Disclosure of Invention
The invention aims to provide a friction kinematic pair equivalent model and a modeling method thereof, which can characterize the friction force characteristics of a broadband, and have a relatively simple form so as to be used for predicting, analyzing, designing and controlling the forced vibration response of a local nonlinear mechanical system containing friction pairs.
In order to achieve the above purpose, the invention provides an equivalent model of a friction kinematic pair and a modeling method thereof, wherein the equivalent model can reflect the high-low frequency characteristics of the friction pair by respectively carrying out statistical analysis modeling and superposition on friction force test data of the friction pair according to the high-low frequency bands;
Aiming at the low-frequency band friction force characteristic of the friction pair, introducing positive pressure input information and deforming a traditional LuGre model to obtain an implicit relation between friction force response and relative speed and positive pressure input of the friction pair;
and introducing positive pressure input information according to the high-frequency band friction characteristic of the friction pair and combining friction self-power spectrum assumption in a power function form to obtain an implicit relation between friction response and positive pressure of the friction pair.
Further, the mathematical equation of the friction kinematic pair equivalent model is as follows:
f(N)=aNb+c
Wherein F (N, v) is the friction force output by the friction pair equivalent model and is a function of the friction pair positive pressure N and the tangential relative movement speed v, z is an introduced intermediate variable, usually a function of time, t represents time and ω represents frequency, s (N, v), F (N) and R (N, ω) are all introduced intermediate functions, R (t) is the friction force time history of a high frequency band, the self-power spectral density of the friction force time history is R (N, ω), the rest parameters are to-be-determined coefficients, and the to-be-determined parameters are determined for the specific friction pair, namely the material of the friction pair and the friction contact surface property, and are to-be-determined uniquely.
Further, the undetermined parameters of the friction pair equivalent model can be identified and determined through friction test data, so that the equivalent model modeling of the specific friction pair can be completed. Specific steps of modeling the equivalent model of any specific friction pair include:
S1, carrying out friction test on a friction auxiliary sample piece and recording a friction test related signal, S2, preprocessing the test signal, obtaining a friction force-relative motion speed, a friction force-positive pressure scatter diagram and a friction force self-power spectrum, S3, determining high-frequency-band friction force model parameters based on a least square method, S4, identifying low-frequency-band friction force model parameters based on a least square method and a genetic algorithm, and S5, overlapping to obtain a complete equivalent model.
The equivalent model provided by the invention can reflect the relation between friction force and positive pressure and relative speed, can express the friction force characteristic of a broadband, and has good applicability.
Detailed Description
Further details are provided below with reference to the specific embodiments.
On the basis of the collection and analysis of friction test signals of a large number of friction sub-standard samples, the friction characteristics are statistically analyzed and observed, the characteristics of the low frequency band are basically consistent with the existing research conclusion, the characteristics of the high frequency band show power function characteristics, namely the friction self-power spectrum decays according to a nonlinear power function law along with the increase of frequency, and generally, the friction of the high frequency band increases along with the increase of positive pressure, but the correlation with the relative movement speed is not obvious. Therefore, the invention provides an equivalent model for superposing the low-frequency band characteristic and the high-frequency band characteristic, namely, the model structure is divided into a low-frequency band and a high-frequency band, and the corresponding frequency band filtering signals of the test data are also adopted in the identification of the model parameters. Aiming at a low-frequency band model, the traditional LuGre model is considered to have clear physical meaning and a simpler form, namely fewer model parameters to be identified, and the friction characteristics which can be represented comprise maximum static friction force which is larger than sliding friction force, pre-sliding displacement hysteresis, stribeck effect and sliding friction time lag, so that the traditional LuGre model is adopted, a positive pressure relation is introduced to carry out proper modification on a model structure so as to represent the more complete friction characteristics of the low-frequency band, a friction self-power spectrum power function model containing positive pressure input is directly constructed aiming at a high-frequency band model, the friction time history of the high-frequency band can be further obtained by combining a random phase in consideration of the randomness of a high-frequency friction signal, and then the complete friction output of the whole frequency band can be obtained by directly superposing the low-frequency band friction in a time domain.
According to the thought, the invention provides a novel broadband friction equivalent model of the friction kinematic pair, which is shown as follows.
f(N)=aNb+c (4)
Equation (1) expresses that the friction force F (N, v) output by the friction pair equivalent model is calculated by the low frequency band partAnd the high-frequency part r (t) is directly overlapped in the time domain and is a hidden function of the positive pressure N of the friction pair and the tangential relative movement speed v, wherein z is an introduced intermediate variable, and is usually a function of time t. The friction force of the low-frequency part is given by equations (2) - (4), the equation (4) expresses the power function relation of the friction force f (N) and the positive pressure, the equation (3) expresses the relation of the friction force s (N, v) and the positive pressure and the relative movement speed, N 0 is the reference positive pressure, the equation (2) gives a differential equation met by the intermediate variable z, and obviously, for a given N, v, z and the first derivative thereof can be solved by the equation (2), so that the friction force time history of the low-frequency part is determined. The friction of the high band part is given by equation (5), where ω represents the frequency, ω 0 is the high and low band demarcation frequency, R (N, ω) is the self-power spectral density function of the friction of the high band part, and the high band friction time history R (t) can be found by inverse fourier transform, taking into account the random nature of the friction of the high band part, assuming a random phase spectrum. The rest parameters in the equations (1) - (5) are the undetermined coefficients of the equivalent model, and are determined for the specific friction pair, namely the material of the friction pair and the friction contact surface property, and the undetermined parameters are required to be determined uniquely.
