An instrument for measuring moisture content of sapwood of living standing trees based on weak capacitance detection method

Abstract

The invention discloses a moisture content measuring instrument for sapwood of living trees based on a weak capacitance detection method. The instrument comprises an FPGA control circuit, a DDS module I, a DDS module II, a NIOS II soft core, an infrared temperature measurement module, an LCD driver, an LCD touch screen, a touch controller, a sine wave circuit I, a sine wave circuit II, a drive cable I, a parallel double-needle probe, a drive cable II, a transimpedance amplifier, an AC amplifier, a multiplication phase-sensitive demodulator, an 8th-order active low-pass filter and an analog-to-digital converter. The measuring instrument takes an AC excitation weak capacitance detection method as a basic principle, combines the drive cable technology and the infrared temperature measurement technology, integrates the system circuit, the LCD touch screen, the connection cable and the parallel double-needle probe into one, and designs the appearance structure to be handheld. The moisture content of the sapwood of living trees can be measured with high precision, and the instrument has a simple structure and low cost. The operator can conveniently operate the instrument when performing field experiments in the field, thereby improving the detection efficiency.

Description

Technical Field

[0001] The invention relates to the field of standing wood moisture content measurement, in particular to a standing wood sapwood moisture content measuring instrument based on a weak capacitance detection method. Background Art

[0002] Forests are an important guarantee for human survival and reproduction, and have important ecological and economic value. Forest resources mainly include various types of wood materials in the timber industry. As a basic industry, the timber industry has made great contributions to my country's economy. As the source of the timber industry, the long-term overall growth and development of standing timber directly affects the timber industry. Water is inseparable from the various physiological activities of trees. Two-thirds of the organisms on the earth are water. Studies have found that the more water in plants, the better. Its content should be within the range of plant growth requirements. This requires regular testing of the water content of trees at different growth stages to study and judge the growth status of trees, provide a basis for the prediction of wood decay and tree irrigation, and then take corresponding measures to achieve the purpose of protecting forest resources.

[0003] Therefore, real-time and accurate measurement of the moisture content of standing timber sapwood is of great significance for the efficient development and utilization of forest resources. Existing moisture content measuring instruments are generally used to measure the mass moisture content of wood below 30%, while the actual mass moisture content of standing timber sapwood is usually above 80%. Standing timber sapwood is essentially a complex porous medium, that is, a combination with a certain structure formed by solid phase (wood), liquid phase (free water) and gas phase (air). In recent years, the dielectric constant method has received more and more attention. The dielectric constant method can be used to measure the mass moisture content of standing timber sapwood because the relative dielectric constant of free water in standing timber sapwood is 81, which is much higher than the relative dielectric constant of wood 2 and the relative dielectric constant of air 1, that is, the dielectric constant of standing timber sapwood is mainly affected by its moisture content. The greater the moisture content, the greater the overall dielectric constant.

[0004] The dielectric constant method can be divided into time domain reflectometry, standing wave ratio method and capacitance method according to the specific working principle. The time domain reflectometry inverts the dielectric constant of the object being measured by measuring the time difference between the incidence and reflection of the electromagnetic pulse on the transmission line, and then obtains the mass water content value according to the empirical formula. This method has the advantages of accuracy, no radiation and continuous measurement, but its circuit is complex, the equipment is expensive and the operation is inconvenient; the principle of the standing wave ratio method is similar to that of the time domain reflectometry, but it no longer uses the time difference between the incidence and reflection of the electromagnetic pulse, but measures the standing wave ratio of the electromagnetic pulse on the transmission line. This method has low cost and is less affected by temperature, but has high requirements for circuit anti-interference design, and the probe impedance calculation also affects its measurement accuracy; the capacitance method is often used to measure crude oil with high water content. There are few reports on the measurement of moisture content in the sapwood of standing trees. This method constructs a capacitor with the measured medium, then detects the capacitance value to obtain the moisture content. Specifically in the field of moisture content detection of standing trees, the moisture content of standing trees of different tree species can be converted into a weak capacitance composed of the probe and the inserted standing tree sapwood by designing an insertion probe structure suitable for the sapwood of standing trees. The capacitance is related to the tree species, moisture content of the standing tree and the insertion depth of the probe. The capacitance values ​​of these insertion probe structures are small, generally in the pF level, so it is necessary to reasonably select a weak capacitance detection device to convert it into an intuitively visible analog or digital signal.

[0005] The domestic detection method of weak capacitance is still immature. The conventional method is to convert the capacitance value into other easily measurable physical quantities through a signal processing circuit. However, the circuit is easily interfered with and produces parasitic capacitance of the same order of magnitude as the measured capacitance value, affecting the output accuracy. Therefore, the capacitance method is not mature in measuring the moisture content of the sapwood of standing trees. It is necessary to study a low-cost, anti-interference, high-precision, easy-to-use standing tree sapwood moisture content measuring instrument based on the weak capacitance detection method. Summary of the invention

[0006] The present invention aims at the deficiencies of the prior art and provides a standing timber sapwood moisture content measuring instrument based on a weak capacitance detection method to achieve high-precision measurement of the standing timber sapwood moisture content.

