160T振动剪切机【平行刀片剪切机】【160吨】【说明书+CAD】
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中文译文 标题:数控简单剪切仪动态土壤测试作者:果皮,Pendo M,辉西斯图尔特,乔纳森P,加州大学洛杉矶分校Venugopal,拉维,Sysendes公司。出版日期:01-01-2007系列:加州大学洛杉矶先前发表的作品发布信息:加州大学洛杉矶先前发表的作品,加州大学洛杉矶附加信息:版权2007年,ASTM,http:/journalsip.astm.org/原始引用:果皮,斯图尔特,摩根,d.h。,Venugopal,r .(2007)。“数控简单剪切设备动态土壤测试”,Geotech。测试日报,ASTM,30(5),368 - 377。数控剪切装置简单动态土壤测试文摘:我们描述的特征简单剪切装置能力运用现实的多向地震荷载对土标本。这个装置,文中称为数控简单剪切(直流ss)装置,包括功能,如伺服液压控制的驱动和真正的数字控制克服控制的局限性,一些之前的动态土壤测试机器。该设备是显示有能力正弦和宽带的复制命令信号在一个广泛的频率和振幅,虽然设备有限控制能力非常小的命令位移(少于大约0.005毫米)。小变形限制结果从噪声介绍了控制系统的反馈信号的模数转换。我们证明,双向命令信号准确地传授与最小交叉耦合,结果从一个创新的多输入多输出的数字控制系统。这个功能的设备演示一系列宽带测试标本非饱和土受到uni -和双向励磁。关键词:数字控制,简单的剪切、动态土壤测试、多向加载介绍直接简单的剪切设备已经成功运用多年来描述静态和动态土属性。这测试方法往往是首选当它是可取的标本来体验光滑、连续的旋转主应力方向在剪切。初始应力可以应用模拟静态字段条件当钢丝增强利用膜,减少横向变形样品(即。,NGI-type配置,Bjerrum和Landva1966)。也许最常见的应用程序的简单剪切试验对仿真的垂直(或近垂直)剪切波传播通过土柱。优点和局限性简单的剪切测试相对于其他类型的实验室测试描述了在其他地方,这里不再重复(比如,菜的et al。1972;沈et al。1978;萨达et al。1982;Vucetic和拉卡斯提花1982;1985;Budhu Bhatia et al。1985;Amer et al。1987;Airey和木材1987;Budhu和Britto 1987;雅et al。1993)。最简单的剪切设备运行在一个水平方向并应用谐波载荷频率一般慢于动态过程,如地震摇晃(如。,Tatsuoka和银1981;Doroudian和Vucetic 1995;和Pfender Lefebvre 1996;瑞艾莫和种子1997;Kusakabe et al。1999年,Hazirbaba和Rathje 2004)。虽然总是有近似参与应用在实验室土壤特性测量对现场条件,对现有设备无法提供快速、多向加载介绍进一步的错误未知的意义当实验室测量土壤属性用于工程模拟。一个数量的简单剪切设备已经被发展调查土壤响应多向加载(如。,石原和山崎1980;雅et al。1993;DeGroot et al . 1996)。加州大学伯克利分校的双向循环简单剪切(UCB-2D)装置是值得注意的,因为它明显减少了机械合规问题,导致相对最高/基地帽摇摆在早期的设备(如。,石原和山崎1980;石原和“1988)。另一个显著的特点UCB-2D设备是燃烧室压力控制,方便背压饱和。校长UCB-2D设备本身的限制,早期的设备,是他们无法应用地震像宽带装货吗在快速位移率。这种限制也存在对大多数单向简单剪切设备。其原因是双重的:(1)气动加载系统使用一个可压缩流体(即。、空气)其中介绍了重大错误的反馈回路在高频段;和(2)数字监督模拟控制器被雇用这有效地限制处理速度和成熟的控制算法。当然,振动台和离心机实验都能使用多方向的地震像加载到土壤模型(即。,派克et al。1975;1998年andYanagisawa Jafarzadeh的说法;Kutter 1995;威尔逊et al . 2004)。然而,直接测量土壤元素的响应(例如,剪切应力剪切应变关系、体积应变和孔隙水压力)在这些类型的实验需要密集的仪表数组,可以影响响应他们的目的是措施,从而使复杂数据解释(如。,困难et al . 2005)。 图1应用的能力,可靠程度的控制,多向加载在一个很宽的频率范围内,土壤元素在实验室是至关重要的推进我们的根本理解动态土壤属性。例如,宽带装载能力是需要调查率影响土壤手稿收到2006年3月1日,1月28日发表2007年,2007年5月在线发表。1研究生研究员,副教授和副主席,分别土木工程系&大学环境工程加利福尼亚,洛杉矶,5731年洛杉矶消沉大厅CA 90095。2工程师,指数失效分析,320年,200套房,欧文戈达德,CA92618年。3创始人和高级咨询顾问工程师,Sysendes,Inc .,1804 m,套件4、蒙特利尔、QC H3H 1硅陶瓷、加拿大的观点和结论包含在本文档的作者和不应被解释为一定代表官方的政策,明示或暗示,美国政府。岩土测试日报,卷。30,5号纸ID GTJ100518可以在www.astm.org368年版权2007年由ASTM国际,100年巴尔港驱动器,PO Box,西Conshohocken C700,PA 19428 - 2959。属性,人们知道这是重要的对于粘土(如。,Lefebvre和Pfender 1996;Sheahan et al . 1996)。此外,效果剪切速率和2 d装载在孔隙压力生成或体积改变行为,或两者兼而有之,是了解较少,需要进一步调查一些土壤类型。为了满足这些研究需要,一个简单的剪切设备现场数控与能力室压力控制和多向激励一直发达。这个装置,文中称为现场数控简单剪切(直流ss)装置合并功能,如伺服液压控制的驱动和真正的数字控制克服的局限性先前的动态土壤测试机器。结果是一个真正独特的简单剪切装置的功能应用宽带(地震像)位移要求土壤标本在两个方向,用最小的交叉耦合水平运动。在本文中,我们描述这个装置和它的功能动态土壤测试。物理描述直流ss装置机械设计的直流ss装置使用了UCB-2D的装置作为一个原型(面包师et al . 1993)。这个直流ss装置是专为了保留的主要特征UCB-2D设备如包含细胞压力的目的背压饱和,有限的机械遵守尊重对简单剪切边界条件(如。,顶部和基板“摇摆”),和双向加载功能。除了这些特性,直流ss装置结合了几种改进设计相对于UCB-2D装置包括:我使用一个三邮报框架与高绩效跟踪轴承(容纳垂直位移的顶罩),以进一步降低摇摆;二世一个伺服液压控制的控制系统,使高频率加载;iii双重轴负载细胞获得职位摩擦剪切载荷的测量。图1显示了大会的直流ss装置。照片的直流ss设备显示在图2。直流党卫军的设备是用来测试圆柱土壤标本与直径10.2厘米或更少。这个标本是位于相对严格的底部和顶部帽(图1、图2(b)和通常局限由一线增强膜。见图2(c),水平(顶部和底部)面临的标本被限制的帽子,它含有细磨石epoxied变成一个休会覆盖整个脸的帽子除了保持嘴唇的铝在边缘。这些帽子提供一个“摩擦”表面同时使排水进入多孔石头如果石头不饱和(石头可以饱和对不排水试验)。这个上盖/样品/下盖堆栈顶部之间的位置和底部适配器板图1所示。底部帽适合一个休息在底部适配器板。适配器板顶部轻轻地降低,这样休息在顶端的适配器板适合舒适地覆盖在顶盖。顶部和底部帽是紧紧在各自的适配器板由三个固定螺丝在每个板。一旦样品是获得两个适配器板之间,三个LVDTs间距分布样品安装在顶部适配器板和固定在板通过螺钉。这个标本是然后合并垂直应力和准备剪切载荷。图2顶部适配器板是一种垂直表,进而是附加到一个垂直负载细胞(图1)。垂直负载转移到标本通过垂直表,这是附加的三个等距线性幻灯片。