Abstract: A capacitive sensor may be used as a proximity detector in an obstruction warning system for road vehicles, e.g. for use when the vehicle is reversing. A digital signal processor 11 sends a sine wave through a sensor RC circuit 1, 7. A sensor plate 3 acts as one plate of a sensor capacitor 1 and the obstruction 45 acts as the other plate 5. Changes in the distance between the car 43 and the obstruction 45 result in changes in the capacitance of the sensor capacitor 1, changing the amplitude and phase of the sine wave output by the sensor RC circuit 1, 7. A reference sine wave, generated by a reference signal circuit 17, 19, 21 is subtracted from the sensor output signal in a subtractor 15. The reference signal has a phase offset from the sensor signal so that the amplitude of the difference signal is highly sensitive to changes in phase of the sensor signal. An additional signal, substantially identical to the sensor signal, is coupled to the output of the sensor RC circuit by a coupling capacitor 41.

Abstract: A capacitive sensor for mounting to a body, particularly a body such as a van door (55). The sensor has a sensor plate (51) to which a first signal is applied. A first guard plate (52) is interposed between the sensor plate (51) and the body (55), and a second guard plate (53) is interposed between the first guard plate (52) and the body (55). The first and second guard plates (52, 53) each have signals applied thereto which are the same or at least similar to the first signal applied to the sensor plate (51). In this way, the second guard plate (53) acts as a rear guard to the first guard (52) and reduces its capacitance to ground. This reduces the current drawn by the first guard (52), allowing it to more accurately track the signal on the sensor and consequently better mask the sensor plate (51) from the body (55).

Abstract: A method of detecting a proximate object using a capacitive sensor system, being measuring output signal data from the sensor over predetermined distance intervals and comparing the output signal data with predetermined comparison data indicative of an ideal sensor output signal sequence. Based on the comparison moderated data is output, such that, where an obtained output signal data value is lower value than a corresponding comparison data value, the moderated output data is given the value of the comparison data value, and otherwise the moderated output signal data is given the value of the obtained output signal data. Therefore the detection of low objects is improved through a comparison with data indicative of an ideal response signal.