1、 Overall analysis

1 system analysis

According to the control tasks that the control system needs to complete, the process and working characteristics of the controlled object, as well as the control process, control law, function and characteristics of the control system are analyzed in detail. It is clear that the input and output physical quantities are switching or analog quantities, and the control stages and their characteristics, and the conversion conditions between stages are clearly divided, Draw the complete work flow chart and the action rhythm table of each executive component.

2. Look at the main circuit

Further understand the process flow and its corresponding actuator and components.

Understand the configuration of input signal and corresponding input relay, output relay and corresponding load.

Without the definition of input / output device and the I / O configuration of PLC, the definition of input / output device and the I / O configuration of PLC should be made according to the I / O wiring diagram or ladder diagram and instruction statement table of PLC. 

4. Understand ladder diagram through I / O wiring diagram of PLC

The I / O connection of PLC is the link between the main circuit and PLC ladder diagram.

1) According to the text symbols of the main contacts of electrical appliances (Contactors and relays) controlled by the main circuit of electrical appliances (such as motor, solenoid valve, electric heater, etc.), the coil of the corresponding programming element can be found in the I / O wiring diagram of PLC, then the output relay controlling the control electrical appliances can be known, and the program segment of the output relay can be found in the ladder diagram or statement table, Mark and explain JP〗

2) According to the input device of PLC I / O wiring diagram and its corresponding input relay, find out the dynamic make contact and dynamic break contact of input relay in ladder diagram (or statement table), and make corresponding marks and instructions. 

2、 Structure analysis of ladder diagram

1. Characteristics of ladder diagram of PLC control system

(1) Input signal and output load of PLC control system

The AC contactor and solenoid valve in the relay circuit diagram are controlled by the output relay of PLC, and their coils are connected to the output end of PLC. Buttons, control switches, limit switches and proximity switches are used to provide control commands and feedback signals to PLC, and their contacts are connected to the input end of PLC.

(2) Processing of intermediate relay and time relay in relay circuit diagram

The functions of the intermediate relay and time relay in the relay circuit diagram are completed by the auxiliary relay and timer in PLC, which have nothing to do with the input relay and output relay of PLC.

(3) Setting up the intermediate unit

In the ladder diagram, if multiple coils are controlled by a contact series / parallel circuit, in order to simplify the circuit, an auxiliary relay controlled by the circuit can be set in the ladder diagram, which is similar to the intermediate relay in the relay circuit.

(4) Treatment of instantaneous contact of time relay

In addition to the contact of delay action, the time relay also has the instantaneous contact of immediate action when the coil is powered on or off. For the time relay with instantaneous contact, the auxiliary relay can be paralleled at both ends of the coil of the corresponding timer in the ladder diagram, and the contact of the latter is equivalent to the instantaneous contact of the time relay.

(5) Establishment of external interlocking circuit

In order to prevent the simultaneous action of the two contactors controlling the forward / reverse direction and causing the short circuit of the three-phase power supply, in addition to the soft interlock circuit composed of the dynamic break contacts in series with the coils of their corresponding output relays in the ladder diagram, the hard interlock circuit should also be set outside the PLC.

2. Structure analysis of ladder diagram

General programming method or sequential function diagram programming method is adopted; Single sequence structure of sequential function diagram or selection sequence structure, parallel sequence structure, start / hold / stop circuit, step sequence control instruction or set / reset instruction. See Chapter 4 and Chapter 5 for this part.

The decomposition of ladder diagram starts from the operation of the main command circuit (such as button), and it is difficult to find the line from the main circuit to the control of electrical appliances (such as contactor).

No matter how complex the ladder diagram is, it is composed of some basic units. According to the composition of the main circuit, the ladder diagram and instruction statement table are decomposed into several basic units corresponding to the electrical appliances (such as motor) of the main circuit by using the reverse reading traceability method, and then analyzed one by one, and finally the links are connected by using the forward reading tracking method.

(1) Configuration and function of button, travel switch and transfer switch

In the I / O wiring diagram of PLC, there are many travel switches and transfer switches, as well as pressure relays and temperature relays. These electrical components have no attraction coil, and their contact action is realized by external force or other factors. Therefore, the external force or factors causing these contact action must be found first. Among them, the travel switch is pressed or released by the mechanical linkage mechanism, while the transfer switch is generally operated by hand, so that the contacts of these travel switches and transfer switches are in different working states during the operation of the equipment, that is, the closing and breaking conditions of the contacts are different, so as to meet different control requirements. This is a key in the process of looking at the drawings.

It is difficult to understand the different working states of the contacts of these travel switches and transfer switches only by looking at the circuit diagram. It is necessary to clarify the use of the travel switches and transfer switches, the mechanical linkage mechanism for operating the travel switches, and the working states of the circuits under different closed or open States of the contacts in combination with the equipment manual and the list of electrical components.

(2) The multi load (e.g. multi motor circuit) is decomposed into single load (e.g. single motor circuit) by reverse reading traceability method

According to the text symbol of the main contact of the control appliance controlling the load in the main circuit, find out the output relay of the contactor coil controlling the load in the PLC I / O wiring diagram, and then find out the coil controlling the output relay and its related circuit in the ladder diagram and instruction statement table, which is the local circuit controlling the load.

