Electrical capacitance proximity sensor

An electrical capacitance proximity sensor with high detector sensitivity has an insulating substrate, a detector electrode and an earth electrode formed in a fixed pattern on one face of the insulating substrate, and a detector circuit which detects an object approaching the detector electrode and the earth electrode by detecting the capacitance between the detector electrode and the earth electrode. The detector electrode is formed in a shape such that it surrounds the earth electrode.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an electrical capacitance proximity sensor which detects an approaching object by changes in capacitance. Priority is claimed on Japanese Patent Application No. 2004-145502, filed May 14, 2004, the content of which is incorporated herein by reference.

2. Description of the Related Art

Conventionally, an electrical capacitance proximity sensor is known (for example, Japanese Unexamined Patent Application, First Publication No. 2000-48694 (Paragraph 0006, 0012, FIG. 1, FIG. 2)), where a detector electrode and an earth electrode are formed on one face (detection face) of an insulating substrate, and a shield electrode is formed on an other face (back face) such that it covers a formation area of the detector electrode and the earth electrode, thereby concentrating electric lines of force on the detection face, so that the effect of an object on the side faces and the back face of the detection face are eliminated.

Such an electrical capacitance proximity sensor is furnished with a detector circuit which includes, an oscillation circuit which oscillates at a frequency based on the capacitance between the detector electrode and the earth electrode, and a judgment circuit which determines the oscillation frequency of the oscillation circuit, and the detector circuit detects changes in capacitance resulting from an object approaching the detector electrode and the earth electrode to thereby determine the presence of an object and distance.

In the above-mentioned conventional electrical capacitance proximity sensor, in anticipation of a shielding effect, the earth electrode is formed in a shape such that it surrounds the detector electrode. However, from experiments performed by the inventor, it has become clear that detection sensitivity is high near the detector electrode, and not near the earth electrode.

SUMMARY OF THE INVENTION

The present invention takes such points into consideration, with an object of providing an electrical capacitance proximity sensor with high detection sensitivity.

An electrical capacitance proximity sensor according to the present invention comprises; an insulating substrate; a detector electrode and earth electrode formed in a fixed pattern on one face of the insulating substrate; and a detector circuit which detects an object when it approaches the detector electrode and earth electrode by detecting the capacitance between the detector electrode and earth electrode, and the detector electrode is formed in a shape in which it surrounds the earth electrode.

In one embodiment of the present invention, the detector electrode is made in a letter C pattern which surrounds the earth electrode. Furthermore, in another embodiment of the present invention, the detector electrode comprises a first detector electrode and a second detector electrode separated symmetrically with the earth electrode as the center, and the detector circuit determines the direction in which the object is approaching from, according to a capacitance between the first detector electrode and the earth electrode, and a capacitance between the second detector electrode and the earth electrode. In yet another embodiment of the present invention, a shield material is formed on an other face of the insulating substrate, covering an entire formation area of the detector electrode and the earth electrode.

According to the present invention, since the detector electrode is formed to surround the earth electrode, the capacitance between the detector electrode and the earth electrode can be made high. As a result, the effects of external noise can be eliminated, and the change in capacitance resulting from the presence of an approaching object can be made high. Therefore, it is possible to provide an electrical capacitance proximity sensor capable of detecting over a wide area.

DETAILED DESCRIPTION OF THE INVENTION

Hereunder is a description of preferred embodiments of the present invention, with reference to the appended drawings.

FIG. 1andFIG. 2show an outline configuration of an electrical capacitance proximity sensor according to an embodiment of the present invention. This proximity sensor comprises a sensor section10and a detector circuit20.

The sensor section10comprises a flexible printed circuit (FPC), a rigid printed circuit (RPC) and the like, which is furnished with; an insulating substrate11composed of an insulating material such as polyethelene terephthalate (PET), polyethelene naphthalate (PEN) or epoxy resin; and a copper, copper alloy or aluminum detector electrode12and earth electrode13formed in a pattern on the insulating substrate11. The earth electrode13is square or rectangular, and the detector electrode12is formed in a letter C-type shape or pattern so that it surrounds the perimeter of the earth electrode13on at least most of both sides and the upper side.

