1. Discussion on the Application of Programmable Logic Controllers in CNC Machine Tool Systems
In the actual design and production of machine tools, the selection of positioning control devices is particularly important to improve the machining accuracy of CNC machine tools. Yonghong FBs series PLCs offer more precise NC positioning than other PLCs, and their program design and debugging are quite convenient. This article proposes a method for using Yonghong PLC's NC positioning control to achieve the control functions of a machine tool's CNC system, which meets control requirements and is practically feasible in actual operation. The overall control system has significant advantages such as clear program design, simple and practical hardware circuitry, high reliability, strong anti-interference capabilities, and a good performance-price ratio. Its hardware and software design concepts can serve as a reference for the design and modification of related CNC machine tools in industrial and mining enterprises.
2. Composition and Working Process of CNC Machine Tools
This example of a CNC machine tool consists of input/output devices, a CNC device, a programmable logic controller (PLC), a servo system, a detection feedback device, and the machine tool host.
Input devices transmit various machining information to the computer. In the early days of CNC machine tools, punched paper tape was the input device, but it is now largely obsolete; currently, keyboards, disks, etc., greatly facilitate information input. Output refers to the output of internal working parameters (including original parameters under normal and ideal working conditions, fault diagnosis parameters, etc.). These parameters are typically output and recorded when the machine tool first starts operating. After a period of operation, the output is compared with the original data to help determine if the machine tool is maintaining normal operation. The CNC device is the core and driving force of the CNC machine tool, completing all machining data processing and calculations, ultimately controlling the various functions of the CNC machine tool. It includes microcomputer circuitry, various interface circuits, a CRT monitor, and corresponding software. The programmable logic controller (PLC) controls the spindle unit, processing speed commands in the program to control the spindle speed; it manages the tool magazine, including automatic tool exchange, tool selection, cumulative tool usage count, remaining tool life, and tool sharpening count; it controls spindle forward/reverse rotation, stopping, precise stopping, coolant switching, chuck clamping/unclamping, and robot tool loading/unloading; it also controls external machine tool switches (limit switches, pressure switches, temperature switches, etc.) and output signals (tool magazine, robot, rotary table, etc.). The detection feedback device consists of detection elements and corresponding circuits, primarily detecting speed and displacement and feeding the information back to the CNC unit to achieve closed-loop control and ensure the machining accuracy of the CNC machine tool.
The preparation process for CNC machining is complex and involves many steps, including understanding the part's structure, process analysis, developing a process plan, programming the machining program, selecting tooling, and determining its usage. Machine tool adjustment mainly includes tool naming, loading the tool magazine, workpiece installation, tool setting, measuring tool positions, and checking the status of various machine parts. Program debugging primarily involves checking and adjusting the program's logic and design rationality. Trial cutting is a dynamic evaluation of the part's machining design; each step requires evaluation of the results from the previous step before proceeding to the next. Only after a successful trial cut can the part be formally machined, and the machined part undergoes quality inspection. The first three steps involve idle time; to improve efficiency, shorter idle time is desirable for optimal machine tool utilization. This indicator directly affects the evaluation of machine tool utilization (i.e., machine tool operating rate).
3. Several problems that need to be solved in CNC machine tool systems
Machine tools consist of both mechanical and electrical components. When designing the overall system, the implementation plan for various machine tool functions should be considered from both electromechanical perspectives. The mechanical requirements of CNC machine tools and the functions of CNC systems are complex, so a harmonious integration of electromechanical and electrical systems is essential to leverage strengths and mitigate weaknesses. The selection of components, assembly, programming, and operation of the machine tool control system should be reasonable, and accuracy and stability must meet usage requirements. To facilitate debugging and maintenance, manual functions are provided for all operations, such as manual fast and slow movement of each axis, high and low speed rotation of the spindle, and switching on/off the cutting fluid and lubrication. PLCs perform sequential or time-based actions according to logical conditions. They also include control functions that perform interlocking protection actions based on logical relationships, independent of sequence or timing. PLCs have become the main product replacing relay circuits and performing sequential control, and their application in the electrical control of machine tools is widespread. Currently, programmable logic controllers (PLCs) are widely used in industrial control systems such as CNC machine tools. The control system of a CNC machine tool can be divided into two parts: digital control and sequential control. Digital control includes continuous control of the position of each axis, while sequential control includes control of auxiliary actions such as spindle forward/reverse rotation, start/stop, tool changing, chuck clamping and loosening, cooling, tailstock operation, and chip removal. Modern CNC machine tools use PLCs instead of relay control to complete logic control, making the machine tool structure more compact, its functions richer, and its response speed and reliability greatly improved.
Based on the absorption of advanced foreign technologies, the CK160 CNC lathe is a small-sized, full-function machine. This CNC lathe adopts the POWERMATE0 CNC system from FANUC Corporation of Japan, equipped with a fully digital AC servo device, and the spindle is controlled by an imported frequency converter. The CNC system has a built-in PLC, providing powerful functions and instructions, making it convenient, flexible, and easy for users to understand.
