Design and implementation of a three-degree-of-freedom manipulator control system based on PLC

Abstract: In order to improve the positioning accuracy of the manipulator in industrial production, a design scheme of a three-degree-of-freedom manipulator control system based on PLC is introduced. The scheme proposes a new idea for the application of stepper motors in manipulator positioning, discusses the hardware structure and software implementation method of the three-degree-of-freedom manipulator control system in detail, and establishes the M("GS configuration environment interface to monitor the operation of the system. The test results show that the system runs stably, has accurate positioning, and has high application value.
Keywords: PLC; three degrees of freedom; manipulator; stepper motor; MCGS configuration environment

0 Introduction
A manipulator is a mechatronic automatic device that can simulate human arm movements and replace human hands to grasp, carry workpieces or operate tools according to set programs, trajectories and requirements. A three-degree-of-freedom manipulator, also known as a 3D robot, can grasp or place objects in three degrees of freedom (horizontal, vertical and rotational). It has the characteristics of a large operating range, good flexibility and wide application.
A programmable controller (PLC) is an electronic control device designed specifically for industrial applications to perform digital operations. It is widely used in industrial control systems due to its high reliability, powerful functions, simple programming and friendly human-computer interaction interface.
A stepper motor is an open-loop actuator that converts electrical pulse signals into angular or linear displacements. In the absence of overload, the speed and stop position of the motor depend only on the frequency and number of pulses of the pulse signal. The existence of this linear relationship, coupled with the fact that stepper motors only have periodic errors but no cumulative errors, makes them widely used in speed, positioning and other control fields.
According to the drive mode, manipulators can be divided into hydraulic, pneumatic, electric and mechanical manipulators. The three-degree-of-freedom manipulator designed in this paper is a hybrid manipulator. It combines the advantages of electric and pneumatic manipulators, saving travel switches and PLC I/O ports while achieving the purpose of simple operation and precise positioning.

1 System structure and movement mode of three-degree-of-freedom manipulator
The three-degree-of-freedom manipulator is of cylindrical coordinate type. Figure 1 is a schematic diagram of the manipulator structure. The left and right movement (horizontal direction) of the manipulator arm is controlled by a telescopic stepper motor, the up and down movement (vertical direction) is controlled by a lifting stepper motor, and the counterclockwise and clockwise rotational movement is controlled by the forward and reverse rotation of the chassis DC motor. The clamping device of the manipulator adopts a joint structure, and its clamping and loosening are driven by air pressure and controlled by a solenoid valve.
The manipulator can move the workpiece from A to B according to the action of the set program. SQ1, SQ2, SQ5, SQ6 are limit switches in the horizontal and vertical directions, and SQ3 and SQ4 are optical proximity switches for the origin position and the end position.

2 Design of three-degree-of-freedom manipulator control system
The three-degree-of-freedom manipulator system is set up with manual working mode and automatic working mode. The automatic mode is divided into four working modes: automatic return to origin, single step, single cycle, and continuous.

2.1 Hardware Design
The main control system uses Mitsubishi FX2N series transistor output PLC, and the stepper motor driver uses SH-20403 module. The external wiring diagram of the manipulator is shown in Figure 2.

The manipulator is at the top and rightmost. When the chassis turns to the optical proximity switch X3 and the clamping device is released, the system is said to be in the origin state. X10 is a manual control button. After pressing this button, the manual operation corresponding to the key switches X20~X27 can be performed. X11~X15 are the buttons for returning to the origin, single step, single cycle and continuous working mode in the automatic mode. Press one of the buttons and then press the start button X0, and the action of the working mode will be automatically executed. The stepper motor will only rotate when there is a pulse signal (Y0) and a direction signal (Y2 or Y3) input. Y6 and Y7 select which stepper motor to send the pulse signal Y0 to. Y5 controls the air supply solenoid valve to realize the clamping and loosening of the clamping device. Y10 and Y11 control the forward and reverse rotation of the chassis DC motor.
2.2 Software Design
Before selecting the single step, single cycle and continuous working mode, the system should be in the origin state. If this condition is not met, the return to origin working mode can be selected, which performs the following operations in sequence: move upward to the upper limit x1 → move rightward to the right limit X2 → rotate clockwise to the optical proximity switch X3 → release the clamping device.
The sequential function diagram of the automatic working mode of the manipulator is shown in Figure 3. The movement speed of the manipulator arm is controlled by the pulse frequency of the input stepper motor, and the distance of the manipulator's descent and left movement is controlled by the number of pulses. The pulse frequency and number of pulses can be set in the program according to the actual situation of the industrial site, and it has repeatable operability.

This system adopts the PLC ladder diagram sequential programming method. The PLSY pulse output instruction is used to output pulses, and the MOV instruction is used to set the number of pulses. After the specified pulse is output, the instruction execution completion flag M8029 is set to 1. Since the PLSY instruction can only be used once, and both stepper motors in the system require pulse input, two external relays are used in the design to solve this problem. The pulse output by Y0 is connected to the moving contacts of the two relays at the same time, and the normally open contacts of the two relays are connected to the pulse input terminals of the two stepper motor drivers respectively. Y6 and Y7 are connected to the control terminals of the two relays, so that the pulse input of the stepper motor can be controlled through Y6 and Y7.

3 MCGS configuration environment of the system
MCGS is a general computer system software serving the field of industrial process control and real-time monitoring, with outstanding features of complete functions, simple operation, good visibility and strong maintainability. Figure 4 shows the MCGS configuration environment interface when the three-degree-of-freedom manipulator system is working, which includes the main interface, manual working mode interface and automatic working mode interface.

4 Conclusion
After debugging, the stepper motor runs without jitter or step loss, and the manipulator system is in good operating condition. The three-degree-of-freedom manipulator system only realizes movement in three degrees of freedom. According to the actual needs of industrial production, it can be further improved. By adding a wrist rotation control device to the elbow of the clamping device, it can become a four-degree-of-freedom manipulator control system. If conditions permit, a touch screen can also be used instead of the MCGS configuration environment to control the manipulator system, which takes up less space, has an intuitive human-computer interaction interface, and is more convenient to operate.