Automatic string tension balancing method

Abstract

The invention discloses a string tension automatic balancing method, and aims to solve the problems of multi-string stress coupling interference, temperature and humidity compensation failure and the like. The method is characterized by comprising the following steps: establishing a mechanical reference mapping system and pre-embedding a sensing network; performing initial decoupling of static tension and frequency to eliminate physical deviation; constructing a multi-chord stress coupling matrix to define an interference coefficient; executing environment factor dynamic compensation calculation; and inputting the intonation deviation value into the coupling matrix based on global optimization logic, calculating a displacement compensation vector and synchronously driving an execution mechanism. According to the technical scheme, the intonation accuracy can be ensured, meanwhile, the overall mechanical state of the musical instrument is optimized, the hidden danger of internal stress of the structure is eliminated, and the service life of the musical instrument is remarkably prolonged.

Description

Technical Field

[0001] This invention relates to the field of musical instrument manufacturing, and more particularly to a method for automatically balancing string tension. Background Technology

[0002] In the fields of modern musical instrument manufacturing and acoustic physics research, stringed instruments occupy an irreplaceable core position in the cultural industry due to their unique acoustic resonance characteristics and rich artistic expressiveness. Whether it's a piano, guitar, violin, or other multi-stringed instruments, their core sound production mechanism originates from the forced vibration of strings under preset tension. The precision and stability of string tension directly determine the instrument's intonation and timbre reproduction, and are also key physical parameters for maintaining the overall structural mechanical balance of the instrument and extending the lifespan of the resonating cavity. With the collaborative development of materials science and microelectromechanical systems (MEMS), precise control and automated management of string tension have become an important research direction for improving instrument performance and maintenance efficiency.

[0003] Currently, existing string tension adjustment technology has mainly gone through two stages of development. The initial stage relied primarily on manual tuning, which involved physically twisting the tuning pegs using a tuning wrench or mechanical knob to change the effective length and internal stress of the string, ensuring its vibration frequency conformed to a predetermined tuning standard. With the intervention of electronic and precision sensing technologies, the industry gradually evolved auxiliary tuning devices based on digital signal processing. These existing technologies mostly use piezoelectric sensors or electromagnetic sensors to pick up the string's vibration signals, analyze them using algorithms such as Fast Fourier Transform (FFT) to obtain real-time frequency values, compare them with preset target frequencies, and then perform mechanical displacement compensation through a drive device. From a technical perspective, this approach alleviates the subjective perception bias of manual tuning to a certain extent, improves tuning efficiency, and has significant practical implications for solving single-dimensional pitch correction problems.

[0004] However, with the increasing complexity of musical instrument applications and the rising demands of performers for accurate pitch retention and structural safety, the existing technical solutions based on single-frequency feedback are gradually revealing deep-seated technical bottlenecks and fundamental limitations. Specifically, string tension is not an isolated physical variable, but rather involves complex cross-physical field coupling with the neck stress, bridge pressure, and the elastic deformation of the soundboard. Under current technological frameworks, automated adjustment devices often focus only on the frequency output of a single string. This piecemeal adjustment logic ignores the distribution and evolution of the combined force exerted on the instrument's structure by multiple strings during tensioning. Furthermore, when the temperature and humidity of the external environment fluctuate drastically, the elastic modulus of the string's metal material and the coefficient of thermal expansion and contraction of the instrument's wooden structure exhibit significant nonlinear differences. This not only leads to frequent pitch deviations but also causes neck twisting, bridge detachment, and even permanent cracking of the soundboard due to uneven tension distribution among the multiple strings.

[0005] A deeper contradiction lies in the fact that existing solutions generally lack a global mechanical balance compensation mechanism. In multi-string instruments, due to the integrity of the physical structure, the tension adjustment of a single string will cause stress transmission through shared support points (such as the bridge or nut), leading to secondary fluctuations in the tension of other strings. This dynamic interference makes it difficult to achieve true steady-state at the system level by relying solely on isolated, discrete PID control algorithms. At the same time, existing sensing and detection methods often struggle to accurately distinguish between tension relaxation caused by natural material creep and frequency drift caused by structural deformation. This limitation in information perception leads the control system to blindly increase tension in order to maintain nominal pitch under complex conditions, potentially inducing fatigue damage to the instrument's structure. The root cause is that existing technology has failed to establish a control model that can decouple the complex relationship between tension, frequency, and structural load in real time, resulting in the system sacrificing the overall mechanical stability of the instrument while pursuing tuning accuracy.

[0006] Therefore, achieving global dynamic coordination of tension among multiple strings while ensuring accurate pitch for each string, and monitoring and mitigating structural imbalances caused by environmental and material factors in real time, has become a core technological challenge in the field of intelligent musical instrument control and precision measurement. Addressing this profound technical contradiction, developing an automatic string tension balancing method that balances acoustic performance and structural safety has extremely urgent and significant engineering application value for improving the reliability of high-end musical instruments and promoting the automation of instrument maintenance. Summary of the Invention

[0007] To address the deep-seated technical contradictions of existing string tension adjustment technologies, such as mechanical structural imbalance caused by single-frequency feedback, inability to decouple multi-string stress coupling interference, failure to compensate for environmental temperature and humidity fluctuations, and structural damage induced by natural material creep, this invention provides an automatic string tension balancing method. This method constructs a global mechanical topology sensing system and establishes a multi-dimensional control model based on cross-physical field coupling, achieving optimal mechanical state of the instrument's overall structure while ensuring the accurate pitch of each string.

[0008] To achieve the above-mentioned objective, the present invention provides a method for automatically balancing string tension, the method comprising the following steps: The first step is to establish a structural mechanics benchmark mapping system for the musical instrument. A sensing network composed of multi-physical quantity sensing units is pre-embedded at key nodes in the instrument's physical structure. These key nodes include the bridge support surface, the internal stress ridge of the neck, and the stress-bearing points of each string pin. The multi-physical quantity sensing units include a high-sensitivity thin-film pressure sensor, a triaxial accelerometer, and an integrated temperature and humidity sensor. The thin-film pressure sensor uses a polycrystalline silicon strain gauge structure, fixed to a stainless steel elastic substrate using a glass micro-fusion process, to collect in real-time the vertical pressure component and horizontal shear force component generated by each string on the bridge. The triaxial accelerometer is located in the central resonant region of the soundboard to pick up the time-domain signal of string vibration. The integrated temperature and humidity sensor is used to monitor the ambient air temperature and relative humidity in real-time, serving as the reference input for subsequent compensation logic.

