Introduction: In the application of programmable logic controllers ( PLCs or PCs), we often encounter the problem of modifying relay control systems. In such cases, we often need to refer to the existing relay control circuit to develop the PC application program. Therefore, when programming, we should pay attention to some differences in the working methods of PC control systems and relay control systems.
Let's look at an example: a relay control circuit is shown in Figure 1.
Because the relay control system operates in a "parallel" manner, and the opening and closing of its contacts requires a certain action time, when the circuit starts, whether the time relay KT can continue to be energized after its delay time expires depends on two parallel action processes: KT's normally closed delay contact opens, KA1 is de-energized, and KA1's normally open contact opens; KT's normally open delay contact closes, KA2 is energized, and KA2's normally open contact closes. The result of these two processes determines the state of KT. Simultaneously, the existence of contact action time causes timing competition in the circuit. Therefore, this circuit cannot operate reliably. If the loop in the dashed box is added, and the normally open contact of KA2 is replaced with the normally open contact of KA3 as shown in Figure 1 (see brackets in Figure 1), the result is that after KT operates, KT itself is de-energized and will not continue to be energized.
We will implement the same circuit using a PC, and the ladder diagram is shown in Figure 2.
The PC operates in a "serial" manner, meaning it scans, executing tasks cyclically, continuously, and sequentially. Simultaneously, the actions of soft contacts in the PC can be considered instantaneous, and they can retain the result of each action until the next scan. This means it has a memory retention function. Following this sequential operation, the ladder diagram's action sequence is as follows: When TIM00 is delayed in a certain scan cycle, then:
1. TIM00 normally closed contact open (OFF), 0000OFF;
2. TIM00 normally open contact closed (ON), 0001 ON;
With the normally open contact 3.0000 OFF and the normally open contact 0001 ON, TIM00 remains energized. Furthermore, regardless of how many additional relay circuits (as shown in the dashed box) are added between 0001 and TIM00, and whether the normally open contact 0001 is replaced with the normally open contact of the last relay in the added circuit, TIM00 will still remain energized.
The same circuit can produce different results depending on the operating principle of a relay control system and a PC control system. Being aware of this can help us avoid unnecessary errors during programming.
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