In the field of modern industrial automation, Programmable Logic Controllers (PLCs) are widely used due to their high reliability and flexibility. PLCs receive input signals, execute logical operations, and output control signals to automate the control of various machines and equipment. This article will introduce how to use PLC to achieve a simple moving action, taking a simple robotic arm as an example, and elaborate on PLC programming and control system design in detail.

I. System Overview

The task of the simple robotic arm is to transport a workpiece from station A to station B. The movements of the robotic arm include lifting up and down, moving left and right, clamping and releasing. To achieve these movements, we need to design a PLC control system that consists of the following components:

  1. Input Devices: including a start button, a left-position detection switch, a right-position detection switch, an up-position detection switch, a down-position detection switch, and a workpiece detection switch.
  2. Output Devices: including a motor M1 controlling the left and right movement of the robotic arm, a motor M2 controlling the up and down lifting of the robotic arm, and a YV coil controlling the clamping and releasing of the robotic arm.
  3. PLC Controller: responsible for receiving input signals, executing logical operations, and outputting control signals.

II. System Hardware Design

1. Input Devices

  • Start Button: Used to initiate the moving action of the robotic arm.
  • Left-Position Detection Switch SQ1: Closes when the robotic arm moves to the left side of station A.
  • Right-Position Detection Switch SQ2: Closes when the robotic arm moves to the right side of station B.
  • Up-Position Detection Switch SQ3: Closes when the robotic arm rises to the highest position.
  • Down-Position Detection Switch SQ4: Closes when the robotic arm descends to the lowest position.
  • Workpiece Detection Switch SQ5: Closes when a workpiece is located at station A.

2. Output Devices

  • Motor M1: Controls the left and right movement of the robotic arm.
  • Motor M2: Controls the up and down lifting of the robotic arm.
  • YV Coil: Controls the clamping and releasing of the robotic arm.

3. PLC Controller

Select a suitable PLC controller, determine the number of input/output points based on system requirements, and configure the corresponding input/output modules.

III. System Software Design

1. Define Symbol Table

Before programming, define a symbol table for easy reference to input/output devices in the program. For example:

  • I0.0: Start Button
  • I0.1: Left-Position Detection Switch SQ1
  • I0.2: Right-Position Detection Switch SQ2
  • I0.3: Up-Position Detection Switch SQ3
  • I0.4: Down-Position Detection Switch SQ4
  • I0.5: Workpiece Detection Switch SQ5
  • Q0.0: Motor M1 (Left Movement)
  • Q0.1: Motor M1 (Right Movement)
  • Q0.2: Motor M2 (Up Movement)
  • Q0.3: Motor M2 (Down Movement)
  • Q0.4: YV Coil (Clamping)
  • Q0.5: YV Coil (Releasing)

2. Write PLC Program

According to the control requirements of the robotic arm, write the PLC program. The control requirements of the robotic arm are as follows:

  • The initial state (home condition) of the robotic arm is parked above station A, with SQ1 and SQ3 both closed.
  • If the home condition is met and SQ5 is closed (workpiece at station A), pressing the start button will cause the robotic arm to follow the steps of “home → descend → clamp → ascend → move right → descend → release → ascend → move left → home”.

The PLC program can be written using Ladder Diagram (LD) language. The following is a simplified LD program example:

// Initialization Phase
Network 1:
|----[ I0.0 ]-----------------( Q0.2 )----( )
| |
|----[ I0.1 ] AND [ I0.3 ] AND [ I0.5 ]--|
| |
|----( )------------------------( M0 )----| // M0 is the start intermediate relay

// Descending Phase
Network 2:
|----[ M0 ] AND [ NOT I0.4 ]----( Q0.3 )----( )

// Clamping Phase
Network 3:
|----[ M0 ] AND [ I0.4 ]--------( Q0.4 )----( )
| |
|----[ Timer T1 ]---------------| // T1 is used for delay to ensure clamping action is completed

// Ascending Phase
Network 4:
|----[ M0 ] AND [ T1 Completion ]------( Q0.2 )----( )

// Moving Right Phase
Network 5:
|----[ M0 ] AND [ NOT I0.2 ]----( Q0.1 )----( )

// Descending Phase (Reaching Station B)
Network 6:
|----[ M0 ] AND [ I0.2 ]--------( Q0.3 )----( )

// Releasing Phase
Network 7:
|----[ M0 ] AND [ I0.4 (new state detection after descending) ]----( Q0.5 )----( )
| |
|----[ Timer T2 ]---------------| // T2 is used for delay to ensure releasing action is completed

// Ascending Phase (Returning to Home)
Network 8:
|----[ M0 ] AND [ T2 Completion ]------( Q0.2 )----( )

// Moving Left Phase (Returning to Home)
Network 9:
|----[ M0 ] AND [ NOT I0.1 ]----( Q0.0 )----( )

// Ending Phase
Network 10:
|----[ M0 ] AND [ I0.1 ] AND [ I0.3 ]------( ) // Reset M0, end the program

Note: The above LD program is for illustration purposes only. Actual programming may require adjustments based on the specific PLC model and robotic arm control requirements. For example, some PLCs may use different instructions and symbols, and some robotic arms may require more complex control logic and timer settings.

3. Debugging and Testing

After writing the PLC program, debugging and testing are required. Connect the PLC to the input/output devices, simulate the actual operation process of the robotic arm, and check whether each action is performed as expected. If issues are found, modify the program and retest until the system can run stably.