[0009] The second step involves initial decoupling of static tension and frequency. During system startup and initialization, the main control unit drives the actuator to apply minute displacement excitation to each string. The actuator includes a miniature brushless DC motor, a high-reduction-ratio planetary gear reducer, and a precision lead screw transmission assembly. During excitation, the thin-film pressure sensor acquires the corresponding static tension increment, while the triaxial accelerometer acquires the free vibration frequency of the string under this minute excitation. Based on the acquired tension and frequency data, and combined with preset string material parameters, the main control unit calculates the effective vibration length and linear density deviation of each string under the current physical state. This process eliminates initial interference caused by material inhomogeneity or installation errors.

[0010] The third step involves constructing a multi-string stress coupling matrix. Given the holistic nature of the instrument's structure, changes in the tension of a single string inevitably lead to a reconstruction of the neck deflection and the stress distribution on the bridge. This method monitors the micro-strain of the neck using fiber optic Bragg grating (FBG) sensors arranged longitudinally inside the neck. When the tension of the first string changes, the system simultaneously records the fluctuations in the readings of the pressure sensors on the remaining strings and the wavelength drift of the neck strain sensors. By traversing the tension adjustment process of all strings, this method establishes a multi-dimensional stress interference matrix. This matrix defines in detail the influence coefficients of the tension adjustment of any single string on the tension of other strings and the overall deformation of the instrument, thus providing a physical basis for subsequent global coordinated adjustment.

[0011] The fourth step involves performing dynamic compensation calculations for environmental factors. The main control unit receives data from the integrated temperature and humidity sensor in real time. Considering the hygroscopic expansion characteristics of the wooden body material, the system stores a database of expansion coefficients for different types of wood. When ambient humidity increases, the main control unit calculates the passive tension increment caused by the dimensional expansion of the instrument's structure. Simultaneously, considering the elastic modulus drift of the metal strings under temperature changes, the system calculates the expected frequency deviation based on a thermodynamic model. The main control unit separates the variables induced by environmental factors from the measured frequency deviation, ensuring that the actuator only compensates for tension changes caused by actual pitch deviation or mechanical relaxation.

[0012] The fifth step involves implementing adaptive adjustment logic based on global optimization. When the system detects that the pitch deviation exceeds a preset threshold, the main control unit does not directly drive the corresponding actuator. Instead, it inputs the target deviation into a pre-constructed multi-dimensional stress interference matrix. Through matrix operations, a set of optimal displacement compensation vectors that simultaneously satisfy the target frequency recovery and homogenize the strain of the neck is calculated. These displacement compensation vectors are allocated to the actuators of each string, and through synchronous and coordinated driving, the resultant force center of the bridge during adjustment is always kept near the geometric centerline of the physical design, avoiding the generation of torsional torque.

[0013] Furthermore, in a preferred embodiment of the present invention, the driving precision of the actuator is controlled at the micrometer level. The reduction ratio of the planetary gear reducer is set to 100:1, and the lead of the precision lead screw is 0.5 millimeters. The brushless DC motor has a built-in high-resolution magnetic encoder with a single-turn pulse count of no less than 4,096. Through microstepping drive technology, the system can achieve minute compensation for string length changes at the ten-nanometer level, thereby suppressing tension fluctuations at an extremely fine scale.

[0014] Furthermore, the method also includes structural health monitoring logic. During continuous operation, the main control unit monitors the average static stress value fed back by the thin-film pressure sensor in real time. If, under the premise of stable frequency, the static stress continues to exhibit a non-linear increasing trend, the system automatically determines that the instrument's structure is at risk of fatigue deformation or cracking at the glued joints. At this time, the system will activate a protection protocol, proactively and slightly reduce the overall tension level, and issue a warning signal through the human-machine interface to prevent irreversible structural damage to the instrument.

[0015] Furthermore, the pressure sensing unit on the bridge support surface adopts a layered packaging structure. Its outermost layer is an acoustic conduction layer with the same hardness as the bridge material, used to ensure the lossless transmission of vibration energy; the middle layer is the aforementioned polycrystalline silicon thin-film strain gauge, used for mechanical signal acquisition; the bottom layer is an electromagnetic shielding layer, using conductive copper foil and grounded, to eliminate the interference of environmental electromagnetic noise on weak voltage signals.

[0016] Furthermore, the vibration signals acquired by the triaxial accelerometer are preprocessed by a high-performance analog front-end circuit. The preprocessing includes differential amplification, fourth-order low-pass Butterworth filtering, and adaptive gain control. The preprocessed signal enters the analog-to-digital conversion module of the main control unit, with a sampling rate set to 48 kHz and a sampling precision of 24 bits. In the digital domain, the system extracts the fundamental frequency information through windowing and discretized spectrum analysis. When the ambient noise level is high, the system automatically invokes correlation detection logic to extract the periodic string vibration components from the complex ambient noise, ensuring accurate pitch feedback even in noisy environments.

[0017] Furthermore, the actuator is connected to the string shaft via a one-way over-limit clutch. During normal adjustment, the clutch is engaged, transmitting torque; when the drive system malfunctions or detects an abnormal external torque input, the clutch automatically disengages, cutting off power transmission and mechanically protecting the strings from breaking.

[0018] Furthermore, the system power management module employs a low-noise linear voltage regulator circuit in conjunction with an energy storage capacitor array to ensure that when the actuator generates a large current demand during instantaneous startup, the power supply voltage fluctuation of the sensing network and the main control unit is less than five millivolts, thereby maintaining the measurement stability of the high-precision sensor.

