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The difference between PLC Control and Relay Control

香港嘉玮科技有限公司   作者:SystemMaster 发布日期: 2016-04-24 15:52:51   点击率:733

The difference between PLC Control and Relay Control. 
According to our many items as well as the long time study to sum up the experience, the following shallowly discusses the PLC control system and the relay control system difference. 
There are many similarities and differences between PLC control system and relay control system. The main differences are as follows: 
(1) from the point of view of control method, the control logic of electric control system adopts hardware wiring, and uses the combination control logic such as series or parallel connection of relay mechanical contacts, which has many connections and is complex, large in volume and large in power consumption. After the system is constituted, It is difficult to change or add features. In addition, the number of relay contacts is limited, so the flexibility and scalability of the electrical control system is greatly limited. PLC uses computer technology, its control logic is stored in the memory in the form of a program, to change the control logic only needs to change the program, so it is easy to change or increase the system functions. The "soft relay" of PLC is essentially the state of the memory cell, so the number of contacts of the "soft relay" is the most unlimited, and the system has good flexibility and expansibility. 
(2) in the way of work, in the relay control circuit. When the power is switched on, all the relays in the circuit are in a restricted state. That is to say, all the relays should be sucked together at the same time. The relay that should not be sucked in is limited by certain conditions and cannot be sucked in. This mode of operation is called parallel mode of operation. And the user program of PLC is executed in a certain order, so the soft relays are all in the periodic cycle scanning connection, and the action order of each relay restricted by the same condition is decided by the scanning sequence of the program. This mode of operation is called serial mode. 
(3) from the view of control speed, the relay control system depends on the action of mechanical contacts to realize the control, and the rate of working children is low. The mechanical contact will also appear to shake the problem. But PLC realizes the control through the program instruction control semiconductor circuit, the speed is quick, the procedure instruction execution time is in the microsecond level, and does not appear the contact jitter between problems. 
(4) from the view of timing and counting control, the electric control system adopts the time-delay action of the time relay to control the time. The delay time of the time relay is susceptible to the change of the environment quiet degree and the Ju degree, and the timing precision is not high. PLC uses semiconductor integrated circuit as timer, clock pulse is generated by crystal oscillator, which has high precision and wide timing range. The user can set the timing value in the program according to the need, which is convenient to modify and is not affected by the environment, and the PLC has the function of counting, but the electric control system generally does not have the function of counting. 
(5) from the reliability and maintainability, because the electrical control system uses the mechanical contact of the big basin, its mechanical wear, electric arc burn and so on, the life is short, the system connection is many, so the reliability and maintainability are poor. PIZ has a long life and high reliability. It also has the function of self-diagnosis. It can detect the fault of itself, show to the operator at any time, and monitor the execution of the control program dynamically. It provides convenience for field debugging and maintenance.

PLC控制与继电器控制的区别

根据我们多个项目以及长时间的学习钻研总结经验,下面浅谈下PLC控制系统与继电器控制系统的区别。
PLC
控制系统与继电器控制系统相比,有许多相似之处,也有许多不同。不同之处主要在以下几个方面:
(1)
从控制方法上看,电器控制系统控制逻辑采用硬件接线,利用继电器机械触点的串联或并联等组合控制逻辑,其连线多且复杂、体积大、功耗大,系统构成后,想再改变或增加功能较为困难。另外,继电器的触点数量有限,所以电器控制系统的灵活性和可扩展性受到很大限制。而PLC采用了计算机技术,其控制逻辑是以程序的方式存放在存储器中,要改变控制逻辑只需改变程序,因而很容易改变或增加系统功能。系统连线少、体积小、功耗小,而且PLC的“软继电器”实质上是存储器单元的状态,所以“软继电器”的触点数最是无限的,PLC系统的灵活性和扩展性好。
(2)
从工作方式上看,在继电器控制电路中.当电源接通时,电路中所有继电器都处于受制约状态,即该吸合的继电器都同时吸合.不该吸合的继电器受某种条件限制而不能吸合,这种工作方式称为并行工作方式。而PLC的用户程序是按一定顺序循环执行的,所以各软继电器都处于周期性循环扫描接通中,受同一条件制约的各个继电器的动作次序决定于程序扫描顺序,这种工作方式称为串行工作方式。
(3)
从控制速度上看,继电器控制系统依靠机械触点的动作以实现控制,工作孩率低.机械触点还会出现抖动问题。而PLC通过程序指令控制半导体电路来实现控制,速度快,程序指令执行时间在微秒级,且不会出现触点抖动间题。
(4)
从定时和计数控制上看,电器控制系统采用时间继电器的延时动作进行时间控制,时间继电器的延时时间易受环境谧度和沮度变化的形响,定时精度不高。而PLC采用半导体集成电路作定时器,时钟脉冲由晶体振荡器产生,精度高,定时范围宽。用户可根据需要在程序中设定定时值,修改方便,不受环境的形响,且PLC具有计致功能,而电器控制系统一般不具备计数功能。
(5)
从可靠性和可维护性上肴,由于电器控制系统使用了大盆的机械触点,其存在机械磨损、电弧烧伤等,寿命短,系统的连线多,所以可靠性和可维护性较差。而PIZ大量的开关动作由无触点的半导体电路完成,其寿命长、可靠性高,PLC还具有自诊断功能,能查出自身的故障,随时显示给操作人员,并能动态地监视控制程序的执行情况,为现场调试和维护提供了方便。