JPH0786425B2 - Physical quantity detector - Google Patents

Description

TECHNICAL FIELD The present invention relates to a physical quantity detector for detecting physical quantities such as temperature, relative humidity and pressure of an atmosphere such as the atmosphere.

[Conventional technology]

As such a detector, a detector shown in FIG. 8 has been proposed in Japanese Patent Laid-Open No. 62-129637. In the figure, reference numeral 1 is an AC bridge circuit, and the bridge circuit 1 is composed of a sensitive element and a fixed resistor whose resistance value changes according to the temperature in the atmosphere, the relative humidity, or both of them. A rectangular wave generation circuit 3 is connected to an input end of the AC bridge circuit 1 via a coupling capacitor 2. The rectangular wave generation circuit 3 has, for example, a duty ratio of 50.
%, A rectangular wave with a constant frequency of 1 KHz is generated at its output end, and this is supplied to the input end of the AC bridge circuit 1 via the coupling capacitor 2.

When the rectangular wave from the rectangular wave generating circuit 3 is input to the input end of the AC bridge circuit 1 via the coupling capacitor 2, the output of the AC bridge circuit 1 is changed in amplitude depending on the temperature of the atmosphere, the relative humidity, or both of them. Outputs an alternating signal that changes.

An AC inverting amplifier circuit 7 is connected to the output terminal of the AC bridge circuit 1 via a coupling capacitor 4. The AC inverting amplifier circuit 7 is composed of an operational amplifier 7a, resistors 7b to 7f and a capacitor 7g. A rectifying circuit 9 is connected to the output terminal of the AC inverting amplifier circuit 7 via a coupling capacitor 10, and the rectifying circuit 9 is composed of a rectifying diode 9a, a smoothing capacitor 9b, and resistors 9c and 9d.

In the above configuration, assuming that the rectangular wave generation circuit 3 is now outputting a rectangular wave having a constant amplitude of 4 V, a duty ratio of 50%, and a frequency of 1 KHz as shown in FIG. 9A, the AC bridge circuit 1 As shown in FIG. 9 (b), an alternating current of ± 2 V is applied to the input end of the through the coupling capacitor 2.

After passing through the AC bridge circuit 1 and the coupling capacitor 4, the AC is amplified by the AC inverting amplifier circuit 7, and the output of the AC inverting amplifier circuit 7 is + as shown in FIG.
A signal with an amplitude depending on the temperature of the atmosphere centering around 2 V, the relative humidity, or both of them is output.

The output signal of the AC inversion amplification circuit 7 is converted into an AC as shown in FIG. 9 (d) via the coupling capacitor 10 and applied to the rectifier circuit 9, and the output terminal 11 of the rectifier circuit 9 is connected to the output terminal 11 of FIG. A DC voltage as shown in (e) is transmitted. Output end
The magnitude of the DC voltage of 11 is a value depending on the temperature of the atmosphere, the relative humidity, or both of them.

Therefore, the temperature of the atmosphere, the relative humidity or the absolute humidity can be detected according to the magnitude of the DC voltage obtained at the output terminal 11 of the rectifier circuit 9.

[Problems to be solved by the invention]

As described above, in the conventional physical quantity detector, in order to perform the above detection by the amplitude of the AC signal extracted from the output end of the AC bridge circuit 1, the AC signal is detected by the rectifying diode 9a connected in series to the coupling capacitor 10. By half-wave rectification, a DC signal with an amplitude according to the temperature of the atmosphere, relative humidity, etc. is obtained.

However, when operating with a single 5V power supply, the output voltage of the operational amplifier 7a of the AC inverting amplifier circuit 7 is limited to approximately 0 to 3.5V, so if the bias point of the AC inverting amplifier circuit 7 is set to 2V, Output amplitude can only be obtained in the range of 2V ± 1.5V. When the output of the AC inverting amplifier circuit 7 thus obtained is passed through the coupling capacitor 10, the output is ±
AC signal of 1.5V can be obtained. Therefore, when this ± 1.5V AC voltage is half-wave rectified, only 0.8V, which is the voltage dropped by the forward voltage (approximately 0.7V) of the rectifying diode 9a, is obtained. This drop is very large compared to ± 1.5V, which prevents a large effective voltage from being obtained. In addition, the linearly amplified signal passes through the diode, which is a non-linear element, so that it becomes non-linear in this part and the temperature characteristic of the diode is −2 mV / ° C.
Was not negligible for the effective voltage of 0.8V, which was a big problem in detection accuracy.

