Portable wood moisture content detector

Abstract

The invention relates to the technical field of moisture content detection, in particular to a portable wood moisture content detector. The device comprises a power supply module, an induction module, a signal processing module and a display, the signal processing module comprises a microcontroller, a moisture content detection circuit and a temperature detection circuit, the moisture content detection circuit detects moisture content through an LC resonance circuit formed by an oscillation chip U1, an inductor L3 and a probe, and the temperature detection circuit utilizes a TL431 constant voltage source and a differential amplification circuit. The temperature sensor is calibrated through the adjustable resistor VR1 and the adjustable resistor VR2, and the microcontroller corrects temperature compensation parameters by combining data of the temperature sensor and the adjustable resistor VR2. According to the invention, through hardware calibration of the moisture content detection circuit and the temperature detection circuit and a segmented nonlinear compensation algorithm, reduction of detection precision can be effectively inhibited when the temperature changes drastically, and full-temperature-range high-precision detection is realized.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of water content detection, in particular to a portable wood moisture content detector. BACKGROUND

[0002] In the field of wood processing, storage and construction, the portable wood moisture content detector becomes the core tool for wood moisture management due to its lightweight, easy operation and on-site rapid detection characteristics. Currently, such devices mainly adopt two technical solutions of resistance method and capacitance method: the resistance method calculates the moisture content by measuring the surface resistance value of wood, which is simple in structure but can only detect the surface moisture and is easily disturbed by temperature and fiber direction; the capacitance method inverses the deep moisture content by sensing the dielectric constant change of wood, which breaks through the depth limitation of the resistance method, but the dielectric constant is seriously affected by temperature, resulting in a significant decrease in detection accuracy with temperature fluctuations.

[0003] When the temperature changes dramatically, the dielectric constant of wood and the temperature show a segmented nonlinear relationship, the dielectric constant in the low temperature zone is negatively correlated with the temperature, and in the high temperature zone, the dielectric constant sharply decreases due to the breaking of hydrogen bonds. The ordinary detection circuit cannot accurately adapt to this characteristic, resulting in large detection error fluctuations in different temperature environments. If a complex circuit architecture is used to achieve high-precision detection, not only the cost and failure rate are greatly increased, but also the increase in components and complex layout seriously hinder the integration and lightweight design of portable devices. SUMMARY

[0004] The present application aims to provide a portable wood moisture content detector to solve the problem of low calibration efficiency and large detection accuracy fluctuations with temperature fluctuations in the detection circuit of traditional portable wood moisture content detectors.

[0005] To achieve the above-mentioned purpose, a portable wood moisture content detector is provided, which comprises a power module, a sensing module, a signal processing module and a display, the signal processing module comprises a microcontroller, a moisture content detection circuit and a temperature detection circuit, wherein: The moisture content detection circuit comprises an oscillation chip U1, one end of an inductor L3 is connected to the pin TANK of the oscillation chip U1, two probes are respectively connected to the two ends of the inductor L3, the probes are inserted into wood to form a probe capacitor Ct, and the probe capacitor Ct and the inductor L3 form an LC resonance circuit; The temperature detection circuit comprises a constant voltage source based on a precision reference source TL431 and a differential amplification operational amplifier U2, the constant voltage source sets a reference voltage through a first adjustable resistor VR1 to provide stable excitation for the temperature sensor, and the differential amplification circuit configures the bridge balance of the temperature sensor through a second adjustable resistor VR2; The microcontroller is connected with the output of the oscillation chip U1 and the output of the temperature detection circuit respectively, and the temperature compensation parameter of the water content calculation is dynamically corrected by using the calibration mechanism of the adjustable resistor VR1 and the adjustable resistor VR2.

