JPH051825Y2 - - Google Patents

Description

[Detailed description of the invention] [Field of industrial application] This invention relates to a capacitance measuring device, and in particular to a scanning type that scans a large number of measurement electrodes and measures the capacitance at each electrode in a time-sharing manner. Regarding multi-point capacitance measuring equipment.

[Conventional technology]

Conventionally, so-called multi-point measurement, in which two or more points are measured using one measuring device, has been carried out in many fields for purposes such as recording and control. A time-sharing method is used in which input is performed while scanning to the side for measurement. Therefore, if multiple capacitance detection electrodes are provided and their outputs are scanned, it is possible to measure capacitance at multiple points using a single capacitance measuring device using the same principle. However, there are still unresolved technical problems as described below, and a multipoint capacitance measuring device with sufficient accuracy and reliability has not yet been put into practical use.

[Problem that the invention attempts to solve]

When simply applying the conventional multi-point measurement technique described above to capacitance measurement and measuring the capacitance of a large number of measurement electrodes in a time-sharing manner, the electrodes other than those connected to the measuring device at each time point are The electrodes are usually left at an undefined potential or simply grounded. however,
If the electrodes are left at an undefined potential or simply grounded, the capacitance between the electrodes will be picked up and the measurement target at each point will be disturbed if the electrodes are close together. , leakage current also flows through the measurement circuit, making it extremely difficult to accurately measure the capacitance of each electrode. In addition, the connection cables between each electrode and the measurement circuit must be individually and strictly shielded to prevent mutual interference and disturbances, but when there are many measurement points, the connection cables become bulky and such strict shielding is necessary. It is not easy to provide and maintain a shield, and simply grounding the shield side does not allow accurate measurement of the capacitance of each electrode, as described above. Furthermore, if a switch such as an analog switch connected to each electrode system for scanning has insufficient opening/cutting characteristics, problems may occur due to mutual interference through switch leakage.

This idea was devised in view of the above circumstances, and its purpose was to create a scanning type that can automatically and accurately measure the capacitance of measurement electrodes arranged at multiple points in a time-sharing manner. An object of the present invention is to provide a multi-point capacitance measuring device.

[Means for solving problems]

In order to solve the above problem, in this invention, a capacitance measurement circuit CM having an AC power supply 4 for measurement is used.
and a plurality of measurement electrode systems 8 each including a capacitance measurement electrode Pn, the capacitance measurement circuit
A first switch means S ₁ sequentially connects the CM and each measurement electrode system 8; and a first switch means S ₁ connects the measurement electrode systems 8 other than the measurement electrode systems 8 sequentially selected by the first switch means S 1 to the capacitance measurement circuit. A second switch means S ₂ is provided to connect to the measurement AC power supply 4 of the CM, and the capacitance measurement circuit CM causes the measurement electrode system 8 selected by the first switch means S ₁ to be connected to the measurement electrode system 8. The measurement current Im flowing from the AC power source 4 is sequentially measured, and at the same time, the second switch means
The configuration is such that the measurement electrode systems 8 other than the measurement electrode system 8 under measurement selected by S ₂ are kept at the same potential as the measurement AC power supply.

[Effect]

In general, it is known that a so-called guard shield or guard circuit is used to eliminate the influence of mutual interference due to leakage current or line capacitance from a measurement target point not only for capacitance but also for current parameters. The principle is explained with reference to Fig. 2. When measuring conductance, susceptance, etc. by applying AC voltage V from power source 1 between electrodes P and E and measuring current Im with current detector 2. ,
Leakage current from connecting cables and other non-measuring points also flows through the current detector 2 and causes large errors in measurement. If the part other than the opposing point is covered with a guard shield G and the same AC voltage V as above is applied to it, no leakage current is generated from the connection cable 3, and the leakage current Il from the guard shield G to the earth E is detected by the current detector. 2
Since only the current Im between the electrodes P and E flows through the detector 2 without passing through it, it is possible to eliminate the influence of leakage current and interference caused by the connection cable 3 and others. As described above, a circuit that applies the same voltage to the guard shield G as that applied to the electrode P is also called a guard circuit for a system including the electrode P.

In the scanning multi-point capacitance measuring device of this invention having the above-described configuration, capacitance measuring electrodes such as the electrode P described above are provided at multiple points, and a plurality of capacitance measuring electrodes each including these electrodes are provided at multiple points. By sequentially switching and connecting the measuring electrode system to the current detector 2, the capacitance of each electrode is measured in a time-sharing manner. A guard circuit as described above is formed in the measurement electrode system other than the system (connected to the current detector 2 in FIG. 2). That is, at a certain point in time, only one measuring electrode system is connected to the capacitance measuring circuit, and all other systems are connected, e.g.
It is connected to point X of the circuit in Figure 2, but at the next point, this one system is disconnected from the measurement circuit and connected to point X, and at the same time, the other electrode system is disconnected from point X and measured. The action of connecting to a circuit,
Repeat for all electrode systems.

〔Example〕

As shown in FIG. 1, the scanning multi-point capacitance measuring device of the embodiment includes a capacitance measuring circuit CM consisting of an oscillator 4 and an ammeter 5 provided as an AC power source for measurement, and n static capacitance measuring circuits CM. Capacitance measurement electrode P ₁ ...P _o is connected to the tip of each coated core wire L ₁ ...L _o of multi-core shielded wire 7, and measurement electrode system 8 is fixed facing ground electrode E, and It consists of a scanning switch circuit 6 consisting of a first scanner S ₁ and a second scanner S ₂ provided between them.