[0007] To achieve the above object, the present invention adopts the following technical solution:

[0008] A moisture content measuring instrument for sapwood of living standing trees based on a weak capacitance detection method, characterized in that it includes an FPGA control circuit, a DDS module I, a DDS module II, a NIOS II soft core, an infrared temperature measurement module, an LCD driver, an LCD touch screen, a touch controller, a sine wave circuit I, a sine wave circuit II, a drive cable I, a parallel double-needle probe, a drive cable II, a transimpedance amplifier, an AC amplifier, a multiplication phase-sensitive demodulator, an 8th-order active low-pass filter and an analog-to-digital converter.

[0009] Furthermore, the DDS module I and the DDS module II have the same internal structure, including a frequency control word, a phase control word, a clock module, a 32-bit accumulator I, a 32-bit register, a 32-bit accumulator II and a waveform memory, and the functions of each internal module and the communication between modules are realized by FPGA through Verilog language programming; the NIOS II soft core is a 32-bit soft core processor inherent in the FPGA, and it communicates bidirectional data with the FPGA through the Avalon bus, the output end of the NIOS II soft core is electrically connected to the input end of the LCD driver, the output end of the LCD driver is electrically connected to the input end of the LCD touch screen, the output end of the LCD touch screen is electrically connected to the input end of the touch controller, and the output end of the touch controller and the output end of the infrared temperature measurement module are both electrically connected to the input end of the NIOS II soft core.

[0010] Furthermore, the sine wave circuit I and the sine wave circuit II have the same internal structure, including a digital-to-analog converter, a differential-to-single-ended circuit, a 7th-order passive low-pass filter and a post-stage amplifier circuit, the output end of the digital-to-analog converter is electrically connected to the input end of the differential-to-single-ended circuit, the output end of the differential-to-single-ended circuit is electrically connected to the input end of the 7th-order passive low-pass filter, and the output end of the 7th-order passive low-pass filter is electrically connected to the input end of the post-stage amplifier circuit.

[0011] Furthermore, the FPGA control circuit outputs a phase control word and a frequency control word to the DDS module I and the DDS module II; the output end of the waveform memory and the output end of the clock module in the DDS module I are both electrically connected to the input end of the digital-to-analog converter in the sine wave circuit I; the output end of the post-stage amplifier circuit in the sine wave circuit I is electrically connected to the input end of the drive cable I; the output end of the drive cable I is electrically connected to the input end of the parallel double-needle probe; the output end of the parallel double-needle probe is electrically connected to the input end of the drive cable II; the output end of the drive cable II is electrically connected to the input end of the transimpedance amplifier; and the transimpedance The output end of the amplifier is electrically connected to the input end of the AC amplifier, the output end of the waveform memory in the DDS module II and the output end of the clock module are both electrically connected to the input end of the digital-to-analog converter in the sine wave circuit II, the output end of the post-stage amplifier circuit in the sine wave circuit II and the output end of the AC amplifier are both electrically connected to the input end of the multiplication phase-sensitive demodulator, the output end of the multiplication phase-sensitive demodulator is electrically connected to the input end of the 8th-order active low-pass filter, the output end of the 8th-order active low-pass filter is electrically connected to the input end of the analog-to-digital converter, and the output end of the analog-to-digital converter is electrically connected to the input end of the FPGA control circuit.

[0012] Furthermore, the drive cable I and drive cable II adopt drive cable technology, that is, the connecting cable adopts a low distributed capacitance double-layer shielded cable, wherein the inner shielding layer of the cable is electrically connected to the instrument ground, and the outer shielding layer of the cable is electrically connected to the earth.

[0013] The beneficial effects of the present invention are:

[0014] (1) The present invention adopts an AC excitation weak capacitance detection method with strong ability to suppress stray capacitance, high resolution, good stability and easy implementation as the basic principle for measuring the moisture content of the sapwood of living trees, and improves the signal generation unit, capacitance conversion unit and signal demodulation unit in the traditional AC excitation weak capacitance detection circuit. A parallel double-needle probe is used as a sensor for measuring the moisture content of the sapwood of living trees, which can convert the change of the moisture content of the sapwood of living trees into the change of capacitance. After being processed by the system circuit, the moisture content of the sapwood of living trees can be displayed in real time on the LCD touch screen.