每个三个线性幻灯片附属于一个单独的帖子,有效防止侧运动和摇摆的垂直表(因此,实际上无花果。1原理的直流ss装置在加利福尼亚大学洛杉矶分校。果皮ET AL。在数控简单剪切仪369来说,标本)。这是一个重要的三邮报框架改善UCB-2D设备,雇佣了一个悬臂系统(垂直表附加到一对跟踪轴承沿相同的墙)。负载应用于垂直表由气动执行机构安装在主框架之外。一个重要的功能的设备,直流党卫军留存从UCB-2D装置是其双向加载功能。横向剪切载荷应用在该基地的标本通过两个独立控制的水平表。底部水平表安装在线性幻灯片附加到主框架的装置,这表是只自由移动一个水平方向。上层水平表也安装在线性幻灯片,这样运动上层表是完全垂直于下表。两个水平表可以控制产生净合成的运动底部适配器板在任何水平方向。加载应用较低水平表通过螺纹棒相连致动器,可以应用张力和压缩。有一个tensioncapable辊之间连接上层表及其执行机构以适应垂直位移下表。负载应用到表loadcells来衡量安装执行机构和表之间。负荷测量由loadcells不相同,那些传授标本由于摩擦而在线性的幻灯片。这个巨大的摩擦加载系统中的表征,观察到相当小(大约2.2 N)。这一点的重要性摩擦负载取决于什么类型的测试,是理想的。这种摩擦负载将产生不准确的(大约0.3 kPa10.2厘米直径试样),代表比例可以忽略不计的剪切应力对大多数应用程序。然而,如果很低应力测量是必要的,帖子摩擦剪切应力可以通过使用一个双轴loadcell测量。这种双轴loadcell适合在顶部之间适配器板和垂直表,一个空间,否则占用垫块。这种双轴loadcell能够测量两个垂直和剪切载荷同时以最小的相声这些通道之间。然而,存在的双轴loadcell介绍系统遵从性(即摇摆和垂直变形),可能是重要的在大中型菌株。因此,大多数测试执行没有双轴loadcell到位。三LVDTs(线性可变差动传感器),安装顶部和底部之间的适配器板,用于测量垂直标本变形。这些位置的LVDTs最小化错误由于机械合规。这三个LVDTs是用,这样相对摇摆的标本在两个方向的加载可以测量。三LVDTs数据的平均值定义试样高度在一个测试。水平变形是衡量两个LVDTs安装在水平表在吗正交的方向。设备的直流党卫军下运作的“应变控制”的条件下,这意味着表位移控制和执行机构力量必须实现这些位移测量。这个运动,可以传授表是有限的不同方面的控制系统对不同频段。在低频率f0 24赫兹,限制因素是峰致动器位移umax = 51毫米。在中间频率0.24 Hzf15赫兹,限制因素是流量的能力伺服阀的Qmax = 158立方厘米/秒。在频率f15赫兹,的制约因素是压力能力的液压泵pmax = 21 MPa。的情况下谐波控制信号,这些数量可以与相关的峰值表动作如下:U(t) = D sin(wt) umax (1)u (t) = D w cos(wt) Qmax/A (2)u (t) = D w2sin(wt)pmaxA/m (3) 图3在ut及其衍生产品描述表位移,速度,和加速度,一个是执行机构的横截面积20.3平方厘米,m是表质量5.7公斤,和频率表运动(按弧度/秒)。相应的峰值位移值,速度和加速度给出了图3。控制系统能产生任何运动,是在的限制线在图3。油柱的理论频率的致动器表系统给出了孔蒂和特龙贝蒂(2000):哪里是体积弹性模量的液压流体1.7106 kPa吗和V是原油的体积是463立方厘米的致动器。身体上,油柱频率代表自然频率的三液压执行器系统,它可以被可视化为表质量与弹簧连接有一个刚度定义的油柱在致动器室。对于命令信号频率油柱频率附近,表现为执行机构可以是有限的由于共振行为(如。,孔蒂和特龙贝蒂2000)。直流ss控制系统如图4中,数字控制系统的直流党卫军设备有两个目的。第一个是提供控制信号直接驱动,驱动液压作动器servovalves为每个轴(直接驱动servovalves有机载控制器,纠正跟踪误差在控制信号在开车前液压致动器)。第二个目的是获得LVDTs数据和loadcells。这里指的物理设备作为直流党卫军是最初开发了一个基于pc的数字监督模拟控制系统。这个控制系统使用PID(比例-积分导数)控制算法,跑在一个Windows操作系统。主要问题控制系统延时处理过程中的反馈信号从仪器(如(LVDT)和一代的命令信号。这限制了设备的能力,准确复制一些命令信号。这些问题尤其急性加载功能涉及快速度和2 d摇动。图4该系统已成功用于以前的测试(例如,用力的et al。2004;重击et al。2005),尽管这些应用程序涉及单向震动和1.0赫兹加载频率,所以控制问题相关的基于pc的系统没有显著的。控制系统目前设备使用一个系统所为硬实时数字控制。主要区别基于pc的数字控制是控制功能被实现在控制器板相对于个人电脑操作系统。这使保证采样频率的内部反馈回路的5 kHz使用位移反馈水平LVDTs,而基于pc的数字监督模拟控制系统通常不能可靠地执行计算需要复杂的控制反馈采样频率高于200赫兹,根据处理器时钟速度,控制算法复杂,数量的后台进行由个人电脑操作系统等。数字控制系统利用两个dSPACE DS1104控制器板。每个板包含一个PowerPC 603 e处理器,四个16位2 s模拟数字(A / D)转换器,四个12位A / D转换器800 ns和8个16位位十年代数模转换(D / A)转换器,除了其他输入/输出端口。这两个板安装在在一个PCI插槽主机电脑但运行他们自己的实时内核(即。,一个操作系统专门针对控制功能)独立于主机电脑的操作系统。一个PID控制算法实现了两个基于pc和硬实时数字控制。这被称为随后随着“PID控制器”。收益为PID控制器是调谐为获得最佳性能试错用阶梯函数命令信号。PID控制器的输出是一个数字电压命令,发送到一个穆格公司电压放大器通过的D / A频道在dSPACE板。电压放大器,反过来,发送一个电压驱动信号到适当的执行机构伺服阀。见图5(一个)、PID控制的两个轴都是独立的,从而控制系统作为一个整体无法补偿对于交叉耦合效应(即:影响,运动沿一个轴在运动沿着第二轴)。图5为了减少交叉耦合效应,数字控制系统是通过引入一个多输入多multipleoutput增强(MIMO)控制算法与PID控制器的接口。见图5(b),该控制器采用线性差动变换器反馈两轴和生成一个补偿指令信号对于每个PID控制器,考虑到交叉耦合效果。控制器的设计和实现为一个离散状态空间系统使用LQG(线性二次-高斯)最优控制方法(富兰克林et al . 1990)。这方法需要估计的数量,反映四个经验系统属性。这是通过使用N4SID系统识别算法(Van Overschee和德沼泽1995)系统辨识算法操作输入-输出数据序列;用于此目的的数据是两个不相关的随机输入(生成的PID控制器)和相应的线性差动变换器输出信号。结合MIMO的控制算法,以及两个PID控制器是指后来的“mimo PID”控制器。设备配置的直流党卫军,MIMO算法可以打开或关闭。因此,要么PID或mimo PID吗数字控制的实验是可能的。数据采集功能无论是模式总结如下:输入运动时间步:不实用的下限;数量的输入运动数据点:不实用的上限;反馈采样频率(即。,内部频率对反馈回路):5 kHz;数据记录频率:上界是5 kHz,可以downsampled作为需要。直流ss系统性能性能的评价系统的直流党卫军(即。,控制器,泵、致动器、伺服阀),两个谐波和宽带地震输入运动被指定为PID控制器和的控制器和由此产生的mimo pid反馈信号测量。单向测试进行评估性能每个轴的独立,并提供基线结果相互影响的评估。双向加载轴之间的交叉耦合进行评估。鸣谢开发的设备支持直流党卫军职业美国国家科学基金会的资助下,第二个作者(NSF获奖号:9733113),亨利Samueli学校的工程和应用科学大学洛杉矶分校和美国地质调查显示,国家地震灾害减少计划,奖号。