Numerical control system is the abbreviation of digital control system. It executes part or all of the numerical control functions according to the control program stored in the computer memory, and is equipped with a dedicated computer system with an interface circuit and a servo drive device. Through the use of digital instructions composed of numbers, characters and symbols to realize the action control of one or more mechanical equipment, it usually controls the position, angle, speed and other mechanical quantities and switch quantities.
Computerized numerical control, abbreviated as CNC, is a system that uses a computer to control machining functions and realize numerical control. The CNC system performs part or all of the numerical control functions according to the control program stored in the computer memory, and is equipped with an interface circuit and a servo drive device to control a dedicated computer system for automatic machining equipment.
The CNC system consists of a numerical control program storage device (from the early paper tape to the magnetic ring, to the magnetic tape, the disk to the general hard disk of the computer), the computer control host (evolving from the special computer to the computer of the PC architecture), and the programmable logic controller ( PLC), spindle drive device and feed (servo) drive device (including detection device).
Due to the gradual use of general-purpose computers, the numerical control system has become more software-based, and PLC is used to replace the traditional machine tool electrical logic control device, making the system smaller, its flexibility, versatility, and reliability are better, and it is easy to realize complexity. The numerical control function of, it is convenient to use and maintain, and has the function of connecting to the network and carrying out remote communication.
The Basic Composition Of Cnc System
There are many types of CNC systems in the world, with different forms, and their composition and structure have their own characteristics. These structural features come 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 complex factors that vary from place to place, they may also have their own merits in design ideas.
For example, in the 1990s, the Dynapath system in the United States adopted a small plate structure with small thermal deformation, which is convenient for plate replacement and flexible combination, while the Japanese FANUC system tends to be a large plate structure, reducing inter-board connectors, which is conducive to system work Reliability. However, no matter what kind of system, their basic principles and composition are very similar. Generally, the entire CNC system consists of three parts, namely the control system, the servo system and the position measurement system. The control system hardware is a special computer system with input and output functions. It performs interpolation operations according to the workpiece program and sends out control instructions to the servo drive system.
The measurement system detects the position and speed of the machine’s linear and rotary motion, and feeds it back to the control system and Servo drive system to modify the control instructions; the servo drive system compares and controls the control instructions 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 machinery to move as required. These three parts are organically combined to form a complete closed-loop control CNC system.
The control system hardware is a special computer with interpersonal interaction function and input and output capabilities including fieldbus interface. The servo drive system mainly includes a servo drive device and a motor. The position measurement system is mainly an incremental displacement encoder that uses a long grating or a circular grating.
(1) Input data machining program
It receives the input parts machining program, decodes and processes the machining instructions and data represented by the standard code, and stores them in a prescribed format. Some systems also need to perform compensation calculations or pre-calculations for interpolation calculations and speed control. Generally, the input data machining program includes three items: input, decoding and data machining.
(2) Interpolation calculation program
The CNC system performs the interpolation and densification operation of the intermediate output point according to the data provided in the workpiece machining program, such as the type of curve, the starting point, the end point, and the predetermined speed. The above-mentioned densification calculation must not only strictly follow the requirements of the given trajectory, but also meet the requirements of the acceleration and deceleration of the smooth motion of the mechanical system. According to the result of the calculation, each coordinate axis is issued with a position command to form a feed motion. This process is called interpolation operation. The calculated position command of the feed motion is controlled and adjusted through the position closed loop, speed loop, and current loop in the CNC or servo system, and the output current drives the motor to drive the worktable or the tool to perform the corresponding movement to complete the machining tasks specified by the program.
CNC system is a typical real-time control method, which is a typical real-time control method.
(3) Management procedures
The management program is responsible for scheduling and managing various programs that serve the machining process, such as data input, data machining, and interpolation operations. The management program also processes interrupts caused by panel commands, clock signals, and fault signals. Under the PC-based hardware structure, the management program is usually implemented under the support of a real-time operating system.
