Review of Research on Vehicle Hydro-Pneumatic Suspension Technology
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摘要:
油气悬架具有缓冲减振、车身姿态调整等功能,其结构复杂、压力冲击大、耐磨性与密封性要求高. 优良的悬架油缸结构、油气悬架系统及控制方法是决定车辆行驶性能的重要条件. 从悬架结构、工作特性与控制方式等方面系统阐述了油气悬架的构成型式,归纳总结油气悬架的分类与原理;基于悬架可控制性角度,从油气悬架的结构设计与优化、数学建模、控制算法与策略等方面论述油气悬架技术,分析现有结构的特点与不足后得出:被动悬架结构简单、技术成熟,但缺乏自适应性;半主动悬架能耗小、成本较低、响应快、可靠性高,但自适应性有限;主动悬架性能优良,但能耗大、成本高、系统结构与控制策略复杂. 总结与展望3种悬架的发展现状和研究方向,为车辆油气悬架设计与控制方法的深入研究和发展提供参考.
Abstract:The hydro-pneumatic suspension has the functions of cushioning and damping, body attitude adjustment, etc., but its structure is complex, with large pressure impact and high wear resistance and sealing. The excellent suspension cylinder structure, hydro-pneumatic suspension system, and control method are the important conditions to determine the driving performance of the vehicle. The type of hydro-pneumatic suspension was analyzed, and the classification and principle of hydro-pneumatic suspension were summarized from the aspects of suspension structure, working characteristics, and control mode. From the perspective of suspension controllability, the hydro-pneumatic suspension technology was discussed in terms of structural design and optimization of hydro-pneumatic suspension, mathematical modelling, and control algorithm and strategy. By analyzing the characteristics and shortcomings of existing structures, it is concluded that passive suspension is simple in structure and mature in technology, but it lacks adaptability. The semi-active suspension has low energy consumption, low cost, fast response, and high reliability, but its adaptability is limited. Active suspension has excellent performance, but it has high energy consumption, high cost, and complex system structure and control strategy. The development status and research direction of the three types of suspension were summarized and prospected, so as to provide a reference for the further research and development of vehicle hydro-pneumatic suspension design and control methods.
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尺寸综合作为机械设计中的经典问题,一直是机构学领域的研究热点[1]. 常见尺寸综合方法主要有解析法[2-5]、优化法[6-9]、数值图谱法[10-12]等. 其中,数值图谱法通过建立各种机构轨迹特征的电子图谱库,把机构的尺寸综合问题转化为数据库的搜索问题,其不仅具有解的多样性强、适用范围广等优点,还能避免尺寸综合中的顺序、分支和回路等缺陷. 随着近几年计算机技术的飞速发展,数值图谱法在机械设计领域受到越来越多研究者的关注与重视[13–15].
目前,使用数值图谱法实现尺寸综合的研究已取得大量成果[16–21]. 但现有研究主要集中在闭环轨迹的尺寸综合,而在许多工程实际中,设计要求通常是非封闭的开环轨迹[22,23]. 与闭环轨迹的尺寸综合相比,开环轨迹综合的设计参数更多、曲线特征提取更加困难、综合过程也更加复杂.
现有使用数值图谱法实现开环轨迹尺寸综合的研究整体上可分为2类:一类是把开环曲线拓展成闭环曲线,然后利用闭环轨迹的尺寸综合方法实现开环轨迹的尺寸综合. 由于对开环轨迹进行拓展,通常只能满足拓展后的整体轨迹精度要求,难以保证开环轨迹部分的尺寸综合精度[24-25]. 另一类是通过引入设计区间参数,并在数值图谱库中分别单独存储轮廓段的形状特征参数及设计区间参数,从而实现开环轨迹的尺寸综合. 如Yu等[26]结合斜率转角与P型傅里叶级数描述四杆机构连杆轨迹曲线,实现开环轨迹的尺寸综合,并开发了一套计算机辅助设计软件. SUN等[27,28]使用Haar小波变换更准确地描述了开环轨迹的形状特征,并以平面四杆机构为例,实现开环轨迹的尺寸综合. 刘文瑞等[29]基于小波特征参数和多维搜索树,提出了非预定相对转动区间轨迹综合方法,实现平面四杆机构非预定相对转动区间轨迹综合问题的求解. 然而,上述方法需要先对目标轨迹进行归一化处理,才能得到与机架位置、机架偏转角度和整体缩放均无关的轨迹形状特征,增加了轨迹综合过程中的计算量. 此外,这些方法还需要将一条轨迹分为多个不同的轮廓段,并在图谱库中分别单独存储这些轮廓段的特征参数才能实现轨迹匹配,导致同一基本尺寸型的机构会占用大量存储量空间,不利于丰富图谱库中的机构类型. Deshpande等[30]提出了基于曲率特征与机器学习的开环轨迹综合方法,该方法虽无需引入额外的区间设计参数,但曲率不仅需要对目标轨迹预先归一化处理,而且对非光滑曲线形状特征描述也较差. 此外,机器学习需要大量时间来训练样本数据,该方法得到的设计结果精度不高,综合过程耗时较长,检索效率较低.