1434 - hg - 98 gr - 00037、05 hqgr0050。这种支持是感激地承认。这个观点和结论包含在这个文档是作者的,不应该解释为一定代表官方的政策,要么表示或暗示,美国政府。帕特里克m史密斯和哈罗德卡斯珀是感谢他们的相当大的贡献发展的物理设备。我们感谢三匿名评论者对他们有用的手稿评论。英文原文Title:Digitally controlled simple shear apparatus for dynamic soil testingAuthor:Duku, Pendo M, Fugro WestStewart, Jonathan P, University of California, Los AngelesVenugopal, Ravi, Sysendes, Inc.Publication Date:01-01-2007Series:UC Los Angeles Previously Published WorksPublication Info:UC Los Angeles Previously Published Works, UC Los AngelesAdditional Info:Copyright 2007, ASTM, http:/journalsip.astm.org/Original Citation:Duku, P.M., Stewart, J.P., Whang, D.H., Venugopal, R. (2007). “Digitally controlled simple shearapparatus for dynamic soil testing,” Geotech. Testing Journal, ASTM, 30 (5), 368-377.Digitally Controlled Simple Shear Apparatus forDynamic Soil TestingABSTRACT:We describe the characteristics of a simple shear apparatus capable of applying realistic multidirectional earthquake loading to soilspecimens. This device, herein termed the Digitally Controlled Simple Shear (DC-SS) apparatus, incorporates features such as servohydraulicactuation and true digital control to overcome control limitations of some previous dynamic soil testing machines. The device is shown to be capableof reproducing sinusoidal and broadband command signals across a wide range of frequencies and amplitudes, although the device has limitedcontrol capabilities for very small command displacements (less than approximately 0.005 mm). The small deformation limitation results from noiseintroduced to the control system from analog-to-digital conversion of feedback signals.We demonstrate that bidirectional command signals can beaccurately imparted with minimal cross coupling, which results from an innovative multiple-input, multiple-output digital control system. Thecapabilities of the device are demonstrated with a series of broadband tests on unsaturated soil specimens subjected to uni- and bidirectional excitation.KEYWORDS: digital control, simple shear, dynamic soil testing, multidirectional loadingIntroduction Direct simple shear apparatuses have been utilized successfully formany years to characterize static and dynamic soil properties. Thismethod of testing is often preferred when it is desirable for thespecimen to experience a smooth and continuous rotation of theprincipal stress directions during shear. Initial stresses can be applied to simulate at-rest field conditions when wire reinforcedmembranes are utilized that minimize lateral distortion of thesample (i.e., the NGI-type configuration, Bjerrum and Landva1966). Perhaps the most common application of simple shear testing has been for the simulation of vertical (or nearly vertical) shearwave propagation through a soil column. Advantages and limitations of simple shear tests relative to other types of laboratory tests have been described elsewhere and are not repeated here (e.g.,Lucks et al. 1972; Shen et al. 1978; Saada et al. 1982; Vucetic and Lacasse 1982; Budhu 1985; Bhatia et al. 1985; Amer et al. 1987; Airey and Wood 1987; Budhu and Britto 1987; Boulanger et al. 1993). Most simple shear apparatuses operate in a single horizontal direction and apply harmonic loading at frequencies which are typically slower than dynamic processes such as earthquake shaking (e.g., Tatsuoka and