Communications: electrical – Condition responsive indicating system – Specific condition
Reexamination Certificate
2000-07-14
2001-09-18
Hofsass, Jeffery (Department: 2632)
Communications: electrical
Condition responsive indicating system
Specific condition
C340S618000, C340S635000, C340S657000, C340S620000, C340S870370
Reexamination Certificate
active
06292104
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a capacitance type detector. More specifically, the present invention relates to a capacitance type detector for near contact or contact detection of an object contained in a container, such as a gas or a powder and granular material, using dielectric constant of the object as a capacitance value.
2. Description of the Background Art
Conventionally, a capacitance type detector has been used when a level of an object, for example, is to be detected, as the capacitance type detector is applicable to wide range of objects to be detected including liquid, powder, sediments in a liquid, water and oil and it is easy to install and has no variable unit.
The wide range of applicable objects, however, is realized by changing a structure of an electrode portion of the detector, or by changing components including addition or exchange of components of a circuitry, for example. Further, as the capacitance is measured based on the dielectric constant of a substance, the detector is susceptible to environmental capacitance such as the capacitance of the container. Accordingly, on-site calibration is necessary before detecting operation when the detector is set in a container, which is quite troublesome. Further, detector operation may be less stable when moisture percentage of the object changes because of moisture or when a small amount of a conductive object happens to be adhered on the detector.
In view of the above described problems, Japanese Patent Laying-Open No. 59-54928 describes a capacitance type material level indicator. The level indicator includes a constant oscillation circuit, an amplifier circuit, a resonance circuit having an electrode body of which capacitance changes in accordance with the object of detection, a phase comparing circuit, an automatic calibration circuit having a switch, and an output circuit. An oscillation signal from the constant oscillation circuit is supplied as an excitation signal through the amplifier circuit to the resonance circuit, phase difference between the oscillation signal and the excitation signal of which phase has been changed by the resonance circuit is detected by the phase comparing circuit, and a detection signal corresponding to the phase difference is applied to the automatic calibration circuit. When the switch is pressed, the automatic calibration circuit changes stepwise the operational characteristic of the resonance circuit so that the detection signal matches a reference signal. Further, application to the object and the calibration operation are facilitated, as the operation which takes place is to change the reference signal so that the reference signal matches the detection signal.
In this method, however, calibration is still necessary, though it is simply a pressing of the switch at the site. When an operator fails to calibrate, correct operation is not ensured. When it becomes necessary to change the place of setting, calibration is necessary every time. Further, it is difficult to utilize the resonance state of the resonance circuit, and therefore detection sensitivity may vary.
Japanese Patent Laying-Open No. 5-87612 describes another example, which is a liquid level detector using a PLL circuit. In this example, a PLL circuit is constituted by a phase comparing circuit, a loop filter (integrating circuit) and a voltage controlled oscillation circuit (VCO circuit), in which a pre-oscillation circuit is oscillated at an oscillation frequency in accordance with the object of detection, the oscillation frequency signal is input to the phase comparing circuit of the PLL circuit, and the frequency is compared with the frequency signal of the VCO circuit, so as to detect the object.
In the second example, however, any of the following predetermined conditions must be satisfied.
(1) When there is no object of detection, the oscillation frequency of the pre-oscillation circuit must be out of the lock range of the PLL circuit, and when the object comes to be in contact or in the vicinity, the oscillation frequency of the pre-oscillation circuit must be set within the locked range of the PLL circuit.
(2) When there is no object of detection, the oscillation frequency of the pre-oscillation circuit must be within the locked range of the PLL circuit, and when the object is brought into contact or in the vicinity, the oscillation frequency of the pre-oscillation circuit must be set outside the locked range of the PLL circuit.
(3) When there is no object of detection and when the object is brought into contact or in the vicinity, the oscillation frequency of the pre-oscillation circuit must be set within the locked range of the PLL circuit.
When the range of variation of the frequency of the pre-oscillation circuit and the locked range of the PLL circuit are to be set to satisfy the limited conditions, the site and the object must be taken into consideration, and therefore, setting is very difficult. In other words, applicable range may be limited.
SUMMARY OF THE INVENTION
Therefore, an object of the present invention is to provide a capacitance type detector which is free of any calibration, easy to handle, free of detection sensitivity variation, usable under wide range of conditions, less susceptible to conductivity of the objects and strong against static electricity.
In summary, the present invention provides a capacitance type detector detecting a level of an object in accordance with a capacitance of the object in a container, in which a resonance circuit is formed by a capacitance formed between an electrode body inserted to the container and the object of detection and a resonance transformer to be tuned with the capacitance; an output signal of the resonance circuit and a reference signal are subjected to phase comparison and a phase difference signal in accordance with the phase difference is output from a phase comparing circuit; and in accordance with the phase difference signal, a control signal to eliminate the phase difference is output from a control circuit. A frequency signal of an oscillation frequency in accordance with the control signal is generated from a variable frequency generating circuit and applied as a reference signal to phase comparing circuit, and the electrode body is driven by the frequency signal from the variable frequency generating circuit. The control signal output from the control circuit or the frequency signal output from the variable frequency generating circuit is compared with the reference value set by a reference value setting circuit, and a detection signal indicative of presence/absence of an object of detection is output. Based on the phase difference signal output from the phase comparing circuit, phase difference between a resonance frequency of the resonance circuit and the oscillation frequency of the variable frequency generating circuit is determined, and when it is determined that there is a phase difference, the phase comparing circuit operates such that the oscillation frequency of the variable frequency generating circuit follows the resonance frequency of the resonance circuit.
Therefore, according to the present invention, a loop is formed by the resonance circuit, the driving circuit, the phase comparing circuit and the variable frequency generating circuit, the resonance frequency of the resonance circuit is searched at the time of power on, and the electrode body is operated with the resonance frequency. Therefore, necessity of calibration by the operator at the site is eliminated, and therefore error caused by manual operation can be prevented, and the efficiency of operation can be improved.
Preferably, the phase difference determining circuit includes a reset circuit setting, when the phase difference signal output from the phase comparing circuit attains a prescribed value or higher, the frequency signal of the variable frequency generating circuit to the lowest frequency within the variable range of the frequency of the variable frequency generating circuit, and a mismatch
Hofsass Jeffery
McDermott & Will & Emery
Nguyen Hung
Nohken Inc.
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