A piezoelectric force sensor static calibration method

Abstract

This invention discloses a static calibration method for a piezoelectric force sensor. The method involves sequentially applying multiple predetermined force levels to the piezoelectric force sensor; continuously acquiring the sensor output signal after conversion by a charge amplifier during and before and after the holding phase of each force level; selecting at least four time points from the acquired output signal time series after completing one loading and unloading cycle of force levels; constructing calibration values ​​based on the output signals corresponding to the four time points; unloading the piezoelectric force sensor to a zero-load state and performing zero-point calibration after the holding phase; and establishing a calibration relationship between force and output signal based on the calibration values ​​corresponding to each predetermined force level. This method does not require modeling the charge amplifier drift mechanism and does not rely on a low-drift, high-performance charge amplifier, effectively suppressing calibration errors caused by charge amplifier drift during the holding phase, reducing calibration system costs, and improving calibration accuracy.

Description

Technical Field

[0001] This invention belongs to the field of sensor technology, specifically relating to a static calibration method for a piezoelectric force sensor. Background Technology

[0002] Piezoelectric force sensors are widely used in mechanical measurement fields such as impact load, dynamic load, and quasi-static load due to their advantages of high sensitivity, wide bandwidth, large dynamic range, and compact structure. Since the electrical signal output by a piezoelectric force sensor is a charge signal, in engineering applications, a charge amplifier is usually needed to convert this charge signal into a voltage signal for subsequent data acquisition, processing, and analysis. Therefore, the calibration process of a piezoelectric force sensor essentially establishes a quantitative correspondence between the external applied force and the output signal after conversion by the charge amplifier.

[0003] In the static or quasi-static calibration of piezoelectric force sensors, a certain holding time is typically required after the target force level is applied to obtain a stable output signal. However, due to the finite time constant of the charge amplifier feedback network and the influence of non-ideal factors such as the operational amplifier input bias current and input bias voltage, the output signal often drifts over time during the holding phase. When the holding time is relatively short compared to the feedback network time constant, this drift usually exhibits linear or approximately linear time-varying characteristics.

[0004] During the static calibration process of progressive loading, the drift error that varies with time will accumulate between each force level, thereby introducing additional systematic errors, causing the zero point of the calibration curve to shift and the linearity to decrease, which in turn significantly affects the accuracy and reliability of the calibration results.

[0005] In existing technologies, the aforementioned drift problem is typically addressed by selecting high-end charge amplifiers with low drift performance or by modeling and compensating for the drift process. However, these methods significantly increase the hardware cost of the calibration system and place higher demands on the performance of the charge amplifier. Furthermore, they are highly dependent on the accuracy and stability of the drift model, are complex to implement in engineering, and struggle to maintain good versatility and robustness under different types of charge amplifiers and operating conditions.

[0006] Therefore, there is an urgent need for a piezoelectric force sensor calibration method that can effectively suppress stage drift error during static calibration without modeling the charge amplifier drift mechanism or relying on a low-drift high-end charge amplifier. Summary of the Invention

[0007] To overcome the shortcomings of existing technologies, this invention provides a static calibration method for piezoelectric force sensors. Multiple predetermined force levels are sequentially applied to the piezoelectric force sensor, each force level including a loading phase and a holding phase. During and before the holding phase of each force level, the sensor output signal, converted by a charge amplifier, is continuously acquired. After completing one loading and unloading cycle of a force level, at least four time points are selected from the acquired output signal time series, with the time interval between the first and second time points equal to the time interval between the third and fourth time points. Calibration values ​​are constructed based on the output signals corresponding to the at least four time points to offset the linear or approximately linear drift components in the output signal that change with time. After completing the holding phase, the piezoelectric force sensor is unloaded to a zero-load state and zero-point correction is performed. A calibration relationship between the force and the output signal is established based on the calibration values ​​corresponding to each predetermined force level. This method does not require modeling the charge amplifier drift mechanism and does not rely on a low-drift, high-performance charge amplifier. It can effectively suppress calibration errors caused by charge amplifier drift during the holding phase, reduce calibration system costs, and improve calibration accuracy, making it suitable for static calibration scenarios of piezoelectric force sensors.