Silver 1981; Doroudian and Vucetic 1995; Lefebvre and Pfender 1996; Riemer and Seed 1997; Kusakabe et al. 1999; Hazirbaba and Rathje 2004). While there are always approximations involved in applying soil properties measured in the laboratory to field conditions, the inability of existing devices to provide rapid, multidirectional loading introduces further errors of unknown significance when laboratory-measured soil properties are used in engineering simulations. A number of simple shear apparatuses have been developed to investigate soil response to multidirectional loading (e.g., Ishihara and Yamazaki 1980; Boulanger et al. 1993; DeGroot et al. 1996). The University of California, Berkeley bidirectional cyclic simple shear (UCB-2D) device is noteworthy since it significantly reduced mechanical compliance issues that caused relative top/base cap rocking in earlier devices (e.g., Ishihara and Yamazaki 1980; Ishihara and Nagase 1988). Another significant feature of the UCB-2D device is chamber pressure control, which facilitates back pressure saturation . The principal limitation of the UCB-2D device, and earlier devices, is their inability to apply earthquake-like broadband loading at rapid displacement rates. This limitation also exists for most unidirectional simple shear devices. The reasons for this are twofold :(1) pneumatic loading systems use a compressible fluid (i.e., air) which introduces significant errors to the feedback loop at high frequencies; and (2) digitally-supervised analog controllers were employed which effectively limit the processing speed and sophistication of the control algorithms. Of course, shaking table and centrifuge experiments are capable of applying multidirectional earthquake-like loading to soil models (i.e., Pyke et al. 1975;Jafarzadeh andYanagisawa 1998; Kutter 1995;Wilson et al. 2004).However, direct measurements of the soil element response (e.g.,shear stress-shear strain relationships, volumetric strain, and porewater pressure) in these types of experiments requires dense instrumentationarrays that can affect the response they are intended to measure, which in turn complicates data interpretation (e.g., Elgamal et al. 2005). The capability of applying, with a reliable degree of control, multidirectional loading across a wide range of frequencies to soil elements in the laboratory is critical to advancing our fundamental understanding of dynamic soil properties. For example, broadband loading capabilities are needed to investigate rate effects on soil properties, which are known to be significant for clays (e.g., Lefebvre and Pfender 1996; Sheahan et al. 1996). Moreover, the effect of shear rate and 2D loading on pore pressure generation or volume change behavior, or both, is less well understood and requires further investigation for some soil types.To meet these research needs, a digitally-controlled simple shear device with capabilities for chamber pressure control and multidirectional excitation has been developed. This device, herein termed the Digitally-Controlled Simple Shear (DC-SS) apparatus incorporates features such as servohydraulic actuation and true digital control to overcome the limitations of previous dynamic soil testing machines. The result is a truly unique simple shear apparatus with the capability