In some problems need to be aware of when programming with PLC - programmable logic

Programmable controller ( PLC) Applications, we often encounter the problem of the relay control system upgrading, we tend to refer to the original relay control circuit to the application of PLC. In programming, therefore, we should pay attention to the PLC control system and relay control system of some different on the way to work.

by relay control system is a 'parallel' way to work, and its contact on-off needs certain action time. So when the starting circuit, time relay KT delay time, KT whether can continue to keep the current state, 'parallel' two actions need to be considered at the same time process: KT normally closed delay off contact, KA1, losing electricity KA1 normally open contacts disconnect; KT normally open delay contact closure, KA2 electricity, KA2 normally open contact closure. The results of the two process function, to determine the status of KT. At the same time, the existence of the action time of the contact, makes the circuit in time sequence. So the circuit can not reliable work.

the same circuit, we use PLC controller to realize. PLC is a 'serial' way to work, that is, in the form of scanning cycle, continuous, sequentially, to perform a detailed program works. In the PLC at the same time, the action of soft contact can be considered to be instantaneous, and it can keep the memory of this action until the next time scanning operation. With memory function.

when in a scan cycle T38 delay to the back, is: 1 Internet T38 normally closed off contact ( 关闭) Q0处。 0 ( No output) ; Network 2 T38 normally open contact closure ( 上） Q0。 1对; Network Q0 in 3. 0 normally open contacts OFF, Q0. 1 normally open contacts ON T38 continue to maintain current state. And whether we at Q0. Between 1 and T38 add how much level of relay circuit and a virtual box in front of the circuit, and put Q0. 1 normally open contacts with the final relay and circuit by the normally open contact, T38 still can continue to keep current status.

in this circuit, we use the start button SB1, SB2 normally open button, while using the stop button SB3 normal close button. So it is design of PLC program inside.

but we design for PLC hardware input, the stop button SB3 cannot like relay control circuit using the normally closed contact but use, is normally open contacts. Why is this? This is because the relay control circuit in the push button switch control contactor coil directly, and in the PLC control program, button switch control is with the corresponding input relay ( I） , and by the input relay to control the contact, can't see the hardware button switch program in soft contact, is quite a turn. Application in the soft contact by the corresponding input relay control, and enter the state of the relay by external components of the input signal ( Push button switch, for example) The control.

suppose this example of the stop button SB3 like relay control circuit, we use the normally closed contact of words, then a PLC electricity, the relay I0 SB3 corresponding to the input. 2 immediately to ON, then the corresponding soft shock I0. 2 will change state normally closed contacts disconnect immediately, if you press the start button SB1, relay output Q0. 0 or Q0. 1 there will be no output, that is, has been to OFF.

pass comprehensive two examples we know that the same circuit, because of the relay control system and PLC control system works on the differences, the two have different results. Take note, when we in programming, will avoid the should not mistake. At the same time learn to PLC such some characteristics, will create more powerful and better application.

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