The rapid development of VLSI (Very Large Scale Integration) digital circuit technology has led to the rapid development of DSPs, progressing through stages from 16-bit fixed-point DSPs to 32-bit floating-point DSPs and parallel DSPs, resulting in significantly improved performance and a substantial decrease in cost. Traditional motor control was generally implemented using analog circuits. Today, competitive market demands higher performance and greater efficiency at lower costs. Therefore, designers have moved towards digital servo control. Automatic control theory and computer technology are the two main pillars of digital servo system technology. The rapid development of automatic control theory has provided digital servo systems with many new advanced control laws and corresponding analysis and synthesis methods. The rapid development of computer technology has made it possible to implement these control laws, greatly improving the steady-state and dynamic performance indicators of servo systems. The Development History of Digital Control Digital control has also undergone the development of microcontrollers. Various microcontrollers such as Z80, 8031, 8052, 8096, and 80196 can all perform motor control tasks. The main differences are their bit width and operating frequency. The development trend is towards increasingly higher frequencies, lower power consumption, and lower costs. Motor systems are inherently complex, requiring a more powerful central processing unit (CPU) and shorter execution cycles. Digital Signal Processors (DSPs), developed in recent years, are perfectly capable of improving overall system performance and reducing costs. With on-chip CAN, ADC, PWM, QEP, and SPI interfaces, DSPs are ideal digital servo control chips. The emergence of DSPs will gradually replace microcontrollers, dominating digital control in the field. Furthermore, there are dedicated DSP chips for motor control, such as TI's 2000 series DSPs, making design increasingly convenient. DSPs feature a Harvard architecture and a three-stage pipeline. TI's newly released TMS320F2812 is the latest DSP for control applications, integrating higher computing power and efficient motor control capabilities, easily implementing complex control strategies. Employing high-performance static CMOS technology with an operating frequency of 150MHz, this DSP features a low-power design, a 32-bit CPU, 128KB of on-chip program and data memory, three 32-bit counters, and two event managers for outputting PWM waves to control the motor. It boasts 12 PWM output channels, an SCI serial communication interface, a CAN bus interface, an MCBSP interface, a 12-bit 16-channel A/D converter with a sampling rate of 12.5msps, and an orthogonal encoded pulse interface. Programmable control via a programming language is possible, making it highly portable and ideal for PWM control of motors. In motor control systems, many advanced methods, such as sensorless field control or current-shape controlled switched reluctance motor control, can be easily implemented on a DSP, significantly reducing costs, losses, EMI, and improving system efficiency and reliability. The basic structure of a DSP -based motor position control system is shown in Figure 1. In Figure 1, M represents the motor, PE represents the power electronic drive unit, and DCS represents the digital control system. The signals input to the digital controller are the reference input and process variables. The process variables are the values ​​of the sensor outputs after A/D conversion, including voltage, current, speed, and position signals. The DCS controls the motor drive through the output interface (OI). If the PE requires an analog input, the OI interface also includes D/A conversion. When driving high-power motors, high-power IGBTs, BJTs, etc., are used, and some isolation interface modules are also required. [align=center]Figure 1 Basic structure diagram of a motor position control system based on DSP[/align] The overall requirements of the motor position control system are nothing more than steady-state accuracy and dynamic response. The sensitivity to external disturbances, the ability to adapt to measurement noise and parameter changes must also be considered, especially reliability and cost. More and more parameters or variables need to be controlled, such as position or speed, acceleration, torque, and current. All these variables change very rapidly, so the sampling time must be small enough (typically between 0.1-1ms). Shannon's theorem provides a detailed discussion of the sampling time. Among these parameters, some can be directly measured, while others can be obtained through estimation. Moreover, the system has many analog input variables. For more complex systems, the number of analog input variables is generally 5 to 8. The microprocessor reads data from the corresponding sensors at the appropriate time, and the accuracy is mainly determined by the accuracy of the sensors and converters. For some specific requirements, the DSP must complete some complex control algorithms within each sampling interval. Position control systems are a typical example of complex systems, even when using conventional PID algorithms. For induction motor control, see Figure 2. As can be seen from Figure 2, there are many analog signals that need to be collected, and the required mathematical calculations are quite complex. At the same time, the DSP also needs to receive or send data with external systems, that is, it has communication capabilities. Because