JPH0239247Y2 - - Google Patents

Description

[Detailed description of the invention] (Industrial application field) The invention aims to calculate the correlation between the moisture contained in the object to be measured (hereinafter referred to as the sample), such as timber, grain, and especially wheat, and the electrical resistance value of the sample. Regarding electrical resistance moisture measuring devices (hereinafter referred to as moisture meters), the electrical resistance value is almost exponential when the moisture content of the sample is as high as 40%. This invention relates to a broadband moisture meter that can linearly display moisture content even if it is small.

(Prior art) In conventional electrical resistance grain moisture meters, the relationship between the electrical resistance value and the grain moisture content exhibits non-linearity in low and high moisture regions, so the display scale is set at equal intervals. Attempts have been proposed to improve reading accuracy. For example, a moisture measuring circuit as shown in FIG. 2 is known. Here, diodes D ₁ and D ₂ are connected in series with the sample R _x to obtain an output Vout that combines the logarithmic characteristics of the forward voltage of the diodes and the logarithmic characteristics of the moisture contained in the grain versus the electrical resistance. The rate display is a linear characteristic. However, with this method, when the moisture content reaches a high moisture range of 30% or more, too much current flows through the sample, and when that current is converted logarithmically, not only does the output become saturated, but it also flows into the diodes _D1 and _D2. The measurement output is affected by the generation of Joule heat due to the current.
Furthermore, a voltage drop occurs due to logarithmic conversion elements such as diodes and transistors. When a constant voltage is applied, the voltage drop changes depending on the flowing current, so the voltage between the sample electrodes connected in series also changes, resulting in an error in logarithmic conversion.

FIG. 3 shows a linearized current/voltage conversion circuit according to Utility Model Application No. 53-72586. The circuit includes an operational amplifier IC ₁ and a diode D ₁ as its negative feedback circuit.
Equipped with a series circuit of D ₂ and resistor R ₃ , operational amplifier IC ₁
The current flowing through the amplifier circuit including
The output voltage is determined by the product of that current and the sum of the resistances D ₁ and D ₂ , which act as linearizers, and the resistance R ₃ .
Therefore, a substantially constant voltage is applied to the sample, and a current substantially the same as the current flowing through the sample flows through the linearizer diodes D ₁ , D ₂ and resistor R ₃ . For this reason, there was a drawback that the output fluctuated due to the influence of Joule heat on the diodes D ₁ and D ₂ in the high moisture range.

(Problems to be Solved by the Invention) The present invention makes it possible to maintain a linear characteristic of the moisture-to-electrical resistance characteristic even in a high moisture area, and in this case, the excessive flow of sample current that occurs in the high moisture area This was done to eliminate problems such as output saturation associated with this.

(Means for Solving the Problems) According to the electrical resistance moisture meter according to the present invention, even if the sample current flowing in the high moisture range is large, the linearity of logarithmic transformation in the correlation between the current and the moisture content is maintained. By dividing the sample current within a range that maintains the current, a current proportional to the sample current is obtained, and by converting this into the current/voltage logarithm, it is possible to measure the moisture content over a wide range and display it on an equally spaced scale. There is.

That is, the present invention includes means I, E _a , and E _b for measuring the electrical resistance of an object to be measured, and a device that is coupled to the resistance measuring means and converts changes in steady state current flowing through the object when a constant voltage is applied to voltage changes. A current/voltage signal that converts into a voltage change signal representing the moisture content corresponding to the electrical resistance value of the object to be measured, and converts the exponential change characteristic of the moisture rate signal into a nearly linear characteristic. A broadband electrical resistance moisture content measuring device comprising logarithmic conversion means IC ₃ , D ₁ , D ₂ and a display means M for displaying the conversion signal outputted from the conversion means as a moisture content, the resistance value measuring means and the current/voltage logarithmic conversion means, and includes an operational amplifier circuit IC ₂ having a current feedback path and for converting an input current into a voltage value, and the operational amplifier circuit has a load resistance.
R ₂ and a feedback resistor R ₁ , the current flowing through the object to be measured is shunted through both resistors at a predetermined resistance ratio, and the shunt current flowing through the load resistor is converted into the current/voltage logarithm in proportion to the resistance ratio. The present invention provides a wide-band moisture content measuring device characterized in that it is added to a means for measuring moisture content.

