METHOD FOR MEASURING VIBRATIONS OF A VIBRATION MACHINE

Description

[0001] The present invention relates to methods for measuring vibrations of a vibrating machine.

[0002] Machines with a vibrating unit can be damaged by the self-generated vibration if the vibration of the vibrating unit is equal to or similar to the natural frequency of the machine. Due to the periodic excitation, more and more energy is transferred to the system. This constructive interference stores energy in the system until a resonance catastrophe occurs. In the worst case, this can destroy the machine.

[0003] Such resonance catastrophes initially become noticeable through deformations of the machine. Deformation is an undesirable change in certain reference points of the machine relative to one another. This can be, for example, compression, stretching, or twisting. Deformations can also occur independently of the natural frequency, for example, if a heavy, one-sided load is applied to the machine and / or the machine is used for purposes for which it was not designed or was only inadequately designed.

[0004] The vibrations can be desired movements of the vibrating machine as a whole (translation) or represent undesired deformations of the vibrating machine.

[0005] Measuring such deformations is difficult because they can be lost in the overall vibration of the machine.

[0006] Previous attempts have been made to visualize such deformations using so-called "stroke cards." These "stroke cards" are attached to the corners and edges of the machine, and a line is then drawn by hand on them during operation. The machine's vibrations can then be approximately determined based on the drawn line.

[0007] It is also known to use stroboscopes, which project a rapidly flickering light onto the machine and, with a suitable frequency, "freeze" the machine's movements. This allows any deformations to be observed visually. A disadvantage of this approach is the difficulty of measuring such a visual observation. Furthermore, deformations in a vibrating machine are to be expected in the millimeter range. In a machine several meters in size, deformations can easily be overlooked. Furthermore, many parts of the machine are difficult or impossible to reach, so deformations can also be overlooked.

[0008] Furthermore, systems are known in which vibration sensors are attached to the vibrating machine. An evaluation unit then receives the data from the vibration sensors and can determine the vibration and resonance behavior of the vibrating machine by analyzing the measured vibrations. Typical vibration sensors in this case are acceleration sensors. One problem here is the complex calibration of the vibration sensors to one another. In order for the evaluation unit to draw the correct conclusions, the measured vibrations of the individual vibration sensors must be correlated with one another. For this, the distance between the individual vibration sensors and their orientation relative to one another must be known. Previously, these parameters were measured manually.

[0009] DE 10 2017 009 373 B3 discloses a mobile device for recording the condition and operating parameters of vibrating machines.

[0010] Furthermore, DE 10 2012 014 277 A1 discloses a mobile device for detecting vibrations on a machine with rotating machine elements, in which the spatial orientation of the sensor measuring direction is detected during a vibration measurement by means of a gyroscope and the vibration measurement can be assigned to the spatial orientation of the sensor accordingly.

[0011] Furthermore, EP 2 320 203 A1 discloses a device for measuring vibrations on a machine with rotating machine elements, comprising an acceleration / inclinometer sensor for measuring acceleration forces resulting from the machine vibrations to be measured and for measuring gravity. Evaluation electronics are configured to determine the orientation of the sensor with respect to gravity from the stationary component of the sensor output and to evaluate the non-stationary components of the sensor output according to the determined sensor orientation.

[0012] The object of the present invention is to calibrate the orientation of vibration sensors to each other easily and quickly.

[0013] A further object of the present invention is to measure vibrations of a vibrating machine simply, accurately and repeatably during operation.

[0014] One or more objects are achieved by the subject matter of the independent claims. Advantageous further developments and preferred embodiments form the subject matter of the dependent claims.

[0015] In one method, vibrations of a vibrating machine, in particular of a vibrating screen and a vibrating conveyor, are measured during operation using a vibration sensor. The vibration sensor detects vibration components of the vibrations of the vibrating machine in three spatial directions of a Cartesian sensor coordinate system. The three vibration components form a vibration vector. In vibration measurement methods, a gravitational vector describing the direction of gravity is determined. Furthermore, a machine vector describing a preferred direction of an implied vibration of the vibrating machine is determined. In addition, a three-dimensional Cartesian measurement coordinate system is created, which is aligned based on the direction of gravity and the preferred direction. The vibration vector is mapped from the sensor coordinate system into the measurement coordinate system.

