Briefly describe the basic principles of PLC control system design

We all know that PLC control is used for actual engineering projects, so the level of system design will directly affect the entire control system and the operating reliability of the equipment! Do you agree?

So how can we design a control system that is stable in operation, reliable in action, safe to use, easy to debug, and easy to maintain based on different control requirements? This is also a problem we need to consider when designing PLC control.

Let's first understand the system design and the principles that should be followed

The engineering design of PLC control system can generally be divided into five basic steps: system planning, hardware design, software design, system debugging, and technical document preparation.

The basic design principles are as follows:

1. First of all, all the actions of equipment, production machinery and production process must be realized;

2. The design is to meet the equipment's requirements for product processing quality and production efficiency;

3. Ensure the safe, stable and reliable operation of the system;

4. Simplify the structure of the control system as much as possible to reduce production costs;

5. Fully improve the automation intensity and reduce labor;

6. Improve operating performance and facilitate future maintenance.

Below we will take the "three-phase induction motor fault alarm control" circuit as an example, and briefly talk about the process of converting the traditional electrical diagram into a ladder diagram, and briefly talk about program design. I believe that everyone will have a deep impression of related circuit conversion or program design in the future.

01 Traditional electrical diagram

A conventional electrical diagram of a known three-phase induction motor fault alarm control circuit is shown in FIG1 .

02 Action Description

1. When the power supply is normal, only the green light gl is on and the motor does not move.

2. Press the start button PB1, the electromagnetic contactor MC will operate, the motor will start immediately, the indicator light RL will light up, and the green light GL will go out.

3. Press the stop button PB2, the electromagnetic contactor MC is powered off, the motor stops running, the indicator light RL goes out, and the green light GL turns on.

4. When the motor is running, due to overload or other faults, the thermal relay th-ry is activated, the motor stops running, the buzzer bz sounds an alarm, the indicator light rl goes out, and the green light gl turns on.

5. Press the button switch pb3, the buzzer bz stops the alarm, the white light wl turns on, the green light gl turns on, and the red light rl turns off.

6. After the fault is eliminated, press the reset lever of the thermal relay th-ry, the white light wl goes out, the green light gl turns on, the red light rl goes out, and the motor can be restarted.

03 I/O Encoding

Using PLC means replacing hardware wiring with software programs. In traditional electrical diagrams, the main circuit cannot be replaced by PLC; the part that can be replaced by PLC is the control circuit. The first step in converting traditional electrical diagrams to ladder diagrams is i/o coding, that is, the input/output components in the traditional electrical diagrams are first determined to correspond to the external input/output terminal numbers in the PLC, and the external input component wiring method is to use a/b contacts, as shown in Table 1.

(a): External connection using a contact method

(b): External connection using b contact method

04 PLC external wiring diagram

After the input/output components are coded by I/O and the external input components are determined to use the A/B contact wiring method, the PLC external wiring diagram is shown in Figure 2. The figure shows the Fengwei Vigor-VB series PLC model, which uses NPN wiring, that is, the 24V terminal and the S/S terminal are connected in parallel.

05 PLC Ladder Diagram

The programming steps for converting traditional electrical diagrams into ladder diagrams are as follows:

(1) Convert the control circuit in the electrical diagram directly into the corresponding ladder diagram. Because the PLC ladder diagram stipulates that the contacts are in front and the output coil must be at the end of the loop. Therefore, the electrical diagram must be redrawn first, and the positions of the contacts and output coils in the diagram must be appropriately changed to meet the requirements of the PLC ladder diagram. The redrawn electrical diagram is shown in Figure 3.

(2) Replace the input/output components in the electrical diagram with the component numbers after the i/o coding. It should be noted here that the CA contact and CB contact of TH-RY should be separated and each become a control circuit, as shown in Figure 4.

(3) Rotate the electrical diagram shown in Figure 4 90° to the left and then flip it vertically (upsidedown) to become a PLC ladder diagram. However, due to:

(1) y1, x0 contacts,

(2) The output coils y1 and y2 do not conform to the general programming software format and must be appropriately corrected, as shown on the right side of Figure 5.

※If you use Visio to draw electrical diagrams, it will be easy to rotate it 90° to the left and then flip it vertically.

(4) The ladder diagram drawn using the programming software, as shown in FIG6 , is exactly the same as the ladder diagram after being appropriately flipped and corrected.

06 Instructions

Converting the ladder diagram into instructions is as follows:

07 PLC conversion wiring and ladder diagram

PLC external input/output wiring and ladder diagram after complete conversion of traditional electrical diagram. PLC external input/output wiring and ladder diagram after using software program to replace hardware wiring, as shown in Figure 7 below.