Abstract: This paper describes the design process of a robotic arm for a shearing machine. It includes analyzing the operational requirements and conditions of the entire system, determining the structural design of the hydraulic system through its operation, analyzing the entire cycle to determine the system's working principle diagram, selecting standard hydraulic components based on system parameter requirements, and completing the hydraulic system assembly drawing. The CNC machine tool control system designed in this paper uses a PLC instead of traditional relay control to achieve automatic control, resulting in a more compact machine tool control structure, richer functionality, and improved reliability and response speed.
Foreword
Hydraulic technology is a relatively new technology, primarily used in automatic control systems that require fast response and accurate action. With the development of atomic energy, space technology, and computer technology, hydraulic technology has permeated various industrial fields and is beginning to evolve towards higher pressure, higher speed, higher power, higher efficiency, lower noise, lower energy consumption, durability, and higher integration. Simultaneously, the control strategies of electro-hydraulic control systems are also constantly evolving. Over the years, they have been continuously developed and improved through various novel control methods, from traditional PID control and adaptive control to variable structure control, robust control, and intelligent control.
Currently, various standard components have been developed and produced abroad, and as a major future producer of industrial robots, standardization is a development trend for China. China's manufacturing industry faces the enormous challenge of transforming towards high-end manufacturing, undertaking advanced international manufacturing, and participating in the international division of labor. Accelerating the research, development, and production of industrial robot technology is a major way for China to seize this historical opportunity. Therefore, the development of my country's industrial robot industry needs to further implement the following: First, industrial robot technology is a major means and pathway for my country to transform from a manufacturing giant to a manufacturing powerhouse. The government should provide more policy and economic support for domestically produced industrial robots, referencing advanced foreign experience and increasing technological investment and upgrades. Second, the national science and technology development plan should continue to strongly support the research, development, and application of intelligent robots, forming a new situation of synchronized and coordinated development of products and automated manufacturing equipment. Third, the quality of some domestically produced industrial robots is already comparable to that of foreign products. Enterprises should not blindly import industrial robots but should conduct comprehensive evaluations and prioritize domestic production. Intelligentization and biomimicry represent the highest stage of industrial robots. With the continuous development of materials, control, and other technologies, laboratory products are increasingly being commercialized and gradually applied in various situations. With the development of mobile internet and the Internet of Things, precision industrial robots with multiple sensors and distributed control will become increasingly common, gradually penetrating all aspects of the manufacturing industry and transforming from manufacturing implementation to service.
With the development and application of large-scale integrated circuits and microprocessor technology, the aforementioned control technologies have also undergone fundamental changes. In the 1970s, a new type of industrial controller emerged—the Programmable Programmable Controller (PLC)—that uses software to implement various control functions and is based on a microprocessor. This type of controller is fully adaptable to harsh industrial environments. Because it combines the advantages of both computer control and relay-contactor control, it is now widely used as a standardized and universal device in industrial control worldwide.
1. Functional Structure of the Shearing Machine
This hydraulic shearing machine is mainly used for shearing sheet metal. Its main unit consists of a feeding mechanism 4, a material rack 5, a pressure block 1, and shears 2. The clamping and shearing of the material are driven by hydraulic cylinders. Electrical limit switches (CS0i~SOS0i) are arranged along the stroke of each working mechanism to transmit signals and achieve automatic control in conjunction with a PLC.
When the shearing machine is in the initial position, the clamping block 1 that holds the sheet metal is in the upper position, and the limit switch SQ2 is opened. The shears 2 are also in the upper position, and the limit switch SQ4 is opened. Limit switches SQ1 and SQ5 are both normally open.
Before the shearing machine enters working mode, the material is placed on the feeding belt. Then, the hydraulic system is started and pressurized to the working pressure, and the feeding mechanism is activated to convey the material forward. When the material reaches the specified shearing length, the limit switch SQI is pressed, stopping the feeding mechanism. The pressure block is driven down by the hydraulic cylinder, thus activating the limit switch SQ2. When the pressure block falls to the position where it presses the material, triggering S03, the shears are driven down by another hydraulic cylinder, activating the limit switch S04. After the shears cut the material, the material falls, and the limit switch SQI resets and disconnects. Each time a piece of material falls onto the rack, SQI activates once, counting once. Simultaneously, the pressure block and shears return to their reset positions, completing one automatic work cycle. The above process is then automatically repeated, achieving automatic control of the shearing machine's operation.
