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速度 检测 系统 设计

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滴液速度检测系统设计,速度,检测,系统,设计

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毕 业 设 计（论 文）任 务 书设计（论文）题目：滴液速度检测系统设计 学生姓名：专业：所在学院：指导教师：职称：发任务书日期：年月日 任务书填写要求1毕业设计（论文）任务书由指导教师根据各课题的具体情况填写，经学生所在专业的负责人审查、系（院）领导签字后生效。此任务书应在毕业设计（论文）开始前一周内填好并发给学生。2任务书内容必须用黑墨水笔工整书写，不得涂改或潦草书写；或者按教务处统一设计的电子文档标准格式（可从教务处网页上下载）打印，要求正文小4号宋体，1.5倍行距，禁止打印在其它纸上剪贴。3任务书内填写的内容，必须和学生毕业设计（论文）完成的情况相一致，若有变更，应当经过所在专业及系（院）主管领导审批后方可重新填写。4任务书内有关“学院”、“专业”等名称的填写，应写中文全称，不能写数字代码。学生的“学号”要写全号，不能只写最后2位或1位数字。 5任务书内“主要参考文献”的填写，应按照金陵科技学院本科毕业设计（论文）撰写规范的要求书写。6有关年月日等日期的填写，应当按照国标GB/T 740894数据元和交换格式、信息交换、日期和时间表示法规定的要求，一律用阿拉伯数字书写。如“2002年4月2日”或“2002-04-02”。毕 业 设 计（论 文）任 务 书1本毕业设计（论文）课题应达到的目的： 1、掌握红外液位传感器的原理与使用2、掌握单片机电路的设计与调试3、学会系统一体化设计的规范与运用 2本毕业设计（论文）课题任务的内容和要求（包括原始数据、技术要求、工作要求等）： 液面警报显示电路设置点滴速度点滴速度步进电机单片机速度探测水位探测监测输液瓶液位和点滴速度。整个系统以单片机为核心，并加以一些外围电路进行辅助，设计液滴速度监测装置和液位报警装置。毕 业 设 计（论 文）任 务 书3对本毕业设计（论文）课题成果的要求包括图表、实物等硬件要求： 1、电路设计原理图2、程序源码3、毕业论文4主要参考文献： 1 曹柏荣,曹琪.基于单片机的便携式心电图仪的研究J . 微计算机信息，2006 ，( 2 ) .2宋雪丽,王虎林,万金领.基于单片机系统的液体点滴速度监控装置设计J.电脑开发与应用.2007(05).3马忠梅,等.单片机的 C 语言应用程序设计M.北京:北京航空航天大学出版社,2000. 4胡汉才.单片机原理及其接口技术M.北京:清华大学出版社,1996. 5 刘辉, 王新辉. 基于CAN总线智能输液系统的设计J,湖南工程学院学报, 2005(3): 4-7. 6 刘辉, 王新辉. 智能输液系统的研究与开发J, 湘潭师范学院学报, 2005(2): 65-67.毕 业 设 计（论 文）任 务 书5本毕业设计（论文）课题工作进度计划：2016.2.25-2.16.3.9 完成开题报告、中英文翻译、论文大纲2016.3.19-2016.4.25 提交论文草稿，4月中旬中期检查2016.4.26-2016.5.6 提交论文定稿2016.5.6-2016.5.13 准备答辩2016.5.13-2016.5.26 答辩，成绩评定，修改完成最终稿所在专业审查意见：通过负责人： 2016 年 1 月18 日 滴液速度检测系统设计【精品】外文翻译速度测量系统摘要 一个速度传感器测量带电体的速度，速度传感器用到E型磁芯，一对有间隔的霍尔效应装置位于磁芯的末端，所有的磁芯与导电运动体毗邻。两个霍尔效应装置的不同电压输出反映带电运动体的速度。图a图b 图图图图速度测量系统发明的起源 此处所述的发明是由美国政府的一名雇员所发明的，可以为了美国政府的目的制造和使用，并且不用为此支付任何版税或说明。这项发明的背景1. 这项发明的使用领域 这项发明涉及到速度测量装置，特别是采用磁性装置来模例测量导电体的速度，包括液体和固体物质。2. 大致描述下 都知道磁测速装置是基于电涡流效应，要么直接改变磁通量，要么通过次要磁通路径改变磁通量.过去这类设备的不足在于他们的效率，信号和exitation能量都很差，所以信号的线性度一直不能令人满意。 因此，本发明的目标是提供一个具有好的效率和良好的线性度的信号且可以和磁轴承相结合的磁性速度传感器，进一步的目标是提供一个允许速度测量元件之间不用考虑组件的耦合效应的装置。摘要 根据这项发明，一个E型磁心与他的三个极密切间隔要测量其速度的导电体。一对霍尔器件或发生器在中心极底端的表面，这些发生器很明显被间隔，最好的间隔是被测量速度的运动机构沿其路线的最长距离。因此，霍耳发电机的位置在由导电体的运动速度引起通量密度最大变化和通量差异最大的位置。这种差异由霍耳电机感知，从它反映运动体的速度。 作为这一发明的特色，其手段是通过改变电输入到励磁线圈引起E型磁芯的磁偏，保持恒定总流量通过磁路。这项发明仅适合用于导电体的速度测量，包括具有磁性的导电体。此外，速度分量可在两个或两个以上的方向上来测量，但有一项简单必要的，磁芯或内核与霍尔效应发生器的定位沿着线速度或速度分量测量的方向。进一步，这项发明可能仅仅是一个离散测量装置，或者它可能与其他磁路相结合，如一个磁轴承或多个轴承。简洁说明这些图 图1a 是这个发明基本原理的示意图 图1b是一系列曲线簇表明由图1a的构造引起的流量分布 图 2是这一发明适用于圆形或球形机构测量的一种形式的示意图 图 3是鉴于图2显示的磁芯结构看到的正常结构的端面 图 4是一个磁支撑旋转球型机构的示意图 图 5是这项发明的速度传感系统预想的示意图详细说明这些图 首先图1a，这是说明了这一发明用来测量高速运动物体10 的基本图，箭头12是方向移动。一个E型磁芯14由线圈16通电，供电装置没有画出。一般来说，供电是支流电。运动体10密切定位在磁芯14和磁性材料的导轨18间，磁芯与运动体，导轨与运动体之间的空间距离通常为1毫米。运动体要么是一个导电体要么表面导电。霍尔效应发电机H1和H2的位置，如图所示，在磁芯14的中心段24。霍尔发生器H1被定位在可以看作磁通量通过磁心中心24的后缘位置，霍尔发生器H2被定位在可以看作磁通量通过磁心中心24的前缘位置，考虑到移动机构10的移动方向。导轨18为了提供更多的流量路径，为系统输出更大的信号，但它不是绝对必要在操作中。 图1b是建立在图1a上用来说明磁感应强度穿过图1a上装置的路线，特别说明E型磁芯两极下的磁感应强度。实线26代表运动体10没有运动时的磁感应，虚线28代表着运动体由选定的速度运动产生的磁感应分布。因此，按照本发明应当指出，霍尔发电机H1和H2位于磁感应强度变化最大处来获得测量最大的感应。 常规方式下霍尔电机H1和H2适当的偏离直流源，他们的输出输入到差分电路32，如桥路，桥路的适当输出输入给给运动体10的速度提供一个显示或者提供一个指示的指示器34。 图2和图3是说明这个发明用于像测量球体40的速度的，这个发明的基本理论和操作将从这些图中得到说明。 如图所示，E磁芯的极面42，44，46的弯曲符合球体40的外表面50，直流源54提供线圈52直流电，由响应位于中心极44的表面中心的霍尔效应发生器58产生的流量感应的直流控制56来控制。