Concrete vibration compactness detection and adaptive frequency modulation system and method

Abstract

The application discloses a concrete vibration compactness detection and self-adaptive frequency modulation system and method, and relates to the technical field of building engineering construction automation. The system comprises a vibration excitation device, a variable frequency driving device and a control unit. The control unit collects the active current component and output voltage of the variable frequency driving device, carries out compensation and normalization processing on the active current component, self-adaptively calibrates a reference threshold value, carries out smooth filtering on the compensated active current component and extracts the time domain first-order derivative feature, identifies the concrete evolution stage based on the sign and amplitude of the first-order derivative feature, and judges that the compact state is reached when the preset condition is met within continuous multiple sampling periods, then controls the variable frequency driving device to smoothly reduce the vibration frequency to the pressure maintaining frequency and maintain the preset time length before stopping the vibration. The system can realize self-adaptive calibration of the threshold value, automatic confirmation of the concrete state and phased regulation and control on the vibration excitation device, solves the problems of reference drift and over-vibration segregation, and improves the engineering quality.

Description

Technical Field

[0001] This invention relates to a concrete vibration compaction detection and adaptive frequency tuning system and method, belonging to the field of construction automation technology. Background Technology

[0002] Concrete vibration is a crucial process in building construction, as its density directly affects the strength, durability, and service life of the concrete structure. Attached vibrators, a commonly used type of compaction equipment, are used to remove air bubbles from the concrete and achieve thorough compaction by attaching the vibrating device to the outside of the formwork and utilizing high-frequency vibration.

[0003] Traditional attached vibrators typically operate at a fixed frequency, and the determination of compaction during the vibration process mainly relies on the experience and visual inspection of construction workers, or on monitoring vibration parameters by placing external mechanical sensors (such as accelerometers and force gauges) on the formwork. However, manual visual inspection is affected by personal experience and its accuracy is difficult to guarantee. Furthermore, the harsh environment at construction sites makes external sensors extremely easy to damage and susceptible to interference from high-frequency resonance of the structure. Summary of the Invention

[0004] The purpose of this invention is to provide a concrete vibration compaction detection and adaptive frequency tuning system and method to solve the problems of easy damage, high cost and susceptibility to interference of external mechanical sensors in the prior art.

[0005] To solve the above technical problems, the present invention includes the following technical solutions:

[0006] A concrete vibration compaction degree detection and adaptive frequency modulation system includes a vibration excitation device and a frequency converter drive device. The vibration excitation device can be attached to a formwork for the concrete to be vibrated. The frequency converter drive device is signal-connected to the vibration excitation device. The system also includes:

[0007] The control unit, which is communicatively connected to the variable frequency drive device, is used to acquire the active current component I of the variable frequency drive device. q and output voltage U, and active current component I q Perform compensation normalization processing;

[0008] The control unit during the initial vibration start-up period T init Within, the reference threshold is adaptively calibrated based on the statistical characteristics of the active current component;

[0009] The control unit performs smoothing filtering on the compensated active current component and extracts its first-order time-domain derivative. ;

[0010] The control unit is based on the first derivative. The symbols and amplitudes are used to identify the stages of concrete evolution, and the concrete is determined to be in a compacted state when preset conditions are met within multiple consecutive sampling cycles.

[0011] After determining that a compaction state has been reached, the control unit controls the variable frequency drive device to smoothly reduce the vibration frequency to the pressure holding frequency and maintain it for a preset time before stopping the vibration.

[0012] Furthermore, the duration T of the initial time period init It lasts from 5 to 10 seconds.

[0013] Furthermore, the smoothing filter employs a moving average algorithm, with a sliding window duration of 0.5 to 2 seconds.

[0014] Furthermore, the vibration frequency of the variable frequency drive device before determining that a compacted state has been reached is 40Hz to 60Hz, and the pressure holding frequency is 20Hz to 40Hz.

[0015] Furthermore, the slope of the smooth reduction in vibration frequency is from 2 Hz / s to 15 Hz / s.

[0016] Furthermore, the control unit is equipped with a pre-trained one-dimensional convolutional neural network model;

[0017] The control unit is also used to perform a short-time Fourier transform on the active current component, extract the harmonic energy distribution spectrum, and input the harmonic energy distribution spectrum into the model to obtain the classification confidence level of the concrete density state. When the judgment confidence level based on the first derivative feature is lower than a preset threshold, the classification confidence level is combined for fusion judgment.