[0015] (2) The present invention combines the driving cable technology with the AC excitation type weak capacitance detection circuit, and utilizes the "virtual short and virtual break" characteristics of the ideal operational amplifier to eliminate the influence of various external interferences and the random parasitic capacitance formed between the core wire and the shielding layer of the connecting cable. In this way, the capacitance value (<5pF) converted by the parallel double-needle probe moisture content sensor can be accurately detected under the influence of the connecting cable with hundreds of pF of parasitic capacitance, thereby improving the measurement accuracy of the moisture content.

[0016] (3) The present invention uses infrared temperature measurement technology to achieve non-contact temperature measurement of the epidermis of living trees, obtains the temperature of the epidermis of living trees, adds this temperature to the inversion model of the moisture content of the sapwood of living trees for temperature calibration, reduces the influence of temperature on the moisture content measurement, and improves the measurement accuracy of the moisture content.

[0017] (4) The present invention integrates the system circuit, LCD touch screen, connecting cable and parallel double-needle probe into one, and designs the appearance structure to be handheld, which has a simple structure and low cost. It is easy for operators to operate when conducting field experiments in the field, thereby improving the detection efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 The system block diagram of the moisture content measuring instrument for sapwood of living standing trees based on the weak capacitance detection method in the present invention.

[0019] Figure 2 It is a structural block diagram of the signal generating unit in the present invention.

[0020] Figure 3 It is a circuit schematic diagram of the capacitance conversion unit and the signal adjustment unit in the present invention.

[0021] Figure 4 It is the equivalent circuit diagram of the driving cable technology in the present invention.

[0022] Figure 5 The present invention is a front view plane structure of a moisture content measuring instrument for sapwood of living standing trees based on a weak capacitance detection method.

[0023] In the figure: 1. Parallel double-needle probe; 2. Double-layer shielded cable; 3. Switch; 4. Cable adapter; 5. Infrared temperature probe; 6. LCD touch screen; 7. Instrument housing; 8. Temperature measurement button; 9. Trunk of living standing tree. DETAILED DESCRIPTION

[0024] The present invention is described in more detail below with reference to the accompanying drawings and embodiments.

[0025] like Figure 1 As shown, the present invention discloses a moisture content measuring instrument for standing wood sapwood based on a weak capacitance detection method, comprising an FPGA control circuit, a DDS module I, a DDS module II, a NIOS II soft core, an infrared temperature measurement module, an LCD driver, an LCD touch screen, a touch controller, a sine wave circuit I, a sine wave circuit II, a drive cable I, a parallel double-needle probe, a drive cable II, a transimpedance amplifier, an AC amplifier, a multiplication phase-sensitive demodulator, an 8th-order active low-pass filter and an analog-to-digital converter.

[0026] The DDS module I and DDS module II have the same internal structure, including a frequency control word, a phase control word, a clock module, a 32-bit accumulator I, a 32-bit register, a 32-bit accumulator II and a waveform memory. The functions of each internal module and the communication between modules are realized by FPGA through Verilog language programming; the NIOS II soft core is a 32-bit soft core processor inherent in the FPGA, and bidirectional data communication is performed between it and the FPGA through the Avalon bus. The output end of the NIOS II soft core is electrically connected to the input end of the LCD driver, the output end of the LCD driver is electrically connected to the input end of the LCD touch screen, the output end of the LCD touch screen is electrically connected to the input end of the touch controller, and the output end of the touch controller and the output end of the infrared temperature measurement module are both electrically connected to the input end of the NIOS II soft core.

[0027] The sine wave circuit I and the sine wave circuit II have the same internal structure, including a digital-to-analog converter, a differential-to-single-ended circuit, a 7th-order passive low-pass filter and a post-stage amplifier circuit. The output end of the digital-to-analog converter is electrically connected to the input end of the differential-to-single-ended circuit, the output end of the differential-to-single-ended circuit is electrically connected to the input end of the 7th-order passive low-pass filter, and the output end of the 7th-order passive low-pass filter is electrically connected to the input end of the post-stage amplifier circuit.

[0028] The FPGA control circuit outputs a phase control word and a frequency control word to the DDS module I and the DDS module II. The output end of the waveform memory and the output end of the clock module in the DDS module I are electrically connected to the input end of the digital-to-analog converter in the sine wave circuit I. The output end of the post-stage amplifier circuit in the sine wave circuit I is electrically connected to the input end of the drive cable I. The output end of the drive cable I is electrically connected to the input end of the parallel double-needle probe. The output end of the parallel double-needle probe is electrically connected to the input end of the drive cable II. The output end of the drive cable II is electrically connected to the input end of the transimpedance amplifier. The output end of the device is electrically connected to the input end of the AC amplifier, the output end of the waveform memory in the DDS module II and the output end of the clock module are both electrically connected to the input end of the digital-to-analog converter in the sine wave circuit II, the output end of the post-stage amplifier circuit in the sine wave circuit II and the output end of the AC amplifier are both electrically connected to the input end of the multiplication phase-sensitive demodulator, the output end of the multiplication phase-sensitive demodulator is electrically connected to the input end of the 8th-order active low-pass filter, the output end of the 8th-order active low-pass filter is electrically connected to the input end of the analog-to-digital converter, and the output end of the analog-to-digital converter is electrically connected to the input end of the FPGA control circuit.