In the ladder diagram and instruction statement table, it is easy to find the coil circuit of the output relay and its power on and power off conditions, but it is not easy to find the power on and power off of the coil and its related circuits, so the reverse reading traceability method can be used to find the following conditions:

1) In the coil circuit of the output relay, the closing and breaking of the contacts of other programming elements in series and parallel are the conditions for the power on and power off of the output relay.

2) From these contacts, we can find out their coil circuits and related circuits. In these coil circuits, there will be contacts of other contactors and relays

3) Keep looking until you find the input relay.

It is worth noting that: when a programming element is powered on or released after power loss, the function status of the front and rear programming elements driven by all the contacts of the programming element should be found out without omission.

Find out the dynamic make contact and dynamic break contact of a programming element in other circuits. These contacts provide conditions for power on and power off of other programming elements, or provide conditions for interlock and interlock, which cause other electrical components to act and drive the actuator.

(3) The single load circuit is further decomposed

The local circuit controlling a single load may still be very complex and needs further decomposition until it is decomposed into basic unit circuits.

(4) Matters needing attention in decomposing circuit

1) If the main shaft of the motor is connected with a speed relay, the motor forms a parking brake circuit according to the speed control principle.

2) If a rectifier is connected in the main circuit of the motor, it indicates that the motor adopts energy consumption braking and stopping circuit.

(5) Set zero as whole, comprehensive analysis

The basic unit circuits are connected in series, and the whole circuit is analyzed by the read in sequence tracking method.

3、 How to read ladder diagram

The process of reading PLC ladder diagram and statement table is the same as the process of PLC scanning users, from left to right, from top to bottom, according to the sequence of program segments.

It is worth pointing out that: in the process of program execution, in the same cycle, the previous logical operation results affect the subsequent contacts, that is, the executed program uses the latest intermediate operation results. But in the same week, the results of the following logical operations do not affect the previous logical relations. The final state of all internal relays (coil on or off, contact on or off) except input relay in the scan cycle will affect the on or off of each contact in the next scan cycle.

Because many readers are familiar with the control circuit of relay contactor, it is suggested to read the control circuit of relay contactor by checking the line and reading the diagram

1) According to the I / O distribution table and ladder diagram of I / O equipment and PLC, find out the input and output relays, and give the text code corresponding to the control circuit of relay contactor.

2) Mark the text code of corresponding input device and output device beside the ladder programming element coil and its contact.

3) The ladder diagram is divided into several basic units. Each basic unit can be a program segment (including an output component) or several program segments (including several output components) of the ladder diagram, and each basic unit is equivalent to the control of relay contactor

A branch of a circuit.

4) The corresponding control circuit of relay contactor can be drawn for each step.

5) When a programming element is powered on, all the dynamic make contacts are closed and the dynamic break contacts are open. When a programming element loses power, all the closed contacts are open (reset), and all the open contacts are closed (reset). Therefore, after the power on and power off of the programming element, it is necessary to find out all the dynamic make contact and dynamic break contact, and analyze their influence on the corresponding programming element.

6) Generally speaking, the ladder can be read from the first natural line of the first program segment. First, the natural behavior program starts the line. Press the start button to turn on an input relay. All the dynamic make contacts of the input relay are closed and the dynamic break contacts are open.

Then find out the programming elements which are affected by the closing of the dynamic make contact and the breaking of the dynamic break contact of the input relay, and analyze the action of these programming elements, so as to determine the function of these programming elements. It is worth noting that some of these programming elements may be powered on immediately, and some may not be powered on immediately, but only prepare for the power on action.

According to the working principle of PLC, when the input terminal is connected with the dynamic contact, if the dynamic contact of the input terminal is closed, the input relay coil corresponding to the input terminal is powered on, and its dynamic contact is closed and the dynamic contact is disconnected; When the input end is connected with the dynamic breaking contact and the PLC is working, if the dynamic breaking contact of the input end does not act, the input relay coil corresponding to the input end is powered on, and its dynamic making contact is closed and the dynamic breaking contact is open. If the dynamic breaking contact is in series with the output relay coil, the output relay coil cannot be powered. Therefore, if the start and stop of the motor is controlled by PLC, if the stop button uses the dynamic break contact, the PLC output relay coil connected with the contactor of the control motor should be connected in series with the dynamic make contact corresponding to the input terminal of the stop button. In the relay contact control, the stop button and the thermal relay both use the dynamic break contact. In order to be consistent with the control circuit of the relay contact control, the dynamic break contact is also used in the PLC ladder diagram. In this way, the stop button and the thermal relay contact connected with the input end must use the dynamic close contact. This must be noted when reading programs.

4、 Example of reading PLC ladder diagram

When analyzing the function of PLC control system, we can think of it as a control box in a relay control system. Its external wiring diagram describes the external wiring of the control box. The ladder diagram or statement table is the internal “circuit diagram” of the control box. The input relay and output relay in the ladder diagram are the “interface relay” connecting the control box with the external world, In this way, we can analyze the PLC control system by analyzing the relay circuit diagram. In the analysis, the contact of the input relay in the ladder diagram can be imagined as the contact or circuit of the corresponding external input device, and the coil of the output relay can be imagined as the coil of the corresponding external load. The coil of external load is controlled not only by ladder diagram, but also by external contact.

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Editor in charge: LQ

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