Multi-channel electrostatic sensor apparatus

Abstract: Disclosed is a multi-channel electrostatic sensor apparatus comprising essentially a single main oscillation circuit; a plurality of pull-in type sub-oscillation circuits which are adapted to synchronize with said main oscillation circuit to generate sub-frequency signals whose frequency is equal to the main oscillation frequency or a multiple thereof; and a plurality of resonance circuits which are sensitive to any change in exterior capacitance when detected by an associated detector for changing its resonator resonance point with respect to the sub-frequency signal. Positive synchronization of the sub-oscillation circuits with the single main oscillation circuit prevents any adverse effect from being caused by the varying stray capacitance in flexible conductors connecting the sub-oscillation circuits to the main oscillation circuit while allowing the relatively free movement of the video heads of a video system, on which magnetic heads the sub-oscillation circuits and subsequent components are attached.

Description: The present invention relates to a multi-channel electrostatic sensor apparatus comprising a plurality of parallel channels each including a sensor circuit which is responsive to the change of an exterior capacitance for outputting a detection signal representing the change of the exterior capacitance. A conventional electrostatic sensor uses a tank circuit having an exterior capacitance in its resonance circuit, thus providing an output signal whose frequency varies with the change of the exterior capacitance. This conventional electrostatic sensor is of low sensitivity, and therefore recently electrostatic sensors have been popular because of their relatively high sensitivity. U.S.Patent No. 4528655 describes electrostatic sensors using a resonance circuit whose resonant frequency is somewhat different from the oscillation frequency of an associated oscillator, and the capacitance of the resonance circuit is allowed to vary, thereby providing an amplitude-modulated signal. As shown in Fig. 1 of the present application, such an arrangement comprises oscillator circuit 1, resonance circuit 2, capacitance detector circuit 3, detection circuit 4 and amplifier 5. The resonator of oscillator circuit 1 has a fixed oscillation frequency f1, whereas the resonance circuit 2 has a resonant frequency fo somewhat different from oscillation frequency f1, as seen from Fig.2. The resonant frequency fo is allowed to change by Δf in response to the change ΔC in a very small capacitance when detected by detector 3, thereby permitting conversion of the change ΔC in the very small capacitance into corresponding voltage changen ΔV. which is amplified and derived. There is a demand for detecting the change in each of a plurality of capacitances and processing a corresponding plurality of detection signals in different ways. For instance, a plurality of electrostatic sensors may be used simultaneously to detect video signals from a plurality of disks in a video system, and a corresponding plurality of signals thus detected subjected to parallel processing. In an attempt to carry- out such parallel processing a parallel arrangement of series connections each comprising oscillator 1, resonance circuit 2, detection circuit 4 and amplifier 5 may be used. Assume that the oscillation frequency of oscillator is as high as 1 GHz, and that capacitance increment is detected at the sensitivity of 1x10⁻⁵ PF. Exterior disturbance and stray capacitances which are liable to appear in the electrostatic sensor circuit included in each channel, however, will make it difficult for each sensor circuit to have the same oscillation frequency. Oscillation frequency difference between adjacent channels causes mutual interference such as a beat frequency between the oscillation frequencies of the channels. For instance, assume that the oscillation frequency of a selected channel is "f", and that the oscillation frequency of an adjacent channel is "F". Then, interference will be caused in the form of a beat frequency of F-f or F+f, adversely reducing the S/N ratio to prevent correct signal processing. The oscillation frequency of oscillation circuit 1 is selected to be 1 GHz, and the oscillation frequencies of adjacent channels can be selected to be sufficiently different to prevent mutual interference between the oscillation frequencies. It is, however, difficult to prevent the oscillation frequencies of adjacent channels producing beat frequencies when there is a plurality of channels and the difficulty will increase with the number of channels. Very complicated production work is required to reduce the interference trouble with recourse to this remedy. In order to eliminate such complicated production work the inventors proposed that a single oscillation circuit 1 is used