[0018] Accordingly, the present invention also provides a method for detecting and adaptively adjusting the compaction density of concrete, which utilizes a variable frequency drive device to drive a vibration excitation device to vibrate the concrete. The method includes the following steps:

[0019] Collect the active current component I of the frequency converter drive device q and output voltage U;

[0020] For the active current component I q Perform compensation normalization processing;

[0021] During the initial period of vibration start-up, based on the active current component I q The statistical characteristics are adaptively calibrated to determine the benchmark threshold;

[0022] The compensated active current component is smoothed and filtered, and its first-order time-domain derivative features are extracted.

[0023] Based on the sign and amplitude of the first derivative characteristics, the concrete evolution stage is identified, and when the preset conditions are met within multiple consecutive sampling periods, it is determined that the concrete has reached a compacted state.

[0024] After determining that a compaction state has been reached, the variable frequency drive device is controlled to smoothly reduce the vibration frequency to the pressure holding frequency and maintain it for a preset time before stopping the vibration.

[0025] Furthermore, the sampling rate for the active current component is 200Hz to 500Hz;

[0026] The duration of the initial period is 5 to 10 seconds;

[0027] The smoothing filter uses a moving average algorithm, with a sliding window duration of 0.5 to 2.0 seconds;

[0028] The vibration frequency of the variable frequency drive device before determining that a compacted state has been reached is 40Hz to 60Hz, and the pressure holding frequency is 20Hz to 40Hz.

[0029] The frequency reduction slope of the smooth reduction of vibration frequency is 2Hz / s to 15Hz / s;

[0030] Furthermore, the method also includes:

[0031] If the compaction state is not determined after the preset maximum time has elapsed since the vibration has started, or if the active current component exceeds the limit by a step, the frequency converter will be forcibly stopped and an alarm signal will be issued.

[0032] Furthermore, the method also includes:

[0033] Perform a short-time Fourier transform on the active current component to extract the harmonic energy distribution spectrum;

[0034] The harmonic energy distribution spectrum is input into a pre-trained one-dimensional convolutional neural network model to obtain the classification confidence of the concrete compaction state.

[0035] When the confidence level of the determination based on the first derivative feature is lower than a preset threshold, a fusion determination is made by combining the classification confidence level.

[0036] This invention, by employing the above technical solutions, possesses the following advantages and positive effects compared to existing technologies: The concrete vibration compaction detection and adaptive frequency tuning system and method provided by this invention, based on electrical parameters, avoids the need to deploy fragile accelerometers or strain gauges in complex and harsh high-vibration construction sites, greatly improving the system's hardware survivability and feasibility for large-scale promotion. Furthermore, this invention completely abandons the traditional judgment method based on "absolute current value," employing "feature derivative extraction (first derivative)..." The system employs a method of "+continuous N-cycle hysteresis confirmation," supplemented by "adaptive calibration" and "grid voltage compensation" during initial startup, thus exhibiting strong anti-interference and anti-false-judgment capabilities (engineering robustness). This system effectively addresses static reference drift caused by sudden changes in the power grid at the construction site, differences in formwork stiffness, and varying concrete mix proportions, significantly reducing the false alarm rate on-site. Furthermore, addressing the pain points of traditional vibrators—such as "instantaneous shutdown" leading to internal shrinkage micro-cracks in concrete and "continuous high frequency" causing coarse aggregate settling and cement paste floating (over-vibration segregation)—this system, after capturing the compaction plateau, enters a "low-frequency pressure maintenance" state by issuing a "smoothing frequency reduction command" in a closed loop. It utilizes low-amplitude oscillations to complete surface slurry lifting and stress release, achieving a leap from "blindly strong vibration" to "intelligent and refined shaping" in the vibration process. Additionally, the system is compatible with a parallel redundant verification scheme based on time-frequency characteristics and lightweight machine learning, ensuring extremely high decision confidence even under extremely low signal-to-noise ratio conditions. Attached Figure Description

[0037] Figure 1 A schematic diagram of a concrete vibration compaction detection and adaptive frequency modulation system;

[0038] Figure 2 This is a schematic diagram of the time history curve of the active current component as a function of vibration time.

