Abstract: This paper introduces the fault diagnosis method of stepper motor drive circuit commonly used in CNC machine tools and gives a diagnostic example of the XK91-2 bearing cage milling machine. Keywords: Stepper motor, fault diagnosis. With the increasing application of CNC machine tools, improving the level of maintenance and repair technology and ensuring a high integrity rate of high-tech equipment has become one of the urgent issues for promoting high technology. Among the faults of such equipment, the fault rate of stepper motor drive and protection circuits is relatively high, and the fault point is also relatively difficult to diagnose. Therefore, this paper discusses the diagnostic method and precautions for stepper motor drive circuit faults. 1 Determining the scope of the fault. Nowadays, most CNC machine tools are controlled by microcomputers. When a fault occurs, the program often terminates normal operation and enters the fault detection and alarm handling program. According to the alarm prompt information, the stage to which the equipment was executed before stopping can be estimated to estimate the fault location. Based on the estimation, measurement preparations can be made, and the relevant interface signals can be observed after restarting the machine tool. If the output interface signal controlling the stepper motor is normal but the motor does not run, and the missing step pulse voltage on the motor winding is checked, then the stepper motor drive circuit fault can be determined. The signals for each winding of the stepper motor can come from a ring distributor, which is controlled by a microcomputer. At this point, the control signals sent by the microcomputer and the output signals of the ring distributor should be further checked for correctness. If the input is correct but the output is incorrect, then the ring distributor is definitely faulty. If the output is normal, then the power drive stage should be checked further. 2. Locating the Ring Distributor Fault: Locating the ring distributor fault involves disconnecting the power drive stage to avoid burning out the power stage due to incorrect input signals. If the output signal waveform of the ring distributor is unavailable, it can be obtained by analyzing the circuit and compared with the observed waveform. If a single pulse input can be used to make the distributor change step by step, statically checking the operation of each logic component, then a multimeter can be used to find the fault point, which is a convenient and easy method. 3. Locating the Power Drive Circuit Fault: The power drive circuit operates under high voltage and high current conditions, and faults often burn out the power transistors. Therefore, the power transistors are generally checked first. This is especially important when a fault causes the protection to trip and the fuse to blow. After discovering a damaged power transistor, further inspection of the related circuitry is necessary. Power should be applied with the power transistor disconnected to verify the drive signal is correct before reactivating the power transistor, preventing further damage to the power transistor and potentially the power supply circuit. In some circuits, the protection circuit may trip and cut off power when the power transistor is disconnected, preventing dynamic testing of other circuits. In such cases, after disconnecting the power transistor, the protection circuit should be disconnected again, and a thorough check should be performed for any faults that could cause a short circuit. Power should only be applied for testing after confirming that a short circuit is unlikely, to prevent further damage to components without protection. After troubleshooting, reconnect the power transistor. If the current is adjustable, set it to the minimum position and gradually increase it to a suitable value after power is applied. The motor should rotate normally. Finally, mount the motor on the machine tool, apply a mechanical load, operate the equipment, and observe for a period of time. If no abnormalities are observed, the repair work can be completed. When observing the voltage between two ungrounded points using an oscilloscope, such as the voltage on the stepper motor windings mentioned in later examples, the oscilloscope's common terminal is connected to the casing. Therefore, the oscilloscope casing should be insulated from ground, and safety precautions should be taken during operation. If the oscilloscope has dual-ended input, the common-mode voltage should be considered to ensure it is within the oscilloscope's allowable range. 4. Troubleshooting Examples: Taking the xK91-2 bearing cage milling machine produced by Quanzhou Machine Tool Factory as an example, this paper analyzes its stepper motor drive circuit and explores dynamic testing methods. These methods are also valuable for locating fault points in other CNC machine tool models. This milling machine is a dedicated CNC milling machine for machining bearing cages, controlled by a TP801A single-board computer. Except for workpiece replacement, all processes are performed automatically. The indexing value can also be freely and conveniently set by a dial switch, improving labor efficiency and machining accuracy. The key to achieving high precision is the accurate control of the stepper motor by the microcomputer to change the indexing head angle. The indexing motor is a 90BF002 type reactive stepper motor with a maximum torque of 40 kg·cm, a step angle of 0.75 degrees, a rated current of 7A, and a five-phase ten-beat operating mode. The five-phase pulse control signal generated by the ring distributor is applied to the stepper motor windings via the drive circuit. The clock pulse of the ring distributor can be provided by an oscillator in the self-test position, while in the