JPH0387641A - Signal processing circuit for humidity sensor - Google Patents

Abstract

PURPOSE:To exactly correct minute non-linearity and a temperature characteristic and to cope with the use under the condition that high accuracy is requested by offsetting the non-linearity in a humidity sensor and a voltage converting circuit with an amplifying circuit. CONSTITUTION:An operational amplifier 19 for which an input resistance 20 and a feedback resistance 21 are connected is used as the amplifying circuit of a final output stage, and also, a diode 22 is connected in parallel to the feedback resistance 21 so that the output side of the operational amplifier 19 becomes the cathode. That is, the diode 22 is biased in the forward direction, and also, voltage setting and a gain allocation are executed in advance so as to operate before a turning-on state, and in accordance with the output voltage of the operational amplifier 19, the small forward current of the diode 22 is varied, and non-linearity of minute hygrosensitive characteristic of a humidity sensor 3, a temperature characteristic, or non-linearity of the detecting circuit itself can be corrected.

Description

DETAILED DESCRIPTION OF THE INVENTION [Objective of the Invention (Industrial Application Field) The present invention relates to a signal processing circuit for a humidity sensor used in, for example, an air conditioner, a humidifier, or the like.

(Prior Art) Most signal processing circuits for humidity sensors convert changes in sensor characteristics into voltage or frequency. When voltage conversion is performed, the output is further converted to A
/D conversion and can be imported into a microcomputer, etc. When frequency conversion is performed, the output is processed by a software counter in a microcomputer or a hardware counter and is taken in as data. - Although the signal processing circuit for frequency conversion has a simpler structure in the membrane, the subsequent processing by the counter tends to be complicated.On the other hand, the signal processing circuit for voltage conversion tends to be complicated, but the subsequent processing is expected to be relatively easy.

FIG. 6 shows a conventional example of a signal processing circuit for converting the output of a humidity sensor into voltage. In this circuit, a signal from an AC signal source 1 is voltage-divided by a variable capacitance humidity sensor 3 and a comparison capacitor 4, and after impedance conversion is performed by an impedance conversion circuit 5, The sample and hold circuit 7 detects the peak voltage, and the DC amplifier circuit amplifies the peak voltage to provide the sensor output. In addition, in Figure 6, 2
is the bias resistance.

Capacitive humidity sensors using polymeric materials were originally
It is characterized by almost linear moisture sensitivity characteristics and low temperature dependence. Therefore, the above-described signal processing circuit is effective when used under normal conditions where the conventional polygonal linear nonlinearity correction means is not required and high accuracy is not required.

(Problem to be solved by the invention) However, even capacitive humidity sensors do not have non-linearity in their humidity-sensing characteristics; they have non-linearity and temperature characteristics, albeit slight, and are highly accurate. This becomes a problem when used under the required conditions. in this case,
In the signal processing circuit described above, no particular consideration is given to measures to correct minute nonlinearities and temperature characteristics. Furthermore, in the signal processing circuit described above, the circuit that detects the capacitance change corresponding to the humidity of the humidity sensor 3 has some nonlinearity in the transfer function itself, and the effect thereof appears in the output characteristics. Therefore, there has been a strong desire for a signal processing circuit that can be used under conditions that require high accuracy.

The present invention has been made in view of the above, and its purpose is to be able to accurately correct minute nonlinearities and temperature characteristics;
An object of the present invention is to provide a signal processing circuit for a humidity sensor that can be used under conditions that require high accuracy.

[Structure of the Invention] (Means for Solving the Problems) In order to achieve the above object, the present invention provides a humidity sensor configured with a polymeric material as a sensitive material and detects a change in humidity as a change in capacitance, and a humidity sensor that detects changes in humidity as changes in capacitance. A voltage conversion circuit that converts the capacitance detected by the sensor into voltage, and an amplifier circuit that amplifies the voltage converted by the voltage conversion circuit and includes a diode in the feedback circuit to offset nonlinearity in the humidity sensor and voltage conversion circuit. The gist is to have the following.

(Function) In the signal processing circuit for a humidity sensor according to the present invention, after the voltage conversion circuit converts the capacitance detected by the humidity sensor into voltage, this converted voltage is used to correct nonlinearity in the humidity sensor and the voltage conversion circuit. The output is amplified by an amplifier circuit with a diode provided in the feedback circuit so as to cancel the signal. That is, the output of the humidity sensor is output with nonlinearity corrected when it is amplified by the amplifier circuit that is the final output stage.

(Example) Hereinafter, the present invention will be explained in detail using the drawings.

