Capacitance detection circuit for testing soil moisture content

Abstract

The invention provides a capacitance detection circuit for testing soil moisture content. The capacitance detection circuit comprises a soil moisture sensing electrode, a CV conversion capacitance detection circuit, an intelligent digital controller, a soil temperature detection circuit and a detection result output circuit, the soil moisture sensing electrode is electrically connected with the C / V conversion capacitance detection circuit; the CV conversion capacitance detection circuit, the soil temperature detection circuit and the detection result output circuit are electrically connected with the intelligent digital controller; the soil moisture sensing electrode transmits capacitance signals to the CV conversion capacitance detection circuit, the CV conversion capacitance detection circuit converts the changed capacitance signals into voltage signals and transmits the voltage signals to the intelligent digital controller, and the intelligent digital controller converts analog voltage signals into digital quantity through an internal ADC and directly outputs moisture content.

Description

Technical Field

[0001] The present invention belongs to the technical field of soil moisture sensing, detection and processing, and in particular relates to a capacitance detection circuit for testing soil moisture content. Background Art

[0002] In this era of the Internet of Everything, with the development of micro-nano technology, artificial intelligence technology and sensor technology, many electronic devices have the ability to perceive the world. The pursuit of more advanced, smarter and more integrated electronic devices has also promoted the development of reliable, high-performance and low-cost sensors.

[0003] Capacitive soil moisture sensor is an important member of the environmental monitoring sensor family. It uses the charging and discharging principle to convert the capacitance that changes with the dielectric constant of the soil to be measured into a quantitative signal such as voltage, current or frequency that is easy to measure and transmit. It has been widely used in agricultural production, atmospheric monitoring, environmental testing, geological monitoring and other measurements. However, in the process of soil moisture detection in the prior art, due to the small capacitance change, generally in the order of pF, the small change of soil moisture is not easy to collect, resulting in low detection accuracy.

[0004] Therefore, the technical problem to be solved by the present invention is that in the process of detecting soil moisture in the prior art, since the change in tiny capacitance is very small, generally in the order of pF, the tiny change in soil moisture is not easy to be collected, resulting in low detection accuracy. Summary of the invention

[0005] In order to solve the technical problem that in the process of soil moisture detection in the prior art, due to the small capacitance change, generally in the pF order, the small change of soil moisture is not easy to collect, resulting in low detection accuracy, the present invention provides a capacitance detection circuit for soil moisture content testing.

[0006] The specific plan is as follows: A capacitance detection circuit for testing soil moisture content comprises a soil moisture sensing electrode, a CV conversion capacitance detection circuit, an intelligent digital controller, a soil temperature detection circuit and a detection result output circuit; the soil moisture sensing electrode is electrically connected to the C / V conversion capacitance detection circuit; the CV conversion capacitance detection circuit, the soil temperature detection circuit and the detection result output circuit are all electrically connected to the intelligent digital controller.

[0007] The soil moisture sensing electrode transmits the capacitance signal to the CV conversion capacitance detection circuit to convert it into a corresponding voltage value. The CV conversion capacitance detection circuit converts the changing capacitance signal into a voltage signal and transmits it to the intelligent digital controller. The intelligent digital controller converts the analog voltage signal into a digital value through the internal ADC and directly outputs the moisture content.

[0008] The device also comprises a storage unit, which is electrically connected to the intelligent digital controller and the soil temperature detection circuit, and the storage unit adopts an EEPROM memory.

[0009] It also includes a power supply module, which is electrically connected to the soil moisture sensing electrode, the C / V conversion capacitor detection circuit, the soil temperature detection circuit, the storage unit, the intelligent digital controller, and the detection result output circuit, and the voltage input range is 5-28V.

[0010] The power supply module includes a DCDC power supply regulator and an LDO linear regulator.

[0011] The CV conversion capacitance detection circuit uses a CV conversion chip with high-precision conversion. The CV conversion chip integrates a programmable gain amplifier, an analog-to-digital collector, a voltage reference, a temperature sensor and a standard IIC digital interface circuit.

[0012] The soil temperature detection circuit adopts a digital temperature sensor.

[0013] The intelligent digital controller adopts the ARM-Cortex M0 core and is the central processing control unit of the entire circuit.

[0014] The detection result output circuit adopts RS485 digital output physical interface.

[0015] The beneficial effects of the present invention are: The present invention provides a capacitance detection circuit for testing soil moisture content, which has high integration and can detect and process tiny capacitances. By converting capacitance signals into voltage signals and continuously sampling the voltage signals for quantization, capacitance changes can be obtained, thereby obtaining soil moisture changes. A temperature sensor is provided to perform heat monitoring and temperature compensation on the circuit, thereby suppressing temperature drift during the period and avoiding the influence of temperature changes, and the detection accuracy is high. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 It is a circuit module structure connection diagram of the present invention.