[0016] Using the method described here, a vibration sensor can be placed anywhere on a vibrating machine without manually measuring its orientation. This also makes it possible to place vibration sensors in difficult-to-reach locations, since attaching them, e.g., using magnets, is usually easier than determining the orientation with an orientation measuring device, such as a spirit level.

[0017] Furthermore, the automatic evaluation of the orientation of the vibration sensor is more accurate than a manual measurement, since the manual measurement can only be carried out using the housing of the vibration sensor, while the automatic measurement uses the measured values of the vibration sensor itself.

[0018] Additionally, the automated method is significantly faster than manual measurement, as it requires only a few vibration cycles. This means that enough data is collected within a few seconds to reliably determine the orientation of the vibration sensor. With manual measurement, however, a user must first approach the sensor and then perform the measurement relative to the vibrating machine, which takes longer than these few seconds.

[0019] The vibration sensor is an acceleration sensor that measures and outputs the acceleration applied to it in three dimensions.

[0020] A vibration within the meaning of the present invention is a wound-up vibration. This can be generated, for example, with an oscillating drive, such as an unbalanced motor. The vibration can be one-dimensional, i.e., along one direction, or two-dimensional, i.e., along two directions. So-called elliptical vibration or circular vibration are known in this case. Vibrations in three directions are also possible.

[0021] The entire oscillating machine oscillates in the same direction, driven by vibration. The method described here exploits this to define the second vector, in addition to gravity, to maintain orientation in three-dimensional space.

[0022] The machine vector describes directly or indirectly the preferred direction of vibration.

[0023] Direct means that when determining the machine vector, the preferred direction runs along the one-dimensional vibration direction, or in the case of multi-dimensional vibrations, along the direction with the greatest deflection.

[0024] Indirect means that the machine vector is determined by a mathematical transformation from the preferred direction. For example, the machine vector can be perpendicular to the preferred direction. This is particularly useful when dealing with a circular vibration in which both directions of vibration are equally pronounced.

[0025] Generally, the machine vector points in the preferred direction. However, it is also possible for the machine vector to be a projection of the preferred direction onto a predetermined plane. This plane is, for example, a plane spanned by two axes of another transition coordinate system, which is explained in more detail below using the exemplary embodiment.

[0026] The mapping of the measurement coordinate system and the mapping of the vibration into the measurement coordinate system can begin partially before the determination of the machine vector or the gravitational vector. In this case, the vibration vector is rotated depending on the already determined machine vector or gravitational vector.

[0027] The direction of gravity, which is described by the gravitational vector, points towards the Earth's center of gravity.

[0028] By specifying the gravitational vector and the machine vector, two vectors are available that are independent of the orientation of the vibration sensor and are fixed in relation to the vibration machine, but can be determined by the vibration sensors.

[0029] To determine the orientation of an object—here the vibration sensor—in space, two vectors are required. These are the gravitational vector and the machine vector.

[0030] This method only works when the vibrating machine is in operation. Only during operation is vibration imposed on the vibrating machine, so that a machine vector can be determined.

[0031] According to the invention, the gravitational vector is determined as the temporal average of the oscillations.

[0032] Gravity is the only acceleration force that acts on the vibration sensor in the same direction over time. The imposed vibrations oscillate around a zero value. These imposed vibrations therefore average to zero over time. A time averaging thus results in a gravitational vector that reflects the direction of the Earth's gravity.

[0033] For temporal averaging, a duration of at least one vibration cycle is required. However, several vibration cycles are preferred. Typically, values of a few seconds are averaged to also average out harmonic and subharmonic vibrations.

[0034] According to the invention, to determine the machine vector, the vibrations are converted into Fourier space using a Fast Fourier Transformation (FFT). The amplitudes of the vibrations determined there at an excitation frequency specified by the vibrating machine form an amplitude vector pointing in the preferred direction, allowing the machine vector to be determined.

[0035] The oscillating machine oscillates at an excitation frequency determined by the vibration. This frequency is usually adjustable and is set by the oscillation drive. If the oscillations are converted into Fourier space using FFT, there are three amplitude values at the position of the excitation frequency, one for each spatial direction of the sensor coordinate system. These three amplitudes represent a vector that encompasses the sum of all vectors in which the vibration occurs. If the vibration is one-dimensional, this amplitude vector corresponds to the vibration vector. If the vibration is two-dimensional and elliptical, the amplitude vector is the addition of the vibration vector along the major axis and the vibration vector along the minor axis of the elliptical vibration. The amplitude vector has a fixed, predetermined relationship to the preferred direction.

