I. 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.
3. Examine the I/O configuration of the control system and the I/O wiring of the PLC.
Understand the input signals and their corresponding configurations, the output relay configurations, and their corresponding loads. If no input/output device definitions or PLC I/O configurations are provided, the input/output device definitions and PLC I/O configurations should be determined based on the PLC I/O wiring diagram or ladder diagram and instruction statement list.
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 (e.g., solenoid valves, electric heaters, etc.) of the electrical appliances, 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.
II. 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
Actuators such as relays and solenoid valves are controlled by the output relays of the PLC, and their coils are connected to the output terminals of the PLC. Buttons, control switches, limit switches, proximity switches, etc., are used to provide control commands and feedback signals to the PLC, and their contacts are connected to the input terminals of the PLC.
(2) Handling of sums 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 three-phase short circuit, 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 (SFM) programming methods; whether to use a single-sequence structure, selection sequence structure, or parallel sequence structure of the SFM; and whether to use start/hold/stop circuits, step sequence control instructions, or set/reset instructions for programming. This section is covered in Chapters 4 and 5.
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:
① 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.
②From these contacts, find their coil circuits and related circuits. In these coil circuits, there will also be contacts of other contactors and relays.
③ Continue searching in this way until you find the input relay (master control device).
It is worth noting that when a programming element is energized and engaged or de-energized and released, the operational states of all the preceding and following programming elements driven by all the contacts of that programming 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
①If a speed relay is connected to the motor spindle, the motor will form a parking brake circuit according to the speed control principle.
②If a rectifier is connected in the main circuit of the motor, it indicates that the motor uses an energy-consumption 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.
III. Specific Methods for Reading Trapezoid 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 contacts; that is, the executed program uses the latest intermediate operation results from the preceding operations. However, within the same cycle, the results of subsequent logical operations do not affect the logical relationships from the preceding operations. The final state (whether the coil is on or off, whether the contacts are open or closed) of all internal relays (except for the input relays) within this scan cycle 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, identify the input and output relays and provide the corresponding text codes for the relay contactor control circuits.
2. Label the corresponding input and output device codes next to the coils 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, it is necessary to identify all its normally open and normally closed contacts and analyze their impact on the corresponding programming element.
6. Generally, ladder diagrams 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 activates a certain input relay, closing all normally open contacts and opening all normally closed contacts of that input relay.
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 a 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 is energized, its normally open contact is closed, and its normally closed contact is open; when the input terminal is connected to a normally closed contact and the PLC is working, if the normally closed contact of the input terminal is not activated, the coil of the input relay corresponding to that input terminal is energized, its normally open contact is closed, and its normally closed contact is 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 PLC output relay coil 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 contact 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.
IV. Examples of Reading PLC Ladder Diagrams
When analyzing the functions of a PLC control system, it can be imagined as a control box within a relay control system. The external wiring diagram describes the external wiring of this control box, while the ladder diagram or statement list is the internal "wiring diagram." The input and output relays in the ladder diagram are the "interface relays" that connect the control box to the outside world. Therefore, the methods used to analyze relay circuit diagrams can be applied to analyze the PLC control system. During analysis, the contacts of the input relays in the ladder diagram can be imagined as the contacts or circuits of the corresponding external input devices, and the coils of the output relays can be imagined as the coils of the corresponding external loads. The coils of the external loads may be controlled not only by the ladder diagram but also by external contacts.