Taking the CK160 as an example, we will now explain the application of PLC in CNC machine tools.
Design of PLC CNC System Control Principle for 2 Lathe
2.1 Lathe operation requirements
Lathes are generally used for machining rotating surfaces and threads. Their operations typically include rapid traverse in the X and Z axes, working feed, and rapid retraction. During machining, they should be able to switch between automatic and manual modes, and perform operations such as turning external diameters and threads; they should also be able to perform intermediate steps.
2.2 Problems that need to be solved by PLC numerical control systems
The operation of a lathe is quite complex, while PLCs are generally only suitable for sequential control of actions. To use a PLC to control lathe movements, three problems must be solved:
1) How to generate signals to drive the servo mechanism and coordinate the X and Z direction movements;
2) How to change the feed system speed;
3) How to achieve internal transmission and change the thread lead when turning threads.
These problems can be solved by combining the PLC, its control module, and the corresponding actuators.
2.3 Control Principle of CNC System
The CNC retrofit of a conventional lathe involves converting the tool post, X, and Z axis feeds to CNC control. Based on the retrofit characteristics, stepper motors are used as servo components, and an open-loop control system is sufficient to meet the requirements. The Z-axis pulse equivalent is 0.01mm, and the X-axis pulse equivalent is 0.005mm. A transistor-output PLC is selected to drive the stepper motors. The pulse signals are generated by programming, and different frequency pulses are generated through the program to achieve speed variation. X and Z axis movements can be manually operated or automatically controlled by the program. The pulse signal for thread cutting is generated by a spindle pulse generator, connected to the PLC input through an AND gate circuit, and then frequency-converted by the PLC program to obtain the pulse with the required lead. Tool post indexing, tool feed, and retraction can be controlled manually or automatically by the program.
3. Feasibility of CNC machine tool retrofit
Converting a conventional machine tool into a CNC machine tool refers to making certain modifications to the mechanical structure of a conventional machine tool and adding a CNC device, thereby enabling the conventional machine tool to perform CNC machining.
The main contents of CNC retrofitting of machine tools are as follows:
One approach is to restore the original functionality. This involves diagnosing and restoring the faulty parts of the machine tool.
The second is NC conversion. This involves adding a digital display device or a numerical control system to a regular machine tool to transform it into an NC machine tool or CNC machine tool.
Thirdly, refurbishment. To improve precision, efficiency, and automation, the mechanical and electrical components are refurbished, the mechanical parts are reassembled and processed to restore their original precision, and the CNC systems that do not meet production requirements are updated with the latest CNC technology.
Fourthly, technological upgrading or innovation. This involves large-scale technological upgrading or innovation based on existing technology to improve performance or grade, or to utilize new processes or technologies. This results in a significant increase in level and grade.
4 Programmable Logic Controller Selection
Based on the motion cycle diagram and technical characteristics of the D-U3710 combination machine tool, the Toshiba EX series programmable controller was selected. Compared with other types of programmable controllers, the Toshiba EX series programmable controller is smaller, consumes less power, has a shorter setup time, is resistant to high current surges, and has extremely high reliability. In particular, it employs a unique graphical LCD programmable reader/writer, allowing for clear and intuitive monitoring and modification of the program on its display. System analysis shows that the D-3701 combination machine tool has 24 input points and 19 output points. Therefore, the basic unit EX-40H of the Toshiba EX series programmable controller, plus the expansion unit EX20, was selected. The EX-40H+EX20 combination has 36 input points and 24 output points, exceeding the system's required I/O count, making the use of EX-40H+EX20 feasible.
After determining the required number of I/Os for the system, it is necessary to further determine the component numbers for the input and output quantities. The component numbers of the input/output relays should match the terminal numbers connected to their corresponding I/Os. Completing this step prepares the material for drawing the hardware wiring diagram. During this process, an address allocation table should be created, indicating the name, code, and assigned component number of each signal. If necessary, the valid states of the signals should also be listed, such as whether they are active on the rising or falling edge, high or low level, etc. For switch input signals, it should also be listed whether they are normally open or normally closed contacts, and under what conditions the contacts are connected or disconnected. After assigning component numbers to the input and output quantities, the next step is to design the hardware wiring diagram of the programmable controller and the schematic diagrams, wiring diagrams, and installation diagrams of other electrical components.
5PLC Working Process
A PLC is a digital electronic system designed for industrial applications. It uses a programmable memory to store instructions for performing logical operations, sequential control, timing, counting, and arithmetic operations, and controls the production processes of various machines through digital and analog inputs and outputs.
The working process of a PLC is the process by which the CPU scans the user program cyclically and executes it sequentially: the execution of the user program is mainly carried out in three stages.