(4) Diagnostic procedures
The function of the diagnostic program is to discover system failures in time during program operation and point out the types of failures. It is also possible to check whether the functions of the main components of the system (CPU, memory, interface, switch, servo system, etc.) are normal before operation or after the failure occurs, and point out the location of the failure.
From the perspective of hardware structure, the CNC system so far can be divided into two stages 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 realize the numerical calculation and logic control required for numerical control, including the first-generation electronic tube numerical control system.
- The second generation is a transistor numerical control system
- The third generation is an integrated circuit numerical control system;
- The fourth 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 is a small computer numerical control system,
- The fifth generation is a microcomputer numerical control system
- The sixth generation is a PC numerical control system
Since the 1990s, the popularization of PC-based computer applications, the rapid advancement of computer CPU and peripheral storage, display, and communication technologies under the PC architecture, and the significant reduction in manufacturing costs have led to the PC-based CNC system becoming the mainstream CNC system. Structural system. The development of the PC numerical control system has formed a “NC+PC” transitional structure, which not only retains the traditional NC hardware structure, but only uses the PC as the HMI. Representative products include FANUC’s 160i, 180i, 310i, 840D, etc. There is also a kind of centralized realization of CNC functions in the form of motion control cards, and the development of PC numerical control systems by adding and expanding NC control boards (such as DSP-based motion control cards, etc.). A typical representative is the PMAC-NC system constructed with PMAC multi-axis motion control cards by DELTA TAU of the United States. Another more revolutionary structure is to use all the software and hardware resources of the PC platform, and only increase the field bus interface necessary for communication with the servo drive and I/O equipment, thereby realizing a very simple hardware architecture.
The Classification Of Related Systems
(1) Point control numerical control system
The control tool moves relative to the workpiece from a processing point to the precise coordinate position between another processing point, and does not control the trajectory of the movement between points, and does not perform any processing during the movement. The equipment of this type of system includes CNC drilling machine, CNC jig boring machine and CNC punching machine.
(2) Linear control numerical control system
Not only to control the precise position of the point and the point, but also to control the tool movement trajectory between the two points to be a straight line, and the tool can be processed at a given feed speed during the movement, and its auxiliary function requirements are also higher than point control There are many numerical control systems, such as it may be required to have functions such as spindle rotation control, feed speed control, and automatic tool exchange. The equipment of this type of control method mainly includes simple CNC lathes, CNC boring and milling machines, etc.
(3) Contour control CNC system
This type of system can strictly control two or more coordinate directions, that is, not only control the stroke position of each coordinate, but also control the movement speed of each coordinate. The movement of each coordinate is coordinated with each other according to the stipulated proportional relationship, and is accurately coordinated for continuous processing to form the required straight line, oblique line or curve, and curved surface. The equipment using this type of control method includes CNC lathes, milling machines, machining centers, electrical machining machines, and special machining machines.
According to the control mode of the servo system, the CNC system can be divided into the following categories:
(1) Open-loop control numerical control system
This type of numerical control system does not have a detection device or a feedback circuit, and uses a stepping motor as the driving element. The feed command (mostly pulse interface) output by the CNC device is amplified by the drive circuit and converted into a current pulse signal that controls the power on/off of each stator winding of the stepper motor, drives the stepper motor to rotate, and then is driven by the machine tool The mechanism (gear box, lead screw, etc.) drives the worktable to move. This method is simple to control and relatively inexpensive. Since the 1970s, it has been widely used in economical CNC machine tools.
(2) Semi-closed loop control numerical control system
The position detection element is installed on the motor shaft end or the screw shaft end, and the actual operating position (linear displacement) of the machine tool table is indirectly calculated through the measurement of angular displacement. Because the closed loop does not include the screw, nut pair and machine tool These large inertia links of the workbench, the errors caused by these links cannot be corrected by the loop, and its control accuracy is not as good as the fully closed-loop control CNC system, but its debugging is convenient, the cost is moderate, and relatively stable control characteristics can be obtained, so in practical applications In this way, it is widely adopted.