to apply broadband (earthquake-like) displacement demands on soil specimens in two directions and with minimal cross coupling between the horizontal motions. In this paper, we describe this device and its capabilities for dynamic soil testing. Physical Description of DC-SS DeviceThe mechanical design of the DC-SS device was developed using the UCB-2D device as a prototype (Boulanger et al. 1993). The DC-SS device was designed to retain the main features of the UCB-2D device such as inclusion of cell pressure for purposes of back pressure saturation, limited mechanical compliance with respect to simple shear boundary conditions (e.g., top and base platen “rocking”), and bidirectional loading capability. In addition to these features, the DC-SS device incorporates several design improvements relative to the UCB-2D device including: _i_ the use of a tri-post frame with high performance track bearings (which accommodate vertical displacements of the top cap) to further reduce rocking; _ii_ a servohydraulic control system to allow for high frequency loading; and _iii_ a dual axis load cell to obtain post-friction shear load measurements. Figure 1 shows the general assembly of the DC-SS apparatus. Photographs of the DC-SS device are shown in Fig. 2. The DC-SS device was designed to test cylindrical soil specimens with a diameter of 10.2 cm or less. The specimen is located between relatively rigid bottom and top caps (Fig. 1, Fig. 2(b) and is typically confined by a wire reinforced membrane. As shown in Fig. 2(c), the horizontal (top and bottom) faces of the specimen are confined by the caps, which contain fine porous stones epoxied into a recess covering the entire face of the cap except for a retaining lip of aluminum around the edge. These caps provide a “frictional” surface while allowing for drainage into the porous stones if the stones are unsaturated (the stones can be saturated for undrained tests). The top cap/specimen/bottom cap stack is positioned between the top and bottom adapter plates shown in Fig. 1. The bottom cap fits into a recess within the bottom adapter plate. The top adapter plate is gently lowered such that a recess within the top adapter plate fits snugly over the top cap. The top and bottom caps are held tightly on their respective adapter plates by three set screws on each plate. Once the specimen is secured between the two adapter plates, three LVDTs equally spaced around the specimen are mounted on the top adapter plate and fixed to the plate by set screws. The specimen is then consolidated by a vertical stress and is ready for shear loading. Above the top adapter plate is a vertical table, which in turn is attached to a vertical load cell (Fig. 1). Vertical loads are transferred to the specimen through the vertical table, which is attached to three equally spaced linear slides. Each of the three linear slides is attached to a separate post, which effectively precludes lateral movements and rocking of the vertical table (and hence, practically speaking, the specimen as well). This tri-post frame is a significant improvement over the UCB-2D device, which employed a cantilever system (vertical table attached to a pair of track bearings along the same wall). Loads are applied to the vertical table by a pneumatic actuator mounted outside