[0017] Figure 2 It is a schematic diagram of the C / V conversion capacitance detection circuit of the present invention.

[0018] Figure 3 It is a controller schematic diagram of the ARM core of the present invention.

[0019] Figure 4 It is a principle diagram of a soil temperature detection circuit of the present invention.

[0020] Figure 5 It is a schematic diagram of the detection result output circuit of the present invention.

[0021] Figure 6 It is a schematic diagram of a DC power supply voltage stabilizing circuit of the present invention.

[0022] Figure 7 It is a schematic diagram of a linear voltage stabilizing circuit of the present invention.

[0023] Figure 8 It is a schematic diagram of the memory cell circuit of the present invention.

[0024] Fig. 9 It is a schematic diagram of the power interface circuit of the present invention.

[0025] Fig.10 It is a circuit principle diagram of the status indicator light of the present invention. DETAILED DESCRIPTION

[0026] The following will be combined with the accompanying drawings in the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the implementation of the present invention, not all of the implementations. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0027] like Figure 1 As shown, the present invention provides a capacitance detection circuit for soil moisture content testing, including a soil moisture sensing electrode, a CV conversion capacitance detection circuit, an intelligent digital controller, a soil temperature detection circuit and a detection result output circuit; the soil moisture sensing electrode is electrically connected to the C / V conversion capacitance detection circuit; the CV conversion capacitance detection circuit, the soil temperature detection circuit and the detection result output circuit are all electrically connected to the intelligent digital controller.

[0028] The soil moisture sensing electrode measures the moisture in the soil. The change in soil moisture content causes the change in the dielectric constant of the soil itself. The capacitance value of the material changes linearly with the change in the dielectric constant. The soil moisture sensing electrode transmits the capacitance signal to the CV conversion capacitance detection circuit and converts it into a corresponding voltage value. The CV conversion capacitance detection circuit converts the changing capacitance signal into a voltage signal and transmits it to the intelligent digital controller. The intelligent digital controller converts the analog voltage signal into a digital value through the internal ADC and directly outputs the moisture content.

[0029] The present invention provides a capacitance detection circuit for testing soil moisture content, which has high integration and can detect and process tiny capacitances. By converting capacitance signals into voltage signals and continuously sampling the voltage signals for quantization, capacitance changes can be obtained, thereby obtaining soil moisture changes. A temperature sensor is provided to perform heat monitoring and temperature compensation on the circuit, thereby suppressing temperature drift during the period and avoiding the influence of temperature changes, and the detection accuracy is high.

[0030] It also includes a storage unit, which is electrically connected to the intelligent digital controller and the soil temperature detection circuit. Figure 8 The schematic diagram of the memory unit circuit is shown. The memory unit adopts EEPROM memory, which is an electrically erasable programmable read-only memory and a non-volatile static memory. The sensor's own information can be saved when the power is off, and the data will not be lost. In the present invention, the EEPROM memory stores the calibration data of the temperature sensor and soil moisture. Specifically, the 24C16 model can be used. The 24C16 is a serial EEPROM with a 16K-bit capacity. It uses an I²C bus interface for data transmission and has the advantages of low power consumption, wide operating voltage range, strong anti-interference ability, write protection function and long life.

[0031] The soil moisture sensing electrode is embedded in the PCB board and is used to contact the soil to detect the moisture in the soil. The two ends of the electrode are respectively connected to the INA / INB pins of the C / V conversion capacitor detection circuit to transmit the detected capacitance signal to the C / V conversion capacitor detection circuit.

[0032] It also includes a power supply module, which is electrically connected to the soil moisture sensing electrode, the C / V conversion capacitor detection circuit, the soil temperature detection circuit, the storage unit, the intelligent digital controller, and the detection result output circuit. The voltage input range is 5-28V and is used to provide a stable power supply.

[0033] The power module includes a DCDC power regulator and an LDO linear regulator, wherein the DCDC power regulator adopts the TPS54302 model, which is used to provide a DC regulated power supply. The circuit schematic is as shown in Figure 6 As shown, the LDO linear regulator uses HT7833 to provide a linear regulated power supply. The circuit schematic is shown in Figure 7 As shown, the linear regulated power supply can provide excellent power supply signal, low ripple output, and reduce the noise of analog signals.

[0034] The present invention adopts TYPE-C interface, such as Fig. 9 As shown in the circuit schematic diagram, the present invention is also provided with a status indicator light to indicate the status, such as Fig.10 The circuit schematic is shown in the figure.