[0006] In the technical solution, the LC resonance loop composed of the oscillation chip U1, the inductor L3 and the probe detects the water content by using the change of the resonance frequency caused by the change of the dielectric constant of the probe capacitor Ct due to the change of the water content of the wood, and selects the high-frequency oscillation chip to utilize the characteristic that the temperature coefficient of the dielectric constant of the high-frequency signal in the wood is more stable, thereby reducing the nonlinear interference when the temperature changes greatly. The temperature detection circuit utilizes the reference source TL431 and the differential amplification operational amplifier U2, the reference end of the reference source TL431 is connected with the adjustable resistor VR1, a precise constant voltage source is formed, the adjustable resistor VR2 is configured to balance the bridge, and the temperature sampling accuracy is ensured. Finally, the microcontroller combines the data of the two, dynamically corrects the temperature compensation parameter by using the calibration mechanism and the segmented nonlinear compensation algorithm, and realizes the high-precision detection in the whole temperature range.

[0007] On this basis, the pin AGC of the oscillation chip U1 is connected with the capacitor C4 and the adjustable resistor Rx, and the other end of the capacitor C4 and the adjustable resistor Rx is grounded.

[0008] In the technical solution, the pin AGC of the oscillation chip U1 is connected with the capacitor C4 and the adjustable resistor Rx, and the pin AGC of the oscillation chip U1 is connected with the capacitor C4 and the adjustable resistor Rx, and the other end of the capacitor C4 and the adjustable resistor Rx is grounded.

[0009] In another technical solution, the cathode of the reference source TL431 is connected with the resistor R8, the other end of the resistor R8 is connected with the input power supply, the reference end of the reference source TL431 is connected with the adjustable resistor VR1, the anode of the reference source TL431 and the other end of the adjustable resistor VR1 are grounded. The pin 1 of the interface P1 is connected with the resistor R3, the other end of the resistor R3 is connected with the positive input end of the operational amplifier U2 and the resistor R5, the other end of the resistor R5 is grounded, the adjustable resistor VR2 and the adjustable end thereof are connected with the resistor R4, and the other end of the resistor R4 is connected with the inverting input end of the operational amplifier U2.

[0010] In the technical solution, the reference source TL431 is connected to an input power supply through a resistor R8, and a reference end is connected to an adjustable resistor VR1 adjustment end, forming a precise constant voltage source, and using the low temperature coefficient characteristic of TL431 to provide stable excitation for the temperature sensor, compared with the traditional resistance voltage division type power supply, the excitation voltage fluctuation is reduced. The interface P1 is connected to the temperature sensor to form a Wheatstone bridge, resistors R1 and R2 and an adjustable resistor VR2 form a bridge arm, and by adjusting VR2, the bridge is in a balanced state, which can eliminate the influence of the sensor lead resistance on the measurement, and the operational amplifier U2 adopts a differential amplification architecture, which uses its high common mode rejection ratio to suppress common mode signals such as environmental electromagnetic interference, and improves the signal-to-noise ratio of the temperature signal.

[0011] Compared with the prior art, the beneficial effects of the present application are: In the portable wood moisture content detector, by using a high-frequency oscillation chip U1, using the characteristic that the dielectric constant temperature coefficient of high-frequency signals in wood is more stable, the non-linear interference when the temperature changes sharply is reduced, and then through the hardware calibration mechanism in the temperature detection circuit, the reference source TL431 sets the reference voltage through the adjustable resistor VR1, and provides stable excitation for the temperature sensor, and the differential amplification circuit configures the bridge balance through the adjustable resistor VR2, and finally the temperature compensation parameters are dynamically corrected by the microcontroller, realizing low error detection in the whole temperature range and increasing the detection accuracy when the temperature changes sharply. BRIEF DESCRIPTION OF DRAWINGS

[0012] Figure 1 The overall structure diagram of the present application is shown in the figure; Figure 2 The structure schematic diagram of the moisture content detection circuit of the present application is shown in the figure; Figure 3 The structure schematic diagram of the temperature detection circuit of the present application is shown in the figure. DETAILED DESCRIPTION

[0013] The technical solutions in the embodiments of the present application will be described clearly and completely in combination with the drawings in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, not all. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor are within the scope of protection of the present application.

