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Introduction to Programmable Logic Controllers (PLC) | Parts and Operation Of PLC

Introduction to PLC:

可编程逻辑控制器，PLC或可编程控制器是用于典型工业机电过程的自动化的数字计算机，例如工厂装配线，娱乐骑行或灯具的机器控制。PLC在许多工业中使用了许多机器。PLC设计用于数码和模拟输入和输出的多种布置，延长温度范围，对电噪声的免疫力，以及振动和撞击的抵抗力。控制机器操作的程序通常存储在电池备用或非易失性存储器中。

•PLC（即可编程逻辑控制器）是一种用于更换机器控制的必要顺序继电器电路的设备。

• The PLC works by looking at its inputs and depending upon their state, turning on/off its outputs. The user enters a program, usually via software, that gives the desired results.

Example of PLC :

• Let’s assume that when a switch turns on we want to turn a solenoid on for 5 seconds and then turn it off regardless of how long the switch is on for.

•我们可以通过简单的外部计时器执行此操作。但如果该过程包括10个开关和螺线管，该怎么办？我们需要10个外部计时器。如果过程还需要计算单独打开的交换机的次数是多少？我们需要很多外部计数器。

有什么需要的plc：

• The bigger the process the more is need for a PLC.

•只需将PLC编程以计算其输入并在指定的时间内转动螺线管。

• The primary reason for designing PLC was eliminating the large cost involved in replacing the complicated relay based machine control systems.

Major Components Of PLC Circuits:

The PLC mainly consists of :

– A CPU,

- 记忆区域，和

– Appropriate circuits to receive input/output data.

• We can actually consider the PLC to be a box full of hundreds or thousands of separate relays, counters, timers and data storage locations.

•输入继电器 - （联系人）：These are connected to the outside world. They physically exist and receive signals from switches, sensors, etc. Typically they are not relays but rather they are transistors.

•内部实用继电器 - （联系人）：These do not receive signals from the outside world nor do they physically exist. They are simulated relays and are what enables a PLC to eliminate external relays. There are also some special relays that are dedicated to performing only one task. Some are always on while some are always off. Some are on only once during power-on and are typically used for initializing data that was stored.

• COUNTERS :These again do not physically exist. They are simulated counters and they can be programmed to count pulses. Typically these counters can count up, down or both up and down.

•定时器：These also do not physically exist. They come in many varieties and increments. The most common type is an on-delay type. Others include off-delay and both retentive and non-retentive types. Increments vary from 1ms through 1s.

•输出继电器 - （线圈）：These are connected to the outside world. They physically exist and send on/off signals to solenoids, lights, etc. They can be transistors, relays, or triacs depending upon the model chosen.

•数据存储：通常，分配给简单地存储数据的寄存器。它们通常用作数学或数据操纵的临时存储。当从PLC移除电源时，它们通常也可以用于存储数据。在上电时，它们仍将在删除电源之前具有相同的内容。

PLC操作：

•PLC通过不断扫描程序。我们可以将此扫描周期视为3个重要步骤。

• Step 1-CHECK INPUT STATUS :

First the PLC takes a look at each input to determine if it is on or off. In other words, is the sensor connected to the first input on? How about the second input? How about the third… It records this data into its memory to be used during the next step.

• Step 2-EXECUTE PROGRAM :

接下来，PLC一次执行一个指令。也许程序表示，如果第一个输入亮起，则应打开第一个输出。由于它已经知道从上一步开始/关闭哪个输入，因此它将能够根据第一输入的状态来确定是否应打开第一个输出。它将在下一步稍后存储执行结果。

•步骤3 - 更新输出状态：

Finally the PLC updates the status of the outputs. It updates the outputs based on which inputs were on during the first step and the results of executing your program during the second step. Based on the example in step 2 it would now turn on the first output because the first input was on and your program said to turn on the first output when this condition is true. After the third step the PLC goes back to step one and repeats the steps continuously. One scan time is defined as the time it takes to execute the 3 steps listed above.

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