Design the PLC ladder diagram based on the circuit diagram of the relay

PLC uses a ladder chart language that is extremely similar to the circuit diagram of the relay. If PLC is used to transform the relay control system, it is a shortcut to design a ladder chart based on the relay circuit diagram. This is because the original relay control system has been used and tested for a long time and has been proved to be able to complete the control functions required by the system. The relay circuit diagram has many similarities with the trapezoid diagram, therefore, the circuit diagram of the relay can be translated into a trapezoid diagram, that is, the external hardware wiring diagram of the PLC and the trapezoid diagram have many points to look like, the function of the relay system. This design method generally does not need to change the control panel and maintains the original external features of the system. The operator does not need to change the long-term operational habits.

1. Basic Methods

The relay circuit diagram is a pure hardware circuit diagram. When it is changed to PLC control, the external wiring diagram and ladder diagram of plc need to be used to equivalent the circuit diagram of the relay. The PLC can be imagined as a control box. Its external wiring diagram describes the external wiring of the control box. The trapezoid diagram shows the internal "circuit diagram" of the control box ", the input and output positions in the trapezoid diagram are the "interface Relays" associated with the external world of the control box. In this way, the PLC control system can be analyzed by analyzing the circuit diagram of the relays. When analyzing the trapezoid diagram, you can imagine the contact of the input bit as the contact of the corresponding external input device, and the coil of the output bit as the corresponding external load. The coil of the external load can be controlled by external contacts in addition to the ladder chart.

The steps for converting the circuit diagram of the relay into the external wiring diagram and ladder diagram of the PLC with the same function are as follows:

1) understand and be familiar with the working principle, technological process and mechanical action of the controlled equipment, and analyze and master the working principle of the control system according to the circuit diagram of the relay.

2) determine the PLC input signal and output load. If the AC contactor and solenoid valve in the relay circuit diagram are controlled by the PLC output position, their coils are at the PLC output end. Buttons, Operation switches, stroke switches, proximity switches, etc. provide PLC digital input signal relays in the circuit diagram of the Intermediate Relays and time relays functions using the PLC internal memory bit and timer to complete, they are irrelevant to the input and output bits of the PLC.

3) Select the PLC model, and select the CPU module, power module, and digital input and output module based on the functions and scale required by the system to configure the hardware, determine the installation location of the input and output modules in the rack and Their starting addresses.

4) determine the address of the input bit and output bit corresponding to the input signal of each digit of the PLC and the output load, and draw the external wiring diagram of the PLC. The addresses of each input and output in the trapezoid diagram depend on the starting address of their module and the terminal number in the module.

5) determine the address of the memory, timer, and counter in the trapezoid diagram corresponding to the intermediate and time relays in the circuit diagram of the relay.

6) Draw a ladder chart based on the above relationship.

1. Notes

Pay attention to the following issues when designing the external wiring diagram and trapezoid diagram of PLC according to the circuit diagram of the relay:

1) The grammar regulations in the ladder chart language should be observed. Due to different working principles, the trapezoid diagram cannot copy some processing methods in the relay circuit. For example, in the relay circuit, the contact can be placed on both sides of the coil, but in the trapezoid diagram, the coil must be placed on the rightmost side of the circuit.

2) properly separate some circuits in the circuit diagram of the relay. When designing the circuit diagram of a relay, the basic principle is to minimize the number of contacts used in the diagram, because this means that the cost is saved, but this will often make the control circuits of some coils intertwined. When designing a trapezoid diagram, the first problem is that the design idea should be clear. The design of the trapezoid diagram is easy to read and understand, rather than saying goodbye to whether to use more contacts, this does not increase the hard work cost, but it takes a little longer to input the program.

3) Minimize the input and output points of the PLC.

The price of PLC is related to the number of I/O points. Therefore, the number of input and output signals is the main measure to reduce the hardware cost.

In the external input circuit of PLC, each input end can be connected to a series parallel circuit consisting of a Common open or closed point or a contact point. PLC cannot identify the structure and contact type of the external circuit, but can only identify the breaking of the external circuit.

4) Handling of time Relays

In addition to contacts with delayed operations, the time relay also has transient contacts that are instantly connected when the coil is powered on. In the trapezoid diagram, the coil at both ends of the timer coil is connected to the Fed bit. Its contact is equivalent to the instantaneous contact of the timer.

5) set the intermediate Unit

In the trapezoid diagram, if multiple coils are controlled by a series parallel circuit of a contact. In order to simplify the circuit, an intermediate unit can be set in the trapezoid diagram, that is, the circuit is used to control a storage space, and its common open contact is used in the control circuit of each coil. This intermediate element is similar to an intermediate relay in a relay circuit.

6) set up an external lock circuit

If the AC contactor being switched by the asynchronous motor is simultaneously operated, it will lead to a short circuit of the three-phase power supply. In order to prevent such an accident, the hardware lock circuit should be set outside the PLC.

7) rated voltage of External Load

The PLC bidirectional Thyristor output module can only drive the load of the rated voltage ac220v. If the coil voltage of the system's original AC contactor is 380 V, it should be changed to a V coil or an external intermediate relay.