Abstract: Linear drive systems are widely used due to their simple mechanical structure. This paper proposes a current detection system for a permanent magnet linear synchronous motor based on a magnetic field balanced Hall current sensor and its implementation scheme. The basic principle of the current detection system, the processing of the current signal, and the overcurrent protection function are analyzed. Experimental results verify the effectiveness of the current detection system. Keywords: Permanent magnet linear synchronous motor; Hall current sensor; Overcurrent protection [b][align=center]Design of the current detection system of the permanent magnet linear synchronous motors CHEN YOUPING, ZHANG YING AI Wu, LIANG CHAOYU[/align][/b] Abstract: The linear motor system is widely used because of its simple structure. This paper presents a feasible scheme of the current detection system based on the magnetic balanced Hall current sensor. The system's basic principle, current signal processing, and overcurrent protector are analyzed respectively. The validity of the current detection system is confirmed by experimental results. Keywords: the permanent magnet linear synchronous motor (PMLSM); Hall current sensor; overcurrent protector 1. Introduction High-speed machining and micro-machining are important development directions in mechanical manufacturing and scientific research. This requires the feed system to have large driving force, high frequency response, and high motion accuracy and displacement stiffness. Permanent magnet linear synchronous motor (PMLSM) has the characteristics of large thrust, low loss, small electrical time constant, fast response speed, high power factor, measurable control parameters, and good control performance. Compared with other high-speed precision feed systems, PMLSM feed system has great advantages. Therefore, PMLSM has broad application prospects in lifting systems, electronic manufacturing industry and high-speed precision CNC systems, and has attracted the attention and research of many scholars [1]. The permanent magnet synchronous linear motor control system in this paper adopts a three-loop control strategy, that is, the position loop, speed loop and current loop are controlled separately. The linear motor position servo control system is shown in Figure 1. Among them, the position loop and speed loop are the outer loops, and the current loop is the inner loop of the servo system. In the three control loops of the servo control system, the current loop is located in the innermost loop and is also the most important link, because in the linear motor vector control system, high-performance thrust control is obtained through the current controller, and the performance of the current loop will directly affect the performance of the outer loop and the entire system. Therefore, it is of great significance to study the current detection system of permanent magnet synchronous linear motor. [align=center] Figure 1 Block diagram of permanent magnet linear synchronous motor control system[/align] 2. Current detection technology based on magnetic field balance Hall current sensor The current detection of permanent magnet linear motor is completed by current sensor module. Since the working current of AC linear motor is relatively large under rated working conditions, about 5A, there are two requirements for current sensor when detecting current signal: First, it generates a signal proportional to the measured current, and the current sensor is required to have high accuracy, fast response time and good working stability to achieve the purpose of real-time control; Second, in order to prevent mutual interference between the main circuit and the control circuit, the main circuit and the control circuit must be isolated. Therefore, the current detection of this system adopts CSN series magnetic field balance current sensor as current detection element [2]-[4], which is characterized by the application of Hall effect closed-loop compensation. It has excellent accuracy, good linearity, low temperature drift, excellent response time, wide bandwidth, frequency range up to, and strong anti-interference ability. It is constructed using the principle of electro-magnetic-electric conversion, integrating current transformer, magnetic amplifier, Hall element and electronic circuit together. It has strong anti-interference ability and fast dynamic response, and is very suitable for detecting motor winding current. The working principle is shown in Figure 2. [align=center] Figure 2 Working principle of Hall current sensor[/align] In Figure 2, there is a magnetic ring with a gap made of soft magnetic material, and a Hall element is placed in the gap. A fixed current I[sub]c[/sub] flows through the Hall element, and a wire passes through the magnetic ring, through which the current to be measured flows. A magnetic field is generated in the magnetic ring and its gap, with a magnetic induction intensity of B. Then the Hall element generates a Hall potential difference V[sub]H[/sub]. (1) Where — K is the Hall coefficient. V[sub]H[/sub] is amplified by amplifier A to obtain a compensation current I[sub]s[/sub]. The current I_s flows through a multi-turn coil wound on a magnetic ring. The magnetomotive force generated by V_H is opposite in direction to the magnetomotive force generated by the current being measured, thus producing a compensation effect and reducing the magnetic field. Because the magnetic induction intensity in the iron core is extremely low, it will not saturate the core, nor will it generate large hysteresis losses or eddy current losses. Because the amplifier has a very large amplification factor, therefore: 3. Current Signal Processing Since the output of the Hall sensor is a current signal I_s, in order to send the current signal to an analog-to-digital (A/D) converter for conversion, the output of the Hall sensor is passed through a measuring resistor R_m with a known resistance value. The voltage drop V_m across R_m is proportional to the current signal I_s, thus linearly converting the current signal into a voltage signal V_m, which can