Abstract : This paper mainly studies the structural composition, control scheme, and hardware and software design of a satellite antenna servo control system. The design scheme makes full use of DSP hardware resources and complex logic arrays to achieve coordinated control of the drive motor and the balancing motor. The system has high speed stability and high-precision scanning cycle, and can operate reliably for a long time in a simulated outer space environment. Keywords : Satellite antenna; Servo control system; Permanent Magnet motor; Polygonal flux linkage locus; Digital signal processor [align=center]Design about Antenna Driver of Secondary Planet System Based on DSP Jia Yaqiong (Hunan Institute of Technology, Hengyang 421008, China)[/align] Abstract : This paper mainly studies the antenna servo control system: system composition control scheme, hardware and software design. The harmonious control of the driver motor and balance motor is realized by making full use of the DSP hardware resources and complicated programmable logic device. The result shows that the system has good speed stability, high precision scan period and can work reliably under the simulative space environment. Keywords : Satellite antenna; Servo control system; Permanent Magnet motor; Polygonal flux linkage locus; DSP 1 Introduction The reliable and stable rotation of the scanning antenna spindle on a satellite is a key and important component of the satellite. Domestically used satellite antenna scanning drive systems all adopt brushless DC motor or stepper motor drive technology. This project specifically designs a novel rare-earth permanent magnet synchronous motor servo drive structure to address the reliability and stability issues of a certain type of satellite. Its main task is to ensure the stable rotation of the onboard antenna while providing accurate angular position measurement data, and transmitting power, remote sensing data, engineering telemetry data, and remote control commands between the rotating and stationary parts. 2 System Overall Design 2.1 System Composition The satellite antenna drive system consists of five parts: a drive motor, a moment of momentum balancing motor, a rotary grating encoder, a servo controller, and a power converter. The system structure block diagram is shown in Figure 1. [align=center] Figure 1 System Composition Block Diagram[/align] Their respective functions are as follows: 1. Drive Motor: The power source for driving the antenna rotation. 2. Momentum Balance Motor: Used to balance the momentum during antenna rotation. 3. Rotary Grating Encoder: Used to measure the angle of antenna rotation and input it to the servo motor drive controller to ensure precise control of the antenna's speed and angle. 4. Servo Controller: Used to control and drive the drive motor and moment of momentum balancing motor for high-precision operation, as well as for information processing and communication with the control subsystem. 5. Power Converter: Used to convert the DC28V on the satellite and transmit it to the rotating antenna and rotating components as power supply. Information Transmission Device: Used to transmit information from the rotating antenna to the fixed equipment on the satellite, and to transmit control signals from the fixed equipment to the rotating antenna. This device ensures reliable information transmission during relative motion. 2.2 System Control Scheme In this system, the drive motor drives the antenna to perform periodic scanning, providing imaging data to the imaging equipment. It requires a position signal at the start of the scanning period, precise scanning period, and theoretically, the scanning antenna should rotate at a strictly uniform speed. Therefore, the drive motor requires both speed control and position control; it is a combination of speed control and position control. The block diagram of the drive motor control principle is shown in Figure 2. [align=center]Figure 2. Control Block Diagram of the Drive Motor[/align] According to the control system schematic, based on the speed requirement, the rotor position of the motor is calculated and a signal is given. This signal is compared with the position feedback signal. If there is an error, a position adjustment algorithm is calculated, and the result is used as the speed input. A speed adjustment algorithm is then performed, and the result is used as the current input. This current is compared with the current feedback signal, and current adjustment is performed. The current adjustment signal serves as the basis for generating the voltage vector amplitude. The PWM waveform allocation is found using a lookup table, and a PWM control waveform is generated. This PWM waveform is applied to the isolated drive circuit to control the motor's rotation. 3. Control System Hardware Design In this servo system, the controller is its core component. It not only processes external signals and provides motor drive signals, but more importantly, it implements the control strategy for the entire system. This system uses the TMS320F240 DSP device as the control core, fully utilizing the F240's high-speed signal processing capabilities and optimized peripheral circuits for motor control. It has advantages such as high control accuracy, strong anti-interference ability, and low cost, providing reliable and efficient signal processing and hardware control for high-performance drive control. The system block diagram is shown in Figure 3. It mainly consists of a DSP (TMS320F240) minimum system module, a drive motor logic control unit, a balance motor logic control unit, an isolated drive unit, a power circuit unit, a current detection and circuit protection unit, a drive motor position feedback unit, a balance motor position feedback