[0008] The technical solution adopted by this invention to solve its technical problem is as follows: Step 1: Load level by level; Multiple predetermined force levels are sequentially applied to a piezoelectric force sensor; the force levels are increased step by step in a predetermined order, and each force level includes a loading stage and a holding stage; in the loading stage, the load is gradually applied from the previous force level to the current force level; in the holding stage, the current force level is kept constant. Step 2: Continuous data acquisition; During the holding phase of each force level and before and after it, the sensor output signal converted by the charge amplifier is continuously acquired to form a time series of the output signal for the stress level. Step 3: Select multiple time points; After completing one loading and unloading of the force level, at least four time points are selected from the output signal time series, such that the time interval between the first time point and the second time point is equal to the time interval between the third time point and the fourth time point. Step 4: Standardization and quantitative construction; Based on the output signals corresponding to at least four time points, a calibration quantity is constructed by combining the output signals of the loading and unloading stages. The multi-time point selection step and the calibration quantity construction step together constitute a symmetrical drift elimination mechanism, which makes the time-related output drift components in the calibration quantity cancel each other out, thereby eliminating the drift error that changes linearly or approximately linearly during the holding stage. Step 5: Unloading and zero-point calibration; After the holding phase of each force level is completed, the piezoelectric force sensor is unloaded to a zero-load state, and the output signal is zero-point calibrated. Step 6: Establish the calibration relationship; Based on the calibration values ​​corresponding to each predetermined force level, a calibration relationship between the external load force and the sensor output signal is established to obtain the calibration curve of the piezoelectric force sensor.

[0009] Preferably, the at least four time points include a first time point before loading, a second and third time points during the loading phase, and a fourth time point after unloading.

[0010] Preferably, the time interval between the first time point and the second time point, and the time interval between the third time point and the fourth time point, are selected and set to be equal when the acquired output signal is post-processed.

[0011] Preferably, the calibration value is obtained by combining the output signals of the loading and unloading phases to offset the time-varying linear or approximately linear drift components in the output signal.

[0012] Preferably, the duration of the holding phase is less than the actual time constant corresponding to the parameters of the charge amplifier feedback network, so that the output drift changes linearly or approximately linearly during the holding phase.

[0013] Preferably, the multiple predetermined force levels are applied sequentially in ascending order, and each force level is maintained for the expected holding time.

[0014] Preferably, after the holding phase of each force level is completed, the piezoelectric force sensor is unloaded and a zero-point calibration operation is performed.

[0015] Preferably, the charge amplifier is preheated before performing the step-by-step loading process to achieve a thermally stable state.

[0016] Preferably, the piezoelectric force sensor is preloaded at least once before the progressive loading.

[0017] Preferably, the duration of the holding phase is less than the actual time constant corresponding to the parameters of the charge amplifier feedback network, so that the output drift changes linearly or approximately linearly during the holding phase.

[0018] The beneficial effects of this invention are as follows: 1) No precise modeling of the charge amplifier drift mechanism is required, the calibration method is simple to implement, and it is highly operable in engineering. 2) By combining continuous data acquisition with multi-time point selection to construct calibration data, the linear or near-linear drift error during the holding period can be effectively offset, thereby improving the accuracy and stability of the calibration results. 3) Significantly reduces the dependence of the calibration process on the low drift performance of the charge amplifier, allowing the use of charge amplifiers with higher drift rates; 4) While ensuring accuracy, effectively reduce the overall cost of the calibration system and enhance its engineering application value; 5) It is suitable for static calibration scenarios of piezoelectric force sensors, and has good versatility and prospects for promotion. Attached Figure Description

[0019] Figure 1 This is a schematic flowchart of the method of the present invention; Figure 2 This is a timing diagram illustrating the loading and holding stages of each force level during the preloading and progressive loading process in an embodiment of the present invention. Figure 3 This is a schematic diagram illustrating the construction of a calibration quantity to eliminate drift components by sampling at multiple time points within a holding period in an embodiment of the present invention. Detailed Implementation

[0020] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0021] The purpose of this invention is to provide a static calibration method for piezoelectric force sensors that is independent of the linear drift of the charge amplifier. Without the need for precise modeling of the charge amplifier drift mechanism or the selection of a low-drift high-end charge amplifier, this method improves the acquisition and processing of the output signal, effectively suppresses linear or near-linear drift errors during the hold-up phase of the calibration process, improves the accuracy and stability of the calibration results, and reduces the overall cost of the calibration system.

