Abstract: CNC machine tools are high-precision, high-efficiency automated machine tools, and PLC -based electrical control systems represent a development trend in CNC machine tools. This paper discusses the design of an electrical control system for a CNC milling machine and explores methods to improve the safety, reliability, machining accuracy, and productivity of CNC machine tools, aiming to provide a reference for the further application of PLCs in CNC machine tools.
CNC machine tools are high-precision, high-efficiency automated machine tools with typical mechatronic systems. The application of PLCs in CNC machine tools is an inevitable trend in machine tool development, improving both the automation level and the machining accuracy and controllability. This project takes a CNC milling machine as an example to explore the electrical control technology and design methods of CNC machine tools.
1. Overview of CNC Systems
The electrical control system of a CNC machine tool consists of a data transmission device, a CNC system, a PLC (Programmable Logic Controller), and a spindle drive system. The data input device inputs instruction information and various application data into the CNC system. The CNC system is the central hub of the CNC machine tool; it decodes and processes all received function instructions, then systematically issues various necessary motion commands and control commands for various machine tool functions until the motion and function are completed. The PLC is the logic control center for all machine tool functions. It logically sequences various motion and function commands from the CNC, enabling accurate, coordinated, and safe operation; simultaneously, it transmits various information and operating statuses from the machine tool to the CNC, allowing the CNC to issue further control commands promptly and accurately, thus achieving control of the entire machine tool. PLC control software is mostly integrated into the CNC system, while PLC hardware in larger systems often adopts a distributed structure. The spindle drive system receives drive commands from the CNC, adjusts speed and torque (power) to output drive signals, and drives the main motor to rotate.
2. Electrical control system of CNC milling machine
The electrical control system of CNC milling machine is mainly implemented through the integration of PLC and CNC via software interface. Generally, the system assigns fixed storage addresses to various communication information between the two, and the system monitors the status of information at all addresses in real time. Based on the current status of each interface signal, it analyzes and judges, and makes further control commands to complete the control of motion or function.
2.1 System Overall Structure
The entire control system consists of two parts: a CNC computer numerical control system and a power supply cabinet. The CNC computer numerical control system is a dedicated numerical control device, composed of a CNC system, input/output interfaces, drive units, and actuators; it is the core of the control system for performing machining operations. The power supply cabinet consists of power circuits, control circuits, and a programmable logic controller (CNC). The system structure block diagram is shown in Figure 1.
Figure 1 Block diagram of electrical control system
2.2 System High-voltage Circuit
The system's high-voltage circuit consists of a power supply section, servo power supply, spindle power supply, cooling motor power supply, and a backup power supply (as shown in Figure 2). The high-voltage control system is located between the CNC device and the machine tool's mechanical and hydraulic components. Its main function is to receive signals from the CNC device for auxiliary operations such as spindle speed change, reversal, start/stop, tool selection and change, indexing and locking of the indexing table, workpiece clamping/unclamping, and coolant on/off. After necessary compilation, logical judgment, and power amplification, these signals directly drive the corresponding actuators (such as electrical, hydraulic, pneumatic, and mechanical components) to complete the actions specified in the instructions, thereby achieving fully automated operation of the CNC machine tool during machining. The power supply section uses an air switch selected according to the system's total power and current. The servo power supply powers the servo power module and is controlled by an AC control circuit. The spindle power supply powers the spindle motor and controls it. The cooling motor power supply is controlled by a PLC.
Figure 2 High-voltage circuit diagram
2.3 System Control Loop
The system control loop mainly consists of a power supply loop, an AC control loop, and a DC control loop.
1) The power supply circuit (as shown in Figure 3) mainly provides AC and DC power to the control section to ensure the system's safe power supply. In the power supply circuit, the high-voltage power is converted through a control transformer to obtain the 220V AC and 24V DC power required by the control circuit. If the system's CNC module and control panel fail to start normally after power-on, first check if the power supply is functioning correctly, and then check each interface circuit.
Figure 3 Power supply circuit diagram
2) The AC circuit (as shown in Figure 4) uses the energization and de-energization of contactors to control the energization and de-energization of the high-voltage circuit servo module, spindle motor, etc. In the AC circuit, PLC programming instructions are used to control the energization of intermediate relays, which in turn control the contactors. Therefore, the PLC plays a core role in this process.