控制电流是为了让穿过中心极44的磁路的磁通量保持恒定。 因为在这一系统中它通常不可能采用固定磁轨，球体40的表面通常构造一个在具有良好磁性的层61在其上的良好的导电材料外层60。另外，表面区域可以是只有具有这两种特性的单层。此外，表面区域可以是只有单一的导电材料层。信号放大器适当的放大霍尔发生器66和68和输出信号。放大器62和64的输出输入到差分电路70做减法运算，最后由与速度成正比的信号指示器72指示。 该系统的机理，旋转球上的磁场产生电压促使电流在球体表面流动。这些电流以反对磁通量的变法流动。其结果是减小我们看到的旋转区域中心极44边缘74磁感应强度，并增加其尾部或离开边缘76的感应强度。在霍尔发生器上由霍尔发生器66和68测得的不同磁感应强度是与球体上速度的速度分量成正比的第一近似值。一种简化的系统分析如下：E型磁芯48中心极的磁感应强度B是：B = (AT1 - AT2）p （1）P是磁通量的一个磁通量管的渗透率，AT1是磁化的线圈16的安培轮流，AT2是由运动转子10产生的安培轮流，反对安培轮流存在磁道管中正在审议。磁芯48下的运动转子中的电压均称的给一个封闭的电路eBv（2）V是转子的速度，由电压产生的转子中的电流是i = e/r （3）r是电流通过转子那部分的电阻，结合（1）（2）（3）有如下关系i (p/r)(AT1 - AT2)v. (4)作用在转子通量管正在审议的安培轮流AT2正比于转子的电流，所以，结合（4）有：AT2 = k(p/r)(AT1 - AT2)v （5）化简这个公式，有AT2 = AT1v/(r/kp+v)（6）因为AT2引起在所示的霍儿电机测量点磁通量变化为B，结合公式（1）磁通量变化为B= AT2p, 结合公式（5）B= AT1pv/(r/kp+v) （7）霍儿发生器产生的两个不同信号在霍尔发生器线性范围内比例于磁感应强度的变化。这样，结合在霍尔发生器在感应环路和放大器的增益因子g，测量信号为：S=gAT1pv/(r/kp+v)（8）这个公式表明如果在v v (9)关于这三个参数 R，K，P，K是一个不能改变多少比例系数（在公式（5）中）。而R，P在适当的设计中能修改。在转子中选择高抗性的电流路径，例如：由一个较大的磁通量空气路径很有用。随后在公式（8）中很明显的信号强度损失能够在足够高的增益因子G和高磁化的E型磁芯中得到补偿。公式（8）和（9）很明显的表明，非线性效应由减小线圈16产生的安培轮流效应的转子10的安培轮流造成的。因此，传感器信号越小，非线性效应越小 ，同时也要更大的扩大信号以获得满意的信号强度。在多数情况下，在一个可以接受的噪声指数内，足够的放大是可以获得的。通过技术说明，电磁子系统的非线性可以通过控制磁通量为恒定值来补偿的，也就是说，通过电流源56来控制通过线圈52的激励电流，电流源反过来由辅助霍尔发电机58控制。后者提供一个信号指示通过E型磁芯的流量规模，其中，通过常规技术，电流控制56控制电源54提供一个电流输入给绕组16来获得恒定的通量。图4和5说明这个发明的使用结合磁轴承80和82，磁轴承具有E型截面并且适用于磁支撑，在球体84上没有任何物理支撑。这个系统支撑的一个例子先前在美国 Pat. No. 3,017,777上发给申请人。磁轴承80和82的磁芯通过偏压源86来控制，通常86是差分源，提供差分电流给磁轴承。进一步说明，为了保证这一发明的系统的测量精度，通过手段是保证通过轴承的磁通量的总和恒定为常数。注意在图4中，霍尔效应发生器H1和H2在磁轴承82的中心心的极端，这是对旋转球体40的方向测量。同样的，霍尔效应发生器H3和H4在磁轴承80的中心柱处。特别说明的是图5，注意到霍尔发生器是有偏差的，通过电源88和一系列可变电阻90，通过霍尔发生器的电流得到调整。每个霍尔发生器的输出电流反馈给一个放大器92，94，96，98。这些放大器的总输出通过他们的一系列相连相加，因此，提供一个总的信号，记作（H1+H2）+（H3+H4），分压器100分得的参考电压减去这个信号，分压器由给霍尔发电机提供偏置的电源88供电，。通过调整分压器，得到由霍尔效应装置感知选定的流量总和时为0的的输出。为了保持速度指示的精确和校准，可取的做法是保持总的通量恒定。因此，通过调节分压器100来实现选定的输出，输出反馈给终端101，从101到电流控制102，102再控制给磁轴承80和82能量的偏流源86的总电流输出。通常情况下，偏流源86给磁轴承80和82提供不同的电流。放大器92和94的离算输出在不同的电路109中相减，放大器96和98的离算输出在不同的电路104中相减，由次产生的不同信号在加法电路106中相加，产生信号（H1+H2）+（H3+H4）。这个由霍尔发电机从磁轴承80和82获得的信号与运动体10沿相当于与霍尔发生器交叉的平面的沿线的速度成比例。通过在放大器107中的合适放大，信号反馈给提供显示或者其他指示的速度显示108。上面已经描述了下我的发明，什么是我主张的：速度传感器测量导电体沿一条运动路径的表面速度的组成：1. 一个E型磁芯具有中心极和两个间隔的外极，两外极的末端配制时密切间隔要测量其速度的带电运动体，两极定位按照运动体的运动方向；一个激励线圈饶在中央极上；第一和第二个霍尔装置沿运动体运动方向上安置在磁芯的中央级的末端表面上，每个霍尔装置有一个偏置输入和信号输出。偏置意思是提供一个偏置电流给磁芯和霍尔装置偏置输入；差分意思是连接两个霍尔装置的输出并对这两个信号相减，产生指示沿着但间隔霍尔装置中心极的外表面的运动体的速度的信号2. 组成在1中已详细的说明，其中：传感器包括第三个霍尔装置，位于磁芯中心极外表面的中央；偏置意思是提供一个偏置电流给激励线圈，包括直流控制，意思是响应第三个霍尔装置的输出，为了使通过磁芯中心极的通量保持不变。3. 在2中所说的速度传感器进一步的组成有放大第一和第二个霍尔装置的输出的放大器，显示是指响应差分的输出，显示带电体的速度。4. 测量沿着一条路径运动的带电体表面速度的传感器组成：第一个和第二个相反定位的E型磁芯调整和定位来磁支撑一个位于磁芯间的旋转球体，每个磁芯有中心级和两个间隔的外极，所有的磁芯位于沿运动体运动方向上；第一个和第二个霍尔效应装置间隔的定位在第一个磁芯中心极的底部，第三个和第四个霍尔效应装置间隔的定位在第二个磁芯中心极的底部；第一个和第三个霍尔效应装置截然相反的定位在运动体的对面，第二个和第四个霍尔效应装置截然相反的定位在运动体的对面；第一个激励线圈绕在第一个磁芯，第二个激励线圈绕在第二个磁芯；偏置意思是提供一个偏置电流给霍尔效应装置和激励线圈，显示是指响应差分的输出，显示带电体的速度。5. 在4中所说的速度传感器进一步的组成有放大第一和第二个霍尔装置的输出的放大器。6. 在5中所说的速度传感器进一步的组成求和的意思是做为一个输出 ，比例于霍尔效应装置输出和的信号；参考信号由求和输出提供一个信号，为了使由E型磁芯总流量为选定的时输出为0；直流输入与参考信号有关，意思是给第一和第二个激励线圈提供一个直流，保持第一和第二个磁芯的磁通量不变。