(Function) Since the potential at the negative input terminal of operational amplifiers IC ₂ and IC ₃ is almost zero, and the input impedance is very large, the current flowing through the sample is equal to the current flowing through the negative feedback resistor, and the load The current I ₂ flowing through the resistor R ₂ is divided into (R ₁ /R ₂ )|I ₁ |. Therefore, a small shunt current flows through the diode circuits D ₁ and D ₂ which are voltage logarithmic conversion circuits, so linearity is maintained even in a high moisture range, and
Prevents overcurrent from flowing in the diode circuit.

(Example) Examples of the present invention will be described below with reference to the drawings.

FIG. 1 shows a measuring circuit of an electrical resistance moisture meter according to the present invention. In the figure, the oscillator 1 is a DC or AC constant voltage source, for example, with a frequency of 80Hz and a voltage E ₀
= Use 8V power supply. Counter electrodes E _a and E _b that contain crushed sample grains, such as wheat, are each
It is connected to the oscillator 1 of the constant voltage source and the inverting input of the operational amplifier IC ₂ . The non-inverting input of amplifier IC ₂ is grounded. 200Ω on the output side of operational amplifier IC ₂
A resistor R ₁ and a load resistor R ₂ of 20KΩ are connected in parallel, and the resistor R ₁ creates a negative feedback circuit for the amplifier IC ₂ .
As a whole, a shunt circuit 2 is constructed which shunts an input current I into currents I ₁ and I ₂ through resistors R ₁ and R ₂ , respectively. The output of the shunt circuit 2 is input to the logarithmic conversion circuit 3 at the next stage using the load resistance R ₂ as a coupling resistance. The configuration of the logarithmic conversion circuit 3 can basically be the one shown in FIG. 2 or another known circuit. For example, in block 3 of Fig. 1, a coupling resistor _R2 is connected to one input of the operational amplifier _IC3 for current/voltage conversion, and a diode element for logarithmic conversion is connected to the feedback path of the amplifier _IC3 .
D ₁ and D ₂ are connected to each other, and on the output side of the amplifier IC ₃ , the sample current is converted into a voltage value, and logarithmically converted and approximately linearized output for moisture display is taken out. In order to further linearize the output, it is amplified via an arithmetic circuit 4 composed of a linearizer or the like, and is displayed on a DC ammeter M on an equal division scale or digitally displayed using a counter display.

To explain the operation and principle of the above circuit, assuming that the electrical resistance value R _x is correlated to the water content of the sample, the current flowing through the sample is I ₁ =E ₀ /R _x . The shunt ratio is determined by the values of resistors R ₁ and R ₂ of the shunt circuit 2. Here, a current I ₂ shunted at 1/100 is applied to the logarithmic conversion circuit 3, but most of the current is passed through the negative feedback diode D. ₁ , flows to _D2 . Letting the output voltage of the logarithmic conversion circuit be V, the reason for this can be expressed and explained using the formula: |I ₂ |=R ₁ /R ₂ |I ₁ |=R ₁ /R ₂・|E ₀ |/R _x Therefore , V = -kT/q (lnI _{2 -} lnIconst) = kT/q (lnR ₁ | E ₀ | /R ₂ - lnIconst - lnR _x ) = -kT/q (lnC - lnR _x ) where q is the charge , k is Boltzmann's constant, T is temperature, and Iconst and C are constants. Also, since lnC=lnR ₁ |E ₀ |/R ₂ lnIconst is constant, it can be set to 0 appropriately by, for example, adding zero point adjustment using the arithmetic circuit 4.
Ignoring this, it can be seen that the output voltage V of the circuit 3 is proportional to the logarithm of the sample resistance R _x . The arithmetic circuit 4 also performs arithmetic operations to correct temperature dependence, which causes measurement errors, and to linearize the output.