[0039] Figure 3 This is a schematic diagram showing the relationship between the active current component and the relative density of concrete.

[0040] Figure 4 This is a schematic diagram of the time history curve showing the change in vibration frequency of the vibrator with vibration time.

[0041] The numbers in the diagram are as follows:

[0042] 1-Formwork; 2-Concrete; 3-Vibration excitation device; 4-Variable frequency drive device; 5-Control unit. Detailed Implementation

[0043] The concrete vibration compaction detection and adaptive frequency tuning system and method provided by the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. The advantages and features of the present invention will become clearer from the following description. It should be noted that the accompanying drawings are all in a very simplified form and use non-precise proportions, and are only used to facilitate and clarify the illustration of the embodiments of the present invention.

[0044] Example 1

[0045] like Figure 1As shown, the concrete compaction density detection and adaptive frequency adjustment system provided in this embodiment includes a vibration excitation device 3, a frequency converter drive device 4, and a control unit 5. The vibration excitation device 3 is an attached vibrator, fixed to the outside of the formwork 1 of the concrete to be vibrated 2 by bolts. The frequency converter drive device 4 is a frequency converter, electrically connected to the vibration excitation device 3. The frequency converter drive device 4 can control the vibration frequency of the vibration excitation device 3 by outputting different current magnitudes. The frequency converter drive device 4 also integrates an active current component I. q And the real-time acquisition function of output voltage U.

[0046] Control unit 5 is an embedded microcontroller, and its workflow is as follows:

[0047] Control unit 5 reads the active current component I inside frequency converter 4 in real time at a sampling rate fs. q The output voltage U, for example, fs, ranges from 200Hz to 500Hz. Specifically, the control unit 5 sends a read command to the inverter every 4ms, and the inverter returns the current I... q And the value of U.

[0048] Control unit 5 adjusts the active current component I based on the output voltage U. q Compensation and normalization are performed. The purpose of compensation and normalization is to eliminate the impact of power grid voltage fluctuations on current measurement at the construction site. In actual construction, the power grid voltage may fluctuate within a certain range. Without compensation, the current measurement value will change with voltage fluctuations, leading to instability in the compaction determination benchmark. Through compensation and normalization, the current value is converted to the equivalent value under the rated voltage, ensuring the consistency of the determination benchmark.

[0049] Control unit 5 adaptively calibrates a reference threshold based on the statistical characteristics of the active current component during the initial period after vibration start-up. Specifically, the first T after vibration start-up... init For the initial calibration period, such as T init The value is taken between 5 and 10 seconds, and the control unit collects the I value within this time period. q A total of 2000 data points were collected. The control unit processed these 2000 data points as follows: First, a sliding window method was used to find the local minimum of Iq. The window length was 50 data points (corresponding to 0.2s). All data points were traversed to find the minimum value within the window. The average of all window minimum values ​​was taken as the valley threshold I of the concrete liquefaction state. valley_est Secondly, the control unit calculates the standard deviation σ of the first derivative of Iq during this time period. dI Finally, the control unit determines the value based on σ. dI Adaptive setting of the threshold for determining if the first derivative returns to zero , , where k εThis is a coefficient. This threshold is used to subsequently determine the state of the concrete. The technical advantage of adaptive calibration is that different concrete mix proportions (such as C30, C40) and different formwork materials (steel formwork, wooden formwork) will lead to significant differences in the current fluctuation characteristics during the initial stage of vibration. Fixed thresholds cannot adapt to various working conditions, while adaptive calibration can dynamically adjust the judgment threshold according to the actual working conditions, greatly improving the accuracy of the judgment.

[0050] Control unit 5 performs smoothing filtering on the compensated active current component and extracts its first-order time-domain derivative features. Specifically, the control unit uses a moving average algorithm to smooth the I... q To perform smoothing and noise reduction, a sliding window of duration W = 1.0 s is used, corresponding to 250 data points. The moving average algorithm can employ existing formulas. The smoothed signal I... q The envelope effectively suppresses high-frequency noise and transient fluctuations. Then, the control unit calculates the transient first derivative of the envelope. First derivative It reflects the rate of change of the active current component, and its physical meaning is the rate of change of concrete stiffness.