FIG. 1 shows a signal processing circuit of a humidity sensor according to an embodiment of the present invention. Its features include the use of an operational amplifier 19 connected to an input resistor 20 and a feedback resistor 21 as the final output stage amplifier circuit;
The diode 22 is connected in parallel to the feedback resistor 21 so that the output side of the feedback resistor 21 serves as a cathode. That is, the diode 22 is biased in the forward direction, and the voltage setting and gain distribution are performed so that it operates before the on state, and the diode 2
By changing the slight forward current of 2, it is possible to correct the slight nonlinearity of the moisture sensitivity characteristics of the humidity sensor 3, the temperature characteristics, or the nonlinearity of the detection circuit itself. In FIG. 1, the same reference numerals as in FIG. 6 indicate the same components.

Next, as an effect of this embodiment, correction processing for nonlinearity and the like in one operational amplifier will be described.

FIG. 2 shows the humidity sensitivity characteristics of the humidity sensor 3, which exhibits nonlinearity in that the output decreases greatly on the low humidity side.

In contrast, depending on the output voltage of one operational amplifier,
Since the minute forward current flowing through the diode 22 changes and the characteristics of the operational amplifier 19 are intentionally made nonlinear, the characteristics of the humidity sensor 3 can be corrected. That is, in this case, the diode 22 turns on only when the output voltage decreases due to the direction of the diode 22, and a forward current flows slightly, reducing the sensitivity and correcting the low humidity side characteristics of the humidity sensor 3. It will be done. Therefore, the characteristic of one operational amplifier is as shown in FIG. 3, and a linear characteristic can be obtained from both characteristics.

FIG. 4 shows the temperature characteristics of the humidity sensor 3. The sensitivity of the temperature sensor 3 tends to decrease on the low temperature side and increase on the high temperature side. On the other hand, the diode 22 has a positive temperature characteristic, and the forward current increases as the temperature rises, so if this characteristic is used, the signal processing circuit can have a temperature characteristic of sensitivity. Therefore, it is possible to correct the temperature characteristics of the sensitivity of the humidity sensor.

Figure 5 shows the nonlinearity of the detection circuit, and its characteristics are upwardly convex, and the capacitance change of the humidity sensor 3 is divided by the comparison capacitor 4 to obtain the output. This is unique to the detection method itself. In response to this, by inserting the diode 22 into the feedback circuit, it is possible to lower the sensitivity of the amplifier circuit on the low humidity side and improve linearity, similar to correcting the characteristics of the humidity sensor 3.

Therefore, according to this embodiment, the linearity of the humidity sensor is ensured by correcting the nonlinearity of the humidity sensor, the temperature characteristics, and the nonlinearity of the detection circuit, so it can meet the requirements for use under high precision conditions. be able to.

Note that by reversing the direction of the diode 22 from that shown in FIG. 1, it is also possible to correct the characteristics of the humidity sensor 3 whose characteristics are degraded on the high temperature side. Furthermore, it is possible to correct these two characteristics simultaneously, and it is also possible to adjust the amount of correction by inserting a resistor or a thermistor in series with the diode.

[Effects of the Invention] As explained above, according to the present invention, after the voltage conversion circuit converts the capacitance detected by the humidity sensor into voltage, the converted voltage is used to offset the nonlinearity in the humidity sensor and the voltage conversion circuit. As shown in the figure, the output is amplified by an amplifier circuit with a diode in the feedback circuit, and the output of the humidity sensor is output with nonlinearity corrected when it is amplified by the amplifier circuit, which is the final output stage. As a result, minute nonlinearity and temperature characteristics can be corrected appropriately, and thus it can be used appropriately under conditions where high accuracy is required.

[Brief explanation of drawings]

FIG. 1 is a diagram showing an embodiment of the present invention, FIGS. 2 to 5 are diagrams for explaining the embodiment, and FIG. 6 is a diagram showing a conventional example. 1... AC signal source 2... Bias resistor 3... Humidity sensor 4... Comparison capacitor 7... Sample hold circuit 9... DC amplifier circuit] 9... Operational amplifier 20... Human resistance 21... Feedback resistance 22... Diode

Claims (1)

[Claims] A humidity sensor configured with a polymeric material as a sensitive material and detects changes in humidity as changes in capacitance, a voltage conversion circuit that converts the capacitance detected by the humidity sensor into voltage, and nonlinearity in the humidity sensor and voltage conversion circuit. 1. A signal processing circuit for a humidity sensor, comprising: a diode provided in a feedback circuit so as to cancel the voltage, and an amplifier circuit for amplifying the voltage converted by the voltage conversion circuit.