Common Faults and Anti-jamming Measures of CNC Machine Power Supply

Due to the large voltage fluctuations in China's industrial power grids, the frequency of power supply faults in the numerical control system is relatively high. Therefore, attention must be paid to the repairs. The power supply is the energy supply part of the circuit board. The power supply is abnormal and the work of the circuit board must be abnormal.

First, the common failure of switching power supply


1, the fuse blows


If the insurance tube is black and spotted when it is blown, it indicates that the circuit has a serious short circuit, which is caused by breakdown of the high-voltage filter capacitor, breakdown of the rectifier tube, and other obvious causes. If the fuse is not black, it is slowly blown and static measurements can be made. It is generally a switch tube in the half-bridge that is broken or defective.


2, fuse constantly, output no voltage


In this case, check for a 300V DC voltage first. If not, the fault occurs before the inverter; if there is 300V high voltage and no output, you can use an oscilloscope to check whether the switch collector has a 20kHz waveform. If the switch is punctured or does not oscillate. High-frequency transformers can open the inverter can cause inverter stop. In addition, the inverter is normal but is protected by an overcurrent or overvoltage circuit at the rear stage so that the output has no voltage. If the 12V main output power supply output is no load, it will cause over-protection and no voltage output.


3, the power output voltage is not accurate


Under normal circumstances, the allowable voltage range of the regulated DC voltage of each control system of the numerical control system is within ± 5% of the rated value. If this range is exceeded, the voltage regulator potentiometer can be adjusted. Adjust the main output voltage level to the standard value. If it cannot be adjusted to the standard value, it may be that the potentiometer is broken or the regulator tube is broken. If only one of the voltages deviates from the larger one, it is likely that the rectifier diode is damaged. Replace the same type of diode as much as possible. Sometimes the switching power supply's load capacity is poor, and the output voltage will also be reduced too much. This may cause the circuit operation point to deviate from the linear region due to the parameter change, such as the amplification gain decreases, and the detection circuit is in a non-linear state.


4, switching power supply repeatedly issued special sound


This is usually caused by a low operating frequency. The oscilloscope can be used to detect pulse width modulators, which are approximately 20 kHz during normal operation. If the capacity of the timing loop capacitor becomes large, it will also cause the oscillation frequency to be too low, causing the power supply to produce a special repetitive noise. The switching power supply does not work properly. Replace the proper capacitor to resume normal operation.


Second, the anti-jamming approach of CNC machine tools


1, using anti-jamming high-quality power


Experience shows that the interference introduced by the power supply is the main source of system interference, and a good quality power supply with good anti-interference performance is the key to improving system reliability.


2, blocking the noise transmission path


The electromagnetic environment used by the numerical control system is generally rather harsh, especially when the nearby large-scale electrical equipment starts and stops, it will generate high-frequency transient noise on the public AC grid and control loop. These noises can enter the system through the input power of the CNC system. Therefore, measures such as filtering, isolation, shielding and protection must be taken to block the noise outside the system.


1) Use a power filter to suppress input power noise


Power filter is a powerful measure to suppress power interference. Currently, there are various types of filters available on the market. From the point of view of anti-jamming, it should be verified whether the insertion attenuation amount meets the requirements. In addition, the actual suppression effect of the filter on noise depends on the method of use. The following three points should be noted:


a. The filter should be installed as close to the power input socket as possible. Use twisted pair cables near the ground potential for the incoming and outgoing wires. The two must be routed separately. They cannot be paralleled and cannot be bundled together.


b. The grounding resistance of the filter should be as small as possible, and it is better to install it directly on the system casing at the closest position to the system grounding terminal. This can better suppress high-frequency common mode noise.


c. The power supply of the servo motor driver, peripheral interface circuit and computer circuit in the numerical control system can be powered by three filters respectively. This can not only suppress the interference of the external power supply, but also suppress the mutual interference between various parts.


2) Isolation transformer with a ratio of 1:1


The isolation transformer is to add a shielding layer between its primary winding and its secondary winding, and ground it together with the iron core to prevent the interference signal from passing through the circuit between the primary and secondary into the DC power supply system. It can effectively suppress the transient strong pulse interference that is invaded by the power grid, making it difficult for the DC or low-frequency interference signal to form inductive noise through conduction.


3) Install the power supply in a metal shield box and install it as far as possible from other parts of the system to reduce the noise interference within the system.


4) Establish power-off protection function


The power supply of the industrial grid is unstable or accidental failure of the system power supply. Sudden loss of power is inevitable. This requires the system to protect the site's data in the event of a power failure. When the voltage returns to normal, the system can resume execution from the point of power loss. The power-down protection scheme of the system can use the RAM (such as FLASH) with power-off protection or the readable and writable EEPROM to save the field data and flag words when the system is powered off.


3, suppress the noise generated by the power supply


1) Suppression of DC regulated power supply noise


The power supply (+5V) of part of the CNC system is composed of a three-terminal integrated voltage regulator. When the circuit has a TTL device, the switching operation time is 5 to 10 ns. Under the action of the transient current and the common impedance, a switching noise is generated on the DC power line. The noise margin of the circuit is reduced, causing the logic circuit and the microprocessor to malfunction. An effective way to reduce the switching noise is to connect a 0.01μF to 0.1μF noise-reducing tantalum capacitor or high-frequency non-inductive filter capacitor between the power supply terminal and the ground terminal of each integrated circuit. This capacitor should be installed when designing the circuit board. On the power supply input side of the integrated circuit, the wiring of the capacitor is minimized.


2) Suppression of switching power supply noise


At present, switching power supplies are widely used in numerical control systems. However, the switching power supply has large noise, wide noise spectrum, and high-frequency radiation interference. These inherent shortcomings cannot be fundamentally eliminated, and isolation, filtering, and shielding can only be used to block the transmission of noise. The specific method is as follows:


a. Reduce the coupling capacitance between the switch-level transistors and the power shield case to reduce noise generation;


b. Connect the switching power supply housing to the casing of the CNC system with an inductor to reduce common-mode noise;


c. Accessing the line filter at the input of the AC power supply not only suppresses the generation of common-mode noise and series-mode noise, but is also effective for external power supply noise.


d. When the switching power supply has multiple loads, the wiring method that separates the load circuits at the power supply should be adopted instead of the method of connecting the load after leaving the switching power supply for a certain distance. When the latter is wired, the distributed capacitances unbalance the lines of the various loads, resulting in large series-mode noise. In addition, the power supply housing and the load circuit are grounded at one point and the ground impedance is as small as possible. Switching power supply to each load circuit using twisted pair connection;


e. The switching power supply needs to supply both high-power load and small-signal load at the same time. Even though their voltages are the same, they must be powered by two separate sets of switching power supplies. The ground wires of these two sets of power supplies must have a common connection point. A common impedance is formed to prevent the two loads from interacting with each other.


4, reasonable grounding and wiring


The working place of the DC power supply in the system should be separated from the power formed by the relays, solenoid valves and its driving power in the system, and they should not be mixed. In addition, the grounding cable should be thick enough and have a low resistance. When laying out power cords, strong and weak currents should be separated and the input and output lines should be separated. According to the size of the current, as far as possible to increase the width of the wire, so that the power line, ground line and the direction of data transmission consistent. The above method is adopted to improve the design of the power supply part of the CNC system, which effectively eliminates the influence of interference and increases the reliability of the entire CNC system.

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