[0051] Control unit 5 identifies the stages of concrete evolution based on the sign and amplitude of the first derivative characteristics, and determines that the concrete has reached a compacted state when preset conditions are met within multiple consecutive sampling periods. Specifically, the control unit uses a three-stage state machine to identify the concrete evolution process. The first stage is the liquefaction period: when... At that time, the concrete is determined to be in the liquefaction stage. For example... Figure 2 As shown, in the initial stage of vibration, the vibration energy causes air bubbles inside the concrete to be expelled, increasing the concrete's fluidity and reducing its stiffness, leading to a decrease in current. The second stage is the consolidation and ramp-up period: when And I q Crossing I valley_est At this point, the concrete is determined to be in the consolidation ramp-up phase. During this time, air bubbles in the concrete gradually dissipate, aggregates rearrange, stiffness begins to increase, and the current rises. The third stage is the compaction plateau phase: when N consecutive sampling periods (e.g., N=10) or confirmation time windows T... confirm Inside, satisfy And the current absolute value (△ is used to prevent misjudgment bias) when the concrete is determined to have reached a dense state. The purpose of the continuous N-cycle confirmation mechanism is to avoid misjudgments caused by transient fluctuations or measurement noise, and to ensure the reliability of the judgment. Figure 2 As shown, the active current component I q The curves showing the change with vibration time clearly demonstrate the three-stage evolution characteristics. For example... Figure 3 As shown, the active current component I q The relationship between the current I and the relative density D of the concrete follows an S-shaped curve. When the density D increases from 75% to 85%, the current I... qIt remains at a low value of 21A to 23A; when the density D increases from 85% to 95%, the current I... q The current Iq rises rapidly to 55A; when the density D reaches more than 95%, the current Iq tends to stabilize.

[0052] After determining that the concrete has reached a compacted state, control unit 5 controls the frequency converter to smoothly reduce the vibration frequency to the holding frequency and maintain it for a preset time before stopping vibration. When the control unit determines that a compacted state has been reached, it immediately sends a smooth frequency reduction command to the frequency converter. The frequency reduction process uses linear ramp control, with the frequency linearly decreasing from the strong vibration frequency of 50Hz to the holding frequency of 30Hz at a reduction slope of 10Hz / s. The holding frequency of 30Hz is maintained for a duration T. hold =10 seconds, during which low-amplitude vibration can complete the surface grouting and stress release of the concrete, avoiding shrinkage micro-cracks caused by sudden shutdown. After the pressure holding period, the control unit sends a shutdown command to the frequency converter, and the vibration process ends. Figure 4 As shown, the frequency control curve of the vibrator over time illustrates the complete frequency conversion control process. From 0 to 25 seconds, the frequency remains constant at 50Hz, indicating the strong vibration phase. From 25 to 27 seconds, the frequency linearly decreases from 50Hz to 30Hz, with a reduction slope of 10Hz / s. From 27 to 38 seconds, the frequency remains constant at 30Hz, indicating the pressure holding phase. At 38 seconds, the frequency drops to 0Hz, and vibration stops.

[0053] Traditional "vibration-time shutdown" methods are prone to causing shrinkage microcracks. This invention, upon identifying the compaction inflection point, does not immediately cut off the power. Instead, it smoothly reduces the frequency to a low-frequency range of 20-40Hz with a slope of 2-10Hz / s. At this point, the excitation force decays quadratically, resulting in a state of slight vibration. Holding the low-frequency pressure for 3-10 seconds effectively completes the surface grouting of the concrete while completely avoiding the settling of coarse aggregate and the floating (segregation) of cement paste caused by continuous high-frequency oscillation.

[0054] It should be noted that the vibration excitation device 3 is considered a macroscopic mechanical sensor. The relationship between the mechanical power of the vibration excitation device and the stiffness and damping of the concrete-formwork system is as follows:

[0055] ;

[0056] in, ;

[0057] In the formula: U is the output voltage, I q F0 is the active current, m is the equivalent mass of the concrete-formwork system, c is the equivalent damping of the concrete-formwork system, k is the equivalent stiffness of the concrete-formwork system, ω is the excitation angular frequency, and f is the vibration frequency of the vibrator.

