Features of PLC Control System Ladder Diagrams
01
Input Signals and Output Loads of PLC Control Systems
The output relays of the PLC control the actuators in the relay circuit diagram, such as AC contactors and solenoid valves, with their coils connected to the output terminals of the PLC. Buttons, control switches, limit switches, proximity switches, etc., provide control commands and feedback signals to the PLC, with their contacts connected to the input terminals of the PLC.
02
Handling Intermediate Relays and Timers in Relay Circuit Diagrams
The functions of intermediate relays and timers in relay circuit diagrams are fulfilled by auxiliary relays and timers inside the PLC, which are independent of the PLC’s input and output relays.
03
Setting Intermediate Units
In a ladder diagram, if multiple coils are controlled by a certain contact series/parallel circuit, to simplify the circuit, an auxiliary relay controlled by that circuit can be set in the ladder diagram, similar to intermediate relays in relay circuits.
04
Handling Momentary Contacts of Time Relays
In addition to the delayed action contacts, time relays also have momentary contacts that act immediately when energized or de-energized. For time relays with momentary contacts, auxiliary relays can be connected in parallel across the timer’s coil in the ladder diagram, with the latter’s contacts equivalent to the momentary contacts of the time relay.
05
Establishing External Interlocking Circuits
To prevent both contactors controlling forward and reverse rotation from acting simultaneously, causing a short circuit in the three-phase power supply, not only should a soft interlock circuit be set in the ladder diagram composed of normally open contacts in series with the coils of the corresponding output relays, but a hard interlock circuit should also be established externally from the PLC.
Structural Analysis of Ladder Diagrams
Whether to adopt general programming methods or sequential function chart programming methods; whether to use single sequence structures or choose sequence structures, parallel sequence structures, using start/hold/stop circuits, step control instructions for programming, or using set/reset instructions for programming.
The decomposition of ladder diagrams starts with the main control circuit (such as buttons), tracing the line to the main circuit control devices (such as contactors) actions, which involves many programming elements and circuits, making it relatively difficult to trace.
No matter how complex a ladder diagram is, it is composed of some basic units. Based on the configuration of the main circuit, using reverse tracing methods, the ladder diagram and instruction statement table can be decomposed into several basic units corresponding to the main circuit’s load devices (such as motors), then analyzed step by step, and finally, the various links can be connected using forward tracing methods.
01
Configuration and Role of Buttons, Travel Switches, and Selector Switches
In the I/O wiring diagram of the PLC, there are many travel switches and selector switches, as well as pressure relays, temperature relays, etc. These electrical components do not have attracting coils; their contacts operate based on external forces or other factors, so it is necessary to first identify the external forces or factors that cause these contacts to operate. Among them, travel switches are actuated or released by mechanical linkage mechanisms, while selector switches are generally operated manually, causing the contacts of these travel switches and selector switches to be in different working states during equipment operation, i.e., the contact closure and disconnection conditions vary to meet different control requirements, which is a key point during the reading process.
It is difficult to clarify the different working states of these travel switches and selector switches’ contacts by merely looking at the circuit diagram. It is necessary to combine the equipment manual and the detailed list of electrical components to clarify the purpose of each travel switch and selector switch, the mechanical linkage mechanism actuating the travel switch, and the working state of the circuit under different closure or disconnection conditions of the contacts.
02
Using Reverse Tracing Methods to Decompose Multi-Load Circuits (e.g., Multiple Motor Circuits) into Single-Load Circuits (e.g., Single Motor)
Based on the main contact symbols of the control devices in the main circuit, find the output relay controlling that load in the PLC’s I/O wiring diagram, then find the coil controlling that output relay and its related circuits in the ladder diagram and instruction statement table, which constitute the local circuit controlling that load.
In the ladder diagram and instruction statement table, it is easy to find the coil circuit of that output relay and its energizing and de-energizing conditions, but it is not easy to find the conditions that cause that coil to energize or de-energize and its related circuits; reverse tracing methods can be used to search for:
The closure and disconnection of other programming element contacts in series or parallel with the output relay coil circuit are the conditions for energizing and de-energizing that output relay.
From these contacts, find their coil circuits and related circuits; there will also be contacts from other contactors and relays in these coil circuits…
Continue searching until the input relay (main control device) is found. It is worth noting that when a certain programming element is energized or de-energized, all the contact states of that programming element’s contacts and the subsequent programming elements must be identified without omission.
Identify the action contacts and break contacts of a certain programming element in other circuits; these contacts provide conditions for the energizing and de-energizing of other programming elements or provide conditions for interlocking, leading to the operation of other electrical components and driving actuators.
03
Further Decomposing Single Load Circuits
The local circuit controlling a single load may still be complex and may need further decomposition until it is broken down into basic unit circuits.
04
Considerations for Circuit Decomposition
If the motor spindle is connected to a speed relay, then the motor is composed of a stop and brake circuit based on speed control principles.
If the main circuit of the motor has a rectifier, it indicates that the motor uses an energy consumption braking stop circuit.
05
Gathering and Comprehensive Analysis
Connect the basic unit circuits in series and analyze the entire circuit using forward tracing methods.
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