CNC machine tools generally consist of three parts: NC control system, servo drive system and feedback detection system. The servo performance required by the CNC machine tool for the position system includes: positioning speed and contour cutting feed rate; positioning accuracy and contour cutting accuracy; surface roughness of finishing; stability under external disturbance. These requirements mainly depend on the static and dynamic characteristics of the servo system. For closed-loop systems, it is always desirable to have a higher dynamic accuracy, that is, when the system has a small position error, the moving parts of the machine will react quickly. The following is a discussion of several aspects of the position control system affecting the machining requirements of CNC machine tools.
1, processing accuracy
Accuracy is a performance metric that machine tools must guarantee. The positional accuracy of the position servo control system largely determines the machining accuracy of the CNC machine. Therefore, positional accuracy is an extremely important indicator. In order to ensure sufficient positional accuracy, on the one hand, the size of the open-loop amplification factor in the system is correctly selected, and on the other hand, the accuracy of the position detecting component is required. Because in the closed-loop control system, it is difficult to distinguish between the error of the detection element itself and the deviation of the detected quantity, and the accuracy of the feedback detection element often plays a decisive role in the accuracy of the system. It can be said that the machining accuracy of CNC machine tools is mainly determined by the accuracy of the inspection system. The minimum amount of displacement that the displacement detection system can measure is called the resolution. The resolution depends not only on the sensing element itself but also on the measuring line. In the design of CNC machine tools, especially high-precision or large- and medium-sized CNC machine tools, the detection components must be carefully selected. The resolution or pulse equivalent of the selected measurement system is generally required to be an order of magnitude higher than the machining accuracy. In short, high-precision control systems must be guaranteed with high-precision sensing components. For example, the accuracy of linear induction synchronizers commonly used in CNC machine tools has reached ±0.0001mm, ie 0.1μm, sensitivity is 0.05μm, repeatability is 0.2μm; and the accuracy of circular induction synchronizer can reach 0.5N, sensitivity is 0.05N , repeatability of 0.1N.
2, open loop magnification
In a typical second-order system, the damping coefficient x = 1/2 (KT) - 1/2, the velocity steady-state error e (∞) = 1 / K, where K is the open-loop amplification factor, which is called Loop gain. Obviously, the open-loop amplification of the system is one of the important parameters that affect the static and dynamic indicators of the servo system.
In general, the magnification of the CNC machine tool servo is taken as 20 to 30 (1/S). The servo system with a range of K<20 is usually called a low-magnification or soft servo system, and is often used for point control. The K>20 system is called a high-magnification or hard servo system and is applied to the contour processing system.
If the surface roughness and accuracy of the machined part are not affected, it is desirable that the step response does not oscillate, that is, the value is larger, and the open-loop amplification factor K is smaller; if it is based on the rapidity of the system, it is hoped that x is small. Some, that is, the open-loop amplification factor is expected to increase, and the increase in the K value can also improve the steady-state accuracy of the system. Therefore, the selection of the K value is a comprehensive consideration. In other words, the higher the magnification of the system, the better. When the input speed is abrupt, the high magnification may cause a sharp change in the output, the mechanical device may be subjected to a large impact, and some may also cause stability problems of the system. This is because system stability has a range of values ​​for K values ​​in higher order systems. Low-magnification systems also have certain advantages. For example, the system adjustment is relatively easy, the structure is simple, the disturbance is not sensitive, and the surface roughness of the processing is good.
3. Improve reliability
CNC machine tools are high-precision, high-efficiency automation equipment. If the fault occurs, the loss will be even greater. Therefore, it is especially important to improve the reliability of CNC machine tools. Reliability is one of the main quantitative indicators for evaluating reliability. It is defined as the probability that a product will perform a specified function under specified conditions and within a specified time. For CNC machine tools, its specified conditions refer to its environmental conditions, working conditions and working methods, such as temperature, humidity, vibration, power supply, interference intensity and operating procedures. The functions here mainly refer to the use of machine tools, such as the various functions of CNC machine tools, servo performance and so on. #p#å‰¯æ ‡é¢˜#e#
Mean Time Between Failure (MTBF) is a repairable device or system that can continue to work after a repair or replacement of a fault. The average time from one fault to the next is commonly used by CNC machines as a measure of reliability. index. Since the reliability of the numerical control device is greatly improved after the microcomputer is used, the reliability of the servo system is relatively prominent. Its failure mainly comes from the servo components and the mechanical transmission part. Generally, the reliability of the hydraulic servo system is worse than that of the electric servo system. The reliability of electromagnetic components such as solenoid valves and relays is poor, and should be replaced by contactless components as much as possible.
At present, CNC machine tools are not very reliable due to the limitations of component quality, process conditions and cost. In order to make the CNC machine tool welcome to the factory, it is necessary to further improve its reliability and thus increase its use value. When designing the servo system, the components must be selected according to the technical requirements and reliability of the design, and screened according to strict test and inspection. In the aspect of mechanical interlocking devices, close attention must be paid to minimize the malfunction caused by mechanical parts.
4, wide range speed
In the machining of CNC machine tools, the servo system requires a sufficiently wide speed range for the feed drive in order to simultaneously satisfy high-speed rapid shift and single-step jog.
Single-step action is an auxiliary work method that is often used in the adjustment of the workbench.
If the servo system achieves smooth feed at low speeds, the speed must be greater than the "dead zone" range. The so-called "dead zone" means that the motor cannot overcome this frictional force and cannot rotate due to the presence of static friction. In addition, due to the mechanical gap, although the motor rotates, the carriage does not move, and these phenomena can also be expressed by "dead zones".
If the dead zone range is a, then the lowest speed Vmin should satisfy Vmin ≥ a, since a ≤ dK, d is the pulse equivalent (mm / pulse); K is the open loop magnification, then
Vmin≥dK
If d = 0.01mm / pulse, K = 30 × 1 / S, then the lowest speed
Vmin≥a=30×0.01mm/min=18mm/min
The maximum speed of the servo system is chosen taking into account the machine's mechanical permissible limits and actual machining requirements. High speeds can increase productivity, but drive requirements are higher. In addition, there is a problem of detection and feedback from the perspective of system control, especially in computer control systems, it must be considered whether the time of software processing is sufficient.
Since fmax=fmax/d
Where: fmax is the pulse frequency of the highest speed, kHz; vmax is the highest feed rate, mm/min; d is the pulse equivalent, mm.
Also set D as the speed adjustment range, D=vmax/vmin,
Fmax=Dvmin/d=DKd/d=DK
Since the reciprocal of the frequency is the interval time of the two pulses, the reciprocal corresponding to the highest frequency fmax is the minimum interval time tmin, that is, tmin=1/DK. Obviously, the system must perform position detection and control operations in hardware or software within tmin. For the highest speed, the value of vmax is constrained by tmin.
A better servo system, the speed range D can often reach 800 ~ 1000. The most advanced level today is that the feed rate is continuously adjustable from 0 to 240 m/min under pulse equivalent d = 1 μm.
5 Conclusion
In the above aspects, the servo performance required by the position servo system of CNC machine tools is analyzed, and the reliability index of the stable operation of the system is proposed. The research results can be used in the design of servo numerical control system, and can also be used to transform existing CNC machine tools. Improve the accuracy of its work.
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