With capacitive proximity sensors, by generation and measurement of electric alternating fields, the approach of an object to the sensor electrodes of the proximity sensor is measured without contact. From the measuring signal functions can be derived, for example switching functions of an electric hand-held device. For the approximation detection without contact of an object to a capacitive proximity sensor different solutions are known from prior art, which distinguish in the way of the signal production and of measurement.
A first principle known from prior art for capacitive approximation detection provides the use of a capacitive proximity sensor with only one electrode. In this measurement system the capacity of the electrode is detected and evaluated in relation to the ground potential of the measuring electronics of the capacitive proximity sensor. This measurement system is also called loading method. If an object, for example a user, approaches the sensor electrode, the capacity at the sensor electrode changes, which can be detected and evaluated accordingly.
Another measurement system known from prior art provides a capacitive proximity sensor with two sensor electrodes. One sensor electrode operates as transmitting electrode and the other sensor electrode as reception electrode. The electric alternating field emitted at the transmitting electrode is coupled into the reception electrode and measured by means of an electrical signal tapped at the reception electrode. This method is also called transmission method, as it measures the transmission between a transmitter and a receiver. In case of approach of a user to the sensor electrodes the alternating electric field formed between the transmitting electrode and the reception electrode changes, which can be measured and evaluated accordingly.
The loading method, which measures the capacitive load against a reference knot (preferably the ground of an electric device), is preferably used in grounded systems. The transmission method, which measures the transmission between a transmitter and a receiver, is preferably used for battery-operated i.e. free-of-ground systems.
The disadvantage of the capacitive proximity sensors known from prior art is on the one hand the dependence of the capacitive proximity sensor on the parasitic capacitive loop over people, for example a user and ground. This means that the measuring signal depends on the grounding conditions of the capacitive sensor on the one hand and on the grounding conditions of the object which approaches the capacitive proximity sensor on the other hand. If the concrete grounding conditions of the capacitive proximity sensor or of the object are not known, a correct measurement of an approach of the object to the capacitive proximity sensor cannot be guaranteed.
On the other hand interference, for example by interfering electric fields, from the environment of the capacitive sensor on the sensor electrodes of the capacitive proximity sensor is problematic, because such interference can have a negative impact on the result of the measurement. Such interference or interfering electric fields can be caused for example by neighbouring electronic components in complex systems, like mobile phones. In order to avoid such interference or interfering electric fields, additional measures must be taken which avoid the coupling of external interfering fields into the sensor electrodes of the capacitive proximity sensor as far as possible, ideally completely. Such measures however lead to an increased structural expenditure, which affects the material and production cost of the capacitive proximity sensor or the electric device in which the capacitive proximity sensor is integrated.
A possibility known from prior art to prevent the coupling of external electric interfering fields into the sensor electrode of the capacitive proximity sensor consists in shielding the sensor electrodes of the proximity sensor against other electronic components in a complex system, which can take place for example with the help of a screen electrode. In this respect it is however disadvantageous that a screen electrode can occupy or must occupy a large amount of space in order to guarantee safe shielding. Especially when using capacitive proximity sensors in electric hand-held devices, for example mobile phones, it is desirable to do without such screen electrodes in order to reach a possibly compact design of the electric hand-held device.