by all channels in common. Specifically, a single oscillation circuit distributes its oscillation frequency signal to the resonance circuit of associated channels. Such an arrangement is shown also in the Patent Abstracts of Japan, vol. 9, No. 177, dated 23rd July 1985, at page 375. Here is disclosed an Electric Field Type Human Body Detector in which each of a plurality of separate oscillation circuits (12a, 12b, 12c) is connected directly to an external oscillator (11) and arranged to oscillate in synchronism with it. This proposed apparatus works well so far as every channel is fixed and stationary. However, assume that the video head of the video system includes the channel comprising a series connection of resonant circuit 2 to amplifier 5 and moves on the video disk of the video system, and is connected to the resonance circuits 2 of all channels via cable or other conductors. If the cables are insufficiently flexible, movement of the video head will be adversely affected to lower the video head's capability of detecting video signals. For the transmission of high frequency signals, coaxial cables are preferably used, but such coaxial cables are so heavy and inflexible that these cables, in fact, cannot be used. In an attempt to facilitate movement of the video head, shielded soft copper wires may be used. The wires, however, will change their shape as the video head moves on the disk, and accordingly the stray capacitance and impedance of the wires will change, thus reducing the amplitude of oscillation signals and causing distortion of the waveforms of oscillation signals. As a result very small changes in capacitance cannot be detected. In an atttempt to prevent the lowering of the oscillation signal an amplifier may be used to compensate for the reduction of the amplitude of oscillation signals. The use of such an extra amplifier, however, will cause undesired oscillation between oscillation circuit 1 and resonance circuit 2, reducing the stability of the capacitance detection system. In view of the above, one object of the present invention is to provide a high sensitivity, multi-channel electrostatic sensor apparatus which is free of beat signals between adjacent channels, and attenuation of oscillation signals from an associated oscillator, which attenuation will adversely affect the performance of the whole apparatus. In order to achieve this object a multi-channel electrostatic sensor apparatus according to the present invention is defined in claim 1. In operation the oscillation signal is supplied from the main oscillator circuit to the sub-oscillator circuit of each channel, which generates a sub-frequency signal of the same frequency as the main oscillation circuit generates or of a multiple of the main oscillation circuit frequency. This sub-frequency signal is applied to an associated resonance circuit, which varies its resonance point with the change in an exterior capacitance for providing a detection signal representing the change in the exterior capacitance. The detection signal is detected and amplified in each channel, and then it will be subjected to signal processing as desired. Multi-channel electrostatic sensor apparatus according to the invention will now be described with reference to the accompanying drawings, in which :- Fig. 1 is a block diagram representing a conventional electrostatic sensor apparatus; Fig.2 shows how a very small capacitance can be detected in the electrostatic sensor; Fig.3 is a block diagram representing a multi-channel electrostatic sensor apparatus according to the present invention; Fig.4 shows some details of a single channel in the multi-channel electrostatic sensor apparatus according to the present invention; and Fig.5 shows diagrammatically the connection between the main oscillation circuit and each of the sub-oscillation circuits. A multi-channel electrostatic sensor apparatus according to one embodiment of the present invention will be described with reference to Fig.3 to 5. As shown in these drawings, it comprises main oscillation circuit 100 and a plurality of electrostatic sensor circuit 13 each comprising pull-in type sub-oscillation circuit 18, resonance circuit 2, detection circuit 4, capacitance detector 11, amplifier circuit 5 and AFC circuit 8. These capacitance sensor circuits 13 are connected to main oscillation circuit 100 in common. As seen from Fig.4, main oscillation circuit 100 uses a dielectric resonator such as a ceramic resonator 10 which is capable of generating a fixed main frequency signal between a very high frequency and a super-high frequency in the range from 1 to 1/50 times the oscillation signal of pull-in type oscillator 18. The dielectric resonator is preferably used, for reducing the size of main oscillation circuit. The main frequency signal is supplied to sub-oscillation circuit 18 after passing through