Aiming at any friction kinematic pair, the model structure provided by the invention is shown in equations (1) - (5), wherein the undetermined coefficient can be determined through experimental identification, so that the modeling of the equivalent model of the friction kinematic pair is completed. The method comprises the following specific steps.
S1, performing a friction test on the friction auxiliary sample piece and recording a friction test related signal.
This step should prepare a standard friction pair test piece for the target friction pair, whose material and contact surface properties (surface hardness, roughness, etc.) are consistent with the target friction pair. The friction test should synchronously record positive pressure, friction force and tangential relative motion displacement or speed signals of the friction pair, and should comprise a pre-sliding state and a macroscopic sliding state of the friction pair. The friction pair reciprocating sliding test can be carried out, and the friction pair unidirectional sliding test can also be carried out. Different friction test modes are selected, and different friction pair friction force test devices or equipment are often required. The invention recommends a triangular wave displacement excited friction pair reciprocating sliding friction test, can rapidly carry out the test and acquire signals such as friction force in a required friction state, can acquire relevant information of a friction pair pre-sliding stage in a reversing section of relative reciprocating sliding, can acquire relevant information of a friction pair macroscopic sliding stage in other stable sliding sections at a constant speed, and is convenient for modeling use. The friction pair relative sliding test is usually carried out under a given positive pressure, each test comprises a stable sliding state with a given speed and a transition state from rest to the given speed, and the positive pressure and the speed are adjusted to obtain friction test signals of various states required by modeling.
S2, preprocessing the test signals, and obtaining a friction force-relative movement speed, a friction force-positive pressure scatter diagram and a friction force self-power spectrum.
And filtering the actually measured friction force signals to obtain low-frequency-band friction force signals and high-frequency-band friction force signals respectively. The relative motion displacement signal and the velocity signal (if only one signal is recorded, the other signal can be obtained by differentiation or integration of the two signals) are mutually referenced with the low-frequency band friction signal, so that the friction force of macroscopic sliding and pre-sliding stages is easily distinguished, and the friction force of a constant-velocity sliding section is also easily distinguished. The friction-relative movement speed scatter diagram can be obtained by using the friction signals of the sliding sections with different constant speeds under the given positive pressure, and the friction-positive pressure scatter diagram can be obtained by using the friction signals of the sliding sections with different constant speeds under the different positive pressures. Similarly, a high-frequency band friction-positive pressure scatter diagram can be obtained by utilizing the high-frequency band friction signal, and a self-power spectrogram can be obtained by carrying out spectrum analysis on the high-frequency band friction signal under a given reference positive pressure.
S3, determining high-frequency band friction model parameters based on a least square method.
And (3) determining undetermined parameters in the equation (5) based on a least square method by using the high-frequency-band friction force-positive pressure scatter diagram and the high-frequency-band friction force self-power spectrogram, so as to determine a frequency spectrum model of the high-frequency-band friction force.
S4, identifying low-frequency band friction model parameters based on a least square method and a genetic algorithm.
Each of the undetermined parameters in equations (4) and (3) can be determined based on the least square method using the friction-positive pressure scatter diagram and the friction-relative movement speed scatter diagram of the low frequency band, respectively. And then the friction force time history and the corresponding relative movement speed time history of the low frequency band and the identified intermediate functions s (N, v) and f (N) are utilized to calculate undetermined parameters sigma 0 and sigma 1 in equations (1) and (2) based on a genetic algorithm (of course, other mature optimization algorithms can be selected).
S5, overlapping to obtain a complete equivalent model.
Taking into consideration the random nature of the friction force of the high frequency band, assuming that the friction force has a random phase spectrum, then utilizing the identified and obtained self-power spectral density function of the friction force of the high frequency band, namely (5), the time course of the friction force, namely the friction force r (t) of the high frequency band, can be obtained through inverse Fourier transform, and then the friction force r (t) of the high frequency band is matched with the friction force of the low frequency band obtained in the step S4And superposing to obtain complete friction force output. So far, all undetermined parameters and functions in equivalent model equations (1) - (5) are determined, and the complete equivalent model of the target friction kinematic pair is obtained.
Examples:
Fig. 1 is a photograph of a typical sample of material constituting a friction pair. Aiming at a specific friction pair standard sample, a self-grinding reciprocating friction testing machine is adopted to conduct friction test, related actual measurement signals are obtained, then an equivalent model of the friction pair is established according to the method, and the friction force predicted by the equivalent model is compared with the actual measurement friction force, so that the effectiveness of the equivalent model is illustrated. The specific implementation and results are as follows.
