In high-performance stable platform servo systems composed of servo motors and servo drivers, real-time acquisition of servo motor angle and speed information is required. High-speed, high-precision sensors and corresponding peripheral circuit designs are essential. Due to the limitations of microcontroller resources, it is difficult to meet the high precision, high computing power, and fast real-time requirements of modern servo systems. In stable platform servo control systems, DSPs have gradually replaced microcontrollers and become the mainstream chip. This design uses TI's 32-bit floating-point DSP chip TMS320F28335, with a working clock frequency of up to 150MHz, possessing powerful computing capabilities and able to complete complex control algorithms in real time. It integrates a wealth of motor control peripheral components and circuits, simplifying the hardware design of the control circuit and improving system reliability.
This study utilizes the new DSP development board ICETEK-F28335-A, along with its EQEP module and photoelectric encoder, to design a solution for measuring the speed of a servo motor. Simultaneously, the digital-to-analog (D/A) converter module on the development board is used to control the servo motor speed through voltage conversion and amplification, realizing a closed-loop system for controlling the servo motor of a stable platform. Practice shows that this system has the advantages of low power consumption, low cost, and simple structure, while also possessing high precision, high resolution, and fast real-time performance, enabling the stable platform servo system to achieve good control results.
1. Servo System Structure of a Stable Platform
The main technology used in the stable platform application is servo control technology. This system realizes speed control of Panasonic MINASA series servo motors. It mainly consists of Panasonic MINASA series servo drivers, servo motors, corresponding photoelectric encoders, TMS320F28335 motion control development board, corresponding ICETEK-5100USB emulator, and peripheral circuits necessary to realize the closed-loop process. The structure of the servo system is shown in Figure 1.
The servo system of the stable platform realizes the speed closed-loop process as follows: The DSP controller calculates the error value of the motor speed by subtracting the speed feedback value from the speed command value given by the host computer. The motor speed control signal is generated by the digital filter (adjustment algorithm) of the drive unit. That is, the D/A module generates analog voltage, which is converted to a voltage range that can control the servo motor, thereby realizing the speed control of the servo motor. The feedback value is based on the quadrature pulse signal fed back by the incremental photoelectric encoder. After optical isolation and shaping, the feedback signal is provided to the eQEP module of TMS320F28335. The collected pulse signal calculates the motor speed according to the M/T counting method and feeds it back to the host computer to realize automatic control, so that the stable platform can isolate the carrier movement and establish a stable reference plane [1]. The power supply module converts the +5V voltage provided by the switching power supply to +3.3V to power the system [2].
2. Hardware Design of the Servo System
2.1 eQEP module of TMS320F28335
The eQEP module of TMS320F28335 is an enhanced quadrature decoding module, mainly used in motion control systems. It provides a direct interface to the encoder. The position, direction and speed information of the motor can be obtained through the eQEP module. The TMS320F28335 provides four pin signals that enter the quadrature decoding module inside the eQEP through the GPIO multiplexer. The QDU (quadrature decoding unit) decodes the direction and pulse of the received quadrature pulse signal from the encoder. After decoding, the position pulse signal and direction signal of the 4 times frequency are obtained and sent to the position counter for pulse counting. The encoder control register QDECCTL is set to quadrature counting mode. The rotation direction is observed by observing the quadrature direction flag bit in the status register QEPSTS. The count is incremented when clockwise and decremented when counterclockwise. The actual position information of the motor can be obtained by reading the value of the position counter QPOSCNT through the program. The position information can be used to perform closed-loop control with the given position information. In addition, the speed information of the motor can be calculated through the QCAP module [3]. The timing logic of quadrature encoding pulse, timer counting pulse and counting direction is shown in Figure 2.
2.2 Interface circuit between photoelectric encoder and TMS320F28335
The encoder signal of the servo system is a differential signal output from the servo driver, while the DSP requires a TTL signal. Therefore, before acquisition, the encoder output signals OA+, OA-, OB+, OB-, OZ+ and OZ- need to be converted. This system uses the AM26LS32 chip to receive the differential signal [4]. The output signal after reception is A, B and Z three-way signals, where the phase difference between A and B signals is 90°. The signal output by the photoelectric encoder is sent to the corresponding pin of the DSPeQEP module after photoelectric isolation and shaping. Its interface circuit is shown in Figure 3. Among them, 6N137 is a high-speed optocoupler chip to realize the isolation between digital signals and analog signals; 74HC14 is a high-speed CMOS inverter to realize the shaping of the input pulse signal. Figure 3 only shows the photoelectric isolation and shaping of the OA+ and OA- signals output by the photoelectric encoder. The signal after photoelectric isolation and shaping is sent to the EQEP1A, EQEP1B and EQEP1I pins of the TMS320F28335 peripheral for quadrature decoding.
Because the DSP development board has a large output impedance, the voltage division causes severe attenuation loss. Therefore, a voltage follower needs to be added before the amplifier circuit to play an impedance matching role, so that the subsequent amplifier circuit can work better.
3. Software Design of Servo System
The system's software debugging and development both adopted the CCSV 3.3 version for TMS320F28335. TI provides a visual window for the software development tool CCS (CodeComposerStudio), which integrates all code generation tools. All of the user's development process is carried out in CCS, including project creation, source code editing, program compilation and debugging. In addition, CCS also provides the real-time operating system DSP/BIOS, which greatly facilitates debugging and development. The DSP program of this system is mainly divided into two modules: the main program and the interrupt service program. The main program module [10] mainly implements the initialization of each functional module, the definition of memory variables and the declaration of interrupt vectors. The interrupt program module mainly implements the setting of relevant registers, reading and latching the pulse count of the eQEP module, the feedback of the detection circuit and the program of the control algorithm. Its software flow is shown in Figure 6.
This paper proposes the design of a stable platform servo system. The eQEP module of the TMS320F28335 DSP chip decodes and counts the pulse signals from the photoelectric encoder to obtain the angle and speed information of the servo motor. This information is then compared with the setpoint from the host computer. An adjustment algorithm is used to generate a voltage signal from the D/A module to control the speed of the servo motor. Research shows that this design has high response speed, stable accuracy, and strong resistance to load disturbances, fully realizing high-precision control of the stable platform. Furthermore, the system exhibits strong robustness and adaptability, verifying the effectiveness of the scheme and providing a high-performance digital solution for various control fields.