high-impedance conversion circuit 6. Sub-oscillation circuit 18 is a pull-in type oscillation circuit which can be put in synchronization with main oscillation circuit 100, providing a sub-oscillation signal whose frequency is equal to the main oscillation signal or a multiple of the frequency of the main oscillation signal. This sub-oscillation signal is applied to resonance circuit 2. Resonance circuit 2 comprises a ceramic resonator 12, which is separated or independent from ceramic resonator 10 of main oscillation circuit 100 and the resonator of sub-oscillation circuit 18. Capacitance detector 11 includes a stylus (which might be substituted by an electrode or other device) to detect any change in a capacitance. Detection circuit 4 is connected to ceramic resonator 12 via coupling capacitor 25. Detection circuit 4 comprises inductance element 21, diode 22, capacitor 23 and resistor 24. The output signal from resonance circuit 2 is applied to detection circuit 4 via coupling capacitor 25. Diode 22, capacitor 23 and resistor 24 together constitute the detection section of detector circuit 4. The detection section performs the envelope detection of the high-frequency signal from resonance circuit 2 to convert to a relatively low frequency signal, which is comparable to the signal from the object under test. Inductance element 21 is connected to the anode of diode 22, thereby preventing the relatively low forward impedance of diode 22 from functioning to the "Q" of resonance circuit 2. Specifically, capacitor 25 and inductance element 21 together constitute a high-impedance circuit to prevent the lowering of the "Q" of resonance circuit 2. Amplifier 5 is composed of transistor 27, resistors and other elements, and amplifier 5 amplifies the signal from detection circuit 4 before applying to a signal processing circuit (not shown) and to AFC circuit 8. AFC circuit 8 is composed of operational amplifier 28, capacitors 29,30, variable resistor 31, resistors 32,35, variable capacitance diode 33, and coupling capacitor 34. The negative-side input terminal of operational amplifier 28 is connected to the output terminal of amplifier 5 whereas the positive-side input terminal of operational amplifier 28 is connected to the slider contact of variable resistor 31. Capacitor 29 is connected between the negative-side input terminal of operational amplifier 28 and the output terminal thereof. One electrode of capactor 30 is connected to the output terminal of operational amplifier 28, and the other electrode of capacitor 30 is grounded. The output terminal of operational amplifier 28 is connected to the output terminal of resonance circuit 2 via resistors 32, 35 and coupling capacitor 34. The cathode of variable capacitance diode 33 is connected between coupling capacitor 34 and resistor 35 whereas the anode of variable capacitance diode 33 is grounded. Operational amplifier 28 of AFC circuit 8 amplifies the signal from amplifier 5. The signal appearing at the output terminal of operational amplifier 28 is smoothed and integrated by the capacitor 29 and becomes nearly a direct current signal having a low frequency. The signal will pass through an integrator circuit made up by capacitor 30 and resistor 32 to be integrated before applied to variable capacitance diode 33. Variable capacitance diode 33 is biased in negative direction. In this particular embodiment a 12 volt power supply it used, and 6V or more bias voltage is applied to variable capacitance diode 33, the operating point of the diode 33 being put thereby at a low negative bias voltage in accordance with the zero voltage. The voltage applied to variable capacitance diode 33 can be adjusted to a dc level by moving the slidable contact on the resistor 31. In other words, the central operating point of the AFC circuit can be set as desired. Variable capacitance diode 33 is responsive to the voltage signal from the integrator circuit for changing its capacitance, and then the change in the capacitance will affect resonance circuit 2 through coupling condenser 34, thus causing the resonant frequency of resonance circuit 2 to vary. Assume that the resonant frequency of resonance circuit 2 is caused to shift rightward from "fo" for any reason, as seen from Fig. 2. Then, disadvantageously, the signal appearing at the output terminal of resonance circuit 2 will fall below Vo out of the Vo-to-V1 range. In such case AFC signal is applied to resonance circuit 2, thereby automatically shifting the resonant frequency fo leftward to prevent the signal appearing at the output terminal of resonance circuit 2 from departing from the Vo-to-V1 range. In this sense AFC unit functions as an AGC (Automatic Gain Control) unit. In Fig. 4, the symbol ▿ is used to indicate grounding. A plurality of sensor circuits 13-1 to 13-n are connected to main oscillation circuit 1′. The