A portable wood moisture content detector

Abstract

The present application relates to the technical field of moisture content detection, in particular to 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 comprises a microcontroller, a moisture content detection circuit and a temperature detection circuit, the moisture content detection circuit detects the moisture content through an LC resonance loop composed of an oscillation chip U1, an inductor L3 and a probe, the temperature detection circuit utilizes a TL431 constant voltage source and a differential amplification circuit, calibrates a temperature sensor through an adjustable resistor VR1 and an adjustable resistor VR2, and the microcontroller corrects temperature compensation parameters in combination with data of both. The present application can effectively inhibit the decrease of detection accuracy when the temperature changes dramatically through hardware calibration of the moisture content detection circuit and the temperature detection circuit and a segmented nonlinear compensation algorithm, and realizes high-precision detection in the whole temperature range.

Description

Technical Field

[0001] This invention relates to the field of moisture content detection technology, and more specifically, to a portable wood moisture content detector. Background Technology

[0002] In the fields of wood processing, warehousing, and construction, portable wood moisture content analyzers have become core tools for wood moisture management due to their lightweight, ease of operation, and rapid on-site testing capabilities. Currently, these devices mainly employ two technical solutions: the resistance method and the capacitance method. The resistance method calculates moisture content by measuring the surface resistance of the wood. While simple in structure, it can only detect surface moisture and is easily affected by temperature and fiber orientation. The capacitance method, on the other hand, inversely determines deeper moisture content by sensing changes in the wood's dielectric constant. Although it overcomes the depth limitations of the resistance method, the accuracy of the detection decreases significantly with temperature fluctuations because the dielectric constant is severely affected by temperature.

[0003] When temperatures change drastically, the dielectric constant of wood exhibits a piecewise nonlinear relationship with temperature. In the low-temperature region, the dielectric constant shows a linear negative correlation with temperature, while in the high-temperature region, the dielectric constant drops sharply due to hydrogen bond breakage. Ordinary detection circuits struggle to accurately adapt to this characteristic, resulting in large fluctuations in detection errors under different temperature environments. If a complex circuit architecture is used to achieve high-precision detection, it not only significantly increases costs and failure rates but also severely hinders the integration and lightweight design of portable devices due to the increased number of components and complex layout. Summary of the Invention

[0004] The purpose of this invention is to provide a portable wood moisture content detector to solve the problems of large fluctuations in detection accuracy with temperature and low calibration efficiency in the detection circuit of traditional portable wood moisture content detectors.

[0005] To achieve the above objectives, a portable wood moisture content detector is provided, comprising a power module, a sensing module, a signal processing module, and a display. The signal processing module includes a microcontroller, a moisture content detection circuit, and a temperature detection circuit, wherein:

[0006] The moisture content detection circuit includes an 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.

[0007] 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.

[0008] 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.

[0009] In the above technical solution, an LC resonant circuit composed of an oscillating chip U1, an inductor L3, and a probe is used to detect moisture content by utilizing the change in the dielectric constant of the probe capacitance Ct caused by the change in wood moisture content, which leads to a shift in the resonant frequency. Simultaneously, a high-frequency oscillating chip is selected to leverage the more stable temperature coefficient of the dielectric constant in wood due to high-frequency signals, reducing nonlinear interference during rapid temperature changes. The temperature detection circuit utilizes a reference source TL431 and a differential operational amplifier U2. The reference terminal of the TL431 is connected to the adjustment terminal of an adjustable resistor VR1, forming a precision constant voltage source. A bridge circuit is balanced using the adjustable resistor VR2 to ensure temperature sampling accuracy. Finally, the microcontroller combines the data from both sources and uses a calibration mechanism and a piecewise nonlinear compensation algorithm to dynamically correct the temperature compensation parameters, achieving high-precision detection across the entire temperature range.

[0010] Based on this, the AGC pin of the oscillator chip U1 is connected to capacitor C4 and adjustable resistor Rx, and the other end of capacitor C4 and adjustable resistor Rx is grounded.

[0011] In this technical solution, the AGC pin of the oscillator chip U1 is connected to capacitor C4 and adjustable resistor Rx to form a gain control circuit. When the moisture content of the wood is too high, causing the Q value of the LC circuit to decrease and the oscillation amplitude to decrease, the AGC circuit adjusts the feedback of the internal gain amplifier of the chip through the adjustable resistor Rx to stabilize the output amplitude within the range of ±3Vpp. The AGC response threshold can also be adjusted according to the loss characteristics of different types of wood.

[0012] In another technical solution, 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.

[0013] 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.