针对上述缺陷,本文提出一种新的开环轨迹尺寸综合方法. 首先,利用弦角描述符表述轨迹的形状特征,在无需对轨迹归一化预处理和引入额外区间设计参数的条件下,即可直接提取具有旋转、平移和缩放不变性的轨迹形状特征. 然后,利用弦角描述符的自包含属性,提出可实现部分匹配的开环轨迹匹配算法. 在此基础上,结合多维尺度缩放与层次聚类算法,建立具有层次聚类结果的平面四杆机构数值图谱库,以实现杆长参数的快速检索,提高图谱库的搜索速度.
1. 开环轨迹的弦角描述符表述及特征
1.1 开环轨迹的弦角描述符表述
弦角描述符是一种基于曲线轮廓特征的形状表述符号[31]. 如图1所示,实线$ {P_1}{P_n} $为一平面四杆机构的运动轨迹,$ P = \left\{ {{P_1},{P_2},\cdots,{P_n}} \right\} $为该迹曲线上的顺序等间距采样点序列集合. 对于曲线上的任意2个非重复采样点$ {P_i} $和点$ {P_j} $,弦角${\theta _{i,j}}$的定义如式(1)~(3)所示.[20]
θi,j={∠(→PiPj,→PjPm),|i−j|>Δ,0,|i−j|⩽Δ, (1) Pm={Pj+Δ,i>j,Pj−Δ,i⩽j, (2) ∠(→PiPj,→PjPm)=|arccos→PiPj⋅→PjPm|→PiPj||→PjPm||. (3) 式中:${\theta _{i,j}}$的取值范围为[0,π];$i、j $为采样点的顺序索引;$\varDelta $为索引位移参数,通常取3~5时效果较好(本文后续计算均取$\varDelta = 4$)[32]. $ {P_m} $为$ {P_i} $和$ {P_j} $之间的另一个采样点.
为使得弦角对机构轨迹形状的描述更加符合人类视觉特性[32],可将$ {\theta _{i,j}} $转化到对数空间中表示,如式(4)所示.
θzh,ij=log(1+θi,j), (4) 式中:${\theta_{{\mathrm{zh}},ij}} $为转化到对数空间中的弦角.
对图1轨迹上的任意2个不同采样点,按照式(1)~式(4)构造其弦角,可得到整个开环轨迹的弦角描述符矩阵$ {\boldsymbol{A}} $为
A=[θzh,1,1⋯θzh,1,k⋯θzh,1,k+s⋯θzh,1,n⋮⋮⋮⋮θzh,k,1⋯θzh,k,k⋯θzh,k,k+s⋯θzh,k,n⋮⋮⋮⋮θzh,k+s,1⋯θzh,k+s,k⋯θzh,k+s,k+s⋯θzh,k+s,n⋮⋮⋮⋮θzh,n,1⋯θzh,n,k⋯θzh,n,k+s⋯θzh,n,n], (5) 式中:$ n $为轨迹采样点数量;$k,s \in [1,n]$,且$ 1 < k < n $,$ 0 < s < n - k $.
1.2 开环轨迹的弦角描述符特征
从上述弦角描述符矩阵的构造过程不难发现,式(5)矩阵的每个元素本质上都是角度对数值. 由于角度不会随曲线的平移、旋转和缩放的变化而改变,式(5)的矩阵具有平移、旋转和缩放不变性.