[0019] Furthermore, the parameter correction process of the multi-string stress coupling matrix is ​​automatically executed during performance breaks. The main control unit determines whether the system is currently in a static state by real-time detection of the amplitude attenuation characteristics of the vibration signal. Within milliseconds of confirming static state, the system executes a rapid mechanical self-calibration process, fine-tuning individual strings and monitoring the global response to update the coefficients in the coupling matrix in real time, adapting to changes in the rheological properties of the material over time.

[0020] Furthermore, the integrated temperature and humidity sensor is set to collect data once per minute. The main control unit uses a moving average filtering algorithm to process the collected environmental data to filter out spurious fluctuations caused by local airflow or instantaneous heat sources. When the rate of change of ambient humidity exceeds a preset slope of 5% per hour, the system will increase the gain coefficient of the control loop to intervene more actively in tension compensation, preventing stress concentration in the wood during intense moisture exchange.

[0021] Furthermore, for strings of different physical specifications in multi-stringed instruments, this method employs segmented tension slope control logic. For the thicker strings in the bass range, the actuator focuses on providing greater output torque and a longer compensation stroke to overcome the material's hysteresis effect; for the thinner strings in the treble range, the actuator focuses on improving dynamic response speed and displacement resolution to finely control minute pitch deviations.

[0022] Furthermore, this method implements a state machine-based task scheduling logic at the software architecture level. This task scheduling logic divides the system's operating state into self-test mode, real-time monitoring mode, active adjustment mode, environmental compensation mode, and safety protection mode. Seamless switching between these modes is achieved based on sensor triggering conditions and internal preset logic. In active adjustment mode, the system has the highest processor resource priority, ensuring the real-time execution of the control law; in real-time monitoring mode, the system enters a low-power operating state, maintaining only necessary sensor data acquisition and frequency capture.

[0023] Furthermore, the FBG sensor inside the neck is protected by a specific encapsulation sleeve. The coefficient of linear expansion of the sleeve material matches that of the neck body material. The sensor is fixed to the neutral layer by epoxy resin dispensing, so that the sensor is only sensitive to the axial tensile and bending strain of the neck, but does not respond to local shear forces perpendicular to the neck direction.

[0024] Furthermore, this method also includes a resonance suppression logic based on energy spectral density. The main control unit analyzes the distribution of higher harmonics during string vibration. If an abnormal resonance peak is found at a specific frequency, it is determined that the structural components of the instrument are loose or the bridge is not in stable contact. At this time, the adjustment system will fine-tune the tension balance ratio, changing the static pressure distribution of each string on the bridge, thereby altering the mechanical impedance characteristics of the system, eliminating noise and optimizing the purity of tone.

[0025] Furthermore, the control system features a wireless communication interface, employing a short-range, low-power communication protocol based on the IEEE 802.15.4 standard. This interface transmits real-time mechanical balance data, pitch deviation trajectories, and environmental evolution curves to external monitoring equipment, facilitating long-term instrument condition assessments by professional technicians.

[0026] Furthermore, during the global coordinated adjustment in step five, the system employs predictive control logic. The main control unit predicts the frequency trend within tens of milliseconds based on the current tension change trend. Through predictive compensation, the system can initiate minute correction actions before the frequency deviation reaches a threshold perceptible to the naked eye or ear. This feedforward compensation mechanism significantly improves the dynamic stability of pitch, enabling the instrument to maintain extremely high frequency consistency even during intense playing.

[0027] Furthermore, the surface of the precision lead screw assembly is covered with a superhard diamond-like carbon (DLC) coating. This coating has an extremely low coefficient of friction and extremely high wear resistance, ensuring that the clearance of the transmission pair does not significantly increase during hundreds of thousands of fine-tuning cycles, thereby maintaining the system's control accuracy over a long period.

[0028] Furthermore, the firmware layer of the control system includes a complete string fatigue life evaluation algorithm. This algorithm comprehensively considers the usage time, tension frequency, tension fluctuation amplitude, and environmental corrosion factor of each string. When the algorithm evaluation result shows that a certain string has reached the physical fatigue critical point, the system will, during the next power-on self-test, reduce the adjustment rate and issue a replacement reminder to avoid the risk of string breakage during performance.

[0029] Furthermore, the signal conditioning circuit of the thin-film pressure sensor is composed of an instrumentation amplifier with high input impedance. Its input impedance is greater than 100 gigahertz, and its common-mode rejection ratio is greater than 120 decibels. This ensures that even with weak charge signals caused by millinewton-level pressure changes, the system can still achieve an extremely high signal-to-noise ratio, providing a reliable data source for precise mechanical decoupling.

[0030] Furthermore, the system-level feedback frequency of the method, i.e., the total time from sensor acquisition to the actuator completing displacement compensation, is controlled within fifty milliseconds. This metric ensures that the system can respond quickly and suppress frequency drift caused by temperature rise when sudden impacts occur in the external environment (such as instantaneous temperature rise caused by strong stage lighting).

[0031] Furthermore, the automatic string tension balancing method provided by this invention, in addition to achieving stress balancing at the hardware level, also implements a "virtual pitch locking" function through software algorithm logic. After the user sets the target tuning (such as equal temperament or just intonation), the system uses the frequency array corresponding to that tuning as a permanent reference for closed-loop control. Even in cases of severe plastic deformation of the string, the system will forcibly maintain the preset frequency by maximizing the stroke margin of the actuator until the physical limit is reached and an alarm is triggered.

[0032] Furthermore, the stress-bearing point structure of each string axis employs an eccentric cam mechanism. This mechanism is connected to the output shaft of a stepper motor, and the angular displacement of the string axis is fine-tuned by changing the rotation angle of the cam. Compared to the traditional worm gear structure, the eccentric cam mechanism has a higher response speed and lower friction loss within a small adjustment range, making it particularly suitable for high-frequency pitch micro-correction.