The detector circuit20, as shown inFIG. 2, comprises: an oscillation circuit21which changes its oscillation frequency according to the capacitance between the detector electrode12and the earth electrode13; a frequency detector circuit22which detects the size of the oscillation output frequency of the oscillation circuit21; a judgment circuit23which discriminates an approaching object, distance and the like based on the size of the frequency detected in the frequency detector circuit22; and a control circuit24which controls these circuits. Here, an alternating voltage corresponding to the oscillation output is applied to the detector electrode12connected to the oscillation circuit21, but the earth electrode13is fixed at earth potential. The frequency detector circuit22can be formed by a frequency-voltage transducer such as a low-pass filter which converts frequency into voltage, and a frequency counter which, for example, counts the rising edges of the oscillation output. The judgment circuit23can be formed by programmable devices such as a voltage comparator and a CPU. The control circuit24is, for example, a circuit for gain adjustment for amplifiers used in the oscillation circuit21, the frequency detector circuit22and the judgment circuit23. It may also be a circuit used for correction for external environment, such as temperature and humidity, and may be formed together with the judgment circuit23.

The above is one example of a detector circuit20, and in addition to the above, for example, an oscillation circuit which oscillates at a constant frequency, a one-shot multivibrator which changes the duty of the output pulse from the oscillation circuit using the capacitance of the detector electrode and the earth electrode, and a low-pass filter which outputs the average value of the output of the one-shot multivibrator, may be used to form the detector circuit.

Next is a description of the effect of the electrical capacitance proximity sensor configured in the above manner.

As shown inFIG. 3, an electrode1of 40 mm sides has formed on its upper side and left and right sides, via a 10 mm gap, an electrode2of width 40 mm in a letter C shape. The electrode1is connected to earth potential and is the earth electrode. The electrode2is the detector electrode and is connected to a fixed feedback path of the circuit inFIG. 2. A flat earth electrode of 30 mm sides was brought close to each of the points A, B, C and D ofFIG. 3, and the relationship between the distance to each of the points and the capacitance between the electrodes1and2was measured. Those results are shown inFIG. 4A. As shown in the figure, when the flat earth electrode of 30 mm sides, smaller than the central electrode1, was brought close to the point A of the electrode1which is the earth electrode, no change in capacitance was observed at all. On the other hand, after being brought closer than a distance of 20 mm to each of the points B, C, D of the letter C shape electrode2, which is the detector electrode, the capacitance itself and its change was large, varying between 6 to 10 pF, thus confirming excellent detector sensitivity.

The earth electrode being brought close to the electrodes1and2ofFIG. 3was changed to one of 50 mm sides, and measurements were conducted as for embodiment 1. Those results are shown inFIG. 4B. In this manner, when the earth electrode being brought close was made larger than the electrode1, even at point A of the electrode1, a slight change in capacitance was observed. Furthermore, after being brought closer than a distance of 20 mm to each of the points B, C and D of the letter C shape electrode2, which is the detector electrode, the change in capacitance became 7.5 to 12 pF. Thus a larger capacitance and capacitance change than embodiment 1 was observed.

COMPARATIVE EXAMPLE 1

In Comparative Example 1, the electrode1was changed to become the detector electrode, and the electrode2was changed to become the earth electrode. Otherwise, measurements were conducted as for embodiment 1. Those results are shown inFIG. 5A.

COMPARATIVE EXAMPLE 2

In Comparative Example 2, the electrode1was changed to become the detector electrode, and the electrode2was changed to become the earth electrode. Otherwise, measurements were conducted as for embodiment 2. Those results are shown inFIG. 5B.

As is clear fromFIGS. 5A and 5B, a change in capacitance was only observed for point A of the electrode1, and the capacitance and its change was small, and in the range of 6 to 7.5 pF in comparative example 1, and 6 to 8 pF in comparative example 2, and both detector sensitivity and detector range were comparatively worse than in embodiment 1 and 2.

COMPARATIVE EXAMPLE 3

As shown inFIG. 6, three electrodes3,4and5, of 40 mm sides were set in a line, leaving a 10 mm gap, in a state where the electrode3was made the earth electrode, the electrode4was made the detector electrode, and the electrode5was made to have nothing connected to it. A flat earth electrode of 30 mm sides was brought close to each of the points A, B and C and the change in capacitance was measured. Those results are shown inFIG. 7A.