working position it is controlled by a pulse sent by a microcomputer, thereby controlling the rotation angle of the indexing motor. As shown in the circuit of the ring distributor in reference [1], the clock pulse is provided by a rectangular wave oscillator composed of a time base circuit 555 (IC210) during self-test. Switch K203 sends the oscillation pulse to each D flip-flop. The ring distributor is mainly composed of five two-input quad AND-OR-NOT gate circuits CD4086 with extended terminals, three dual D flip-flops, MC14013 and a hex inverter MN4069. The generated control signal is output to the drive power stage by the integrated Darlington circuit MC1413P. All five CD4086 are connected as two-input two AND-OR-NOT circuits. Switch K202 controls the forward and reverse rotation of the motor. When rotating forward, K202 sends a low level to pins 2 of IC204 to IC208, disabling their pins 1 and 2 AND gate. Simultaneously, the low level is sent to IC209-4, inverted to a high level, making pins 5 and 6 of IC204-IC208, and pin 5 of the AND gate, high. Based on the two-input AND-OR-NOT logic relationship, the logic level of the D input of IC201-1 is [logic level missing]. Since pin 6 of IC204 is connected to IC202-2, the D input of IC201-1 is at the same logic level as the D input of IC202-2. Similarly, the D input of IC201-2 is at the same logic level as the D input of IC203-1, the D input of IC202-1 is at the same logic level as the D input of IC201-1, the D input of IC202-2 is at the same logic level as the D input of IC201-2, and the D input of IC203-1 is at the same logic level as the D input of IC202-1. During reset, K201 introduces a high level to the set "1" terminal S of IC201-1, IC201-2, and IC202-1, and the reset "0" terminal R of IC202-2 and IC203-1. Therefore, after reset, the logic levels of the Q terminals of IC201-1 to IC203-1 are sequentially 11100. Using this as the initial state, and based on the previously derived logic relationship, the five-phase waveform can be derived step by step. Test the actual waveform when K201 is in the working position. If it does not match the analysis, K203 can be switched to "external" and the microcomputer interface can be disconnected. Connect a resistor of about 1kΩ to +5V at this point, which will make the ring counter work step by step. Use an oscilloscope, logic probe, or multimeter to check the logic level at each point after each step. Check whether the relationship between the input and output logic levels of each device matches the device to find the faulty device. The analysis and inspection method is the same during reversal. The power drive circuit of this machine tool consists of an identical five-phase circuit, but this article only analyzes one phase. Since the stepper motor operates under high current and high voltage conditions, its power supply circuit is equipped with two types of protection: overcurrent protection and winding energization timeout protection. During an overcurrent event, the voltage generated by R406 triggers the thyristor Kp401 to conduct, bypassing the base current of BG404 and causing BG403 to cut off the fault current, thus achieving overcurrent protection. The power-on timeout protection is mainly completed by the timer circuit 556 (IC101-1). A positive pulse from the ring distributor cuts off D101. If D102 is off, BG101 conducts, which on one hand turns on optocoupler BG103, applying +80V to the motor winding Ψ[sub]A[/sub] via BG104 and BG105. On the other hand, BG102 and R123 charge C402. If BG101 conducts for too long, C402 reaches the action threshold, causing terminal 1 of IC101-1 to discharge, D108 to conduct, triggering KP402 to conduct via the optocoupler. Similar to KP401 conducting, this causes BG403 to cut off, interrupting the load current. During normal circuit operation, the positive pulse from the ring distributor also cuts off D103, thus turning on BG107 and BG108, providing power to the winding Ψ[sub]A[/sub]. The current provides a path, and the voltage generated by the winding current through R116 is applied to the control electrode of IC101-2. Since the inductor current of Ψ[sub]A[/sub] increases over time, the voltage on R116 can reach the discharge threshold of IC101-2 at a certain moment. Terminal 13 of IC101-2 discharges, D102 conducts, causing BG101 and BG102 to be cut off, and the charge of C402 discharges to ground through R123 and R104. As long as each process is relatively short, the timeout protection will not operate. Furthermore, the cutoff of BG101 causes BG103 to be cut off, and through BG104 and BG105, Ψ[sub]A[/sub] is de-energized, limiting the winding current. Afterward, IC101-2 resets and energizes Ψ[sub]A[/sub] again, forming an oscillating current with limited amplitude on Ψ[sub]A[/sub]. The difficulty in diagnosing this part of the circuit lies in the fact that BG105 and BG108 are easily burned out when a fault occurs. Therefore, it is not advisable to connect these two power transistors when the drive signal is abnormal. However, at this time, there is no current in Ψ[sub]A[/sub], which leads to IC101-2 not discharging, and B101 and BG102 conducting for a long time, causing the timeout protection to activate, making it impossible to dynamically check this part of the circuit. Therefore, under the premise of disconnecting these two power transistors, disconnect D108, disable the timeout protection, power on and check the circuit. After it is normal, connect the good power transistors, connect D108, adjust R1101 to the lowest voltage division position, start powering on, and then gradually adjust RW101 to increase the current to the normal value. After observing that the operation is normal, install the motor, apply the actual mechanical load, and operate according to the procedure. If the operation is normal, the maintenance work is over.