为进一步说明,图2(a)给出了一条平面开环轨迹曲线示例,图2中的横坐标和纵坐标分别表示轨迹在图1坐标系下的x轴坐标值和y轴坐标值. 图2 (b)为图2 (a)曲线水平向右平移0.5的结果;图2 (c)为图2 (a)曲线逆时针旋转90°后的结果;图2 (d)为图2 (a)曲线缩小一半后的结果.
对图2的曲线分别按照式(5)构造其弦角描述符矩阵,并绘制其灰度图,如图3所示. 对比图3(a) ~(d)的结果容易看出,图3 (a)~(d)的结果完全相同,结果表明:使用弦角描述符表示开环轨迹的形状特征,无需进行归一化处理,可大大减少轨迹形状特征提取过程的计算量.
除此之外,通过式(5)还可以发现,曲线的弦角描述符矩阵中包含了该曲线部分轮廓段的形状信息. 例如,设目标轨迹$ {P^*}{\text{ = }}\left\{ {{P_k},{P_{k + 1}},\cdots,{P_{k + s}}} \right\} $是机构轨迹$ P $的一部分,对于该目标轨迹,按照式(1)~式(4)可构造其弦角描述符矩阵$ {{\boldsymbol{A}}^*} $为
A∗=[θzh,k,k⋯θzh,k,k+s⋮⋮θzh,k+s,k⋯θzh,k+s,k+s], (6) 对比式(6)和式(5)结果不难看出,$ \boldsymbol{A}^{*} $为$ \boldsymbol{A} $中对角线上截取从第$ k $个元素到第$ k + s $个元素所形成的子矩阵,即一条轨迹的弦角描述符矩阵包含了该条轨迹的所有局部轮廓段信息,即式(5)具有自包含属性.
为进一步说明,图4给出了2条开环轨迹. 其中,图4(a)的轨迹${P_{\text{S}}}$是图4(b)轨迹$ {P_{\text{W}}} $的下半部分. ${P_{\text{S}}}$、$ {P_{\text{W}}} $的弦角描述符矩阵${{\boldsymbol{A}}_{\text{S}}}$、${{\boldsymbol{A}}_{\text{W}}}$的灰度图分别如图4(c)和图4(d)所示. 对比图4(c)和图4(d)的结果发现,${{\boldsymbol{A}}_{\text{S}}}$为${{\boldsymbol{A}}_{\text{W}}}$的红色虚线框所示部分,该结果表明:若在轨迹$ {P_{\text{W}}} $中找到与${P_{\text{S}}}$最相似的部分,只需要在${{\boldsymbol{A}}_{\text{W}}}$中找到与${{\boldsymbol{A}}_{\text{S}}}$最相似的部分,即可得到该机构轨迹中找到与目标轨迹最相似的轮廓段. 其具体匹配算法将在下一节中给出.
对于开环轨迹的尺寸综合,目标轨迹往往只与机构轨迹的某一小部分形状相似. 因此,利用弦角描述符的自包含属性,将轨迹用弦角描述符表示后,只需要在机构轨迹的弦角描述符中找到与目标轨迹弦角描述符最相似的部分,便可实现开环轨迹的部分匹配.
图2~4表明:使用弦角描述符表示开环轨迹的形状特征,只需用一个矩阵就能存储该条轨迹的所有轮廓段形状特征,其结果不仅与机构的机架位置、机架偏转角度、整体缩放比例均无关,还具有自包含属性,可实现开环轨迹的部分匹配.
2. 开环轨迹的弦角描述符匹配
利用1.1节得到的弦角描述符矩阵,2条轨迹的匹配将直接转化为2个矩阵的匹配. 即在机构轨迹的弦角描述符矩阵中,沿着对角线找到与目标轨迹弦角描述符矩阵较为相似的子矩阵,从而实现开环轨迹的匹配.
值得注意的是,受轨迹采样点数量的影响,目标轨迹采样点数目可能出现比机构轨迹采样点数目更多的情况,即目标轨迹采样分辨率比机构轨迹采样分辨率高. 因此,为了在轨迹匹配过程中避免采样分辨率的影响,实际匹配时可先对目标轨迹进行重采样,以确保目标轨迹采样点数量比机构轨迹采样点数量少.
本文给出的基于弦角描述符的开环轨迹部分匹配具体过程如下:
步骤1 给定目标轨迹的采样点数$\alpha $,对目标轨迹进行重采样,使得目标轨迹的采样点数少于机构轨迹的采样点数$\beta $.