the main frame.An important feature of the DC-SS device that was retained from the UCB-2D device is its bidirectional loading capability. Horizontal shear loads are applied at the base of the specimen through two independently controlled horizontal tables. The bottom horizontal table is mounted on linear slides attached to the main frame of the apparatus, and this table is free to move in only one horizontal direction. The upper horizontal table is also mounted on linear slides such that the movement of the upper table is exactly perpendicular to the lower table. The two horizontal tables can be controlled to produce net resultant movements of the bottom adapter plate in any horizontal direction. Loads are applied to the lower horizontal table by threaded rods that are attached to an actuator that can apply tension and compression. There is a tensioncapable roller connection between the upper table and its actuator to accommodate perpendicular displacements of the lower table. The loads applied to the tables are measured by loadcells mounted between the actuators and the tables. The loads measured by the loadcells are not identical to those imparted to the specimen due to friction in the linear slides. The magnitude of the frictional load within the system was characterized and observed to be quite small (approximately 2.2 N). The significance of this frictional load is dependent on what type of testing is desired. This frictional load will produce inaccuracies of approximately 0.3 kPa (for a10.2-cm diameter specimen), which represents a negligible percentage of the shear stress for most applications. However, if very low stress measurements are needed, post-friction shear stresses can be measured by using a dual-axis loadcell. The dual-axis loadcell fits in between the top adapter plate and the vertical table, a space which is otherwise occupied by a spacer block. The dual-axis loadcell is capable of measuring both the vertical and shear loads simultaneously with minimal cross talk between these channels. However, the presence of the dual-axis loadcell introduces system compliance (i.e., rocking and vertical deformations) that may be significant at medium to large strains. Therefore, most tests are performed without the dual-axis loadcell in place. Three LVDTs (linear variable differential transducers), mounted between the top and bottom adapter plates, are used to measure the vertical specimen deformations. These locations of LVDTs minimize errors due to mechanical compliance. The three LVDTs are used so that relative rocking of the specimen in either direction of loading can be measured. Data from the three LVDTs are averaged to define specimen height during a test. Horizontal deformations are measured by two LVDTs mounted to the horizontal tables in orthogonal directions. The DC-SS device operates under “strain-control” conditions, meaning that table displacements are controlled and the actuator forces required to achieve those displacements are measured. The motions that can be imparted to the tables are limited by different aspects of the control system for different frequency bands. At low frequencies (f0.24Hz), the limiting factor is the peak actuator displacement (Umax=51 mm). At intermediate frequencies (0.24Hzf15Hz) the limiting factor is the