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通讯作者:

李伟,E-mail:liwei@main.xidian.edu.cn

中图分类号:O302

文献标识码:A

文章编号:1672-6553-2022-21(10)-034-010

DOI:10.6052/1672-6553-2022-067

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目录contents

    摘要

    本文研究了在FOPID控制器控制下的广义Van Der Pol随机系统瞬态概率密度函数和可靠性函数变化情况.首先,引入广义谐和函数,将快变变量转换为慢变变量,并利用分数阶微积分的性质,获得了FOPID控制器在慢变变量形式下的新表达式.在此基础上,由于径向基神经网络具有准确性高,易于求解高维问题,求解速度快等优势,所以我们应用径向基神经网络分别对该随机系统所满足的前向和后向柯尔莫哥洛夫方程进行求解,得到随机系统的瞬态概率密度函数和可靠性函数.最后,通过分析控制器中分数阶导数和分数阶积分对Van Der Pol随机系统响应和可靠性的影响,我们得到结论,分数阶控制器一定程度上会增强系统的响应,并导致分岔.

    Abstract

    In this paper, the transient probability density function and reliability function of generalized Van Der Pol stochastic system controlled by FOFOPID controller are studied. Firstly, the generalized harmonic function is introduced to transform the fast variable into the slow variable, and the new expression of FOFOPID controller in the form of slow variable is obtained by using the properties of fractional calculus. On this basis, because the radial basis function neural network has the advantages of high accuracy, easy to solve high-dimensional problems, fast solving speed and so on, we apply the radial basis function neural network to solve the forward and backward Kolmogorov equations satisfied by the stochastic system respectively, and obtain the transient probability density function and reliability function. Finally, by analyzing the influence of fractional calculus in the controller on the response and reliability of Van Der Pol stochastic system, we obtain the conclusion that fractional order controller can enhance the response of the system to a certain extent and lead to bifurcation.

  • 引言

  • 随着分数微积分在理论和应用方面的快速发展,分数阶微积分由于对粘弹性材料的完美模拟而扩展到随机动力学系统的模型[1-3].因此,分数阶随机动力系统的分析成为近年来的热点问题之一.

  • 由于分数微积分的成就,经典PID控制器引入到动力学系统之中,用于控制飞行器的振动[4]、磁悬浮系统的稳定[5]、随机系统可靠性的控制[6]等.虽然系统可以得到较好的控制,但可调节参数较少,仍会有一定局限性,从而Podlubny[7]提出名为FOPID控制器,是一种更有效的控制工具,近年来逐渐用于一些确定性微分方程[8-10]中的控制系统稳定性.此外FOOPID控制器还会应用在各个系统之中,其中,Riyad[11]分别将PID和FOPID控制器应用到核能源系统中,得到该系统下两个控制器的最优控制参数,在自动智能化光伏系统中,Firas[12]加入FOPID控制器用于稳定传感器的电流和电压;Bapayya[13]将FOPID控制器引入到传统无刷直流电机系统中,用于控制该电机的速度和转矩.作为传统积分阶PID控制器的新推广,FOPID控制器包括线性比例算子、分数阶积分器和分数阶微分器,并且其中有五个可调参数以实现期望的控制目的.许多应用[14-16]证明,FOPID控制器在分数阶动态系统的控制方面比经典PID控制器表现得更好.

  • 系统中的随机振动在结构工程[17]、航空工程[18],土木工程[19]等存在噪声激励(如受强气流干扰的飞机)的情况下的随机振动通常被建模为数学中的分数随机微分方程,噪声对动力学的累积影响可以分别通过关于系统响应的平稳或瞬态概率密度或随机可靠性来测量,它们相应地由Fokker-Planck-Kolmogorov(FPK)方程或者Backward-Kolmogorov(BK)方程进行分析描述.因此,获得系统响应知识的重要任务是求解FPK和BK方程.

  • 通常,这两个方程都是具有非线性和变系数项的偏微分方程(FPK方程是具有归一化条件、初始条件和边界条件的前向Kolmogorov方程,但BK是具有初始条件和边缘条件的后向Kolmagorov方程),它们必须基于一些基于网格的技术进行数值求解.经典计算方法之一是有限元法[20-23].有限元方法高度依赖于计算域的网格具体化,网格的稀疏性和密集性极大地影响了计算的准确性和效率.另一种常用的求解FPK和BK方程的方法是蒙特卡罗模拟(MCS)[24],该方法基于大数定律,需要大量采样路径和长轨迹才能获得系统响应.MCS是一种无网格方法,与边界条件无关.然而,它在很大程度上取决于生成的数据量和长时间的消耗,MCS方法的解决方案有时不可靠或不太准确.

