Proximity sensor

Details Proximity sensor Proximity sensor

2000-03-06

2002-02-19

Mullen, Thomas (Department: 2632)

Communications: electrical

Condition responsive indicating system

Specific condition

C180S272000, C280S735000, C340S561000

Reexamination Certificate

active

06348862

ABSTRACT:

TECHNICAL ART
The instant invention generally relates to proximity sensors, and more particularly to proximity sensors that are responsive to an electric-field-influencing media.
BACKGROUND OF THE INVENTION
A variety of systems have been developed to suppress the passenger air bag in dangerous situations. Systems have included sensors used alone or in combinations. Sensor technologies used include:
a) Infra-red sensors
b) Passive infra-red sensors (heat detectors)
c) Ultrasonic sensors
d) Capacitive sensors (usually using a different type of capacitive sensor)
e) Weight sensors (including various sensor technologies and measurement methods)
f) Child seat “tag” sensors
Usually two or more of these sensors are used together in an attempt to identify child seats, small occupants, empty seats, large occupants and out-of-position occupants. The more sensors that are used, the better the chance for a high performance system. The costs of systems that use many sensors however, can become prohibitively high because of the large number of components and the increased assembly complexity of the vehicle.
The prior art also teaches the use of capacitive sensing to detect the presence, proximity, or position of an occupant. U.S. Pat. No. 3,740,567 teaches the use of electrodes incorporated into the base and back of the seat respectively, together with a capacitance responsive circuit, for purposes of discriminating between human occupants and animals or packages resting on an automobile seat. U.S. Pat. No. 3,898,472 teaches an occupant detection apparatus which includes a metallic electrode which is disposed to cooperate with the body of an automobile to form an occupant sensing capacitor, together with related circuitry which senses variations in the associated capacitance responsive to the presence of an occupant. U.S. Pat. No. 4,300,116 teaches the use of a capacitive sensor to detect people proximate the exterior of a vehicle. U.S. Pat. No. 4,796,013 teaches a capacitive occupancy detector wherein the capacitance is sensed between the base of the seat and the roof of the vehicle. U.S. Pat. No. 4,831,279 teaches a capacity responsive control circuit for detecting transient capacitive changes related to the presence of a person. U.S. Pat. Nos. 4,980,519 and 5,214,388 teach the use of an array of capacitive sensors for detecting the proximity of an object. U.S. Pat. No. 5,247,261 teaches the use of an electric field responsive sensor to measure the position of a point with respect to at least one axis. U.S. Pat. No. 5,411,289 teaches the use of a capacitive sensor incorporated into the back rest of the seat to detect occupant presence. U.S. Pat. No. 5,525,843 teaches the use of electrodes incorporated into the base and back of the seat for purpose of detecting the presence of an occupant, whereby the electrodes are substantially insulated from the vehicle chassis when the detection circuit is active. U.S. Pat. Nos. 5,602,734 and 5,802,479 teach an array of electrodes mounted above the occupant for purposes of sensing occupant position based upon the influence of the occupant on the capacitance amongst the electrodes. U.S. Pat. No. 5,166,679 teaches a capacitive proximity sensor with a reflector driven at the same voltage as to sensing element to modify the sensing characteristic of the sensor. U.S. Pat. No. 5,770,997 teaches a capacitive vehicle occupant position sensing system wherein the sensor generates a reflected electric field for generating an output signal indicative of the presence of an object. U.S. Pat. Nos. 3,943,376, 3,898,472, 5,722,686, and 5,724,024 also teach capacitive-based systems for sensing occupants in motor vehicles.
In addition to methods taught by the above referenced U.S. Patents, the prior art also teaches various means of measuring capacitance, as for example given in
the Standard Handbook for Electrical Engineers
12
th
edition, D. G. Fink and H. W. Beaty editors, McGraw Hill, 1987, pp. 3-57 through 3-65 or in Reference Data for Engineers: Radio, Electronics, Computer, and Communications 7
th
edition, E. C. Jordon editor in chief, Howard W. Sams, 1985, pp. 12-3 through 12-12, both included herein by reference.
The technical paper “Field mice: Extracting hand geometry from electric field measurements” by J. R. Smith, published in IBM Systems Journal, Vol. 35, Nos. 3 & 4, 1996, pp. 587-608, incorporated herein by reference, describes the concept of Electric Field Sensing as used for making non-contact three-dimensional position measurements, and more particularly for sensing the position of a human hand for purposes of providing three dimensional positional inputs to a computer. What has commonly been referred to as capacitive sensing actually comprises the distinct mechanisms of what the author refers to as “loading mode”, “shunt mode”, and “transmit mode” which correspond to various possible electric current pathways. In the shunt mode, a voltage oscillating at low frequency is applied to a transmit electrode, and the displacement current induced at a receive electrode is measured with a current amplifier, whereby the displacement current may be modified by the body being sensed. In the “loading mode”, the object to be sensed modifies the capacitance of a transmit electrode relative to ground. In the transmit mode, the transmit electrode is put in contact with the user's body, which then becomes a transmitter relative to a receiver, either by direct electrical connection or via capacitive coupling.
There are many technologies—including ultrasonic, active infrared, and passive infrared—that have been used for sensing the position of an occupant in a motor vehicle. Some of the problems associated with ultrasonic sensors include: poor reflections off some clothing types, relatively slow pulse propagation times, susceptibility to acoustic noise, and blockage by a newspaper. Some of the problems associated with active infrared sensors include: poor reflections off some clothing types, signal saturation as a result of extreme sunlight conditions, and blockage by a newspaper. Some of the problems associated with passive infrared sensors include: poor signal as a result of occupant's clothing, poor contrast because of ambient temperature, and difficulty in making direct distance measurements.
In an elementary capacitive sensor a metal electrode is connected to a capacitance measuring circuit. Whereas many variations of capacitive sensors are taught in the related art for the occupant sensing application, one problem with known capacitive sensors is a relatively low signal to noise ratio that can be caused by uncertainty in measurement offsets, particularly when sensing very low levels of capacitance, for which the noise resulting from signal drift can overwhelm the signal in static situations where the target is at a distance that would cause only a small change in the capacitance. This can limit the useful range of position measurements that can be reliably sensed by the sensor. Drifts in the offset may result from drift in the measurement electronics or in the connection to the sensing electrode. Furthermore, known single electrode capacitive sensors measure a single value representing a summation of the total capacitance to ground in all directions, including to the rear of the sensor, from which it is difficult or impossible to obtain an accurate idea of an object's location relative to sensor.
SUMMARY OF THE INVENTION
The instant invention overcomes the above-noted problems by providing—in a first aspect—a proximity sensor for sensing an electric-field-influencing media within a region of space, comprising a first electrode, at least one second electrode, a first oscillatory signal operatively connected to the first electrode, at least one second oscillatory signal operatively connected to the at least one second electrode, and a circuit operatively connected to the first electrode for sensing a third signal from the first electrode and for generating a measure of the proximity of the electric-field-influencing media to the first electrode. Th

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