MFG Guide

Anti-interference Design of Economical Lathe CNC System

Starting with the noise transmission path and the disturbed equipment, some methods to improve the anti-interference ability of the economical lathe CNC system are proposed, that is, to suppress the noise source, block the noise transmission path, and improve the anti-interference ability of the core part of the CNC system—the computer. Practice has proved that the flexible use of these methods according to the specific situation can effectively improve the anti-interference ability of the economical lathe CNC system: economical lathe; CNC system; electromagnetic noise; anti-interference. The electromagnetic noise inside and outside the numerical control system often interferes with the normal operation of the numerical control system, and even causes malignant accidents, and the interference can be eliminated after many repetitions and a great price. Therefore, the anti-interference design of the numerical control system is particularly important.

Under normal circumstances, there are three conditions for electromagnetic interference, p: the noise source that sends out noise; the path for transmitting electromagnetic noise; the victim device that is subject to electromagnetic interference. In order to ensure that the CNC system is free from interference in the electromagnetic environment, corresponding measures must be taken from the design stage: suppress the noise source and directly eliminate the cause of the interference; block the noise transmission path; improve the anti-interference ability of the core part of the CNC system—the computer .

1 Inhibition of noise sources that interfere with the noise of the numerical control system can be divided into external noise and internal noise. For external noise, it is impossible to eliminate it directly, only try to avoid it, and for internal noise, try to eliminate it as much as possible. Some measures to eliminate internal and external noise are discussed below 1.1 Eliminate the transient noise generated when switching inductive loads Switching relays, contacts Inductive loads such as devices and motors will generate high surge voltages, with a peak value of up to several thousand volts, and its pulse power is sufficient to damage semiconductor devices. In addition, the high-order harmonics contained in the surge voltage will invade the numerical control system through the distributed capacitance and insulation resistance between the power lines and wires, and interfere with the normal operation of the microcomputer. This transient noise can be suppressed by connecting diodes in parallel or connecting RC absorption circuits, such as connecting diodes in parallel on the DC relay coils that control the main motor of the machine tool and the electric tool post motor, and connecting RC absorption circuits in parallel on the contactor coils and motor windings. Practice has proved that the suppression effect of this method is very obvious. In addition, the thyristor zero-crossing switch can be used instead of the AC contactor, which can more completely eliminate the transient noise. 1.2 Eliminate the DC power supply noise. Most economical lathe CNC system computer power supply (+ 5V) is composed of a three-terminal integrated voltage regulator, the circuit is simple and reliable, but the ability to suppress the power noise from the external AC power grid is very weak, and the multi-stage filter circuit shown can be used to strengthen the filtering. The middle capacitor C2C4C6 and inductor Li are used to suppress normal mode noise.

When there are TTL devices in the circuit, the switching action time is 5-10ns. Under the action of transient current and common impedance, switching noise is generated on the DC power line, which reduces the noise tolerance of the circuit and leads to logic circuits and microprocessors. An effective way to reduce switching noise by misoperation is to insert a 0.01~0.1F noise-limiting tantalum capacitor or high-frequency non-inductive filter capacitor between the power terminal and the ground terminal of each integrated circuit. Install this capacitor on the power input side of the integrated circuit, and shorten the wiring of the capacitor as much as possible. 1.3 Eliminate the noise of switching power supply At present, switching power supply is widely used in numerical control systems, but the noise of switching power supply is large, and the noise spectrum is wide and high. Frequency radiation interference is serious. These inherent shortcomings cannot be eliminated fundamentally. Only measures such as isolation, filtering and shielding can be used to block the transmission of noise. The specific method is as follows: Reduce the coupling between the switching transistor and the power shield Capacitors to reduce the generation of noise Connect the switching power supply casing with the numerical control system casing with an inductance coil to reduce common mode noise.

Connecting a line filter to the AC power input end can not only suppress the generation of common mode noise and normal mode noise, but is also effective for external power supply noise. The circuit filter circuit is as shown. Among them are common mode chokes. And capacitor C2C3 are used to suppress common mode noise, and inductor L2 and capacitor G are used to suppress normal mode noise.

Add a common mode choke to the output to suppress common mode noise. The circuit is shown in the figure. The medium capacitor G should use tantalum capacitors or high-frequency non-inductive capacitors to suppress normal mode noise.

Install the switching power supply in a metal shielded box and install it as far as possible from other parts of the system, which can reduce the radiation interference of noise in the system. 2 Block the noise transmission path The electromagnetic environment at the use site of the numerical control system is generally harsh, especially in the vicinity When large-scale electrical equipment is started and stopped, high-frequency transient noise will be generated on the public AC power grid and control loops. These noises enter the system mainly through the input power supply, input signal lines and output control lines of the CNC system. Therefore, filtering, isolation, and shielding must be taken to block the noise from the outside of the system. Some noises that cannot be eliminated inside the system should also be eliminated. Take filtering, isolation and shielding measures to deal with. 21 Use power supply filter to suppress input power noise. Common mode choke coil LiL2 capacitor C3G attenuates common mode noise, and capacitor CiC2G attenuates normal mode noise. In addition, the filter has a practical effect on noise. The suppression effect also depends on the method of use. Pay attention to the following 3 points: the filter should be installed as close as possible to the power input socket, the incoming and outgoing wires should be twisted-pair and wired close to the ground potential. The two must be routed separately, not parallel. , And can not be bundled together. The grounding resistance of the filter should be as small as possible. It is best to install it directly on the system chassis at the position closest to the system ground terminal, which can better suppress high-frequency common-mode noise.