  • 由于深度学习算法的快速发展,神经网络已广泛应用于图识别[25]、语言处理[26]、天气预报[27]、智能驾驶[28]等领域.数学家还尝试使用神经网络来获得FPK方程的解.例如,Xu[29]使用人工神经网络(ANN)求解一类随机系统响应的平稳概率密度函数(PDF),他们建立的算法不需要任何插值和坐标变换.Li[30]将神经网络应用于随机肿瘤免疫模型,用于求解FPK方程,以讨论肿瘤的平均首次通过时间(MFPT)问题.但这些神经网络求解中都有高维求解困难,耗时长等问题.为了解决上述问题,且由于多个高斯函数可以逼近任何概率分布,因此Wang[31]提出运用高斯径向基神经网络求解瞬态概率密度.

  • 本文进一步将高斯径向基神经网络应用到含有FOPID控制器的广义VDP系统之中.将偏微分方程(FPK方程和BK方程)与边界条件作为该神经网络的损失函数,使用初始条件求解系统初始参数,通过线性求和的方式构造该神经网络,以此来直接求得各个时刻的精确解.达到有效求解各个时刻瞬态概率密度函数和可靠性函数的目的.

  • 1 含有FOPID控制器的广义VDP随机系统的近似

  • 本文考虑了高斯白噪声激励下的广义VDP随机系统,旨在通过FOPID控制器研究系统的瞬态响应和可靠性.该系统对应的微分方程形式为:

  • x¨+β1-β2x2+β3x4x˙+ω02x=εu(x,x˙)+W(t)
    (1)
  • 其中ω0是广义VDP系统的固有频率,βii=1,2,3)是常数,ε是一个标量参数,也是FOPID控制器的系数.Wt)是一个均值为零,相关函数是Rτ)=E[WtWt+τ)]=2τ)的高斯白噪声,Dτ分别表示噪声强度和相关时间.uxx˙)=k1xt)+k2Dαxt)+k3Iβxt)是FOPID控制器,Dαxt)是α阶的分数阶导数,Iβxt)是β是阶的分数阶积分.

  • 在不同领域对于分数阶导数和分数阶积分有多种定义,在这些定义中,Caputo定义在随机动力系统中更常用,因为Caputo定义只需要具有良好物理意义的整数阶导数.因此,我们也将在本文中考虑Caputo意义下的分数阶导数和分数阶积分.

  • α阶的分数阶导数定义如下:

  • (2)
  • β阶的分数阶积分定义为:

  • Iβx(t)=1Γ(β)0t x(τ)(t-τ)1-βdτ,β>0
    (3)
  • 其中Γ(·)为Gamma函数,且Γx=0 tx-1e-tdt.

  • β1β2β3ε同阶即系统受到的阻尼力为弱阻尼且高斯白噪声激励为弱激励时,系统为准守恒系统,响应为准周期状态.该条件下,可以应用广义谐和函数将系统的快变变量转化为慢变变量:

  • x(t)=a(t)cosφ(t)x˙(t)=-a(t)ω0sinφ(t)
    (4)
  • 其中φt)=ω0t+θt),at)和θt)系统响应的振幅和相位,均为慢变变量.由于分数阶导数和积分定义的复杂性,通常情况下针对0<αβ<1的情况进行讨论.该情况下,应用Gamma函数的性质Γ(α)Γ(1-α)=π/sinπα代替其定义式(2),分数阶导数Dαxt)可以用二重积分表示.进行变换后,该分数阶导数可以重写为一个新的函数ψyt)的广义积分,其表达式如下[6]

  • ψ(y,t)=sinπαπαa(t)ω02cosφ(t)-ω0y1αsinφ(t)ω02+y2α+c1e-ty1α
    (5)
  • 常数c1可由ψy,0)=0求出.再将ψyt)的表达式代入Dαxt=0 ψytdy中得到

  • Dαx(t)acosφω0αcosπα2-asinφω0αsinπα2+ctαΓ(α+1)
    (6)
  • Dαxt=DnIn-αxt=In-αDnxt,分数阶积分由慢变变量表示为:

  • Iβx(t)asinφω0-βsinπβ2+acosφω0-βcosπβ2+sinπβπct1-βΓ(1-β)
    (7)
  • 将公式(4)回代到公式(9)和公式(10)中,得到分数阶导数和积分项可等效记为经典阻尼力和恢复力之和,FOPID控制器uxx˙)近似等价为u~xx˙