main oscillation of high frequency is connected to sub-oscillation circuits 18-1 to 18-n. A multi-channel electrostatic sensor apparatus according to the present invention can be provided on a single circuit board as a whole. However, if it is used in detecting video signals from a video system equipped with a plurality of disks, each sensor channel 13-1 to 13-n of the multi-channel electrostatic sensor apparatus may be fixed to each corresponding video head of the video system, and the main oscillation circuit 100 is commonly arranged for all sensor channels, and is connected thereto via the distributer of high-frequency gyrator 15 and associated conductors. Specifically, the main oscillation signal is distributed to "n" sensor circuits 13-1 to 13-n by high-frequency gyrator 15 and the main oscillation signal is supplied to the sub-oscillation circuit 18 of each sensor circuit via semi-rigid coaxial cables or wave guides 16 and flexible shielded wire 19, as seen from Fig.5. Signals appearing at the output terminals of amplifiers 5-1 to 5-n of sensor circuits 13-1 to 13-n may be subjected to parallel processing. Sometimes, these output signals may be divided in pairs, and differential output between each pair is subjected to a desired processing. The operation of a multi-channel electrostatic sensor apparatus according to the present invention will be described as follows: As seen from Fig.2, sub-oscillation frequency f1 of each sub-oscillation circuit 18-1 to 18-n is set somewhat apart from the resonant frequency fo of each resonance circuit 2. Main oscillation circuit 100 generates main frequency signal at the frequency of 1 GHz. Then, each sub-oscillation circuit 18-1 to 18-n is put in synchronism with the main oscillatin signal of 1 GHz at the ratio 1-to-1 to 1-to-N to generate a sub-oscillation signal, for instance, at the same frequency as the main oscillation signal (1 GHz). When objects 14-1 to 14-n move, very small capacitance increments or changes will be caused to be detected by styluses 17-1 to 17-n of detection sections 11-1 to 11-n with the result that the operating point or resonant point of resonant circuits 2-1 to 2-n change by Δf-1 to Δf-n. In the first sensor circuit 13-1 sub-oscillation signal f1 is multiplied by the resonant frequency increment Δf-1, and in the second sensor circuit 13-2 sub-oscillation signal f1 is multiplied by the resonant frequency increment Δf-2, and so forth, thus obtaining amplitude-modulated signals. In this particular embodiment the main and sub-oscillation signals have a super-high frequency such as 1 GHz, and then, the modulated signal in each sensor channel has a super-high frequency varying about the center frequency of 1 GHz in the range corresponding to movement of objects 14-1 to 14-n and such super-high frequency signals are applied to corresponding detection circuits 4-1 to 4-n in which these signals are subjected to envelope detection, thus being converted to signals in the signals range of objects 14-1 to 14-n 8 (in this particular embodiment, 3MHz). The signals thus coverted are amplified by amplifiers 5-1 to 5-n to be directed to signal processing circuits (not shown) and to AFC circuits 8-1 to 8-n. In AFC circuits 8-1 to 8-n these signals are reduced to nearly dc level by a smoothing effect provided by capacitors 29 and 30, and then such dc signals are integrated by an integrator circuit to a desired level before being applied to variable capacitance diode 33. The capacitance of variable capacitance diode 33 changes with the dc signal when applied thereto, thus permitting appropriate adjustment resonant frequency fo in resonance circuit 2. As may be understood form the above, "n" sensor circuits 13-1 to 13-n have an oscillation circuit 100 in common, and the sub-oscillation circuits 18-1 to 18-n of all sensor circuits are synchronized with the main oscillation circuit to generate signals, for instance, at the same frequency as the main oscillation frequency, not causing any difference or shift in frequency in each sub-oscillation circuit. As a result there will be no interference due to beat frequency which otherwise would appear in adjustment sub-oscillation circuits, and a very small capacitance change can be detected without any problem. In case a multi-channel sensor apparatus according to the present invention is used for detecting video signals from a video apparatus, movement of the video will cause flexible shielded wires 19 to vary in shape (see Fig.5), accordingly varying the stray capacitance of shielded wires 19. Irrespective of the change of stray capacitance, however, it is assured that sub-oscillation circuits 18-1 to 18-n generate signals, for instance, at the same frequency as main oscillation circuit 100 because of complete synchronization being guaranteed, and therefore, a very small capacitance change can be detected even