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基于RFID的AGV定位与导引研究

Research on AGV Positioning and Guiding Based on RFID

【作者】 卢少平

【导师】 吴耀华;

【作者基本信息】 山东大学 , 控制理论与控制工程, 2011, 博士

【摘要】 AGV(Automated Guided Vehicle)是一种可广泛运行于仓库、码头、生产车间等场所的无人驾驶自动搬运车,是现代物流技术的代表性设备,其关键技术之一是实现自动定位与导引。目前,AGV多采用激光制导、惯性制导等技术,实现毫米级的定位成本昂贵。同时,由于光污染等现场因素,使得其广泛应用受到局限。而RFID(Radio Frequency Identification)作为一种自动识别技术,除实现多目标、超视距识别外,利用其电磁特性实现定位己受到广泛关注。特别是随着物联网概念的兴起和其应用技术的深入发展,作为其关键技术的RFID将更加普及,相关技术与设备的应用成本将越来越低廉。研究基于RFID的AGV定位与导引技术和方法,具有理论和现实意义。本文研究了AGV现行定位与导引技术存在的不足以及不同场景下AGV的定位与导引要求;通过对RFID系统原理的深入研究,分析了现行RFID定位理论与方法存在的不足;分别针对远场和近场RFID系统,首次提出了“基于方向函数求解的误差抑制定位法”和“基于磁场等场线的精确覆盖定位法”,并对上述方法进行了分析与证明。同时,依据上述方法提出可实施的基于远场有源、远场无源和近场RFID的AGV定位与导引系统设计方案,并进行系统实现与仿真。(1)“基于方向函数求解的误差抑制定位法”与应用研究针对现行远场RFID系统定位中,采用接收信号强度(RSSI)及其路径损耗模型定位精度严重偏低的问题,本文首次提出了“基于方向函数求解的误差抑制定位法”:依据读写器天线测得的标签信号强度值,可以确定一个对应于方向函数的等场线方程,求解三个以上的等场线方程确定待测目标的位置,实现定位;通过对影响接收信号强度的各种原因的分析,消除和抑制产生误差的因素,提高定位精度。针对现行的路径损耗模型只考虑RSSI与距离的关系而忽视天线方向性特性对RSSI影响的现象,首次提出了基于方向函数的远场电场功率等场线求解方法,从理论上消除产生误差的根源;环境反射产生的多径效应是室内环境下影响接收信号强度的主要因素,本文首次提出利用第一零点功率波瓣角小于入射角余角的方法消除主反射信号进而抑制多径效应产生的误差;通过增加可控的反射器来抑制天线后侧的反射影响及控制信号覆盖范围来抑制天线远端方向的反射影响;首次从微观上考虑跳频对信号强度的影响并提出抑制办法;分析了电磁环境对无源系统产生的影响,并提出了抑制措施。针对有源RFID系统,分析了Friis传输方程及其影响因素,提出了标签车载移动、读写器天线阵列固定布置的AGV定位与导引系统设计方案。车载标签采用8单元道尔夫-切比雪夫天线阵,并由AGV直接供电,解决有源系统信息发送时间间隔长不能实时定位和电池输入功率变化产生定位误差的问题;读写器天线采用道尔夫-切比雪夫天线阵加平板反射器,抑制垂直反射和水平反射引起的误差;按读写器读得的同一标签的四个最大天线信号强度有效值,确定等场线方程组,并两两求解,选择6个有效值,取几何中心为最后定位位置,以提高定位精度。该系统可用于自由路径导引和固定路径导引,且适合于多车辆运行的环境,并有计算量少、成本低廉的特点。针对无源系统,分析了雷达反向散射方程及其影响因素,特别是标签散射面积的影响,提出了读写器车载移动、标签阵列固定布置的AGV定位与导引系统设计方案。标签采用普通对称振子无源标签、竖直等间距布置在车辆行驶路径的一侧或两侧。车载读写器移动天线采用道尔夫-切比雪夫天线阵加夹角反射器得到线极化定向信号,并抑制垂直反射、水平反射和环境反射;读写器可以有效覆盖到三个以上的标签,根据识读标签及其信号强度计算每个标签相对读写器天线的等场线方程,联立求解得到AGV的位置,实现定位与导引。该系统适合于环境比较复杂的场合,用于固定路径导引。(2)“基于磁场等场线的精确覆盖定位法”与应用研究针对近场耦合RFID系统的工作原理和基本特点,以及近场RFID系统一般只作识读性定位的现状,本文首次提出了“基于磁场等场线的精确覆盖定位法”:根据AGV定位与导引的要求,按照一定的条件设计出近场天线,水平安装在AGV底部,使其覆盖在地面的磁场等场线形成一个精确稳定的圆。利用正多边形的外接圆与其几何中心重合的几何特性,以正三角形等间距布置标签,使满足启动标签识读的阅读器天线磁场等场线圆恰好覆盖标签三角形,实现精确定位。首次提出并证明了圆环天线线圈产生的磁场沿其垂直方向的等场面平行截线是一个圆;通过对空间磁场等场线极值点与距离的关系进行分析,证明存在一个合适的区域,其等场线圆是一个稳定的圆;通过分析垂直方向的磁场与线圈直径、距离的关系,证明该圆是一个精确的圆;研究了等场线圆的半径以及标签间距、标签尺寸等对定位精度的影响。按照上述方法,根据AGV系统运行要求设计了基于近场RFID的AGV定位与导引系统:在车辆底部装载不同直径的RFID读写器天线,其中行驶天线选择大直径圆环线圈,使其有较大的覆盖范围,用于车辆行驶的定位与导引,以达到高效低成本的目的;停位天线由小直径圆环线圈组成,用于AGV的精确定位和姿态校正;与此对应,地面等间距布置相同型号的标签,行驶区间采用适用大直径行驶天线的大三角形稀疏标签阵,停位区间采用适用于小直径天线的小三角形密集标签阵,并进行了系统仿真、测试。通过上述理论研究与实验,实现了AGV的高精度定位。实验表明,定位精度可达到毫米级,满足行驶与停位要求。由于构成该定位和导引系统的核心装置RFID标签和读写器的成本低廉,使得AGV的成本大大降低,同时,不受光污染等因素的应用局限。该系统可以用于自由路径和固定路径AGV的定位与导引。