VELOCllY MEASURJ2MENT SYSTEM ABSTRACT A velocity sensor for sensing the speed of a moving conductive body employing an E-shaped magnetic core having a pair of spaced Hall effect devices positioned on the end of the central core, the ends of all cores being arranged adjacent to the path of the moving conductive body. The difference in output voltage registered by the two Halleffect devices is indicative of the speed of the conductive body. 6 Claims,6 Drawing Figures U.S.Patent June 6, 1978 Sheet 1 of 3 4,093,917 U.S.Patent June 6, 1978 Sheet2 of 3 4 093,917 U.S.Patent June 6, 1978 Sheet 3 of 3 4,093,917 VELOCFTY MEASURFMENT SYSTEM ORIGIN OF THE INVENTION The invention described herein was made by an employee of the United States Government, and may be manufactured and used by or for the Government of the United States for governmental purposes without the payment of any royalties thereon or therefor. BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to speed measurements devices, and particularly to an apparatus employing magnetic means for analog measurement of the speed of an electrically conductive body,either of solid or liquid substance. 2. General Description of the Prior Art Most known magnetic speed measuring devices are based upon eddy current effects, either changing the magnetic flux directly or by causing changes in a secondary flux path. The difficulty in the past with such devices has been that their efficiency, or signal vs. exitation energy, has beenpoor and that their signal linearity has generally been unsatisfactory. Accordingly, it is an object of this invention to provide an improved magnetic type speed sensor with a good efficiency and satisfactory signal linearity which may be combined with magnetic bearings. A further object of this invention is to provide a device which permits the measurement of speed components without cross coupling effects as between components. SUMMARY OF THE INVENTION In accordance with this invention, an E-shaped magnetic core is positioned with its three poles closely spaced from a conductive body, the speed of which is to be monitored. A pair of Hall effect devices or generators are positioned on the surface of the end of the central pole, these generators being significantly spaced, preferably spaced a maximum distance measured along the line of movement of the body to be monitored in speed. Thus, the Hall generators are positioned where the flux density shows its greatest change resulting from the velocity of the conductive body and at points of greatest difference in flux. This difference is thus sensed by the Hall generators, and from it the speed of the body is indicated. As a feature of this invention, means are provided to maintain a constant total flux through the magnetic circuit involved by varying an electrical input to an energizing coil supplying magnetic bias to the E-shaped magnetic core. The invention is adaptable for use for measuring the speed of bodies which are simply electrically conductive, as well as being both Conductive and having a magnetic property. Further, speed components may be measured in two or more directions, it being simply necessary to orient the magnetic core or cores with Hall effect generators positioned along a line which speed or a speed component is to be measured. Still further, this invention