Example to explain the design steps of PLC control system

The main contents of system design:

① Formulate technical conditions for control system design

② Select the electrical transmission form and actuators such as motors and solenoid valves

③ Select the PLC model

④ Prepare the PLC input/output allocation table or draw the input/output terminal wiring diagram

⑤ Write the software specification according to the requirements of the system design, and then use the corresponding programming language (commonly used ladder diagram) for program design

⑥ Understand and follow user cognitive psychology, attach importance to the design of human-computer interface, and enhance the friendly relationship between people and machines

⑦ Design operating tables, electrical cabinets and non-standard electrical components

⑧ Write design instructions and operating instructions

PLC Hardware System Design

(1) Selection of PLC model

① Selection of input/output points

② Selection of storage capacity

③ Selection of I/O response time

④ Select according to the characteristics of the output load

⑤ Choice between online and offline programming

⑥ Select the model based on whether it is networked or not

⑦ Selection of PLC structure

(2) Assign input/output points

① Determine the I/O channel range

② Determine the internal auxiliary relay

③ Assign timer/counter

PLC software system design methods and steps

(1) PLC software system design method

① Graphical programming

a. Ladder diagram method

b. Logical flow chart method

c. Sequential Flowchart Method

d. Step sequence control method

② Empirical Programming

③ Computer-aided design programming

(2) Steps in PLC software system design

① Divide system tasks into blocks

② Prepare the logical relationship diagram of the control system

③ Draw various circuit diagrams

④ Compile PLC program and perform simulation debugging

⑤ Make console and control cabinet

⑥ On-site debugging

⑦ Prepare technical documents and conduct on-site trial operation

Control unit input terminal wiring

Keep the input line as far away from interference sources such as high voltage lines and power lines as possible.

Do not connect input devices to terminals with "."

The built-in DC power output of the AC PLC can be used for input; when the DC power output power of the DC PLC is insufficient, an external power supply can be used.

Do not connect external power to the output terminals of the built-in DC power supply of the AC type PL.

Do not connect input power supplies in parallel, and do not connect these power supplies in parallel to other power supplies.

Control unit output terminal wiring

Keep the output line as far away from interference sources such as high voltage lines and power lines as possible.

Do not connect output devices to terminals with "."

Each "COM" terminal is independent, so each output terminal can be output independently or in parallel. When each load uses a different voltage, the independent output method is used; when each load uses the same voltage, the common output method can be used.

When multiple loads are connected to the same power supply, use a short-circuit chip model AFP1803 to short-circuit their "COM" terminals.

If the output terminal is connected to an inductive load, the corresponding protection circuit must be connected according to the different load conditions. Connect an RC series circuit at both ends of the AC inductive load; connect a diode protection circuit at both ends of the DC inductive load; connect a bleeder resistor at the output end with a low current load to avoid interference from leakage current. The above protection devices should be installed within 50cm from the load.

There is no fuse in the PLC internal output circuit. To prevent output short circuit caused by load short circuit, a fuse should be installed in the external output circuit or an emergency stop circuit should be designed.

Example: The role of PLC in machine tool control

Process requirements:

The four-station combination machine tool consists of four working slides, each with a processing power head, forming four processing stations. In addition to the four processing stations, there are four auxiliary devices, namely, a fixture, a loading and unloading manipulator, and a feeder, as well as a cooling and hydraulic system, totaling four parts. The process requirements are that the loading manipulator automatically loads the material, and the four processing power tool heads of the machine tool process a part at the same time, completing one part at a time, and the processed part is automatically taken away by the unloading manipulator. It is required to have three working modes: fully automatic, semi-automatic, and manual.

The following figure is a state flow chart of the fully automatic working cycle and semi-automatic working cycle of the combined machine tool control system. In the figure, S2 is the initial state, and the conditions for driving it are that all slides and auxiliary devices are in place, the fixture is in a loose state, there are parts to be processed on the material channel, and the lubrication system is working normally.

Combined machine tool fully automatic and semi-automatic working process:

Loading Press the start button, the loading robot moves forward, sends the part to the fixture, and the fixture clamps the part. At the same time, the feeding device feeds the material, then the loading robot returns to its original position, and the unloading device returns to its original position

Processing: Four working slides move forward, and four processing power heads process simultaneously, washing the end faces and drilling the center holes. After processing is completed, each working slide returns to its original position.

Unloading The unloading robot moves forward to grab the part, the clamp is released, the unloading robot returns to its original position and takes away the processed part.

This completes a work cycle. If the pre-stop function is selected, the machine tool automatically stops at the original position after each cycle is completed, realizing a semi-automatic working mode; if the pre-stop function is not selected, the machine tool automatically starts the next work cycle, realizing a fully automatic working mode.

PLC selection

The electrical control system of the four-station modular machine tool has 42 input signals and 27 output signals, all of which are switch quantities. The external input components include: 17 detection components, 24 push button switches, and 1 selection switch; the external output components include: 16 solenoid valves, 6 contactors, and 5 indicator lights.

Based on the number and type of I/O signals and control requirements, and taking into account many factors such as maintenance, transformation and economy, it was decided to use the FX2N-64MR host and an output expansion unit FX-16EX, which has a total of 48 input points and 32 output points of the host, meeting the control requirements.

I/O address numbering: 42 input signals and 27 output signals are divided into their respective functional types, and correspond to the PLC I/O terminals one by one, and the addresses are arranged. A table comparing external I/O signals and PLC I/O terminal addresses is listed, as shown in the table.

PLC ladder diagram program

The ladder diagram of the PLC control system of the four-station modular machine tool includes the initialization program, manual adjustment program and automatic working program. Figure 2 is the ladder diagram program of the four-station modular machine tool in full-automatic and semi-automatic working modes. It is written in STL stepping instructions, and the program is simple and clear. Figure 3 Ladder diagram of the initialization program of the four-station modular machine tool.