1) Input sampling sequentially reads the state of all input signals in a scanning manner and stores this state in the input image register. During the program execution phase and the output refresh phase, the contents of the input image register do not change with the actual signal changes.
2) During the program execution phase, the program scans each instruction in a top-to-bottom and left-to-right order, reads relevant data from the input image register and the output image register, performs corresponding calculations, and stores the calculation results back into the output image register.
3) Output refresh: After all instructions have been executed, the status (on/off) of all output relays in the output image register is transferred to the output latch during the output refresh phase and output in a certain way to drive the external load.
Information exchange between 6PLC and external devices of CNC machine tools
The information exchange between the PLC, the system, and the machine tool includes the following four parts:
1) Machine tool to PLC
The switch signals from the machine tool side are sent to the PLC through the PLC input interface. Except for a very few signals, the meaning of most signals and the PLC address (X address) they occupy can be defined by the PLC programmer.
2) PLC to machine tool
The PLC controls the machine tool through signals...
Programmable Logic Controllers (PLCs) are widely used in control systems; without them, a control system is like a body without a soul. To enhance understanding of PLCs, this article will explore why they can replace relay control technology. If you are interested in PLCs, please continue reading.
In the industrial automation industry, programmable logic controllers (PLCs) serve as the foundation for most automation technologies, bringing about unprecedented and profound changes to industrial control systems. Compared to traditional relay-based industrial control systems, PLC-based industrial control systems offer unparalleled advantages in operation, control, efficiency, and precision. While relay control equipment used in industrial control systems has not been completely eliminated, the emergence of PLCs has fundamentally altered the design philosophy of industrial control system designers.
The application of relays in various fields of home and industrial control is becoming increasingly widespread. Compared to previous products, thanks to advancements in modern technology, today's relays are much safer and more reliable. However, this also brings corresponding problems. Most control relays are prone to failure under prolonged wear and tear or fatigue conditions. Furthermore, relay contacts are susceptible to arcing, which can sometimes melt and cause malfunctions, leading to serious consequences. Moreover, for devices using hundreds of relays, the control box is inevitably large and heavy. Under full load operation, large relays generate significant heat and noise, while also consuming substantial amounts of energy. In addition, relay control systems require manual wiring and installation; even simple modifications require considerable time, labor, and resources for modification, installation, and adjustment.
Programmable logic controllers (PLCs) are known for their small size and powerful functions. They can not only easily perform functions such as sequence logic, motion control, timing control, counting control, digital operation and data processing, but also establish connections with digital and analog quantities of various production machines through input/output interfaces.
1. Process Control
Process control is the most fundamental and widely used application of PLCs. Process control refers to the automatic sequential operation of various actuators in a production process according to control signals, following the order of the process flow. With advantages such as flexible programming design, high speed, high reliability, low cost, and convenient maintenance, it can completely replace traditional relay and contactor control systems in single-unit control, multi-unit control, and production process control. It mainly controls the operation of mechanical moving parts based on operation buttons, limit switches, other field-issued command signals, and sensor signals, realizing automated production line control. Typical applications include escalator control, automatic opening and closing of solenoid valves on pipelines, and sequential start-up of belt conveyors. For example, our factory's raw material mixing system utilizes the sequential control function of a PLC.
2. Control closed-loop process
In the past, process control simulation was achieved using PID controllers composed of hardware circuits. Now, PLC control systems can be used with selected simulation control modules. The system's functions are completed by software, and its accuracy is determined by the number of bits, which is not affected by the software. Therefore, it is more reliable and easier to implement complex and advanced control. It can simultaneously control multiple control loops and multiple control parameters, such as temperature, flow rate, pressure, and speed.
3. Control the movement position
PLCs can support the control and management of CNC machine tools. In the machining industry, programmable controllers and computer numerical control (CNC) are integrated to complete the position control of machine tools. Their function is to accept input information from input devices, process and calculate it, and send corresponding pulses to the driver through stepper motors or servo motors, so that the machine can move according to a predetermined trajectory to complete the control of multi-axis servo motors.
4. Monitor and manage the production process
PLCs can connect to peripheral devices such as display terminals and printers via communication interfaces. A display is an intelligent device containing a microchip as a human-machine interface (HMI). Combined with a PLC, it can replace multiple control buttons, selector switches, indicator lights, and simulated production process screens on the control panel, as well as multiple intermediate relays and terminal blocks within the control panel. All operations can be performed through the operating software on the display. The PLC can collect and process production process data on the display screen and can display parameters in binary, decimal, hexadecimal, and ASCII characters. The color changes of icons reflect the operating status of field equipment, such as valve opening/closing, motor start/stop, and position switch status. PID loop control uses a comprehensive method, including data and bar graphs, to reflect changes in the production process. Operators can adjust parameters through parameter settings, search for data records through data queries, and save relevant production data by printing, facilitating future production management and process data analysis.
