A SENSOR FOR INDICATING CHANGES IN THE PRESENCE OF PERSONS OR OBJECTS

Abstract: The invention is characterized in that the sensor (1, 17, 18) includes at least two sheets of electrically conductive material (2, 3; 10, 11) mutually spaced on an electrically non-conductive carrier sheet (4; 12) or the like, and by a sensing circuit (5) which functions to sense changes in capacitance between the sheets of electrically conductive material (2, 3; 10, 11).

Description: A SENSOR FOR INDICATING CHANGES IN THE PRESENCE OF PERSONS OR OBJECTS The present invention relates to a proximity-type sensor. There is a great need for sensors, or detectors, that contain no movable parts, that are soft and pliable, and that can be produced in shapes and sizes that suit the field of use for which they are intended. Several different types of proximity sensors for indicating the presence of people or objects are known to the art. Examples of such sensors include different types of membrane contacts, infrared detectors, etc. These known sensors, however, are more or less complicated and in certain cases expensive. There is also a need for an inexpensive and simple contactless sensor that is able to detect, or sense, the presence of a person or an object without coming into contact therewith. It is also desired to be able to produce very thin sensors of this kind. The present invention relates to a sensor that can sense the presence of a person or an object either without or by coming into contact with said person or said object, that can be made very thin, that lacks movable parts, and that can be designed to suit the area of use for which it is intended. Accordingly, the present invention provides a sensor for indicating changes in the physical presence of persons or objects, and is characterized in that the sensor includes at least two electrically conductive sheets of material that are spaced mutually apart on an electrically non-conductive carrier sheet or the like; and in that a sensing circuit is provided for sensing changes in the capacitance between electrically conductive sheets. The invention will now be described in more detail with reference to exemplifying embodiments of the invention and also with reference to the accompanying drawing, in which - Figures 1 and 2 illustrate the principles of a sensor according to a first embodiment; - Figures 3 and 4 illustrate the principles of a sensor according to a second embodiment; - Figure 5 illustrates an alternative embodiment; and - Figure 6 illustrates an electronic circuit in block form. Figure 1 is a cross-sectional view of a first embodiment of an inventive sensor 1. Figure 2 shows the sensor of Figure 1 from above. The sensor is intended to indicate a change in the proximity of persons or objects. The inventive sensor includes at least two sheets of electrically conductive material 2, 3 mounted in mutually spaced relationship on a carrier plate 4 or like substrate that is not electrically conductive. The sensor also includes a sensing or detecting circuit 5 which is designed to detect changes in capacitance between the sheets of electrically- conductive material 2 , 3. In one highly preferred embodiment of the sensor, the electrically-conductive sheets are comprised of a polymeric material that includes a powdered electrically-conductive substance. This polymeric material will preferably be a carbon-containing material. Such materials are easily bent and can be obtained from Minnesota Mining and Manufacturing, Inc. (3M), U.S.A., among other suppliers. This material has been found to have good electrically conductive properties for the purpose concerned. In the case of the Figure 1 embodiment, the sheets 2, 3 are juxtaposed on the carrier plate 4. The sensor may also include a covering 19. The carrier plate and also the covering, when provided, will preferably be comprised of an electrically non-conductive plastic material. The sensor can be made very thin, for instance given a thickness of about 1 millimetre. When an object or a person comes into the close proximity of the two sheets 2, 3, or in contact therewith, the capacitance between the sheets 2, 3 will change. For instance, if the capacitance between the sheets 2, 3 is 30 pF when no person or object is in the proximity of the sensor, the capacitance will increase to, e.g., 200 pF when a person's hand comes close to the elements. The change in capacitance is thus a significant change. Figure 6 illustrates in block form a sensing circuit which is designed to sense or detect changes in capacitance between the two sheets. The circuit 5 includes a voltage device 6 which functions to apply an electric voltage between the sheets 2, 3, and a suitable kind of capacitance measuring device^ The circuit also includes a suitable microprocessor 8 for detecting those changes in capacitance that occur. The microprocessor 8 has a signal output 9 via which the processor delivers an electric signal when a certain predetermined change in capacitance occurs. This signal can be used to control some other device, for instance an alarm, an indicator lamp, etc., with which the inventive sensor is used. This predetermined change in capacitance must, of course, be adapted to the size of the sensor and its anticipated use. For instance, if the sensor is intended to detect the close presence of a person's hand, such as mentioned above, a capacitance change of 50 pF may be suitable as the predetermined capacitance change. According to one preferred embodiment, the sensing circuit 5 is designed to deliver an electric signal 9 when the capacitance changes to a predetermined value within a predetermined time period. This embodiment prevents slow capacitance changes caused by external circumstances from producing an electric signal. One such circumstance may, for instance, be a change in the relative humidity. A sensor according to this first embodiment may conveniently be used in beds, such as to indicate when a patient leaves his/her bed and when the patient lies on his/her bed. Ideally, the sensor will be placed in the centre of the bed and transversely to its longitudinal axis. In this case, the sensing circuit may be constructed to deliver a signal solely when the value of the change in capacitance is so high as to correspond to when a patient lays on or leaves his/her bed. The inventive sensor can also be used as a switch, where the signal from the sensing circuit controls an electric contact means or some other electric circuit. One appropriate use of the sensor in this respect is in the seats of automotive vehicles, e.g. cars and limousines, so as to indicate when a vehicle seat is occupied. The sensor is used to deliver a seat-belt warning signal. Figures 3 and 4 illustrate a second embodiment of the invention, in which the two sheets 10, 11 are placed one above the other. Figure 3 is a cross-sectional view while Figure 4 shows the sensor from above. The sheets are mutually separated by an intermediate sheet 12 of electrically conductive compressible material. This material may, for instance, be a plastic foam material. In the case of this embodiment, the intermediate sheet 12 may form said carrier sheet. However, the covering or carrying sheet 13, 14 may be provided on one or both sides of the sensor. In the case of this second embodiment, capacitance changes will occur between the sheets 10, 11 as a result of changes in the distance between said elements. A sensor constructed in accordance with this embodiment may be placed beneath a carpet or mat on which people walk or stand, such that a significant change in capacity will occur when a person's foot is placed on the carpet or mat. The elements 10, 11 are coupled to the sensing circuit 5 in the same manner as described with reference to the elements 2, 3. In the case of another embodiment illustrated in Figure 5, the sensor includes two or more mutually different fields 15 comprised of sheets of electrically conductive material, where the capacitance is measured between mutually adjacent fields or within each field. Each field may either consist in a sensor of the design illustrated in Figures 3 and 4, or the capacitance can be measured between pairs of fields 15, 16 in accordance with the embodiment illustrated in Figures 1 and 2. This embodiment thus enables a plurality of mutually different capacitance changes to be measured with one and the same sensor 17. Each field in an embodiment according to Figure 1 or each pair of fields in an embodiment according to Figure 3 is herewith connected to the sensing circuit in Figure 5. When two or more capacitances shall be measured in one and the same sensor, the sensing circuit will preferably be designed to scan respective capacitances in a successive order and therewith detect the occurrence of any capacitance change that may have taken place. The sensor arrangement shown in Figure 5 may be designed to indicate a direction, for instance the walking direction of a person. When a sensor according to Figure 5 is placed under a doormat and the sensor is sufficiently large to be actuated by at least two steps of a person walking on the mat, the sensor arrangement may be adapted to indicate whether a person is entering or leaving a store or shop, for instance. The fact that the sheets are comprised of a plastic material enables them to be bent or flexed. Because the sensors can be bent or flexed and given any appropriate shape as seen from above, the sensors can be constructed to suit all purposes in the present context. The sensor may also be used as an anti-pinch device in connection with elevator doors or other automatic doors. The sensor may also be used as part of a burglar alarm system, by placing the sensor beneath or adjacent to valuable objects, such as museum objects. Quite another area of use of the sensor, and then primarily a sensor according to the first embodiment, is as a liquid leakage monitor or liquid level monitor. A significant change in capacitance is obtained when liquid comes into the close proximity of the two sheets or in contact therewith. It will be obvious that the sensor can be constructed in a manner different to that shown and described. For instance, the various fields may have shapes other than a square or a rectangular shape. Furthermore, the carrier plate of the first embodiment can be a rigid plate when application of the sensor so requires. The coverings in the second embodiment can also be made rigid. The invention shall not therefore be considered to be limited to the aforedescribed and illustrated exemplifying embodiments thereof, since variations and modifications can be made within the scope of the following Claims.

