The Design of DC Servo Motor Controller System Based on a DSP Controller

The servo control technique is one of the key technologies in the field of control like robots. In order to meet the demands of servo control for robots, a DSP-based DC servo motor control system was developed. Firstly, this paper promoted the design of the controller and analyzed the overall structure and function of the system. Then the hardware of the system was introduced and the key components were chosen and demonstrated; the classical PID algorithm was selected as the core algorithm of servo control and the design of the system program was elaborated. Finally, an experiment was conducted on the hardware platform with software systems.

Introduction

In many automatic control systems, especially in fields such as medical treatment and robots, precise control of parameters like position, speed, torque and high reliability are required. Due to its characteristics of smooth speed regulation and easy control, DC Motor speed control systems are widely used in automation equipment and production departments which have high requirement on control such as CNC machine tools and industrial robots. The trend of the development of the servo motor control area is the embedded control system with DSP as its core controller for its great function of input and output (I/O) and capability of high speed digital signal processing. So it is meaningful and promising to develop a servo control system of DC motor with high performance and reliability.

This paper developed a DC servo motor control system based on a DSP controller after analyzing the control demand of the robot in our lab and taking the experience of other researchers domestic and abroad. The system consisted of the driver and controller for the DC servo motor. The experimental results showed that the system could run with high precision and reliability in either speed mode or position mode and could keep parameters like PID parameters saved despite the loss of power.

Hardware Structure

The entire DC servo motor control system includes the host computer, motor driving part, parameter storage, and signal sampling and processing part. The structure diagram of the system is illustrated in the following picture. The function of the host computer is sending instructions via RS485 bus to the controller in order to modify the parameters or run the motor; The core part of the controller is the minimum system based on a DSP chip, which is specially developed by TI company for the application of motor; The DC servo motor driving part is a H-bridge driving circuit made up of a driving chip and four power; a photoelectric encoder is used as the signal sampling unit which measures the speed of the motor; A data memory chip is utilized to save parameters like speed, position, accelerator and PID parameters.

DSP minimum system and peripheral circuits. The hardware circuits are important parts of the system, based on which the software system can run normally to fulfill the control task. This part presents the circuit design of the functional modules. As is shown in the following picture, the hardware of the system is made up of the following parts: DSP minimum system, motor driving unit, storage unit, and upper host computer, and the main parts are the DSP minimum system and industrial DC servo motor driving unit.

The DSP minimum system is the core of the control system, which generates control signals calculated according to the feedback signal and control algorithm. The industrial DC servo circuit of the minimum system includes a power supply unit, timer unit, SRAM, and JTAG interface. The peripheral circuits compromise a storage unit and a communication interface unit. A nonvolatile ferro-electric memory (FM24CL32) is used to store the parameters that the system needs to run correctly.

The RS485 bus is one of the widely used buses in industrial sites for its mature technology and easy manipulation. So the RS485 bus is used in this system to communicate with the host PC. At the same time, the circuit of the CAN bus is reserved as another way to communicate. Motor driving circuit. The motor driving circuit is one of the most important circuits in the system, which directly determines the performance of the motor.

Software Design

A well-performed control system has not only reliable hardware but efficient and useful software as well. The following picture illustrates the closed-loop control diagram of the system. The system includes a speed loop and a position loop. The calculated position result is firstly converted to speed and finally changed into voltage loaded on the motor.

Control Algorithm. Since 1903, the PID algorithm has experienced great development in industrial areas. As is presented in the following picture, the PID driver has a simple structure with three parameters: Proportion (P), Integration (I), and Differential (D) and the parameters are easy to change. During the long period of its usage, the technology of PID drivers becomes more and more mature and accumulates a lot of experience. PID driver overcomes the shortcoming that the mathematical model of the object is hard to get and it's easy to implement this algorithm into software. So the PID algorithm is used in this control system.

The Main Program. The software is designed in a modular way and the program is made up of initialization, PWM generation, QEP decoder, Timer module, IIC module, and SCI module. In this way, we can improve the efficiency of the servo system and reduce the risk of failure.
Servo Control Program. The servo control program is executed in the timer period interrupt program as the closed-loop control function. During the servo period, the control amount is calculated according to the control algorithm and real-time system parameters and it is sent to the PWM driving function to drive the motor. As illustrated in the following picture, there are 2 states in position mode. The first is that the path is relatively long and needs proceedings of acceleration, uniform, and deceleration; the second is that the path is short enough so it needs a period of acceleration and deceleration.

During each servo cycle, firstly, the real speed and position are detected and the function mode is selected. Secondly, the error between the real and given speed and position is calculated and the control signal is generated to drive the motor to minimize the error. In the speed mode, the motor is accelerated and kept to the given speed.