[0022] To achieve the above objectives, the present invention adopts the following technical solution: a static calibration method for a piezoelectric force sensor that is independent of the linear drift of the charge amplifier, the method being applicable to static calibration conditions, comprising the following steps: 1) Gradual loading step. Multiple predetermined force levels are sequentially applied to the piezoelectric force sensor, with the force levels increasing progressively in a predetermined order. Each force level includes a loading phase and a holding phase. In the loading phase, the load is gradually increased from the previous force level to the current force level; in the holding phase, the current force level is maintained constant. 2) Continuous acquisition step. During the holding phase of each force level and before and after it, the sensor output signal after conversion by the charge amplifier is continuously acquired to form a time series of the output signal for the stress level; 3) Multi-time point selection step. After completing one loading and unloading of the force level, at least four time points are selected from the output signal time series, such that the time interval between the first time point and the second time point is equal to the time interval between the third time point and the fourth time point; 4) Calibration Quantity Construction Step. Based on the output signals corresponding to the at least four time points, a calibration quantity is constructed by combining the output signals of the loading and unloading phases. The multi-time point selection step and the calibration quantity construction step together constitute a symmetrical drift elimination mechanism, which makes the time-related output drift components in the calibration quantity cancel each other out, thereby effectively eliminating the drift error that changes linearly or approximately linearly during the holding phase. 5) Unloading and Zero-Point Calibration Steps. After completing the holding phase of each force level, the piezoelectric force sensor is unloaded to a zero-load state, and the output signal is zero-point calibrated. 6) Calibration relationship establishment steps. Based on the calibration values ​​corresponding to each predetermined force level, establish the calibration relationship between the external load force and the sensor output signal to obtain the calibration curve of the piezoelectric force sensor.

[0023] The duration of the hold phase is less than the actual time constant corresponding to the parameters of the charge amplifier feedback network, which makes the output drift linear or approximately linear during the hold phase.

[0024] Example: I. Calibration System Composition and Experimental Preparation; In this embodiment, the piezoelectric force sensor is mounted on the loading axis of the force standard machine, so that the force direction of the sensor is consistent with the loading direction of the standard machine. The piezoelectric force sensor is connected to the data acquisition system through a charge amplifier, which is used to convert the charge signal output by the sensor into a voltage signal.

[0025] Before starting the calibration experiment, the entire calibration system should be preheated to ensure that the charge amplifier and related electronic components reach a thermally stable state. Preferably, the preheating time should be no less than 30 minutes. After preheating, the range of the charge amplifier should be set, and the system should be confirmed to be in normal working condition.

[0026] II. Preloading process; In this embodiment, as Figure 2 As shown, before formal calibration, a static load of its rated range was applied to the piezoelectric force sensor, and multiple pre-loading cycles were performed. During each pre-loading process, the load was gradually applied to the rated force and held for a certain period of time, then unloaded to a zero-load state, and a certain period of time was waited after unloading before the next pre-loading was performed.

[0027] The above preloading process can effectively stabilize the mechanical structure of the sensor and the working state of the charge amplifier, providing stable initial conditions for the subsequent calibration process.

[0028] III. Step-by-step loading and retention phase settings; After the preloading process is completed, the formal calibration process begins.

[0029] Multiple force levels are applied sequentially to the piezoelectric force sensor according to a predetermined force level sequence. Each force level includes a loading phase and a holding phase. During the loading phase, the load is gradually increased from the previous level to the current force level; during the holding phase, the current force level is maintained, and the output signal is acquired during this phase.

[0030] In this embodiment, the holding time of each force level is the same, and the holding time is relatively short relative to the time constant of the charge amplifier feedback network, so that the drift of the output signal during the holding phase mainly exhibits linear or approximately linear variation characteristics.

[0031] Figure 2 The temporal sequence of the loading and holding phases at each force level during preloading and progressive loading is illustrated.

[0032] IV. Continuous data acquisition and selection of multiple time points; During the loading and holding phases of each force level, as well as before and after them, the sensor output signal converted by the charge amplifier is continuously acquired to form a time series of the output signal for each stress level.

[0033] After completing one load and unload of force level, such as Figure 3 As shown, the time series of the output signal is post-processed, and at least four time points are selected, including: the first time point P1 before loading, the second time point P2 and the third time point P3 during the holding phase after loading, and the fourth time point P4 after unloading.

[0034] During the selection process, the time intervals between the first time point P1 and the second time point P2, as well as the time intervals between the third time point P3 and the fourth time point P4, are made the same. This ensures that the time-varying drift component in the output signal can be effectively offset when constructing the calibrated quantity.

[0035] At each of the above time points, the corresponding output signals are denoted as follows: and .

[0036] V. Standardization and Drift Elimination; During the hold phase, the charge amplifier output signal contains a time-varying drift component. A calibration metric is constructed by combining the output signals corresponding to the at least four time points.

[0037] In this embodiment, the effective output during the loading phase is represented as:

[0038] The effective output of the unloading phase is:

[0039] in, This represents the drift rate of the charge amplifier output signal during the hold phase.

[0040] By combining the effective output values ​​of the loading and unloading phases, a calibration value for calibration is constructed:

[0041] in, and To disregard the loading and unloading forces that exhibit linear drift, and To correct the effective voltage after linear drift.

[0042] In the calibration value thus constructed, the time-varying linear or approximately linear drift components cancel each other out, making the calibration value independent of the drift rate of the charge amplifier.