[0029] The drive cable I and drive cable II adopt the drive cable technology, that is, the connecting cable adopts a low distributed capacitance double-layer shielded cable, wherein the inner shielding layer of the cable is electrically connected to the instrument ground, and the outer shielding layer of the cable is electrically connected to the earth.

[0030] The parallel double-needle probe in the present invention and the medium between the double-needle probe are equivalent to a capacitor C x The capacitance of the capacitor is positively correlated with the moisture content of the sapwood of the inserted living tree. Therefore, the present invention adopts the AC excitation type weak capacitance detection method as the basic principle of measuring the moisture content of the sapwood of the living tree. The AC excitation type weak capacitance detection method first converts the capacitance signal into an AC amplitude modulated voltage signal, and then performs AC amplification and demodulation on the amplitude modulated signal to obtain a DC voltage output. The detection circuit mainly includes three parts: a signal generating unit, a capacitance conversion unit and a signal demodulation unit.

[0031] The signal generating unit is composed of a DDS module I, a DDS module II, a sine wave circuit I and a sine wave circuit II. Since the internal structures of the DDS module I and the DDS module II are the same, and the internal structures of the sine wave circuit I and the sine wave circuit II are the same, one of them is now described in detail. Figure 2As shown, this embodiment is a detailed flow chart of a sine wave signal generating unit, which is composed of a DDS module and a sine wave circuit. Its basic principle is direct digital frequency synthesis technology (DDS). The core of the DDS system is a 32-bit phase accumulator, which is composed of a 32-bit accumulator I and a 32-bit register. For each clock pulse from the clock module, the 32-bit accumulator I adds the frequency control word data with the accumulated phase data output by the 32-bit register, and sends the added result to the data input end of the 32-bit register. The 32-bit register feeds back the new phase data generated by the 32-bit accumulator I after the previous clock pulse to the input end of the 32-bit accumulator I, so that the 32-bit accumulator I continues to add the frequency control word data under the action of the next clock pulse. Then, the 32-bit phase accumulator performs linear phase accumulation under the action of the clock pulse. When the 32-bit phase accumulator accumulates the full amount, an overflow will occur, completing a cycle of action. This cycle is a frequency cycle of the DDS synthetic signal. The overflow frequency of the 32-bit phase accumulator is the frequency of the output signal. The 32-bit accumulator II adds the phase control word data and the accumulated phase data output by the 32-bit register again under the action of each clock pulse, so that the signal of the entire cycle has a phase delay, thereby realizing the phase control of the DDS synthetic signal.

[0032] The waveform memory is essentially a phase / amplitude converter, which mainly completes the conversion of the signal's phase sequence to the amplitude sequence. It stores the sampled code value of a periodic sinusoidal signal with the phase as the address, including the digital amplitude information of a periodic sinusoidal wave, and each address corresponds to a phase point in the range of 0° to 360° in the sinusoidal wave. The data output by the 32-bit accumulator II is used as an address to address the sinusoidal lookup table, which maps the input address phase information into a sinusoidal amplitude signal, driving the digital-to-analog converter to output a pair of differential sinusoidal analog signals. The signal output by the digital-to-analog converter is a current signal, so a differential-to-single-ended circuit is needed to convert the differential signal into a single-ended signal, and at the same time, the current signal is converted into a voltage signal output to improve the circuit's load capacity. When using DDS technology, due to its limited phase and amplitude resolution, there will be stray noise in its output. In order to obtain a clean, low-jitter signal, a 7th-order passive low-pass filter is connected to the output of the differential-to-single-ended circuit to filter the signal. In order to output a higher voltage value and prevent the subsequent circuit from affecting the filter, a subsequent amplifier circuit is connected to the output of the 7th-order passive low-pass filter to amplify the amplitude of the sinusoidal wave signal to the amplitude required by the system.

[0033] From the above analysis, it can be seen that the signal generating unit composed of DDS module I, DDS module II, sine wave circuit I and sine wave circuit II can output two sine wave signals with adjustable frequency and phase. In this embodiment, the digital-to-analog converter can use AD9764 chip, the differential-to-single-ended circuit and the post-amplifier circuit can use OPA690 chip, and the parameters and simulation of the 7th-order passive low-pass filter can be designed and completed by the filter design software Filter Solutions 2011. This part of the circuit principle is not the core technology of the present invention and will not be described in detail.

