According to the relay circuit diagram design of PLC ladder diagram

Use PLC and relay circuit diagram similar ladder diagram language. If modified relay control system with PLC, ladder diagram according to the relay circuit diagram design is a shortcut. This is because the original relay control system after a long time of use and test, have been shown to complete the control function of the system requirements, and relay circuit diagram and ladder diagram there are many similarities, so can relay circuit diagram 'translate' into ladder diagram, in which external PLC hardware wiring diagram and ladder diagram have many want to, the function of relay system. This kind of design method generally do not need to change the control panel, keep the external characteristic of the original system, operators need not change the habit of operation in long term. 1, the basic method of relay circuit diagram is a pure hardware circuit diagram. Instead of PLC control, it need external wiring diagram and ladder diagram with PLC to equivalent relay circuit diagram. Might think PLC as a control box, the outer wiring diagram describes the wiring outside the control box, ladder diagram is the internal control box 'circuit diagram', ladder diagram of the input and output of the control box is contact with the outside world 'interface relay', so that you can use the method of analysis of the relay circuit diagram for the analysis of PLC control system. In relation to the ladder diagram can enter a contact to imagine the corresponding external input device contact, will output a coil to imagine the corresponding external load coil. The coil of the external load, except under the control of the ladder diagram is right can be controlled by external contacts. Will be converted into function the same PLC controller relay circuit diagram of the external wiring diagram and ladder diagram procedure is 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 controller. Button, switch and travel switch, proximity switch operation, such as the provision of PLC digital input signal relay circuit diagram of the function of intermediate relay and time relay with PLC internal memory and timer to complete, it has nothing to do with PLC's input and output. 3) Choose PLC model, based on the system the required functionality and scale to choose the CPU module, power module and digital input and output module, the hardware configuration, determine the input and output modules in the rack installation position and their starting address. 4) Determine the PLC digital quantity input signal and output load corresponding to the input and output bits address, draw the PLC external wiring diagram. Each input and output address in the ladder diagram depends upon their terminals in the starting address of the module and the module number. 5) Determine the corresponding with the intermediate relay circuit diagram, time relay ladder diagram of memory, and timer, counter address. 6) Draw the ladder diagram according to the corresponding relation. 2, matters needing attention according to the design of PLC relay circuit diagram of the external wiring diagram and ladder diagram should pay attention to the following questions: 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. In the relay circuit, for example, the contact can be placed on both sides of the coil, but in ladder diagram, the coil must be put in the most the right side of the 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 the contact number, because it means 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 say goodbye to care about whether to use a few contact, because it will not increase the cost of them, just need to spend a little time when input program. 3) To minimize the PLC input and output points. The price of PLC I/O points, so points of input and output signal is the main measures to reduce the hardware cost. In the external input circuits of PLC, each input can be normally open or normally closed points, also can contact point of series-parallel circuit. PLC can't identify the external circuit structure and contact type, can only identify the external circuit on and off. 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 the ladder diagram, can be in timer on both ends of the coil and the fed is a coil, the contact is equal to the instantaneous moving contact of the timer. 5) Set the middle unit in ladder diagram, if more than one coil is 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. This element is similar to the intermediate relay relay circuit in the middle.
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