Claims: CLAIMS 1. A sensor for indicating changes in the physical presence of persons or objects, characterized in that the sensor (1, 17, 18) includes at least two sheets of electrically conductive material (2, 3; 10, 11) that are spaced mutually apart on an electrically non-conductive carrier sheet (4; 12) or the like; and in that the sensor includes a sensing circuit (5) which is adapted to detect changes in capacitance between the sheets of electrically conductive material (2, 3; 10, 11). 2. A sensor according to Claim 1 , characterized in that the electrically conductive sheets (2, 3; 10, 11) are comprised of a polymeric material that includes a pulverized electrically conductive material, preferably carbon. 3. A sensor according to Claim 1 or 2 , characterized in that the sheets (2, 3) are juxtaposed on said carrier sheet. 4. A sensor according to Claim 1 or 2 , characterized in that the sheets (10, 11) are placed one above the another; and in that said materials are separated by an intermediate sheet (12) of electrically non-conductive material, said material being compressible. 5. A sensor according to Claim 1, 2, 3 or 4, characterized in that the sensor includes two or more different fields (15, 16) comprised of sheets of electrically conductive material, said capacitance being measured between mutually adjacent fields within each field. 6. A sensor according to any one of the preceding Claims, characterized in that the sensing circuit is designed to deliver an electric signal when the capacitance is changed by a predetermined value within a predetermined time period.

Inventors: RUDEKE JAN, RUDEKE, JAN
Applicants: RUDEKE JAN, RUDEKE, JAN
Application year: 1998
Main classification: G08B13
IPC codes: G08B13