[0034] The capacitance conversion unit and the signal conditioning unit are the core technologies of the present invention. The capacitance conversion unit adopts an AC excitation capacitance conversion circuit, including a transimpedance amplifier and an AC amplifier. The transimpedance amplifier is used to detect the induced current generated on the capacitance sensor probe and convert it into a voltage. The performance index of the transimpedance amplifier directly affects the capacitance conversion accuracy. Figure 3 As shown, in order to achieve the purpose of high-precision detection, the transimpedance amplifier in this embodiment adopts ADI's ADA4530-1 femtoampere (fA) input bias current operational amplifier, whose maximum input bias current is 20fA, maximum offset voltage is 50μV, and maximum offset voltage drift is 0.5μV / ℃. As the first-stage circuit of the capacitance conversion unit, the resistance stability and noise level of the feedback resistor Rf of the transimpedance amplifier directly affect the capacitance conversion accuracy and signal-to-noise ratio. At the same time, if high-sensitivity capacitance conversion is to be achieved, the feedback resistor Rf must be very large. According to the theory of thermodynamics, the thermal noise voltage density of the resistor is proportional to the square root of the resistance value. To solve this problem, this embodiment adopts a T-type feedback resistor network structure instead of the feedback resistor Rf to achieve the circuit function, as shown in FIG. Figure 3 As shown in the circuit schematic diagram of the mid-impedance amplifier, it can be seen that the T-type resistor feedback network structure is equivalent to using resistors R1, R2 and R3 to replace the feedback resistor Rf, and its equivalent feedback resistor Rf is:

[0035]

[0036] It can be seen from formula (1) that the large-value resistor of 4.14MΩ can be replaced by three small-value resistors R1=100kΩ, R2=1kΩ, and R3=40kΩ. Compared with the direct use of the large-value resistor of 4.14MΩ, the noise voltage density is reduced by about 90%. It can be seen that the T-type network structure can effectively reduce the thermal noise of the feedback circuit while using small-value resistors to replace large-value resistors. Therefore, this method can be used to reduce the noise of the feedback circuit in the AC excitation capacitor conversion circuit, thereby improving the signal-to-noise ratio of the circuit.

[0037] Since the parallel double-needle probe for measuring the moisture content of the sapwood of living trees is at a certain distance from the detection system circuit, the connecting cable between the probe and the detection system usually requires certain shielding measures to eliminate various external interferences and the influence of random parasitic capacitance formed between the core wire and the shielding layer of the connecting cable. The present invention adopts the driving cable technology, also known as the double-layer shielding equipotential technology, which is a potential tracking technology. Its basic idea is: the connecting cable adopts inner and outer double-layer shielding, and the driving circuit is used to make the inner shielding layer and the shielded core wire have the same potential, and the outer shielding layer is connected to the ground to play a shielding role. Figure 3 As shown in the driving cable I and driving cable II, one probe of the parallel double-needle probe is connected to the output end of the front-stage sine wave circuit I through a low distributed capacitance double-shielded cable, and the other probe of the parallel double-needle probe is connected to the reverse input end of the transimpedance amplifier through a low distributed capacitance double-shielded cable. The inner shielding layer of the two double-shielded cables is connected to the instrument ground, and the outer shielding layer is connected to the ground. The final equivalent circuit is as follows Figure 4 As shown in the figure, capacitors C7 and C9 are parasitic capacitors between the cable core and the inner shielding layer, capacitors C8 and C10 are parasitic capacitors between the inner shielding layer and the outer shielding layer of the cable, and capacitors C7 and C8 are directly driven by the first sinusoidal wave signal SINg output by the signal generating unit, and have no effect on the AC signal applied to the probe, so they have no effect on the subsequent capacitance detection circuit; capacitor C9 is connected to pin 8 of the transimpedance amplifier, and according to the "virtual short and virtual break" characteristics of the ideal operational amplifier, pin 1 is connected to the instrument ground, then pin 8 is also connected to the instrument ground, and the two ends of capacitor C9 are equipotential, so the capacitance of C9 is zero, and capacitor C10 is the capacitor between the instrument ground and the earth, so the AC signal transmitted from the probe to pin 8 is not affected by capacitors C9 and C10.