The power supply of the peripheral interface circuit of the stepping motor driver and the computer circuit inside the numerical control system can be powered by three filters respectively, which can not only suppress the external power interference, but also suppress the mutual interference between various parts. 22 Use a shielded transformer to suppress the input power Most of the internal circuits of the noise-economical CNC system need to be powered by a transformer. The common transformer has a relatively large coupling capacitance between the primary and the secondary and weaker suppression of common mode noise, while a shielding layer is inserted between the primary and the secondary. The shielding transformer has a strong suppression effect on common mode noise. The principle is as shown in the primary common mode noise voltage, C2 is the distributed capacitance between the primary, secondary and shielding layer, and C3 is the secondary to ground capacitance. The ground impedance of the shielding layer is Z, so that the common mode noise voltage V2 coupled to the secondary is generally very small, so V2 is greatly reduced compared to V, but as the frequency increases, the impedance of G decreases. As Z increases, the ability to suppress noise decreases, so the ground wire of the shielding layer should be short and thick. However, the single-layer shielded transformer shown has a weak ability to suppress normal mode noise. The shield winding can be added and connected to the primary end, and the normal mode noise can be filtered out through the distributed capacitance between the primary coil and the shield winding.

23 The isolated input and output lines of the input and output lines may also bring noise into the system. For example, the input and output signal lines for the control of the electric tool post are the channels through which the surge noise of the tool post enters the system. The input and output interface circuits of the switch are as shown. The optocoupler has a good suppression effect on common mode noise, and G can suppress normal mode noise.

In the actual processing of the lathe, in order to ensure that the threads are not messy, the spindle encoder must output the same number of pulses to the system every time the spindle rotates. However, when the system is connected to an AC inverter or other strong interference sources, the encoder’s The output pulse will be severely disturbed or even “eaten”. Therefore, the input interface of the encoder pulse should use a differential comparator receiver to often suppress the mode noise, and also use a high-speed optocoupler for isolation to suppress common mode noise. The circuit is as shown.

A dedicated ground terminal should be set on the chassis. Short and thick wires should be used to directly connect the power supply filter, shielding transformer, and computer shield to the ground terminal. The smaller the ground resistance, the stronger the suppression of high-frequency noise. 3 Improve The anti-interference ability of the computer The anti-interference ability of the computer itself in the numerical control system is determined by the computer’s circuit design and software design. Reasonably adopt some software and hardware measures to reduce the computer’s noise sensitivity. Once the interference occurs, the computer can automatically respond. To minimize the impact of interference.

3.1 Optimal circuit design Most economical numerical control systems use MCS-51 series single-chip microcomputers and expand some peripheral interface circuits to realize them. From the perspective of anti-interference and power saving, CMOS series chips should be used as much as possible, such as 80C3180C3227C51282C55 and so on.

The input ports of the single-chip microcomputer and 8255 should be equipped with pull-up resistors. In particular, the integration and shaping circuit should be added to the interrupt entry, and the current driver should be added to the output port to enhance the anti-interference ability of the input and output ports. When there are too many expansion chips of the single-chip microcomputer, the bus driver should be added as much as possible to choose large-scale integrated circuits, reduce the number of components used, and reduce the area of ​​the printed circuit board. When designing the circuit board, the layout should be reasonable. On the printed circuit board, widen the power and ground wiring as much as possible, use bypass capacitors reasonably to reduce common impedance noise and switching noise. 3.2 Software anti-interference measures Anti-interference is also an important task of software design. As long as the computer can operate normally, the software anti Interference measures will be very effective.

For example, if the input signal is sampled multiple times, when the results of multiple samples are the same, the input is considered valid; the output signal is scanned multiple times; the logo and the backup of the processing program are set in the RAM area, and the logo is found to be damaged and the backup program is used to restore Processing program 1 Wang Wenxi. Machine tool digital adjustment technology M. Beijing: China Science and Technology Press, 1992189-20ft 3 Xu Jiazheng. Grid interference and monitoring. Electronic Measurement Technology, 1992 (4): 23-27. (Responsible Editor Zhang Qiujuan) 24 Suppression of mutual interference between various parts of the numerical control system. Economical lathe numerical control systems can generally be divided into four parts: computer driver, interface and power supply. 2 in space They should be separated as much as possible to reduce mutual interference, especially the computer part should be installed separately in a fully enclosed shield to reduce radiation interference. The interface circuit should be installed as close as possible to the input and output sockets of the system to reduce external noise lines in the system. Internal wiring distance.

Yibangzun-Common mode interference caused by poor grounding of the outer shell and mutual interference between the internal circuits of the system rightsreserved, 1994-2 (Jiannan Jiang Anning Chinese! Al 2 The connection between the drive interface circuit and the computer The wire should be twisted pair or shielded wire and routed separately from the power wire, motor wire and other noise wires. The computer driver and interface should be powered by a different power filter, shielded transformer, and the control signal sent from the computer to the driver should also go through the photocoupler. Isolation 25 Optimized grounding design The correct grounding can better suppress the transmission of noise. For the small-scale CNC system, the internal circuit should be floating, that is, the grounding of the computer, the drive and the external interface are isolated from each other and insulated from the chassis. The greater the insulation resistance, the better

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