Feed servo systems are automatic control systems that use the position and speed of moving parts as control variables. They are widely used in industries such as CNC machine tools , and the diagnosis and handling of their faults is always an important aspect. Let's take a look at the types of faults in feed servo systems and how to diagnose and handle them!
● Fault type and cause diagnosis ●
(1) Overtravel
An overtravel alarm will occur when the feed motion exceeds the software-set soft limit or the hard limit determined by the limit switch. The alarm message will typically be displayed on the CRT. The fault can be resolved and the alarm cleared by referring to the CNC system manual.
(2) Overload
Overload alarms will be triggered when the feed motion load is too high, there are frequent forward and reverse movements, or the transmission chain is poorly lubricated. These alarms will typically be displayed on a CRT screen indicating servo motor overload, overheating, or overcurrent. Simultaneously, indicator lights or digital displays on the feed drive unit in the power control cabinet will indicate drive unit overload and overcurrent information.
(3) Moving
The causes of jaywalking during feeding include: ① unstable speed measurement signal, such as a faulty speed measurement device or interference with the speed feedback signal; ② unstable or interfered speed control signal; ③ poor contact at the wiring terminals, such as loose screws. When jaywalking occurs at the instant of reversal between forward and reverse motion, it is generally due to the backlash in the feed drive train or excessive gain in the servo system.
(4) Crawling
When this occurs during the startup acceleration phase or low-speed feed, it is generally caused by factors such as poor lubrication of the feed drive chain, low servo system gain, and excessive external load. Of particular concern is the coupling connecting the servo motor and the ball screw. Loose connections or defects in the coupling itself, such as cracks, can cause the ball screw and servo motor to rotate out of sync, resulting in inconsistent feed speeds and a creeping phenomenon.
(5) Vibration
Machine tools may vibrate when running at high speeds, which can trigger overcurrent alarms. Machine tool vibration problems are generally speed-related, so the speed loop should be checked; that is, any speed-related issue should be investigated by checking the speed regulator. Therefore, vibration problems should be traced back to the speed regulator. The troubleshooting should primarily focus on three aspects: the input signal, the feedback signal, and the speed regulator itself.
① First, check the signal input to the speed regulator, i.e., the setpoint signal. The setpoint signal is issued by the position deviation counter, converted into an analog signal VCMD by the D/A converter, and then sent to the speed regulator. Check if this signal has a vibration component. If it has only one cycle of vibration signal, it can be confirmed that the speed regulator is not the problem, but rather the problem lies with the upstream stage. That is, the problem should be found in the D/A converter or the position deviation counter. If they are normal, then turn to checking the tachogenerator and servo motor for problems.
② Inspect the tachogenerator and servo motor. When the machine tool vibrates, it indicates that the machine tool speed is oscillating, and the waveform fed back by the tachogenerator will also oscillate. Observe whether its waveform shows regular large fluctuations. At this time, it is best to measure whether there is an accurate proportional relationship between the machine tool's vibration frequency and the motor's rotational speed. If the vibration frequency is four times the motor speed, then the motor or tachogenerator should be considered to be faulty. First, check whether the motor is faulty, and check its carbon brushes and commutator surface condition. If there are no problems, then check the tachogenerator.
③ Speed regulator malfunction. If the above methods fail to completely eliminate vibration or even provide any improvement, the speed regulator itself should be considered as the cause. The speed regulator board should be replaced or the regulator should be replaced and the waveforms thoroughly inspected.
④ Check the relationship between vibration frequency and feed rate. If they are proportional, apart from machine tool resonance, it is mostly caused by poor interpolation accuracy of the CNC system or excessively high position detection gain, requiring interpolation adjustment and detection gain adjustment. If it is unrelated to feed rate, possible causes include: mismatch between speed control unit settings and machine tool, improper adjustment of speed control unit, excessive speed loop gain of the axis, or faulty printed circuit board of speed control unit.
Example: Troubleshooting X-axis oscillation
Fault phenomenon: On a CNC machine tool equipped with FANUC, the X-axis load of the machining center sometimes suddenly rises to 80%, and the X-axis motor hums loudly at the same time; at other times it is normal.
Analysis and Handling Process: On-site observation revealed that the X-axis motor was humming at a low frequency, indicating low-frequency oscillation of the X-axis. The causes of this oscillation include:
① The axis position loop gain is inappropriate;
②The mechanical parts have large clearances, the transmission chain has poor rigidity, and there is jamming;
③ The load inertia is relatively large.
Upon inspection, the X-axis position gain remained unchanged, and the load was normal. This machine tool had been undergoing heavy cutting operations, resulting in a large X-axis backlash, which had just been compensated. The X-axis backlash compensation parameter was checked and found to be 0535, with a set value of 250. A dial indicator measured the actual X-axis backlash at 0.22, indicating overcompensation. The X-axis oscillation only disappeared after the set value was changed to 200.