[0038] The amplitude of the AC voltage signal output from the transimpedance amplifier changes very little, which is not conducive to signal conditioning. Interference can easily enter and drown out useful signals, so it is necessary to perform AC amplification to provide a signal of appropriate amplitude for the key signal demodulation unit at the subsequent stage. Figure 3 As shown, in this embodiment, the AC amplifier adopts the instrument differential amplifier AD620 of ADI Company, whose differential mode input impedance is 10GΩ||2pF, common mode input impedance is 10GΩ||2Pf, maximum offset voltage is 50μV, and maximum offset voltage drift is 1μV / ℃. In addition to high performance, its biggest advantage is that the amplification factor is determined by only one resistor, involving fewer components, so the component drift problem has little effect on the performance of the module, and does not involve the consistency problem of the resistor components in the ordinary negative feedback circuit, and only needs to select the resistor R that determines the amplification factor G The gain of AD620 is:

[0039]

[0040] In this embodiment, R G = R4 = 51kΩ, with an accuracy of 0.1%. To reduce interference and improve the accuracy of the capacitance conversion unit, a DC blocking capacitor C2 is placed at the input of the AC amplifier during design, so that the voltage drift caused by temperature drift in the previous stage cannot be transmitted to the AC amplifier and amplified. The output voltage u of the AC amplifier g for:

[0041]

[0042] In formula (3), u o is the output voltage of the transimpedance amplifier, u e is the voltage of the first sine wave signal SINg output by the signal generating unit and input into the transimpedance amplifier after passing through the parallel double-needle probe. Substituting into formula (3) we get:

[0043]

[0044] in, From formula (4), we can know that the parallel double-needle probe is equivalent to the measured capacitance C x The sine wave signal u is converted by the AC excitation capacitor conversion unit e The amplitude modulation of the modulated signal u g With the sine wave excitation signal u e There is a 90° phase delay between them.

[0045] The signal demodulation unit uses a multiplication phase-sensitive demodulation method to realize the modulation signal u in the AC excitation capacitor conversion unit. g The demodulation includes a multiplicative phase-sensitive demodulator and an 8th-order active low-pass filter, such as Figure 3 As shown, the multiplication phase-sensitive demodulator in this embodiment adopts a precision, high-speed four-quadrant analog multiplier AD734 of ADI Company, which has a wide signal input range, a full-scale exponentiation calculation error of less than 0.1%, stable operation, little influence by temperature and power supply voltage fluctuations, and low noise: at 10Hz~100MHz, the signal-to-noise ratio reaches 70dB, and the X1 pin of AD734 is connected to the modulation signal, that is, the output signal u of the AC amplifier. g , the Y1 pin is connected to the sine wave reference signal SINr, the X2, Y2, U1, U2, U3 and Z2 pins are connected to the instrument ground respectively, the Z1 pin is connected to the W pin, the DD and ER pins are left floating, and the VP and VN pins are connected to the positive and negative 15V power supply respectively. Under this configuration, the modulation signal and the reference signal can be directly multiplied, and the transfer function of the circuit is:

[0046]

[0047] In formula (5), u gis the output voltage of the AC amplifier, u r The second sine wave signal SINr is output by the signal generating unit, u p is the output signal of the multiplication phase-sensitive demodulator, let Substituting formula (4) into formula (5) yields:

[0048]

[0049] Calculated using the product-sum-difference formula of the sine and cosine functions:

[0050]

[0051] It can be seen from formula (7) that the DC voltage signal containing the measured capacitance information can be obtained by filtering out the frequency component represented by the high-frequency term in the formula using a low-pass filter. At the same time, the phase difference of the two sinusoidal signals output by the signal generating unit is adjusted so that the second sinusoidal signal SINr has a 90° phase delay relative to the first sinusoidal signal SINg. At this time, the demodulation efficiency is the highest, and the output after filtering by the low-pass filter is:

[0052]

[0053] In formula (8), u m This is the DC voltage output finally obtained by the AC excitation weak capacitance detection circuit, which is equivalent to the capacitance to be measured C of the parallel double-needle probe. x Directly proportional.

[0054] The low-pass filter in this embodiment is an 8th-order active low-pass filter. Since the operating frequency of this system is 20kHz, it can be seen from formula (7) that in order to obtain the DC component, the required 8th-order active low-pass filter is mainly used to attenuate the high-order harmonics with a frequency of 40kHz, and the signal at 40kHz is at least attenuated to one ten-thousandth of the fundamental frequency signal to suppress the influence of high-frequency harmonics. This embodiment uses the LTC1164-5 chip of LINEAR Company to realize the filtering function, which integrates an 8th-order active low-pass filter and has a clock-adjustable cut-off frequency. The ratio of the clock frequency to the cut-off frequency is 100:1. Figure 3 As shown in the figure, the 1MHz clock signal CLK / 1M is input from the 11th pin of the chip, which can configure the LTC1164-5 chip as an 8th-order active low-pass filter with a cutoff frequency of 10kHz. The output signal u of the multiplication phase-sensitive demodulator is input from the 2nd pin of the chip. p , Uout output from pin 9 of the chip is the DC voltage u in formula (8) m .