步骤2 对重采样后的目标轨迹和机构轨迹分别按照式(5)构造其对应的弦角描述符矩阵$ {{\boldsymbol{A}}_\alpha } $和$ {{\boldsymbol{A}}_\beta } $.
步骤3 在机构轨迹的弦角描述符矩阵$ {{\boldsymbol{A}}_\beta } $中,以对角线上不同元素$k$为起点,截取与$ {{\boldsymbol{A}}_\alpha } $大小相同的子块,得到$ {{\boldsymbol{A}}_\beta } $的子矩阵$ {\boldsymbol{A}}_\beta ^{(k)} $,并利用二维互相关系数计算每个子矩阵$ {\boldsymbol{A}}_\beta ^{(k)} $与$ {{\boldsymbol{A}}_\alpha } $的相似度$ \rho (\alpha ,k) $,如式(7)所示.
ρ(α,k)=|Cov(Aα,A(k)β)√D(Aα)√D(A(k)β)|, (7) 式中:$ D({{\boldsymbol{A}}_\alpha }) = \displaystyle\sum\limits_{i = 1}^\alpha {\displaystyle\sum\limits_{j = 1}^\alpha {{{\left( {{{\boldsymbol{A}}_{\alpha{ij}} } - \overline {{{\boldsymbol{A}}_\alpha }} } \right)}^2}} } $,$ D({\boldsymbol{A}}_\beta^{(k)}) = \displaystyle\sum\limits_{i = 1}^\alpha \displaystyle\sum\limits_{j = 1}^\alpha {{{\left( {{\boldsymbol{A}}_{\beta{ij}} ^{(k)} - \overline {{\boldsymbol{A}}_\beta ^{(k)}} } \right)}^2}} $,$ {\mathrm{Cov}}( {{\boldsymbol{A}}_\alpha },{\boldsymbol{A}}_\beta ^{(k)}) = \displaystyle\sum\limits_{i = 1}^\alpha \displaystyle\sum\limits_{j = 1}^\alpha \left( {{\boldsymbol{A}}_{\alpha{ij}} } -\overline {{{\boldsymbol{A}}_\alpha }} \right)\times \left( {{\boldsymbol{A}}_{\beta{ij}} ^{(k)} - \overline {{\boldsymbol{A}}_\beta ^{(k)}} } \right) $,$ \overline {{{\boldsymbol{A}}_\alpha }} $和$ \overline {{\boldsymbol{A}}_\beta ^{(k)}} $分别为矩阵$ {{\boldsymbol{A}}_\alpha } $与$ {\boldsymbol{A}}_\beta ^{(k)} $中所有元素之和的平均值.
$ \rho (\alpha ,k) $的范围为[0,1],且$ \rho (\alpha ,k) $越接近1,表示目标轨迹形状与机构轨迹该部分的轮廓段形状越相似;反之,$ \rho (\alpha ,k) $越接近0,表示相似度越低.
步骤4 逐渐增加$\alpha $值,并重复步骤2~3,直到$\alpha = \beta $. 记${\rho _{\max }} = \max \{\rho (\alpha ,k)\}$,并记录${\rho _{\max }}$对应$k$与$ \alpha $的值,即可得到机构轨迹中与目标轨迹最相似轮廓段的起始位置与长度.
上述匹配过程的流程如图5所示. 为进一步说明,图6给出了1条目标轨迹和1条机构轨迹,其中目标轨迹形状是机构轨迹形状的一部分,但目标轨迹有251个采样点,机构轨迹有50个采样点. 根据上述匹配流程,得到$ \rho (\alpha ,k) $的计算结果如图7所示. 从图7中可以看出,当$ k = {\text{16}} $且$ \alpha = {\text{25}} $时,$ \rho $取得最大值
0.9968 . 这说明对于图6(a)的目标轨迹,在图6(b)的机构轨迹中,从第16个采样点开始,且长度为25个采样的机构轨迹轮廓段与该目标轨迹最为相似.为方便直接观测,图8为机构轨迹的匹配轮廓段结果. 从图8中可以看出,得到的匹配轮廓段与目标轨迹几乎完全重合. 该结果表明:该匹配算法不仅能够实现开环轨迹的部分匹配,而且对于不同采样分辨率的目标轨迹与机构轨迹同样适用.
表1为图8匹配结果所用时间,为方便对比,表中还给出了其他常见的描述符如B样条曲线描述符、曲率描述符、傅里叶描述符的匹配所用. 其中,归一化的计算采用文献[16]的方法.
