AB 罗克韦尔 可编程控制器 1756-PC75 PLC模块 通讯模块

AB 罗克韦尔 可编程控制器 1756-PC75 PLC模块 通讯模块

AB 罗克韦尔 可编程控制器 1756-PC75 PLC模块 通讯模块

(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井联等组合成控制逻辑，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系统的灵活性和可扩展性好。

（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看

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。

AB 罗克韦尔 可编程控制器 1756-PC75 PLC模块 通讯模块

(1) From the control method

The control logic of the electrical control system uses hardware wiring, and uses the series or well connection of the relay mechanical contacts to form a control logic, which has many and complex connections, large volume and large power consumption. After the system is formed, it is difficult to change or increase the function. In addition, the number of relay contacts is limited, so the flexibility and scalability of the electrical control system is greatly limited. The PLC uses computer technology, its control logic is stored in the memory in the way of a program, to change the control logic only need to change the program, so it is easy to change or increase the system function. The system connection is less, the size is small, the power consumption is small, and the PLC "soft relay" is essentially the state of the memory unit, so the "soft relay" contacts are the most unlimited, and the PLC system has good flexibility and scalability.

(2) From the way of work

In the relay control circuit. When the power supply is switched on, all relays in the circuit are in a restricted state, that is, the relays that are drawn are drawn at the same time. The relay that should not be drawn is limited by some conditions and cannot be drawn, and this working mode is called parallel working mode. The PLC user program is cyclic execution in a certain order, so the soft relays are in a periodic cycle of scanning on, subject to the same conditions of each relay action order determined by the program scanning order, this mode of work is called serial mode of work.

(3) From the control speed

The relay control system relies on the action of mechanical contacts to achieve control, and the working rate is low. Mechanical contacts will also have a shaking problem. The PLC controls the semiconductor circuit through the program instruction to achieve control, the speed is fast, the program instruction execution time is in the microsecond level, and there is no contact jitter problem.

(4) From the perspective of timing and counting control

The electric control system uses the time delay action of the time relay to control the time. The time delay time of the time relay is easy to be affected by the shape and sound of the change of the environmental silence and the intensity, and the timing accuracy is not high. The PLC uses a semiconductor integrated circuit as a timer, and the clock pulse is generated by a crystal oscillator, which has high precision and wide timing range. The user can set the time value according to the need in the program, easy to modify, not subject to the shape of the environment, and the PLC has the function of counting, and the electrical control system generally does not have the function of counting.

(5) From the reliability and maintainability

Because the electrical control system uses a large pot of mechanical contacts, it has mechanical wear, arc burn, etc., short life, and many connections of the system, so the reliability and maintainability is poor. A large number of switching actions of PIZ are completed by contact-free semiconductor circuits, which has a long life and high reliability. PLC also has a self-diagnostic function, which can detect its own faults and display them to the operator at any time, and can dynamically monitor the execution of the control program, which provides convenience for on-site debugging and maintenance.