COMPARATIVE EXAMPLE 4

For the same free electrodes3,4and5as comparative example 3, the same measurements as comparative example 3 were performed in a state where the electrode3was made the earth electrode, the electrode5was made the detector electrode, and the electrode4was made to have nothing connected to it. Those results are shown inFIG. 7B.

As is clear fromFIGS. 7A and 7B, a change in capacitance was only observed for points B and C of the detector electrode, and the capacitance itself was small, in the range of 3.5 to 6.5 pF and 3.8 to 6.8 pF.

As mentioned above, according to the present embodiment of the electrical capacitance proximity sensor, the effect where this has a high detector sensitivity, and is strong against external noise, can be demonstrated.

Furthermore, inFIG. 1, to increase the detector sensitivity of the detector electrode12, a shield which faces the back face of the electrode is effective.

FIG. 8is an I–I′ cross-sectional view of the sensor section10ofFIG. 1. The undersurface of the insulating substrate11has a shield material14formed such that it covers the entire formation area of the detector electrode12and the earth electrode13. Therefore, the effect of the electric lines of force on the back of the insulating substrate can be eliminated, and the noiseproof property can be improved.

FIG. 9is an example where an insulating material16with a shield material14formed thereon, is set on the underside of the insulating substrate11via a spacer15composed of insulating material. That is to say, when the insulating substrate11is, for example, a membrane sheet of about 75 μm, the detector electrode12and the shield material14may be too close together, and the electric lines of force may become too concentrated between them. Therefore, by putting a spacer15between them, the electric lines of force on the upper surface can be increased and detection sensitivity can be further increased.

FIG. 10is an example in which the shield material14ofFIG. 8is lengthened to a height to enclose not only the underside of the insulating substrate11, but also the electrodes12and13.FIG. 11is an example in which the shield material14and the insulating material16ofFIG. 9have been lengthened to a height to enclose not only the sides of the spacer15, but also the electrodes12and13. By covering the sides of the electrodes12and13with the shield material14as in these examples, it is possible to eliminate the effects of the underside even more completely.

FIG. 12andFIG. 13are drawings showing the outline specification of a proximity capacitance sensor according to another embodiment of the present invention. In this embodiment, there is a difference to the previous embodiment in that the detector electrode which constitutes a sensor section30is separated into two detector electrodes, and a detector circuit40determines the approach of an object, distance, and direction of movement, with the two detector electrodes.

The sensor section30is furnished with an insulating substrate31, detector electrodes32aand32bin a pattern formation on the insulating substrate31, and an earth electrode33. The earth electrode33is square or rectangular, and the detector electrode32ais formed such that it surrounds the earth electrode33on its left side and the left half of its upper side, and the detector electrode32bis formed such that it surrounds the earth electrode33on its right side and the right half of its upper side, in respective letter L shapes.

The detector circuit40, as shown inFIG. 13, comprises an oscillation circuit41awhich changes its oscillation frequency according to capacitance between the detector electrode32aand the earth electrode33; an oscillation circuit41bwhich changes its oscillation frequency according to capacitance between the detector electrode32band the earth electrode33; frequency detector circuits42aand42bwhich detect the size of the respective oscillation output frequencies of the oscillation circuits41aand41b;a judgment circuit43which determines the approach of an object, distance, and direction of movement and the like based on the size of the frequency detected from the frequency detector circuits42aand42b;and a control circuit44which controls the circuits.

In this embodiment, when an object moves from left to right inFIG. 12, the capacitance of the detector electrode32bchanges after the capacitance of the detector electrode32achanges, and conversely, when an object moves from right to left inFIG. 12, the capacitance of the detector electrode32achanges after the capacitance of the detector electrode32bchanges, thereby enabling the direction of movement of the object to be detected.

The present invention is not limited to the embodiments stated above.

For example, for the shape of the electrodes, a pattern as shown inFIG. 14with a circular earth electrode51, and an annular detector electrode52which surrounds the circumference of the earth electrode51arranged on concentric circles except for the area connecting the earth electrode to the detector circuit may be used. Moreover, as shown inFIG. 15, a pattern where an earth electrode61of a rectangular wave type shape is surrounded on at least most of three of its four perimeter sides by a corresponding rectangular type detector electrode62may be used.

In short, the present invention is characterized by the detector electrode being formed in a shape which surrounds the earth electrode, and various modifications are possible without departing from the spirit or scope of the present invention.