Electricity: measuring and testing – Impedance – admittance or other quantities representative of... – Lumped type parameters
Patent
1996-11-25
1999-09-14
Do, Diep N.
Electricity: measuring and testing
Impedance, admittance or other quantities representative of...
Lumped type parameters
324674, 324681, 340562, G01R 2726
Patent
active
059528354
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to proximity detectors and in particular, to a non-contact proximity detector which relies upon a capacitive proximity effect to detect the presence of an object.
BACKGROUND ART
Proximity detectors are known in the art and three basic types of proximity detectors exist, namely electromagnetic, optical and ultrasonic. Electromagnetic proximity detectors may be subdivided into inductive and capacitive types. Most inductive proximity detectors respond to the presence of any metal object, although ferrous metals give the best response (about two-thirds higher than for non-ferrous metals).
One common principal of operation for inductive proximity detectors involves the use of an eddy current killed oscillator (ECKO). The oscillator is in the form of a tuned LC tank circuit amplifier. A sensor coil and ferrite core form the inductor. The amplifier provides just enough positive feedback to make the circuit oscillate in the RF range. When a metal object is positioned near the detector, the RF field causes eddy currents to flow in the surface of the metal object. When the metal object reaches a predetermined proximity to the detector, (depending on the detector sensitivity), the load caused by the eddy currents is enough to disable the oscillator. A level detector is connected to the oscillator for producing an output signal. Usually, a certain amount of hysteresis (differential travel) is built into the detector. The purpose of the hysteresis is to prevent chatter when the metal object is positioned right at the oscillation threshold point of the oscillator.
Although these inductive proximity detectors are suitable for detecting the presence of a metal object, when using these types of detectors to detect the presence of a portion of an individual, such as a hand, the inductive detection methods yield non-linearities that result in large errors. Because the human hand represents an irregular reflective surface, optical and ultrasonic proximity detectors are also unsuitable.
Capacitive proximity detectors have been used to detect the presence of a human hand. In these prior art capacitive proximity detectors, differential capacitive arrangements have been implemented wherein the centre plate of the capacitive arrangement is formed by the hand to be detected. Human tissue exhibits certain electrical properties (such as relatively low impedance to surrounding grounds at 100 kHZ), which are sufficient to make such arrangements practical. In capacitive arrangements of this nature, the centre plate (constituted by the hand) is maintained at ground potential and two fixed capacitors are provided in a complex bridge configuration. Unfortunately, these prior art capacitive proximity detectors are bulky, complex and have significant power requirements.
It is therefore an object of the present invention to provide a novel proximity detector.
DISCLOSURE OF THE INVENTION
According to one aspect of the present invention there is provided a non-contact proximity detector comprising: to be sensed is adjacent said plate, said capacitor changing capacitance as said object approaches said plate; and output representative of the presence of said object when said object is within a predetermined proximity to said sensing plate.
According to another aspect of the present invention there is provided a non-contact proximity detector to detect the presence of a human appendage comprising: appendage to be sensed is adjacent said plate, said capacitor changing capacitance as said appendage approaches said plate; and output representative of the presence of said appendage when said appendage is within a predetermined proximity to said sensing plate.
Preferably, the detection means includes an oscillator responsive to the change in capacitance for generating an oscillating signal having a magnitude which varies as a function of the capacitance and signal conditioning means responsive to the oscillating signal and generating output representative of the presence of the object or appendage wh
REFERENCES:
patent: 3067364 (1962-12-01), Rosso
patent: 3675051 (1972-07-01), Mioduski
patent: 4887024 (1989-12-01), Sugiyama et al.
patent: 4994793 (1991-02-01), Curtis
patent: 5287086 (1994-02-01), Gibb
patent: 5465091 (1995-11-01), Nishino et al.
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