The first scanner _S1 of the scanning switch circuit 6 is
The output terminal P connected to the oscillator 4 output is sequentially connected to the measurement electrode via the ammeter 5 of the capacitance measurement circuit CM.
P ₁ ... is switched and connected to P _o , but the second scanner ₂ is
At the same time, the oscillator 4 of the capacitance measurement circuit CM
Guard terminal connected to output and measuring electrode P ₁ ...
...Sequentially disconnect from P _o . In this way, at the moment when the output terminal is connected to the measuring electrode P ₁ , only the electrode P ₁ is disconnected from the guard terminal while all other electrodes P ₂ ... P _o are connected to the guard terminal. At this moment, the electrode
P ₂ ...P _o and core wire L ₂ ...L _o have the same potential,
A guard circuit is formed for the system including the electrode _P1 , and only a current indicating the capacitance at the electrode _P1 flows through the ammeter 5. Incidentally, the shield 9 of the shield wire 7 is always connected to the guard terminal, and also forms a guard circuit. Thereafter, the capacitance is similarly measured from electrode P ₂ to P _o , and then scanning is repeated again starting from electrode P ₁ .

The scanning type switch circuit 6 can be constructed using a semiconductor analog switch, a mercury contact switch, etc., but as shown _{in FIG} . An example of a switch circuit is shown below. In FIG. 3, the gate input of each analog switch 10 of the first scanner S ₁ is connected to four different output terminals of the decoder 11, and the gate input of each analog switch 12 of the second scanner S ₂ is connected to four different output terminals of the decoder 11, respectively. Similarly, it is connected to the decoder 11 via the NOT circuit 14. Further, the input terminals of the analog switches 10 and 12 of the first and second scanners S ₁ and S ₂ are respectively connected to the output terminal and the guard terminal of the capacitance measuring circuit CM, and the corresponding input terminals of the analog switches 10 and 12 of the first and second scanners S ₁ and S ₂ are The output terminals of the analog switches 10 and 12 are commonly connected to corresponding measurement electrodes P ₁ ...P ₄ .
The decoder 11 is driven by the counter 13 and each output terminal becomes H level one by one.
The analog switch 10 of the first scanner _S1 connected to the level output terminal closes and the capacitance measuring device CM
One measurement electrode P ₁ to which the output terminal corresponds...
In synchronization with the connection to P ₄ , the corresponding analog switch 12 of the second scanner S ₂ is opened, and its electrode is disconnected from the guard terminal. That is, at each point in time of scanning, only one analog switch 10 of the first scanner S ₁ is closed, and only one analog switch 10 of the second scanner S ₂ is closed.
Then, only the corresponding analog switch 12 is opened, and the other three are closed.

[Effect of idea]

As explained above in detail, according to the scanning multi-point capacitance measuring device of this invention, there is no need to use separate shield wires for each measuring electrode, and the electrodes are close to each other. In this case, it is possible to prevent the capacitance between the electrodes from being picked up, eliminate mutual interference due to leakage when the scanning switch is opened, and perform accurate measurements. Furthermore, according to this invention, as shown in FIG. 4a, the measuring electrodes P ₁ ... P _o
If the electrodes P _i are closely spaced, or if a guard circuit is not formed even if the electrodes P However, due to the action of the guard circuit, the electric lines of force become parallel,
Another effect is that the measurement range by each electrode is accurately determined.

[Brief explanation of the drawing]

Figure 1 is a circuit diagram of an embodiment of the scanning multi-point capacitance measuring device of this invention, Figure 2 is a circuit diagram for explaining the principle of the guard circuit, and Figure 3 is a circuit diagram of the scanning type switch circuit. Circuit diagram of one embodiment, Figure 4 a and b
FIG. 2 is a schematic diagram showing the distribution state of electric lines of force in the measurement electrode section. 4...Oscillation circuit, 8...Measurement electrode system, CM...
...Capacitance measurement circuit, G...Guard terminal, P ₁ ...
...P _o ...capacitance measurement electrode, E ...ground electrode,
S ₁ ... first scanner, S ₂ ... second scanner, L ₁ ...
L _o ...Sheathed core wire.

Claims (1)

[Scope of utility model registration request] Capacitance measurement circuit with measurement AC power supply 4
The capacitance measurement circuit is connected between the CM and a plurality of measurement electrode systems 8 each including a capacitance measurement electrode Pn.
A first switch means _S1 sequentially connects the CM and each measurement electrode system 8, and a first switch means _S1 connects the measurement electrode systems 8 other than the measurement electrode systems 8 sequentially selected by the first switch means S1 to the capacitance measurement circuit. A second switch means _S2 is provided to connect to the measurement AC power source 4 of the CM, and the capacitance measurement circuit CM connects the measurement electrode system ₈ selected by the first switch means S1 to the measurement electrode system 8. The measurement current Im flowing from the AC power supply 4 is sequentially measured, and at the same time, the second switch means S ₂
The measurement electrode system 8 during the measurement selected by
A scanning multi-point capacitance measuring instrument characterized in that the measuring electrode system 8 other than the measuring electrode system 8 is maintained at the same potential as the measuring AC power source 4.