表 1 轨迹匹配过程所用时间Table 1. Time consumption of path matching process描述符 归一化
时间/ms相似度
计算时间/ms总时间/ms 减少时间
百分比/%弦角描述符 13 13 B样条曲线描述符 12 11 23 43 曲率描述符 12 8 20 35 傅里叶描述符 12 9 21 38 从表1中可以看出,由于弦角描述符无需对轨迹进行归一化处理,在轨迹匹配过程中的时间只有相似度计算时间,因而所花费的总时间相较于其他描述符的时间都要少. 因此,利用弦角描述符可减少匹配过程的计算量,从而缩短时间.
3. 平面四杆机构图谱库的建立与查询
本节以平面四杆机构为例,通过建立机构的尺寸型及弦角描述符图谱库,并结合多维尺度缩放变换和层次聚类算法对图谱库进行压缩和聚类,以对实现图谱库快速查询.
3.1 平面四杆机构图谱库的建立
本文所用平面四杆机构尺寸型参数示意如图9所示. 其中,${l_0}$为机架长度,${l_1}$为驱动杆杆长,${l_2}$为连杆杆长,${l_3}$为连架杆杆长,$Q$为连杆上生成运动轨迹的端点,${l_4}$和$\phi $分别为用来确定$Q$点位置的长度与角度.
为避免数据冗余,可采用量纲一(相对杆长),即取${r_0} = {l_0}/{l_0} = 1$,${r_1} = {l_1}/{l_0}$,${r_2} = {l_2}/{l_0}$,${r_3} = {l_3}/{l_0}$,${r_4} = {l_4}/{l_0}$以及$\phi $作为存储参数. Deshpande等[22]研究表明:当${r_1}$~$ {r_3} $接近1时,机构的运动轨迹形状对${r_1}$~${r_3} $变化敏感度更高. 因此,在生成不同尺寸型的机构轨迹时,为丰富图谱库中轨迹形状,并尽可能减少相似的尺寸型以节省图谱库存储空间,使${r_1} $~${r_3} $在1附近非均匀变化. 具体而言,令${r_1} $~${r_3} $的变化满足对数正态分布,如式(8)所示. ${r_4}$的变化满足正态分布,如式(9)所示.
lnY∼N(0,0.6),Y=r1,r2,r3, (8) r4∼N(0,0.2). (9) 并让$\phi $在$[ - 2{\text{π}} ,2{\text{π}} ]$内均匀变化,以此生成
16000 组不同尺寸型的平面四杆机构. 需要注意的是,生成的平面四杆机构中既有曲柄机构,也有双摇杆机构.对于每组参数的机构,让驱动杆逆时针转动,其中双摇杆机构的转动范围可通过文献[33]计算得到,以此得到不同尺寸型平面四杆机构点$Q$的运动轨迹,每个轨迹有50个等间距均匀采样点. 得到机构运动轨迹后,按照1.1节的方法,计算每条轨迹的弦角描述符矩阵,从而得到
16000 组50 × 50维的弦角描述符矩阵.对于给定的目标轨迹,计算出弦角描述符矩阵,并按第2节方法在图谱库中找出与该弦角描述符最相似的矩阵块,即可在数据库中匹配识别出目标轨迹机构尺寸型.
3.2 图谱库的降维与聚类
为提高图谱库检索效率,本文采用层次聚类算法对图谱库进行自动分类,以提高图谱库检索效率. 同时,为避免聚类过程中因维数过高出现“维度诅咒”问题[34],聚类前先使用多维尺度缩放变换[35](MDS)对弦角描述符矩阵进行降维处理.
3.2.1 弦角描述符的MDS降维
本文使用经典MDS算法对本文图谱库中弦角描述符矩阵进行降维,其基本流程如下.
步骤1 对于每个弦角描述符矩阵,将其展开为
2500 × 1维的向量,从而将整个图谱库转化为2500 ×16000 维的数据集,记该数据集为$ X \in {\mathbb{R}^{h \times g}} $,其中,$h = 2\;500$为样本原始维度,$g = 16\;000$为样本数量.步骤2 利用欧式距离定义数据集$X$中任意两样本的距离,得到$X$的距离相似度矩阵${\boldsymbol{D}}$,如式(10)所示.