of the high execution efficiency and rich peripheral interfaces of the DSP, it has been widely used in many fields (communication, image, speech recognition, robot control, etc.), especially in the field of servo control, where servo systems designed with DSP technology are becoming increasingly widely used [1,2,3,7,8]. It can be seen that the DSP has become a recognized industry focus of technology that will grow exponentially. [align=center]Figure 2 Control Block Diagram of Induction Motor[/align] System Design This paper focuses on the specific characteristics of the TMS320F2812, fully utilizing its functions to design a multi-axis servo control board for the motor. It features up to 16 AD sampling channels (12-bit) and can meet multiple input ranges, 8 DA outputs (14-bit), and CAN, SPI, SCI, and RS422 communication interfaces, as well as orthogonally encoded isolated input channels. It also includes 24 digital input and 24 digital output interfaces. During the design process, a large-scale programmable device (CPLD) is used for address decoding and peripheral channel addressing, simplifying circuit design, reducing the number of components, and increasing control flexibility. This results in a new type of hardware that combines all the advantages of a general-purpose DSP processor with the advanced performance of a CPLD. By fully utilizing the speed of this DSP, advanced control algorithms such as PID+FF control, fuzzy control, and neural network control can also be implemented. Because DSPs are programmable, tasks can be time-division multiplexed, reducing system hardware size and improving system design flexibility and reliability. Simultaneously, the system is open and upwardly compatible, readily adopting and absorbing new technologies and algorithms. The communication port also facilitates networked control, enabling the control of large systems such as theodolites' servo control, receiving commands from the main control computer, encoders, radar guides, or GPS. The power management TMS320F2812 uses dual power supplies: 3.3V for the external interfaces and 1.8V for the core chip. However, 5V is typically provided, necessitating voltage conversion. The TPS767D318PWP power converter chip is used to convert 5V to 3.3V and then to 1.8V, with a maximum output current of 1A per channel. See Figure 3. [align=center] Figure 3 Power Circuit Schematic[/align] For the communication circuit, to increase reliability and enhance anti-interference capabilities, an isolated interface circuit was designed. The communication schematic is shown in Figure 4. [align=center]Figure 4 SCI Interface Schematic[/align] The isolation circuit uses a 6N137 optocoupler for isolation, achieving speeds up to 10 Mbps to meet different transmission rate requirements. Optocoupler isolation is also used for the CAN bus interface. As the interface with the encoder, the DSP needs to receive the encoder's pulse signals as position and speed feedback signals; this part of the optocoupler isolation circuit also uses the 6N137 optocoupler. In situations requiring signal exchange with strong external signals or high-voltage, high-current main circuits, isolation is considered to reduce external interference and increase reliability. Simultaneously, each input/output port circuit is appropriately designed with fault tolerance, ESD protection, and short-circuit protection, taking into account various interference characteristics. The TMS320F2812 A/D converter is a 12-bit, 16-channel A/D converter with a sampling rate of 12.5 msps, divided into two groups: AD0-AD7 in one group and AD8-AD15 in another. Each group has a dedicated input terminal. The event manager can configure the ADC as two independent 8-channel modules or connect them in series to form a 16-channel module. ADCin0 is the analog input, and ADCina0 is the signal input to the analog input port of the DSP. To meet the requirements of multi-range analog voltage input, J5, R66, R67, R74, and R76 in the circuit design shown in Figure 5 are used to select the input range, satisfying R66=R74=4R67 and R76=2R67. The three positions of J5 are used to set different input ranges. When connected to 1 and 2, it represents a ±5V input range; when connected to 5 and 6, it represents a ±10V input range. The DSP itself has 12 PWM outputs. When servo motor units require PWM signal drive, these output ports can be used for control. For some applications requiring analog output, this board also includes an 8-channel DA converter, using an AD7841 chip containing 8 14-bit DACs in a single package. The output voltage settling time is 31μs. When the reference is ±5V, it has a full-scale output of ±10V, which can be directly connected to some servo drive units. [align=center]Figure 5 Schematic diagram of one of the analog input channels[/align] PCB electromagnetic compatibility design: Due to the numerous interfaces and intersecting network lines, the layout and functional unit differentiation are key considerations before designing the PCB. It is worth noting that the routing between the DSP and CPLD, and between the CPLD and the input/output interfaces, needs to be considered. The network can be readjusted to facilitate PCB routing; otherwise, the intersecting traces will make routing difficult, requiring the addition of vias. During routing, all signal lines should be classified first, distinguishing between control lines, data, address buses, and I/O interface lines. Clock and sensitive signal lines should be routed first, followed by high-speed