Therefore, the present invention is intended to provide a physical quantity detector having a simple structure and improved detection accuracy.

[Means for solving problems]

As shown in the basic configuration diagram of FIG. 1, a physical quantity detector made according to the present invention to solve the above-mentioned problems includes a rectangular wave generating means A for generating a rectangular wave having a constant frequency and a constant amplitude.
It has an element whose resistance value changes according to the physical quantity in the atmosphere, the rectangular wave from the rectangular wave generating means A is input to the input end via the coupling capacitor B, and the amplitude is changed to the output end according to the physical quantity. An AC bridge circuit C that outputs a changing AC signal, an AC amplifier circuit D that amplifies the AC signal from the AC bridge circuit C, and a bias point of the AC amplifier circuit D as a reference voltage. Output period of the AC amplifier circuit D corresponding to a predetermined period of a rectangular wave generated by the A / D conversion means E for sampling the output signal of the AC amplifier circuit D and converting it into a digital signal. It is characterized in that the physical quantity is detected by the output of the conversion means E.

[Work]

In the above configuration, when the rectangular wave having the constant frequency and the constant amplitude generated by the rectangular wave generating means A is input to the AC bridge circuit C via the coupling capacitor B, the AC bridge circuit C responds to the physical quantity in the atmosphere. AC signal of different amplitude is output. The AC signal from the AC bridge circuit C is amplified by the AC amplifier circuit D and input to the A / D conversion means E. The A / D converter uses the bias point of the AC amplification circuit D as a reference voltage, and outputs a period of the AC amplification circuit D corresponding to a predetermined period of the rectangular wave generated by the rectangular wave generation means A, for example, a period of L level or H level. The output signal of the AC amplifier circuit D is sampled and converted into a digital signal.

Therefore, it is possible to detect, for example, temperature, relative humidity, absolute humidity or pressure of the atmosphere by the digital signal obtained at the output of the A / D conversion means E.

〔Example〕

 Embodiments of the present invention will be described below with reference to the drawings.

FIG. 2 shows a frost formation amount detector as an embodiment of the physical quantity detector according to the present invention. In the figure, the same parts as those described above with reference to FIG. 8 are designated by the same reference numerals.

In FIG. 2, the AC bridge circuit 1 has a humidity sensitive element 1a. The moisture-sensitive element 1a has a structure as shown in FIG. 3, and a moisture-permeable film 1a-2 such as Japanese paper is provided so as to cover the entire surface of the element body 1a-1, and the element body 1a-1 is provided.
An electrode 1a-3 is provided on 1a-1, and a connection terminal 1a-4 is connected to the electrode 1a-3. The moisture permeable film 1a-2 prevents the dew condensation of water vapor on the surface of the element body 1a-1, and enables the moisture sensitive element 1a to operate normally at a temperature of 0 ° C. or less.
FIG. 4 shows the characteristics of the humidity sensitive element 1a, in which the resistance value R (Ω) changes with relative humidity RH (%) and temperature.

1b is a first resistor connected in parallel with the humidity sensitive element 1a,
The second and third resistors 1c and 1d are connected in series between one of the connection points a between the humidity sensitive element 1a and the first resistor 1b and the ground, and the humidity sensitive element 1a and the first resistor 1b are connected in series. The other end of the connection point B is an AC input terminal, and the connection point C of the first and second resistors 1c and 1d is an output terminal.

In the AC bridge circuit 1 including the humidity sensitive element 1a and the resistors 1b to 1d, when a rectangular wave having a constant amplitude and a constant frequency is input to the AC input terminal (b), water vapor contained in an arbitrary atmosphere is contained in the atmosphere. The values of the resistances 1b to 1d are set with respect to the humidity sensitive element 1a so as to send an output having an amplitude corresponding to the amount of frost formation per unit time in an atmosphere of a predetermined lower temperature to the output end c. . In particular, the first resistor 1b connected in parallel with the humidity sensitive element 1 determines the rate of change in output, that is, the slope, and the semi-fixed resistor 1d which constitutes the third resistor 1d together with the fixed resistor 1d-1. -2 is used to raise or lower the output level by its adjustment.

