Share dry goods! What are the common faults and troubleshooting methods of the machine tool feed servo system?
Fault formation and cause diagnosis
(1) Overtravel
When the feed motion exceeds the soft limit set by the software or the hard limit determined by the limit switch, an overtravel alarm will occur. Generally, the alarm content will be displayed on the CRT. According to the CNC system manual, the fault can be eliminated and the alarm can be cleared.
(2) Overload
When the load of the feed motion is too large, frequent forward and reverse movements, and the lubrication status of the transmission chain is poor, overload alarms will be triggered. Generally, alarm information such as servo motor overload, overheating, or overcurrent will be displayed on the CRT. At the same time, on the feed drive unit in the high-voltage cabinet, indicator lights or digital tubes will indicate information such as overload and overcurrent of the drive unit.
(3) Bouncing
The phenomenon of bouncing occurs during feeding, and the reasons include: ① unstable speed measurement signal, such as speed measurement device failure, speed feedback signal interference, etc.; ② The speed control signal is unstable or interfered with; ③ Poor contact of wiring terminals, such as loose screws. When the movement occurs at the moment of reversal from forward motion to reverse motion, it is generally caused by the reverse clearance of the feed transmission chain or excessive gain of the servo system.
(4) Crawling
When it occurs during the acceleration phase or low-speed feed, it is generally caused by factors such as poor lubrication of the feed transmission chain, low servo system gain, and excessive external load. It is particularly important to note that the coupling used for connecting the servo motor and ball screw may not be synchronized with the rotation of the ball screw and the servo motor due to loose connections or defects in the coupling itself, such as cracks, resulting in varying feed speeds and crawling phenomena.
(5) Vibration
When the machine tool is running at high speed, vibration may occur, and an overcurrent alarm may occur. The vibration problem of machine tools generally belongs to speed problems, so it is necessary to search for the speed ring, that is, for any speed related problems, the speed regulator should be searched. Therefore, vibration issues should be addressed by looking for speed regulators. Mainly search for faults from three aspects: given signal, feedback signal, and speed regulator itself.
① Firstly, check the signal input to the speed regulator, which is the given signal. The given signal is emitted by the position deviation counter, converted into an analog quantity VCMD by a D/A converter, and then sent to the speed regulator. It is necessary to check if there is any vibration component in this signal. If it only has one cycle of vibration signal, it can be confirmed that there is no problem with the speed regulator, but rather with the previous stage. That is, the problem should be found in the D/A converter or position deviation counter. If everything is normal, turn to finding the problem with the tachometer generator and servo motor.
② Check the speed measuring generator and servo motor. When the machine tool vibrates, it indicates that the speed of the machine tool is oscillating, and the waveform feedback from the speed measuring generator must also be oscillating. Observe whether its waveform shows regular ups and downs. At this point, it is best to measure whether there is an accurate proportional relationship between the vibration frequency of the machine tool and the speed of the motor rotation. For example, if the vibration frequency is four times the speed of the motor, it should be considered that there is a fault in the motor or speed measuring generator. Firstly, check for any faults in the motor, check the surface condition of its carbon brushes and commutator. If there are no problems, then check the tachometer generator.
③ Speed regulator malfunction. If the above methods cannot completely eliminate vibration and even have no improvement, the problem with the speed regulator itself should be considered. The speed regulator board should be replaced or the waveforms in each part should be thoroughly checked after replacement.
④ Check the relationship between vibration frequency and feed rate. If the two are proportional, apart from the resonance of the machine tool, most of them are caused by poor interpolation accuracy or high position detection gain of the CNC system, which requires interpolation adjustment and detection gain adjustment. If it is not related to the feed rate, the possible reasons are: the setting of the speed control unit does not match the machine tool, the speed control unit is not adjusted well, the speed loop gain of the axis is too large, or the printed circuit board of the speed control unit is poor.
Example: Fault repair of X-axis oscillation
Fault phenomenon: A CNC machine equipped with FANUC OMC sometimes experiences a sudden increase in X-axis load to 80% on the machining center, while the X-axis motor makes a buzzing sound; Sometimes it's normal again.
