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Small, medium and large CNC system





Small, medium and large CNC system
A numerical control system is a special computer system that performs some or all of the numerical control functions according to the control programs stored in the computer memory, and is equipped with interface circuits and servo drive devices. The movement control of one or more mechanical equipment is realized by using digital instructions composed of numbers, characters, and symbols. The mechanical quantities and switching values controlled are usually position, angle, speed, and so on.
brief introduction
The abbreviation of digital control system, known as Numerical Control System in English, was developed and evolved in parallel with computers in the early days. It is used to control automatic processing equipment, and is composed of electronic tubes, relays, and other hardware to form a dedicated controller with computing power. It is called Hardware Numerical Control (Hard NC). After the 1970s, separated hardware and electronic components were gradually replaced by more integrated computer processors, known as computer numerical control systems.
Computerized numerical control (CNC) systems are systems that use computers to control machining functions and achieve numerical control. CNC system is a dedicated computer system that performs some or all numerical control functions based on control programs stored in computer memory, and is equipped with interface circuits and servo drive devices for controlling automatic processing equipment.
The CNC system consists of numerical control program storage devices (from early paper tapes to magnetic rings, to magnetic tapes, magnetic disks, and general-purpose hard disks for computers), computer control hosts (from specialized computers to computers with PC architecture), programmable logic controllers (PLC), spindle drive devices, and feed (servo) drive devices (including detection devices).
Due to the gradual use of general-purpose computers, numerical control systems have increasingly taken on the color of software. PLC has been used instead of traditional machine tool electrical logic control devices, making the system smaller, more flexible, versatile, and reliable. It is easy to implement complex numerical control functions, and is also convenient to use and maintain. It also has the function of connecting to the network and conducting remote communication.
software structure 
(1) Input data processing program
It receives the input part processing program, decodes and processes the processing instructions and data represented by standard codes, and stores them in a prescribed format. Some systems also require compensation calculations, or pre calculations for interpolation operations and speed control. Generally, input data processing programs include input, decoding, and data processing.
(2) Interpolation calculation program
The CNC system performs interpolation densification operations for intermediate output points based on the data provided in the workpiece processing program, such as the type, starting point, end point, and predetermined speed of the curve. The above densification calculation should not only strictly comply with the given trajectory requirements but also meet the requirements for acceleration and deceleration of the smooth motion of the mechanical system. According to the calculation results, position commands for forming feed motions are issued to each coordinate axis. This process is called interpolation. The calculated position command for the feed motion is controlled and adjusted through the position closed-loop, speed loop, and current loop in the CNC or servo system. The output current drives the motor to drive the workbench or tool to perform corresponding motions, completing the processing tasks specified in the program.
CNC system is a typical real-time control method, which performs both interpolation and processing.
(3) Management Procedure
The management program is responsible for scheduling and managing various programs that serve the processing process, such as data input, data processing, and interpolation operations. The management program also handles interrupts caused by panel commands, clock signals, fault signals, etc. In a PC based hardware architecture, management programs are typically implemented with the support of real-time operating systems.
(4) Diagnostic Procedures
The function of a diagnostic program is to promptly identify system faults during program operation and indicate the type of fault. You can also check whether the functions of the main components of the system (CPU, memory, interface, switch, servo system, etc.) are normal before operation or after a fault occurs, and indicate the location of the fault.
hardware structure 
From the perspective of hardware structure, CNC systems can be divided into two stages up to now and a total of six generations. The first stage is the numerical logic control stage, which is characterized by not having a CPU and relying on numerical logic to achieve the numerical calculation and logical control required for CNC, including the first generation of electronic tube CNC systems, the second generation of transistor CNC systems, and the third generation of integrated circuit CNC systems; The second stage is the computer control stage, which is characterized by the direct introduction of computer control, relying on software calculations to complete the main functions of numerical control, including the fourth generation of small computer numerical control systems, the fifth generation of microcomputer numerical control systems, and the sixth generation of PC numerical control systems.
Since the 1990s, the popularization of PC based computer applications, the rapid progress in computer CPU and peripheral storage, display, and communication technologies under PC based architectures, and the significant reduction in manufacturing costs have led to the gradual emergence of PC based CNC systems as mainstream CNC system architectures. The development of PC numerical control systems has formed a "NC+PC" transitional structure, which retains the traditional NC hardware structure and only uses the PC as an HMI. Representative products include FANUC's 160i, 180i, 310i, 840D, etc. There is also a category that focuses on the realization of numerical control functions in the form of motion control cards, and develops PC numerical control systems by expanding the number of NC control cards (such as motion control cards based on DSP). The typical representative is the PMAC-NC system constructed by DELTA TAU Company of the United States using a PMAC multi-axis motion control card. Another more revolutionary architecture is to fully utilize the software and hardware resources of the PC platform, adding only the field bus interfaces necessary for communication with servo drives and I/O devices, thereby achieving a very concise hardware architecture.
important factor
The functional applicability of CNC systems is undoubtedly an important limiting factor for the design and selection of CNC machine tools. The following factors are important factors that must be considered in selecting a numerical control system.
(1) Driving ability
The power range and matching motor range of the servo are also different in the solutions of different CNC suppliers. First of all, preliminary screening should be conducted based on the type and power range of the motor that can be matched. In particular, it is important to note whether the CNC machine tool scheme includes torque motors, linear motors, whether the electric spindle is a synchronous or asynchronous electric spindle, the rated and overload current requirements of the above motors, and the maximum rotational speed requirements of the electric spindle.
(2) Full closed-loop demand and dual drive demand
CNC machine tools, especially large and heavy CNC machine tools, mostly have full closed-loop and dual drive requirements. In the full closed-loop control scheme, it is necessary to choose between distance coded gratings, absolute value gratings, and ordinary incremental gratings. At the same time, the CNC system should also support corresponding feedback signal access.
(3) Five axis control requirements
The 5-axis machine tool needs to specify whether it requires 5-axis linkage or only 5-sided machining, and the corresponding selection of CNC system functions is also different. For example, for the processing of five-sided boxes, RTCP is usually not required, and there is a large choice. At the same time, the NC system supplier must carefully consider the export license, after-sales service, and technical support that may be involved in the five-axis function.
(4) Production system requirements
Networked support for CNC systems has become a necessary condition for production system integration. For CNC machines that are to be incorporated into production systems with a high degree of automation, it is necessary to clarify that the CNC system has corresponding access solutions, including direct access to advanced CNC systems through PLC input and output points, and opening up internal data of the CNC system to users in accordance with the OPC standard. In addition, for production systems, automated online workpiece detection and tool detection are also necessary functions to support.

Copyright: NIIC (suzhou) Su ICP Bei No. 2020061042-1

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