[0014] In this technical solution, the reference source TL431 is connected to the input power supply via resistor R8, and the reference terminal is connected to the adjustment terminal of the adjustable resistor VR1, forming a precision constant voltage source. Utilizing the low temperature coefficient of the TL431, a stable excitation is provided for the temperature sensor, reducing excitation voltage fluctuations compared to traditional resistor-based voltage divider power supplies. The adjustable resistor VR1 is used to calibrate the reference voltage. When ambient temperature or component aging causes the TL431 output to drift, adjusting VR1 can control the excitation voltage error, ensuring the linearity of the temperature sensor.

[0015] Interface P1 connects to the temperature sensor to form a Wheatstone bridge. Resistors R1, R2 and adjustable resistor VR2 form bridge arms. By adjusting VR2, the bridge is balanced, which can eliminate the influence of the sensor lead resistance on the measurement. 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 improve the signal-to-noise ratio of the temperature signal.

[0016] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0017] This portable wood moisture content detector utilizes a high-frequency oscillation chip U1, taking advantage of the more stable temperature coefficient of the dielectric constant in wood due to the high-frequency signal. This reduces nonlinear interference during rapid temperature changes. Furthermore, through a hardware calibration mechanism in the temperature detection circuit, the reference source TL431 is set with a reference voltage via an adjustable resistor VR1 to provide stable excitation for the temperature sensor. The differential amplifier circuit is configured with bridge balance via an adjustable resistor VR2. Finally, the microcontroller dynamically corrects the temperature compensation parameters, achieving low-error detection across the entire temperature range and increasing detection accuracy during rapid temperature changes. Attached Figure Description

[0018] Figure 1 This is a block diagram of the overall structure of the present invention;

[0019] Figure 2 This is a schematic diagram of the moisture content detection circuit of the present invention;

[0020] Figure 3 This is a schematic diagram of the temperature detection circuit of the present invention. Detailed Implementation

[0021] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0022] Example 1

[0023] Please see Figure 1As shown, the purpose of this embodiment is to provide a portable wood moisture content detector, including 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 integrates a high-frequency oscillator chip U1, an operational amplifier U2, and peripheral circuits. The inductor L3 is directly connected to the TANK pin of the oscillator chip U1 via a short path to optimize high-frequency characteristics. The bottom layer fixes the microcontroller U3 and the display. Adjustable resistors VR1 (reference voltage calibration), VR2 (bridge balance calibration), and Rx are vertically mounted on the edge of the PCB. The adjustment terminals of adjustable resistors VR1 and VR2 correspond to the calibration holes reserved in the outer shell. After debugging, they are sealed and fixed with sealing material to prevent accidental activation.

[0024] The adjustable resistor Rx is a digital potentiometer, and its I... 2 The interface C connects to the microcontroller U3, which adjusts the equivalent resistance in real time using its built-in algorithm. When the temperature is below 28℃, the microcontroller U3 controls the adjustable resistor Rx to set the resistance to 220Ω (low temperature adaptation mode). By reducing the resistance value, the gain of the oscillator chip U1 is increased, compensating for the decrease in the Q value of the LC circuit at low temperatures and maintaining stable output amplitude. When the temperature reaches or exceeds 32℃, the microcontroller U3 controls the adjustable resistor Rx to set the resistance to 1000Ω (high temperature compensation mode). This increases the resistance value and reduces the chip gain, preventing excessive signal strength and distortion at high temperatures. Simultaneously, the microcontroller algorithm compensates for the sudden drop in dielectric constant caused by hydrogen bond breakage in the wood.

[0025] The sensing module consists of two parallel carbide probes, each 3mm in diameter and 40mm in length, with an 8mm spacing between them. They are mechanically secured and connected to the main circuit board via shielded cables. The temperature sensor is integrated into one of the probes, with insulating and thermally conductive material filling the space between the sensor and the probe's metal body. The temperature sensor leads are designed to withstand interference as they extend through the probe.

[0026] The power module houses a rechargeable battery located at the bottom of the device's handle cavity and connects to the circuitry via flexible contacts. The voltage regulator circuit employs a tiered power supply architecture, providing stable voltages to both the digital circuitry and the high-frequency oscillation circuitry. The main body of the casing is made of lightweight engineering plastic, with a display embedded in the front cover, a removable battery compartment on the rear cover, and the handle area covered with anti-slip material. The probe interface features a sealed structure to meet protection requirements. The entire device boasts a compact design, with critical areas covered by electromagnetic shielding.

