Capacitive sensor guard diagnostics from redundant coupling measurement

A method of operating a capacitive sensing device comprising at least one sense-guard capacitive sensor and a capacitive measurement circuit that is configured for determining a complex impedance of an unknown capacitance from a complex sense current flowing in the at least one sense-guard capacitive sensor, the at least one sense-guard capacitive sensor including a first electrically conductive sense electrode and a first electrically conductive guard electrode and at least a second electrically conductive sense electrode, which is galvanically separated from the first sense electrode, and at least a second electrically conductive guard electrode, wherein each of the guard electrodes is proximally arranged to at least one of the sense electrodes and is galvanically separated from each of the sense electrodes, said method comprising at least steps of:

• providing (52) an alternating electrical measurement signal to a first one of the at least two sense electrodes,
• by operating the capacitive measurement circuit, determining (54) a first complex sense current flowing in a second one of the at least two sense electrodes,
• switching off (56) the alternating electrical measurement signal to the first one of the at least two sense electrodes,
• commencing (58) to provide an alternating electrical measurement signal to the second one of the at least two sense electrodes,
• by operating the capacitive measurement circuit, determining (60) a second complex sense current flowing in the first one of the at least two sense electrodes,
• calculating (62) a difference of imaginary parts of the determined first and second complex sense currents,
• comparing (64) the calculated difference with at least one predetermined threshold for the difference of imaginary parts, and
• based on an outcome of the step (64) of comparing, determining (66) a status of the galvanic connection of at least one of the guard electrodes.