Digital Servo Drive

Digital Servo Drive Technical Overview

To achieve high-performance control of AC permanent magnet synchronous motors, this Digital Servo Drive is developed based on a high-speed DSP and an intelligent power module (IPM).

Experimental validation confirms precise signal tracking and excellent dynamic response, making it ideal for high-end automation systems.

Industry Evolution & Performance Requirements

Modern servo systems require extreme precision, stability, and reliability across advanced applications.

DSP + IPM architecture enables higher integration, modularity, and superior control.

DSP Control System

High-speed processing enables vector control, ensuring precision, fast response, and system stability.

IPM Power Module

Integrated IGBT with PWM control ensures efficient conversion and built-in protection mechanisms.

Servo Control Circuit Hardware Desgn

The servo control circuit is the core part of the AC servo drive, and its main functions are as follows:

Receives command and feedback signals;
Runs closed-loop servo control algorithms;
Generates PWM output signals;
Controls motor operation for high performance;
Provides protection against overvoltage, undervoltage, overcurrent, and IPM failures through real-time detection and processing of various fault information.

Based on modular design, the servo control circuit utilizes a DSP+FPGA hardware architecture.

The DSP chip mainly realizes high-performance control of the motor, including the following functions

(1) It has a 32-bit single-precision floating-point arithmetic unit, which can process motor vector control algorithms with large computational load and signal filtering algorithms with high real-time requirements, etc., to achieve high-performance control of the motor;

(2) It has an eCAN communication module, which can realize communication with the host computer, receive control instructions and send various operating and fault states of the driver;

(3) It has multiple independently programmable multiplexed general-purpose input and output interfaces (GPIO), which can realize I0 control of the driver;

4) It has a 12-bit A/D converter, which can realize analog input control;

(5) It has an IC bus module, which can realize communication with E2PROM, store the control parameters and historical faults of the driver;

(6) It has a high-resolution pulse width modulation (PWM) module, which can easily realize PWM control of the motor;

(7) It has a parallel bus external interface (XINTF), which can realize communication with the FPGA chip, receive status information such as position, speed and current, and fault information such as overvoltage, undervoltage, overcurrent and IPM fault.

The FPGA chip mainly processes position, voltage, current, and fault signals, including the following functions

(1) Receive and process the motor's position information through the peripheral RDC module;
(2) Receive and process the motor's three-phase current and three-phase voltage information through the peripheral A/D module;
(3) Send the processed status information and fault information to the DSP chip;
(4) Receive the PWM signal sent by the DSP chip, interlock it, and output it;
(5) Receive bus overvoltage, undervoltage, three-phase overcurrent, and IPM fault signals, and implement protection by blocking the PWM output signal in the event of a fault.

Servo Control Software Design

The servo control software runs on a DSP chip, primarily receiving commands and feedback status information. It then implements three closed-loop control loops for the motor's position, speed, and current using corresponding control algorithms. The closed-loop control principle block diagram is shown in Figure 3. PI control algorithms are designed for all three closed-loops based on the error between commands and feedback. The current control algorithm employs field-oriented vector control for the motor. Clark and Park transforms are used to transform the three-phase current in the stationary ABC coordinate system into two-phase current in the rotating dq coordinate system. A closed-loop algorithm is used to regulate the motor's dq axis currents, which are then adjusted using the inverse Park transform, the inverse Clark transform, and PWM. The stator's three-phase voltage is adjusted to achieve motor control.

The servo control software primarily consists of a main program and an interrupt routine. The main program first initializes the system, configuring register parameters for each control chip module and system parameters. It then performs a self-test of the driver's status to determine if all components are functioning properly. If the self-test fails, the fault indicator flashes as an alarm. If the self-test passes, power is connected and the circuit enters a loop state. Once in the loop state, the control chip first receives control commands. Upon receiving a run command, the timer interrupt is enabled. The timer interrupt subroutine executes the motor control algorithm, driving the motor. Upon receiving a stop command, the timer interrupt is disabled, stopping the motor. After each timer interrupt subroutine completes, it checks for fault signals such as overvoltage, undervoltage, and overcurrent. If a fault is detected, the PWM output signal is immediately disabled and the fault indicator flashes as an alarm. If no fault is detected, the circuit loops and waits for the next timer interrupt.

The timer interrupt subroutine is the core of the servo drive control algorithm, completing the motor position, speed, and current control algorithms. The timer interrupt period is 0.1ms, which determines the PWM output. The frequency of the output signal is also the period of the current loop. After entering the timer interrupt subroutine, the motor's position, speed, and current information are first read. Then, depending on the operating mode, the position, speed, or current command is read and the corresponding control algorithm is executed. Finally, the PWM signal that controls the motor is output.

Experimental Results

The developed digital AC servo drive has an output power of approximately 6 kW. To investigate the control performance of this digital AC servo drive, a Tonghang servo motor with a rated power of 5.5 kW and a rated speed of 3200 rpm was used.

Experiment 1: Tracking a step signal at rated speed. The experimental results are shown in the figure.

As can be seen from the figure, the motor's rise time when tracking a 3200 r/min step signal is approximately 80ms, with an overshoot of 1.5%. The peak-to-peak speed fluctuation in steady state is less than 5 r/min. This demonstrates that the designed driver has excellent dynamic performance and steady-state accuracy.

Experiment 2: Tracking a sinusoidal signal at rated speed. The experimental results are shown in the figure. As can be seen from the figure, the motor can well track a sinusoidal signal with an amplitude of 3200 r/min and a period of 1s, with a peak tracking error of less than 20 r/min. This demonstrates that the designed driver can drive the motor to track both step and sinusoidal signals while maintaining constant control parameters, demonstrating excellent adaptive performance.