Capacitive sensing system configured for using heating element as antenna electrode

摘要

A capacitive sensing system for a heating element comprises a capacitive detector connectable to the heating element and a common mode choke for connecting the heating element with a heating current supply. The detector drives an alternating current into the heating element and produces an output depending on the capacitive load, which the alternating current is subject to. Depending on the object sensed, the load varies in a range between a minimum and a maximum value. The choke represents an inductive load in parallel of the capacitive load. The capacitive load and the inductive load contribute to a complex impedance dominated by the inductive load. A compensation capacitor arranged parallel to the choke represents an additional capacitive load also contributing to the complex impedance. The compensation capacitor is dimensioned such that the sum of additional capacitive load and maximum value amounts to at least 50% of the inductive load.

主权项

1. Capacitive sensing system for being connected to a heating element producing heat upon electrical current being caused to flow across the heating element, said capacitive sensing system comprising
a capacitive detector connectable to said heating element, configured for driving an alternating current into said heating element and for producing an output depending on a capacitive load acting on said alternating current between said heating element and a reference node, said capacitive load being variable in a range between a minimum value and a maximum value depending on an object to be sensed; a common mode choke for connecting said heating element with a heating current supply, said common mode choke representing an inductive load in parallel of said capacitive load, said capacitive load and said inductive load contributing together to a complex impedance; wherein, within said complex impedance, said inductive load dominates said capacitive load, and wherein said capacitive sensing system comprises at least one compensation capacitor arranged in parallel of said common mode choke, said compensation capacitor representing an additional capacitive load also contributing to said complex impedance, said compensation capacitor dimensioned such that the sum of said additional capacitive load and said maximum value amounts to at least 50% of said inductive load.