[0027] The core of the signal processing module consists of a microcontroller U3, a moisture content detection circuit, and a temperature detection circuit. In the moisture content detection circuit, the oscillator chip U1 uses a high-frequency oscillator chip of model MC1648. The TANK pin of the oscillator chip U1 is connected to one end of inductor L3, and two probes are connected to the two ends of inductor L3 respectively. When the probes are inserted into the wood, a probe capacitance Ct is formed. This capacitance Ct and inductor L3 together form an LC resonant circuit. The VCC pin of the oscillator chip U1 is grounded and filtered through parallel capacitors C1 and C2. The OUT pin is grounded through capacitor C3 and outputs a frequency signal to the counter pin of the microcontroller U3 through a signal conditioning circuit. The signal conditioning circuit includes an RC low-pass filter network and a Schmitt trigger. The low-pass filter network is used to filter out high-frequency noise, and the Schmitt trigger is used to shape the quasi-sine wave or non-ideal square wave output by the oscillator into a digital square wave signal with steep edges, thereby improving the accuracy of the microcontroller's frequency counting and its anti-interference capability. Meanwhile, the BIAS pin is connected to the other end of the inductor L3 and the grounding capacitor C5, while the AGC pin is grounded through capacitor C4 and adjustable resistor Rx to maintain stable 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.

[0028] The temperature detection circuit uses a precision reference source TL431 to construct a constant voltage source. The cathode of the TL431 reference source is connected to the input power supply via resistor R8, the anode is directly grounded, and the other end of the adjustable resistor VR1 is grounded. The reference terminal is connected to the adjustment terminal of the adjustable resistor VR1 to set a precise constant voltage value. By adjusting the resistance value of the adjustable resistor VR1, and according to the standard operating equation (i.e., the voltage divider formula) of the TL431 reference voltage source, its reference output voltage Vref is set. During factory calibration, Vref is monitored using a precision voltmeter, and the adjustable resistor VR1 is adjusted to accurately reach the design value (2.500V), providing a stable and accurate excitation voltage for the temperature sensor.

[0029] The TL431 reference source supplies power to the temperature sensor (pin 1 of interface P1) via resistor R1, and simultaneously forms one arm of a Wheatstone bridge via resistor R2 and adjustable resistor VR2. Interface P1 connects to the temperature sensor (PT1000), forming the other arm of the bridge. During calibration (performed at a reference temperature of 25°C), the resistance of the adjustable resistor VR2 is adjusted, and the balance condition equation of the Wheatstone bridge is applied to bring the bridge to a balanced state. At this point, the bridge output is zero or at its minimum voltage, eliminating the influence of sensor lead resistance and initial offset, ensuring accurate zero-point temperature measurement.

[0030] The bridge output signal is transmitted to the non-inverting input of operational amplifier U2 via resistor R3, and is connected to the inverting input of operational amplifier U2 via resistor R4. The output of operational amplifier U2 is connected to the ADC pin of microcontroller U3 via resistor R7, diode D1 and filter capacitor C9.

[0031] This configuration eliminates the influence of the temperature sensor lead resistance and improves the temperature signal signal-to-noise ratio to over 60dB through the differential amplification architecture of operational amplifier U2, ensuring linear characteristics of temperature and voltage conversion.

[0032] This invention achieves dynamic temperature compensation through a collaborative mechanism of hardware calibration and software algorithms. When the probe is inserted into the wood, the probe capacitance Ct formed by the inductor L3 and the wood medium constitutes an LC resonant circuit. The oscillation signal generated by this circuit is converted into a square wave signal by the high-frequency oscillation chip U1 and output to the microcontroller U3. The microcontroller U3 measures the current oscillation frequency in real time through its built-in counter and calculates its relative offset to a pre-stored reference frequency. Simultaneously, the PT1000 temperature sensor integrated inside the probe outputs a temperature signal through a Wheatstone bridge. After being conditioned by the differential operational amplifier U2, the signal is converted into a digital temperature value T (unit: °C) by the ADC module of the microcontroller U3.

[0033] The microcontroller U3 dynamically executes a piecewise nonlinear compensation algorithm based on the real-time temperature T:

[0034] When the real-time temperature T is below 28℃, the low-temperature linear compensation model is activated, and its expression is:

[0035] ;

[0036] in:

[0037] α is the low-temperature compensation coefficient (determined by the calibration values ​​of adjustable resistors VR1 and VR2).

[0038] 25 represents the reference temperature point (unit: °C).

[0039] This model accurately corresponds to the linear negative correlation between the dielectric constant of wood and temperature in the low-temperature region. For every 1°C increase in temperature, the compensation amount decreases by α units.

[0040] When the temperature T reaches or exceeds 32℃, the system switches from the adjustable resistor Rx to the high-temperature nonlinear correction model:

[0041] ;

[0042] Where: β and γ are high temperature correction parameters (stored in the FLASH memory of U3 after factory calibration);

[0043] e is the natural constant (approximately 2.71828).

[0044] This exponential function is specifically designed to compensate for the nonlinear and dramatic change in dielectric constant caused by the breaking of hydrogen bonds in wood above 32°C.

