基于51单片机的射频识别系统软硬件设计及射频知识

基于51单片机的射频识别系统软硬件设计 一、系统设计概述 该系统基于51单片机控制，以无线通信为基础，搭建一个简单的射频识别系统。该系统有如下优点：①识别时，无需接触；②识别时间短；③错误识别的概率相对较小；④有良好的扩展性。这种系统进行功能扩展后可用于停车场、交通道路管理、智能物业管理等多种场合。 系统设计主要分阅读器、应答器、线圈3部分。阅读器采用晶振1和晶振2，分别提供应答器的功率驱动信号和数字调制信号。晶振1产生的振荡信 号经过带通滤波器处理后，进行功率级放大，并通过匹配网络进行阻抗变换，以最大效率从线圈发射出去，为应答器提供所需的工作能量。晶振2产生的信号经过低 通滤波器滤除高频杂波后，送往开关电路；手动设置的信息，南单片机转换为相应的控制信号，控制开关的通断，从而形成2ASK调制信号，与应答器进行通信。 此外，阅读器还需将线圈上接收到的应答信号滤波放大并检波，最终获取有效信息，并由串口读取。图1给出阅读器结构。

晶振产生振荡信号后，经低通滤波器去除高频杂波，送往开关电路，作为2ASK的载波信号。控制开关电路通断的信号由操作者通过拨码开关手动 设置，并由单片机读取后产生相应的控制信号。同时，应答器通过线圈接收功率驱动信号，待整流滤波后，得到直流电平，然后作DC-DC变换，以获取最终所需 的直流电平，供整个应答器部分工作。图2给出了应答器结构。 二、 硬件电路 1、阅读器与应答器2ASK调制解调电路 2ASK的载波信号是2MHz正弦波，它由有源晶振产生的方波经过低通滤波器得到。数字调制信号从CPU的串口输出，经过模拟开关 MX7501控制信号通断，产生了2ASK信号。Ll，L2，C1和C2构成了二阶巴特沃兹低通滤波器，输出为近似正弦波的2 MHz信号。R1与R4使LC滤波器阻抗匹配。当EN为0时，OUT为0；反之为S1通道信号。图3给出阅读器的2ASK调制电路。 图4给出了解调电路。通过匹配网络的2ASK信号幅度低，噪声大，需要经过三级处理才能解调出数字信号。首先通过OP37中高速运算放大器 将信号放大5倍，再经过高速比较器MAX910中的比较器A，将毛刺状信号加宽，减小噪声，D／A输出经L1，C1和C2组成的低通滤波器，取直流分量， 这相当于包络检波。LC低通滤波器的截止频率为480 HZ。最后将该信号进行比较整形，得到数字调制，再经QB输入到单片机串口。 2、发射电路 有源晶振产生的8 MHZ方波经带通滤波器取出8 MHz的正弦信号，并通过功率放大器后输出到线圈上。图5中，L3，L4，C7和C8构成了二阶8 MHz的巴特沃兹带通滤波器，用以滤除谐波分量。VQ1为集射极跟随器，用以调节R3，使其静态工作电流约为1．5 mA，该级电路起到与信源隔离的作用。VQ2为丙类放大器，调节R7和VQ2的基极偏压使其工作在丙类放大状态。改变C2使其谐振，此时电源电流最小。再 调节R4和VQ2的基极偏压，使电源电流更小，输出幅度更大，以处于丙类放大状态。L1为线圈，同时起到谐振电感的作用。C6为线圈的等效电容，经测量， 约为34 pF；C3为外接电容，它能使并联回路谐振在8 MHz。谐振时，L1上的电压可达45V。阅读器和应答器发射的另一个信号是2ASK信号，它经过运算放大后，通过匹配网络直接连接到线圈上。 3、应答器电源电路设计 应答器的整流滤波电路采用单相桥式整流滤波电路。T1为两个线圈，C1并接在应答器线圈两端，在8 MHz并联谐振下，其输入幅度最大。VD1～VD4采用IN5817型整流二极管，要求整流8 MHz的信号。正常时，C2两端电压基本稳定，即整流电流全部通过负载R1，因此RL越大，其电压也越大。这就要求负载尽量少，负载输入阻抗尽量大。C2 的电容值不易太大或太小，太大则吸纳的电荷越多，使输出电压就越小；导致滤波效果不佳。因此，这里取C2=47μF。在耦合信号幅度一定时，该电路的功率 驱动能力是固定的。实测中发现，当输入频率为8 MHz时，输出电压为5 V；当负载频率为1 kHz时。输出电压为4．25 V。由此可知，其驱动功率P=U2/RL=18 mW，应答器功耗必须小于该值。L1和L2进一步滤波，也能利用感应电势的作用防止电流大幅度突变，其电路如图6所示。 三、系统软件设计 1、 软件思想 该系统软件设计的基本原理是：首先通过单片机控制阅读器，发送数字基带信号，经过2ASK调制后，再由天线发射出去。应答器经天线收到调制 信号后，进行2ASK解调，解调信号通过串行口送到单片机。单片机验证信号进行处理后，将EEPROM中存储的信息读取出来，并送向调制器，调制信号再经 天线发送出去。阅读器收到返回的信号后将信号进行解调送回单片机。单片机将信号进行解码等操作，来分析返回的信号，以达到识别物品的目的。 2、程序流程图 复位后，阅读器端查寻是否有相应物品。若有，则阅读器端控制调制信号的产生。同时，允许将信号发送出去。应答器对接收到的相应信号进行解调，并检查所接收信息是否错误，并作相应处理。程序流程见图7。 RFID基础知识 ······················································································· 7 RFID应用领域 ····················································································· 13 RFID相关术语 ····················································································· 14 标签 ······································································································· 17 RFID读写设备基本介绍······································································ 20 RFID读写器 ························································································· 24 RFID知识进阶 ····················································································· 25 RFID工作频率的分类 ········································································· 29 RFID中间件知识 ················································································· 33 如何保护RFID内部信息····································································· 40 RFID天线知识 ····················································································· 45 电子标签耦合 ······················································································· 49 电子标签的制作及封装 ········································································ 52 射频标签通信协议简介 ········································································ 53 射频标签内存信息的写入方式 ···························································· 54 RFID工作频率指南和典型应用 ·························································· 54 从传统条码到RFID ············································································· 57 射频技术和条码的比较 ········································································ 62 RFID标签能否取代条码技术 ······························································ 63 使用高频标签会对人体有辐射危害吗 ················································ 64 RFID面临的问题 ················································································· 64

RFID基础知识 1．什么是RFID RFID是Radio Frequency Identification的缩写，即射频识别。常称为感应式电子晶片或近接卡、感应卡、非接触卡、电子标签、电子条码，等等。 一套完整 RFID系统由 Reader 与 Transponder 两部份组成 ,其动作原理为由 Reader 发射一特定频率之无限电波能量给Transponder,用以驱动Transponder电路将內部之ID Code送出，此时Reader便接收此ID Code。 Transponder的特殊在于免用电池、免接触、免刷卡故不怕脏污，且晶片密码为世界唯一无法复制，安全性高、长寿命。 RFID的应用非常广泛，目前典型应用有动物晶片、汽车晶片防盜器、门禁管制、停车场管制、生产线自动化、物料管理。RFID标签有两种：有源标签和无源标签。