then be sent to the computer after analog-to-digital conversion. [align=center]Figure 3 Current Detection Signal Processing Circuit[/align] In this system, the current analog input signal processing circuit of the ADC module is shown in Figure 3. Here, I[sub]u[/sub] is one of the current signals output by the Hall sensor after detecting the three-phase AC current of the linear motor. LF353N is an amplifier integrated chip, used as a voltage follower at X201A to improve the sampled current output signal; and as an inverter at X201B. By adjusting the effective resistance value of potentiometer R203, it can be ensured that when the current signal I[sub]u[/sub] changes within a predictable amplitude range, the output signal CH1A of the processing circuit is within a voltage range of ±5V, thus meeting the requirements of the analog-to-digital converter for the input analog voltage signal. Acquiring the converted voltage signal is an important part of current detection. The DSP chip TMS320LF2407A has a built-in 10-bit analog-to-digital converter with a built-in sampling and holding circuit, but it can only receive unipolar signals of 0V-3.3V. At the same time, since the system has high requirements for current detection accuracy, the Maxim 14-bit MAX125 converter is used as the core of the signal acquisition unit. The eight input channels of the MAX125 all have ±17V input fault protection circuits, and the sampling voltage range is -5V to +5V, which is very suitable for the application of DSP-based linear motor current data detection system. In this system, the two-phase current signals I[sub]u[/sub] and I[sub]v[/sub] detected by the Hall sensor are converted into voltage signals CH1A and CH2A by the processing circuit, and then sent to the MAX125 chip for analog-to-digital conversion. Since the linear motor windings are connected in a star configuration, according to the principles of motors: (3) After detecting I[sub]u[/sub] and I[sub]v[/sub], I[sub]w[/sub] can be calculated. Therefore, in hardware design, only any two phases of the three-phase AC power need to be detected and converted from digital to analog, thus reducing hardware costs. 4. Overcurrent and Protection The starting current of a linear motor is very large, or malfunctions in the control circuit or drive circuit may cause short circuits in the output circuit, resulting in excessive current flowing through the insulated bipolar transistors (IGBTs) in the motor drive system. This can cause the current flowing through the IGBT to exceed its rated current and burn it out. Therefore, a circuit is needed to quickly detect excessive current and take appropriate overcurrent protection measures when the IGBT is subjected to excessive current, shutting it off in time to prevent damage. A Hall effect sensor is used to detect the bus current. When an overcurrent signal is generated, it is transmitted to the DSP control system, causing an interrupt (power drive protection interrupt input) and shutting off the control signal waveform. The overcurrent and overvoltage fault detection circuit is shown in Figure 4. In the figure, capacitor C acts as a filter, and potentiometer V[sub]R[/sub] provides a reference voltage. Adjusting V[sub]R[/sub] can change the power supply voltage value that triggers the protection circuit. After I[sub]u1[/sub] and I[sub]w1[/sub] are compared by a comparator, their output signals are sent to a logic chip via optocouplers. After being processed by other overvoltage (three-phase AC voltage) detection signals, the SPDPING signal is generated and sent to the DSP to block the DSP's PWM port. All PWM output pins are in a high-impedance state, and an interrupt request is simultaneously sent to the DSP core to notify the CPU of an abnormal situation. [align=center]Figure 4 Overcurrent Protection Detection Circuit[/align] 5. Experimental Results The parameters of the three-phase AC permanent magnet synchronous linear motor, which is the controlled object, are shown in Table 1. Table 1 Parameters of the Permanent Magnet Synchronous Linear Motor In the entire permanent magnet synchronous linear motor control system, the sampling period of the current loop is 0.1ms, the sampling period of the speed loop is 0.5ms, and the sampling period of the position loop is 1ms. Experimental results show that after adopting the current detection system proposed in this paper, the control performance of the permanent magnet synchronous linear motor system has a small overshoot and a fast adjustment speed. It can achieve precise control of the thrust current even in the presence of interference, as shown in Figure 5. [align=center] Figure 5 Current during the start-up process of permanent magnet synchronous linear motor[/align] 6. Conclusion In the closed-loop control system of permanent magnet synchronous linear motor, high-performance thrust control is obtained through the current controller. The performance of the current loop will directly affect the performance of the entire control system. Therefore, this paper proposes a current detection, processing and overcurrent protection system based on a magnetic field balance Hall current sensor. Experiments show that the permanent magnet synchronous linear motor using this current detection system can accurately control the thrust current, thereby obtaining high control accuracy. Innovation: A current detection, processing and overcurrent protection system based on a magnetic field balance Hall current sensor is proposed. On this basis, the permanent magnet synchronous linear motor can achieve accurate control of the thrust current. References [1] Xu Yuetong, Fu Jianzhong, Chen Zichen, Thrust fluctuation and experimental study of permanent magnet linear synchronous motor. [2] Li Liang, Que Peiwen, Chen Liang. Application of new Hall sensor in current detection. Instrumentation Technology and Sensors, 2005 (4), 3-5. [3] Lu Guanghua. Research on the application of magnetically compensated Hall sensor in power electronics. Instrumentation Technology, 2003 (9), 46-47. [4] Deng Zhongyi. Design of DC current detection circuit using Hall sensor. Microcomputer Information, 2003 (8), 61-62.