when sensor circuicts 13-1 to 13-n may be moved along with the video heads of the video system. The common use of a main oscillation circuit in a multi-channel electrostatic sensor apparatus permits substantial reduction of the whole sensor apparatus in size and weight. The present invention should not be limited to the embodiment described above because various modifications are possible within the scope of the present invention, as for instance follows: Also, in this particular embodiment as shown in Fig. 5 a high-frequency gyrator is used as distributor 15. The distributor 15 may be composed of strip lines if the space permits. In this particular embodiment the main oscillation frequency signal is directed to sub-oscillation circuits 18-1 to 18-n via wave guides 16 and flexible shielded wires 19. However, the shapes of flexible shielded wires 19 may vary with movement of the video heads on which sensor circuit are fixed with the result that the stray capacitances of flexible shielded wires 16 cause significant deformation of signals. In an attempt to prevent such adverse effect bandpass filters may be provided between main oscillation circuit 100 and sub-oscillation circuits 13-1 to 13-n, thereby preventing distortion of signals, attenuation of signals or mismatching of impedance. In this particular embodiment sub-oscillation circuits 18-1 to 18-n are designed to generate signals at the same frequency as the main oscillation frequency. Sub-oscillation circuits 18-1 to 18-n may be of pull-in type whose frequency is a multiple of the frequency of the main oscillation circuit 100. If the main oscillation frequency is selected to be 10 or more GHz, and if sensor circuits 13-1 to 13-n are fixed to the video heads of a video system, allowing sensor circuits 13-1 to 13-n to move along with the video heads as seen from Fig.5, there is a fear of adverse effects such as the distortion or an attenuation of signals, which are caused by allowing the shapes of flexible shielded wires 19 to vary while following the video heads. In an attempt to reduce such adverse effects, the main frequency of main oscillation circuit 100 is selected to be 2,4,6 .... times less than sub-frequency of sub-oscillation circuits 13-1 to 13-n, for example, the main oscillation frequency of the main oscillation circuit is selected to be 100 KHz and the oscillation frequency of the sub-oscillation circuits 18-1 to 18-n is selected to be 1 GHz, thus permitting the main oscillation frequency to be lowered below the frequency at which little or no adverse effect will be caused by the varying shape of cables or flexible shielded wires 19, thereby permitting determination of a very small capacitance change. In this particular embodiment, main oscillation circuit 100 is separated from sub-oscillation circuits 13-1 to 13-n. However, main oscillation circuit 100 may be included in a selected sensor circuit. A multi-channel electrostatic sensor apparatus according to the present invention uses an oscillation circuit in common and a plurality of pull-in type sub-oscillation circuits, thus permitting the sub-oscillation circuits to operate at the same frequency as the main oscillation circuit or at a multiple of the main oscillation frequency. This arrangement has the effect of preventing appearance of beat signals caused by interference between adjacent sensor circuits, thus permitting detection of a very small capacitance change. In case sensor circuits 13-1 to 13-n on the video heads of a video system are connected to main oscillation circuit 100 by flexible shielded wires 19 to permit relatively free movement of sensor circuits 13-1 to 13-n, the stray capacitance will vary with the change of the shape of flexible shielded wires 19, and the impedance matching can be hardly obtained. Sub-oscillation circuit 18, however, is guaranteed to generate signals at the same frequency as the main oscillation circuit or at a multiple of the main oscillation frequency. This permits detection of video signals at an increased sensitivity, compared with a conventional electrostatic detection apparatus. A multi-channel electrostatic sensor apparatus according to the present invention uses a single main oscillation circuit in common, and accordingly the size and weight of the whole system is substantially reduced, compared with a conventional multi-channel electrostatic sensor apparatus using a main oscillation circuit allotted to each sensor circuit. As described above, there is no fear of interference between main and sub-oscillation circuits, and therefore adjacent sensor channels can be put close to each other, accordingly permitting increase of the density with which sensor circuits are arranged. The present invention should no be limited to the embodiment described above because various modifications are possible within the scope of the present invention.

