As large enterprises become increasingly automated, the demands on servo motors are also rising. As a crucial component in automatic control systems, the reliability, accuracy, and response speed of motors directly impact the performance of the control system, making them an indispensable element. Therefore, servo motor DDC (Distributed Dynamics Control) has significant application value and broad prospects.
Traditional control methods for servo motors
Since the externally provided motor speed is an analog signal, an A/D module is often needed to convert it into a digital signal. The servo motor driver then receives the digital signal and controls the motor's operation. This traditional method of controlling servo motor drivers has been used for a long time. However, its reliability is not high, and it suffers from a certain degree of zero drift. Furthermore, considering the cost of A/D modules, this method is relatively expensive.
Direct digital control of servo motors
With the rapid upgrades in computer technology and the widespread adoption of Microsoft's Windows operating system and visual programming tools such as Visual Basic and Visual C++, more and more industrial sectors are using industrial control computers (ICCs) for automated control. Communication between the ICC and its slave devices not only provides operators with a user-friendly interface but also makes operation simple and easy to learn, with reliability comparable to manual control.
Direct digital control (DC) of servo motors involves direct communication between an industrial PC and the servo motor driver. The industrial PC transmits digital signals to the servo motor driver via serial communication, thereby controlling the motor's operation. Because it's direct digital signal control, zero drift is eliminated, improving reliability. Furthermore, it eliminates the need to convert analog signals to digital signals, reducing the need for an A/D module and lowering costs. Many manufacturers now equip their servo motor drivers with communication ports and protocols. These protocols are simple and reliable, allowing users to easily complete communication programs. This enables direct motor control from the industrial PC, allowing for convenient modification of various parameters, such as motor speed, according to process requirements.
Application Examples
The DDC (Digital Direct Current) controller for servo motors is applied to the monitoring system of the SZ (Small Inch) fiber optic cable production line. This production line uses Panasonic's Minasa series AC servo motor drivers. These drivers have built-in serial communication ports and detailed communication protocol specifications. The industrial PC can directly communicate with the driver via its built-in COM1 and COM2 ports. The monitoring system is required to control the motor from the industrial PC, enabling smooth acceleration and deceleration. It should provide a user-friendly interface for easy parameter modification and real-time display of the motor's current status.
Communication Protocol
The driver provides many parameter numbers, subdividing various functions. Among them, the parameters related to communication control include:
Parameter number 05h
Internal and external speed control (internal setpoint is 1, external setpoint is 0) requires an internal setpoint for data transmission to be valid; this is a prerequisite for using communication control. The factory default setting is 0.
Parameter number 53h
Internal speed: The communication transmits the required motor speed to this parameter.
Address: When multiple drives need to communicate, they can be connected in series via RS2485. Different addresses are used to distinguish them, ranging from 01h to 0fh, and can be set and viewed on the surface of the drive.
This system uses only one driver, and communication is implemented using an RS2232 with address 01h. The data transmission format is shown in Figure 2 (taking the transmitted value as 278 as an example).
Software implementation
The software portion was developed using Microsoft's object-oriented programming tool, Visual C++. Microsoft provides a serial communication control for Windows; communication can be easily achieved by setting the properties of this control. Since this monitoring system is based on a document rather than a dialog box class, the communication control needs to be created and the port opened in the onCreate() function first.
Actual measurement results
During normal production, the motor speed is 975 r/min, and the speed change during startup is shown in Figure 3. As can be seen from Figure 3, the speed change trend is linear, meeting production requirements. The acceleration time is less than 6 seconds, resulting in minimal overshoot and higher stability, with an error of less than 0.01 seconds.
• By using communication control, the driver directly receives digital signals, eliminating the need for analog-to-digital conversion and saving an A/D module. This method is low-cost.
The basic idea of DDC for servo motors is to directly use digital control, which can completely eliminate the zero drift effect caused by analog control and greatly improve reliability.
• Controlling the driver from the industrial computer requires a maximum communication time of 80ms. This short time allows the driver to immediately start the motor after receiving the data, meeting actual production requirements.
• Based on the manufacturer's actual process requirements, parameters can be flexibly adjusted on the industrial control computer, and motor control can be improved compared to traditional control methods. Through communication, the integral process can be manually controlled, effectively achieving the control requirements.
The direct digital control (DC) of servo motors allows for the convenient implementation of complex control algorithms, which is impossible with analog control. Direct digital control of servo motors has wide applications in various fields, offering excellent control performance, improved system reliability and speed, and promising future prospects.