[0014] Embodiment 1 Please refer to Figure 1The embodiment aims to provide a portable wood moisture content detector, which comprises a power module, a sensing module, a signal processing module and a display. The signal processing module adopts a double-layer PCB layout, the top layer of which is integrated with a high-frequency oscillation chip U1, an operational amplifier U2 and peripheral circuits, wherein the inductor L3 is directly connected to the TANK pin of the oscillation chip U1 through a short path to optimize the high-frequency characteristics, and the bottom layer is fixed with a microcontroller U3 and a display. The adjustable resistors VR1 (reference voltage calibration) and VR2 (bridge balance calibration) and the adjustable resistor Rx are vertically assembled at the edge of the PCB, the adjustable resistors VR1 and VR2 are adjusted at the corresponding reserved calibration holes of the shell, and after debugging, the adjustable resistors VR1 and VR2 are fixed by sealing materials to prevent accidental touch.

[0015] The adjustable resistor Rx is a digital potentiometer, and its I 2 The C interface is connected to the microcontroller U3, and the microcontroller U3 adjusts the equivalent resistance value in real time through a built-in algorithm. When the temperature is lower than 30℃, the microcontroller U3 controls the adjustable resistor Rx to set the resistance value to 220Ω (low-temperature adaptation mode), reduces the resistance value to increase the gain of the oscillation chip U1, compensates for the decrease of the Q value of the LC loop at low temperature, and maintains the stability of the output amplitude; when the temperature reaches or is higher than 30℃, the microcontroller U3 controls the adjustable resistor Rx to set the resistance value to 1000Ω (high-temperature compensation mode), increases the resistance value to reduce the chip gain, avoids the distortion caused by the too strong signal at high temperature, and compensates for the sudden drop of the dielectric constant of wood due to the breaking of hydrogen bonds.

[0016] The sensing module is composed of two parallel hard alloy probes, the diameter of which is 3mm, the length of which is 40mm, and the distance between the probes is 8mm. The probes are strictly fixed by a mechanical structure and are connected to the main circuit board through a shielding cable. The temperature sensor is integrated in one of the probes, and the space between the probe metal body and the temperature sensor is filled with insulating and heat-conducting materials. The temperature sensor lead is designed to pass through the probe to resist interference.

[0017] The power module is built-in rechargeable battery, which is placed at the bottom of the handle cavity of the device and connected to the circuit through elastic contact. The voltage stabilizing circuit adopts a hierarchical power supply structure, which provides stable voltage for digital circuits and high-frequency oscillation circuits respectively. The main body of the shell is made of lightweight engineering plastic, the front cover is embedded with the display, the rear cover is provided with a detachable battery compartment, the handle area is covered with anti-slip material, the probe interface is provided with a sealing structure to meet the protection requirements, and the whole machine adopts a compact design, and the key areas are covered with electromagnetic shielding structure.

[0018] The core of the signal processing module is composed of a microcontroller U3, a moisture content detection circuit and a temperature detection circuit. In the moisture content detection circuit, the oscillation chip U1 uses a high-frequency oscillation chip with model MC1648, the TANK pin of the oscillation chip U1 is connected to one end of the inductor L3, and the two probes are respectively connected to the two ends of the inductor L3, when the probes are inserted into the wood, a probe capacitor Ct is formed, which together with the inductor L3 forms an LC resonance circuit, the VCC pin of the oscillation chip U1 is connected to the ground through the parallel connection of the capacitor C1 and the capacitor C2 for filtering, the OUT pin is connected to the ground through the capacitor C3 and outputs a frequency signal to the counter pin of the microcontroller U3 through the signal conditioning circuit, the signal conditioning circuit includes an RC low-pass filter network and a Schmidt trigger, the low-pass filter network is used to filter out high-frequency noise, and the Schmidt trigger is used to shape the sine wave or non-ideal square wave output by the oscillator into a digital square wave signal with steep edges, so as to improve the accuracy and anti-interference ability of the frequency count of the microcontroller. At the same time, the BIAS pin is connected to the other end of the inductor L3 and the ground capacitor C5, and the AGC pin is connected to the ground through the capacitor C4 and the adjustable resistor Rx to maintain the stability of the oscillation amplitude, the oscillation frequency of the LC circuit changes with the moisture content of the wood, and the microcontroller U3 calculates the initial moisture content value by monitoring the frequency offset.

