summary:
Servo drives, also known as servo controllers or servo amplifiers, are controllers used to control servo motors. Their function is similar to that of a frequency converter for a regular AC motor. They are part of a servo system and are primarily used in high-precision positioning systems. Generally, they control the servo motor through position, speed, and torque to achieve high-precision positioning of the transmission system. Currently, they represent a high-end product in transmission technology.
1. Introduction
With the emergence of fully digital AC servo systems, AC servo motors are increasingly being used in digital control systems. To adapt to the development trend of digital control, most motion control systems use fully digital AC servo motors as actuators. Control is achieved using pulse trains and direction signals.
The fundamental concept of servo control is accurate, precise, and rapid positioning. Frequency conversion is an essential internal component of servo control, and servo drives also incorporate frequency conversion (for stepless speed regulation). However, a significant difference lies in how servo control achieves closed-loop control, whether through current, speed, or position loops. Furthermore, the construction of servo motors differs from that of ordinary motors, designed to meet the requirements of rapid response and accurate positioning. Currently, most commercially available AC servo motors are permanent magnet synchronous AC servos. However, due to manufacturing limitations, it's difficult to achieve high power outputs with these motors; synchronous servos exceeding tens of kilowatts are extremely expensive. Therefore, when field applications allow, asynchronous AC servos are often used. In these cases, many drives are high-end frequency converters with encoder feedback closed-loop control. Ultimately, the essence of servo control is accurate, precise, and rapid positioning; once these requirements are met, the debate between servo and frequency converter becomes irrelevant.
2. Differences between the three control methods
Generally, servo drives have three control modes: speed control, torque control, and position control.
Speed control and torque control both use analog signals. Position control is achieved by sending pulses. The specific control method used depends on the customer's requirements and the desired motion function. If you have no requirements for the motor's speed or position, and only need a constant torque output, then torque mode is the obvious choice. If you have certain accuracy requirements for position and speed, but are not very concerned about real-time torque, torque mode is less convenient, and speed or position mode is better. If the host controller has good closed-loop control capabilities, speed control will be more effective. If the requirements are not very high, or there are basically no real-time requirements, position control does not place high demands on the host controller. In terms of servo drive response speed, torque mode has the least computational load and the fastest response to control signals; position mode has the greatest computational load and the slowest response to control signals. When there are high requirements for dynamic performance during motion, real-time adjustments to the motor are needed. In this case, if the controller itself has a slow processing speed (such as a PLC or a low-end motion controller), position control should be used. If the controller has a fast processing speed, the position loop can be moved from the driver to the controller using speed control, reducing the workload of the driver and improving efficiency (for example, most mid-to-high-end motion controllers). If there is a better host controller, torque control can also be used to move the speed loop away from the driver. This is generally only possible with high-end dedicated controllers, and in this case, there is no need to use a servo motor at all.
1. Torque Control: Torque control is achieved by setting the output torque of the motor shaft through external analog input or direct address assignment. For example, if 10V corresponds to 5Nm, then when the external analog input is set to 5V, the motor shaft output will be 2.5Nm. If the motor shaft load is below 2.5Nm, the motor rotates forward; if the external load is equal to 2.5Nm, the motor does not rotate; and if the load is greater than 2.5Nm, the motor rotates in reverse (typically occurring under gravity loads). The set torque can be changed in real-time by altering the analog input setting, or by changing the corresponding address value via communication. This method is primarily used in winding and unwinding devices where strict requirements on material stress are required, such as wire winding devices or fiber optic drawing equipment. The torque setting must be adjusted continuously according to changes in the winding radius to ensure that the material stress does not change with the winding radius.
2. Position Control: Position control typically determines the rotation speed by the frequency of externally input pulses and the rotation angle by the number of pulses. Some servo systems can also directly assign speed and displacement values via communication. Because position control allows for very strict control over both speed and position, it is generally used in positioning devices. Applications include CNC machine tools, printing machinery, etc.
3. Speed Mode: Rotation speed can be controlled via analog input or pulse frequency. With an external PID control system connected to a higher-level controller, speed mode can also be used for positioning, but the motor position signal or the position signal from the direct load must be fed back to the higher-level controller for calculation. Position mode also supports direct load external loop position signal detection. In this mode, the encoder at the motor shaft end only detects the motor speed, and the position signal is provided by the detection device at the direct final load end. This reduces errors in the intermediate transmission process and increases the overall positioning accuracy of the system.
