This paper introduces a high-performance all-digital AC spindle servo drive system. The system uses an intelligent power module (IPM) as the inverter switching element and a high-performance DSP (ADMC401) as the control core. The control algorithm employs rotor magnetic field-oriented slip frequency vector control, achieving current control, speed control, and position control. Operational results show that the system has high speed regulation performance, low-speed load characteristics, and good dynamic and static performance, achieving spindle positioning and C-axis functionality. Slip Frequency Vector Control When the d-axis of the rotor flux linkage rotating coordinate system is selected in the direction of the rotor flux linkage vector, and the rotor flux linkage is controlled to be constant in steady state, it is necessary to control the slip angular frequency ωs, rotor flux linkage ψdr, electrical angular frequency ω1, and rotor flux linkage angle θe, respectively: Where: ωm—rotor mechanical angular frequency; p—pole pair number. The electromagnetic torque equation of the AC asynchronous spindle motor is: Where ids and iqs are the stator current components on the d and q axes, respectively; l1, l2, and m12 are the stator and rotor inductances and mutual inductance, respectively; r2 is the rotor resistance; ω1 and ωs are the electrical angular frequency and slip angular frequency, respectively; and p = d/dt is the differential operator. This achieves linear torque control, but the state equations for ids and iqs still contain coupling terms involving the product of ids, iqs, and ω1. From the perspective of current controller design, it is necessary to take decoupling control measures to facilitate controller design. Therefore, a voltage feedforward compensation method is needed in the dq coordinate system to achieve independent control of the torque and excitation components of the motor stator current. The voltage feedforward compensation control is shown in the following equation: where is the leakage flux coefficient, uds and uqs are the stator voltage components on the d and q axes, and u′ds and u′qs are the outputs of the magnetic field and torque current controllers. This removes the coupling term of the stator current component in the dq coordinate system during steady-state operation, and the controller can be designed independently as a first-order system. The system control structure block diagram is shown in Figure 1. System Structure The structure of the all-digital AC spindle servo drive system with a digital signal processor as the control core is shown in Figure 2. As can be seen from the figure, the system mainly consists of the following parts: control circuit, main circuit (including rectifier and inverter), and switching power supply circuit. The ADM401 high-performance digital signal processor uses a 16-bit fixed-point DSP, the ADSP2171, as its core and integrates a rich set of peripheral controllers onto a single chip. Its instruction execution speed is 26 miPs. The main on-chip peripherals include: • A high-precision 8-channel A/D converter (ADC) with 12-bit conversion accuracy, supporting two-channel synchronous sampling and sequential sampling, with a conversion time of less than 2μs. • A three-phase 16-bit PWM generator (PWM) with programmable switching cycle, dead time, and minimum pulse width limit. It features single-refresh and double-refresh modes, and can generate symmetrical and asymmetrical PWM waveforms, as well as space vector PWM waveforms. The PWM output can be locked by external pins or software programming, and it has strong fault protection capabilities. The on-chip incremental encoder interface unit (EIU) features a programmable pulse input filter, enabling functions such as frequency multiplication, direction detection, and counting. Combined with a dedicated encoder event timer, it facilitates m/t method speed detection, providing high-precision position and speed feedback for high-performance motor control. The ADMC401 is the core of the entire system, used for vector transformation, current loop, speed loop, position loop control, SVPWM signal generation, and various fault protection handling. To achieve rapid real-time control, the system adopts a dual-CPU structure of microcontroller + DSP. Therefore, the system's control tasks are divided: the DSP handles vector control and closed-loop control with high real-time requirements; the FLASH-based microcontroller 89C8252 handles management tasks with lower real-time requirements, such as control parameter setting, keyboard processing, status display, and serial communication; and the FPGA handles parallel data exchange between the microcontroller and DSP, external I/O signal management, position pulse command processing and counting, fault signal processing, and spindle encoder counting. The system supports multiple input methods, including analog speed input, digital speed input, pulse input, and control via a host computer. Main Circuit and Switching Power Supply Circuit The main