Reference numeral 5 is a central processing unit (CP) that includes an arithmetic unit (ALU), ROM, RAM, I / O port, A / D converter, etc.
U), and the output port PA _{0 of the} CPU 5 has a pull-up resistor 5a
And a rectangular wave having a duty ratio of 50% and a frequency of 1 KHz is transmitted from the output port PA ₀ . The rectangular wave is inverted by the inverter 5b and then applied to the AC input terminal B of the AC bridge circuit 1 via the coupling capacitor 2.

An AC inverting amplifier circuit 7 is connected to the AC output terminal C of the AC bridge circuit 1 via a coupling capacitor 4, and the AC inverting amplifier circuit 7 includes an operational amplifier 7a and a resistor 7b.
~ 7d, capacitor 7e, bias power supply 7h.

Reference numeral 6 is a load resistance connected between the output of the operational amplifier 7a which is the output of the AC inverting amplifier circuit 7 and the ground.
Output signal from 7a is input pin of CPU5 A / D converter AN ₀
Has been entered in.

The bias power supply 7h of the AC inverting amplifier circuit 7 is connected to the reference voltage input terminal V _AREF of the A / D converter of the CPU 5.

Note that V _CC of CPU5 is + 5V power supply terminal, V _SS is GND terminal, AV _CC
Is the + 5V power supply pin of the A / D converter, and AV _SS is the GND pin of the A / D converter.

In the above configuration, the CPU 5 outputs a rectangular wave having a duty ratio of 50% and a frequency of 1 KHz to its output port PA ₀ and applies it to the input of the inverter 5b. CPU5 output port PA ₀
The output voltage characteristic of is the TTL characteristic as shown in FIG. 5 (a) and the output current is small, but the inverter 5b inverts the rectangular wave from the output port PA ₀ and outputs it to the output of FIG. As shown in b), a rectangular wave with a constant amplitude of about 5 V is generated, and the output current is increased to 4 mA.

The rectangular wave shown in FIG. 5 (b) is converted to ± 2.4 V AC as shown in FIG. 5 (c) by passing through the coupling capacitor 2 and input to the AC input terminal B of the AC bridge circuit 1. It The alternating current input to the alternating current input terminal (b) of the bridge circuit 1 passes through the alternating current bridge circuit 1 and the coupling capacitor 4 and then is amplified by the alternating current inverting amplification circuit 7, and the output of the alternating current inverting amplification circuit 7 is +2. A signal whose amplitude changes around 55V according to the amount of frost per unit time is output.

Load resistance 6 connected to the output of the AC inverting amplifier circuit 7
Is a load for discharging current for guaranteeing the output of the op amp 7a at 0V, whereby the output of the operational amplifier 7a, which is the output of the AC inverting amplifier circuit 7, is 0 to 3.5 around + 2.55V. Outputs in the V range are obtained.

For example, when ± 1.5V AC is input to the input terminal of the AC inverting / amplifying circuit 7, if the gain of the circuit 7 is set to -1, the output is centered at + 2.55V as shown in Fig. 5 (d). As a result, an asymmetric rectangular wave of + 1.05V to + 3.5V is obtained.
This is because the output of the operational amplifier 7a is saturated at + 3.5V, but the voltage of the input terminal AN ₀ of the A / D converter is sampled only when the output level of the output port PA ₀ is L level. If so, it does not affect the signal processing. The voltage applied to the input terminal AN ₀ is sampled and taken in by the A / D converter as shown in FIG. 5 (e).

The A / D converter in the CPU 5 changes into an 8-bit digital signal of an analog signal in the range from the voltage 0V of the GND terminal AV _SS to the voltage + 2.55V of the reference voltage input terminal V _AREF . The converted digital signal value is stored in the conversion result register CR ₀ in the CPU5.
Stored in.

The data stored in the register CR ₀ corresponds to the frost formation amount per unit time, but since it is inverted, the data in the register CR ₀ is transferred to the arithmetic unit (ALU),
Here, if the complement of 1 is taken, it is possible to obtain a digital value according to the amount of frost formation per unit time as shown in the table below.

The digital value according to the magnitude of the amount of frost per unit time obtained as described above is the value of the heat pump type air conditioner previously proposed by the applicant of the present application in, for example, JP-A-61-268938. In the frost control device, it can be used to generate a signal for starting the defrosting operation when the frost formation amount reaches a predetermined value, and therefore, the arithmetic processing for this is performed on the digital value. .

When the defrosting control device is configured by using a CPU, the defrosting control device can be caused to function as the CPU 5 of the frost amount detector, so that the cost can be reduced.