[0033] Furthermore, when dealing with multi-string interference, the method considers not only the transmission of static forces but also the cross-string coupling of dynamic vibration energy. When the main control unit detects that a vibration at a certain frequency significantly excites the parasitic vibration of another string, the system will fine-tune the tension of the non-voicing string to deviate from the resonant frequency, thereby improving the acoustic clarity and timbre separation of the instrument.

[0034] Furthermore, the data transmission link of the sensing network adopts a fully differential bus architecture, with ferrite beads and transient voltage suppressors (TVS) connected in series on key signal lines. This engineering design effectively blocks interference from external static electricity, electromagnetic pulses, and ground loop currents on sensitive control loops, ensuring that the system maintains high reliability even in complex electromagnetic environments (such as large-scale performance venues).

[0035] Furthermore, the core clock frequency of the control unit is no less than 180 MHz, and it has a hardware floating-point unit (FPU). This enables complex matrix decoupling operations and environmental compensation algorithms to be processed within one control cycle, eliminating control logic lag caused by computational delays and ensuring the smoothness of adjustment actions.

[0036] Furthermore, the system's enclosure is made of aerospace-grade aluminum-magnesium alloy, machined using high-precision CNC machine tools. The surface of the enclosure undergoes hard anodizing treatment, which not only provides excellent mechanical strength and corrosion resistance, but also provides secondary electromagnetic shielding for the internal microprocessor through the Faraday cage effect formed by the metal enclosure itself.

[0037] The beneficial effects of this invention are: This invention provides an automatic string tension balancing method that, through in-depth analysis of mechanical characteristics and cross-physical field information fusion, completely transforms the isolated and one-sided adjustment mode of traditional tuning techniques. Its core innovation lies in establishing a complete physical model of the musical instrument, integrating environmental perception, materials mechanics, vibration acoustics, and precision motion control. This method not only eliminates the structural stress hazards caused by multi-string tension coupling, significantly extending the instrument's service life, but also maintains near-perfect pitch stability under extreme environmental fluctuations. From an engineering perspective, the connections between the modules described in this invention are clear, the technical specifications are specific, and sensor accuracy, motor execution dynamics, and environmental anti-interference capabilities are fully considered. This system-level technical solution provides a solid theoretical foundation and practical framework for the intelligent evolution of musical instruments, greatly enhancing the reliability and artistic expression of high-end instruments in professional performances and long-term collections.

[0038] Furthermore, the automatic string tension balancing method possesses self-learning characteristics during long-term operation. The main control unit maintains a time-evolving database of characteristic parameters, recording the mechanical response history of specific instruments under different seasons and climatic conditions. Through trend analysis of historical data, the system can autonomously optimize the weights of the temperature and humidity compensation algorithm, enabling each instrument to develop a personalized control logic tailored to its own wood grain and structural characteristics. This deep optimization based on individual differences further enhances the system's adaptability to complex climate changes.

[0039] Furthermore, the lead screw of the actuator employs a preloaded double-nut structure to completely eliminate backlash in the transmission system. During forward and reverse adjustment switching, this mechanical structure ensures a one-to-one linear relationship between the displacement command and the physical execution. Combined with the encoder's fully closed-loop feedback, the system achieves error-free adjustment across the entire stroke range, which is crucial for multi-cycle calibration of multi-stringed instruments.

[0040] Furthermore, the signal acquisition module of the thin-film pressure sensor employs a 24-bit high-resolution analog-to-digital converter and is equipped with a built-in programmable gain amplifier (PGA). The system automatically adjusts the gain factor according to the rated range of the string tension, ensuring that the sampled signal is always within the optimal quantization range of the analog-to-digital converter. This design detail ensures that the system achieves the same order of magnitude measurement resolution whether the string tension is extremely high or low, such as when changing strings.

[0041] Furthermore, the control unit achieves a sensorless load sensing function by monitoring the current waveform of the actuator. When abnormal high-frequency ripples or sudden spikes appear in the current waveform, the system determines that the mechanical transmission components have experienced poor lubrication or foreign object jamming. This function, as a redundant monitoring method, corroborates the data from physical sensors, forming a multi-layered fault diagnosis system.

[0042] Furthermore, the sensing network employs flexible printed circuit board (FPC) technology in its wiring design. The FPC is arranged close to the internal contours of the instrument body, saving installation space and avoiding resonance noise that might occur with traditional wires within the resonance cavity. The circuit board surface is coated with a special sound-absorbing and damping material, further reducing the impact of electrical components on the instrument's original sound quality.

[0043] Furthermore, this method establishes a balance between handling rapid tuning needs and maintaining structural stability. When the user triggers a rapid key change command, the system prioritizes frequency response speed by sacrificing instantaneous stress balance; once the pitch stabilizes, the system switches to background balance mode, smoothly restoring the optimal mechanical distribution of the neck and bridge through subtle, slow compensation actions. This phased task execution strategy balances the performer's immediate experience with the long-term protection of the instrument.

[0044] In summary, this invention constructs a complete, adaptive string tension balancing system with deep security protection capabilities through precise hardware sensing layout, rigorous cross-physics mathematical modeling, and closed-loop high-precision execution logic. It represents a significant technological advancement in resolving the multi-dimensional coupling contradictions raised in the background technology, contributing a systematic engineering solution to improving the precision and intelligence level of musical instrument manufacturing. By real-time monitoring and precise coordination of key physical variables such as pressure, displacement, frequency, temperature, and humidity, this invention ensures that musical instruments maintain a high degree of unity between acoustic performance and structural safety in complex real-world application environments, possessing broad market prospects and extremely high industry application value. Attached Figure Description

[0045] Figure 1 This is a schematic diagram of the overall structure of an automatic string tension balancing method according to the present invention; Figure 2 This is a schematic diagram of the connection between the actuator and the string axis in this invention; Figure 3 This is a flowchart illustrating the steps of an automatic string tension balancing method according to the present invention.

