introduction
Since the 1960s, when robotic arms were first developed as a product, their application has continuously evolved. The most typical development is their application in the healthcare industry, fulfilling the urgent need for rapid, high-volume sample data in hygiene testing. However, in the healthcare field, robotic arms using single-enzyme colorimetric methods and filter structures resulted in expensive reagents, limiting market development. With advancements in science and technology, robotic arm design has broken free from the limitations of single reagents, heating, and filters. Industrial robotic arms can be categorized by their drive mechanism: motor-driven, mechanically driven, hydraulically driven, and pneumatically driven. Motor-driven robotic arms offer high speed and strong grip, but suffer from difficulty in speed control; hydraulically driven robotic arms provide significant grip but are slow; while pneumatically driven robotic arms offer relatively high speed but weak grip.
With the development of industrial mechanization and automation, and the inherent advantages of pneumatic technology, pneumatic robots have been widely used in various industries for production automation. Over the past two decades, the application areas of pneumatic technology have expanded rapidly, especially in various automated production lines. The combination of programmable logic controller (PLC) technology and pneumatic robot technology has resulted in a higher degree of automation, more flexible control methods, and more reliable performance in the entire control system. The rapid development of pneumatic robots and flexible automated production lines has placed higher demands on pneumatic technology. The introduction of microelectronics technology has promoted the development of proportional servo technology, and the development of modern control theory has enabled pneumatic technology to move from on/off control to closed-loop proportional servo control, continuously improving control accuracy.
With the development of robotic arm technology, coupled with the application of PLC control and motor control technologies, general-purpose robotic arms suitable for industrial automation production have been widely used. PLCs replace a large number of intermediate and time relays with software, leaving only a small amount of hardware related to input and output. Wiring can be reduced to 1/10 to 1/100 of that in relay control systems, greatly reducing failures caused by poor contact. High reliability is a key performance characteristic of electrical control equipment. PLCs, employing modern large-scale integrated circuit technology and rigorous manufacturing processes, utilize advanced anti-interference technology in their internal circuits, resulting in high reliability. Due to their simple structure, convenient control, and accurate positioning, pneumatic robotic arms are widely used in automated production lines. This article will introduce a pneumatic handling robotic arm controlled by a PLC and stepper motor.
1. Introduction to Pneumatic Robots
The pneumatic manipulator mainly consists of five parts: cylinders, parallel mechanisms, hinges, fixed grippers, and movable grippers. The manipulator designed in this paper features a 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 1 shows a schematic diagram of the cylinder-driven manipulator structure, and Figure 2 shows the action sequence diagram of the pneumatic manipulator.
Figure 1 Schematic diagram of the robotic arm structure
Figure 2. Cylinder action sequence diagram
1.1 Implementing agency
The actuator includes components such as hands, wrists, arms, and columns, and some also have a walking mechanism.
1. Hands
This refers to the component that comes into contact with the object. Due to different forms of contact, hands can be categorized into clamping and suction types. In this project, we adopt a clamping hand structure. A clamping hand consists of fingers (or grippers) and a force transmission mechanism. The fingers are the components that directly contact the object. Common finger movement forms include rotary and translational types. Rotary fingers have a simple structure and are easy to manufacture, hence their widespread use. Translational fingers are less common because their structure is more complex; however, when clamping round parts, changes in the workpiece diameter do not affect the position of their axis, making them suitable for clamping workpieces with a large diameter variation. The finger structure depends on the surface shape of the object being grasped, the grasped part (outer contour or inner hole), and the object's weight and size. Common finger shapes include flat, V-shaped, and curved surfaces; fingers can be externally clamping or internally supporting; finger types include two-finger, multi-finger, and two-finger double-finger types. The force transmission mechanism generates clamping force through the fingers to complete the task of clamping and releasing the object. Commonly used force transmission mechanisms include: sliding lever type, connecting rod lever type, inclined plane lever type, gear and rack type, lead screw nut and spring type, and gravity type.
2. Wrist
It is a component that connects the hand and arm, and can be used to adjust the position (i.e., posture) of the object being grasped.
3. Arms
The arm is a crucial component that supports the object being grasped, the hand, and the wrist. Its function is to drive the fingers to grasp the object and move it to a designated location according to predetermined requirements. The arm of an industrial robot typically consists of components that drive its movement (such as hydraulic cylinders, pneumatic cylinders, rack and pinion mechanisms, linkage mechanisms, screw mechanisms, and cam mechanisms) working in conjunction with a drive source (such as hydraulic, pneumatic, or electric motors) to achieve various arm movements.
