JPS6146779B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a high-frequency resistance moisture meter, and more particularly to an electric circuit for a moisture meter for measuring moisture in an object to be measured (hereinafter referred to as a sample). Various electrical moisture measuring instruments have been developed in the past. These include DC resistance moisture meters that measure changes in resistance due to moisture using compressed electrodes, capacitive moisture meters that measure changes in dielectric constant due to moisture, high frequency resistance moisture meters that measure high frequency loss due to moisture, and moisture meters that measure changes in resistance due to moisture. There is a microwave moisture meter that measures the absorption of microwaves. Generally, in a conventional high frequency resistance type moisture meter, a moisture detection electrode detects a change in high frequency resistance, and the detection resistor is combined with a coil to indicate moisture by a change in Q. However, since active elements such as vacuum tubes and transistors are used to detect resistance due to moisture, there are large variations in characteristics, and in order to obtain a moisture measurement circuit with uniform characteristics, there are severe restrictions on the selection of active components. However, the circuit configuration was complicated and calibration was required for each manufactured circuit, which increased manufacturing costs and made mass production difficult. An object of the present invention is to provide a high frequency resistance type moisture measuring circuit based on a new principle different from conventional moisture detection methods. The basic concept of the present invention is to connect the electric resistance R taken out from the detection electrode of the sample in parallel with the resonant circuits L and C, and to generate a transient output by switching the resonant circuits on and off using high frequency switching means. The purpose is to integrate the attenuated transient current after rectifying it, and measure the integrated value as a moisture value representing the sample moisture. Focusing on the fact that the attenuation area of the cut-off transient current of the resonant circuit changes due to the high-frequency resistance value R, which is inversely proportional to the amount of moisture in the sample, the moisture value of the sample is obtained by integrating the attenuation area. An electrode connected to the resonant circuit is brought into contact with the sample. The electrical resistance between the electrodes changes in correlation with the moisture content of the sample, and when the moisture content is small, the electrical resistance is large (Fig. 1 (b) R
= R ₁ ), the decay of the transient current output when the resonant circuit is interrupted is slow, the moisture content is large, and therefore the resistance is small (R
= R ₂ ), the attenuation is fast. Based on this relationship, the moisture value of the sample is determined from the decay rate of the transient current. In the present invention, the passive elements of the coil and capacitance that constitute the resonant circuit are basically involved in moisture measurement, so the characteristics are relatively stable.
Manufacturing costs are reduced. Embodiments of the present invention will be described below with reference to the drawings. FIG. 2 is a block circuit diagram showing an embodiment of the moisture measuring circuit according to the present invention. In Figure 2, 1
0 is an electrode portion that comes into contact with the sample and exhibits resistance according to the water content thereof. Reference numeral 12 denotes an oscillation detection section, which includes a resonant circuit made up of a coil L, a capacitor C, and a high-frequency resistance component of the electrode section 10, and a switching circuit that operates this circuit intermittently. 13 is a rectifier circuit, 1
4 is an integrator, 15 is an O adjustment, sensitivity adjustment, and buffer circuit, 16 is a mixing amplification section, and 17 is a moisture display section. 22 is a temperature detection section provided in the electrode section 10 for detecting the temperature of the sample. The principle of the present invention will be explained again with reference to FIG. Figure 1a shows the basic resonant circuit of the present invention, where L is the inductance of the coil L, C is the capacitance of the capacitor, r ₀ is the effective internal resistance of the coil L,
R is an externally connected high-frequency resistor, I is the coil current when the switch SW is closed, and E is the DC power supply.
In the circuit shown in the figure, when the switch SW is closed and a steady state is established, a current I flows through the coil L. When the switch SW is opened, the electromagnetic energy represented by 1/2 LI ² stored in the coil moves to the capacitor C and is stored as electrostatic energy, that is, 1/2 Cv ² . Here, v is the voltage across L, C, and R. This energy conversion is performed alternately at the frequency of L and C.

The electrical energy gradually decreases to zero and becomes stable due to the resistance component r ₀ and the Joule heat loss of R 2 . Here, the attenuation curve of voltage v is expressed as v=KIe〓 ^t (1). However, K is a constant determined by the circuit constant, α=-Rr ₀ C+L/2RCL Therefore, in the first equation, if the externally connected high-frequency resistance R is large, the attenuation degree of v will be as shown in Figure 2b. become more gradual,
The smaller R is, the steeper it becomes. If the area surrounded by this envelope and the vertical axis is electrically determined, the high frequency resistance R
It can be taken as a signal. When the electrode plate of the detection electrode is connected to the above R and a sample containing moisture is brought into contact with it, R corresponding to the moisture value of the sample can be measured. FIG. 3 shows a specific circuit configuration of the block diagram of FIG. 2. In FIG. 3, 11 is a resonant circuit;
This is a precision timer IC that drives the switching transistor TR ₂ , which operates the LC intermittently, at an accurate cycle. For example, the NE555 type is used. _TR1 is a switching transistor that switches on and off the operation of transistor _TR2 . When transistor TR ₂ cuts off the current flowing to coil L according to the on/off state of TR ₁ ,
LC resonant circuit (tank circuit) has a frequency of 700KHz, for example