【Abstract】 AGV (Automated Guided Vehicle) is a kind of unmanned automated guided vehicles which can be widely used in warehouses, docks and manufacturing facilities. It is a representative technology and equipment in the field of logistics and automation. One of the key technologies involved in AGV is the automatic positioning and guiding. Currently, Most AGV use laser guidance or inertial guidance technologies. It is very expensive to achieve millimeter-level positioning. Meanwhile, Factors existing at the application sites such as light pollution limit its applications. While existing positioning and guiding technologies are expensive in manufacturing and limited in their applications, Radio Freqency Identification (RFID), as an automatic identification technology, in addition to its usage for multi-target and beyond visual range recognition, the usage of its electromagnetic properties to enable positioning has attracted widespread attention. Especially with the emergence of the concept of the Internet of Things and the further development of its application, RFID, as the key technology, will become more and more popular and the relevant technologies and equipments will cost significantly less. This research explores RFID-based positioning and guiding technology used by AGV, which has theoretical and practical significance.This paper examines the deficiencies of current AGV positioning and guiding technologies and the AGV positioning and guiding requirements under different scenarios. Based on in-depth study of the RFID system theoretical basis and analysis of the deficiencies of current RFID positioning theories and methods, we first propose two new approaches, "Error Suppression Positioning Based On Solving Orientational Correlation Function" and "Precise Coverage Positioning Based On Magnetic Field Strength Contours", for far-field and near-field RFID systems respectively. We analyze and proof these two approaches, based on which practical implementation design of AGV positioning and guiding systems, for active far-field, passive far-field and near-field RFID respectively, are suggested followed by simulations. (1) Errors Suppression Positioning Method By Solving Orientational Correlation FunctionExisting Far-field RFID systems adopt more frequent use of Received Signal Strength Indicator (RSSI) and the path loss model for positioning, which has major problem of low accuracy. This paper first proposes a new method called "Error Suppression Positioning By Solving Orientational Correlation Function":according to the signal strength of tags measured by the reader antenna, a field strength contour equation can be determined which corresponds to an Orientational Correlation Function. The positioning can be achieved by solving more than three such field strength contour equations. In the system design and implementation process, based on the analysis of a variety of factors that would possibly weaken the received signal strength, we improve the positioning accuracy by eliminating or suppressing the error factors.Existing path loss model only takes the relationship between RSSI and distance into consideration and overlooks the impact of antenna directivity on RSSI. We first propose a method that solves far-field electric field strength contour equations based on Orientational Correlation Function, which eliminates errors from their origin theoretically. Multi-path effect produced by environmental reflection is the main factor that influences the received signal strength negatively in indoor environment. We propose to eliminate the main vertical reflected signals by taking advantage of the fact that the First Null Beamwidth (FNBW) is less than the complementary angle of incidence angle, and thereby eliminate multi-path interference. The horizontal reflection is suppressed by increasing the reflector and controlling the signal coverage. For the first time, a suppressing approach is proposed based on a microscopic consideration of the impact frequence hopping has on signal strength. Besides, we proposed suppression measures after analyzing the impact of the electromagnetic environment on the passive systems.For active RFID systems, after the analysis of the Friis transmission equation and its influence factors, we suggest a layout design of AGV positioning and guiding system via arranging tags carried by vehicles and fixing reader antenna arrays. The vehicle tags adopt 8-cell Duelfer-Chebyshev antenna array and they are all directly supported by the AGV’s power. In this way, the data transmission interval is shortened to meet the requirement of real-time positioning. It also avoids the