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输入输出点数对价格有直接影响。每增加一块输入输出卡件就需增加一定的费用。当点数增加到某一数值后,相应的存储器容量、机架、母板等也要相应增加,因此,点数的增加对选用、存储器容量、控制功能范围等选择都有影响。在估算和选用时应充分考虑,使整个控制系统有较合理的性能价格比。
分析了电磁干扰及其对PLC控制系统干扰的机制,指出在工程应用时必须综合考虑控制系统的抗干扰性能,并结合工程提出了几种有效的抗干扰措施。
1概述
随着科学技术的发展,PLC在工业控制中的应用越来越广泛。PLC控制系统的可靠性直接影响到工业企业的安全生产和经济运行,系统的抗干扰能力是关系到整个系统可靠运行的关键。自动化系统中所使用的各种类型PLC,有的是集中安装在控制室,有的是安装在生产现场和各电机设备上,它们大多处在强电电路和强电设备所形成的恶劣电磁环境中。要提高PLC控制系统可靠性,一方面要求PLC生产厂家用提高设备的抗干扰能力;另一方面,要求工程设计、安装施工和使用维护中引起高度重视,多方配合才能完善解决问题,有效地增强系统的抗干扰性能。
2电磁干扰源及对系统的干扰
2.1干扰源及干扰一般分类
影响PLC控制系统的干扰源与一般影响工业控制设备的干扰源一样,大都产生在电流或电压剧烈变化的部位,这些电荷剧烈移动的部位就是噪声源,即干扰源。
干扰类型通常按干扰产生的原因、噪声干扰模式和噪声的波形性质的不同划分。其中:按噪声产生的原因不同,分为放电噪声、浪涌噪声、高频振荡噪声等;按噪声的波形、性质不同,分为持续噪声、偶发噪声等;按噪声干扰模式不同,分为共模干扰和差模干扰。共模干扰和差模干扰是一种比较常用的分类方法。共模干扰是信号对地的电位差,主要由电网串入、地电位差及空间电磁辐射在信号线上感应的共态(同方向)电压迭加所形成。共模电压有时较大,特别是采用隔离性能差的配电器供电室,变送器输出信号的共模电压普遍较高,有的可高达130V以上。共模电压通过不对称电路可转换成差模电压,直接影响测控信号,造成元器件损坏(这就是一些系统I/O模件损坏率较高的主要原因),这种共模干扰可为直流、亦可为交流。差模干扰是指作用于信号两较间的干扰电压,主要由空间电磁场在信号间耦合感应及由不平衡电路转换共模干扰所形成的电压,这种让直接叠加在信号上,直接影响测量与控制精度。
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