  • u~(x,x˙)=k1+k2ω0αcosπα2+k3ω0-βcosπβ2x+k2ω0α-1sinπα2-k3ω0-β-1sinπβ2x˙
    (8)
  • 从而系统(1)的等效随机系统为:

  • x¨+β1-β2x2+β3x4x˙+ω02x=εu~(x,x˙)+W(t)
    (9)
  • 2 高斯径向基神经网络

  • 考虑到随机系统的响应和可靠性主要通过求解FPK方程和BK方程得到瞬态概率密度函数和可靠性函数进行研究,且任何平滑的概率密度函数都可以应用足够多的高斯函数以任意精度来进行逼近[32],所以本文结合高斯函数的优势,引入高斯径向基神经网络对此问题进行求解讨论.

  • 2.1 神经网络结构

  • 假设是一个很小的时间步长.在第步中,我们将瞬态概率密度或可靠性函数fkΔτx0)表示为f~x0ωk.我们用具有高斯激活函数的神经网络解来表示:

  • f~x0,ω(k)=i=1N1 ωi(k)gx0,μi,Σi
    (10)
  • 其中是高斯函数的个数,模型参数形成一个向量ωk=ω1kω2kωN1kT.解的时间依赖性是通过系数ωk)的时间依赖性进行表现. gx0μiΣi是一个高斯函数形式的神经元,其均值为μi和协方差矩阵Σi.

  • gx,μi,Σi=1(2π)2σi22exp-12σi2x-μi2=12πσi1exp-12σi12x1-μi12×12πσi2exp-12σi22x2-μi22
    (11)
  • 图1为该二维系统下高斯径向基神经网络结构图,其中gi表示gx0μiΣi.

  • 图1 高斯径向基神经网络结构图

  • Fig.1 Structure diagram of Gaussian radial basis function neural network

  • 2.2 瞬态概率密度损失函数

  • 由于概率密度函数只在有限区间内取值不为零,所以在高斯函数系数确定和采样时我们选择区间D=[-3,3]×[-5.5,5.5]进行.将该区间分成间隔为0.1的网格,每个网格中点都是一个高斯函数的均值点,且网格间隔为方差值.对于x的采样,选择在该固定区间上随机均匀采样,且采样点个数N2与高斯函数个数关系约为N2=4N1.

  • 随机微分方程(9)的瞬态响应可由前向柯尔莫哥洛夫确定性偏微分方程来解释:

  • pt=-x(x˙p)-x˙(cp)+D2x˙2p=LFPK[p]
    (12)
  • 其中

  • c=-β1-εk2ω0α-1sinπα2+εk3ω0-β-1sinπβ2-β2x2+β3x4x˙-ω2xω2=ω02-εk1-εk2ω0αcosπα2-εk3ω0-βsinπβ2

  • RBFNN解f~[xωk]需满足式(12),f~t求偏导进行差分计算得到:

  • f~t=1τ{f~[x,ω(k+1)]-f~[x,ω(k)]}+εt
    (13)
  • εt为截断误差,将式(13)代入到式(12)中,可以得到神经网络误差e[xωk)]为

  • e[x,ω(k)]=f~[x,ω(k-1)]+i=1N1 si(x)ωi(k)k=1,2,
    (14)
  • 其中

  • si(x)=-gx,μi,Σi+τLFPKgx,μi,Σi
    (15)
  • 瞬态概率密度函数在每个时刻R2空间求积分都为1,所以RBF-NN解应满足:

  • R2 f~[x,ω(k)]dx=1
    (16)
  • 又由于神经网络激活函数为高斯函数,根据高斯函数的性质,瞬态概率密度归一化条件可以表示为:

  • (17)
  • RBF神经网络的损失函数被定义为积分均方误差和归一化条件求和的形式:

  • J[c(k)]=j=1N2 12e2xj,ω(k)+λ(k)i=1N1 ωj(k)-1xjD
    (18)
  • 其中ck=ωTkλkT为参数集,我们通过最小化损失函数J[ck)],得到在归一化条件下均方误差最小解.定义矩阵S=sil=sixlG=Gil=gixl将式(14)带入式(18)中,进一步得到损失函数的矩阵形式:

  • J[c(k)]=cTAc(k)+cTBc(k-1)-bλ+d(k-1)
    (19)
  • 式中ARN1+1×N1+1SRN1×N2构成,向量bλRN1+1×1和d(k-1)的具体表达式如下:

  • A=112SST1000, B=0SGT0000

  • bλ=[0,00,1]Tdk-1=j=1N2 f~2xjωk-1 最小化J[ck)],对ck)求导令导数为0:

  • c(k)J=A+ATc(k)+Bc(k-1)-bλ=0
    (20)
  • 根据多次计算模拟的经验,当采样点x的个数为N2高斯函数个数的四倍及以上时,矩阵A+AT为可逆矩阵,因此,当采样点个数足够多时,参数ck)可以由以下方程求得:

  • c(k)=A+AT-1bλ-Bc(k-1)
    (21)
  • 初始时刻t=0,考虑初始点为x0=(0,0),概率密度函数为δ函数,即在初始点处为1,为此,我们将初始时刻近似看做高斯函数,将(0,0)点处的高斯函数权重设为1,来进行迭代计算k时刻的ck),以便求得时刻的概率密度函数.

  • 2.3 可靠性损失函数

  • 可靠性或者首次穿越时间概率是衡量随机系统是否能够安全可靠地工作的一个重要问题.一般来说,随机可靠性的目的是确定一个由噪声激发的动力系统在给定的时间间隔内保持在一个预先指定的安全域内的概率.

  • 在随机振动领域,有关于单自由度可靠性函数安全域的类型有单边型,双边型和包络型如图1.由于求解方法相同,本文只针对包络型安全域问题进行讨论,设置安全域范围为S=[-2,2]×[-2,2],在S上设置间隔为0.05的网格,高斯函数的均值为各个网格的中点,方差为间隔值.在求解可靠性的问题上,我们将采样分成两部分,一部分在安全域内S随机均匀取值N2个点,一部分在安全域的边界S取值M2个点.

  • 图2 安全域类型(a)单边型;(b)双边型;(c)包络型

  • Fig.2 Security domain type (a) unilateral; (b) bilateral type; (c) envelope type

  • 该系统可靠性函数可求解下式BK方程得到:

  • Rt0=-x˙0x0R-a2x˙0R-D2x˙02R=LBK[R]
    (22)
  • 与计算概率密度函数步骤相同,将RBF神经网络解对时间求偏导并进行差分,移项后得到神经网络误差e0x0ωk

  • e0x0,ω(k)=f~x0,ω(k-1)+i=1N1 si0x0ωi(k)
    (23)
  • 式中

  • e0x0,ω(k)=f~x0,ω(k-1)+i=1N1 si0x0ωi(k)
    (24)
  • RBFNN求解可靠性函数时除了需要满足BK方程之外,还需满足下面的边界条件和初始条件:

  • si0x0=-gx0,μi,Σi+ΔτLBKgx0,μi,Σi
    (25)
  • 于是我们将BK方程误差值e0x0ωk和边界条件构成可靠性函数的损失函数:

  • f~ω(k),x0=0,x0Sf~ω(0),x0=1,x0S
    (26)
  • 当使得损失函数J0最小时,即神经网络解在满足各个时间段边界条件的情况下,BK方程的均方误差项达到最小.将式(23)带入式(26)中,损失函数J0进一步化简写成矩阵形式为:

  • J0ω(k),x0=12ωTA0+λ(k)Rbω(k)+ωTB0ω(k-1)+d0(k-1)
    (27)
  • A0RN1×N1B0RN1×N1,被定义为与slx0i)有关的矩阵,RbRN1×N1被定义为与gly0i)有关的矩阵.具体定义如下:

  • S0=sli0=sl0x0iG0=Gli0=gx0i, μl, Σl, x0iSGb=Glj0=gy0i, μl, Σl, y0iSA0=S0S0T, B=S0G0T, Rb=GbGbTd0 (k-1) =12ωT (k-1) G0G0Tω (k-1)

  • 将损失函数J0[ωk),x0]最小化,即分别对ωk)和λk)求偏导数令其等于0知:

  • J0ω(k)=A0+λ(k)Rbω(k)+B0ω(k-1)=0
    (28)
  • J0λ(k)=12ωTRbω(k)=0
    (29)
  • 由式(29)可知,ωk)为Rb零空间的任意向量,于是令ZRb的零空间,Z不为方阵,有ωk)=Zvk),vk)为任意向量,于是求解ωk)的问题转化为求解vk)的问题,将ωk)=Zvk)带入式(28)中求解得到vk)的迭代公式为:

  • v(k)=-ZTA0Z-1ZTB0Zv(k-1)
    (30)
  • 针对可靠性函数初始条件,对初始时刻的ω(0)进行求解,且ω(0)仍为Rb零空间的任意向量即满足可靠性函数的边界条件,构造初始时刻的损失函数为:

  • J00=j=1N2 12Rx0j, ω (0) -12

  • =12v(0)TZTG0G0TZv(0)-v(0)TZTG0e+12eTe,x0jS
    (31)
  • 其中eRN2×1且每个元素ei都为1.最小化J00可以得到v(0):

  • v(0)=ZTG0G0TZ-1ZTG0e
    (32)
  • 同样地,当采样点个数足够大时,约为高斯函数个数的4倍时,ZTA0ZZTG0G0TZ均为可逆矩阵.根据式(30),式(32)和ωk)=Zvk)得到各个时刻的ωk)后,带入式(10)即可得到随机系统各个时刻的可靠性函数.

  • 3 分数阶微积分对随机系统的控制

  • 图2第一行是t=5s时刻下,αβ分别为0.1、0.5、0.9时的概率密度函数图,可以看到随着FOPID控制器中分数阶导数和分数阶积分的同时增大,PDF极值先变小再变大,极值由1到3再到1的过程,产生分岔.随着时间的增大,中点极值越来越小,两侧极值逐渐产生,并且概率密度函数几乎不再产生变化,如第二行t=30s时刻所示,系统的瞬态概率密度趋向平稳概率密度.当αβ同时增大时,概率密度函数极值由一个峰变为三个峰最后变为两个峰.

  • 图3 t=5s和t=30s时刻不同分数阶导数和分数阶积分下的瞬态概率密度图

  • Fig.3 Transient probability density diagram under different fractional derivative and fractional integral at t=5s and t=30s

  • 图4 分数阶导数和分数阶积分分别变化下的平稳概率密度截面图

  • Fig.4 Section of stationary probability density with fractional derivative and fractional integral changing respectively

  • 固定分数阶积分β=0.5,令分数阶导数α变化时,平稳概率密度函数截面图如图3左,可以看到随着α越来越大,pxx˙)在x˙=0的截面图取值越来越小,且产生随机分岔现象,极值点个数由1个变为3个,且有变为2个的趋势.固定分数阶导数α=0.5,分数阶积分β发生变化时,平稳概率密度函数截面图如图3所示,变化幅度不大,平稳概率密度函数一直是三个峰的状态,但概率密度值随着β的变大越来越小.

  • 设置安全域为包络型安全域下可靠性函数的变化相对于系统概率密度函数的变化不大,图4展示了在不同时间下,可靠性函数随分数阶导数和分数阶积分的变化情况.

  • 图5 t=Tt=2T时刻不同分数阶导数和分数阶积分下的可靠性函数图

  • Fig.5 Reliability function diagram under different fractional derivative and fractional integral at t=T and t=2T

  • 不同分数阶导数和分数阶积分下,可靠性函数在固定时刻2T时的截面图如图5所示,可以看到αβ同时增大时,可靠性函数值也略有增加.

  • 图6 分数阶导数和积分同时变化下可靠性函数截面图

  • Fig.6 Section of reliability function under simultaneous change of fractional derivative and integral

  • α=0.5,β=0.5情况下,可靠性函数随时间的变化情况如图5,随着时间的增大,系统穿越安全域的可能性即首次穿越概率也就越大,因此在安全域内的概率即可靠性函数也就越来越小,从初始时刻的1逐渐减小到0.

  • 图7 可靠性函数随时间变化图

  • Fig.7 Graph of reliability function versus time

  • 4 结论

  • 本文对高斯白噪声激励下含有FOPID控制器的广义VDP系统的响应和可靠性,通过高斯径向基神经网络进行求解讨论,并分析了不同分数阶导数和分数阶积分值下随机系统的性质.

  • 首先,通过广义变换将分数阶导数和分数阶积分近似等价于恢复力和阻尼力和的形式,然后提出高斯径向基神经网络分别对随机系统的FPK方程和BK方程进行求解,得到其瞬态概率密度函数和可靠性函数.最后讨论了分数阶导数和分数阶积分对可靠性函数和概率密度函数峰值的影响.

  • 结果表明,分数阶微积分对概率密度影响较大,且会导致分岔,当分数阶微积分同时增大时,概率密度中心峰值减小,两侧峰值显现并增大.基于此可知,通过调整FOPID控制器中的分数阶微积分参数,随机系统响应的演化可得到理想控制.FOPID控制器在本文参数下对可靠性函数的影响微小,但随分数阶微积分同时增大,可靠性函数仍有微小增大,且可靠性函数随着时间的增加单调递减到0.

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