[0019] The temperature detection circuit uses a precision reference source TL431 to build a constant voltage source, the cathode of the reference source TL431 is connected to the input power supply through the resistor R8, the anode is directly grounded, the other end of the adjustable resistor VR1 is grounded, the reference end is connected to the adjustable resistor VR1 to set the precise constant voltage value, and the resistance value of the adjustable resistor VR1 is adjusted, and the standard working equation of the TL431 reference voltage source (i.e. the divider formula) is used to set the reference output voltage Vref. During factory calibration, the Vref is monitored by a precision voltmeter, and the adjustable resistor VR1 is adjusted to accurately reach the designed value (2.500V), so as to provide a stable and accurate excitation voltage for the temperature sensor.

[0020] The reference source TL431 supplies power to the temperature sensor (PIN1 pin of the interface P1) through the resistor R1, and forms one arm of the Wheatstone bridge through the resistor R2 and the adjustable resistor VR2. The other arm of the bridge is formed by connecting the interface P1 to the temperature sensor (PT1000). During calibration (at the reference temperature of 25℃), the resistance value of the adjustable resistor VR2 is adjusted, and the balance condition equation of the Wheatstone bridge is applied to make the bridge reach a balanced state. At this time, the bridge output is zero or minimum voltage, which eliminates the influence of the sensor lead resistance and initial offset, and ensures the accuracy of the temperature measurement zero point.

[0021] The bridge output signal is transmitted to the positive input terminal of the operational amplifier U2 through the resistor R3, and the inverting input terminal of the operational amplifier U2 is connected through the resistor R4, and the output terminal of the operational amplifier U2 is connected through the resistor R7, the diode D1 and the filter capacitor C9 in series, and then connected to the ADC pin of the microcontroller U3 through the analog-to-digital converter.

[0022] The configuration can eliminate the influence of the temperature sensor lead resistance, and the temperature signal signal-to-noise ratio is improved to more than 60dB through the differential amplification architecture of the operational amplifier U2, and the linear characteristics of temperature and voltage conversion are ensured.

[0023] The present application realizes dynamic temperature compensation through the cooperative mechanism of hardware calibration and software algorithm. When the probe is inserted into wood, the probe capacitor Ct formed by the inductor L3 and the wood medium constitutes an LC resonant circuit. The oscillation signal generated by the circuit is converted into a square wave signal output to the microcontroller U3 through the high-frequency oscillation chip U1. The microcontroller U3 measures the current oscillation frequency in real time through the built-in counter, and calculates the relative offset thereof with respect to the pre-stored reference frequency. At the same time, the PT1000 temperature sensor integrated in the probe outputs a temperature signal through a Wheatstone bridge, which is conditioned by the differential amplification operational amplifier U2, and then converted into a digital temperature value T (unit: ℃) by the ADC module of the microcontroller U3.

[0024] The microcontroller U3 dynamically executes a segmented non-linear compensation algorithm according to the real-time temperature T: When the real-time temperature T is lower than 30℃, a low-temperature linear compensation model is enabled, and the expression is ; Wherein: α is the low-temperature compensation coefficient (determined by the calibration values of the adjustable resistor VR1 and the adjustable resistor VR2); 25 represents the reference temperature reference point (unit: ℃).

[0025] This model accurately corresponds to the linear negative correlation characteristics of the dielectric constant of wood in the low-temperature region with temperature, and the compensation amount decreases by α units for every 1℃ increase in temperature.

[0026] When the temperature T reaches or exceeds 30℃, the system is switched to a high-temperature non-linear correction model by the adjustable resistor Rx: ; Wherein: β and γ are high-temperature correction parameters (fixedly stored in the FLASH memory of U3 after factory calibration); e is the natural constant (approximately equal to 2.71828).

[0027] This exponential function specially compensates for the nonlinear dramatic change in dielectric constant above 30℃ due to the breaking of hydrogen bonds in wood.

[0028] In the transition temperature zone of 28℃≤T<32℃, to prevent numerical mutation caused by model switching, a weighted fusion algorithm is adopted: ; wherein: w is the weight coefficient, which is dynamically calculated as w=(32-T) / 4, and T is the real-time temperature value.