may solely be a discrete measurement device, or it may be combined with other magnetic circuitry, such as that of a magnetic bearing or bearings. BRIEF DESCRIPTION OF THE DRAWINGS FIG. la is a schematic illustration of the basic concept of this invention. FIG. lb is a series of curves illustrating flux distribution resulting from the configuration shown in FIG. la FIG. 2 is a schematic illustration of a form of this invention adapted to measure the surface speed of a circular or spherical body. FIG. 3 is a view ofthe magnetic core structure shown in FIG. 2 as seen when viewing the structure normal to the pole faces. FIG. 4 is a schematic illustration of a magnetic support for a rotating spherical body. FIG. 5 is a schematic illustration of a speed sensing system as contemplated by this invention. DETAILED DESCRIPTION OF THE DRAWINGS Referring fvst to FIG. la, there is shown a basic illustration of this invention as employed to measure the speed of a moving body 10,moving in the direction of arrow 12.An E-shaped magnetic core 14 is energized by a winding 16which is powered by means not shown . Typically, the power supply would be a direct current power supply. Moving body 10 is closely positioned between E core 14and a yoke of magnetic material 18, the spacing between the core and moving body and the yoke and moving body typically being approximately one millimeter. Moving body 10 would either be of a conductive material or have a conductive surface on it. Hall effect generators H1 and H2 are positioned, as shown, on the phase of center leg 24 of E core 14,Hall generator H1 being positioned on what may be deemed the trailing flux edge of center core 24, and Hall generator H2 behg positioned on what may be deemed the leading flux edge of center core 24, considering the direction of movement of moving body 10.Yoke 18 unctions to provide increased flux path for greater signal output of the system, but it is not absolutely necessary for operation. FIG. lb is positioned under FIG. la to illustrate the magnetic flux density across the circuit of the device shown in FIG. la, and particularly to illustrate the magnetic flux density under the poles of the E-shaped magnetic core 14.Solid line 26represents the magnetic induction when moving body 10 is at rest, and dashed line 28 represents the induction distribution occurring for a selected velocity of moving body 10.Thus, it will be noted that in accordance with this invention, Hall effect generators H1 and H2 are placed at the position of maximum flux density variation to obtain maximum sensitivity of the measuring arrangement. Hall generators H1 and H2 appropriately biased by a direct current source in a conventional manner, and their outputs are applied to difference circuit 32,such as a bridge, and its output, appropriately scaled, is applied to indicator 34which displays or otherwise provides an indication of the speed of movement of body 10. FIGS. 2and 3illustrate an embodiment of this invention as applied to the measurement of the speed of sphere 40, and the basic theory of operation of this invention will be explained with respect to these figures. As