The first step is to install the friction pair standard sample piece on a special friction testing machine, load different positive pressures, set different speeds to perform triangular wave displacement excitation, enable the friction pair sample piece to perform tangential reciprocating relative motion, record the time course of friction force and relative displacement, and typical friction force and relative displacement actual measurement signals are shown in figure 2. During the test, the ambient temperature is 20+/-5 ℃ and the humidity is 75% -85%. The positive pressure and relative velocity selected for the test conditions are shown in table 1.
TABLE 1 test conditions
Selecting test data of different speeds under 30N pressure, carrying out 2Hz low-pass filtering on the friction time history, comparing the relative displacement with the speed time history, solving a friction average value for a friction signal in a constant-speed sliding stage to obtain a friction-speed scatter diagram, selecting test data of 3mm/s speeds under different positive pressures, carrying out 2Hz low-pass filtering on the friction time history to obtain a friction-positive pressure scatter diagram, selecting 30N,3mm/s test data, intercepting the friction time history in one direction of reciprocating motion for removing the fundamental wave influence of the reciprocating motion, subtracting a direct current component, carrying out power spectral density estimation on the direct current component to obtain a friction power spectral density curve of a high-frequency band part, and carrying out high-pass filtering (cut-off frequency is 2 Hz) on the friction time history, and then carrying out spectral analysis on a filtered signal to obtain the friction power spectral density curve of the high-frequency band part, wherein the results are basically the same.
And thirdly, establishing a high-frequency part friction model. That is, based on the least square method, the undetermined parameters in the self-power spectrum model (i.e. equation (5)) are obtained according to the actually measured power spectrum density curve of the friction force of the high-frequency part, fig. 3 shows the comparison between the calculated curve and the actually measured curve of the friction force self-power spectrum model of the high-frequency part, and then the friction force time history of the high-frequency part is obtained by means of inverse fourier transform assuming that a random phase spectrum (uniform distribution) exists.
And fourthly, establishing a low-frequency part friction model. Firstly, based on a least square method, respectively utilizing a friction force-positive pressure scatter diagram and a friction force-speed scatter diagram which are obtained by analyzing measured signals, respectively fitting a f (N) =aN b +c friction force-positive pressure curve and a friction force-positive pressure curveThe friction force-speed curve is used for solving undetermined parameters a, b, c, mu c、μs、vc、σ2 and delta s, and the fitting effects of a friction force-speed scatter diagram and a friction force-positive pressure scatter diagram of a friction test of a PC/ABS-PC/ABS material auxiliary sample are shown in fig. 4. Here, the reference positive pressure is taken to be 30N. And then adopting a genetic algorithm, taking the error standard deviation of the friction time history calculated by the low-frequency-band friction model and the low-pass filtering time history of the experimental measured friction force 2Hz under the working conditions of 30N and 3mm/s as an objective function, and iterating out the parameter sigma 0、σ1 when the objective function is minimum, namely determining the low-frequency-band part friction model, wherein the comparison between the low-frequency-band part friction model prediction and the actual measured time history is shown in FIG. 5.
And fifthly, directly superposing the friction force of the low-frequency part and the friction force of the high-frequency part in a time domain to obtain the total friction force of the friction pair equivalent model, namely, all undetermined parameters in friction pair equivalent model equations (1) - (5) are determined, modeling of the friction pair equivalent model is completed, and by using the equivalent model, all intermediate functions and intermediate variables in equations (1) - (5) and finally equivalent model output required to be calculated, namely, friction force time history, can be calculated as long as the model input, namely, the time history of positive pressure and relative speed (comprising the condition of constant) is given. FIG. 6 shows the effect of comparing the measured time course of friction with the predicted time course of friction of an equivalent model under typical conditions of a friction test of a secondary sample of PC/ABS-PC/ABS material.
Obviously, the PC/ABS-PC/ABS material friction pair equivalent model established by the method can accurately predict the friction force time course, and in fact, the friction pair equivalent model has similar effects for other typical material friction pairs, and FIG. 7 shows comparison of typical actual measured friction force time course and equivalent model prediction time course, the sizes and change trends of the two are basically consistent, so that the equivalent model well represents the dynamics characteristics of the friction pair.
It should be noted that, in the present invention, unless explicitly specified and limited otherwise, terms such as "mounted," "connected," "fixed," and the like are to be construed broadly and include, for example, either fixedly connected or detachably connected or integrally connected, either directly or indirectly connected through an intermediate medium, and also include communication between the two elements. The specific meaning of the above terms in the present invention can be understood by those of ordinary skill in the art according to the specific circumstances.
The foregoing is merely exemplary embodiments of the present application, and specific structures and features that are well known in the art are not described in detail herein. It should be noted that modifications and improvements can be made by those skilled in the art without departing from the structure of the present application, and these should also be considered as the scope of the present application, which does not affect the effect of the implementation of the present application and the utility of the patent. The protection scope of the present application is subject to the content of the claims, and the description of the specific embodiments and the like in the specification can be used for explaining the content of the claims.