[0058] As concrete consolidates, the equivalent damping c decreases, while the equivalent stiffness k surges, leading to a reduction in the mechanical impedance modulus. Consequently, the vibration excitation device needs to draw a larger active current I. q To maintain a constant excitation force. When the concrete reaches a dense state, the equivalent stiffness k reaches a stable state (remains unchanged), I q The curve stops its monotonically increasing trend and enters a plateau phase, i.e., the derivative... .

[0059] In low signal-to-noise ratio environments (such as easily resonant light steel formwork), single time-domain derivative determination may suffer from insufficient confidence. Preferably, a lightweight one-dimensional convolutional neural network (1D-CNN) model (model size <2MB) is deployed in parallel in the control unit and stored in the control unit's memory. This is achieved by sampling at a rate >250Hz. q The signal undergoes a short-time Fourier transform to extract the harmonic energy distribution spectrum, which is then input into the neural network. When the decision conditions of the rule-based state machine are ambiguous, the confidence scores of the "loose / appropriate / excessive vibration" classifications output by the machine learning are used as the fusion decision criteria.

[0060] Example 2

[0061] This embodiment provides a method for detecting the compaction density of concrete and adaptive frequency modulation. The method utilizes a variable frequency drive to drive a vibration excitation device to vibrate the concrete, and includes the following steps:

[0062] Collect the active current component I of the frequency converter drive device q and output voltage U;

[0063] For the active current component I q Perform compensation normalization processing;

[0064] During the initial period of vibration start-up, based on the active current component I q The statistical characteristics are adaptively calibrated to determine the benchmark threshold;

[0065] The compensated active current component is smoothed and filtered, and its first-order time-domain derivative features are extracted.

[0066] Based on the sign and amplitude of the first derivative characteristics, the concrete evolution stage is identified, and when the preset conditions are met within multiple consecutive sampling periods, it is determined that the concrete has reached a compacted state.

[0067] After determining that a compaction state has been reached, the variable frequency drive device is controlled to smoothly reduce the vibration frequency to the pressure holding frequency and maintain it for a preset time before stopping the vibration.

[0068] In one specific embodiment, the sampling rate of the active current component is 200Hz to 500Hz; the duration of the initial time period is 5 seconds to 10 seconds; the smoothing filter uses a moving average algorithm, and the duration of the sliding window is 0.5 seconds to 2.0 seconds; the vibration frequency of the frequency converter before determining that a compaction state has been reached is 40Hz to 60Hz, and the voltage holding frequency is 20Hz to 40Hz; the frequency reduction slope of the smoothing reduction of the vibration frequency is 2Hz / s to 15Hz / s.

[0069] In one specific embodiment, the concrete vibration compaction detection and adaptive frequency adjustment method further includes: when the compaction state is not determined after the preset maximum time exceeds the start of vibration, or when the active current component exceeds the limit by a step, the frequency conversion drive device is forcibly stopped and an alarm signal is issued.

[0070] In one specific embodiment, the concrete vibration compaction detection and adaptive frequency modulation method further includes:

[0071] Perform a short-time Fourier transform on the active current component to extract the harmonic energy distribution spectrum;

[0072] The harmonic energy distribution spectrum is input into a pre-trained one-dimensional convolutional neural network model to obtain the classification confidence of the concrete compaction state.

[0073] When the confidence level of the determination based on the first derivative feature is lower than a preset threshold, a fusion determination is made by combining the classification confidence level.

[0074] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0075] The embodiments described above are merely illustrative of several implementations of the present invention, and while the descriptions are specific and detailed, they should not be construed as limiting the scope of the invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these modifications and improvements all fall within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the appended claims.