[0035] The CV conversion capacitance detection circuit uses the latest CV conversion chip with high-precision conversion. The CV conversion chip integrates a programmable gain amplifier, an analog-to-digital collector (ADC), a voltage reference (bandgap), a temperature sensor (Temperature Sensor) and a standard IIC digital interface circuit to convert the input capacitance signal into a voltage signal, preparing for subsequent signal processing and quantization.

[0036] After years of development, the capacitance reading circuit has produced a variety of structures, such as frequency type, switched capacitor type, etc. The present invention adopts a voltage-type capacitance digital converter to meet the actual application needs, that is, a capacitance-voltage converter and an analog-to-digital converter are combined into a capacitance digital converter for soil moisture content testing. The capacitance data is read simultaneously through the fast conversion capability of the intelligent digital controller and the capacitance converter, and converted into soil moisture content and outputted, which reduces the sampling time of soil moisture content and improves the measurement accuracy of soil moisture content.

[0037] like Figure 2 The figure shows the schematic diagram of the C / V conversion capacitor detection circuit. CAP+ / CAP- is the capacitor to be tested, Cref is the external reference capacitor, and the pins INA / INB are connected to the two ends of the soil test capacitor electrode respectively. Square wave signals with opposite phases are input to the pins INA and INB respectively. The capacitance signal of CAP is converted into a voltage signal. By continuously sampling and quantizing the voltage signal, the continuous change of CAP can be tracked. The ADC inside the C / V conversion capacitor detection circuit converts the available capacitance value into the corresponding digital value, which is transmitted to the intelligent digital controller through the IIC digital interface circuit to obtain the digital capacitance value corresponding to the soil moisture content.

[0038] The soil temperature detection circuit uses a digital temperature sensor. In this embodiment, the digital temperature sensor uses the SHT30 model, which is a high-precision, high-reliability temperature and humidity sensor. It uses MEMS technology to integrate temperature and humidity sensors on the same chip, and can measure the temperature and humidity of the environment at the same time. The temperature error can reach ±0.5°C, which is used for thermal monitoring and temperature compensation of soil temperature. Figure 4 The figure shows the principle diagram of soil temperature detection circuit. The present invention adopts 16-bit digital output, can accurately collect the real-time temperature of environmental soil, provide basis for moisture algorithm compensation, and has the advantages of high resolution and high precision.

[0039] Temperature compensation means that as the soil temperature changes, the dielectric constant of the soil, that is, the soil capacitance value, will change. The soil temperature can be sensed by a temperature sensor to calculate the compensated soil moisture content value, thereby avoiding the impact of soil temperature changes on the detection results and improving the detection accuracy.

[0040] The intelligent digital controller adopts the advanced ARM-Cortex M0 core, and is internally integrated with advanced multiplication and division operators and digital interface circuits, which can accurately calculate the output results of the circuit and is the central processing control unit of the entire circuit.

[0041] The intelligent digital controller adopts the microcontroller STM32G031G8U6 with ARM core. Figure 3 The circuit diagram of the controller STM32G031G8U6 is shown, with a 32-bit Cortex™-M0 CPU, a maximum operating frequency of 72MHz, single-cycle multiplication and hardware division, 64K bytes of flash program memory, a 12-bit analog-to-digital converter, which can be expanded to 16 bits with oversampling, up to 9 communication interfaces and 7 timers. It is currently a high-performance microcontroller used to control various analog modules, adjust the working mode, receive capacitance quantization signals and temperature signals, store and process digital signals, and output them through various output interfaces.

[0042] The detection result output circuit adopts RS485 digital output physical interface, such as Figure 5 The figure shows the principle diagram of the detection result output circuit of the present invention. The RS485 interface device can specifically adopt THVD1410, which has a long output distance, strong anti-interference ability, and can be easily compatible with various network transmission protocols. The communication protocol in the present invention adopts the industrial modbusRTU protocol standard format.

[0043] In terms of implementation methods, there are many types of analog-to-digital converters (ADCs): successive approximation type, integral type, voltage-frequency conversion type, ∑-Δ type ADC, etc. Successive approximation logic is simply a circuit implementation of binary division. It starts from the most significant bit (MSB) of the circuit, and each time the reference voltage (REF) is divided into half of the original through the digital-to-analog converter (DAC), and then compared with the input voltage. The result is fed back to the digital-to-analog converter (DAC) through digital logic to complete the conversion of one bit. Therefore, a simple N-bit successive approximation register analog-to-digital converter (SAR-ADC) only needs N clock cycles to complete the entire conversion. The present invention uses a 12-bit analog-to-digital converter (ADC).

