Electrical transmission or interconnection systems – Personnel safety or limit control features – Interlock
Patent
1990-05-17
1991-11-05
Miller, Stanley D.
Electrical transmission or interconnection systems
Personnel safety or limit control features
Interlock
307494, 328 5, H03K 0000, G06G 712, G01N 2700
Patent
active
050633066
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to proximity sensing devices, and in particular capacitive sensing devices.
Prior art devices used to detect the proximity of a person or object, have as their principle of operation, the utilization of a change in capacitance of the sensing component altering the frequency of an oscillator, and the detection of this change in frequency as indicative of the presence of a person or object.
Such frequency dependent devices have had limited application particularly in situations where compact size is required and where environmental influences and objects, not being the desired subjects for detection, interfere with the frequency detection and generate undesired and spurious results.
This is found to be the case for capacitive sensing devices such as switches or keys that have been used in keyboards and the like where there is a plastic or glass overlay. The presence of the overlay, tinting, moisture, dust, and finger grease have been known to affect the frequency of oscillation resulting in inaccurate detection.
In an effort to try and overcome the aforementioned disadvantages, the present invention provides a specially adapted capacitive sensor and the detection of a change in a differentiated signal caused by the proximity of a person, (or other capacitive object), and particularly, the accurate detection of a persons finger through glass or plastic on a capacitive sensor according to the invention.
In the case of a keyboard, where a number of sensors are required, the present invention further provides a method of scanning the sensors for the differentiated signal and any change thereto, which minimises interference from the scanning circuitry.
It has been found that the detection of a differentiated signal and any changes thereto rather than detection of a frequency change, provided advantages in that such previous environmental disturbances from cover plates, dust moisture etc., have a minimal or no effect upon the differentiated signal.
The specially adapted capacitive sensor comprises, a planar capacitor having an outside area and a central plate. The planar capacitor is preferably etched directly on a printed circuit board, and this contributes to the space and cost savings of the device.
It has been found that the size of the central plate affects the required dielectric distance between the outside area and central plate for the proper functioning of the capacitor as part of the proximity device. For a one finger size central plate a ratio of 1/3 between the central plate size and the dielectric distance is preferable. For a two finger size central plate, a ratio of 2/3 is preferably.
The layout of the planar capacitor can also be determined alternatively, and preferably, by the ratio between the outer perimeter of the dielectric between the outer area and central plate, and the perimeter of the central plate. A preferable ratio of about 2.1 has been determined for proper functioning.
Preferably connected to the back of the central plate of the planar capacitor, are a transistor, preferably bipolar, and a bias resistor for the transistor in a configuration such that the circuit behaves as an electronic differentiator of the drive signal. The connection to the back of the central plate allows a user's finger to approach the sensor without obstruction, and also the configuration is preferred as it minimises electrical interference whilst being as close as possible to the plate for a usable amount of signal. The gain is set for maximum output without introducing finger noise. (It has been found that finger poise manifests itself as a 50 hertz oscillation).
The drive signal, generated externally, is applied preferably to the outside area of a planar capacitor, and picked up by the central plate. The drive signal needs to be a pulse with a sharp leading edge and preferably is a square wave pulse. It has been found that a CMOS driver circuit provides a suitable quality pulse.
As the circuit is behaving as an electronic differentiator, the square wave input results in a corre
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Intellect Electronics Ltd.
Miller Stanley D.
Phan Trong
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