signal lines, and finally general signal lines. Since the DSP operates at a very high frequency, reaching 150MHz internally, reflections, high-frequency crosstalk, electromagnetic interference, and heat distribution must be fully considered in the PCB design, and necessary measures should be taken to ensure signal quality. When designing a PCB board, some capacitors are added near the circuit, chip and power circuit to meet the requirements of low power noise and low ripple when the digital circuit is working. Decoupling capacitors, bypass capacitors and energy storage capacitors are added in appropriate positions to improve signal quality. The grounding is also carefully handled. This design is a 4-layer board (distributed as signal, ground, power and signal). The design follows the 3W and 20H principle (3W principle: the distance between traces must be three times the width of a single trace; 20H principle: the physical size of the printed circuit board should be 20h smaller than the physical size of the nearest ground plane, where h is the distance between the two printed circuit boards). The area of ​​the current loop is minimized as much as possible, and the system uses only one reference plane. The ground of analog signal and digital signal is handled by bridging [9]. Signal integrity is fully considered during PCB design, and effective control measures are taken to address signal delay, reflection, crosstalk, and ground bounce (when numerous digital signals on the PCB are switched synchronously (such as the CPU's data bus and address bus), impedance on power and ground lines causes synchronous switching noise, and ground bounce noise also occurs on the ground line). Electrostatic discharge (ESD) is also a critical issue. ESD can damage circuits and corrupt data, leading to system resets, crashes, and program errors. The impact of ESD on the entire system is severe and must be carefully considered. For component selection, ESD-insensitive devices should be used whenever possible. Isolation of input/output circuits or protection with TVS diodes, proper grounding, large areas of PCB copper foil, minimum spacing, and multiple decoupling capacitors for large-scale integrated circuits all contribute to preventing ESD interference. The designed PCB is shown in Figure 6. The system block diagram of the entire DSP control system is shown in Figure 7. The 12-channel PWM output can be directly used for up to three DC motors (each brushed DC motor requires four PWM signals). If the configured motor driver requires fewer PWM signals, more motors can be controlled. The 16-channel A/D input can accept different input ranges by configuring appropriate jumpers. Motor voltage, current, and other analog signals can be processed by conditioning circuits before entering the DSP's sampling circuit for AD sampling. The 8-channel D/A output has output operational amplifiers for voltage conversion and protection circuits, which can be used to control the control variables of analog input devices. The 6-channel quadrature encoder signal input, after level conversion by isolation circuits, enters the DSP. The DSP uses a program to control the counting of quadrature pulses to obtain position and speed signals. An 8-channel RS422 communication capability is designed, implemented using two 16C554 chips, and interface converted using SP489 and SP487 interface chips. Optical isolation circuits and ESD protection circuits are designed for SCI and CAN communication. The voltage levels of the I/O input/output ports have been level-converted, with a high level of 5V. There are 16 unisolated inputs and 8 isolated inputs; the 24 isolated outputs are provided. All I/O ports are indicated by LEDs for easy fault diagnosis. To simplify circuit design, a large-scale integrated circuit (CPLD) is used, which can be programmed online via JTAG. [align=center] Figure 7: Block diagram of the DSP control system[/align] Experiment The designed PCB board needs to ensure normal operation. The clock signals or crystal oscillator output signals of each chip must be normal and not interfered with or interfere with other devices. Therefore, the chip clocks need to be carefully arranged when designing the PCB traces. Practical experience shows that the clock signals of this board meet the design requirements, with good waveform quality and no glitches. See Figures 8 and 9. The next step is to implement the servo control algorithm using the designed control board to control the elevation and azimuth motors of the large theodolite and communicate with various subsystems to achieve high-precision target tracking. [align=center]Figure 8 xclkout output waveform and DSP crystal oscillator output signal[/align] Conclusion The entire control system has rich interfaces and communication functions, and can be configured as a multi-axis servo control system. For example, in the servo system of a theodolite, it can communicate with encoders, dimming and focusing systems, main control computers, capture televisions, measuring televisions, infrared systems, and digital communication systems, etc., and control the position and speed loops of the azimuth and pitch motors in real time to complete the target tracking task, return relevant information in real time, and cooperate with the main control computer to obtain the target's azimuth, elevation information, and motion attitude. In addition, this multi-axis servo control system can also be used in robot control, CNC and other fields, and has broad application prospects.