(6) The servo motor does not rotate.
In addition to the speed control signal, the CNC system also inputs a servo enable control signal to the feed drive unit, typically a DC +24V relay coil voltage. ① Check if the CNC system outputs a speed control signal; ② Check if the enable signal is active. Observe the I/O status using a CRT monitor and analyze the machine tool PLC ladder diagram (or flowchart) to determine the starting conditions of the feed axis, such as whether lubrication and cooling are met; ③ For servo motors with electromagnetic brakes, check if the electromagnetic brake is released; ④ Feed drive unit malfunction; ⑤ Servo motor malfunction.
(7) Position error
When the servo axis movement exceeds the position tolerance range, the CNC system will generate an alarm for excessive position error, including following error, contour error, and positioning error. The main causes are: ① The system's set tolerance range is too small; ② The servo system gain setting is improper; ③ The position detection device is contaminated; ④ The cumulative error of the feed transmission chain is too large; ⑤ The balancing device (such as a balancing cylinder) is unstable during the vertical movement of the spindle head.
(8) Drift
When the command value is zero, the coordinate axis still moves, causing a position error. This is eliminated through drift compensation and zero-speed adjustment on the drive unit.
(9) Failure to return to reference point
Reference point retrieval faults are generally divided into two categories: those that cannot find the reference point and those that cannot accurately locate the reference point. The former is usually caused by a failure in the signal generated by the reference point deceleration switch or the zero-position pulse signal, which can be determined by checking the zero mark of the pulse encoder or the zero mark of the grating ruler. The latter is caused by an improper setting of the reference point switch stop position, which requires readjusting the stop position.
(10) The servo motor rotates automatically after being powered on.
The main reasons are: ① incorrect polarity of position feedback; ② position offset of coordinate axis due to external force; ③ malfunction of driver, tachogenerator, servo motor or system position measurement circuit; ④ faulty motor or driver.
●Fault diagnosis of feed drive●
01 DC Feed Drive
PWM speed control uses a pulse width modulator to control the switching time of a high-power transistor. The speed control signal is converted into a square wave voltage of a certain frequency and applied to the two ends of the armature of a DC servo motor. By controlling the width of the square wave, the average voltage across the armature is changed, thereby controlling the armature current and ultimately the speed of the servo motor. Thyristor speed control, on the other hand, uses a speed regulator to control the conduction angle of the thyristor. By changing the size of the conduction angle, the voltage across the armature is altered, thus achieving speed control.
1. CRT alarm display fault
For FANUC systems, the CRT displays servo alarms 400-457 (servo system error alarms) and 702-704 (overheat alarms). The causes of overheat alarms include:
① Harsh cutting conditions and increased friction torque of the machine tool cause the overload relay in the main circuit to trip;
② The servo motor current is too high during cutting or the transformer itself is faulty, causing the servo transformer thermal control switch to activate;
③ A short circuit or poor insulation inside the armature of the servo motor, demagnetization or detachment of the permanent magnet of the motor, and a faulty motor brake can cause the thermal control switch inside the motor to activate.
2. Faults indicated by the alarm indicator light
For alarm lights on the printed circuit boards of the speed control units in FANUC and Siemens systems, their functions, causes of failure, and troubleshooting methods can be found in the corresponding machine tool manuals.
3. Fault with no alarm display
① Machine tool malfunction. The speed feedback signal is a positive feedback signal, which often occurs during maintenance and debugging, usually due to incorrect connection of the cable signal line.
② Machine tool vibration. This is caused by incorrect settings of system parameters related to position control, such as incorrect settings for the command ratio (CRM) and detection ratio (DMR). Check the machine tool vibration period. If the vibration period changes with the feed speed, especially during rapid traverses accompanied by large impacts, it is often due to a faulty speed measuring device, such as poor contact of the tachogenerator brushes on the servo motor. If the vibration period does not change with the feed speed, adjust the gain potentiometer to reduce the gain and observe whether the vibration decreases. If it decreases, and the vibration frequency is between tens and hundreds of hertz, which is the machine tool's natural vibration frequency, it can be resolved through the relevant settings on the printed circuit board. If the vibration does not decrease, then the printed circuit board is faulty.
③Low positioning accuracy. In addition to large errors in the machine tool feed transmission chain, this is also related to the low gain of the servo system. Adjust the gain potentiometer to increase the gain and see if the fault can be eliminated.
④ Excessive noise during motor operation. This may be due to poor surface roughness or damage to the commutator of the servo motor; oil or dust intruding into the brushes or commutator; or axial movement of the motor.
⑤ The servo motor does not turn. The permanent magnet of the motor has fallen off; for servo motors with electromagnetic brakes, the brake has failed and cannot disengage after power is applied.
