When an AC servo system controls an AC motor in a machine tool, it is necessary to measure the three-phase current of the AC motor. The common method is to directly measure the three-phase current using current sensors. Based on the connection method of the motor windings, at least two current sensors are required, with Hall effect sensors being commonly used. These sensors are typically complex to manufacture, expensive, and bulky; current sampling requires two-way A/D conversion, and strict synchronous conversion is required. This paper proposes a novel design method for a Hall effect-free current sensor servo system. This method can measure the three-phase current of the AC motor using a simple and inexpensive resistor, reducing the cost of the servo system and optimizing its structure. The principle of the current detection method without a Hall effect current sensor: Define a switching quantity sa. When the power transistor in the upper arm of phase a is turned on, let sa = 1; when the power transistor in the lower arm of phase a is turned on, let sa = 0. Similarly, define phases b and c. With a resistor connected in series on the DC bus, the stator current based on the switching state can be expressed as follows: idc = ia when (sa, sb, sc) = (1,0,0) idc=-ia when (sa, sb, sc)=(0,1,1) idc=ib when (sa, sb, sc)=(0,1,0) idc=-ib when (sa, sb, sc)=(1,0,1) idc=ic when (sa, sb, sc)=(0,0,1) idc=-ic when (sa, sb, sc)=(1,1,0) idc=0 when (sa, sb, sc)=(1,1,1) idc=0 when (sa, sb, sc)=(0,0,0) Figure 1 gives an example of a switching state: when (sa, sb, sc) = (0,1,1),idc=-ia [align=center] Figure 1 Schematic diagram of DC side single resistor current detection[/align] Based on the above diagram, the phase current (ia) is the DC bus current. All three phase currents can be measured on the DC bus, but (sa, sb, sc) are different when measuring different phase currents. Implementation method of current detection in digital AC servo without Hall element current sensor. The digital AC servo system without Hall element current sensor designed in this paper uses the DSP (Digital Signal Processor) TMS320F240 from Texas Instruments as the controller. The TMS320F240 uses the space vector method to generate PWM. During PWM generation, the motor voltage vector at any given time falls within one of six zones, as shown in Figure 2: [align=center] Figure 2 Motor Voltage Vector Synthesis Diagram[/align] At any given time, the motor voltage vector can be represented by two vector elements on adjacent basic space vectors: t0 = tp - t1 - t2, uout is the motor voltage vector at any given time, tp is the period of the PWM carrier, and t1 and t2 correspond to the switching modes of ux and ux+60, respectively. The zero vector is used to balance the switching cycle of the transistor. As shown in Figure 3, in a DSP-based digital AC servo system, when performing current detection, t1 can be defined as the duration of the voltage vector that appears first in a PWM cycle, and t2 as the duration of the voltage vector that appears later. [align=center]Figure 3. Effect diagram of space vector PWM period t1 and t2[/align] In the symmetrical PWM modulation method, the first half of the cycle consists of vector states (0, 0, 0), (0, 0, 1), (1, 0, 1), and (1, 1, 1) in sequence. The second half of the cycle includes the same vector state sequence, but in reverse order. Current measurements are taken during periods t1 and t2 to obtain the two-phase current values. The third-phase current can be obtained from the equation: ia + ib + ic = 0. In the example above, the vector state at t1 is (0, 0, 1), so the measured phase current is: ic = idc. Design of a Hall-element-free digital AC servo system In the design of an AC servo system, it is necessary to continuously monitor the three-phase current of the motor. The usual method is to couple a Hall-element current sensor to the motor's power line to measure the three-phase current. This often results in a very complex and costly servo system structure. The structure of a Hall-element-free digital AC servo system is shown in Figure 4. [align=center] Figure 4 Structure diagram of digital AC servo system without Hall element current sensor[/align] When performing current detection, a sampling resistor is connected in series with the DC bus, and the current flowing out of the inverter will pass through it. An operational amplifier is used to collect the voltage across the resistor, and its output is used as the input of the DSP analog-to-digital converter, with a range of 0 to 5V. The requirements for the current sampling resistor are firstly low power consumption, and secondly, the collected voltage value should be adapted to a reasonable analog-to-digital converter sampling gain. Conclusion Compared with other traditional AC motor digital AC servo systems, this paper proposes a digital AC servo system without Hall element current sensor, which is convenient for current detection and reduces costs; this servo system avoids the need for the motor power cable to be wound around the Hall element for current detection, thereby optimizing the structure of the machine tool position and speed AC servo system. This digital AC servo system design method can also be used in AC spindle drive systems and frequency converter systems, and this design method has a very good application prospect. References [1] Wang Chengyuan et al., Vector Control AC Servo Drive Motor, Machinery Industry Press, 1995. [2] Dan Teodorescu, Speed ​​Control Transmission System for Permanent Magnet Synchronous Motor, Foreign Electrical Automation, 1992. [3] Griva G., Profumo F., Abrate M., Tenconi A., Berruti D., Wide Speed ​​Range DTC Drive Performance with New Flux Weakening Control, IEEE Annual PE Specialists Conference, 1998, pp. 1599-1604. [4] Leebyeong-seok, Krishnanr., Adaptive Stator Resistance Compensator for High Performance Direct Torque Controlled Induction Motor Drives, IAS Annual Meeting, 1998, pp. 423-430. [5] Texas Instruments, Implementation of Vector Control for PMSM Using the TMS320F240 DSP, Literature Number: Spra494, 1998.