Analysis and handling process: On site observation found that the frequency of the buzzing sound of the X-axis motor is relatively low, so it is judged that there is low-frequency oscillation in the X-axis. The reasons for oscillation include: ① inappropriate gain in the axis position loop; ② There is a large gap in the mechanical part, poor rigidity of the transmission chain, and jamming; ③ The load inertia is relatively large. After inspection, the gain of the X-axis position remains unchanged and the load is also normal. Due to continuous heavy cutting, this machine tool has a large X-axis clearance and has undergone clearance compensation. After checking the X-axis clearance compensation parameter 0535, the set value is 250, and the actual clearance of the X-axis measured with a dial gauge is 0.22, indicating a compensation transition; The X-axis oscillation was not eliminated until the set value was changed to 200.
(6) Servo motor not rotating
The CNC system inputs not only the speed control signal to the feed drive unit, but also the servo enable control signal, usually DC+24V relay coil voltage. ① Check if the CNC system has speed control signal output; ② Check if the enable signal is connected. By observing the I/O status through CRT, analyze the ladder diagram (or flowchart) of the machine tool PLC to determine whether the starting conditions of the feed shaft, such as lubrication, cooling, etc., are met; ③ For servo motors with electromagnetic braking, it is necessary to check whether the electromagnetic braking is released; ④ Feed drive unit malfunction; ⑤ Servo motor malfunction.
(7) Position error
When the servo axis motion exceeds the position tolerance range, the CNC system will generate an alarm for excessive position error, including tracking error, contour error, and positioning error. The main reasons are: ① the tolerance range set by the system is small; ② Improper gain setting of servo system; ③ The position detection device is contaminated; ④ The accumulated error of the feed transmission chain is too large; ⑤ The balance device (such as the balance cylinder) is unstable when the spindle box moves vertically.
(8) Drift
When the command value is zero, the coordinate axis still moves, causing positional errors. Eliminated through drift compensation and zero speed adjustment on the driving unit.
(9) Failure of the reference point
The failure of the reference point is generally divided into two categories: inability to find the reference point and inability to find the reference point. The first type of fault is generally the failure of the signal generated by the reference point deceleration switch or the zero position pulse signal. It can be determined whether there is a fault by checking the zero mark position of the pulse encoder or the zero mark position of the grating ruler; The latter type of fault is caused by improper setting of the reference point switch block position, which needs to be readjusted.
(10) The servo motor automatically rotates after starting up
The main reasons are: ① incorrect polarity of position feedback; ② Due to external forces, the coordinate axis has shifted in position; ③ Defective driver, tachometer generator, servo motor or system position measurement circuit; ④ The motor or driver is faulty.
Fault diagnosis of feed drive
1, DC feed drive
PWM speed regulation is the use of a pulse width modulator to control the switching time of high-power transistors. Convert the speed control signal into a fixed frequency square wave voltage and apply it to both ends of the armature of a DC servo motor. By controlling the width of the square wave, the average voltage at both ends of the armature is changed to control the armature current and thus control the speed of the servo motor. Thyristor speed regulation is the use of a speed regulator to control the conduction angle of the thyristor. By changing the magnitude of the conduction angle, the voltage at both ends of the armature is changed to achieve the purpose of speed regulation.
1. CRT has a fault with alarm display. For the FANUC system, the servo alarms displayed by CRT are servo system error alarms for numbers 400-457 and overheat alarms for numbers 702-704. The causes of overheating alarm include: ① harsh cutting conditions and increased friction torque of the machine tool, which causes the overload relay in the main circuit to act; ② During cutting, the current of the servo motor is too high or the transformer itself malfunctions, causing the thermal control switch of the servo transformer to act; ③ Short circuit or poor insulation inside the armature of the servo motor, demagnetization or detachment of the permanent magnet of the motor, and poor braking of the motor can cause the thermal control switch inside the motor to operate.
2. The fault indicated by the alarm indicator light can be found on the printed circuit board of the speed control unit in FANUC and Siemens systems. The function, cause of the fault, and troubleshooting method can be found in the corresponding machine tool manual.