[0029] The final moisture content calculation is completed through the core formula: ; wherein: k is the moisture content conversion coefficient (stored in U3 after calibration); Δf / fo represents the relative offset of the current oscillation frequency f relative to the reference frequency fo; b(T) is the temperature compensation term mentioned above; MC represents the compensated moisture content value (unit: %).

[0030] The compensated moisture content value is calculated and the result is output to the display. In the workflow, the LC circuit frequency change after the probe contacts the wood is converted into a digital signal by the oscillation chip U1, and the temperature signal is amplified and filtered by the operational amplifier U2. The microcontroller U3 fuses the two-way data to execute the compensation algorithm. Through the cooperative mechanism of hardware calibration adjustable resistors VR1 and VR2 and software segmented compensation, the detection error is reduced in the temperature range of-10℃ to 60℃.

[0031] Example 2 Based on the temperature compensation architecture and segmented nonlinear algorithm, combined with the capacitance method calibration method, the optimized moisture content detection system is verified in the full temperature range. The experimental sample is a poplar block with a size of 10cm×10cm×10cm. The moisture content gradient is set to 10%, 30%, 50%, and 70%. The constant temperature box is set to-10℃, 25℃, 40℃, and 60℃ respectively to simulate the typical high and low temperature environment in wood processing.

[0032] In the hardware calibration link, the adjustable resistor VR1 is adjusted to accurately lock the TL431 reference voltage at 2.500V±1mV, eliminating the influence of temperature drift on the sensor excitation voltage. At the same time, the adjustable resistor VR2 is adjusted to make the PT1000 bridge reach the balance state at 25℃. The measured common-mode rejection ratio is improved to 62dB, and the temperature sampling error is compressed to ±0.3%.

[0033] At the algorithm execution level, the microcontroller U3 automatically controls the resistance value of the digital potentiometer (i.e. adjustable resistor Rx) according to the measured temperature to select a compensation mode. When a low-temperature environment (below 30℃) is detected, the Rx resistance value is set to 220Ω (low-temperature adaptation mode), the gain of the oscillation chip is increased, and the problem of signal attenuation caused by low temperature is effectively compensated. When a high-temperature environment (above 30℃) is detected, the adjustable resistor Rx resistance value is set to 1000Ω (high-temperature compensation mode), the chip gain is actively reduced, and the signal overload distortion caused by high temperature is prevented. For the critical transition region of 28℃ to 32℃, the microcontroller directly uses a weighted fusion algorithm to ensure smooth transition of the temperature critical point measurement value.

[0034] The calibration results show that the determination coefficient of the water content calculation formula under the 25℃ reference is 0.991. The full-temperature range test data are as follows: Table 1 Full-temperature range test results Temperature (°C) Moisture content (%) Uncompensated error (%) Compensated error (%) Algorithm contribution value (%) Compensation type -10 30 -8.2 -1.3 +6.9 Low temperature linear compensation 25 50 +0.7 +0.2 -0.5 Reference point does not need compensation 40 50 +6.5 +0.9 -5.6 High temperature exponential suppression 60 50 +11.7 -1.1 -12.8 Hydrogen bond breaking correction Experiments show that the temperature compensation mechanism compresses the full-temperature range detection error from ±11.7% to ±1.3%, which improves the accuracy by 89% compared to the non-compensation working condition (-8.2%—+11.7%). Under the condition of 60℃ high temperature, the contribution value of the compensation algorithm is-12.8%, which effectively offsets the dramatic change effect of the dielectric constant, and verifies the synergistic value of hardware calibration and segmented algorithm.

[0035] The above shows and describes the basic principles, main features and advantages of the present application. Those skilled in the art should understand that the present application is not limited to the above examples, and the above examples and descriptions in the specification are only preferred examples of the present application and are not intended to limit the present application. Without departing from the spirit and scope of the present application, various changes and improvements can be made to the present application, and these changes and improvements all fall within the scope of the claimed present application. The scope of protection of the present application is defined by the appended claims and their equivalents.