Next, the operation performed by the CPU 5 according to a predetermined program will be described with reference to the flowcharts of FIGS. 6 (a) and 6 (b).

CPU5 starts executing the program when the power is turned on,
The necessary initialization is performed in the first step S1. Then the process proceeds to step S2, determines whether or not the output ports PA ₀ is 0, and waits until YES is determined in step S2, that is, the output port PA ₀ becomes 0. Step S2
If the determination is YES, the process proceeds to step S3 and the input port A
The analog signal output from the AC inverting amplifier circuit 7 is read from N ₀ , and the read analog signal is continued.
In S4, it is converted into a digital signal by the A / D converter in CPU5, and the converted digital value is registered in CPU5.
Store in CR ₀ . Then proceed to step S5, where CPU5
In the arithmetic unit (ALU) in the above, a one's complement is obtained for the digital value in the register CR ₀ . The one's complement obtained in step S5 is processed for use in the next step S6, and after the processing is completed, the process returns to step S2.

During execution of the main routine described above, a timer interrupt is performed at fixed time intervals of 0.5 ms. In the timer interrupt, in the first step S10, it is determined whether or not the output port PA ₀ is 0. If the determination is YES, the output port PA _{0 is set} to 1 in step S11 and then the process returns to the main routine.
When the determination is NO, the output port PA _{0 is set} to 0 in step S12, and the process returns to the main routine. As a result, the output port PA ₀ becomes 0.1 every 0.5 ms, and a rectangular wave with a frequency of 1 KHz is generated.

In the above embodiment, the rectangular wave from the output port PA ₀ is passed through the inverter 5b before the coupling capacitor 2
However, if a buffer is used instead of the inverter 5b, the sampling period by the A / D converter is the period when the rectangular wave of the output port PA ₀ is at the H level. It is clear that

Further, as the bias power supply 7h of the AC inverting amplifier circuit 7,
The structure shown in FIG. 7 can be preferably applied. That is, the semi-fixed resistor 7h-1 and the operational amplifier 7h-2
It is possible to set the bias point to a desired 2.55V by adjusting the semi-fixed resistor 7h-1 by using

[Effect]

As described above, according to the present invention, the output signal is sampled by the AC amplifier circuit in the period corresponding to the predetermined period of the rectangular wave and converted into the digital signal, and the physical quantity in the atmosphere is detected by the value of the digital signal. Therefore, it is not necessary to perform rectification by a diode as in the conventional case, and the detection accuracy is improved.

[Brief description of drawings]

FIG. 1 is a block diagram showing a basic configuration of a physical quantity detector according to the present invention, FIG. 2 is a block diagram showing an embodiment of a physical quantity detector according to the present invention, and FIG. 3 is a concrete example of a sensitive element in FIG. 4 is a partially cutaway perspective view showing the dynamic structure, FIG. 4 is a graph showing the relative humidity-resistance value characteristic of the humidity sensitive element with temperature as a parameter, and FIG. 5 is a waveform diagram showing the waveform of each part in FIG. FIG. 6 is a flow chart showing the work performed by the CPU in FIG. 2, FIG. 7 is a circuit diagram showing a concrete example of a part of FIG. 2, and FIG. 8 is a diagram showing an example of a conventional physical quantity detector. FIG. 9 is a waveform diagram showing the waveform of each part in FIG. A ... Square wave generation means, B ... Coupling capacitor, C ... AC bridge circuit, D ... AC amplification circuit (AC inverting amplification circuit), E ... A / D conversion means (A / D converter).

Claims (1)

[Claims] 1. A rectangular wave generating means for generating a rectangular wave having a constant frequency and a constant amplitude, and an element whose resistance value changes in accordance with a physical quantity in an atmosphere, wherein the rectangular shape is provided at an input end through a coupling capacitor. A rectangular wave from the wave generating means is input, an AC bridge circuit that outputs an AC signal whose amplitude changes according to a physical quantity to an output end, an AC amplifier circuit that amplifies the AC signal from the AC bridge circuit, and the AC An output period of the AC amplifier circuit corresponding to a predetermined period of the rectangular wave generated by the rectangular wave generating means, using the bias point of the amplifier circuit as a reference voltage. The output signal of the AC amplifier circuit is sampled and converted into a digital signal. A physical quantity detector comprising: a / D conversion means, and a physical quantity is detected by an output of the A / D conversion means.