4. Columns
The column is the component that supports the arm, and it can also be part of the arm itself. The arm's rotation and lifting (or pitching) movements are closely related to the column. Sometimes, the column of a robotic arm can also move laterally as needed for the job; this is called a movable column.
5. Walking mechanism
When industrial robots need to perform operations over long distances or expand their application range, a roller-type walking mechanism can be installed on the base. This mechanism can consist of rollers, tracks, and other components to enable the overall movement of the industrial robot. Roller-type robots are available in both tracked and trackless versions. A separate mechanical transmission device is required to drive the rollers.
6. Base
The base is the foundation of the robot arm. All components of the robot arm's actuator and drive system are mounted on the base, thus serving a supporting and connecting function.
1.2 Drive System
The drive system consists of power devices, adjustment devices, and auxiliary devices that drive the movement of the industrial robot's actuators. Commonly used drive systems include hydraulic transmission, pneumatic transmission, and mechanical transmission. The control system governs the movement of the industrial robot according to specified requirements. Currently, the control system of industrial robots generally consists of a program control system and an electrical positioning (or mechanical stop positioning) system. There are two types of control systems: electrical control and jet control. It governs the robot's movement according to a prescribed program.
It can also memorize the instructions given to the robotic arm by people (such as the sequence of actions, movement trajectory, movement speed and time), and issue instructions to the actuators according to the information of its control system. When necessary, it can monitor the actions of the robotic arm and issue an alarm signal when there is an error or malfunction.
1.3 Action Flow
In the initial position, the robotic arm opens to a certain angle, that is, the initial angle of cylinder 2 is 0, and cylinder 1 has a certain offset angle.
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. Design of motion control system
The control system is the system that directs the movement of an industrial robot according to specified requirements. Currently, the control system of an industrial robot generally consists of a program control system and an electrical positioning (or mechanical stop positioning) system. There are two types of control systems: electrical control and jet control. It directs the robot to move according to a prescribed program, memorizes the instructions given to the robot (such as the sequence of actions, movement trajectory, movement speed, and time), and issues instructions to the actuators based on the information from its control system. When necessary, it can monitor the robot's movements and issue alarm signals when errors or malfunctions occur.
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 3 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. 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. Sensors
The concept of a proximity switch sensor: Among various types of switches, there is a component that has the ability to "sense" objects approaching it—a displacement sensor. The purpose of a proximity switch is to control whether the switch is on or off by utilizing the sensitivity of the displacement sensor to approaching objects. When an object moves towards the proximity switch and approaches within a certain distance, the displacement sensor "senses" it, and the switch will activate. This distance is usually called the "detection distance." Different proximity switches have different detection distances.
For different materials and detection distances, different types of proximity sensors should be selected to ensure a high performance-price ratio within the system. Therefore, the following principles should be followed during selection:
(1) When the object to be detected is a metallic material, a high-frequency oscillation type proximity sensor should be selected. This type of proximity sensor is most sensitive to iron-nickel and A3 steel objects. Its detection sensitivity is low for aluminum, brass and stainless steel objects.
(2) When the object to be detected is a non-metallic material, such as wood, paper, plastic, glass and water, a capacitive proximity sensor should be selected.
(3) When metal and non-metal objects need to be detected and controlled at a distance, photoelectric proximity sensors or ultrasonic proximity sensors should be selected.
(4) When the detection object is metal, if the detection sensitivity requirement is not high, a low-cost magnetic proximity sensor or Hall effect proximity sensor can be selected.
Figure 4 Schematic diagram of the proximity switch sensor
4. 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 grips 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 are interlocked.
The hydraulic circuit wiring was completed on the Siemens hydraulic PLC control experimental platform according to the hydraulic air circuit. The S7-300 was programmed using STEP7. The experimental simulation results are shown in Figure 5.
Figure 5 Simulation results
5. Conclusion
This design features a pneumatic universal robotic arm. Compared to dedicated robotic arms, universal robotic arms offer variable degrees of freedom and adjustable control programs, thus broadening their applicability. Utilizing pneumatic transmission, it boasts rapid action, sensitive response, overload protection, and ease of automatic control. It exhibits excellent adaptability to various working environments, unaffected by environmental changes in transmission and control performance. Resistance loss and leakage are minimal, preventing environmental pollution. Furthermore, it is cost-effective. Employing PLC control offers advantages such as high reliability and flexible program modification. Whether performing time-based, stroke-based, or mixed control, it can be achieved through PLC programming. The program can be modified according to the robotic arm's action sequence, further enhancing its versatility.
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