shown, pole faces 42,44,and 46 of E core 48 are curved to conform with that of the outer surface 50 of sphere 40.DC exitation is provided coil 52 from DC current source 54, controlled by current control 56, in turn responsive to flux sensed by Hall effect generator 58,positioned in the center of the face of center pole 44. Current is thus controlled to maintain a constant flux in the magnetic circuit through center pole 44. Since it is normally not possible in such a system to employ a fixed magnetic yoke, the surface region of sphere 40 would typically be constructed of an outer layer 60which is of a good electrical conductive material formed over layer 61, which is of a good magnetic material. Alternately, a single layer having both of these properties may be employed. As another alternate, the surface region may simply be of a conductive material, and signal amplifiers 62 and 64 are employed as shown to appropriately amplify the outputs of Hall generators 66 and 68. The outputs of amplifiers 62 and 64 are applied to and subtracted by difference circuit 70, and the resulting signal, which is proportional to velocity, is indicated by indicator 72. To examine the operation of the system, voltages induced by the magnetic field on the rotating sphere cause electric currents to flow in the conducting surface of the sphere. These currents areoriented in such a way that they oppose magnetic flux change. The result is a decrease in magnetic flux density as seen from the rotating sphere at the leading or on-coming edge 74 of center pole 44, and an increase in flux density on its trailing or leaving edge 76. The difference in flux densities measured by Hall generators 66 and 68 is a first approximation proportional to the speed component of the sphere in the plane of the Hall generators. A simplified analysis of the properties of the system follows: The magnetic flux density B under the center pole of 30 E-shaped core 48 is: B = (AT1 - AT2）p （1）Where p is the permeance of a magnetic flux tube of the 35 magnetic flux, AT1 are the magnetization ampere turns of winding 16, and AT2, are the ampere turns produced by the moving armature 10, opposing the ampere turns AT1, in the magnetic flux tube under consideration. The voltage in moving armature 10 under core 48 is for a closed electrical circuit given by the proportionality eBv（2） where v is the speed of armature 10. The resulting current in the armature, caused by voltage e,is i = e/r （3）Where r is the resistance of the current path in the armature. Substituting equations (1) and (2) in (3) shows the relationship i(p/r)(AT1 - AT2)v. (4)The armature ampere turns AT2 , acting on the flux tube under consideration are proportional to the armature currents; thus, with equation (4), it follows that AT2 = k(p/r)(AT1 - AT2)v （5）or resolving this equation for AT2, one obtains AT2 = AT1v/(r/kp+v)（6）Since AT2 , causes the change in flux densityB at the measuring points of the Hall generators as shown with equation (l), this flux change is B= AT2p, and with equation (5): B= AT1pv/(r/kp+v) （7）The difference of the signals from the two Hall generators is within the linearity