positioning error caused by the fluctuation of battery input power. Meanwhile, reader antennas adopt Duelfer-Chebyshev plus plate reflector to suppress the errors caused by vertical and horizontal reflection. The same tag’s four largest antenna signal strength read by the readers can determine the field strength contour equations. After solving the equation set and selecting the adjacent six numerical values, the geometric center is computed for the final positioning. This system can be used for AGV guiding in both fixed path and free path circumstances. It is suitable for multi-vehicle operation environment. It is featured by low calculation load and low cost.For passive RFID systems, the radar backscatter equation is analyzed along with its influence factors, especially the impact of the tags’scattering area. Based on this analysis, we suggest an AGV positioning and guiding system design which is featured by mobile readers and fixed-layout tag array. We adopt the ordinary symmetric oscillator passive tags, which are vertically arranged and equally spaced on one or both sides along the vehicles’route. We adopt the Duelfer-Chebyshev antenna array together with reflector with angles, which obtains the linear polarization in order to inhibit vertical, horizontal and environmental reflection. Besides, each reader can cover three or more tags at one time. With the tags’reading and their relative signal strength, AGV can be located and guided by solving the field strength contours equations established by every tag and its reader’s antenna. This system is suitable for fixed-route guidance in complex environment.(2) Precise Coverage Positioning Based On Magnetic Field Strength Contours MethodBased on the principle and basic characteristics of near-field coupling RFID systems, and the fact that near-field RFID systems are previously used only for positioning through recognition, this work first propose a method called "Precise Coverage Positioning Based On Magnetic Field Strength Contours". According to the requirements of AGV’s positioning and guiding, a special near-field antenna is designed to be installed at the bottom of a AGV and generate stable and accurate circular magnetic field contours covering the ground. By taking advantage of the geometric characteristics that the geometric center of an equilateral polygon and its circumcircle coincide, equally spaced tags are arranged in the shape of an equilateral triangle and the reader antenna is adjusted to cover the tags arranged in the triangle shape. As such, accurate positioning is achieved.This work studies and proofs that the magnetic field contours generated by a circular antenna coil along its central vertical line is a circle. By analyzing the relationship between field extrema and the distance to the center, we proof that there exists a suitable area, within which the field contour is a stable circle. After analyzing the relationship between magnetic field vertical component, the distance to the coil, and coil diameter, we proof that the contour is an accurate circle. We also study the positioning accuracy dependency on the radius of the field contour circle, the spacing and size of tags.Using the method as described above, an AGV positioning and guiding system is designed for near-field RFID to meet the operation requirements. Reader antennas with different diameters are mounted beneath the vehicle. Large-diameter driving antenna coils are used for coarse positioning with greater aerial coverage in order to achieve cost-effectiveness. Small-diameter stopping antenna coils are used for precise positioning and posture correction. Correspondingly, same types of tags are arranged on the ground with equal spacing. Large triangular arrays are used at driving zones to fit the large-diameter antennas. High density tagging of smaller triangular arrays is used at stopping zones. Simulation and test are conducted for the design.This work established theoretical foundation and conducted simulation for achieving high accuracy requirement of AGV. The simulations demonstrate that the highest achievable positioning accuracy can reach millimeter range. This could make AGV systems autonomously driving and stopping without auxiliary devices, meeting application requirements. The low cost of RFID tags and readers which are the core devices of positioning and guiding system leads to low manufacturing cost of AGV and free AGV from the application limitation imposed by factors such as light pollution. The system can be used for both fixed and free route guidance.

  • 【网络出版投稿人】 山东大学
  • 【网络出版年期】2011年 12期
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