unit, a permanent magnet synchronous motor, and a brushless motor. When the antenna drive controller receives the remote control command to turn on or off rotation from the information processing and control subsystem, it simultaneously starts or brakes the drive motor and the balance motor. Based on the principle of angular momentum balance, the speed of the balance motor is calculated from the speed of the drive motor, so that the balance motor tracks the drive motor, thereby achieving momentum balance. The remaining angular momentum during stable operation of the drive mechanism is not greater than 0.02 Nms, and not greater than 0.1 Nms during start-up and braking. [align=center] Figure 3 System Module Block Diagram[/align] 3.1 DSP Interface Circuit The TMS320F240 and its interface circuit are shown in Figure 4. It mainly includes the memory expansion, reset pin, JTAG pin configuration, and clock module pin configuration, which are briefly introduced below. The memory expansion is primarily due to the limited internal storage capacity of the TMS320F240, and also to facilitate program downloading to external high-speed SRAM during debugging, avoiding frequent writes to the on-chip EPPROM. The memory expansion utilizes the high-speed static RAM chip CY7C199, with a storage capacity of 32k bytes, a 15-bit address bus, and an 8-bit data bus. In this system, two CY7C199 chips are used to form a 32k-word high-speed memory. The CY7C199's data access cycle is 10ns, while the TMS320F240's CPU cycle is 50ns. Therefore, the ready pin, used to generate the wait signal, does not need to be connected to the memory; it is directly connected to a high level via a resistor. Other pin configurations are as follows: ① Pins related to the clock source module. Since this system uses an external crystal oscillator, /OSCBYP is pulled high via a resistor, and XTAL1/CLKIN is connected to the 4MHz external crystal oscillator. XTAL2 is connected to the other end of the crystal oscillator. ② Pins related to system reset. Power reset uses the /PORESET pin, which is connected in an RC circuit. The system resets when a low-to-high change occurs on the pin. /RS functions the same as /PORESET when used as an input, so it is pulled high directly. In the diagram, the VCCP programming voltage is connected high for debugging and flash programming, so the watchdog reset function can be disabled. After debugging, VCCP is grounded to prevent interference from causing accidental operation of the program and watchdog. ③ Pins related to the JTAG interface. Program download is done through the JTAG interface, which is connected to the parallel port of the PC or a dedicated board interface through a conversion circuit (emulator). In addition to power and ground, the DSP's JTAG interface has 7 pins, of which EMLO and EMLl need to be pulled high, and the other pins TDI, TDO, TMS, TCK, and TRST are directly connected to the emulator. [align=center] Figure 4 Schematic diagram of TMS320F240 and its interface circuit[/align] 3.2 Dead time generation circuit The actual power device's turn-on and turn-off are not instantaneous, but require a certain amount of time. If two power transistors on one bridge arm conduct simultaneously, the current will far exceed the rated value due to the generally low impedance of the power transistors, damaging the devices. To prevent this, a small time interval, typically 10-30µs, is usually added between the turn-on signals of the upper and lower arms in the system control logic timing. The TMS320F240's full comparator unit has a dead-time generator, and an external dead-time circuit is used for system design flexibility and reliability. The dead-time circuit is composed of resistors, diodes, and capacitors, as shown in Figure 5. The input square wave signal is in, and the ideal output pulse signal is out, the width of which is determined by the values ​​of the resistors and capacitors. When the input transitions from low to high, the capacitor charging current generates a voltage across the resistor, and the charging time is the pulse width. After charging is complete, the output becomes low. When the input transitions from high to low, the capacitor discharging current is short-circuited through the diode, so the output remains low. [align=center]Figure 5 Dead Zone Circuit and Ideal Waveform[/align] 3.3 Current Detection Circuit Since vector control is required, the winding current of the three phases of the motor must be detected to achieve current loop control of the drive motor and the compensation motor. The three-phase current of the motor is inverted through the switching transistors, so in actual detection, only the DC bus current at the front end of the motor inverter bridge needs to be measured to reflect the motor current. Therefore, as shown in Figure 6, the Nanjing Zhongxu Hol DC current sensor HNC025A is selected to detect the bus current. The current is then amplified using a sampling resistor and an AD620 amplifier from AD company, and after RC low-pass filtering, it is connected to the external pin of the A/D converter integrated in the TMS320F240. (Note that under normal circumstances, T = RC > 5t, where t is the power transistor turn-off time, resulting in the best filtering effect; therefore, the selection of RC parameters is also very important). [align=center]Figure 6 Current Detection Circuit[/align] 3.4 Voltage Detection Circuit Since the power supply is provided by solar cells in space, the power supply will fluctuate around 28V. Considering that the compensation motor is a DC brushless motor, its speed is particularly high when the antenna is stable. When the supply voltage fluctuates significantly, the system needs to meet the momentum balance condition, possessing characteristics such as fast