Foreword
Modular machine tools are high-efficiency, automated, specialized machining equipment designed for specific workpieces and processes. These machines typically allow multiple tools to operate simultaneously and feature automatic cycle functionality. Modular machine tools evolved from general-purpose and special-purpose machine tools with the continuous development of the machinery industry. General-purpose machine tools generally use a single tool for machining, resulting in low automation, long auxiliary time, and low production efficiency. However, they can be readjusted to adapt to changes in the workpiece. Special-purpose machine tools can perform multi-tool cutting, have a higher degree of automation, simpler structure, and higher production efficiency. However, special-purpose machine tools have long design and manufacturing cycles, high costs, and lower reliability. Since special-purpose machine tools are designed for a specific workpiece and a specific process, they become unusable when the product is improved or the workpiece's structure and dimensions change slightly. Modular machine tools were developed by combining the advantages of general-purpose and special-purpose machine tools.
Modular machine tools are typically composed of standard general-purpose components and machining-specific components. The power unit is driven by an electric motor or a hydraulic system, and the electrical system controls the automatic cyclic operation. They are typical mechatronic or mechatronic-hydraulic integrated automated machining equipment. Common modular machine tools include standard general-purpose components such as powered slides, various machining power heads, and rotary worktables, which can be driven by electric motors or hydraulic systems. When these standard general-purpose power components are combined to form a modular machine tool, the machine tool's control circuit can be constructed by combining the control circuits of each power component through specific interconnecting circuits.
The control of a multi-powered machine tool typically involves three aspects: first, the jogging and resetting control of the power components; second, the semi-automatic cycle control of the power components; and third, the fully automatic batch processing cycle control. The machine tool used in this paper is a four-station machine tool, consisting of four slides, each carrying a machining power head, forming four machining stations. In addition to the four machining stations, it also includes fixtures, loading/unloading robots and feeders, four auxiliary devices, and a cooling and hydraulic system, totaling 14 parts.
1. Brief Description of Hydraulic System Characteristics
With the continuous development and advancement of hydraulic technology and the expanding application areas and scope, the requirements for system flexibility and various performance aspects are becoming increasingly stringent. Traditional system designs that focus on completing predetermined actuator cycles and are limited by static system performance are far from meeting these demands. Therefore, it is essential for modern hydraulic system design researchers to study the dynamic characteristics of systems, understand and master the dynamic working characteristics and parameter changes of hydraulic systems, in order to improve the system's response characteristics, control accuracy, and operational reliability.
The dynamic characteristics of a hydraulic system refer to its properties as it moves from a previous equilibrium state to a new one. These characteristics are primarily caused by changes in the transmission and control systems, as well as external disturbances. During this process, the performance of each system parameter over time determines the quality of the system's dynamic characteristics. The dynamic characteristics mainly manifest as stability (the instantaneous peak and fluctuation of pressure in the system) and transient response quality (the response quality and speed of the actuators and control mechanisms).
The main research methods for the dynamic characteristics of hydraulic systems include transfer function analysis, simulation, experimental research, and digital simulation. Digital simulation utilizes computer technology to study the dynamic characteristics of hydraulic systems. First, a digital model of the hydraulic system's dynamic process—the state equations—is established. Then, the time-domain solutions of the main variables in the system during the dynamic process are obtained on a computer. This method is applicable to both linear and nonlinear systems, and can simulate the changes in various system parameters under the action of input functions. This provides a direct and comprehensive understanding of the system's dynamic process, allowing researchers to predict the dynamic performance of the hydraulic system during the design phase. This enables timely verification and improvement of the design results, ensuring the system's performance and reliability. It offers advantages such as accuracy, adaptability, short cycle time, and low cost.
2. System Principles and Modeling
2.1 Modeling of Electro-hydraulic Position Servo System
Establishing the transfer function of an electro-hydraulic servo system requires establishing functional expressions for components such as the servo valve and hydraulic cylinder based on physical laws such as the continuity equation and Newton's laws, and then solving these expressions simultaneously to derive the system's transfer function. Specifically, this involves establishing the servo valve equation, the servo valve flow equation, the continuity equation, and the force balance equation.
2.1.1 Servo Valve Equation
Servo valves are devices with complex, high-order nonlinear characteristics. In practice, servo valves can often be simplified and equivalent to a first-order system (low frequency) or a second-order system (high frequency):
Figure 1. Simulink dynamic simulation model of the system
3. Hardware Design
3.1 Structural Selection
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.
3.2 I/O Point Selection Principles
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.
Based on 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 points on the main unit. This is sufficient to meet the requirements of 42 inputs and 27 outputs, with a certain margin of safety.
3.3 Input/Output Design
An input/output signal address table is a table that lists inputs and outputs, assigning them corresponding addresses and names for use during software programming and system debugging. As this design demonstrates, buttons, limit switches, and detection elements in the control circuit are all input devices of the PLC. The PLC's output control objects are mainly those in the control circuit.
The main actuators in this design are contactors, solenoid valves, and indicator lights. Specifically: 12 limit switches; 24 pushbuttons; 1 selector switch; 5 detection elements; 16 solenoid valves; 6 contactors; and 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 operation panel design
The control system's operation panel is composed of master control elements that issue control commands to the PLC control system. In this design, there are a total of 42 input elements: 24 buttons (SB), 5 detection elements (YJ), 12 limit switches (SQ), and 1 selector switch. Based on the control requirements for the machine tool's operating mode, the panel should include a selector switch (1SA) and a pre-stop button. Since the corresponding buttons issue control commands to drive the corresponding components of the machine tool in manual adjustment mode, buttons 5SB-24SB should be provided on the panel. For the control of starting, stopping, and handling lubrication faults of the machine tool, a start button (2SB), a total stop button (1SB), and a lubrication fault clearing button (4SB) should be provided on the operation panel. Other input elements are detection elements and are not included in the operation panel. Based on the above summary, the control system operation panel is shown in Figure 2 below.
Figure 2 Control system operation panel diagram
5. Conclusion
As a new generation of industrial control devices, PLCs (Programmable Logic Controllers) offer advantages such as high development flexibility, simple wiring, convenient installation, and strong anti-interference capabilities, making them an ideal choice for controlling complex production equipment like 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 precision requirements of workpieces. PLCs hold a crucial position in machine tool CNC system design. Long-term operation of this machine tool has demonstrated that the entire system is rationally designed, possesses high control precision, operates reliably, improves the level of production automation, and reduces the labor intensity of operators.
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