[0046] The attached diagram is labeled as follows: 1. Bridge; 2. Neck; 3. Tuning pin; 4. Thin-film pressure sensor; 5. Triaxial accelerometer; 6. Integrated temperature and humidity sensor; 7. Miniature brushless DC motor; 8. Planetary gear reducer; 9. Precision lead screw drive assembly; 10. Main control unit; 11. Fiber Bragg grating sensor; 12. One-way over-limit clutch; 13. Eccentric cam mechanism; 14. Flexible circuit board; 15. Encapsulation housing. Detailed Implementation

[0047] The present invention will be further described below with reference to the accompanying drawings and specific embodiments. The illustrative embodiments and descriptions herein are used to explain the present invention, but are not intended to limit the present invention.

[0048] like Figure 1-3 As shown, in the overall architecture of the automatic string tension balancing method of this invention, the core logic revolves around closed-loop control of physical field coupling perception and precise motion execution. First, a multi-dimensional sensing network is constructed at key physical topological nodes of the musical instrument structure. Specifically, a high-rigidity pressure sensor array is embedded at the junction of the bridge support surface and the soundboard. The thin-film pressure sensor adopts a polycrystalline silicon strain gauge structure, which is firmly fixed to a 0.5 mm thick stainless steel elastic substrate using advanced glass micro-fusion technology. This structure can not only withstand the enormous static pressure generated by string tension but also capture pressure fluctuations at the millinewton level. The thin-film pressure sensor is protected by a layered encapsulation structure. Its outermost layer is covered with a hard conductive layer whose hardness matches the acoustic impedance of the bridge wood material (such as ebony or rosewood), ensuring that the string vibration energy can be transmitted to the soundboard without loss, without causing tone degradation due to the sensor's intervention. At the bottom of the pressure sensor, an electromagnetic shielding layer made of conductive copper foil is reliably grounded, aiming to shield electromagnetic noise interference from the environment outside the signal conditioning circuit.

[0049] Furthermore, precision grooves are longitudinally carved along the stress ridges inside the neck to embed fiber Bragg grating (FBG) sensors. These FBG sensors are fixed within a specific encapsulation sleeve using an epoxy resin dispensing process. The sleeve material is a composite material with a linear expansion coefficient matching the height of the neck wood. Because the FBG sensors are located near the neutral layer of the neck, they can accurately sense the microscopic axial tensile and bending strains caused by changes in string tension. Simultaneously, miniature torque sensing units are installed at the stress-bearing points of each tuning pin, i.e., the bearing positions where the tuning pins contact the headstock. A triaxial accelerometer is installed in the central resonant region of the soundboard to capture the time-domain acceleration signals generated by string vibrations in real time. In addition, the sensing network includes an integrated temperature and humidity sensor, with a sampling frequency set to once per minute, providing the system with environmental baseline compensation data.

[0050] At the execution level, the automatic string tension balancing method provided by this invention employs an actuator based on a miniature brushless DC motor. The miniature brushless DC motor incorporates a high-resolution magnetic encoder with 4,096 pulses per revolution. Combined with a high-reduction-ratio planetary gear reducer (100:1 ratio) and a precision lead screw drive assembly with a lead of 0.5 mm, the system achieves a displacement compensation resolution at the 10-nanometer level at the physical level. The surface of the precision lead screw assembly is coated with a diamond-like carbon (DLC) coating, resulting in an extremely low coefficient of friction and effectively eliminating mechanical wear during long-term operation. The actuator is connected to the string pin via a one-way over-limit clutch. This design ensures that power transmission can be automatically cut off in the event of a drive system malfunction, mechanically protecting the strings.

[0051] The specific implementation steps of the automatic string tension balancing method of the present invention are as follows: First stage: The system performs instrument structural mechanics benchmark mapping. The main control unit acquires the static mechanical distribution map of the instrument in the current environment through a sensing network. At this time, the thin-film pressure sensor records the vertical component force and horizontal shear force applied by each string to the bridge. Second stage: Initial decoupling of static tension and frequency is performed. During system initialization, the main control unit instructs the actuator to sequentially apply tiny displacement pulses (e.g., axial displacement of five micrometers) to each string, and the resulting free vibration frequency changes of the strings are captured by a triaxial accelerometer. By analyzing the relationship between tension increment and frequency change rate, the main control unit automatically identifies the effective vibration length and linear density parameters of each string, thereby eliminating calculation deviations caused by material inhomogeneity.

[0052] The third stage involves constructing a multi-string stress coupling matrix. This is a complex systems engineering process designed to quantify the impact of adjusting a single string on the overall structure. Specifically, when adjusting the tension of the first string, the main control unit simultaneously monitors the changes in the readings of the pressure sensors corresponding to the remaining strings, as well as the wavelength drift of the fiber Bragg grating sensor inside the neck. By traversing the adjustment process of all strings, the system generates a multi-dimensional stress interference matrix. This matrix defines the strain transfer coefficient of the instrument's structure. Furthermore, the main control unit receives data from the integrated temperature and humidity sensor in real time, proceeding to the fourth stage of dynamic environmental factor compensation calculation. The system integrates a database of moisture expansion coefficients for different types of wood (such as spruce and maple). When the ambient humidity changes significantly, the main control unit calculates the passive tension increment caused by moisture absorption and expansion of the instrument body, and subtracts it as a feedforward variable in the control logic to ensure that the adjustment action only addresses the actual pitch deviation.

[0053] In the fifth stage, an adaptive adjustment logic based on global optimization is implemented. When the system detects that the pitch deviation exceeds a preset threshold (e.g., plus or minus one cent), the main control unit initiates a global coordinated adjustment program. At this point, instead of simply driving the corresponding motor, the deviation of all strings is substituted into a multi-dimensional stress interference matrix for calculation. The calculation result is a set of synchronously executed displacement compensation vectors. By coordinating the synchronous action of the actuators of each string, the center of the resultant force on the bridge is always locked on the geometric center line of the physical design during the adjustment process. This adjustment method avoids the torsional stress concentration on the neck caused by traditional individual string tuning, greatly protecting the structural stability of the instrument.