02 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, because AC servo motors contain precision detectors, collisions or impacts may cause malfunctions. During maintenance, the following checks should be performed: ① Check for any mechanical damage; ② Check if rotating parts can be turned normally by hand; ③ If a brake is present, check if the brake is functioning properly; ④ Check for any loose screws or gaps; ⑤ Check if the motor is installed in a humid environment, with drastic temperature changes, or in a dusty place.
2. Installation Precautions for AC Servo Motors. After maintenance, the following points should be noted when installing the servo motor:
①Because the waterproof structure of the servo system is not very tight, if cutting fluid, lubricating oil, etc. seep into the interior, it will cause a decrease in insulation performance or a short circuit in the winding. Therefore, care should be taken to avoid the splashing of cutting fluid as much as possible.
② When the servo motor is installed on the gearbox, when adding lubricating oil, the oil level in the gearbox must be lower than the output shaft of the servo motor to prevent lubricating oil from seeping into the interior.
③ When fixing servo couplings, gears, timing belts, and other connecting parts, under no circumstances should the force acting on them exceed the permissible radial or axial load.
④ Connect the servo and control circuits correctly according to the instructions.
3. Common faults of AC servo motors. These include the following:
① Rotor position detection device malfunction. When the Hall switch or photoelectric pulse encoder malfunctions, it will cause loss of control and vibration during feeding.
② Use a multimeter or bridge to measure the DC resistance of the armature winding to check for open circuits, and use a megohmmeter to check if the insulation is good.
③ Separate the rotor from the mechanical device and rotate it by hand. Under normal circumstances, you will feel resistance. After rotating it to a certain angle, release your hand and the rotor will return to its original position. If the rotor can rotate several times continuously and stop freely when rotated by hand, the motor is damaged. If it cannot be rotated by hand or does not return after rotation, the mechanical part may be faulty.
④ Replacement of the pulse encoder. If the pulse encoder of the AC servo is faulty, it should be replaced.
03 Stepper motor drive
Stepper motor drives are the most commonly chosen servo drive systems in open-loop control systems. Open-loop feed systems have a simple structure, are easy to debug, maintain, and use, are reliable, and are inexpensive. They were once widely used in machine tools where high precision is not required.
During use, the stepper motor drive system has the following common faults:
1. Motor overheating alarm. This may be due to an excessively harsh working environment, such as excessively high ambient temperature; improper parameter selection, such as excessive current or exceeding the phase current; or the need to reset the parameters.
2. During operation, the motor stops rotating after a high-pitched scream. Specifically, during processing or operation, the driver or stepper motor emits a piercing scream. Possible causes include: the input pulse frequency being too high, causing a stall; this can be resolved by reducing the input pulse frequency; or the sudden change frequency of the input pulse being too high, which can also be resolved by reducing the sudden change frequency of the input pulse.
3. The machine stops during operation, and a sudden stop occurs under normal working conditions.
4. Stepper motor loses steps or takes multiple steps. This fault may cause the axis of the stepper motor drive system to suddenly stop during operation, and then continue to move.
5. Uneven operation and vibration. This manifests in the machining process as vibration marks on the workpiece and a large surface roughness.
●Fault Diagnosis and Handling●
The following are some common treatment methods:
(1) Repair or replace components.
(2) Reinstall or reconnect the relevant connections.
(3) Adjust the relevant parameters.
The CNC system, PLC, and servo drive system all have many modifiable parameters to adapt to the requirements of different machine tools and different working conditions. These parameters not only match the electrical systems with the specific machine tool but also optimize the various functions of the machine tool. Therefore, any change in parameters (especially analog parameters) or even their loss is unacceptable; however, changes in the mechanical or electrical performance caused by the long-term operation of the machine tool can disrupt the initial matching and optimization states, requiring readjustment of one or more relevant parameters to eliminate the problem.
This method places high demands on maintenance personnel, who must not only have a thorough understanding of the main parameters of the specific system, that is, know its address and be familiar with its function, but also have rich experience in electrical debugging.
(4) Special treatment method.
Modern CNC systems have entered a PC-based and open development stage, with increasingly rich software content, including system software, machine tool manufacturer software, and even user-defined software. Due to unavoidable problems in the design of software logic, some fault states, such as system crashes, cannot be analyzed. For such faults, special methods can be used to handle them, such as powering off the entire machine, pausing briefly, and then powering it on again; sometimes this can eliminate the fault.
Maintenance personnel can discover patterns or other effective methods through long-term practice. When fault analysis results focus on a specific printed circuit board, due to the increasing integration of circuits, pinpointing the fault to a specific area or even a single component is extremely difficult. To shorten downtime, if the same spare parts are available, the spare parts can be replaced first, and then the faulty board can be inspected and repaired. However, before removing the old board and replacing it with a new one, it is essential to carefully read the relevant documentation, understand the requirements and operating procedures, and then proceed to avoid causing more serious problems.
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