NC System

The NC system is the abbreviation of numerical control system.

NC System

A dedicated computer system performs numerical control functions and is equipped with an interface circuit and a servo drive device. The control program is stored in the computer’s memory.

The motion of one or more mechanical devices is controlled by digital instructions consisting of numbers, words, and symbols.

The system typically controls mechanical and switching quantities, such as position, angle, and speed.

Basic composition

There are various types of numerical control systems in the world, each with its own unique form and structure. These structural features are determined by the original design requirements and the hardware and software engineering design concepts. The design ideas may also vary among manufacturers based on historical development factors and local influences.

For instance, the Dynapath system in the United States in the 1990s had a compact plate structure with low thermal deformation, allowing for easy replacement and flexible board combination. Meanwhile, the Japanese FANUC system is designed with a large-board structure that reduces inter-board connectors, enhancing system reliability.

Despite the differences, all numerical control systems have similar basic principles and components. Typically, the CNC system consists of three main parts: the control system, the servo system, and the position measurement system.

The control system hardware is a dedicated computer system with input and output capabilities. It performs interpolation operations based on the machining program and sends control commands to the servo drive system. The measurement system detects the position and speed of the machine’s linear and rotary motion and sends feedback to the control system and servo drive system to correct the control commands. The servo drive system then compares and controls the control commands and feedback information, driving the servo motor to move the machine as required.

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The control system hardware also has human interaction capabilities and includes a dedicated computer and fieldbus interface input and output. The servo drive system comprises the servo drive and motor. The position measurement system mainly uses incremental displacement encoders with long or circular gratings.

These three components are integrated to form a complete closed-loop controlled CNC system.

CNC system hardware structure

From a hardware structure perspective, the numerical control system has evolved through six generations so far, which can be divided into two phases.

The first phase is the numerical logic control phase, where the system relies on numerical logic to perform the numerical calculations and logic controls required for numerical control, without a central processing unit (CPU). This phase includes the first-generation electronic tube numerical control system, the second-generation transistor numerical control system, and the third-generation integrated circuit numerical control system.

The second phase is the computer control stage, which introduces computer control and relies on software calculations to complete the main functions of the CNC. This phase includes the fourth-generation small computer numerical control system, the fifth-generation microcomputer numerical control system, and the sixth-generation PC numerical control system.

The widespread adoption of computer applications for PC architecture, the rapid advancement of computer CPUs and peripheral technologies, and the significant reduction in manufacturing costs have made PC-based CNC systems the most popular CNC systems since the 1990s.

The development of PC numerical control systems has resulted in a “NC+PC” transitional structure, which retains the traditional NC hardware structure and uses the PC only as the human-machine interface (HMI). Representative products include FANUC’s 160i, 180i, 310i, 840D, among others.

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Another approach focuses on implementing CNC functions in the form of motion control cards and augmenting NC control boards with PC control systems. A typical example is the PMAC-NC system developed by the US DELTA TAU Company using the PMAC multi-axis motion control card.

A more revolutionary architecture is to use the full hardware and software resources of the PC platform, and only add the fieldbus interface necessary for communication with servo drives and I/O devices, resulting in a compact hardware architecture.

CNC system software structure

(1) Input Data Processing Program

The input data processing program receives and processes the input part of the processing program. It decodes the processing instructions and data represented by standard code and processes the data in a prescribed format. Some systems also perform compensation calculations or pre-calculations for interpolation operations and speed control. The input data processing program typically includes input, decoding, and data processing.

(2) Interpolation Calculation Program

The CNC system performs the interpolation and densification operations of intermediate output points based on the data provided in the machining program, such as the type of curve, the starting and ending points, and the predetermined speed. The densification calculation must not only follow the given trajectory requirements but also meet the requirements for smooth motion acceleration and deceleration of the mechanical system.

The interpolation calculation program issues a position command for each coordinate axis to form a feed motion based on the calculation result. This process is called interpolation. The position command for calculating the feed motion is controlled by the position closed loop, speed loop, and current loop in the CNC or servo system. The output current drives the motor to complete the machining tasks specified by the program.

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The CNC system is a real-time control method that performs interpolation calculations in real-time.

(3) Management Procedures

The management program is responsible for scheduling and managing various programs, such as data input, data processing, and interpolation operations. It also handles interrupts caused by panel commands, clock signals, fault signals, and more. Under the PC hardware structure, the management program is typically implemented with the support of a real-time operating system.

(4) Diagnostic Procedures

The diagnostic program detects system failures in real-time during program execution and indicates the type of fault. It can also check the proper functioning of the main components of the system (CPU, memory, interface, switch, servo system, etc.) before operation or after a fault occurs and indicate the location of the fault.

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