(3) Fully closed-loop control numerical control system
The position detection device is installed on the machine tool table to detect the actual operating position (linear displacement) of the machine tool table, and compare it with the command position (or displacement) calculated by the CNC device, and use the difference for adjustment control. The position control accuracy of this kind of control method is very high, but because it puts the screw, nut pair and machine tool table in the closed loop, the connection stiffness of the entire system becomes worse. Therefore, it is difficult for the system to achieve a high level during debugging. Gain, that is, easy to produce oscillations.
(1) CNC system for turning and milling
CNC system for CNC lathe control and CNC system for machining center control. This type of CNC system belongs to the most common CNC system. FANUC uses T and M to distinguish the two major types. Siemens configures different programming tools on a unified CNC kernel: Shopmill and shopturn to distinguish them. The biggest difference between the two is: the turning system requires that the distance between the tool point and the axis of the lathe can be reflected at any time to express the radius of the current workpiece, or multiplied by 2 to express the diameter; the turning system has various fixed cycles for turning threads; The turning system supports the switching of the spindle and the C axis, and supports the programming of the end-face rectangular coordinate system or the cylindrical coordinate system of the slewing body, and the numerical control system needs to be transformed into polar coordinates for control; and the milling numerical control system requires more programming of complex curves and curved surfaces Machining capabilities, including five-axis and inclined plane processing, etc. With the increasing popularity of turning and milling technology, CNC systems are required to have both turning and milling functions, such as Dalian Koyo’s GNC60/61 series CNC system.
(2) Grinding CNC system
Special CNC system for grinding machine control. FANUC is distinguished by G code, Siemens must configure the function. The main difference from other numerical control systems is to support the access of the workpiece online measuring instrument. The measuring instrument mainly monitors whether the size is in place and informs the numerical control system to exit the grinding cycle. The grinding CNC system also supports the dressing of the grinding wheel, and the corrected grinding wheel data is included in the CNC system as the tool data. In addition, the PLC of the grinding numerical control system also needs to have a strong temperature monitoring and control loop, and it also requires the ability to connect with vibration monitoring and ultrasonic grinding wheel cut-in monitoring instruments to work together. For non-circular grinding, the CNC system and servo drive require higher dynamic performance on the feed axis. For some non-circular machining (such as cams), due to the high precision and high finish requirements of the machined surface, the CNC system also needs special treatment for curve smoothing technology.
(3) Numerical control system for special processing
In order to adapt to special processing, such systems often need special motion control processing and actuator control. For example, parallel machine tool control needs to add the corresponding parallel structure decoupling algorithm to the conventional CNC motion control algorithm; wire cutting processing needs to support retreat along the path; punching cutting machine tool control requires the C-axis to maintain the punching head in the tangent attitude of the motion track; Gear processing requires the numerical control system to achieve an electronic gear ratio relationship or expression relationship that conforms to the law of gear formation; laser processing requires a constant distance between the laser head and the plate; electrical processing requires a numerical control system to control the discharge power supply; laser processing requires The numerical control system controls the laser energy.
(1) Economical CNC system
Also known as a simple CNC system, it usually uses a stepper motor or a servo drive with a pulse train interface. It does not have position feedback or position feedback does not participate in position control; it can only meet the general accuracy requirements of processing, and can process simple straight lines and oblique lines. , Arc and threaded parts, the microcomputer system used is a single-board computer or a single-chip computer system; usually it does not have user-programmable PLC functions. The positioning accuracy of the equipped machine tool is usually above 0.02mm.
(2) Popular CNC system
The numerical control system between the simple numerical control system and the high-performance numerical control system. Its characteristics are that the number of linkage axes is less than 4 (including 4 axes), closed-loop control (servo motor feedback information participates in control), and has pitch error compensation and cutting tools. Management function, support users to develop PLC function.
