Abstract : This paper introduces the structure, working principle, and control sequence of a pneumatic handling robot controlled by a Siemens PLC and stepper motors. By analyzing the process flow of the pneumatic robot, an electrical control system based on Siemens 300 is designed. The positioning controller of the stepper motor is controlled by allocating input/output interfaces to the PLC. Experimental results show that the output overshoot of the robot system controlled by this system is significantly reduced, and the positioning accuracy is significantly improved. Therefore, this system has the advantages of simple control, reliable operation, and accurate positioning.
Keywords : PLC; pneumatic manipulator; position control; stepper motor
Intermediate Classification Number : TP 9 Document Identification Code: B
0 Introduction
Industrial robots can be classified according to their driving method into motor-driven, mechanism-driven, hydraulic-driven, and pneumatic-driven robots. Motor-driven robots have high movement speed and large gripping force, but suffer from the drawback of difficulty in speed control; hydraulic-driven robots have large gripping force, but slow speed; while pneumatic robots have relatively high speed, but small gripping force.
Robotic arms are an inevitable product of industrial production. They are automated technological devices that mimic some functions of the human upper limbs, transporting workpieces or holding tools according to predetermined requirements. They play a vital role in realizing industrial automation and promoting further industrial development. Therefore, they possess strong vitality and are widely valued and welcomed. Practice has proven that industrial robotic arms can replace heavy manual labor, significantly reducing workers' labor intensity, improving working conditions, and increasing labor productivity and automation levels. Robotic arms are effective for handling heavy workpieces and performing long-term, frequent, and monotonous operations in industrial production. Furthermore, they can operate under high temperature, low temperature, deep water, space, radioactive, and other toxic or polluted environments, further demonstrating their superiority and promising broad development prospects. Hydraulic robotic arms have advantages such as speed and efficiency. These robotic arms are hydraulically driven, controlling the extension and retraction of the arm, the rotation of the wrist, and the grasping and releasing movements of the hand; the up-and-down movement is controlled by a motor.
Robotic arms can be classified into parallel mechanisms, swing mechanisms, rotary mechanisms, multi-point mechanisms, and articulated mechanisms according to their working principles. Currently, most robotic arms used in industry employ composite mechanisms. This paper introduces a hydraulically driven industrial robotic arm and conducts hydraulic circuit design, simulation, PLC motion control, and mechatronics integration simulation experiments for its four main movements: extension, grasping, retraction, and release.
With the development of robotic arm technology, coupled with the application of PLC and motor control technologies, general-purpose robotic arms suitable for industrial automation have been widely used. Pneumatic robotic arms, due to their simple structure, convenient control, and accurate positioning, are widely used in automated production lines. This article introduces a pneumatic handling robotic arm controlled by a PLC and stepper motor.
1. Working principle of pneumatic manipulator
Figure 1 shows a schematic diagram of an industrial cylinder-driven robotic arm. This structure is characterized by its pneumatic cylinder drive, integrating a parallel mechanism and a single-sided swing mechanism. To overcome the shortcomings of pneumatic drive, a differential circuit is used for the rapid advance process, and a synchronization circuit and a pressure regulating circuit are used for the working process. Figure 2 shows the cylinder action sequence.
Figure 1. Schematic diagram of the robotic arm structure
1, 2 - Cylinder, 3 - Parallel mechanism, 4 - Hinge, 5 - Fixed claw, 6 - Movable claw
Figure 2. Cylinder action sequence diagram
Functionality implementation:
Extend. Cylinders 1 and 2 move to the right simultaneously, with the hydraulic circuit employing a differential circuit and a synchronous circuit;
Grab. Cylinder 1 stops working, cylinder 2 moves to the right, and the circuit uses a voltage regulating circuit.
To retract. The opposite of the action of extending.
Release. The opposite of the grasping action.
In the initial position, the manipulator opens to a certain angle, that is, the initial angle of cylinder 2 is 0, and cylinder 1 has a certain offset angle. The pneumatic principle of the pneumatic manipulator is shown in Figure 3.
Figure 3. Pneumatic principle diagram of the pneumatic manipulator
The working process of this pneumatic manipulator is as follows: after the workpiece is pushed into the storage platform, the pneumatic manipulator arm extends forward, the forearm descends, the pneumatic fingers clamp the workpiece, the forearm rises, the arm retracts, the arm rotates to the right to the position, the arm extends forward, the forearm descends, the gripper releases and places the workpiece into the feed port, the forearm rises, the arm retracts, the manipulator rotates to the left to return to its original position, waits for the next workpiece to arrive, and then repeats the above actions.