Generates a damped output of [formula]. The switching period is approximately 3,000 to 4,000 Hz, and is determined based on the temperature characteristics of the coil L and rectifier diode _D1 . The high frequency resistor R is connected at high frequency from both ends of the coil L via a coaxial cable to the electrode plate of the electrode section 10 that shows the moisture resistance due to the sample, and is configured so that the resistance of the sample is applied in parallel to the coil L. There is. The decay current of the transient phenomenon caused by LC appearing at both ends of the tank circuit is subjected to envelope detection with a time constant of 3 μsec by the rectifier circuit 13 consisting of the diode D ₁ , the capacitor C ₇ , and the resistor R ₉ , and a pulsating current decay waveform is generated. be done. The successive sawtooth wave detection outputs are integrated and averaged by an integrating circuit 14 consisting of a resistor _R10 and a capacitor _C8 . The time constant of the integrating circuit 14 is determined to be sufficiently larger than the switching period of the transistor _TR2 , and is, for example, 30 msec. The output of the integrating circuit 14 represents the high frequency resistance R that correlates to the moisture content of the sample according to the principle described above. The operation of converting the moisture value of the sample from the resistance value R obtained from the correlation between moisture and resistance of the sample can be performed by a conventional method. That is, in order to correct the temperature of the sample moisture, the sample temperature is detected from the temperature detection section 22, a correction value is converted, and a signal representing the value is generated. On the other hand, the output of the integrating circuit 14 is applied to one input terminal of the operational amplifier OP ₂ of the temperature correction calculation circuit 16 via the buffer amplifier OP ₁ , and a temperature correction signal is applied to the other input terminal. Op amp OP ₂
The output is a temperature-compensated moisture value output, and the resistance
It is displayed on the meter of the display section 17 through _R21 .
In block 15, the buffer amplifier _OP1 also serves as a zero adjustment function for the meter display, and a sensitivity adjustment circuit for the potentiometer _VR2 is provided between the buffer amplifiers OP2 and _OP2 . Capacitor C ₉ and resistor R ₁₇ integrate the ever-changing moisture signal current to further stabilize it. The temperature detection circuit 22 includes a diode D ₈ built into the contact electrode section 10 as a temperature detection sensor. _D8 is connected to a constant current circuit having a resistor _R13 , and a constant current is passed through it. The forward voltage of diode _D8 changes with temperature. The changing voltage is amplified by a differential amplifier circuit using an operational amplifier OP ₃ and input to the operational amplifier OP ₂ of the mixing amplification section 16 . The output of the operational amplifier OP ₂ is displayed on the display section 17 as a temperature-corrected moisture value. Since the power supply circuit is a common one, it will be omitted. Samples compatible with the present invention are hygroscopic materials,
Therefore, this circuit is suitable for measuring moisture in wood, paper, grain, fertilizer, etc. According to the present invention, a high-frequency resistance moisture meter includes a resonant circuit consisting of an LC tank circuit connected in parallel with an electrode section in order to detect the moisture content of a sample, and a resonant circuit configured to intermittent the resonant circuit to generate a transient phenomenon. and an integrating circuit for integrating the attenuated output of a transient phenomenon that changes according to a high frequency resistance that is inversely proportional to the moisture content of the sample due to the operation of the switching circuit, and the output of the integrating circuit is used as the moisture value of the sample. I'm looking for it. Therefore, the basic circuit for detecting high-frequency resistors corresponding to moisture does not require active elements with large variations in characteristics, and is achieved with L and C tank circuits, reducing the number of parts and reducing manufacturing costs. Ru. Furthermore, the characteristics are uniform and stable against temperature changes.

[Brief explanation of the drawing]

Figures 1a and b are a basic circuit diagram and an output waveform diagram for explaining the principle of the present invention, respectively, Figure 2 is a block circuit diagram showing an embodiment of the present invention, and Figure 3 is a concrete example of Figure 2. This is a typical circuit diagram. L... Coil, C... Capacitor, R... High frequency resistor, 10... Sample electrode section, 11... Timer,
12... Oscillation detection section, 13... Rectifier circuit, 14...
...Integrator, 16...Mixing amplifier section, 17...Display section, 22...Temperature detection section, _TR1 , _TR2 ...Transistor, _D1 ...Diode, _C8 ...Integrator capacitor, _R10 ... ...integrator resistance.

Claims (1)

[Claims] 1. In a high-frequency resistance moisture meter that detects the resistance of a sample and measures the moisture content of the sample by utilizing the correlation between the moisture content of the sample and the electrical resistance, there is a method for extracting the moisture-related resistance of the sample. a detection electrode section, a resonant circuit connected to the electrode section, a switching circuit that operates the resonator intermittently for a predetermined period of time to generate a transient phenomenon output, and a switching circuit that generates a transient phenomenon output in response to the operation of the switching circuit. It includes an integrating circuit that rectifies and then averages the phenomenon output, and that the integrating circuit integrates the attenuation characteristic output corresponding to the high frequency resistance value that is inversely proportional to the sample moisture, so that the integrated value represents the sample moisture signal. Features: High frequency resistance type moisture meter. 2. In claim 1, a sample temperature detection element is provided which is integrated into the detection electrode section, and the detected temperature signal is added to the moisture signal of the integration circuit to perform temperature correction of the moisture signal. A high frequency resistance type moisture meter characterized by: