【Technology】NC System

The NC system is the abbreviation of numerical control system.

A dedicated computer system that performs some or all of the numerical control functions and is equipped with an interface circuit and a servo drive device according to a control program stored in the computer memory.

One or more mechanical device motion controls are implemented by digital instructions consisting of numbers, words, and symbols.

It usually controls the mechanical quantity and switching quantity such as position, angle and speed.

Basic composition

There are many kinds of numerical control systems in the world, each with different forms, and their respective structures have their own characteristics.

These structural features are derived from the basic requirements of the initial design of the system and the engineering design ideas of hardware and software.

For different manufacturers, based on historical development factors and the influence of different factors that vary from place to place, the design ideas may also have their own advantages.

For example, in the 1990s, the Dynapath system in the United States adopted a small plate structure with small thermal deformation, which facilitated the replacement and flexible combination of boards.

The Japanese FANUC system tends to be a large-board structure, reducing the inter-board connectors, which makes it beneficial to the reliability of the system.

However, no matter which system, their basic principles and composition are very similar.

Generally, the entire CNC system consists of three major components, namely 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.

The interpolation operation is performed according to the machining workpiece program, and a control command is issued to the servo drive system.

The measuring system detects the position and speed of the linear and rotary motion of the machine and feeds it back to the control system and the servo drive system to correct the control commands.

The servo drive system compares and controls the control commands from the control system and the feedback information of the measurement system, controls the PWM current to drive the servo motor, and the servo motor drives the machine to move as required.

These three parts are organically combined to form a complete closed-loop controlled CNC system.

The control system hardware has a human interaction function and has a dedicated computer including fieldbus interface input and output capabilities.

The servo drive system mainly includes servo drive and motor.

The position measuring system is mainly an incremental displacement encoder using long grating or circular grating.

CNC system hardware structure

From the perspective of hardware structure, the numerical control system can be divided into two stages for six generations so far.

The first phase is the numerical logic control phase.

Its characteristic is that it does not have a CPU, and relies on numerical logic to realize the numerical calculation and logic control required for numerical control.

Including the first generation is the electronic tube numerical control system, the second generation is the transistor numerical control system, and the third generation is the integrated circuit numerical control system.

The second stage is the computer control stage, which is characterized by direct introduction of computer control and relying on software calculation to complete the main functions of the CNC.

Including the fourth generation is a small computer numerical control system, the fifth generation is a microcomputer numerical control system, and the sixth generation is a PC numerical control system.

Since the beginning of the 1990s, the popularization of computer applications for PC architecture, the rapid advancement of computer CPUs and peripheral storage, display, and communication technologies under the PC architecture, and the significant reduction in manufacturing costs have led to the structure system which makes the PC-based CNC systems become the increasingly popular CNC system.

The development of the PC numerical control system has formed a “NC+PC” transitional structure, which retains the traditional NC hardware structure and only uses the PC as the HMI.

Representative products include FANUC’s 160i, 180i, 310i, 840D, etc.

There is also a class that will focus on the implementation of CNC functions in the form of motion control cards, and develop PC control systems by augmenting NC control boards (such as DSP-based motion control cards).

Typical representatives are the PMAC-NC system constructed by the US DELTA TAU Company using the PMAC multi-axis motion control card.

Another more revolutionary architecture is to use all the hardware and software resources of the PC platform, only to increase the fieldbus interface necessary for communication with servo drives and I/O devices, thus achieving a very compact hardware architecture.

CNC system software structure

(1) Input data processing program

It receives the input part processing program, decodes the processing instructions and data represented by the standard code, and processes the data in a prescribed format.

Some systems also need to perform compensation calculations or pre-calculate for interpolation operations and speed control.

Typically, input data processing programs include input, decoding, and data processing.

(2) Interpolation calculation program

The CNC system performs the interpolation and densification operation of the intermediate output points according to the data provided in the workpiece machining program, such as the type of the curve, the starting point, the end point, and the predetermined speed.

The above-mentioned densification calculation must not only strictly follow the requirements of a given trajectory but also meet the requirements of smooth motion acceleration and deceleration of the mechanical system.

According to the calculation result, a position command for forming a feed motion is issued to each coordinate axis.

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 in the servo system.

The output current drives the motor to drive the table or the tool to perform corresponding movements to complete the machining tasks specified by the program.

The CNC system is a typical real-time control method that performs interpolation while performing calculations.

(3) Management procedures

The management program is responsible for scheduling management of various programs that are serviced by the processing process, such as data input, data processing, and interpolation operations.

The hypervisor also handles interrupts caused by panel commands, clock signals, fault signals, and so on.

Under the PC hardware structure, the hypervisor is usually implemented with the support of a real-time operating system.

(4) Diagnostic procedures

The function of the diagnostic program is to detect the failure of the system in time during the running of the program and indicate the type of fault.

It is also possible to check whether the main components of the system (CPU, memory, interface, switch, servo system, etc.) function properly before operation or after a fault occurs, and indicate the location where the fault occurred.