(3) High-end CNC system
Generally refers to multi-channel (two or more) CNC equipment control capabilities, dual-drive control, 5-axis and above interpolation linkage function, bevel processing, spline interpolation, two-way pitch error compensation, straightness and perpendicularity Error compensation, tool management and tool length and radius compensation functions, high static accuracy (resolution 0.001μm, the minimum resolution is 1nm) and high dynamic accuracy (follow-up error within 0.01mm), high speed and complete PLC control function CNC system.
The functional applicability of the CNC system is undoubtedly an important restrictive factor for the design and selection of CNC machine tools. The following factors are important factors that must be considered when choosing a CNC system.
(1) Driving ability
The power range of the servo and the range of the supporting motor in the solutions of different CNC suppliers are also different. First of all, it should be initially selected from the type of motor that can be matched and the power range. In particular, pay attention to whether the CNC machine tool plan includes torque motors, linear motors, and electric spindles are synchronous electric spindles or asynchronous electric spindles, the rated current requirements and overload current requirements of the above-mentioned motors, and the maximum speed requirements of the electric spindle.
(2) Fully closed loop demand and dual drive demand
Most of CNC machine tools, especially large and heavy CNC machine tools, have full closed loop and dual drive requirements. In the full-closed-loop control scheme, it is necessary to choose between distance-coded grating, absolute value grating, and ordinary incremental grating, and the numerical control system should also support the corresponding feedback signal access.
(3) Five-axis control requirements
Five-axis machine tools need to be clear whether five-axis linkage or only five-sided machining is required, and the corresponding selection of CNC system functions is also different. For example, for the processing of five-sided cabinets, RTCP is usually not required, and the choice is relatively large. At the same time, the export license, after-sales service, and technical support must also be carefully considered by the supplier of the CNC system for the five-axis function.
(4) Production system requirements
The networked support of CNC system has become a necessary condition for production system integration. For CNC machine tools that are to be incorporated into a highly automated production system, it must be clear that the CNC system has corresponding access solutions, including low-level PLC input and output points directly connected to the high-level CNC system built-in OPC server, according to OPC standards The user opens the internal data of the CNC system. In addition, for production systems, automated online workpiece inspection and tool inspection are also functions that must be supported.
1. Position ring
This is the key link for the CNC system to issue a control command and compare it with the feedback value of the position detection system to further complete the control task. It has a high working frequency and is connected with peripherals, so it is prone to failure.
Common faults are:
- ①Position control ring alarm: It may be that the measurement loop is open; the measurement system is damaged, and the internal damage of the position control unit.
- ②Move without issuing instructions, it may be caused by excessive drift, positive feedback, malfunction of the position control unit; damage of the measuring element.
- ③Failure of the measuring element, generally manifested as no feedback value; the machine tool cannot return to the reference point; the possible cause of the alarm when the pulse is missed at high speed is that the grating or the reading head is dirty; the grating is broken.
2. Servo drive system
The servo drive system is associated with the power grid, mechanical system, etc., and it has been in frequent startup and operation states during work, so this is also the part with more failures.
3. Power supply part
The power supply is the energy support part that maintains the normal operation of the system. The direct result of its failure or failure is the shutdown of the system or the destruction of the entire system. Generally in European and American countries, there are relatively few such problems, and there are not many considerations in this aspect of the design. However, in China, due to large power fluctuations, poor quality, and hidden interference such as high-frequency pulses, plus artificial Factors (such as sudden power failure, etc.). These reasons can cause power failure monitoring or damage. In addition, some operating data, setting data and processing programs of the CNC system are generally stored in RAM memory. After the system is powered off, it is maintained by the backup battery or lithium battery of the power supply. Therefore, the downtime is relatively long, and plugging in or out of the power supply or memory may cause data loss and make the system inoperable.