This invention relates to sensors, and more particularly, to capacitive proximity sensors.
Capacitive sensors using a single plate capacitive proximity detector are known. The publication entitled: Capacitive Sensors-Design and Applications (ISBN#0-7803-5351-X) by Larry K. Baxter, 1st ed. (IEEE, Piscataway, N.J., 1997) describes one known capacitive proximity sensing system. Typically, a balance is achieved such that the system is stable. The balance is disrupted, for example, when a third object projects itself into the system, thereby altering a previous capacitance. The net result is a disruption of the balance. The balance can be achieved by using a bridge circuit with the proximity detecting capacitor in one arm of the bridge and a second capacitor, that has been adjusted to null the output of the bridge, in the other arm. Alternatively, the system can consist of two virtually identical oscillators that are independent of each other. Each of the two identical oscillators generates a signal with a frequency, dependent on a capacitance, that is virtually identical to the other oscillator. Thus, when one capacitance is changed, the balance between the two frequencies is disrupted and the disruption can be measured, for example, by way of an electronic device.
To understand the instant invention, some related background is necessary. Capacitance is the linear coefficient that relates the charge induced on one electrode to the potential applied to a different electrode. There is also self-capacitance that relates to the charge on an electrode to its own potential. For example, consider a situation with N electrodes, all electrically isolated from one another. Electrode i has charge Qi and potential Vi. The charge on electrode i can be expressed as:                               Q          i                =                              ∑                          j              =              1                        N                    ⁢                      xe2x80x83                    ⁢                                    C              ji                        ⁢                                          V                                  j                  ⁢                                      xe2x80x83                                                              .                                                          (        1        )            
Here, Cji is the capacitance of electrode i with respect to electrode j. Given the geometry, the capacitance can be calculated. In electrostatics it is known that for any geometry, Cji=Cij.
One embodiment of the invention is a capacitive sensor which includes a first electrode that has at least one conducting surface being disposed for receiving an alternating current signal, and a second electrode that has at least one conducting surface being disposed to generate an input signal for a detecting device. The sensor further includes a middle electrode that has at least one conducting surface placed between the first electrode and the second electrode, as well as being grounded to a ground. The at least one conducting surface of the middle electrode, the at least one conducting surface of the first electrode, and the at least one conducting surface of the second electrode being contained within a dielectric material holding within itself the first electrode, the second electrode and the middle electrode.
Another embodiment of the invention is a capacitive proximity sensing system which includes a first electrode that has at least one conducting surface being disposed for receiving an alternating current signal, and a second electrode that has at least one conducting surface being disposed to generate an input signal. The sensing system further includes a middle electrode that has at least one conducting surface placed between the first electrode and the second electrode, as well as being grounded to a ground. The at least one conducting surface of the middle electrode, the at least one conducting surface of the first electrode, and the at least one conducting surface of the second electrode being contained within a dielectric material holding within itself the first electrode, the second electrode and the middle electrode. The sensing system also includes an oscillator coupled to the first electrode generating the alternating current signal, and a detector that receives the input signal. An output of the detector is representative of the capacitance between the first electrode and the second electrode. Other detection methods known in the art may be used in place of the synchronous detector, such as envelope detection.