[0043] VI. Unloading and Zero-Point Calibration; After the current force level is held, the piezoelectric force sensor is unloaded to a zero-load state, and the output signal is zero-point calibrated.

[0044] During the calibration process of progressive loading, it is preferable to perform unloading and zero-point correction operations after each force level is completed in order to reduce the impact of zero-point drift on the calibration results of subsequent force levels.

[0045] VII. Establishing calibration relationships and repeating experiments; After completing the loading, holding, and unloading processes for all predetermined force levels, a calibration relationship between the external loading force and the sensor output signal is established based on the calibration values ​​corresponding to each force level, thus obtaining the sensor calibration curve.

[0046] To improve the reliability and repeatability of the calibration results, it is preferable to repeat the above-mentioned step-by-step loading and four-point measurement process multiple times, and to perform statistical analysis on the results of multiple experiments to obtain the final calibration curve.

[0047] Through the above embodiments, the present invention can effectively eliminate linear or near-linear drift errors in the output signal during the holding phase without modeling the drift mechanism of the charge amplifier or relying on a low-drift high-side charge amplifier. Therefore, the present invention allows the use of a charge amplifier with a high drift rate, significantly reducing the cost of the calibration system while ensuring calibration accuracy, and is suitable for static calibration scenarios of piezoelectric force sensors.

Claims

1. A static calibration method for a piezoelectric force sensor, characterized in that, Includes the following steps: Step 1: Load level by level; Multiple predetermined force levels are sequentially applied to a piezoelectric force sensor; the force levels are increased step by step in a predetermined order, and each force level includes a loading stage and a holding stage; in the loading stage, the load is gradually applied from the previous force level to the current force level; in the holding stage, the current force level is kept constant. Step 2: Continuous data acquisition; During the holding phase of each force level and before and after it, the sensor output signal converted by the charge amplifier is continuously acquired to form a time series of the output signal for the stress level. Step 3: Select multiple time points; After completing one loading and unloading of the force level, at least four time points are selected from the output signal time series, such that the time interval between the first time point and the second time point is equal to the time interval between the third time point and the fourth time point. Step 4: Standardization and quantitative construction; Based on the output signals corresponding to at least four time points, a calibration quantity is constructed by combining the output signals of the loading and unloading stages. The multi-time point selection step and the calibration quantity construction step together constitute a symmetrical drift elimination mechanism, which makes the time-related output drift components in the calibration quantity cancel each other out, thereby eliminating the drift error that changes linearly or approximately linearly during the holding stage. Step 5: Unloading and zero-point calibration; After the holding phase of each force level is completed, the piezoelectric force sensor is unloaded to a zero-load state, and the output signal is zero-point calibrated. Step 6: Establish the calibration relationship; Based on the calibration values ​​corresponding to each predetermined force level, a calibration relationship between the external load force and the sensor output signal is established to obtain the calibration curve of the piezoelectric force sensor.

2. The static calibration method for a piezoelectric force sensor according to claim 1, characterized in that, The at least four time points include a first time point before loading, a second and third time points during the loading phase, and a fourth time point after uninstallation.

3. The static calibration method for a piezoelectric force sensor according to claim 1, characterized in that, The time interval between the first time point and the second time point, and the time interval between the third time point and the fourth time point, are selected and set to be equal when the acquired output signals are post-processed.

4. The static calibration method for a piezoelectric force sensor according to claim 1, characterized in that, The calibration value is obtained by combining the output signals of the loading and unloading phases to offset the time-varying linear or approximately linear drift components in the output signal.

5. The static calibration method for a piezoelectric force sensor according to claim 1, characterized in that, The duration of the holding phase is less than the actual time constant corresponding to the parameters of the charge amplifier feedback network, so that the output drift changes linearly or approximately linearly during the holding phase.

6. The static calibration method for a piezoelectric force sensor according to claim 1, characterized in that, The multiple predetermined force levels are applied sequentially in ascending order, with each force level maintained for a predetermined duration.

7. The static calibration method for a piezoelectric force sensor according to claim 1, characterized in that, After each force level holding phase is completed, the piezoelectric force sensor is unloaded and a zero-point calibration operation is performed.

8. The static calibration method for a piezoelectric force sensor according to claim 1, characterized in that, Before performing the step-by-step loading process, the charge amplifier is preheated to achieve a thermally stable state.

9. The static calibration method for a piezoelectric force sensor according to claim 1, characterized in that, Before progressive loading, the piezoelectric force sensor is preloaded at least once.

10. The static calibration method for a piezoelectric force sensor according to claim 1, characterized in that, The duration of the holding phase is less than the actual time constant corresponding to the parameters of the charge amplifier feedback network, so that the output drift changes linearly or approximately linearly during the holding phase.

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