[0036] This fixed reference can be designed in such a way that the amplitude vector is set equal to the preferred direction.

[0037] Alternatively, this fixed reference can also include a rotation. Such a fixed rotation means that the determined amplitude vector is rotated in a specific way. This can be, for example, such that the vector is rotated 90° along an axis, mirrored, or mapped onto a given axis.

[0038] The fixed reference can also be achieved by having the machine vector be a mapping of the amplitude vector to a predetermined plane. This can be achieved, for example, by replacing one of the vector elements, such as the one for the z-axis, with zero.

[0039] According to a first embodiment, a first axis of the measuring coordinate system runs parallel or antiparallel to the gravitational vector.

[0040] This first axis is usually the Z-axis and generally describes the height.

[0041] According to the first embodiment, a second axis of the measuring coordinate system runs along the part of the machine vector that is orthogonal to the first axis.

[0042] It is usually not the case that the machine vector is perpendicular to the gravitational vector. Therefore, both vectors cannot easily form an orthogonal coordinate system. Therefore, only the portion of the machine vector orthogonal to the first axis is chosen to form the second axis of the measurement coordinate system.

[0043] According to an alternative embodiment, the steps are reversed, so that the first axis of the measurement coordinate system runs parallel or antiparallel to the machine vector, and the second axis of the measurement coordinate system runs along the portion of the gravitational vector that is orthogonal to the first axis. Preferably, the position of the vibration sensors is determined by a radio location system, in particular a satellite location system, such as a global positioning system (e.g., GPS, GLONASS, Galileo, Beidou). The radio location system can also be a mobile radio network, with the sensor exchanging bearing signals with the nearest neighboring transmitters of the mobile radio network.

[0044] This makes it particularly easy to transfer the vibration sensors to the corresponding coordinate system without manually determining their position. It also simplifies determining the sensor spacing.

[0045] Preferably, timestamps are added to the vibration measurement at regular intervals.

[0046] These timestamps can be relative, for example, indicating the microseconds that have elapsed since the last full hour, or absolute, representing the absolute time. These timestamps are contained in the oscillation signals. To enable the sensors to provide absolute timestamps, they have a clock that can be synchronized with an external clock.

[0047] Preferably, the inaccuracy of the clock is not greater than 400 µs, preferably 200 µs and in particular 100 µs.

[0048] Preferably, several vibration sensors are aligned.

[0049] If multiple vibration sensors are aligned, the measured vibrations are also aligned and related to each other. This makes it possible to analyze the measured vibrations in relation to each other.

[0050] For example, it is possible to measure deformations of the vibration machine by subtracting the measured vibrations of two vibration sensors aligned with each other.

[0051] Preferably, the spatial assignment of the vibration sensors to each other is maintained during alignment.

[0052] This means that two vibration sensors that were originally spaced a certain distance apart will maintain this distance even after alignment. Two vibration sensors that are 3 m apart will remain 3 m apart even after alignment.

[0053] Preferably, these distances are predetermined.

[0054] The predetermined sensor spacing allows for absolute representation of deformations between two vibration sensors. Thus, it makes a difference whether the deformation has an amplitude of 2 mm and the sensors are spaced 5 cm or 5 m apart.

[0055] In addition, the distance makes it possible to display the deformation or vibrations in relation to the dimensions of the vibrating machine.

[0056] Preferably, one of the vibration sensors is selected as a reference vibration sensor and the reference vibration sensor defines the origin of a deformation coordinate system.

[0057] The deformation coordinate system therefore vibrates relative to the surrounding space. However, if the vibration sensors are viewed in this deformation coordinate system, the translational movement is eliminated, and the movements displayed in the deformation coordinate system represent a deformation.

[0058] Preferably, all relative vibration signals are calculated with respect to the reference vibration sensor.

[0059] This makes it possible to track a change in deformation originating from the reference vibration sensor. For example, a deformation in the area around the reference vibration sensor may be more likely to correspond to a stretch, while in the area further away, it may be more likely to correspond to a twist.

[0060] Preferably, the reference vibration sensor is arranged as close as possible to the vibration source.