[0054] To further enhance system stability and reliability, the data conditioning circuit of the sensing network consists of a high-input-impedance instrumentation amplifier with an input impedance greater than 100 gigahertz and a common-mode rejection ratio (CMRR) as high as 120 decibels. The analog signals acquired by the triaxial accelerometer are processed by a fourth-order low-pass Butterworth filter and adaptive gain control circuitry, and then digitized by a 24-bit high-resolution analog-to-digital converter at a sampling rate of 48 kHz. The core clock frequency of the main control unit operates at over 180 MHz, ensuring that complex matrix operations can be completed within a 50-millisecond system feedback cycle.

[0055] The automatic string tension balancing method of this invention also includes a structural health monitoring function. The main control unit continuously compares the static stress value fed back by the thin-film pressure sensor with the historical reference value. If it finds that the frequency remains stable but the static stress shows a continuous nonlinear increase, the system will automatically determine that the instrument body structure may be at risk of cracking or delamination, and then actively reduce the global tension level and issue an early warning. In terms of power management, the system adopts a low-noise linear voltage regulator circuit in conjunction with a large-capacity energy storage capacitor array to ensure that the current transients when the actuator starts will not interfere with the voltage reference of the sensor, and the voltage fluctuation is strictly controlled within five millivolts.

[0056] In terms of engineering implementation details, this method also employs resonance suppression logic based on energy spectral density. The main control unit analyzes the high-order harmonic distribution of the vibration signal in real time. If abnormal resonance is detected at a specific frequency, it is determined that the structural component is loose. At this time, the adjustment system will adjust the tension ratio of each string to change the overall mechanical impedance characteristics of the system, thereby eliminating noise. For string fatigue management, the system has a built-in evaluation algorithm based on cumulative damage theory, which comprehensively considers tension fluctuation frequency, environmental corrosion factor, and usage time. When a string approaches its fatigue limit, the system will automatically limit its adjustment rate to prevent the risk of string breakage.

[0057] The technical effects of the present invention are quantitatively demonstrated below through specific embodiments and comparative examples.

[0058] In one specific embodiment of the invention, the experimental subject was an all-solid wood acoustic guitar. In the experimental environment, the ambient humidity was artificially and rapidly increased from 50% to 80% and maintained for 48 hours. The system was set to automatic balancing mode. In the corresponding comparative example, a traditional single-frequency feedback tuner was used in conjunction with a manual mechanical knob for adjustment, with the frequency manually calibrated every four hours.

[0059] The experimental data are shown in the table below: Table 1: Comparison of Technical Performance Data of Embodiments and Comparative Examples of the Invention

[0060] The data analysis in Table 1 clearly shows that the automatic string tension balancing method provided by this invention achieves a significant leap in several core technical indicators. Specifically, under extreme humidity fluctuations, the frequency deviation of the embodiment is controlled within 0.15 cents, almost reaching one percent of the limit of human hearing, while the comparative embodiment, lacking multi-string coupling compensation and environmental feedforward logic, has a frequency deviation as high as 12.4 cents. More significantly, this embodiment reduces the maximum local strain of the neck by 75% through dynamic decoupling of the multi-dimensional stress interference matrix, effectively mitigating the risk of creep in wood under long-term high tension.

[0061] Furthermore, when handling rapid tuning needs, this method switches the system to active adjustment mode through task scheduling logic, with pitch restoration taking only 0.8 seconds. Regarding the actuator's driving details, a space vector pulse width modulation (SVPWM) algorithm is used to silently control the brushless motor, ensuring that electromagnetic noise during adjustment is lower than background ambient noise. The application of flexible printed circuit boards (FPCs) ensures that the sensing network does not introduce additional physical resonance within the resonance cavity.

[0062] Regarding its long-term self-learning capabilities, the feature parameter library maintained within the main control unit continuously refines the wood swelling model for specific musical instruments over time. By analyzing trends in data collected over the past few months, the system can autonomously optimize the weighting coefficients in the temperature and humidity compensation algorithm. For example, after a complete seasonal transition, the system's prediction accuracy for the drift in the string's elastic modulus caused by ambient temperature will be further improved.

[0063] Furthermore, in the physical implementation of the method of the present invention, the eccentric cam mechanism between the actuator and the tuning pin exhibits extremely high dynamic response within a small range of adjustment. When the main control unit detects the vibrato action of the performer, the system automatically enters an inertial lock state. Through the correlation detection logic in the high-performance analog front-end circuit, the transient frequency shift generated by the performance is separated from the long-term static tension fluctuations, thus avoiding system malfunctions. This intelligent recognition of performance dynamics ensures that the instrument can maintain extremely high frequency consistency even in complex performance environments.

[0064] In summary, the automatic string tension balancing method of this invention achieves a technological leap from single-frequency feedback to global mechanical optimization control by constructing a multi-physics sensing and execution system encompassing pressure, strain, vibration, temperature, humidity, and precise displacement. Its engineering design fully considers the signal-to-noise ratio of signal processing, the backlash error of mechanical transmission, the rheological properties of materials, and the suppression of environmental interference, providing a scientific, rigorous, and engineeringable technical solution for the intelligent maintenance of high-end musical instruments. This method not only fundamentally solves the tuning dilemma caused by multi-string coupling but also significantly extends the physical lifespan and acoustic performance of musical instruments through preventative structural monitoring and environmental compensation, demonstrating extremely high industrial application value and technological advancement.

[0065] In implementing the method described in this invention, differentiated damping control strategies are employed for strings with different tension levels. When adjusting high-tension bass strings, the control algorithm increases the system's proportional gain to overcome the greater static friction; while when adjusting low-tension treble strings, a differential control term is added to prevent overshoot caused by adjustment overshoot. This refined control strategy ensures smoothness in pitch adjustment across the entire frequency range.

[0066] Furthermore, the stepper motor of the actuator employs silent drive technology. By using a space vector pulse width modulation (SVPWM) algorithm to sinusoidally control the winding current, electromagnetic noise and vibration during motor operation are effectively eliminated. This feature is particularly important in recording studios or quiet performance environments, ensuring that the automated adjustment process does not cause any audio background interference to the acoustic recording.

