Robust ‘hands on steering wheel’ classification based on a relative measurement system

A method of operating a capacitive sensing device that includes:

• at least one capacitive sensor having at least one sense electrode,
• a measurement signal source configured to provide an alternating electric measurement signal comprising at least three fixed predefined signal frequencies at least to the at least one sense electrode,
• an impedance measurement circuit configured to determine an unknown complex impedance of the at least one sense electrode from a response to the provided electric measurement signal,
• the method comprising at least the following steps:
• operating the measurement signal source to provide an alternating electric measurement signal comprising at least three fixed predefined signal frequencies to the at least one sense electrode,
• by the impedance measurement circuit, measuring a response to the alternating electric measurement signal to determine an unknown complex impedance of the at least one sense electrode,
• the method further comprising at least the following steps that are to be executed for each of the at least three fixed predefined signal frequencies:
• determining a predefined number of momentary values of the unknown complex impedance,
• determining statistical quantities and signal parameters regarding the determined momentary values,
• eliminating from the determined momentary values a frequency portion up to a predefined hand touch movement lower limit frequency,
• eliminating from the determined momentary values a frequency portion from a predefined high-frequency limit down to a predefined hand touch movement upper limit frequency for obtaining remaining determined momentary values,
• excluding the respective determined momentary values if at least one determined statistical quantity exceeds a predefined critical threshold value,
• using the eliminated frequency portion up to the lower limit frequency of the determined momentary values for calculating a momentary reference value for the unknown complex impedance,
• and further comprises the steps of
• using the remaining determined momentary values for determining a signal classification by checking a fulfillment of at least one predefined condition that includes the momentary reference value for the unknown complex impedance, and
• generating an output signal that is representative of the result of the step of checking the fulfillment of the at least one predefined condition.