[0061] The vibration source is usually a vibration motor.

[0062] In this context, as close as possible means that the reference vibration sensor is positioned no further than 50 cm, preferably no further than 25 cm and in particular no further than 10 cm from the vibration source.

[0063] The vibration source is the primary reason why deformation can occur in a vibrating machine. Furthermore, the strongest vibrations occur at the vibration source. By subtracting these vibrations by placing the reference vibration sensor as close as possible to the source, the deformation of the vibrating machine caused by the vibration can be tracked.

[0064] Preferably, if more than two vibration sensors are used, they span an area or a room.

[0065] If a space is spanned, deformation can be determined along all three spatial axes. If only a surface or a line is spanned, deformations can only be determined within the surface or along the line.

[0066] A system for determining the orientation of a vibration sensor on a vibrating machine or vibrating conveyor during operation is designed to carry out a method described above. The system comprises at least one vibration sensor and an evaluation unit.

[0067] The invention is explained in more detail below with reference to the examples shown in the drawings. The drawings schematically show: Figure 1 shows a side view of a vibrating machine with a vibration sensor and an evaluation device, Figure 2 shows a side view of a vibrating conveyor trough with attached vibration sensors and vibration drive, Figure 3 shows various coordinate systems with respect to the gravitational force and the direction of vibration, Figure 4 shows a method for measuring vibrations of a vibrating machine in a flow chart, Figure 5 shows a method of rotating the vibration vector with respect to the gravitational vector in a flow chart, and Figure 6 shows a method of rotating the vibration vector with respect to the machine vector in a flow chart.

[0068] An embodiment of a system for resonance analysis of a vibrating machine 1 comprises a vibrating conveyor 2, a vibration detection device 3 and an evaluation device 4 ( Figure 1 ).

[0069] The vibratory conveyor 2 is designed for conveying bulk materials, such as castings. However, any other application is also possible. The vibratory conveyor 2 comprises a vibratory conveyor trough 5 with side walls 8 and a vibratory drive 6.

[0070] The vibrating conveyor trough 5 is designed with a conveyor floor (not shown) arranged in a vibrating frame. The conveyor floor transports the bulk material. A vibrating drive 6 is connected to the vibrating frame and sets it in vibrating motion.

[0071] The oscillating drive 6 is aligned at a predetermined angle to the conveyor floor (not shown) and causes it to oscillate in a predetermined direction.

[0072] The oscillating drive 6 can be adjusted with regard to the oscillation frequency.

[0073] The vibratory drive is arranged at a predetermined angle a to the vibratory conveyor trough 5.

[0074] The vibrating conveyor trough 5 is aligned at a predetermined angle β with respect to gravity.

[0075] The conveying behavior can be influenced by adjusting the angle α, β and / or the frequency.

[0076] The oscillating drive 6 comprises an unbalance motor, which in this embodiment is a three-phase motor with an unbalance weight (not shown) with an adjustable radius at one shaft end. The amplitude of the generated oscillation can be changed by manually adjusting the unbalance. The frequency is determined by the motor speed.

[0077] In the present embodiment, two counter-rotating drives are used. A single motor would produce a circular motion rather than a linear oscillation.

[0078] The vibration detection device 3 has at least one vibration sensor 7. The vibration sensor 7 is an acceleration sensor that measures vibrations of the vibratory conveyor 2 in all three spatial axes. Each vibration sensor 7 measures the vibration along three sensor axes specified by the vibration sensor 7. The sensor axes are recorded on the vibration sensors 7. The sensor axes are permanently linked to the orientation of the vibration sensor 7. If the vibration sensor 7 rotates, the sensor axes also rotate.

[0079] The vibration sensor 7 and the vibration drive 6 have a certain angle to the vibrating conveyor trough 5 ( Figure 2 ).

[0080] The vibration sensor 7 is connected to the evaluation device 4 via a radio link. Wired connections are also possible.

[0081] The vibration sensor 7 is designed as a so-called "Micro-Electro-Mechanical System" (MEMS).

[0082] The connection between the vibration detection device 3 and the evaluation device 4 can be made via radio, for example Bluetooth, WLAN, ZigBee, Z-Wave or via a mobile network, or can be cable-based, for example via a LAN network.