3. Differences between servo drives and frequency converters
AC servo technology itself borrows from and applies frequency conversion technology. It achieves this by mimicking the control method of a DC motor through frequency conversion PWM, building upon the servo control of a DC motor. In other words, an AC servo motor inevitably involves a frequency conversion stage: frequency conversion first rectifies the 50-60Hz AC power into DC power, then uses various gate-controlled transistors (IGBTs, IGCTs, etc.) to invert it into a frequency-adjustable waveform similar to a sine or cosine wave through carrier frequency and PWM adjustment. Because the frequency is adjustable, the speed of the AC motor is also adjustable (n=60f/p, where n is the rotational speed, f is the frequency, and p is the number of pole pairs).
1.2.1 Frequency Inverter:
Simple frequency converters can only regulate the speed of AC motors. In this case, open-loop or closed-loop control is possible depending on the control method and the frequency converter itself; this is the traditional V/F control method. Many modern frequency converters use mathematical models to convert the three phases of the AC motor's stator magnetic field (UVW) into two current components that control both motor speed and torque. Most well-known brands of frequency converters capable of torque control use this method. Each phase of UVW requires a Hall effect current sensor, and the sampled feedback forms a closed-loop negative feedback current loop with PID regulation. ABB frequency converters have proposed a different direct torque control technology; please refer to relevant materials for details. This allows control of both motor speed and torque, with speed control accuracy superior to V/F control. Encoder feedback is optional; when added, control accuracy and response characteristics are significantly better.
1.2.2 Servo Driver:
Regarding drives: Servo drives, building upon variable frequency technology, employ more precise control techniques and algorithms in their internal current, speed, and position loops (which variable frequency drives lack). Functionally, they are significantly more powerful than traditional variable frequency drives, primarily enabling precise position control. Speed and position are controlled by pulse sequences sent from a host controller (some servo drives integrate control units or directly set position and speed parameters via bus communication). The drive's internal algorithms, faster and more accurate calculations, and superior electronic components make it superior to variable frequency drives.
Regarding the motor: The materials, structure, and manufacturing process of servo motors are far superior to those of AC motors driven by frequency converters (general AC motors or various variable frequency motors such as constant torque and constant power motors). This means that when the driver outputs a power supply with rapidly changing current, voltage, and frequency, the servo motor can respond accordingly. Its response characteristics and overload resistance are far superior to those of AC motors driven by frequency converters. This significant difference in motor characteristics is the fundamental reason for the performance difference between the two. It's not that frequency converters cannot output such rapidly changing power signals, but rather that the motor itself cannot react in time. Therefore, the internal algorithm of the frequency converter includes overload protection settings to protect the motor. Of course, even without these settings, the output capacity of the frequency converter is still limited; some high-performance frequency converters can directly drive servo motors!
1.2.3 AC Motors:
AC motors are generally divided into synchronous motors and asynchronous motors.
1. AC synchronous motor: The rotor is made of permanent magnet material. So when it rotates, the rotor also changes its speed in response to the change of the rotating magnetic field of the stator. Moreover, the rotor speed is equal to the stator speed, so it is called "synchronous".
2. AC asynchronous motor: The rotor is composed of induction coils and materials. After rotation, the stator generates a rotating magnetic field. This magnetic field cuts the stator's induction coils, inducing a current in the rotor coils. This, in turn, generates an induced magnetic field in the rotor. The induced magnetic field follows the changes in the stator's rotating magnetic field, but the change in the rotor's magnetic field is always less than the change in the stator's. Once they equal, there is no longer a changing magnetic field cutting the rotor's induction coils, and no induced current appears in the rotor coils. The rotor's magnetic field disappears, the rotor stalls, and a speed difference with the stator is created, allowing it to regain induced current. Therefore, a key parameter in AC asynchronous motors is slip, which is the ratio of the speed difference between the rotor and stator.
3. Corresponding to AC synchronous and asynchronous motor frequency converters, there are corresponding synchronous frequency converters and asynchronous frequency converters. Servo motors also have AC synchronous servos and AC asynchronous servos. Of course, AC asynchronous frequency converters are more common in frequency converters, while AC synchronous servos are more common in servos.
4. Application
Because frequency converters and servos differ in performance and functionality, their applications also differ:
1. In applications where speed and torque control requirements are not very high, frequency converters are generally used. Some frequency converters also use a closed-loop control system with position feedback signals added to the host computer for position control, but the accuracy and response are not high. Some frequency converters now also accept pulse sequence signals to control speed, but they don't seem to be able to directly control position.
2. In applications requiring strict position control, only servo motors can achieve this. Furthermore, the response speed of servo motors is far greater than that of frequency converters. Servo control is also used in some applications with high precision and response requirements. In almost all motion applications where frequency converter control is feasible, servo motors can replace them. The key differences are twofold: first, servo motors are significantly more expensive than frequency converters; second, there's the power difference: frequency converters can reach hundreds of kilowatts or even higher, while servo motors only reach tens of kilowatts at most. However, with continuous improvements in servo motor technology, power outputs are gradually reaching several hundred kilowatts.