flow rate capacity of the servo-valve (_Qmax=158 cm3/s). At frequencies (f15 )Hz, the limiting factor is the pressure capacity of the hydraulic pump (Pmax=21 MPa)_. For the case of harmonic control signals, these quantities can be related to the peak table motions as follows: Ut=DsinwtUmax (1)Utdx=Dwcos(wt)Qmax/A (2) U(t)dx2=-Dw2sin(wt)PmaxA/m (3)where U(t) and its derivatives describe the table displacement, velocity, and acceleration, A is the cross-sectional area of the actuator( 20.3 cm2), m is the table mass( 5.7 kg), and _ is the frequency of table motion (in radians/s). The corresponding peak values of displacement, velocity, and acceleration are given in Fig. 3. The control system is capable of producing any motion that lies below the limit lines in Fig. 3.DC-SS Control SystemAs illustrated in Fig. 4, the digital control system for the DC-SS device serves two purposes. The first is to provide control signals to direct drive servovalves that drive hydraulic actuators for each axis (direct drive servovalves have an onboard controller that corrects tracking errors in the control signal before driving the hydraulic actuators). The second purpose is to acquire data from the LVDTs and loadcells. The physical device referred to here as DC-SS was originally developed with a PC-based digitally-supervised analog control system. This control system used a PID (Proportional- Integral-Derivative) control algorithm that ran within a Windows operating system. The principal problem with that control system was latency in the processing of feedback signals from instruments (such as an LVDT) and the generation of command signals. This limited the ability of the device to accurately replicate some command signals. These problems were especially acute for loading functions involving fast velocities and 2D shaking. The system was successfully used in previous testing (e.g., Whang et al. 2004; Whang et al. 2005), although those applications involved unidirectional shaking and a 1.0 Hz loading frequency, so control problems associated with the PC-based system were not significant.The control system for the present device uses a system referred to as hard real-time digital control. The principal difference from PC-based digital control is that the control functions are implemented on the controller board as opposed to a PC operating system. This enables guaranteed sampling frequencies for the internal feedback loop of 5 kHz using displacement feedback from the horizontal LVDTs,whereas PC-based digitally-supervised analog control systems typically cannot reliably execute the computations required for complex control at feedback sampling frequencies higher than 200 Hz, depending on the processor clock speed, control algorithm sophistication, number of background processes handled by the PC operating system, etc. The digital control system utilizes two dSPACE DS1104 controller boards. Each board contains a PowerPC 603e processor, four 16-bit 2 _s analog-to-digital (A/D) converters, four 12-bit 800 ns A/D converters and eight 16 -bit 10 _s digital-to-analog (D/A) converters, in addition to other input/output ports. The two boards are mounted in PCI slots in a host PC but run their own real-time kernel (i.e., an operating system specifically tailored for control functions) independent