Relay control circuit how to convert PLC ladder diagram method and steps in detail

transform the relay control system using PLC, because the original relay control system after long-term use and test, have been shown to complete the control function of the system requirements, and the relay circuit diagram and ladder diagram on the representation and analysis method have many similarities, so you can according to the relay circuit diagram design of ladder diagram, converting the relay circuit diagrams have the same function of PLC external hardware wiring diagram and ladder diagram. This design method is generally do not need to change the control panel, keep the original features of the system, and the operators don't have to change the habit of operation in long term, and thus become a practical and convenient design method.

1。 Conversion methods and steps of

relay circuit diagram is a pure hardware circuit diagram, it changed to PLC controller control, need to use PLC controller external wiring diagram and ladder diagram to equivalent relay circuit diagram, the specific methods and steps are as follows:

( 1) Understand and familiar with charged equipment working principle, technological process and the mechanical action, according to the relay circuit diagram analysis and master the working principle of control system.

( 2) To determine the PLC input signal and output load. Relay circuit diagram of ac contactor and electromagnetic valve actuator with the output of the PLC to control, the coil in the output of the PLC. Button, switch and travel switch, proximity switch operation, such as the provision of PLC relay of the digital quantity input signals. The function of the circuit diagram of the intermediate relay and time relay with PLC internal memory and timer to complete, they with PLC's input, output, has nothing to do.

( 3) Determine the corresponding with the intermediate relay circuit diagram, time relay ladder diagram of a memory, and timer, counter address, input and output components and ladder diagram components corresponding relation.

( 4) Draw the ladder diagram according to the corresponding relation.

2。 Ladder diagram design considerations for

according to the design of PLC relay circuit diagram of the following problems should be paid attention to when the external wiring diagram and ladder diagram:

( 1) Shall comply with the ladder diagram language rules of grammar. Due to different working principle, ladder diagram cannot copy some processing methods of relay circuit.

( 2) The appropriate separation of some circuit of relay circuit diagram. A basic principle of design of relay circuit diagram is to minimize the chart used in contact this means that the cost savings, but it tends to make some coil control circuit are intertwined. When designing ladder diagram design train of thought to the first question is clear, the design of ladder diagram easier to read and understand, is not particularly care about whether to use a few contact, because it does not increase the cost of hardware, only to lose the program need to spend a little time.

( 3) To minimize the PLC and dots. PLC prices associated with points, so the input and output signals of the points is the main measures to reduce the hardware cost.

( 4) Time relay processing time relay in addition to the time delay of contact, and the coil electricity snap on at the moment of contact. In ladder diagram, the timer can be in memory a coil in parallel on both ends of the coil, the contact is equal to the instantaneous moving contact of the timer.

( 5) Set the middle unit, in the ladder diagram, if multiple coils are controlled by a contact series-parallel circuit. In order to simplify the circuit, in the middle of the ladder diagram can be set in the unit, which USES the circuit to control a storage place, in the coil used in the control circuit of the normally open contacts.

( 6) Set up external interlock circuit, due to the action time of the software, even in the ladder diagram has been completed interlock, to ensure that no action at the same time, also set up outside the PLC hardware interlock circuit.

( 7) External load rated voltage, PLC output bidirectional thyristor module normally only load rated voltage is ac220v, ac contactor of the system should be replaced by 220 v voltage of the coil.

3。 Motors can be started star Angle conversion cases

( 一) Star delta starter relay control principle diagram b) Star delta starter PLC ladder diagram

figure 1 relay control circuit transformation ladder diagram example

you can see by the picture, the ladder diagram can be appropriate setting of the intermediate links, Auxiliary relay m1) To make the program more concise; Components use there is no limit to the number, the on-off state does not constitute a hardware costs, so in the design of ladder diagram t0 timer kept electricity state; For km1 ( y1） With km2 ( y2） In addition to ladder diagram interlock besides hardware interlock, to make sure it is not at the same time conduction.

the paper discusses the necessity from relay control circuit of PLC ladder diagram, transformation methods and steps, the problems need attention in the design of the transformation and so on related content, at the same time in an asynchronous electric star triangle starting control specific conversion process. Anyway we design out of date should pay attention to the ladder diagram is PLC program, is a kind of software, and the relay circuit is composed of hardware circuit, ladder diagram and the relay circuit is has essential difference. Transformation should not only find the two things in common, and to see the working mechanism of different, only in this way can accurately completed by relay control circuit to the transformation of PLC ladder diagram.

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