Abstract : This paper introduces a high-performance servo system driven by a brushless DC torque motor. Through rational design of hardware and software, good servo system performance is achieved. Keywords: brushless DC torque motor, servo system, PWM, rotary transformer [align=center]The Study on High Performance Servo System Driven by Brushless DC Torque Motor Wang Mulan Gu Shenggu[/align] Abstract : This paper introduces a kind of high performance servo system driven by a brushless DC torque motor. We achieve good dynamics and static system indexes with the rational design of hardware and software Keywords : brushless DC torque motor, servo system, PWM, rotary transformer 1 Introduction A servo system, also known as a position follow-up system, requires fast, stable, and accurate position control. Ordinary servo motors usually have high speed and low torque. When used as actuators to drive loads in a system, they must pass through a gear reduction device. However, due to the influence of backlash, the accuracy and stability of the system often decrease. Therefore, in order to reduce or even eliminate errors, simplify the system structure, improve accuracy and stability, and achieve the goal of using less or no reducers, a torque motor was developed and applied. It features low speed, high torque, and high precision, which can meet the higher performance requirements of servo systems. Furthermore, the use of brushless electronic commutation to replace the original mechanical commutation can further improve the system's reliability, while also conforming to the development trend of "brushless, rare-earth, and synchronized" systems. 2 Hardware Design 2.1 Basic Principles The principle block diagram of the brushless DC torque motor servo system is shown in Figure 1. In Figure 1, the input angle position command signal is sent by the system host computer through the dual-port RAM. It is compared with the current position digital signal measured by the feedback circuit to obtain the position deviation signal. After processing by the position correction stage P(s), it serves as the given signal for the speed loop. This signal is then compared with the actual speed to obtain the speed deviation. After calculation by the speed correction stage H(s) and D/A conversion, it is output to the PWM commutation logic stage. This logic stage integrates the rotor position signal from the motor commutation sensor to form a corresponding trigger pulse, controlling the switching of the power devices and creating a rotating magnetic field in the motor to drive the load to the required position. [align=center] Figure 1 Servo System Principle Block Diagram[/align] 2.2 The square-wave brushless DC torque motor used in the brushless DC torque motor system is specially developed. Its main electrical performance parameters are as follows: Voltage DC60V; Maximum current 6 A; Phase resistance 5.5 N; Maximum speed 380 r/min; Peak stall torque 5 N·m; Position sensor is photoelectric. Its working principle is as follows: When the motor is in a certain position, the photoelectric sensor gives a corresponding switching logic signal, which generates a trigger pulse to drive the power transistor, making the corresponding winding of the motor conduct, and the internal magnetic field makes the motor rotate to a new position. When the position sensor receives a new set of logic signals, it triggers another set of power transistors to make the motor windings in another conduction state. This cycle repeats, generating a rotating magnetic field in the motor, making the motor run. A photoelectric position sensor is provided at the end of the motor. When the light is blocked, the output is "0", and when the light is not blocked, the output is "1". The position encoding of rotation in one direction is shown in Table 1. [align=center]Table 1 Power transistor conduction status corresponding to motor position state (forward rotation A6=0)[/align] 2.3 Commutation logic circuit and power amplifier circuit are shown in Figure 2. The commutation signal, PWM control signal and forward/reverse control signal sent by the photoelectric position sensor are shaped and synthesized, and used as the address signal of EPROM (2732). The corresponding power transistor conduction phase sequence signal is found from it. The output is after inversion, shaping, and driving. The corresponding logic relationship is shown in Table 1. According to the phase sequence listed in Table 1, the rotation vector diagram of the internal magnetic field of the motor can be drawn as shown in Figure 3. The power amplifier circuit used in the system is the intelligent power integrated circuit PWR-82333 produced by DIX Corporation of the United States. It is a hybrid integrated module containing logic connection circuit, drive circuit, overcurrent/overvoltage/undervoltage protection circuit, power conversion circuit (DC/DC), power tube and freewheeling circuit, etc. The power switching device is IGBT, with a rated voltage of 500 V, a rated current of 30 A, and a switching frequency of 25 kHz. Here, the 6 IGBT tubes in this module are used to realize the switching control of the three-phase bridge circuit. 