01
Overall Analysis
1. System Analysis
Based on the control tasks required by the control system, a detailed analysis is conducted on the technological process and working characteristics of the controlled object, as well as the control process, control laws, functions, and characteristics of the control system. It is determined whether the input and output physical quantities are switching quantities or analog quantities, the various stages of control and their characteristics are clearly defined, the transition conditions between stages are specified, and a complete workflow diagram and a schedule of actions for each actuator are drawn.
2. Examine the main circuit.
To gain a deeper understanding of the technological process and its corresponding actuators and components.
3. Examine the I/O configuration of the PLC control system and the I/O wiring of the PLC.
Understand the input signals and the configuration of the corresponding input relays, the configuration of the output relays, and the corresponding loads they are connected to.
If no input/output device definitions and PLC I/O configurations are provided, the input/output device definitions and PLC I/O configurations should be made based on the PLC I/O wiring diagram or ladder diagram and instruction statement table.
4. Understand the ladder diagram by referring to the PLC's I/O wiring diagram.
The I/O wiring of a PLC is the link between the main circuit and the PLC ladder diagram.
(1) Based on the text symbols of the main contacts of the main circuit control electrical appliances (contactors, relays) of the electrical appliances (such as motors, solenoid valves, electric heaters, etc.), find the coil of the corresponding programming element in the I/O wiring diagram of the PLC, and then find the output relay that controls the control electrical appliance. Then find the program segment of the output relay in the ladder diagram or statement list, and mark and explain it.
(2) Based on the input devices and their corresponding input relays in the PLC I/O wiring diagram, find the normally open and normally closed contacts of the input relays in the ladder diagram (or statement list) and make corresponding markings and explanations.
02
Structural Analysis of Trapezoid Diagrams
1. Characteristics of ladder diagrams in PLC control systems
(1) Input signals and output loads of the PLC control system
In relay circuit diagrams, actuators such as AC contactors and solenoid valves are controlled by the output relays of the PLC, with their coils connected to the PLC's output terminals. Buttons, control switches, limit switches, proximity switches, etc., are used to provide control commands and feedback signals to the PLC, with their contacts connected to the PLC's input terminals.
(2) Handling of intermediate relays and time relays in relay circuit diagrams
The functions of intermediate relays and time relays in relay circuit diagrams are accomplished by auxiliary relays and timers inside the PLC, and they are independent of the PLC's input relays and output relays.
(3) Set up intermediate units
In a ladder diagram, if multiple coils are controlled by a series/parallel circuit of a certain contact, an auxiliary relay controlled by that circuit can be set in the ladder diagram to simplify the circuit. The auxiliary relay is similar to an intermediate relay in a relay circuit.
(4) Handling of instantaneous contacts of time relays
In addition to time-delayed contacts, time relays also have instantaneous contacts that activate immediately when the coil is energized or de-energized. For time relays with instantaneous contacts, an auxiliary relay can be connected in parallel across the coil of the corresponding timer in the ladder diagram; the contacts of the auxiliary relay are equivalent to the instantaneous contacts of the time relay.
(5) Establishment of external interlocking circuit
To prevent the two contactors controlling forward/reverse rotation from operating simultaneously and causing a short circuit in the three-phase power supply, in addition to setting up a soft interlock circuit in the ladder diagram consisting of normally closed contacts connected in series with the coils of their corresponding output relays, a hard interlock circuit should also be set up outside the PLC.
2. Structural Analysis of Trapezoid Diagrams
Whether to use general programming methods or sequential function chart programming methods; whether to use single-sequence structures, selection sequence structures, or parallel sequence structures of sequential function charts; whether to use start/hold/stop circuits, step sequence control instructions, or set/reset instructions for programming.
The decomposition of a ladder diagram starts from the operating master circuit (such as a button), and traces the wires to the main circuit control electrical appliance (such as a contactor). It involves many programming components and circuits, making it relatively difficult to find the correct path.
No matter how complex a ladder diagram is, it is composed of basic units. Based on the structure of the main circuit, the ladder diagram and instruction statement list are decomposed into several basic units corresponding to the electrical appliances (such as motors) of the main circuit using the reverse reading and tracing method. Then, each link is analyzed one by one, and finally, the links are connected in sequence using the forward reading and tracing method.
(1) Configuration and function of pushbuttons, limit switches and changeover switches
PLC I/O wiring diagrams contain numerous limit switches, changeover switches, pressure relays, and temperature relays. These electrical components lack coils; their contact operation relies on external forces or other factors. Therefore, it's crucial to first identify the external forces or factors causing these contact actions. Limit switches are activated or deactivated by mechanical linkages, while changeover switches are typically operated manually. This results in the contacts of these limit switches and changeover switches being in different working states during equipment operation—that is, different closed and open states—to meet various control requirements. This is a key aspect of interpreting PLC diagrams.
The different operating states of the contacts of these limit switches and changeover switches are difficult to understand by simply looking at the circuit diagram. It is necessary to refer to the equipment manual and the list of electrical components to clarify the purpose of the limit switch or changeover switch, the mechanical linkage mechanism that operates the limit switch, and the operating state of the circuit when the contacts are in different closed or open states.
