1, open-loop servo system working principle
Whenever the numerical control device issues a command pulse signal, the rotor of the stepping motor is rotated by a fixed angle, and the lathe tool rest moves a certain distance. The open-loop servo system does not have a tool holder only detection device, and does not compensate or correct the mechanical transmission error. The accuracy of the displacement of the tool holder completely depends on the step angle accuracy of the stepping motor and the precision of the gear pair and the screw nut in the transmission gap. Therefore, it is difficult to ensure high position control accuracy for such a system. At the same time, due to the influence of the performance of the stepping motor, its speed is also limited. However, such a system has a simple structure, convenient debugging, reliable operation, stable performance, and low cost, and is therefore widely used in economical CNC lathes with less precise requirements.
2. Working principle of closed-loop servo systemThe numerical control device sends a displacement command pulse, the motor tool and the mechanical transmission device move the tool holder, and the position detector installed on the tool holder converts the mechanical displacement into electrical quantity, and the feedback is input to the input signal and the input signal is compared to obtain the difference. After amplification and transformation, the tool holder is finally driven to reduce the error. If the input signal is continuously generated, the tool post continues to follow the input signal. The position detection device of the closed-loop servo system is installed on the tool holder of the machine tool, and the actual position of the tool holder is detected and compared with the instructions of the numerical control device. Oh, the difference is used to control, and thus a high positioning accuracy can be achieved. Achieving higher speeds, it is widely used in large-scale and precision machine tools requiring high demand in Kyoto. As the system has increased detection, comparison, and feedback devices, the structure is more prideful, and there are many stability factors. It is difficult to debug and maintain.
3, semi-closed-loop servo system working principleThe detection assembly is installed at the shaft end of the motor or the end of the screw shaft. The displacement of the moving part is indirectly calculated by detecting the angle of the ball screw of the servo mechanism, and then it is fed back to the comparator of the numerical control device and compared with the displacement of the original input command. The difference after comparison is compensated and controlled so that the displacement part supplements the displacement until the difference is eliminated. The measuring device of the semi-closed-loop servo system does not include the transmission mechanism such as the screw nut mechanism and the gear mechanism, and the transmission error of these transmission mechanisms still affects the displacement accuracy of the moving parts. However, the semi-closed servo system arranges the inertial tool holder out of the closed loop. The system debugging is easy and the stability is good. The accuracy, speed, and dynamic characteristics that can be transmitted to the open-loop servo system are the majority of small and medium-sized CNC machine tools. use.
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 desirable that the system has a high dynamic accuracy, ie, when the system has a small position error, the moving parts of the machine tool will react quickly. The following discusses several aspects of the position control system's influence on the machining requirements of CNC machine tools.
1, processing accuracyAccuracy 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 magnificationIn a typical second-order system, the damping coefficient x = 1/2 (KT) - 1/2, and the steady-state velocity error e (∞) = 1/K, where K is the open-loop amplification, which is often referred to as engineering. Ring gain. 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). Usually, the servo system with K range of 20 is called low magnification or soft servo system, and it is used for point control. The K 20 system 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, improve reliabilityCNC machine tools are a kind of high-precision, high-efficiency automation equipment. If a failure occurs, the loss is even greater. Therefore, it is particularly 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 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. Nextpage
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 of speedIn the processing of CNC machine tools, the servo system is required to have a sufficiently wide speed range in order to simultaneously satisfy the 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 zonesâ€.
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 ConclusionIn the above aspects, the servo performance required for the position servo system of numerical control machine tools was analyzed, and the reliability index of the stable operation of the system was proposed. The research results can be used for the design of servo numerical control systems, and can also be used for the transformation of existing numerical control machine tools. Improve the accuracy of its work.
The numerical control servo system is an important part of the numerical control machine tool and is used to realize the feed servo control and spindle servo control of the numerical control machine tool. The role of the numerical control servo system is to receive the instruction information from the numerical control device, and after power amplification and shaping processing, the linear displacement or angular displacement movement of the machine tool execution component is converted. As the CNC servo system is the last link of the CNC machine tool, its performance will directly affect the technical indicators such as the precision and speed of the CNC machine tool.
Therefore, the servo drive device of the numerical control machine tool is required to have good rapid response performance, accurately and sensitively track the digital command signal issued by the numerical control device, and can faithfully execute the instructions from the numerical control device to improve the dynamic follow-up characteristics and static of the system. Tracking accuracy. The measuring element detects the actual displacement value of each coordinate axis of the CNC machine tool and inputs it to the numerical control device of the machine tool via the feedback system.
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