Mechanical actuators in the form of keys, buttons, switches, and other actuators have long been used for the entry of operation commands and parameters to machines, such as household appliances. Various issues, however, have led to the replacement of mechanical keys or buttons with data entry components that electrically sense data input, such as capacitive switches. One of the issues leading to the replacement of mechanical actuators with capacitive switches is the wear and tear of moving parts in mechanical actuators. Because capacitive sensing does not require moving parts for data input, the capacitive switches have a longer life. Moreover, capacitive switches are typically sealed with a protective membrane. The membrane helps protect the switches from environmental conditions, such as dirt or spillages. This protection makes capacitive sensors especially suitable in devices that need to be cleaned regularly, as the sensor is not exposed to cleaning agents or other foreign substances. Thus, capacitive switches are especially useful in household appliances that are exposed to dirt, spillage, and other contaminations.
A typical capacitive switch includes a pair of electrical conductors separated by a dielectric, such as air. Each conductor may be, for example, a metal trace that is applied as a layer on a printed circuit board (PCB). An excitation source is connected to one of the two conductors. The conductor coupled to the excitation source is sometimes called an emitter and the other conductor is sometimes called a receiver. An electrical field is formed between the emitter and the receiver. Most of the electrical field is concentrated between the emitter and the receiver on the PCB, however, a fringe field extends out of the PCB from the emitter. The capacitance of the capacitive switch may be measured with a capacitance measurement circuit. The measurement circuit may be a frequency counter that is coupled to the output of a relaxation oscillator that has the capacitive switch coupled to one of its inputs. The capacitive switch is also coupled to a current source and a discharge switch is coupled to the other oscillator input. The discharge switch opens and closes in response to the signal on the output of the oscillator. The charging and discharging of the capacitor through the discharge switch generates a signal having a frequency that is related to the capacitance of the switch. By counting the frequency during a fixed period, a value for the untouched switch may be determined. In other known devices, a sigma-delta capacitance-to-digital converter senses the capacitance formed between the two conductors across the dielectric and generates a digital data value corresponding to this capacitance.
When a human finger approaches a capacitive switch, the capacitance of the person's finger affects the capacitance of the switch. The change in the switch capacitance affects the frequency of the relaxation oscillator or the value generated by a sigma-delta capacitive-to-digital converter. The change in capacitance may be detected by the change in the value generated by the converter or the frequency count measured during the fixed time period. By comparing the change to some threshold value, touching of a capacitive switch may be detected. Thus, touching a capacitive switch is analogous to depressing a mechanical switch. Identification of the touched key provides an appliance with data for control of the appliance.
The above description assumes that the capacitance of a capacitive switch remains relatively constant as long as the switch is not touched. However, capacitance of a switch may change in response to ambient air changes, such as temperature and humidity, as the air is the dielectric for the capacitor. If the capacitance of a switch changes significantly, it may approach the threshold that indicates the switch has been touched. If the capacitance reaches or passes through the threshold, the appliance may process data corresponding to the key even though a user has not touched the key. The erroneous detection of a key being touched in response to ambient air changes is commonly called ghosting. Ghosting may cause an appliance to respond to false commands. Therefore, reducing the likelihood that an appliance processes commands corresponding to a capacitive switch in response to environmental condition changes is desirable.