2.4 Rotary Transformer and Demodulation Circuit The position feedback signal in the system is measured by a sine and cosine rotary transformer. It is a specially developed high-precision small frame size, low-voltage excitation brushless rotary transformer. The main electrical performance indicators are: excitation 2 V/2kHz, turns ratio 1, electrical error ≤ ±3, phase shift ≤ ±4. The RDC (Resolver to Digital Converter) interface chip demodulates and transforms the sine and cosine signals containing load position information from the resolver to obtain angular position signals, which are directly sent to the servo computer for control calculations. Additionally, it provides an analog speed signal of the load rotation, which is then sent to the computer via A/D conversion. The integrated chip used here is the RDC2S8O from ANA1 OG DEVICE. Its main specifications are: resolution 10/12/14/16 bits, reference signal 2 V/2 kHz, resolver output signal 2 V/2 kHz, accuracy ±2 ars-min, power supply -5 V/±12 V, power consumption 0.3 W. It should also be noted that the electrical zero point of the resolver and the mechanical zero point of the load must be consistent; otherwise, it will affect the accuracy and control of the entire system. This means there is a zeroing process during load assembly. 3 Software Design 3.1 Establishment of the Object Model Considering the influence of the acceleration borne by the load, the peak torque is required to reach 20 N·m, and the rotation angle range is 120°. (60°) Based on the constraints of the base structure, and through optimized design, a precision backlash-free gear system was installed between the reducer output shaft and the load, and between the reducer output shaft and the rotary transformer, to achieve the purpose of driving and detection. The entire object is divided into two loops: azimuth and pitch. Modeling it using physical methods would be quite complex. Here, HP's 3562 A dynamic signal analyzer is used to directly approximate the object as a first-order inertial element. According to the amplitude characteristic method, the open-loop transfer function of the servo system speed loop is (-4/(1-0.26)). It is equivalent to the open-loop transfer function between " and 0" shown in Figure 1. 3.2 Correction Design Since the system is required to have high dynamic tracking performance, while the requirements for static positioning accuracy and overshoot are relatively low, conventional lead and lag links are selected for series correction design. The speed loop correction link is calculated to be 3.3 Control Program Flowchart The servo system control cycle is 5 ms. The entire control algorithm is completed by 80386. The required input data and control state are exchanged with the radar main computer through a 2K capacity dual E-port RAM. The main program is actually an interrupt service program. Its flowchart is shown in Figure 4. 4 Experimental Results The servo system was successfully developed using the above hardware and software. The test results and actual use also show that the expected purpose has been basically achieved. The main measured performance indicators of a certain servo system are: maximum angular velocity 1j2°/s, maximum angular acceleration 3820°/s, servo bandwidth > 4 The system features a Hz refresh rate, a transient response time of 0.335 s, an overshoot of 16.25, an oscillation frequency ≤ 1, and an overload capacity of 7 g. Furthermore, various filtering techniques are employed to suppress various interference signals. A linear extrapolation algorithm is used to further improve the system's speed, and dead-zone and nonlinearity compensation are employed to achieve higher accuracy. Detailed design information for this part will be provided separately. References: 1. Gu Shenggu, *Fundamentals of Electric Motors and Drives*, Beijing: Machinery Industry Press, 1981. 2. Chen Boshi, *Automatic Control Systems for Electric Drives*, Beijing: Machinery Industry Press, 1992. 3. AD Company Description: Vartable Resulution Monolrthn Resolver-to-Dignal Converter-2SRO. Research on High-Performance Servo System for Brushless DC Torque Motor Drive.pdf