Claims

1. A portable wood moisture content detector, comprising a power module, a sensing module, a signal processing module, and a display, characterized in that: The signal processing module includes a microcontroller, a moisture content detection circuit, and a temperature detection circuit, wherein: The moisture content detection circuit includes a high-frequency oscillation chip U1. The pin TANK of the oscillation chip U1 is connected to one end of an inductor L3. The two ends of the inductor L3 are respectively connected to two probes. The probes are inserted into the wood to form a probe capacitor Ct. The probe capacitor Ct and the inductor L3 form an LC resonant circuit. The temperature detection circuit incorporates a constant voltage source based on the precision reference source TL431 and a differential amplifier U2. The constant voltage source sets the reference voltage through the first adjustable resistor VR1 to provide a stable excitation for the temperature sensor. The differential amplifier circuit configures the bridge balance of the temperature sensor through the second adjustable resistor VR2. The microcontroller is connected to the output of the oscillation chip U1 and the output of the temperature detection circuit, respectively. It uses the calibration mechanism of adjustable resistors VR1 and VR2 to dynamically correct the temperature compensation parameters for moisture content calculation.

2. The portable wood moisture content detector according to claim 1, characterized in that: The VCC pin of the oscillator chip U1 is connected to the input power supply. The VCC pin of the oscillator chip U1 is also connected to capacitor C1. Capacitor C1 is connected in parallel with capacitor C2. The other ends of capacitors C1 and C2 are grounded. The OUT pin of the oscillator chip U1 is connected to capacitor C3. The other end of capacitor C3 is grounded. The OUT pin of the oscillator chip U1 is connected to the counter input pin of the microcontroller through a signal conditioning circuit.

3. The portable wood moisture content detector according to claim 2, characterized in that: The BIAS pin of the oscillator chip U1 is connected to the other end of the inductor L3 and the capacitor C5. The other end of the capacitor C5 is grounded. The AGC pin of the oscillator chip U1 is connected to the capacitor C4 and the adjustable resistor Rx. The other ends of the capacitor C4 and the adjustable resistor Rx are grounded.

4. The portable wood moisture content detector according to claim 1, characterized in that: The cathode of the reference source TL431 is connected to resistor R8, the other end of resistor R8 is connected to the input power supply, the reference terminal of the reference source TL431 is connected to the adjustment terminal of the adjustable resistor VR1, and the anode of the reference source TL431 and the other end of the adjustable resistor VR1 are grounded.

5. The portable wood moisture content detector according to claim 4, characterized in that: The other end of resistor R8 is also connected to resistors R1 and R2. The other end of resistor R1 is connected to pin PIN1 of interface P1. The other end of resistor R2 is connected to adjustable resistor VR2. The other end of adjustable resistor VR2 is connected to pin PIN2 of interface P1. The two pins of interface P1 are respectively connected to the two ends of the temperature sensor.

6. The portable wood moisture content detector according to claim 5, characterized in that: The PIN1 pin of interface P1 is connected to resistor R3. The other end of resistor R3 is connected to the non-inverting input of operational amplifier U2 and resistor R5. The other end of resistor R5 is grounded. The adjustable resistor VR2 and its adjustment terminal are both connected to resistor R4. The other end of resistor R4 is connected to the inverting input of operational amplifier U2.

7. The portable wood moisture content detector according to claim 6, characterized in that: The output terminal of the operational amplifier U2 is connected to resistor R7. The other end of resistor R7 is connected to the anode of diode D1 and capacitor C9. The cathode of diode D1 and the other end of capacitor C9 are grounded. The other end of resistor R7 is also connected to the analog-to-digital conversion input pin of the microcontroller through an analog-to-digital converter.

8. The portable wood moisture content detector according to claim 1, characterized in that: The sensing module includes a probe and a temperature sensor. There are two probes arranged in parallel. The temperature sensor is integrated inside one of the probes and electrically isolated by an insulating thermally conductive material.

9. The portable wood moisture content detector according to claim 1, characterized in that: The power module includes a lithium battery, a power switch, and a voltage regulator circuit. The lithium battery supplies power to each module through the voltage regulator circuit.

10. The portable wood moisture content detector according to claim 1, characterized in that: The microcontroller's data output terminal is connected to the display for transmitting moisture content and temperature data.

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