[0044] The basic structure of the successive approximation analog-to-digital converter SS-ADC includes a digital-to-analog converter, a comparator, a digital counter and a memory. The working principle is to generate a ramp voltage through the digital-to-analog converter, which increases monotonically and is compared with the input analog signal. When the input signal is equal to the ramp voltage, the comparator is triggered, and the value of the digital counter is locked, indicating that one bit of quantization is completed. This process is repeated until all bits of quantization are completed. Due to its simple structure, small footprint and low power consumption, it is widely used.

[0045] The main disadvantage of SS-ADC is its slow conversion speed. In particular, when the bit width increases, the conversion time will increase significantly. For example, when the ADC clock frequency is 100MHz, an 8-bit SS-ADC requires a conversion time of 0.2 seconds. In order to increase the speed of SS-ADC, SAR ADC and SS-ADC can be used in combination, that is, a SAR / SSADC structure can be used. For example, the SAR ADC structure can be used for the upper bits, and the SS-ADC structure can be used for the lower bits. This can take into account the requirements of speed and accuracy. This structure not only improves the conversion speed, but also reduces the area and power consumption.

[0046] The working principle of SAR-ADC is based on the "compare-shift" method. It uses a digital-to-analog converter (DAC) to generate a reference voltage and compares the input signal with the reference voltage through a comparator. According to the comparison result, SAR will gradually adjust the value of the reference voltage until a digital value that matches the input signal is found. This process starts from the most significant bit (MSB) and gradually moves to the least significant bit (LSB). After each bit is determined, SAR will update the output of the DAC to make it closer to the actual value of the input signal. When all bits are determined, SAR outputs the final digital value in parallel or serial mode. This value represents the digital representation of the input analog signal. The main advantages of SAR-ADC include high performance, low power consumption, and suitability for small-size packages. It is particularly suitable for systems with strict size requirements. The algorithm is simple, easy to implement, and has good compatibility with digital CMOS processes.

[0047] The technical means disclosed in the scheme of the present invention are not limited to the technical means disclosed in the above-mentioned implementation mode, but also include technical schemes composed of any combination of the above-mentioned technical features. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications are also regarded as the protection scope of the present invention.

Claims

1. A capacitance detection circuit for soil moisture content testing, characterized in that: It includes soil moisture sensing electrode, CV conversion capacitor detection circuit, intelligent digital controller, soil temperature detection circuit and detection result output circuit; The soil moisture sensing electrode is electrically connected to a C / V conversion capacitor detection circuit; The CV conversion capacitor detection circuit, the soil temperature detection circuit, and the detection result output circuit are all electrically connected to the intelligent digital controller; The soil moisture sensing electrode transmits the capacitance signal to the CV conversion capacitance detection circuit to convert it into a corresponding voltage value. The CV conversion capacitance detection circuit converts the changing capacitance signal into a voltage signal and transmits it to the intelligent digital controller. The intelligent digital controller converts the analog voltage signal into a digital value through the internal ADC and directly outputs the moisture content.

2. The capacitance detection circuit for soil moisture content testing according to claim 1, characterized in that: The device also comprises a storage unit, which is electrically connected to the intelligent digital controller and the soil temperature detection circuit, and the storage unit adopts an EEPROM memory.

3. The capacitance detection circuit for soil moisture content testing according to claim 1, characterized in that: It also includes a power supply module, which is electrically connected to the soil moisture sensing electrode, the C / V conversion capacitor detection circuit, the soil temperature detection circuit, the storage unit, the intelligent digital controller, and the detection result output circuit, and the voltage input range is 5-28V.

4. The capacitance detection circuit for soil moisture content testing according to claim 3 is characterized in that: The power supply module includes a DCDC power supply regulator and an LDO linear regulator.

5. The capacitance detection circuit for soil moisture content testing according to claim 1, characterized in that: The CV conversion capacitance detection circuit uses a CV conversion chip with high-precision conversion. The CV conversion chip integrates a programmable gain amplifier, an analog-to-digital collector, a voltage reference, a temperature sensor and a standard IIC digital interface circuit.

6. The capacitance detection circuit for soil moisture content testing according to claim 1, characterized in that: The soil temperature detection circuit adopts a digital temperature sensor.

7. The capacitance detection circuit for soil moisture content testing according to claim 1, characterized in that: The intelligent digital controller adopts the ARM-Cortex M0 core and is the central processing control unit of the entire circuit.

8. The capacitance detection circuit for soil moisture content testing according to claim 1, characterized in that: The detection result output circuit adopts RS485 digital output physical interface.

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