Sensor electronics of the type previously mentioned are used, for example, in order to detect the approach of the hand of an operator (driver) to the outer door handle of the vehicle. When the approach of the hand of the operator is detected, control electronics provided in the vehicle initiate an inquiry by radio of an ID encoder (electronic key) carried by the operator, i.e., a transponder or radio transmission receiver that the operator carries (for example, in a clothing pocket or in a briefcase). This inquiry by radio is made, for example, with aid of transmission of a radio signal in a frequency range of, for example, 125 kHz, i.e., an LF frequency range, whereby a coil is used for emitting that, for example, can be provided inside the door handle. If the ID encoder is in the reception range of the transmission coil, it receives the radio signal, acknowledges a challenge to identify, and sends a response signal in a higher frequency range (for example, in the Megahertz frequency range), which includes a code identifying the ID encoder. The control device in the vehicle receives this response, checks the code, and then opens the door lock with the aid of the door handle activated by the operator. The sensor detecting the approach of the operator is utilized in particular so that an inquiry signal does not constantly have to be emitted by the transmitter coil. The inquiry signal is transmitted only when the approach of a body part of the operator is detected.
A capacitive approach sensor is usually used for detection of the approach of a body part of the operator to the door handle. A capacitive approach sensor of this type is, for example, described in publication EP 1 339 025 A1 or in publication EP 1 235 190. A basic principle of the sensor is, for example, described in publication U.S. Pat. No. 5,730,165 A.
In order to improve the sensitivity of a capacitive sensor having a sensor electrode or to extend the range of the sensor, t was proposed that a shielding metallic surface be provided between the sensor electrode and the solid metallic ground surface of the vehicle (there may also be a ground surface in a circuit inside the door handle), wherein the potential of this shielding metallic surface is tracked by means of a voltage tracking circuit (without the two electrodes being connected with each other). Active shielding of this type is, for example, described in publication EP 0 518 836 A1.