The left/right rotation of the pneumatic manipulator is mainly achieved by stepper motor drive. This system uses a three-phase hybrid stepper motor and a microstepping stepper motor driver as the driving and positioning devices for the manipulator's rotation. For the manipulator's positioning needs, an inductive sensor is installed on the manipulator's base as a reference sensor, and limit switches are installed at the extreme positions of the manipulator's left/right rotation.
2 PLC controller
A PLC is the core component of a control system, analogous to the human brain, controlling all our actions. Its structure generally consists of a central processing unit, power supply, and input/output sections. Figure 4 shows a typical PLC block diagram. PLC software is simple to learn, easy to use and maintain; it integrates electrical control, electrical transmission, and electrical instrumentation; it boasts high reliability and strong anti-interference capabilities; and PLC networks offer a high performance-to-price ratio, making it one of the three pillars of modern industrial automation. Currently, some of the world's most renowned PLC companies include Mitsubishi and Panasonic from Japan, AB from the United States, and Siemens from Germany, each company's products possessing unique features.
The programmable controller used in this system is a Siemens 300 series PLC, specifically model 315-2PN/DP (as shown in Figure 3). The performance parameters and module parameters of 315-2PN/DP are shown in Table 1 below:
Table 1 Performance parameters of Siemens 315-2PN/DP and modules
PLC model | 315-2PN/DP | |||
Input power | AC 220V | |||
Output power | DC 24V | |||
DI module input port | DO module output port | |||
type | DC input | type | Relay output | |
Points | 4 PM | Points | 8 o'clock | |
Operating voltage | 24V | Operating voltage | 24V | |
Work instructions | led | |||
Connection method | Terminal block (M3.5 screws) | |||
3 PLC Motion Control and Simulation
The PLC ladder diagram is programmed using the Siemens S7-300 programming interface. I0.0 is the power switch, I0.1 is the cylinder 2 limit switch, I0.4 is the cylinder 1 limit switch, I0.2 is the cylinder 2 limit switch, I0.3 is the cylinder 1 limit switch, M0.0 and M0.1 are intermediate relays, Q0.1 and Q0.2 are solenoid valves YA1 and YA2, Q0.3 and Q0.4 are solenoid valves YB1 and YB2, and Q0.5 and Q0.6 are solenoid valves YC1 and YD2. When power switch I0.0 is turned on, intermediate relay M0.0 self-locks, and Q0.1 and Q0.3 are simultaneously energized, enabling the robotic arm to extend synchronously. When the fixed gripper contacts an object, the normally closed contact of limit switch I0.4 opens, Q0.1 is de-energized, cylinder 1 stops, and cylinder 2 continues to move. When the movable gripper firmly grasps the object, the normally closed contact of limit switch I0.1 opens, and the normally open contact closes, de-energizing Q0.1 and Q0.3, while energizing Q0.2, Q0.4, Q0.5, and Q0.6, causing cylinders 1 and 2 to retract simultaneously. When cylinder 1 reaches the initial position, the normally closed contact of limit switch I0.3 opens, cylinder 1 stops, and cylinder 2 continues to move to release the object. When cylinder 2 reaches the initial position, the normally closed contact of limit switch I0.2 opens, completing one cycle. Intermediate relays M0.0 and M0.1 in the ladder diagram achieve interlocking.
The hydraulic circuit wiring was completed on the Siemens hydraulic PLC control experimental platform according to Figure 3. The S7-300 was programmed using STEP 7, and the experimental simulation results are shown in Figure 5.
Cylinder A 1 Cylinder B 2
Figure 5 Simulation results
4. Conclusion
Hydraulic circuits for the four actions of the robotic arm—extending, grasping, retracting, and releasing—were designed, simulated, and programmed using PLC. The pneumatic handling robotic arm, controlled by a PLC and cylinders, demonstrates a simple principle and reliable performance. Simulations were conducted on a Siemens 300 series PLC-controlled hydraulic test bench; the simulation results prove that the robotic arm is easy to control and provides accurate positioning. In practical applications, the robotic arm's motion flow can be modified as needed, making it highly practical.
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