[0067] Furthermore, the FBG sensor array inside the neck employs serial multiplexing technology. Multiple wave gratings are connected in series via a single optical fiber, utilizing wavelength division multiplexing (WDM) principles to transmit strain information from different locations within the same loop. This greatly simplifies the physical wiring structure inside the neck, maximizing the integrity and bending strength of the neck's wooden structure.

[0068] Furthermore, the system's central control unit possesses a powerful abnormal data cleaning function. When acquiring frequency signals, if transient, unstable frequency fluctuations are encountered due to the performer's large vibrato or heavy strikes, the system automatically determines this as invalid input through a logic threshold and maintains the current tension lock state to prevent the system from generating invalid following actions. The system will only activate correction logic if the frequency deviation persists within a preset stable time window.

[0069] Furthermore, the temperature and humidity compensation unit also includes a predictive model for predicting the hysteretic strain of wood caused by environmental changes. Since moisture migration within the wood exhibits a significant time lag, the system analyzes the humidity evolution curve over the past few hours to fine-tune the tension baseline in advance, thus pre-compensating for impending physical deformation. This proactive control strategy further reduces the fluctuation range of pitch under complex weather conditions.

[0070] Furthermore, the system's power input integrates surge suppression and polarity protection circuits. In the event of a voltage surge or reverse polarity connection from the external power adapter, the protection circuit can cut off the main circuit power supply within microseconds, ensuring that the precision sensor array and expensive microprocessor chip are not damaged.

[0071] Furthermore, the gearbox of the actuator is filled with high-performance grease. This grease maintains stable viscosity characteristics over a wide temperature range of -40°C to 120°C, ensuring that the dynamic response time of the regulating system remains consistent regardless of whether it is the frigid winter in the north or the hot summer in the south.

[0072] Furthermore, the calibration data of the multi-chord stress coupling matrix is ​​stored in non-volatile memory (EEPROM). Even after a system power failure, key mechanical parameters, environmental compensation factors, and structural health records will not be lost. Upon power-up, the system can quickly load the historical optimal configuration, achieving a rapid hot start within milliseconds.

[0073] Through the aforementioned multi-dimensional optimization of engineering details, this invention provides an automatic string tension balancing method that, while ensuring legal certainty, presents a rigorous, complete, and feasible high-end technical concept. Each technical step and hardware selection is designed to address a specific technical problem, and the components form a logical closed loop, collectively supporting a new, industrial-grade standard for musical instrument maintenance and management. This invention not only overcomes the technical limitations of simple pitch adjustment but also elevates musical instrument maintenance to the level of structural mechanics and materials science, providing a final solution to the long-standing industry problem of multi-string coupling.

[0074] The technical solutions of the present invention are not limited to the specific embodiments described above. Any technical modifications made in accordance with the technical solutions of the present invention fall within the protection scope of the present invention.

Claims

1. A method for automatically balancing string tension, characterized in that, The method is achieved by constructing a closed-loop control system consisting of a main control unit (10), a sensing network distributed at key nodes of the instrument structure, and actuators connected to each tuning peg (3). The method includes the following steps: The first step is to establish a mechanical reference mapping system for the instrument structure: the initial mechanical distribution data is collected in real time by multi-physical quantity sensing units preset at the support surface of the bridge (1), the internal force ridge line of the neck (2) and the stress bearing point of each string pin (3). Among them, the vertical pressure component and horizontal shear force component generated by each string on the bridge (1) are collected by the thin film pressure sensor (4) set on the support surface of the bridge (1), the time domain signal of the string vibration is picked up by the triaxial accelerometer (5) set in the resonant area of ​​the center of the soundboard, and the ambient temperature and relative humidity are collected as reference input by the integrated temperature and humidity sensor (6). The second step is to perform initial decoupling of static tension and frequency: the main control unit (10) drives the actuator to perform a small displacement excitation on each string. During the execution, the corresponding static tension increment is collected by the thin film pressure sensor (4), and the free vibration frequency of the string under the small excitation is collected by the triaxial accelerometer (5). The main control unit (10) calculates the effective vibration length and linear density deviation of each string under the current physical state in combination with the preset string material parameters, so as to eliminate the initial interference caused by material inhomogeneity and installation error. The third step is to construct a multi-string stress coupling matrix: using fiber Bragg grating sensors (11) arranged longitudinally inside the neck (2) to monitor the micro-strain of the neck (2), when adjusting the tension of a single string, the system synchronously records the fluctuation of the readings of the remaining strings on the thin-film pressure sensor (4) and the wavelength drift of the fiber Bragg grating sensor (11). By traversing the tension adjustment process of all strings, a multi-dimensional stress interference matrix is ​​established to define the influence coefficient of the tension adjustment of any string on the tension of other strings and the overall deformation of the instrument. The fourth step is to perform dynamic compensation calculation for environmental factors: The main control unit (10) receives the data from the integrated temperature and humidity sensor (6) in real time, calculates the passive tension increment of the piano body structure caused by humidity change based on the database of the moisture expansion coefficient of the wooden material of the piano body, and calculates the expected value of frequency deviation caused by the elastic modulus drift of the metal strings under temperature change based on the thermodynamic model, and separates the variables induced by the above environmental factors from the measured frequency deviation in order to determine the real tension change caused by the mechanical relaxation of the strings; The fifth step is to implement the adaptive adjustment logic based on global optimization: when the pitch deviation exceeds the preset threshold, the main control unit (10) inputs the target deviation into the multidimensional stress interference matrix, and obtains the optimal displacement compensation vector that satisfies the target frequency recovery and makes the strain of the neck (2) uniform through matrix operation. The displacement compensation vector is then distributed to the actuators of each string for synchronous and coordinated driving, so that the center of the resultant force on the bridge (1) during the adjustment process is always kept at the geometric center line of the physical design.

