. The field lines of the electric field, which are generated by the sensor electrode 2, are strongly influenced by disturbances of this type, in particular conductive layers, and no longer sufficiently extend into the detection range.

(11) For this reason, it is important to detect small capacitance changes, even when the base capacitance that is present at all times is variable.

(12) The invention is applied thereby, in that a displacement of the operating point for the circuit is possible depending on the detected measurement values.

(13) FIG. 2 shows a corresponding schematic circuitry. It can be seen that the circuitry is borrowed, in sections, from a simple current source circuit or even a current mirror.

(14) The variable capacitance C.sub.S of the sensor electrode that is to be detected is depicted in the diagram as a variable capacitor. This capacitor is coupled, by way of example, with a damping resistor R0, and with a first switch device SW1. The switch device SW1 can be activated electronically, and can couple the sensor electrode, thus the capacitance C.sub.S, to an operating voltage U.sub.0. In another switching setting, this coupling is released by means of the switch device SW1, and the sensor electrode is coupled to the evaluation circuit for evaluation.

(15) As is shown in the illustration, when the sensor electrode having a capacitor C.sub.S is coupled to the evaluation circuit 10, the discharging of the sensor electrode represents the input current side of the current mirror. For this reason, a collector terminal and a base terminal are connected to the transistor T1 and can be coupled to the sensor electrode. If an input current flows through the transistor T1, then a base-emitter voltage is set, which is linked to the input current.

(16) The base terminals for the two transistors T1 and T2 are connected. In the second current path of the evaluation circuit, formed by the sample and hold or support capacitor C.sub.H, the transistor T2 and the auxiliary resistor R.sub.2 are likewise coupled to the supply voltage U.sub.S. A voltage U.sub.ADC can be accessed. This voltage serves as a measure for the capacitance C.sub.S after accumulation of a charge on the capacitor C.sub.H over numerous measurement cycles.

(17) The invention has, however, compensation capacitors parallel to the first current path of the current mirror. In this exemplary embodiment, two further switch devices SW2 and SW3 are coupled to associated compensation capacitors C.sub.1 and C.sub.2. Depending on the switch settings of the switches SW2 and SW3, the capacitors can be connected to either a ground potential or a floating potential at their terminals, located at the bottom of the capacitors in the depiction. In the phases in which the switch SW1 and the sensor electrode are coupled to the operating voltage U.sub.0, the capacitors C1 and C2 can be coupled to the supply voltage U.sub.S.

(18) In order to evaluate the capacitance C.sub.S, the switch SW1 is actuated for a number of switchings, such that the capacitance C.sub.S is always recharged, and is discharged via the evaluation circuit.

(19) The steps thereby are as follows: coupling the capacitor C.sub.S to the operating voltage U.sub.0 by means of a switch device SW1, by means of which C.sub.S is charged in accordance with the capacitance of the sensor electrode; decoupling the capacitor C.sub.S from the operating voltage U.sub.0 by means of the switch device SW1, by means of which C.sub.S maintains the existing charge; coupling the capacitor C.sub.S to the evaluation circuit 10, wherein a charge compensation occurs between C.sub.S and the capacitors C.sub.1 and C.sub.2, wherein the charge flow is dependent on the charge states of C.sub.1 and C.sub.2, as well as the potentials coupled thereto via SW2 and SW3 (the switches SW1 and SW2 or SW3, respectively, are switched at the same time); the charge portion that has not be compensated for, thus the charge portion from CS that does not flow toward C1 and C2, is conducted through the first, left-hand path of the current mirror, and generates a corresponding current in the second, right-hand path; the current flow generated in the right-hand path results in a charge change at C.sub.H, which is correlated to the uncompensated charge quantity from C.sub.S; SW1 is again actuated for charging CS, and the process starts again, wherein a charge is accumulated at C.sub.H, which can be called up in the design of the voltage U.sub.ADC; the steps are repeated eight to ten times, and the voltage U.sub.ADC is then A/D converted and interpreted as a representative value for the charge at C.sub.H; C.sub.H is discharged from the accumulated charge, and a new measurement cycle is begun.

(20) The voltage U.sub.ADC that is obtained is dependent on the number of switchings as well as the supply voltage U.sub.S and the relationship between the capacitors C.sub.S and C.sub.H, as well as the relationships between the resistors R1 and R2. The quantitative relationships can be adapted to the respective application by means of simple circuit simulations and the expert knowledge regarding current mirrors.

(21) If it has been detected, based on the determined values, that the detected voltage value U.sub.ADC does not move in the configuration range of the circuit, thus a variable capacitance can only be insufficiently precisely determined, the switches SW2 and SW3 can be set to various ranges and combinations, such that either both capacitors C1 and C2 are kept at floating potentials, one of the capacitors is coupled to the ground potential, or both capacitors are coupled to the ground potential.

(22) Based on the diagram, it is immediately apparent that the connecting of the capacitors C1 and C2 in addition to the capacitor C.sub.S affects the evaluation circuit, in that the capacitance that is to be detected is again displaced to the configuration region.

(23) In this manner, sensitive detections are possible, even in adverse environmental conditions, in particular in the event of a coating of the sensor electrode with, e.g., salt crusts or dirt. It is thus essential that the compensation range can be adjusted in situ, depending on which measurement range displacement is required.

(24) In a further development of the invention, larger numbers of compensation capacitors can also be incorporated.

(25) An alternative design of the invention is depicted in FIG. 3. A charge compensation is obtained here in that a compensation capacitor having a fixed capacitance C.sub.12 is used. This capacitor, however, can be coupled to an adjustable voltage U.sub.1, the compensation voltage. Accordingly, with this embodiment, the charge compensation is achieved via the variable compensation voltage U.sub.1, and not via the change in the capacitor capacitances. The compensation voltage U.sub.1 is preferably defined as U.sub.1U.sub.0 thereby. The voltage causes a variable voltage deficit or voltage surplus at the compensation capacitor. Depending on the voltage that is set, the coupled compensation capacitor can thus accommodate different charge quantities, which are branched off prior to the current mirror thereby, and do not result in a charging of the support capacitor.

(26) In the scope of the invention, a combination of the compensation methods presented herein can also be implemented, such that numerous capacitors can thus be connected thereto, of which one or more are coupled to adjustable voltage sources. As a result, a particularly precise compensation adjustment is made possible. The adjustable voltage sources are controlled thereby according to known methods, in order to adjust the charge quantities to the compensation capacitors, such that the sensitivity of the measurement circuit is restored to the desired measurement range.

(27) In the scope of the invention, deviations, in particular regarding the number of sensed capacitors and compensation capacitors, are possible. Instead of the depicted individual sensed capacitor (C.sub.S) in the exemplary embodiments, numerous sensor electrodes, or sensor capacitors, respectively, having allocated switch devices, can be coupled to the same evaluation circuit. Furthermore, the part of the circuit that is designed as a current mirror circuit can also be varied within the scope of the known design alternatives for current mirror circuits.