2. Hydraulic System Design
2.1 Selection of Hydraulic Circuit
After determining the hydraulic actuators, the main circuits that have a decisive influence on the main performance are determined according to the overall characteristics and working requirements of the equipment. The hydraulic system of the robot mainly has three main motion circuits for telescopic motion, lifting motion and rotational motion.
2.2 Comprehensive Analysis of Hydraulic Components and Hydraulic Circuits
Hydraulic circuit integration involves grouping and merging selected hydraulic circuits, adding necessary hydraulic components and auxiliary circuits, and defining a complete hydraulic transmission system. Simultaneously, attention should be paid to: a. simplifying the system structure as much as possible and reducing the use of unnecessary hydraulic components; b. ensuring the reliability and safety of the entire hydraulic system's movements, eliminating interference between components and mechanisms; c. maximizing system efficiency and minimizing unnecessary power consumption; and d. using standard and universally applicable hydraulic components whenever possible.
2.3 Drive Design Requirements
1) Meets the requirements for the sequence of actions of industrial robots. Each action in the sequence is controlled by the electronic control system, which sends signals to control the corresponding electromagnets, and the actions are performed step by step according to the program.
2) The telescopic arm of the robotic arm is mounted on the lifting boom, with a gripper installed at the front end. Following instructions from the control system, it automatically changes the position of the workpiece. The extension and retraction should be smooth and flexible, with quick movements, accurate positioning, and coordinated operation.
3) The control system design must meet the operational logic requirements of the telescopic boom, and the selection of hydraulic cylinders and their control components must meet the power requirements and motion time requirements of the telescopic boom.
This design uses a hydraulic transmission control method, which has the following advantages compared to other transmission methods:
1) Hydraulic transmission can achieve stepless speed regulation during operation, which is convenient and has a relatively large speed regulation range;
2) Under the same power conditions, hydraulic transmission devices are smaller in size, lighter in weight, have lower inertia, and a more compact structure, and can transmit larger forces or torques.
3) Hydraulic transmission operates smoothly, responds quickly, has minimal impact, and can start, brake, and reverse at high speeds. The reversing and rotational motion of a hydraulic transmission device can reach 500 times per minute.
4) Hydraulic transmission devices are relatively simple to control and adjust, easy to operate and labor-saving, and easy to automate. When used in conjunction with electrical control, they can achieve complex sequential actions and remote control.
5) Hydraulic transmission devices easily implement overload protection; if the system is overloaded, the oil returns to the tank via the relief valve. Because it uses oil as the working medium, it is self-lubricating, resulting in a relatively long service life.
6) Hydraulic transmission is easy to serialize, standardize, and generalize, which is beneficial for design, manufacturing, and widespread use.
7) Hydraulic transmission easily achieves rotary and linear motion, and the arrangement of components is flexible.
8) In hydraulic transmission, the heat generated by power loss can be carried away by the flowing oil, thus avoiding excessive temperature rise in certain local parts of the system.
3. PLC Selection
Choosing the right PLC model plays a crucial role in improving the technical and economic performance of the PLC control system. The basic principles for selecting a model are to ensure reliability, ease of maintenance and use, and the best price-performance ratio, while meeting functional requirements.
(1) Structural selection
PLCs are mainly classified into integrated and modular types.
Integrated PLC: The average price of each point in an integrated PLC is cheaper than that of a modular PLC, and its size is relatively small. It is generally used in small control systems where the system process is relatively fixed, the environmental conditions are good, and the maintenance is minimal.
Modular PLCs: Modular PLCs offer flexible and convenient function expansion. They provide a wide range of choices in terms of the number of input and output points, the ratio of input points to output points, and the types of modules available. They are also easy to maintain and are generally used in more complex control systems.