2. The automatic string tension balancing method according to claim 1, characterized in that, In the first step, the thin-film pressure sensor (4) adopts a polycrystalline silicon strain gauge structure and is fixed on a stainless steel elastic substrate using a glass micro-melting process. The thin-film pressure sensor (4) adopts a layered packaging structure, which includes: The outermost acoustic transmission layer has the same hardness as the bridge (1) material, which is used to ensure the lossless transmission of vibration energy; The middle layer is equipped with the polycrystalline silicon strain gauge for mechanical signal acquisition. The bottom layer is an electromagnetic shielding layer made of conductive copper foil and grounded, used to eliminate the interference of environmental electromagnetic noise on electrical signals.

3. The automatic string tension balancing method according to claim 1, characterized in that, In the second step, the actuator includes a miniature brushless DC motor (7), a planetary gear reducer (8), and a precision lead screw drive assembly (9). The miniature brushless DC motor (7) has a built-in high-resolution magnetic encoder with a single-turn pulse count of not less than 4,096. The reduction ratio of the planetary gear reducer (8) is set to 100:

1. The lead of the precision lead screw drive assembly (9) is 0.5 mm, and its surface is covered with an ultra-hard diamond-like carbon (DLC) coating. The actuator is connected to the stringer (3) through a one-way over-limit clutch (12). When the drive system malfunctions or detects an abnormal external torque input, the one-way over-limit clutch (12) automatically disengages.

4. The automatic string tension balancing method according to claim 1, characterized in that, In the third step, the fiber Bragg grating sensor (11) is protected by a sleeve with a linear expansion coefficient matching the material of the neck (2) and is fixed in the neutral layer inside the neck (2) by epoxy resin dispensing. The fiber Bragg grating sensor (11) adopts an array layout and combines wavelength division multiplexing technology to transmit strain information at different positions in a single fiber loop. The main control unit (10) determines whether the instrument is in a static state by real-time monitoring of the amplitude attenuation characteristics of the vibration signal, and performs a mechanical self-calibration process within milliseconds after confirming that the instrument is static. By fine-tuning a single string and monitoring the global mechanical response, the coefficients in the multidimensional stress interference matrix are updated in real time.

5. The automatic string tension balancing method according to claim 1, characterized in that, In the fourth step, the sampling frequency of the integrated temperature and humidity sensor (6) is set to once per minute. The main control unit (10) uses a moving average filtering algorithm to process environmental data and establishes a prediction model for the hysteresis strain of wood caused by environmental changes. The tension benchmark is finely adjusted in advance by analyzing the historical humidity evolution curve. When the rate of change of environmental humidity exceeds the preset slope of five percent per hour, the system automatically increases the gain coefficient of the control loop.

6. The automatic string tension balancing method according to claim 1, characterized in that, In the fifth step, the globally optimized adaptive adjustment logic adopts feedforward predictive control. The main control unit (10) predicts the frequency trend in the next tens of milliseconds based on the current tension change trend, and initiates pre-compensation correction action before the frequency deviation reaches the human ear recognition threshold. For strings of different physical specifications, a segmented tension slope control logic is adopted: for thick strings in the bass range, the proportional gain of the system is increased to overcome static friction and provide a large output torque; for thin strings in the treble range, a differential control term is added to improve the dynamic response speed and suppress pitch overtuning.

7. The automatic string tension balancing method according to claim 1, characterized in that, The method also includes structural health monitoring logic: the main control unit (10) monitors the average static stress value fed back by the thin film pressure sensor (4) in real time. If the static stress shows a nonlinear growth trend under the premise of stable frequency, the system automatically determines that the structure of the instrument body has undergone fatigue deformation or the glued parts have cracked, and starts the protection protocol to actively reduce the global tension level and issue an early warning signal. At the same time, the main control unit (10) has a built-in string fatigue life evaluation algorithm. It comprehensively considers the string usage time, tension frequency, tension fluctuation amplitude and environmental corrosion factor. When a string reaches the physical fatigue critical point, it issues a replacement reminder through the human-machine interface.

8. The automatic string tension balancing method according to claim 1, characterized in that, The data transmission link of the sensing network adopts a fully differential bus architecture and is arranged in accordance with the internal contour of the instrument body using a flexible circuit board (14). The surface of the flexible circuit board (14) is coated with sound-absorbing damping material. The signals of the thin-film pressure sensor (4) and the triaxial accelerometer (5) are preprocessed by an analog front-end circuit composed of a high input impedance instrumentation amplifier. The preprocessing includes differential amplification, fourth-order low-pass Butterworth filtering and adaptive gain control. The preprocessed signal enters a 24-bit resolution analog-to-digital conversion module with a sampling rate set to 48 kHz. The fundamental frequency information is extracted in the digital domain through discretized spectrum analysis.

9. The automatic string tension balancing method according to claim 1, characterized in that, The stress bearing point structure of each string axis (3) adopts an eccentric cam mechanism (13). The eccentric cam mechanism (13) is connected to the output shaft of the stepper motor. By changing the rotation angle of the cam, the angular displacement of the string axis (3) is slightly corrected. The system also includes resonance suppression logic based on energy spectral density: the main control unit (10) analyzes the distribution of high-order harmonics in the vibration process of the string. If an abnormal resonance peak is found at a specific frequency, the tension of the non-sounding string is finely adjusted to make it deviate from the resonance frequency, thereby changing the mechanical impedance characteristics of the system to eliminate noise.

10. A method for automatically balancing string tension according to any one of claims 1 to 9, characterized in that, The main control unit (10) adopts a state machine-based task scheduling logic to divide the operating state into self-test mode, real-time monitoring mode, active adjustment mode, environmental compensation mode and safety protection mode; the system uses a low-noise linear voltage regulator circuit in conjunction with an energy storage capacitor array for power management to ensure that the power supply voltage fluctuation during the instantaneous start-up of the actuator is less than five millivolts; the system encapsulation shell (15) is made of aerospace-grade aluminum-magnesium alloy by CNC machining, and the surface is hard anodized and forms a Faraday cage effect shield; The calibration data of the multidimensional stress interference matrix, environmental compensation factors, and structural health records are all stored in non-volatile memory to achieve millisecond-level rapid hot start after the system is powered on again.

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