In the industrial control industry, "servo" generally refers to an AC servo system . In engineering practice, "servo" usually means servo drive. However, a servo drive and a servo motor are an inseparable system, connected by encoder cables and power cables. Typically, the servo drive receives control commands from the controller and then drives the servo motor via the power cable. The real-time position of the servo motor is fed back to the servo drive via the encoder cable, forming a closed-loop control. Clearly, in this mode, the servo drive merely acts as an amplifier. This is the operating mode of most servos, such as those from Yaskawa, Fuji, Panasonic, Mitsubishi, Delta, etc.
Some servo drives have built-in controller functions, allowing for internal programming to achieve motion control and advanced motion control functions such as electronic cams and phase synchronization. Lenze servos are a prime example; alternatively, Danfoss, CT, and other frequency converters can also achieve this function by installing motion control cards.
Clearly, the servo motor host control discussed in this article mainly refers to the first mode, where the servo driver operates in amplifier mode. In this case, the host computer is the PLC , motion controller, or CNC system. If we compare the servo driver to an engine, then the host computer is an advanced autonomous driving system. Regardless of the type of host computer used, the host computer and servo driver generally communicate via pulse and communication methods.
1. Pulse mode
The host computer controls the servo driver by sending pulses. In this method, the pulse frequency controls the speed, and the number of pulses controls the position. Similarly, the servo driver also sends pulse counts to inform the host computer of the servo motor's position and speed.
For example, if we agree that a servo motor rotates once every 10,000 pulses, then when the host computer sends 10,000 pulses, the servo motor rotates once, achieving position control. If the host computer sends all 10,000 pulses within one minute, the speed of the servo motor is 1 r/min ; if it sends them all within one second, the speed of the servo motor is 1 r/s, or 60 r/min.
Low-end PLCs, CNC systems, and various microcontroller systems generally adopt this mode because it is simple, easy to implement, and inexpensive. Obviously, as the number of servo axes increases, the disadvantages of this control method become apparent: the cost of the host computer hardware increases, wiring becomes more complex, and pulses are easily lost if the EMC in the field is poor. Therefore, this mode is generally used for four axes or less. Thus, most PLCs control two or three axes, and very few PLCs can achieve four axes.
2. Communication methods
Communication methods were specifically developed to address the shortcomings of pulse methods and have become a development trend. They send the number of pulses and pulse frequency to the servo driver via communication. This method can not only transmit the position information of the servo motor, but also various status information, such as the current and torque of the servo motor, as well as the fault codes of the servo driver. Obviously, when there are many axes, the advantages of this method are self-evident.
Due to the unique nature of motion control, different manufacturers have launched their own motion control buses, some open and some closed, such as CANopen, and CANmotion, CANlink, MECHATROLINK-II, CCLink, and others developed based on it. With the development of industrial Ethernet technology, Ethernet-based motion control buses have also emerged, such as EtherCAT , ProfinetNet, and MECHATROLINK-III. There are also fiber optic-based buses like SERCOS and SSCNETⅢ/H.
Although there are many forms of communication, they generally solve real-time problems, which are crucial for motion control. From an application development perspective, pulse and communication are essentially the same; only the form of signal transmission changes.
