CNC machine tools generally consist of three parts: NC control system, servo drive system and feedback detection system. The servo performance required by the numerical control machine tool for the position system includes: positioning speed and contour cutting feed speed; positioning accuracy and contour cutting accuracy; finish machining surface roughness; stability under external disturbances. These requirements mainly depend on the static and dynamic characteristics of the servo system. For a closed-loop system, it is always desired that the system has high dynamic accuracy. That is, when the system has a small position error, the moving parts of the machine tool will react quickly. The superior performance of the Guangzhou Wade servo fully meets the performance requirements in this respect. The following discusses several aspects of the position control system's influence on the machining requirements of CNC machine tools.
1. The accuracy of machining accuracy is a performance indicator that the machine tool must guarantee. The position accuracy of position servo control system largely determines the machining accuracy of CNC machine tools. Therefore, position accuracy is an extremely important indicator. In order to ensure sufficient position accuracy, on the one hand, the size of the open loop amplification in the system is selected correctly, and on the other hand, the position detection element is required to provide accuracy. Because in the closed loop control system, it is difficult to distinguish between the error of the detection element itself and the detected amount, 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 displacement that a displacement detection system can measure is called resolution. The resolution depends not only on the detection 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 elements must be carefully selected. The resolution or pulse equivalent of the selected measurement system generally requires an order of magnitude higher than the machining accuracy. In short, high-precision control systems must have high-precision detection components as a guarantee. For example, the accuracy of the commonly used linear induction synchronizer in CNC machine tools can reach ±0.0001mm, ie 0.1 μm, the sensitivity is 0.05 μm, and the repeatability is 0.2 μm; while the accuracy of the round induction synchronizer can reach 0.5N, and the sensitivity is 0.05N. , repeatability 0.1N.
2, open-loop amplification In a typical second-order system, the damping coefficient x = 1/2 (KT)-1/2, the speed of steady-state error e (∞) = 1 / K, where K is the open-loop amplification, Open loop gain is often referred to as engineering. Obviously, the open-loop amplification of the system is one of the important parameters affecting the static and dynamic index of the servo system.
Under normal circumstances, the magnification of the CNC machine tool servo is taken as 20 to 30 (1/S). A servo system with a range of K>20 is usually called a low magnification or soft servo system and is used for point control. The system with K<20 is called a high magnification or hard servo system and is applied to contour processing systems.
In order to not affect the surface roughness and precision of machined parts, it is hoped that the step response will not oscillate, that is, the requirement is to take a larger value, and the open loop amplification factor K is smaller; if the system starts from the rapidity, it is hoped that x will choose small Some, that is, you want to increase the number of open-loop gains, while increasing the value of K can also improve the steady-state accuracy of the system. Therefore, the selection of K value is a problem that must be considered comprehensively. In other words, the higher the magnification of the system, the better. When the input speed changes abruptly, high magnification may cause drastic output changes, mechanical devices are subject to greater impact, and some may cause system stability problems. This is because the stability of the system in the high-order system requires a range of values for the K value. Low-magnification systems also have certain advantages, such as easier system adjustment, simple structure, insensitivity to disturbances, and good surface roughness for machining.
3, to improve reliability CNC machine tools is a high-precision, high-efficiency automation equipment, if the failure of the loss is greater, so to improve the reliability of CNC machine tools is particularly important. Reliability is one of the main quantitative indicators for evaluating reliability. It is defined as the probability that a product will fulfill a specified function under specified conditions and within a specified period of 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 control the use of machine tools, such as various functions of CNC machine tools, servo performance, etc.
Mean time between failure (failure) (MTBF) refers to the failure of repaired or replacement parts can continue to work repairable equipment or systems, from the average time to the next failure, CNC machine tools commonly used as a quantitative measure of reliability index. As the numerical control device adopts a microcomputer, its reliability is greatly improved, so the reliability of the servo system is relatively prominent. Its fault mainly comes from servo components and mechanical transmission parts. Usually, the reliability of the hydraulic servo system is worse than that of the electric servo system. The reliability of electromagnetic elements such as solenoid valves and relays is poor, and they should be replaced with non-contact elements.
At present, the reliability of numerically-controlled machine tools is not high due to component quality, process conditions, and expenses. In order to make CNC machine tools welcome in factories, their reliability must be further enhanced to increase their use value. When designing the servo system, components must be selected in accordance with the design's technical requirements and reliability, and rigorous testing and inspection should be conducted. In mechanical interlocking devices, etc., close attention must be paid to minimizing the failure caused by mechanical components.
4. Wide-range speed regulation In the processing of CNC machine tools, the servo system requires a sufficiently wide speed range in order to simultaneously satisfy high-speed quick-moving and single-step jogging.
Single-step point action is an auxiliary work style often used in the adjustment of the table.
If the servo system achieves smooth feed at low speed, the speed must be greater than the "dead zone" range. The so-called "dead zone" refers to the fact that due to the presence of static friction, the system can't overcome this friction and cannot rotate because of the small input. In addition, due to the existence of mechanical gaps, although the motor rotates, the carriage does not move, and these phenomena can also be expressed as “dead zones”.
Let the dead zone range be a, then the minimum speed Vmin, should satisfy Vmin ≥ a, because 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 choice of the maximum speed of the servo system must take into account the mechanical limits of the machine tool and the actual machining requirements. While high speeds can increase productivity, the drive requirements are even higher. In addition, from the perspective of system control, there is also a problem of detection and feedback, especially in computer control systems, it must consider whether the software processing time is enough.
Since fmax = fmax/d
In the formula: fmax is the highest speed pulse frequency, kHz; vmax is the highest feed rate, mm/min; d is the pulse equivalent, mm.
Also set D for the speed range, D = vmax / vmin,
Fmax = Dvmin/d = DKd/d = DK
Since the reciprocal of the frequency is the interval time of two pulses, the reciprocal corresponding to the highest frequency fmax is the minimum interval time tmin, ie, 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 limited by tmin.
A better servo system, speed range D can often reach 800 to 1000. The most advanced level today is the continuous adjustment of the feed rate from 0 to 240 m/min with a pulse equivalent d=1 μm.
5. Conclusions The above-mentioned aspects have analyzed the servo performance required for the position servo system of CNC machine tools, and proposed the reliability indicators for the stable operation of the system. The results of this study can be used for the application of Wade's servo numerical control system, and can also be used for existing applications. The transformation of CNC machine tools to improve its accuracy.