Figure 4 High-voltage control circuit
The control circuits in the AC control loop, such as tool clamping, spindle cooling, spindle lubrication, and Z-axis brake, are also controlled by PLC control signals (as shown in Figure 5).
Figure 5 Spindle clamping control circuit
3) DC Control Circuit (as shown in Figure 6): Since intermediate relays are generally driven by a 24V power supply, they utilize PLC signals to control the intermediate relay group, thereby controlling the contactor operation sequence. 3. PLC Input/Output Control System: PLCs have significant advantages such as high reliability, strong anti-interference capability, small construction workload, convenient maintenance, small size, light weight, and low energy consumption. Using PLCs to control CNC machine tools is increasingly becoming a trend.
3.1 PLC Control Process of CNC Machine Tools
First, determine the PLC input and output signals, and identify which machine tool signals (such as signals from buttons, limit switches, relay contacts, and contactless switches) need to be input to the PLC, and which signals (such as relay coils, indicator lights, and other execution circuits) need to be output from the PLC to the CNC machine tool. This will allow you to calculate the number of PLC input and output lines and the allocation of I/O addresses.
Figure 6 DC control circuit
3.2 PLC Input/Output Signals
Figure 7 shows the input and output signals of the PLC system. The corresponding actions are controlled by allocating system input and output (IO) signals. Input signals include tool change, tool clamping, air pressure alarm, axis zeroing, axis positive and negative limit signals, spindle speed arrival signal, and external operation permission signal. Output signals are controlled according to the program, and the program can be modified to change the output signals or IO allocation as needed. Output signals include tool forward/reverse rotation, tool change position, spindle enable, cooling on, servo enable, servo power enable, and spindle tension/relaxation. The output signals can also be expanded.
Figure 7 PLC Input/Output Signals
Figure 8 shows the overall block diagram of the Huazhong CNC system design, which reflects the connection methods of the input and output interfaces.
Figure 8 Overall block diagram of the CNC system
4. Analysis of common faults in CNC systems
To prevent or avoid injury or damage to operators, the machine tool itself, and the workpiece during CNC machine tool processing, emergency stop and overtravel protection are typically required. The emergency stop button is used to immediately stop the CNC machine tool or cut off the main power supply to the power unit (such as the servo drive) in case of an emergency in the CNC system or machine tool. When the CNC system issues an automatic alarm, the emergency stop button must be pressed; after reviewing the alarm information and troubleshooting, the emergency stop button can be released to reset the system and restore normal operation. The hardware control circuit for emergency stop and overtravel protection of the CNC machine tool is shown in Figure 9.
Figure 9 Hardware control loop diagram for emergency stop and overtravel protection
If the machine tool remains in an emergency stop state and cannot be reset, the following are possible reasons: 1) Electrical issues, such as an open circuit in the emergency stop circuit, a damaged limit switch, or a damaged emergency stop button. 2) Incorrect system parameter settings, causing abnormal input/output of system signals or failure to meet reset conditions, resulting in an emergency stop fault; the PLC software may not be sending reset information to the system. Check the KA intermediate relay and the PLC program. 3) The information required for system reset specified in the PLC is not being met, such as servo power supply readiness and spindle drive readiness. Check the power module, power module wiring, and the servo power supply circuit breaker. 4) Errors in the PLC program.
5. Conclusion
The electrical control system designed in this project uses a PLC to control the forward and reverse rotation of the CNC milling machine tool, spindle clamping, and spindle cooling, thereby improving the flexibility and reliability of the machine tool. Through modifications to the mechanical components and electrical control system of the PLC-controlled CNC machine tool, productivity was increased, and automation of the CNC machine tool was achieved.
This article introduces the electrical control system of a CNC milling machine, illustrating the crucial role of a PLC in CNC systems. With the development of CNC technology, the logic processing capabilities of PLCs have become increasingly sophisticated. Therefore, the CNC module must work in concert with the PLC control module to maximize the PLC's precise control capabilities.