3. Fault without alarm display: ① Machine tool out of control. The speed feedback signal is a positive feedback signal, which often occurs during maintenance and debugging, and is usually caused by incorrect connection of cable signal lines Machine tool vibration. System parameter settings related to position control are incorrect, such as incorrect settings for command multiplier CRM and detection multiplier DMR. Check the vibration period of the machine tool. If the vibration period of the machine tool changes with the feed rate, especially when moving rapidly, it is accompanied by large impacts, which is often due to a malfunction of the speed measuring device, such as poor contact of the speed measuring generator brush on the servo motor; If the vibration period of the machine tool does not change with the feed rate, adjust the gain potentiometer to reduce the gain and observe whether the vibration is weakened. If it weakens and the vibration frequency is from tens of Hertz to hundreds of Hertz, which is the natural vibration frequency of the machine tool, it can be solved through the relevant settings on the printed circuit board. If the vibration does not weaken, it is a fault in the printed circuit board Low positioning accuracy. In addition to the large error in the feed transmission chain of the machine tool, it is also related to the low gain of the servo system. Adjust the gain potentiometer and increase the gain to confirm whether the fault can be eliminated Excessive noise during motor operation. The surface roughness of the servo motor commutator is poor or damaged; Oil or dust entering the brushes or commutator; There is axial displacement of the motor. ⑤ The servo motor does not rotate. The permanent magnet of the motor is detached; The servo motor with electromagnetic brake failed to disengage after being powered on due to brake failure.
2, AC feed drive
1. Basic inspection of AC servo motors. In principle, AC servo motors do not require maintenance as they are not damaged. However, due to the presence of precision detectors in AC servo motors, collisions and impacts may cause malfunctions. During maintenance, the following checks should be carried out: ① whether there is any mechanical damage; ② Whether the rotating part can be rotated normally by hand; ③ Is the brake normal for those with brakes? ④ Are there any loose screws or gaps? ⑤ Is it installed in a damp, temperature sensitive, and dusty area.
2. Installation precautions for AC servo motors. After the repair is completed, pay attention to the following points when installing the servo:
① Due to the less tight waterproof structure of the servo system, if cutting fluid, lubricating oil, etc. seep into the interior, it will cause a decrease in insulation performance or winding short circuit. Therefore, attention should be paid to avoiding cutting fluid splashing as much as possible;
② When installing the servo motor on the gearbox, when adding lubricating oil, it should be noted that the lubricating oil level in the gearbox must be lower than the output shaft of the servo to prevent lubricating oil from seeping into the interior.
③ When fixing connecting components such as servo couplings, gears, and synchronous belts, under no circumstances should the force acting on them exceed the allowable radial and axial loads.
④ According to the instructions, make the correct connection between the servo and control circuits.
3. Common faults of AC servo motors. Including the following points:
① The rotor position detection device is faulty. When the Hall switch or photoelectric pulse encoder malfunctions, it can cause loss of control and vibration in the feed.
② Use a multimeter or bridge to measure the DC resistance of the armature winding, check for open circuits, and use a megohmmeter to check for good insulation.
③ Separate from the mechanical device and rotate the rotor by hand. Under normal circumstances, there may be resistance. After turning at an angle, release the hand and the rotor will return; If the rotor can rotate continuously for several turns and come to a free stop by hand, then the motor is damaged; If it cannot be turned by hand or does not return after rotation, there may be a malfunction in the mechanical part.
④ Replacement of pulse encoder. If the pulse encoder of the AC servo is faulty, the pulse encoder should be replaced.
3, Stepper motor drive
Stepper motor drive is the most commonly used servo drive system in open-loop control systems. The structure of the open-loop feed system is relatively simple, convenient for debugging, maintenance, and use, reliable in operation, and low in cost. It has been widely used in machine tools with generally low precision requirements.
During use, the stepper motor drive system has the following common faults:
1. Motor overheat alarm. Perhaps the working environment is too harsh and the ambient temperature is too high; Improper parameter selection, such as excessive current, exceeding phase current, and the ability to reset parameters.
2. During work, if the driver or stepper motor emits a piercing scream and does not rotate after screaming, the specific situation is during processing or operation. The possible reason is that the input pulse frequency is too high, causing blockage, which can be eliminated by reducing the input pulse frequency; The sudden modulation frequency of the input pulse is too high, which can be eliminated by reducing the sudden modulation frequency of the input pulse.
3. During the work process, if the vehicle stops suddenly under normal working conditions.
4. If the stepper motor loses step or steps multiple times, the possible phenomenon caused by this fault is that during operation, a certain axis of the stepper motor drive system suddenly stops, and then continues to move.
5. Uneven operation with shaking. Reflected in the machining process is that the workpiece has vibration patterns and a high surface roughness.