FIG. 4 shows the electrodes E _a ,
The relationship between the equivalent resistance R _x and the meter output when measured by inserting various standard resistances between E _{and b} is shown.
Figure 5 shows the results of measuring the moisture content using the above device when the sample was dried as wheat, and in rectangular coordinates, the vertical axis represents the moisture content, and the horizontal axis represents the meter reading in 100 equal divisions. Displayed. Here, the non-linear part of the characteristic shown is
It is obvious to those skilled in the art that the relationship can be converted into a linear relationship by performing appropriate calculations using the calculation circuit 4.

(Effect of the invention) As mentioned above, conventionally, a logarithmic conversion circuit that combines an operational amplifier and a diode as shown in FIG.
It is necessary to keep the applied voltage low, and satisfactory accuracy cannot be achieved in materials such as grains, which have extremely complex internal structures and must take into account effects such as polarization phenomena. In addition, it was impossible to measure a moisture range of 30% or more. In fact, with conventional moisture meters, the upper limit of moisture measurement is about 30%, and the sample resistance value is 2 to 3 KΩ. However, moisture
At 40%, the sample resistance becomes as small as about 200Ω,
Since the range of resistance change is extremely large, the desired measurement results cannot be obtained simply by using a constant current source or lowering the applied voltage. However, according to the present invention, an operational amplifier is provided at the front stage of the conventional current/voltage conversion circuit, and the sample current is shunted by the feedback resistor and load resistor.
Grain moisture content over a wide range of 40% or more can be measured with high accuracy. Furthermore, by providing a linearizer between the current/voltage conversion circuit and the display device, it is also possible to have a linear relationship between the moisture content converted into an electric signal and the display scale. Note that instead of using an operational amplifier in the shunt circuit 2, the electrodes E _a , E _b
When using a parallel circuit of resistors in series with the sample electrode, it is necessary to keep the voltage between the sample electrodes constant, so it is necessary to reduce the parallel combined resistance. In this case, it was found that the influence of the next-stage operational amplifier IC ₃ became large, and the desired shunt measurement current could not be obtained. The device of the present invention is
It is also possible to measure high moisture samples such as wood other than grains, and its usefulness is great.

[Brief explanation of the drawing]

Fig. 1 is a circuit diagram of a broadband moisture meter device showing an embodiment of the present invention, Figs. 2 and 3 are circuit diagrams for explaining the principle of a conventional electrical resistance moisture meter, and Figs. FIG. 5 is a diagram of measurement characteristics by the apparatus of FIG. 1. R _x is the sample, D ₁ and D ₂ are logarithmic conversion diode elements, IC ₁ and IC ₂ are operational amplifiers, IC ₃ is FET input operational amplifier, R ₁ is feedback resistance, R ₂ is load resistance, and 1 is power supply oscillator. represents.

Claims (1)

[Claims for Utility Model Registration] (1) A means for measuring the electrical resistance of an object to be measured, and a device coupled to the resistance measuring means that converts changes in steady-state current flowing through the object to voltage when a constant voltage is applied. current/voltage logarithmic conversion to convert the exponential change characteristic of the moisture content signal into a nearly linear characteristic. and a display means for displaying a converted signal outputted from the converting means as a moisture content, the device comprising: a means for measuring a resistance value, and a display means for displaying a converted signal outputted from the converting means as a moisture content, the apparatus comprising: a means for measuring a resistance value; The operational amplifier circuit has a load resistance and a feedback resistance, and has a predetermined resistance ratio between the two resistances. A wide-band moisture content measuring device characterized in that a current flowing through an object is shunted and a shunted current flowing through the load resistor is applied to the current/voltage logarithm conversion means in proportion to the resistance ratio. (2) Utility Model Registration The measuring device according to claim 1, wherein the constant voltage is supplied by a DC or AC power source.