[0083] The vibration sensor 7 is connected to the vibratory conveyor 2 via an attachment device. This connection can be fixed (e.g., by screws) or detachable (e.g., by adhesive strips or a clamping mechanism).

[0084] The evaluation device 4 can be configured as a computer, a conventional smartphone, or a tablet. The evaluation device has a receiving module for receiving the data from the vibration sensor 7. The evaluation device 4 processes the data from the vibration sensor 7 using an additional software application, also called an app.

[0085] The evaluation device 4 can also comprise a display device which optically outputs the acquired and further processed data.

[0086] The following explains the procedure for determining the alignment of a vibration sensor on a vibrating machine ( Figure 5 ).

[0087] The procedure for measuring vibrations of a vibrating machine is explained below ( Figure 4 ). The process begins with step S1.

[0088] In the next step (S2) an oscillation vector S =(S_x, S_y, S_z) measured with the vibration sensor 7. The vibration vector S is located in the sensor coordinate system k1.

[0089] Then a gravitational vector G determined (step S3). For this purpose, a mean value vector (M_x, M_y, M_z) is formed from the vibration sensor (S_x, S_y, S_z). The mean value vector forms the gravitational vector G in the coordinate system k1 ( Figure 3a ).

[0090] To calculate the average, a time average of a few seconds, typically 2 to 60 seconds, is calculated. However, choosing longer intervals is also beneficial. At least 10 times the inverse of the excitation frequency should be selected to allow for a reasonable Fourier transformation later.

[0091] In the following steps S4-S10 the oscillation vector S in relation to the gravitational vector G turned.

[0092] The rotation begins with step S4 ( Figure 5 ).

[0093] This is followed by step S5, in which an angle W_xz of the gravitational vector G in the xz-plane.

[0094] The angle W_xz is calculated using the mean values M_x and M_z with an arctangent function.

[0095] In the following step (S6), the oscillation components S_x, S_y, S_z of the oscillation vector S are transformed into a new coordinate system k2, which is a transition coordinate system, by coordinate transformation via rotation with the angle W_xz. S _ y _ k 2 = S _ y .

[0096] Then, in step S7, a mean value M_z_k2 is calculated from the transformed oscillation component S_z_k2 of the transformed oscillation vector S_k2.

[0097] Subsequently, the angle W_yz of the gravitational vector G calculated in the Y_k2, Z_k2 plane (step S8) ( Figure 3b ). The angle W_yz is calculated using the mean values M_y, M_z_k2 with an arctangent function.

[0098] In the next step (S9), the oscillation components S_y_k2 and S_z_k2 of a transformed oscillation vector S_k2 are transformed into a new coordinate system k3, which is a transition coordinate system, by coordinate transformation via rotation with the angle W_yz. Here, S_x_k3 = S_x_k2. The rotation of the oscillation vector S with respect to the gravitational vector ends with step S10.

[0099] Subsequently, in step S11, the machine vector is formed ( Figure 4 ).

[0100] For this purpose, an amplitude vector A with the components A_x, A_y and A_z. The amplitude vector Ais determined by transforming the transformed vibration components S_x_k3, S_y_k3, S_z_k3 into Fourier space using a fast Fourier transformation (FFT). Subsequently, the amplitudes A_x, A_y, and A_z are determined in the Fourier spectrum of the individual vibration components at the operating frequency of the vibration drive 6.

[0101] The amplitude vector A runs along a preferred direction.

[0102] The machine vector M is given by (A_x, A_y, 0) This is projection of the amplitude vector A to the XY plane of the coordinate system k3 ( Figure 3c ).

[0103] In the following step S12, a three-dimensional Cartesian measurement coordinate system k4 is formed. Here, the Z-axis of k4 points in the opposite direction of the gravitational vector G. The X-axis of the measuring coordinate system runs along the machine vector M.

[0104] In steps S13-S17, the transformed oscillation vector S_k3 is related to the machine vector M rotated ( Figure 6 ).

[0105] The rotation begins with step S13.

[0106] In the next step (S14) the angle W_xy of the machine vector M in the X_k3, Y_k3 plane via its components A_x and A_y using an arctangent function. Subsequently, in step S15, the transformed vibration components S_x_k3 and S_y_k3 are transformed into the measurement coordinate system k4 by coordinate transformation via rotation with the angle W_xy. Here, S_z_k4 = S_z_k3.