circuit of the system adopts a modular design. The rectifier power supply and the AC-DC-AC voltage source inverter are connected through a common DC bus. The rectifier power supply uses a diode rectifier module. A soft-start circuit is also designed to reduce the impact of high voltage on the DC smoothing capacitor of the main circuit. The inverter uses a PM 75CVA 120 intelligent power module. In the system's fault protection circuit, it is equipped with protections against main circuit overvoltage, undervoltage, overheating, overload, abnormal braking, photoelectric encoder feedback disconnection, and communication failure. Fault signals are detected by a combination of hardware and software. Once a protection signal is detected, the PWM drive signal is immediately blocked via software or hardware logic. The system uses magnetically balanced Hall current sensors to sample the two-phase current feedback (ia, ib) to obtain real-time current information. The system's control power supply uses a switching power supply, with the TO P224 power switching device selected. For the power supply of the spindle motor's photoelectric encoder, considering the potentially large voltage drop on its feedback signal line, which could affect the reliability of the feedback signal, a separate DC-DC converter with feedback regulation is used for power supply. Experimental Results and Control Function Implementation The all-digital AC spindle servo drive system experiment used an AC variable frequency spindle motor: rated power 3.7kW/5.5kW; rated current 9A; rated synchronous speed 1500r/min; rated torque 24.6nm. The photoelectric encoder mounted on the motor had a resolution of 1000p/r. The current control sampling period was 100μs, the PWM switching frequency was 10kHz, and the speed loop and position loop sampling periods were 500μs. The output limit of the speed loop was 1.2 times the rated current. Constant Torque Control When the AC spindle servo drive system operates below the base speed, the system should exhibit constant torque characteristics. Especially at low speeds, the system should output a considerable torque and maintain smooth operation. Figure 3 shows the speed response curve and current waveform when the system is given a speed of 1000r/min. The results show that the system response is relatively fast. Figure 4 shows the torque characteristics of the system when operating below the base speed, indicating that the system has a wide range of load torque output capabilities. Constant Power Control When the AC spindle motor operates above its base speed, the system automatically enters a field weakening state. During this constant power operation, the stator voltage should remain constant, and the rotor flux linkage should be reduced to achieve field weakening control. According to the slip frequency vector control principle, the stator flux linkage current can be controlled according to the following formula: Where ωbase is the rated synchronous speed of the induction motor. When the motor enters the field weakening range, in order to ensure a sufficient electromagnetic torque output, the torque current limit value of the speed regulator pi needs to be adjusted. The adjustment method is as follows: Where i*qslimit is the output limit value of the speed regulator, imax is the maximum allowable output current of the system, and k is the adjustment coefficient, generally chosen to be less than 1, to prevent potential stall problems during high-speed startup. Figure 5 shows the speed response curve when the system is given a speed of 5500 r/min. Spindle Orientation Control and C-Axis Function Using a position encoder mounted on the spindle motor, high-speed electrical spindle orientation control can be easily achieved, allowing the spindle to accurately stop at a specified position. The spindle orientation position can be easily set via communication or manual operation. The experimental results of high-speed spindle orientation control are shown in Figure 6; the orientation process is completed within 1.2 seconds. The C-axis function can be implemented by switching between speed control and position control. The all-digital AC spindle servo drive system proposed in this paper fully utilizes the high-speed computing power and rich on-chip peripheral resources of the DSP ADMC401, ensuring real-time control and achieving high-precision speed regulation and positioning requirements. It features a wide speed range, high control accuracy, good dynamic and static performance, and comprehensive protection functions. The system's speed range reaches 1:1000, and the constant power range is 1:4 to 12. References [1] Qin Yi, Li Junyuan. Fundamentals of Modern AC Motor Control Technology. Guangdong Science and Technology Press, 1993 [2] DW Novotny, Ta Lipo. Vector control and dynamics of AC drives. Clarendon Press. Oxford, 1996 [3] BK Bose. Modern power electronics and AC drives. Beijing: Machinery Industry Press, 2003 [4] Bi Chengen. Modern CNC Machine Tools. Beijing: Machinery Industry Press, 1991