[0055] The measurement of moisture content of sapwood of living trees is affected by ambient temperature, while the temperature of trunks of living trees is affected by ambient temperature, but it is different from ambient temperature. In order to study the relationship between moisture content and temperature more accurately, the present invention adopts infrared temperature measurement technology to measure the temperature of trunks of living trees in a non-contact manner, and uses the relative dielectric constant ε of sapwood of living trees to measure the moisture content of sapwood of living trees. r The relationship between the temperature T of the trunk of the living tree and the inversion model of the moisture content of the sapwood of the living tree is calibrated. The present invention measures the relative dielectric constant ε of the sapwood of the living tree on site. r_f Relative dielectric constant ε of standing wood sapwood adjusted to 25℃ r_a , and the relationship is:

[0056] ε r_a =ε r_f +0.3572(T-25)-8.250×10 -4 (T-25) 2 -1.000×10 -6 (T-25) 3 (9)

[0057] Formula (9) can be used to perform temperature calibration on the measurement of moisture content of sapwood of living trees, thus improving the measurement accuracy of the instrument.

[0058] The parallel double-needle probe in the present invention and the medium between the double-needle probe are equivalent to a capacitor C x The size of the capacitor is related to the length L, diameter D, spacing d of the double-needle probe and the dielectric constant ε of the medium between the double-needle probes. Its capacitance is:

[0059]

[0060] In formula (9), ε=ε0ε r_f , ε0 is the dielectric constant of vacuum, ε0=8.85×10 -12 F / m, ε r_f is the relative dielectric constant of the medium between the double-needle probes. The medium between the double-needle probes is the sapwood of standing trees. The sapwood of standing trees is essentially a complex porous medium, that is, a combination of solid phase (wood), liquid phase (free water) and gas phase (air) with a certain structure. The relative dielectric constant of free water in the sapwood of standing trees is 81, which is much higher than the relative dielectric constant of wood (2) and the relative dielectric constant of air (1). That is, the relative dielectric constant of the sapwood of standing trees is mainly affected by its moisture content. The greater the moisture content, the greater the overall relative dielectric constant. Then, during the on-site measurement, according to the parameters of the parallel double-needle probe and the capacitance value obtained by the weak capacitance detection method, the relative dielectric constant ε of the measured standing tree sapwood is calculated by formula (10): r_f , the relative dielectric constant ε of the sapwood of standing trees at 25℃ is calculated by formula (9): r_aThen, the volume moisture content of the sapwood of the measured living tree can be calculated based on the inversion model of the moisture content of the sapwood of the living tree (11).

[0061] θ=-0.235+5.41×10 -2 ε r_a -1.30×10 -3 ε r_a 2 +1.171×10 -5 ε r_a 3 (11)

[0062] In formula (11), θ is the volume moisture content of the sapwood of the living tree, and the unit is (V / V)%. Different tree species have different inversion models. Different moisture content inversion models need to be used to measure the moisture content of different tree species. Formula (11) is the inversion model of red pine species.

[0063] The conversion relationship between mass moisture content W and volume moisture content θ is:

[0064]

[0065] In formula (12), ρ w is the density of water, ρ S The unit of dry soil density and mass moisture content W is (m / m)%.

[0066] like Figure 1 As shown, the DC voltage u output from the 8th order active low pass filter is mAfter being converted into digital signals by the digital-to-analog converter, they are transmitted to the FPGA control circuit. The FPGA transmits the data of these digital signals to the NIOS II soft-core processor through the Avalon bus. The infrared temperature measurement module sends the temperature data to the NIOS II soft-core processor. Then, equations (9), (10), (11) and (12) are written into the NIOS II soft-core processor through C language programming to automatically process the data. Then, the processed moisture content, temperature and other data are displayed on the LCD touch screen by driving the LCD driver. At the same time, the μCOS-III system is installed on the NIOS II soft-core processor to realize the human-computer interaction interface. The touch controller is used to realize the touch function of the LCD touch screen. The operator sets each module on the human-computer interaction interface through the touch screen, thereby completing the measurement of the moisture content of the sapwood of the living standing wood. In this embodiment, the FPGA control circuit adopts the EP4CE10F17C8N chip, the analog-to-digital converter can adopt the AD9226 chip, the LCD touch screen can adopt the TK043F1508 LCD screen, the LCD driver can adopt the NT35510 chip, the touch controller can adopt the FT5x16 chip, and the infrared temperature measurement module can adopt the MLX90614-ESF-DCH chip. The temperature measurement range is -70°C to 380°C. The circuit principle of this part is not the core technology of the present invention and will not be described in detail.