For modular machine tools, an integrated PLC is preferable.
(2) I/O point selection principle
The average price of a PLC's I/O point is relatively high, so the number of I/O points should be selected rationally. The goal is to use the fewest possible I/O points while meeting control requirements, but a certain margin must be left. Typically, the number of I/O points is determined based on the actual input/output signal requirements of the controlled object, plus a 10%-20% margin.
The four-station combination machine tool control system, composed of a PLC, has 42 input signals, all of which are switching signals. These include 17 detection elements, 24 push-button switches, and 1 selector switch.
The electrical control system has 27 output signals, including 16 solenoid valves, contactors for six motors, and 5 indicator lights. Based on the selection principle for I/O points, considering a 10%-20% margin, the number of input points can be selected as 46-50, and the number of output points as 29-33.
(3) Determine the PLC model and expansion modules.
Based on (1) and (2) and the actual number of input points for the PLC model, an FX2N-64MR main unit and a 16-point input expansion module (FX-16EX) are selected, resulting in a total of 32+16 input points. The output points are the same as the 32 main unit's input points. This is sufficient to meet the requirements of 42 inputs and 27 outputs, with a certain margin.
1.1 Input/output signal allocation
An input/output signal address table lists inputs and outputs, assigning their corresponding addresses and names for use during software programming and system debugging. As this design shows, buttons, limit switches, and detection elements in the control circuit are all input devices of the PLC. The PLC's output controls primarily the actuators in the control circuit; in this design, these mainly include contactors, solenoid valves, and indicator lights. According to the input/output signal table of the electrical control system:
1. Number of input components
12 limit switches
24 buttons
Select switch 1
5 detection elements
2 Number of output components
16 solenoid valves
6 contactors
5 indicator lights
Based on the PLC model selected for this design, the input and output components are assigned to the PLC's input and output interfaces.
4. PLC Control System Software Design
Based on the control and process requirements of this design, and according to the action sequence of the hydraulic shearing machine and the tasks completed in each step, the work cycle can be obtained.
This hydraulic shearing machine's PLC control has two operating modes: manual and automatic. Manual mode is used for debugging, while automatic mode is used for continuous production. When the operating mode switch is set to manual, a specific process can be run via the corresponding button switch on the control panel. When the operating mode switch is set to automatic, the PLC software supports automatic work cycles and allows for manual intervention in case of unexpected malfunctions, including emergency shutdown.
To save input contacts and facilitate functional expansion as needed, the transmitting element for both manual and automatic operations is connected to the same input contact on the PLC. In automatic operation mode, manual operation can be disabled via the manual/automatic selector switch S and the PLC's internal auxiliary relay, which is controlled by software.
5. Design the state transition diagram and ladder diagram program for the PLC control system.
A state transition diagram is a graphical representation of the complete working process, function, and characteristics of a control system. It is an important tool for analyzing and designing control programs for circuit systems. The state transition diagram for this design is constructed according to the following steps.
1. Draw a state transition diagram according to the control requirements and processing technology of the hydraulic shearing machine.
2. Define state transition conditions on the drawn state transition diagram using PLC input points or other components.
3. According to the function table of electrical actuators provided by the electrical control system, draw electrical actuators that realize the control function of each state and action command on the state flowchart, and define these electrical actuators with the corresponding PLC output point numbers.
The corresponding ladder diagram program is derived from the state transition diagram. There is a one-to-one correspondence between the two. Please see Figure 1 for the ladder diagram program.
Figure 1. Ladder diagram program of PLC control system
6. Conclusion
In actual production, hydraulic devices are widely used in industrial production to improve productivity and reduce labor intensity, as they can perform repetitive actions without fatigue. As a new generation of industrial control devices, PLCs (Programmable Logic Controllers) are characterized by high development flexibility, simple wiring, convenient installation, and strong anti-interference capabilities, making them an ideal choice for controlling complex production equipment such as four-station combination machine tools. The adoption of PLCs reduces machine tool failure rates, saves significant maintenance costs, improves overall machine reliability, and ensures the accuracy requirements of workpieces.