[0045] In the transition temperature range of 28℃≤T<32℃, a weighted fusion algorithm is used to prevent numerical abrupt changes caused by model switching:

[0046] ;

[0047] Where w is the weighting coefficient, which is dynamically calculated according to w=(32-T) / 4, and T is the real-time temperature value.

[0048] The final moisture content calculation is completed using the core formula:

[0049] ;

[0050] Where: k is the moisture content conversion coefficient (stored in U3 after calibration);

[0051] Δf / fo represents the relative offset of the current oscillation frequency f relative to the reference frequency fo;

[0052] b(T) represents the temperature compensation term mentioned above;

[0053] MC represents the moisture content after compensation (unit: %).

[0054] The compensated moisture content value is calculated and the result is output to the display. In the workflow, after the probe contacts the wood, the frequency change of the LC circuit is converted into a digital signal by the oscillation chip U1, while the temperature signal is amplified and filtered by the operational amplifier U2. The microcontroller U3 integrates the two data streams to execute the compensation algorithm. Through the collaborative 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℃.

[0055] Example 2

[0056] Based on a temperature compensation architecture and a piecewise nonlinear algorithm, combined with a capacitance calibration method, the optimized moisture content detection system was validated across the entire temperature range. Poplar wood blocks measuring 10cm×10cm×10cm were used as samples in the experiment. Moisture content gradients of 10%, 30%, 50%, and 70% were set, and four temperature points of -10℃, 25℃, 40℃, and 60℃ were set in the constant temperature chamber to simulate typical high and low temperature environments in wood processing.

[0057] In the hardware calibration stage, the TL431 reference voltage was precisely locked at 2.500V±1mV by adjusting the adjustable resistor VR1 to eliminate the influence of temperature drift on the sensor excitation voltage. At the same time, the adjustable resistor VR2 was adjusted to make the PT1000 bridge reach a balanced state at 25℃. The measured common-mode rejection ratio was improved to 62dB and the temperature sampling error was compressed to ±0.3%.

[0058] At the algorithm execution level, the microcontroller U3 automatically controls the resistance value of the digital potentiometer (i.e., the adjustable resistor Rx) to select the compensation mode based on the measured temperature. When a low-temperature environment (below 28℃) is detected, the Rx resistance value is set to 220Ω (low-temperature adaptation mode) to increase the gain of the oscillation chip and effectively compensate for signal attenuation caused by low temperature. When a high-temperature environment (above 32℃) is detected, the adjustable resistor Rx resistance value is set to 1000Ω (high-temperature compensation mode) to actively reduce the chip gain and prevent signal overload distortion caused by high temperature. For the critical transition region from 28℃ to 32℃, the microcontroller directly adopts a weighted fusion algorithm to ensure a smooth transition of the measured value at the temperature critical point.

[0059] The calibration results show that the coefficient of determination for the moisture content calculation formula at a reference temperature of 25℃ is 0.991. The full-temperature range test data are as follows:

[0060] Table 1. Full Temperature Range Test Results

[0061]

[0062] Experiments show that the temperature compensation mechanism reduces the detection error across the entire temperature range from ±11.7% to ±1.3%, improving accuracy by 89% compared to the uncompensated condition (-8.2% to +11.7%). At a high temperature of 60℃, the compensation algorithm contributes -12.8%, effectively offsetting the drastic change in dielectric constant, thus verifying the synergistic value of hardware calibration and the segmented algorithm.

[0063] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely preferred examples and are not intended to limit the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the present invention as claimed. The scope of protection of the present invention 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. 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 capacitor C1 and capacitor 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. 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. The microcontroller dynamically executes a piecewise nonlinear compensation algorithm based on the real-time temperature T: When the real-time temperature T is below 28℃, the low-temperature linear compensation model is activated, and its expression is: ； in: α is the low-temperature compensation coefficient; 25 represents the reference temperature point, in °C. When the temperature T reaches or exceeds 32℃, the system switches from the adjustable resistor Rx to the high-temperature nonlinear correction model: ； Where: β and γ are high-temperature correction parameters; e is a natural constant; In the transition temperature range of 28℃≤T<32℃, a weighted fusion algorithm is used to prevent numerical abrupt changes caused by model switching: ； Where: w is the weighting coefficient, which is dynamically calculated according to w=(32-T) / 4, and T is the real-time temperature value; The final moisture content calculation is completed using the core formula: ； Where: k is the moisture content conversion factor; Δf / fo represents the relative offset of the current oscillation frequency f relative to the reference frequency fo; b(T) is the temperature compensation parameter mentioned above; MC represents the compensated moisture content value, in units of %.

2. 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.

3. The portable wood moisture content detector according to claim 2, 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.

4. The portable wood moisture content detector according to claim 3, 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.

5. The portable wood moisture content detector according to claim 4, 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.

6. 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.

7. 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.

8. 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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