Servo system failures account for a high percentage of all machine tool failures. Because servo systems involve many components, are diverse in type, and have unique technical principles, maintenance and diagnosis are difficult. Therefore, it is necessary to summarize some fault diagnosis methods. The movement and positioning of the coordinate axes of a CNC machine tool are accomplished by a position servo system. Position servo systems generally use closed-loop or semi-closed-loop control. A characteristic of (semi-)closed-loop control is that a failure in any component can lead to inaccurate, unstable, or malfunctioning system positioning. Diagnosing positioning faults becomes crucial for maintenance. Decomposing and judging the system based on its control principle and interface characteristics has become an effective method. This article introduces position loop and speed loop diagnostic methods using maintenance examples. 1. Position Loop Fault Diagnosis If the position feedback and speed feedback of the position servo system each use a separate feedback device, the control action of the position loop can be disconnected, allowing the speed loop to run independently, in order to determine whether the fault originates in the position loop or the speed loop. Disconnecting the control action of the position loop can be done in two ways: Mechanical disconnection, i.e., disconnecting the transmission connection between the position feedback encoder and the servo motor. Electrical disconnection, i.e., disconnecting the connection between the position feedback encoder and the system. If it is necessary to disable the position feedback disconnection alarm, the position feedback input signal line should be connected as shown in the diagram. During maintenance testing in open-loop position mode, no movement commands should be given to the axis under test; otherwise, the servo motor will malfunction. Example 1: A CK6140A CNC lathe exhibits chatter marks on the boring surface. After ruling out mechanical and tooling factors, the X-axis servo system is inspected. The machine tool's CNC system is a FANUC 3T, and the servo amplifier is a FANUC H-series DC servo. Irregular vibrations are observed on the X-axis during stops and slow movements, initially indicating a gap in the connection between the X-axis position encoder and the leadscrew, or an unstable speed loop. The encoder coupling is checked and found to be normal. Since the X-axis servo system has two encoders, one for position feedback and one for speed feedback, the mechanical connection between the position feedback encoder and the servo motor can be disconnected for further diagnosis. First, support the X-axis slide with a support, and remove the drive belts from the X-axis motor and leadscrew. When the machine tool is started, the X-axis runs in open-loop position mode. Under the zero-drift effect of the servo amplifier, the motor rotates slowly (if the motor barely rotates, the bias potentiometer RV2 on the control board can be adjusted appropriately). At this time, the motor rotates to a certain fixed angle, and there is always a jerking phenomenon. This indicates that the speed loop is basically stable. This may be due to a short circuit in the commutator at a certain angle, causing a momentary drop in speed, resulting in the motor jerking. After carefully cleaning the motor commutator and brushes, the motor runs smoothly. Reconnecting the system, the X-axis returns to normal. Example 2: The Z-axis movement of the [ST5BZ>CH-102 CNC lathe exhibits a jerking phenomenon, with the overshoot becoming more severe at higher speeds. When stopped, the servo diagnostic screen shows that the Z-axis tracking error is stable and close to zero. The machine tool's CNC system is a SIMENS 810GA2, and the servo system is a SIMENS 610. The system position feedback and speed feedback each use an encoder. The initial judgment is that the servo amplifier is overshooting or the system parameters are incorrectly set. First, adjusting system parameters MD2501 (servo gain) and MD2601 (multiple gains) was ineffective. For further diagnosis, the Z-axis position feedback connector was disconnected and the power was turned off. Since the CNC alarm on this machine tool does not affect servo power-on, the feedback disconnection alarm can be left unshielded. The Z-axis servo drive enable control terminal was shorted with a wire, and then a 1.5V battery was used to apply approximately 0.5V to the Z-axis servo amplifier speed command terminal via a potentiometer. The machine was powered on, and the Z-axis moved smoothly, indicating the fault was in the position feedback stage. The position feedback encoder was manually rotated; the connection felt secure and undamaged. The X-axis and Z-axis position feedback connectors and speed command control lines were swapped, but the fault remained in the Z-axis. At this point, the fault was considered to be in the Z-axis position feedback. The Z-axis position feedback encoder was removed, and a screw on the coupling spring was found to be loose. After repair, the fault was eliminated. If position and speed feedback are handled by a single feedback element, and the position feedback signal is converted into a speed control signal via a conversion circuit, then a flexible judgment must be made based on the specific characteristics of the system hardware and the fault information. Example 3: The CK6150A Z-axis exhibits a sudden, rapid, and uncontrolled movement, accompanied by a TGLS alarm on the H-series DC servo board. The fault is unstable; powering off and on again may restore normal operation. Possible causes of the TGLS alarm include: power line disconnection or reversed connection; no speed feedback or positive feedback; mechanical lockup. Since the Z-axis servo motor speed feedback signal is sent from the position feedback encoder signal at the motor's tail to the CNC mainboard and obtained after F/V conversion by the hybrid IC module, and since the system consistently lacks position feedback alarms, the initial assessment is a problem with the connection between the CNC to the servo amplifier cable and the control board. The cable and speed control board were checked and found to be normal. Because the time from the fault occurrence to the servo protection shutdown is only one or two seconds, it is difficult to observe the presence or absence of the speed feedback signal using an oscilloscope or multimeter. Further analysis revealed that while the position feedback encoder signal level was normal, the lack of movement in the A and B phases caused the aforementioned fault. Therefore, the Z-axis position encoder was replaced, and the machine tool returned to normal operation. This could be due to a loose encoder grating disk, resulting in relative displacement between it and the shaft, or a near-failure of the encoder's internal light source diodes, causing the A and B signals to remain unchanged. 2. Speed ​​Loop Fault Diagnosis Test the speed control unit in open-loop speed mode. This method requires familiarity with the system hardware to avoid accidental damage to components. Example 1: A repaired FB15B-2 DC servo motor malfunctioned after installation on a machine tool. The phenomenon indicated abnormal speed feedback; inspection revealed that the tail tachometer motor brushes and leads were normal. To test the performance of the tachometer motor, the following operations should be performed: Secure the motor reliably, connect the power line, but not the feedback line; Remove the S20 short-circuit jumper on the FANUC H-series servo board to disable the TGLS alarm enable; Turn on the power, and the servo amplifier runs in open-loop speed mode, with the motor operating at a high speed of 2000 r/min. At this time, the tachometer motor output voltage is only 6V, while the normal value is 1.4V, indicating that the servo motor's tachometer motor is faulty. Replacing the tachometer motor restores normal machine tool operation. Example 2: A DM3600 CNC lathe experienced a spindle speed issue, reaching a maximum of only 50 r/min, while the load torque display showed a high reading. The machine tool's CNC system is a Mitsubishi M3/L3, and the spindle servo amplifier model is FR-SF-2-11K-T. Possible causes include: excessive load; a faulty spindle drive power module or control module; or abnormal speed feedback. The mechanical transmission was checked and found to be good. Voltage measurements at various test points on the control module and power module readings were normal. The feedback cable between the spindle motor and drive unit, as well as the drive operating parameters, were also checked and found to be normal. The drive unit operating parameter P00 was set to 1, and a spindle operation command was given. The motor ran at low speed in open-loop speed control, and the load torque was observed to be almost zero, indicating abnormal speed feedback. The speed feedback waveform was observed with an oscilloscope; there was no A-phase waveform. The cover above the motor was opened, revealing the PLC output circuit board. The small connector on the circuit board was reconnected, and the A-phase waveform was found to be normal. The system was restored to closed-loop operation, and the spindle ran normally.