of the Hall generators proportional to the change in flux density. Thus, with a gain factor g in the sensing loop of the Hall generators and amplifiers, the measured signal is S=gAT1pv/(r/kp+v)（8）This equation shows that the signal S is proportional to the armature velocity v if v v (9)Of the three parameters k, r, andp, k is a proportionality value (equation (5) which cannot be changed very much, whereas r and p can be modified by proper design. Selection of a high resistance of the current path in the armature and of a low permeance, e.g., by a larger air path for the magnetic flux, are helpful. The subsequent loss in signal strength as evident from equation (8) can be compensated for by a sufficiently high gain factor g as well as by a higher magnetization of the E-shaped core. The results demonstrated by equations (8) and (9) are quite evident if one considers that the non-linearity effect is caused by ampere turns in armature 10 which reduces the effect of exitation ampere turns provided by winding 16. Thus, the smaller the sensor signal the smaller will be the non-linear effect, and the more amplification which will be necessary to obtain a desired signal strength. In most instances, sufficient amplification at an acceptable noise figure can be obtained. The non-linearity inherent in the electromagnetic subsystem can be compensated for by control of the magnetic flux to a constant value by the technique illustrated, that is, by controlling the energizing current to coil 52 by means of current control 56, in turn controlled by auxiliary Hall generator 58. The latter provides a control signal indicative of the magnitude of flux through E-shaped core 48, whereby, by means of conventional techniques, current control 56 controls power source 54 to provide a current input to winding 16 to maintain a constant flux. FIGS. 4 and 5 illustrate the employment of this invention with magnetic bearings 80 and 82, which have an E-shaped cross section and which are adapted to magnetically support and position sphere 84 without any physical bearing surface. An example of such a system of support is shown in U.S.Pat. No. 3,017,777 previously issued to the applicant. The cores of magnetic bearings 80 and 82 are controlled by means of bias source 86, typically a differential type source supplying current differentially to the magnetic bearings. As will be further explained, means are provided to assure that the sum of the magnetic fluxes to the bearings is held constantin order to assure accuracy of the measurement system of this invention. As will be noted in FIG. 4, Hall effect generators H1and H2 are positioned on extreme edges of the center pole of magnetic bearing 82, this being with respect to the direction of measurement of rotation of sphere similarly, Hall effect generators H3 and H4 are so positioned on the center pole of magnetic bearing 80. Referring particularly to FIG. 5, it will be noted that the Hall generators are biased, in series, by means of power source 88 and series variable resistor 90, by which the bias current to the Hall generators may be adjusted. The current output of each Hall generator