(2) Use the reverse reading and tracing method to decompose multi-load (such as multi-motor circuit) into single-load (such as single-motor circuit) circuits.
Based on the text symbols of the main contacts of the control electrical appliances that control the load in the main circuit, find the output relay of the contactor coil that controls the load in the PLC's I/O wiring diagram. Then, find the coil that controls the output relay and its related circuits in the ladder diagram and instruction statement table. This is the local circuit that controls the load.
In the ladder diagram and instruction statement table, it is easy to find the coil circuit of the output relay and its energization and de-energization conditions. However, it is not easy to find the energization and de-energization of the coil and its related circuits. The reverse reading and tracing method can be used to find them:
●The closing or opening of the contacts of other programming elements connected in series or parallel in the output relay coil circuit is the condition for the output relay to be energized or de-energized.
●From these contacts, locate their coil circuits and related circuits. Within these coil circuits, you will find contacts of other contactors and relays...
● Continue searching in this manner until the input relay (master control device) is found. It is important to note that when a programming element is energized and engaged or de-energized and released, the operational states of all preceding and following programming elements driven by all contacts of that element should be identified without omission.
Identify the normally open and normally closed contacts of a programming element in other circuits. These contacts provide conditions for the energization or de-energization of other programming elements, or for interlocking or interlocking, causing other electrical components to operate and driving the actuators.
(3) Further decompose the single-load circuit
The local circuitry controlling a single load may still be very complex and needs to be further decomposed until it is broken down into basic unit circuits.
(4) Precautions for disassembling the circuit
●If a speed relay is connected to the motor spindle, the motor will form a parking brake circuit according to the speed control principle.
●If a rectifier is connected in the main circuit of the motor, it indicates that the motor uses an energy-saving braking and stopping circuit.
(5) Integrate the fragments and conduct a comprehensive analysis.
Connect the basic unit circuits in series and analyze the entire circuit using the sequential reading and tracing method.
03
Specific methods for reading trapezoidal diagrams
The process of reading PLC ladder diagrams and statement lists is the same as scanning user processes with the PLC: read the diagrams segment by segment from left to right and top to bottom, in the order of program segments.
It is worth noting that during program execution, within the same cycle, the results of preceding logical operations affect subsequent operations; that is, the executed program uses the latest intermediate results from preceding operations. However, within the same cycle, the results of subsequent logical operations do not affect preceding logical relationships.
The final state of all internal relays (whether the coil is on or off, whether the contacts are on or off) during this scan cycle, except for the input relay, will affect the on/off state of each contact in the next scan cycle.
Since many readers are familiar with relay contactor control circuits, it is recommended to use the method of reading relay contactor control circuits by checking the wiring diagram and following these steps to view the ladder diagram:
(1) Based on the I/O allocation table and ladder diagram of the I/O devices and PLC, find the input and output relays and give the corresponding text codes of the relay contactor control circuit.
(2) Mark the corresponding input and output device codes next to the coil and contacts of the ladder diagram programming element.
(3) Decompose the ladder diagram into several basic units. Each basic unit can be a program segment (containing one output element) or several program segments (containing several output elements) of the ladder diagram, and each basic unit is equivalent to a branch circuit of the relay contactor control circuit.
(4) The corresponding relay contactor control circuit can be drawn for each step.
(5) When a programming element is energized, all its normally open contacts close and all its normally closed contacts open. When a programming element is de-energized, all its closed normally open contacts open (reset), and all its open normally closed contacts close (reset). Therefore, after a programming element is energized or de-energized, all its normally open and normally closed contacts must be identified, and their impact on the corresponding programming element must be analyzed.
(6) Generally, the ladder diagram can be read starting from the first line of the first program segment. The first line is the program start line. Pressing the start button connects a certain input relay, and all normally open contacts of the input relay close and all normally closed contacts open.
Next, identify the programming elements affected by the closing of the normally open contact and the opening of the normally closed contact of the input relay, and analyze what actions these programming elements will take to determine their functions. It is worth noting that some of these programming elements may operate immediately upon energization, while others may not operate immediately but are merely preparing for their energization.
As can be seen from the working principle of PLC, when the input terminal is connected to the normally open contact, if the normally open contact of the input terminal is closed when the PLC is working, the coil of the input relay corresponding to that input terminal will be energized, its normally open contact will close, and its normally closed contact will open.
When the input terminal is connected to a normally closed contact and the PLC is operating, if the normally closed contact of the input terminal does not operate, the coil of the input relay corresponding to that input terminal will be energized, its normally open contact will close, and its normally closed contact will open. If the normally closed contact is connected in series with the output relay coil, the output relay coil will not be energized.
Therefore, when using a PLC to control the start and stop of a motor, if the stop button uses a normally closed contact, the coil of the PLC output relay connected to the contactor controlling the motor should be connected in series with the normally open contact corresponding to the input terminal connected to the stop button.
In relay contact control, both the stop button and the thermal relay use normally closed contacts. To maintain consistency with the control circuit of relay contact control, normally closed contacts are also used in the PLC ladder diagram. Therefore, the stop button and thermal relay contacts connected to the input terminals must use normally open contacts. This point must be noted when reading the program.
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