speed and wide speed range. Therefore, the system detects the DC side voltage in real time, and the monitored voltage value is sampled, converted, and sent to the DSP's A/D conversion module. The maximum DC side output value is 30V. This voltage is divided by voltage divider resistors to generate a voltage signal within the range of 0-5V. Considering that ordinary optocouplers can only achieve isolation of switching quantities and cannot be used for DC or AC analog quantities, a linear optocoupler is used. The system uses HP's precision linear optocoupler HCNR200, which has a bandwidth greater than 200kHz and high stability. When the temperature changes by one degree Celsius, the resulting error does not exceed ±0.005%/℃. The internal structure of the HCNR200 can be divided into two parts. One part is the coupling between the input LED and the output diode. Changes in the current through the input LED cause changes in its brightness, which in turn causes a proportional change in the current through the output diode; the other part is the coupling between the input LED and the feedback diode. Changes in the current through the feedback diode reflect changes in the brightness of the input LED, and its function is to compensate for nonlinear distortions caused by temperature and other factors. This system uses a typical circuit of HCNR200, as shown in Figure 7. [align=center] Figure 7 Voltage Detection Circuit[/align] 3.5 Hardware Anti-interference Design In control systems, there are many electromagnetic interferences (EMI). On the one hand, the motor itself generates electromagnetic interference signals; on the other hand, the semiconductor components in the inverter operate in a switching manner, generating a large number of high-frequency harmonics. In addition, other external devices will also bring various interference signals into the system. These interference signals are transmitted to the control circuit through "field" or "path". If effective measures are not taken to avoid them, the circuit or program may malfunction. In the hardware design of this system, the following aspects are mainly considered for suppressing interference: ① Suppression of power supply noise interference. Generally, this kind of interference cannot be completely overcome; the amplitude of the interfering pulses can only be minimized. A common practice is to connect a 10-100μF electrolytic capacitor across the power input, along with a small capacitor in parallel to filter high frequencies. Adding an RC low-pass filter to the low-frequency signal transmission path can significantly weaken various high-frequency interference signals. This system specifically uses an EMI filter module at the 28V power input, and connects a 10μF capacitor and a 0.1μF capacitor in parallel at both the 5V±12V power inputs to filter out glitches in the power supply. ② Because the process channel is connected to external devices, both digital and analog input/output channels are channels for interference to enter. To cut off this channel, firstly, the common ground wire between the external devices and the process channel must be removed to achieve electrical isolation; secondly, the layout of components and their routing should be rationally designed to reduce distributed capacitance, stray electromagnetic fields, and suppress the generation of various interference noises. 4. Control System Software Design The system software is generally divided into four main modules: communication module, motor speed control module, fault handling, and A/D sampling. The communication module communicates with the information processing and control subsystem for telemetry parameters and remote control commands. The motor speed control module implements the control algorithms for the drive motor and balancing motor. Fault handling implements software protection. A/D sampling completes current sampling and voltage monitoring, realizing PID regulation of the current. The main program module completes the rotor position measurement for drive motor startup, provides the startup space vector, calls the initialization program, determines whether to send scan cycle data, and calls the speed adjustment program. The block diagrams are shown in Figures 8 and 9, respectively. [align=center] Figure 8 Initialization Program Flowchart Figure 9 Main Program Flowchart[/align] 5. Conclusion Through the statistical graph and curve analysis of the drive motor scan cycle of the satellite antenna drive control system in steady state, it can be seen that the steady-state scan cycle error range is within ±0.5ms, with a center value of 1.70052ms. And through the test curves given by the test system, it can be seen that the start-up time, start-up and braking residual momentum torque and steady-state speed stability of the satellite antenna servo control system all meet the technical requirements proposed by the system. References: [1] Zhang Xiongwei et al. DSP Integration Development and Application Examples [M] Beijing: Electronic Industry Press, 2002 [2] Yu Bin. Design of Servo Control System for Permanent Magnet Brushless DC Motor Based on DSP [J]. Motor and Control Application, 2007.7 [3] Yu Bin. Design of Control System for Three-Phase Brushless DC Motor Based on DSP [J]. Industrial and Mining Automation, 2007.5 [4] Yu Bin. Research on Intelligent Control System for Brushless DC Motor Based on DSP [J]. Control and Transmission, 2007.4 Author Introduction : Jia Yaqiong (1982-), female, from Changzhi, Shanxi, Master, major research direction: circuit and system and DSP technology. Email: [email protected] Tel: 0734-6098825 Address: Department of Electrical and Information Engineering, Hunan Institute of Technology, No. 14 Leigongtang, Hengyang City, Hunan Province, China Postcode: 421008