is fed to one of amplifiers 92, 94, 96, and 98. The sum of the outputs of these amplifiers are added by connecting them in series, and thus there is provided a sum signal which is appropriately labelled as (H1 +H2) +(H3 + H4), from which is subtracted a reference voltage obtained from voltage divider 100, in turn powered from power supply 88 furnishing biasing power to the Hall generators. By adjustment of this voltage divider, an output is obtained which is adjusted to be 0 for a condition of a selected flux sum detected by the Hall effect devices. This selected state would typically be a selected operating flux state for the magnetic bearings. In order to maintain accuracy and calibration of speed indication, it is desirable that this value of total flux be maintained. Thus, the selected output achieved by adjustment of voltage divider 100 as aforesaid is fed to terminal 101(FIG. 5) and from terminal 101(FIG. 4) to current control 102, and it in turn controls the total current output of bias source 86, which supplies power to magnetic bearings 80 and 82. Typically, bias source 86 would differentially apply a current to magnetic bearings 80 and 82. Discrete outputs of amplifiers 92 and 94 are subtracted in difference circuit 109, and the discrete outputs of amplifiers 96 and 98 are subtracted in difference circuit 104, and the thus obtained difference signals are summed in sum circuit 106 to provide a signal representative of (H1 +H2) +(H3-H4). This signal obtained from the Hall generators in both magnetic bearings 80 and 82 is proportional to the speed of movement of sphere 80 along a line corresponding to a plane intersecting the Hall generators. After proper 50 amplification in amplifier 107, the signal is fed to speed indicator 108, which provides a display or other indication of speed. Having thus described my invention, what is claimed is: 1.A velocity sensor for measuring the surface speed of a conductive body along a path of movement comprising: an E-shaped magnetic core having a central pole and two spaced outer poles, the ends of said poles being configured to be closely spaced from the condutive surface of a moving body the speed of which is to be measured, said poles being positioned, in line, along said path of movement of said body; an energizing coil coupled to said central pole; first and second Hall effect devices positioned in a spaced relation along said path of movement of said body on the end surface of said central pole of said core, and each said Hall effect device having a bias input and signal output; bias means for applying a biasing current to said energizing coil and to the bias input of said Hall effect devices; and difference means connected to the signal outputs of said Hall effect devices for subtracting the signal outputs and providing a signal indicative of the velocity of a conductive body moving across but spaced from the end surfaces of said poles and said Hall effect devices. 2.The ombination as set forth in wherein: said sensor further comprises a third Hall effect device centrally positioned on the end surface of the central pole of said core; and said bias