Claims

1. A concrete vibration compaction density detection and adaptive frequency modulation system, comprising a vibration excitation device and a frequency converter drive device, wherein the vibration excitation device can be attached to a formwork of the concrete to be vibrated, and the frequency converter drive device is signal-connected to the vibration excitation device, characterized in that, Also includes: The control unit, which is communicatively connected to the variable frequency drive device, is used to acquire the active current component I of the variable frequency drive device. q and output voltage U, and active current component I q Perform compensation normalization processing; The control unit during the initial vibration start-up period T init Within, the reference threshold is adaptively calibrated based on the statistical characteristics of the active current component; The control unit performs smoothing filtering on the compensated active current component and extracts its first-order time-domain derivative. ; The control unit is based on the first derivative. The symbols and amplitudes are used to identify the stages of concrete evolution, and the concrete is determined to be in a compacted state when preset conditions are met within multiple consecutive sampling cycles. After determining that a compaction state has been reached, the control unit controls the variable frequency drive device to smoothly reduce the vibration frequency to the pressure holding frequency and maintain it for a preset time before stopping the vibration.

2. The concrete vibration compaction density detection and adaptive frequency tuning system according to claim 1, characterized in that, The duration T of the initial period init It lasts from 5 to 10 seconds.

3. The concrete vibration compaction density detection and adaptive frequency tuning system according to claim 1, characterized in that, The smoothing filter uses a moving average algorithm, with a sliding window duration of 0.5 to 2 seconds.

4. The concrete vibration compaction density detection and adaptive frequency tuning system according to claim 1, characterized in that, The vibration frequency of the variable frequency drive device before determining that a compacted state has been reached is 40Hz to 60Hz, and the pressure holding frequency is 20Hz to 40Hz.

5. The concrete vibration compaction density detection and adaptive frequency tuning system according to claim 1, characterized in that, The frequency reduction slope for the smooth reduction of vibration frequency is from 2 Hz / s to 15 Hz / s.

6. The concrete vibration compaction density detection and adaptive frequency tuning system according to claim 1, characterized in that, The control unit is equipped with a pre-trained one-dimensional convolutional neural network model. The control unit is also used to perform a short-time Fourier transform on the active current component, extract the harmonic energy distribution spectrum, and input the harmonic energy distribution spectrum into the model to obtain the classification confidence level of the concrete density state. When the judgment confidence level based on the first derivative feature is lower than a preset threshold, the classification confidence level is combined for fusion judgment.

7. A method for detecting the compaction density of concrete and adaptive frequency modulation, wherein the method utilizes a variable frequency drive device to drive a vibration excitation device to vibrate the concrete, characterized in that, Includes the following steps: Collect the active current component I of the frequency converter drive device q and output voltage U; For the active current component I q Perform compensation normalization processing; During the initial period of vibration start-up, based on the active current component I q The statistical characteristics are adaptively calibrated to determine the benchmark threshold; The compensated active current component is smoothed and filtered, and its first-order time-domain derivative features are extracted. Based on the sign and amplitude of the first derivative characteristics, the concrete evolution stage is identified, and when the preset conditions are met within multiple consecutive sampling periods, it is determined that the concrete has reached a compacted state. After determining that a compaction state has been reached, the variable frequency drive device is controlled to smoothly reduce the vibration frequency to the pressure holding frequency and maintain it for a preset time before stopping the vibration.

8. The method for detecting and adaptively adjusting the compaction density of concrete according to claim 7, characterized in that, The sampling rate for collecting the active current component is 200Hz to 500Hz; The duration of the initial period is 5 to 10 seconds; The smoothing filter uses a moving average algorithm, with a sliding window duration of 0.5 to 2.0 seconds; The vibration frequency of the variable frequency drive device before determining that a compacted state has been reached is 40Hz to 60Hz, and the pressure holding frequency is 20Hz to 40Hz. The frequency reduction slope for the smooth reduction of vibration frequency is from 2 Hz / s to 15 Hz / s.

9. The method for detecting and adaptively adjusting the compaction density of concrete according to claim 7, characterized in that, Also includes: If the compaction state is not determined after the preset maximum time has elapsed since the vibration has started, or if the active current component exceeds the limit by a step, the frequency converter will be forcibly stopped and an alarm signal will be issued.

10. The method for detecting and adaptively tuning the compaction density of concrete according to claim 7, characterized in that, Also includes: Perform a short-time Fourier transform on the active current component to extract the harmonic energy distribution spectrum; The harmonic energy distribution spectrum is input into a pre-trained one-dimensional convolutional neural network model to obtain the classification confidence of the concrete compaction state. When the confidence level of the determination based on the first derivative feature is lower than a preset threshold, a fusion determination is made by combining the classification confidence level.

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