D=[d11⋯d1j⋯d1g⋮⋮⋮di1⋯dij⋯dig⋮⋮⋮dg1⋯dgj⋯dgg], (10) 式中:$dij=‖Xi−Xj‖,i,j∈{1,2,⋯,g}$.
步骤3 计算互相关矩阵${\boldsymbol{B}}$. ${\boldsymbol{B}}$的元素$ {b_{ij}} $($i, j \in \{1,2,\cdots,g\}$)通过式(11)计算.
bij=−12T1+12gT2+12gT3−12g2T4, (11) 式中:$ {T_1} = d_{ij}^2 $,$ {T_2} = \displaystyle\sum\limits_{i = 1}^g {d_{ij}^2} $,$ {T_3} = \displaystyle\sum\limits_{j = 1}^g {d_{ij}^2} $,$ {T_4} = \displaystyle\sum\limits_{i = 1}^g {\displaystyle\sum\limits_{j = 1}^g {d_{ij}^2} } $.
步骤4 对${\boldsymbol{B}}$进行特征值分解,得到
B=VΛVT=(Λ1/2VT)T(Λ1/2VT), (12) 式中:$ {\boldsymbol{\varLambda }} $为特征值组成的对角矩阵,$ {\boldsymbol{V}} $为特征值对应特征向量组成的矩阵.
步骤5 按特征值从大到小重新排列$ {\boldsymbol{\varLambda }} $和$ {\boldsymbol{V}} $中元素,并选取前$l$个特征值和特征向量$(l < h)$,分别得到降维特征值矩阵$ {\boldsymbol{\varLambda}_{{\mathrm{JW}}}} $和降维特征向量矩阵$ {\boldsymbol{V}_{\mathrm{JW}}} $,从而最终得到$X$在$l$维空间的降维结果${\boldsymbol{Z}}$,如式(13)所示.
Z=ΛJW1/2VJWT (13) 表2给出$ {\boldsymbol{\varLambda }} $中最大的前10个特征值,从表2中可以看出,第1个特征值远大于其他特征值. 因此,对于本文的图谱库,只要取$l \geqslant 1$,$ {\boldsymbol{Z}} \in {\mathbb{R}^{l \times g}} $即可正确表征数据集$X$中绝大部分样本之间的相对位置关系. 为兼顾计算效率与降维结果精度,本文后续计算取$l{\text{ = 2}}$.
表 2 ${\boldsymbol{\varLambda }}$中最大的前10个特征值Table 2. Top 10 largest eigenvalues in ${\boldsymbol{\varLambda }}$编号 1 2 3 4 5 6 7 8 9 10 特征值 127984 34480 27102 19090 7166 4120 3694 3068 2586 1847 3.2.2 弦角描述符的层次聚类
本文采用层次聚类算法中的凝聚聚类[36]方法来创建聚类树,实现对${\boldsymbol{Z}}$中的样本进行自动分类. 对弦角描述符压缩特征${\boldsymbol{Z}}$进行聚类的基本流程如下:
步骤1 将${\boldsymbol{Z}}$中每个样本当作1个单独的簇,此时每个簇的中心即为该样本点自身.
步骤2 计算任意2个簇中心的欧式距离,并找出距离值最小的2个簇,合并这2个簇,得到新簇中心$\left( {{{\textit{z}}_x},{{\textit{z}}_y}} \right)$为
zx=zx1+zx2+⋯+zxγγ, (14) zy=xy1+xy2+⋯+yyγγ, (15) 式中:r为簇中的样本编号,$\gamma \leqslant g$;${{\textit{z}}_{x1}}、{{\textit{z}}_{{\text{x2}}}}、 \cdots 、{{\textit{z}}_{x\gamma }}$和${{\textit{z}}_{y1}}、{{\textit{z}}_{{\text{y2}}}}、 \cdots 、{{\textit{z}}_{y\gamma }}$分别为该簇对应到$ {\boldsymbol{V}_{\mathrm{JW}}} $中的第1列和第2列特征向量.
步骤3 计算新簇与其余簇的距离,再次合并距离最近的2个簇,并按照式(14)~(15)计算合并后的簇中心. 反复重复这一过程,直到将${\boldsymbol{Z}}$中所有样本都合并为一个簇.