of the host PCs operating system.A PID control algorithm was implemented for both PC-based and hard real time digital control. This is referred to subsequently as the “PID controller”. Gains for the PID controller are tuned by trial-and-error for optimal performance using a step function command signal. The output of the PID controller is a digital voltage command that is sent to a Moog voltage amplifier via one of the D/A channels on the dSPACE board. The voltage amplifier, in turn, sends a voltage drive signal to the appropriate actuator servovalve. As illustrated in Fig. 5(a), PID control of the two axes are independent, and hence the control system as a whole is unable to compensate one axis on the motion along the second axis). In order to minimize cross-coupling effects, the digital control system was enhanced by introducing a multiple-input multipleoutput (MIMO) control algorithm that interfaces with the PID controllers. As illustrated in Fig. 5(b), this controller uses LVDT feedback from both axes and generates a compensated command signal for each of the PID controllers, taking into account cross-coupling effects. The controller is designed and implemented as a discretetime state space system using the LQG (Linear-Quadratic- Gaussian) optimal control method (Franklin et al. 1990). This method requires the estimation of four empirical quantities that reflect system properties. This is accomplished using the N4SID system identification algorithm (Van Overschee and De Moor 1995). System identification algorithms operate on input-output data sequences; the data used for this purpose were two uncorrelated random inputs (generated by the PID controllers) and the corresponding LVDT output signals. The combination of the MIMO control algorithm and the two PID controllers is referred to subsequently as the “MIMO-PID” controller. The DC-SS device is configured so that the MIMO algorithm can be turned on or off. Hence, either PID or MIMO-PID digital control of experiments is possible. Data acquisition capabilities for either mode are summarized below: Input motion time step: no practical lower limit; Number of input motion data points: no practical upper limit; Feedback sampling frequency (i.e., the internal frequency for the feedback loop): 5 kHz; Data logging frequency: upper bound is 5 kHz, can be downsampled as needed.DC-SS System PerformanceTo evaluate the performance of the DC-SS system (i.e., controller, pump, actuators, and servo-valves), both harmonic and broadband earthquake input motions were specified to the PID controller and the MIMO-PID controller and the resulting feedback signals were measured. Unidirectional tests were performed to evaluate the performance of each axis independently, and to provide baseline results for evaluating interaction effects. Bidirectional loading was performed to evaluate cross-coupling between axes. AcknowledgmentsThe development of the DC-SS device was supported by a CAREER grant from the National Science Foundation to the second author (NSF Award No. 9733113), the Henry Samueli School of Engineering and Applied Science at UCLA, and the U.S. Geological Survey, National Earthquake Hazards Reduction Program, Award Nos. 1434-HG-98-GR-00037 and 05HQGR0050
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