Claims: A multi-channel electrostatic sensor apparatus comprising a main oscillator circuit (100) for generating main oscillation signals, a plurality of sub-oscillation circuits (13) adapted to synchronise with said main oscillation circuit (100) for generating sub-oscillation signals, and a plurality of resonance circuits (2) each having an independent resonator (12) separated from said main oscillation circuit (100) and said sub-oscillation circuits (18), said resonance circuits (2) being sensitive to any change of exterior capacitance detected by an associated detector (11) to change the resonance point thereof, and a plurality of sensor circuits (13) each comprising a sub-oscillation circuit (13) and a resonance circuit (2) are provided using the same main oscillation circuit (100) in common for a plurality of channels, whereby connection means are provided for connecting the main oscillation circuit (100) to each of the sub-oscillation circuits (18) to supply said main oscillation signal thereto and comprising a distributor (15) and separate conductors connecting the distributor (15) to each separate sub-oscillation circuit (13), characterised by a high impedance conversion circuit (6) connected between the main oscillation circuit (100) and the distributor (15) and operable to isolate the main oscillation circuit (1CO) from any impedance changes occurring in any sub-oscillation circuit (18). A multi-channel electrostatic sensor apparatus as claimed in Claim 1 characterised in that the conductors connecting the distributor (15) to each separate sub-oscillation circuit (13) comprise a coaxial cable (16) and a shielded wire (19). A multi-channel electrostatic sensor apparatus as claimed in Claim 1 characterised in that the conductors connecting the distributor (15) to each separate sub-oscillation circuit (13) comprise a wave guide (16) and a shielded wire (19). A multi-channel electrostatic sensor apparatus as claimed in any one of Claims 1 to 3 characterised in that each sensor circuit (13) includes a detector circuit (4) for detecting the output of the resonance circuit (2), and amplifier circuit (5) for amplifying the output of said detection circuit (4) and an AFC circuit (8) for controlling the rescnance point of said resonance circuit (2). A multi-channel electrostatic sensor apparatus as claimed in any on of the preceding claims characterised in that the main oscillation circuit (100) and each said resonance circuit (2) includes a ceramic resonator (10, 12). A multi-channel electrostatic sensor apparatus as claimed in any one of Claims 1 to 5 characterised by band pass filters connected between said main oscillator circuit (100) and each said sub-oscillation circuit (18). A multi-channel electrostatic sensor apparatus as claimed in any one of the preceding claims characterised in that said sub-oscillation signal has a frequency equal to the oscillation frequency of said main oscillation circuit (100). A multi-channel electrostatic sensor apparatus as claimed in any one of Claims 1 to 6 characterised in that said sub-oscillation signal has a frequency which is a multiple of the oscillation frequency of said main oscillation circuit (100). A multi-channel electrostatic sensor apparatus as claimed in any one of the preceding claims characterised in that the distributor (15) comprises a high frequency gyrator.

Inventors: MASUDA NOBORU, OOSAWA TETSUO, MASUDA, NOBORU, OOSAWA, TETSUO
Applicants: MURATA MANUFACTURING CO, MURATA MANUFACTURING CO., LTD.
Application year: 1991
Main classification: G01V3
IPC codes: G01V3, G01R29, G01R27