3.3 图谱库查询具体过程
综合前面内容,利用弦角描述符图谱库查询机构尺寸型的过程具体如下:
步骤1 根据3.1节方法,生成参数已知的平面四杆机构尺寸型,并按照2.1节方法,计算尺寸型弦角描述符矩阵,建立图谱库.
步骤2 按照3.2节方法对图谱库弦角描述符矩阵进行降维与聚类,建立图谱库聚类树,并按式(14)~(15)计算聚类树每层的簇中心.
步骤3 给定设计要求的$\rho_{\mathrm{design}}$值和机构数量$M$,选取聚类中心附近的$k $个样本,分别按式(7)计算其与目标轨迹弦角描述符的相似度.
步骤4 判断是否有$ {\rho _{\max }} \geqslant \rho_{\mathrm{design}} $的匹配轮廓段,若存在,则将该轮廓段作为轨迹匹配结果,并存储其对应的尺寸型参数作为机构设计参数. 若不存在,则进入聚类树下一层.
步骤5 重复步骤3和步骤4,直到在图谱库中检索出$M$个匹配的机构尺寸型.
上述检索过程流程如图10所示. 本文在每个聚类中心附近取10个样本进行匹配,在工作频率为3.9 GHz的Intel i3-7100处理器、内存大小为8 Gb的计算机上展开试验,图谱库的建立约耗时10 s,图谱库降维和聚类共耗时约
2000 s,每个聚类中心的平均检索时间约为15 s. 值得指出的是,花费在降维和聚类上的时间成本是一次性的,这是由于图谱库中机构尺寸型确定后,机构运动轨迹的弦角描述符也随之确定,图谱库的聚类结果也就完全确定了下来.4. 平面开环轨迹尺寸综合算例
为验证上述尺寸综合过程的有效性,本节分别以人体站坐康复训练中下肢髋关节机构和上肢肩关节机构的概念设计为例,对平面四杆机构进行尺度综合.
4.1 算例1 下肢髋关节机构尺寸综合
在站坐康复训练中,髋关节的运动轨迹如图11所示(x、y分别为横、纵坐标),该轨迹是一条类似于S形的光滑曲线,这也是本算例中尺度综合所用的目标轨迹.
利用本文的尺寸综合方法,并指定设计要求$\rho_{\mathrm{design}}=0.997\;0$,机构数量$M = 6$,得到6组平面四杆机构的尺寸型参数,如表3所示. 从表3中可以看出,这6组平面四杆机构的$ {\rho _{\max }} $均大于设计要求的$\rho_{\mathrm{design}}=0.997\;0$,说明这本文方法得到的尺寸综合结果满足设计要求. 图12进一步给出6组机构的运动轨迹及匹配轮廓段,其中绿色曲线表示机构的运动轨迹,红色圆点曲线表示匹配的轮廓段,图12(a)~(f)分别对应于表3中编号1~6的机构. 从图12中也容易看出,这些匹配轮廓段与图11(a)的轨迹曲线十分接近,从而进一步证明了本文方法的有效性.
表 3 算例1的尺寸综合结果Table 3. Dimensional synthesis results of example 1编号 ${r_1}$ ${r_2}$ ${r_3}$ ${r_4}$ $\phi $ ${\rho _{\max }}$ 1 1.3587 3.2236 3.8587 1.2522 − 2.2294 0.9981 2 1.0514 1.1401 2.2410 2.9365 − 0.7038 0.9980 3 0.6868 1.1227 0.7865 0.4678 0.4194 0.9974 4 0.5852 0.8668 0.9214 0.1351 − 0.7159 0.9974 5 0.3917 0.6009 1.5018 1.1475 − 0.2561 0.9973 6 0.7365 1.8271 1.6517 0.3828 0.8777 0.9973 4.2 算例2 上肢肩关节康复机构尺寸综合
肩关节的运动轨迹在站坐康复训练中的轨迹如图13 所示,该轨迹是一条L形的非光滑轨迹. 也是尺寸综合的目标轨迹.
利用本文的尺寸综合方法,并指定设计要求$\rho_{\mathrm{design}}=0.996\;0$,机构数量$M{\text{ = 6}}$,从而得到6组平面四杆机构的尺寸型参数,如表4所示,6组机构的运动轨迹及匹配轮廓段如图14 所示. 结合表4和图14不难看出,每组机构${\rho _{\max }}$均大于设计要求,所得到的轮廓段与肩关节轨迹也都十分相似. 该结果表明:对于非光滑开环轨迹曲线,本文提出的尺寸综合方法也能得到较好结果.