[0107] This is followed by step S16, in which the orientation of the X and Y axes is corrected. At this point, it is still unknown whether the X_k4 axis and the Y_k4 axis span a right-handed or left-handed coordinate system. This can be calculated using the phase difference between the components of the machine vector A_x and A_z. If a left-handed coordinate system is present, this can be corrected by changing the sign of the X or Y axis.

[0108] The rotation is completed with step S17.

[0109] This also ends the mapping of the oscillation vector S from the sensor coordinate system k1 into the measuring coordinate system k4 (S18).

[0110] The rotations in steps S4-S10 and S13-S17 correspond to the mapping into the measuring coordinate system.

[0111] According to the invention, the oscillation signals S_x, S_y, S_z are rounded to an integer number of the excitation frequency periods. This improves the mean value calculation for determining the gravitational vector G. Reference symbol

[0112] 1System for resonance analysis of a vibrating machine 2Vibration machine 3Vibration detection device 4Evaluation device 5Vibration conveyor 6Vibration drive 7Vibration sensor 8Side cheek

Claims

1. Method for measuring vibrations of a vibrating machine (1), in particular a vibrating screen or a vibrating conveyor (2), during operation, in which the vibrating machine (1) vibrates at an excitation frequency predetermined by a vibrating drive (6), with a vibration sensor (7) which detects vibration components (S_x, S_y, S_z) of the vibrations of the vibrating machine (1) in three spatial directions of a Cartesian sensor coordinate system (kl), wherein the three vibration components (S_x, S_y, S_z) form a vibration vector (S) and the following steps are carried out: - determining a gravitational vector (G) which describes the gravitational direction, wherein the gravitational vector is determined as the time average of the vibrations (S_x, S_y, S_z) of the vibrating machine (1) detected by the vibration sensor, wherein the vibration signals (S_x, S_y, S_z) are rounded to an integer number of the periods of the excitation frequency, - determining a machine vector (M) which describes a preferred direction of an imposed vibration of the vibrating machine, wherein the vibrations (S_x, S_y, S_z) are converted into the Fourier space by a Fast Fourier transformation in order to determine the machine vector, and the amplitudes, determined there, of the vibrations of an excitation frequency specified by the vibrating machine form an amplitude vector (A) which points in the preferred direction, - forming a three-dimensional Cartesian measurement coordinate system (k4) which is aligned on the basis of the gravitational direction and the preferred direction, wherein a first axis of the measurement coordinate system (k4) extends parallel or antiparallel to the gravitational vector (G) and a second axis of the measurement coordinate system (k4) extends along the portion of the machine vector (M) which is orthogonal to the first axis, or a first axis of the measurement coordinate system (k4) extends parallel or antiparallel to the machine vector (M) and a second axis of the measurement coordinate system (k4) extends along the portion of the gravitational vector (G) which is orthogonal to the first axis, and - mapping the vibration vector (S) from the sensor coordinate system (k1) into the measurement coordinate system (k4).

2. Method according to claim 1, characterized in that the position of the vibration sensor (7) is ascertained by a global positioning system, e.g. GPS, GLONASS, Galileo, Beidou.

3. Method according to any one of claims 1 or 2, characterized in that time stamps are added to the measurement of the vibrations at regular intervals.

4. Method according to any one of claims 1 to 3, characterized in that several vibration sensors are aligned.

5. Method according to claim 4, characterized in that the spatial assignment of the vibration sensors to one another is retained during the alignment.

6. Method according to claim 4 or 5, characterized in that one of the vibration sensors (7) is selected as the reference vibration sensor and the reference vibration sensor defines the origin of the measurement coordinate system (k4).

7. Method according to claim 6, characterized in that all vibration signals are calculated with respect to the reference vibration sensor.

8. Method according to any one of claims 6 or 7, characterized in that the reference vibration sensor is arranged as close as possible to and no further than 50 cm from the vibration drive (6), preferably no further than 25 cm and in particular no further than 10 cm.

9. Method according to any one of claims 6 to 8, characterized in that, if more than two vibration sensors (7) are used, they span a surface or a space.

10. System for determining the alignment of a vibration sensor on a vibrating machine or a vibrating conveyor during operation, which is designed to carry out a method according to claims 1 to 9, wherein the system comprises at least one vibration sensor (7) and an evaluation unit.