[0067] like Figure 5 As shown, this embodiment is a front view plane structure of a moisture content measuring instrument for sapwood of living trees based on weak capacitance detection method, including a parallel double needle probe 1, a double-layer shielded cable 2, a switch 3, a cable adapter 4, an infrared temperature measuring probe 5, an LCD touch screen 6, an instrument housing 7 and a temperature measuring button 8. The use process of the present invention is as follows: first, long press the switch 3 to turn on the instrument, set the tree species to be measured and the probe parameters by touching the LCD touch screen 6, select the position of the trunk of the living tree to be measured, and then adjust the temperature according to the length, diameter and other parameters of the parallel double needle probe 1. Two holes that can completely cover the parallel double-needle probe 1 are drilled on the trunk 9 of the living tree with an electric drill, and then the parallel double-needle probe 1 is inserted into the holes on the trunk 9 of the living tree. At this time, the measuring instrument is held by hand so that the infrared temperature measuring probe 5 is facing the epidermis of the trunk of the living tree. After pressing the temperature measuring button 8, the LCD touch screen 6 of the measuring instrument displays the temperature of the trunk of the living tree at the position to be measured and the volume moisture content, mass moisture content and dielectric constant of the sapwood of the living tree, thereby completing the measurement of the moisture content of the sapwood of the living tree.

[0068] The embodiments described above are merely descriptions of preferred implementation modes of the present invention and are not intended to limit the scope of the present invention. Without departing from the design spirit of the present invention, various modifications and improvements made to the technical solutions of the present invention by ordinary technicians in this field should all fall within the protection scope determined by the claims of the present invention.

Claims

1. A moisture content measuring instrument for sapwood of living trees based on weak capacitance detection method, characterized in that: Including FPGA control circuit, DDS module I, DDS module II, NIOSII soft core, infrared temperature measurement module, LCD driver, LCD touch screen, touch controller, sine wave circuit I, sine wave circuit II, drive cable I, parallel double needle probe, drive cable II, transimpedance amplifier, AC amplifier, multiplication phase-sensitive demodulator, 8th-order active low-pass filter and analog-to-digital converter; The drive cable I and drive cable II adopt the drive cable technology, that is, the connecting cable adopts a low distributed capacitance double-layer shielded cable, wherein the inner shielding layer of the cable is electrically connected to the instrument ground, and the outer shielding layer of the cable is electrically connected to the earth.

2. The moisture content measuring instrument for sapwood of living standing trees based on weak capacitance detection method according to claim 1, characterized in that: The DDS module I and DDS module II have the same internal structure, including a frequency control word, a phase control word, a clock module, a 32-bit accumulator I, a 32-bit register, a 32-bit accumulator II and a waveform memory. The functions of each internal module and the communication between modules are realized by FPGA through Verilog language programming; the NIOSII soft core is a 32-bit soft core processor inherent in the FPGA, and it communicates bidirectional data with the FPGA through the Avalon bus. The output end of the NIOSII soft core is electrically connected to the input end of the LCD driver, the output end of the LCD driver is electrically connected to the input end of the LCD touch screen, the output end of the LCD touch screen is electrically connected to the input end of the touch controller, and the output end of the touch controller and the output end of the infrared temperature measurement module are both electrically connected to the input end of the NIOS II soft core.

3. The moisture content measuring instrument for sapwood of living standing trees based on weak capacitance detection method according to claim 1, characterized in that: The sine wave circuit I and the sine wave circuit II have the same internal structure, including a digital-to-analog converter, a differential-to-single-ended circuit, a 7th-order passive low-pass filter and a post-stage amplifier circuit. The output end of the digital-to-analog converter is electrically connected to the input end of the differential-to-single-ended circuit, the output end of the differential-to-single-ended circuit is electrically connected to the input end of the 7th-order passive low-pass filter, and the output end of the 7th-order passive low-pass filter is electrically connected to the input end of the post-stage amplifier circuit.

4. The moisture content measuring instrument for sapwood of living standing trees based on weak capacitance detection method according to claim 1, characterized in that: The FPGA control circuit outputs a phase control word and a frequency control word to the DDS module I and the DDS module II. The output end of the waveform memory and the output end of the clock module in the DDS module I are electrically connected to the input end of the digital-to-analog converter in the sine wave circuit I. The output end of the post-stage amplifier circuit in the sine wave circuit I is electrically connected to the input end of the drive cable I. The output end of the drive cable I is electrically connected to the input end of the parallel double-needle probe. The output end of the parallel double-needle probe is electrically connected to the input end of the drive cable II. The output end of the drive cable II is electrically connected to the input end of the transimpedance amplifier. The output end of the device is electrically connected to the input end of the AC amplifier, the output end of the waveform memory in the DDS module II and the output end of the clock module are both electrically connected to the input end of the digital-to-analog converter in the sine wave circuit II, the output end of the post-stage amplifier circuit in the sine wave circuit II and the output end of the AC amplifier are both electrically connected to the input end of the multiplication phase-sensitive demodulator, the output end of the multiplication phase-sensitive demodulator is electrically connected to the input end of the 8th-order active low-pass filter, the output end of the 8th-order active low-pass filter is electrically connected to the input end of the analog-to-digital converter, and the output end of the analog-to-digital converter is electrically connected to the input end of the FPGA control circuit.

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