表 4 算例2的尺寸综合结果Table 4. Dimensional synthesis results of example 2编号 ${r_1}$ ${r_2}$ ${r_3}$ ${r_4}$ $\phi $ ${\rho _{\max }}$ 1 0.6485 0.7565 0.7875 2.7891 1.1536 0.9974 2 0.2227 0.7548 0.3097 0.8841 1.2532 0.9969 3 0.6797 0.3256 0.2244 0.3311 − 1.2038 0.9968 4 1.2501 0.6843 0.4309 0.7100 − 1.7501 0.9968 5 0.6241 1.0639 1.1351 1.9733 − 0.6183 0.9967 6 1.3592 1.2685 0.5040 1.8007 − 1.4115 0.9967 综合算例1和算例2的结果可以看出:无论是光滑开环轨迹还是非光滑开环轨迹,利用本文提出的尺寸综合方法,都能得到满足设计要求的尺寸综合结果,从而验证了本文方法的有效性.
5. 结 论
1) 提出了基于弦角描述符的开环轨迹形状特征提取方法. 利用弦角描述符的旋转、平移及缩放不变性,可直接得到与机架的位置、方位及大小均无关的开环轨迹形状特征,从而无需对轨迹归一化预处理和引入区间设计参数,减少了提取轨迹特征的计算量,缩短计算时间.
2) 使用多维尺度缩放法将弦角描述符压缩为2维特征,并结合层次聚类算法,建立了
16000 组平面四杆机构的数值图谱库. 利用聚类树的层次链接关系,避免了对数据库遍历检索,从而提高图谱库检索效率.3) 康复训练用的肩关节机构和髋关节机构设计案例结果表明:本文方法对光滑轨迹和非光滑轨迹都能够得到满足设计要求的尺寸综合结果,验证了该方法的有效性.
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表 1 悬架控制理论分类及特点
Table 1. Classification and characteristics of suspension control theories
控制理论 控制特点 天棚阻尼 根据簧载质量速度反馈实现阻尼控制,计算简便、响应快、鲁棒性强,广泛应用于半主动悬架,但只考虑幅频特性、忽略相频,致使传递函数评价悬架性能存在不确定性,在具体应用中存在局限性[17-18] 模糊控制 基于专家经验准则、模糊规则及隶属函数,以“if-then”为控制逻辑处理系统不确定性参数控制问题,对模型精度要求低、适应性强、鲁棒性好且易于理解,但精度低且因主观经验影响控制效果,对其实际应用造成局限性[19] 最优控制 在已知运动方程及控制范围前提下,通过性能指标参数优化实现其指标函数的最优调控,此方法目的明确且计算速度快,包括线性、非线性最优调控及H∞最优控制等,但实际悬架含有许多不确定因素,因而难以达到预期控制性能[20-21] 自适应控制 因路面工况、环境等不确定因素会影响油气悬架性能,而自适应控制基于数学模型,依据实际路面激励的变化对悬架系统参数进行实时合理地自动调节,以保持良好工作性能,但控制方案复杂、运算量大,有模型参考自适应和自校正控制两类[22] PID(proportional integral derivative)控制 控制器输入输出间无需精确数学模型,通过对输入量的比例、积分、微分参数修定可得合理的输出量,该方法简单实用,但控制效果较差且参数整定影响系统响应速度[23] 遗传算法 基于自然选择与遗传机理,通过选择、交叉与变异寻找问题最优解,搜索过程简单、覆盖面大,适应度函数选取不当易陷入局部最优解且容易过早收敛、效率低 神经网络 依据动物神经元的大规模信息处理、传递存储等特点而提出,具备信息并行处理、容错能力强与自适应学习等特点,多用于处理系统复杂非线性问题,可逼近任意连续函数[24] 变结构滑模控制 系统结构不固定,可依据系统当前的信息调整其结构,改变控制规则,具备操作简单、响应迅速、对干扰不敏感,通过调整滑模面或控制律参数可增强其控制性能[25] 复合控制 依据不同控制方法的特点以及被控对象的工作特性,通过各方法性能匹配,实现系统单个或者多目标控制,充分发挥各自优点并弥补其缺陷,具备优良控制性能及稳定性,常用模糊 PID、模糊滑模控制、模糊神经网络等 -
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