The word "servo" originates from the Greek word for "slave." People envisioned a "servo mechanism" as a docile tool, obeying the commands of control signals. It remains stationary before a signal arrives; it immediately begins to move after the signal arrives; and it automatically stops when the signal disappears. This "servo-slave" behavior is why it is called a servo system .
Servo definition:
(1) Servo system: is an automatic control system that enables the output of motion actuators such as the position, orientation, state, and torque of an object to change in accordance with any change in the input quantity (or given value).
(2) In an automatic control system, a system that can respond to control signals with a certain degree of accuracy is called a follow-up system, also known as a servo system. To ensure the accuracy of this instantaneous response, there are usually position, speed, and torque sensors for feedback comparison, also known as closed-loop control.
The main task of a servo motor is to amplify, transform, and regulate power according to the requirements of control commands, so as to make the closed-loop control of the torque, speed, and position output by the drive device very flexible and convenient.
Simply put, servo control is the control of a motion system's position, time, and force at every moment, including when and where it is located and how much force it outputs.
If the motion system is driven by a motor, then the relationship between the motor's position and its input current is the problem that the servo control system needs to solve. This involves determining the appropriate voltage and current (including phase) input to the motor at what position; this is called position loop and current loop control. The change in position over time is the velocity loop, and the change in velocity loop is acceleration and jerk. From physics, we know that acceleration corresponds to force (e.g., gravitational acceleration G). The motor's output force is driven by the input voltage and current (including phase). Therefore, from the perspective of the motor input, the current loop equals the force; from the perspective of feedback from the motor sensors, the acceleration equals the force. By obtaining position and acceleration information from the feedback of the sensors on the motor, and comparing it with the control input, a closed-loop servo control system is formed.
A rotary encoder is a rotary position sensor that outputs incremental pulse signals (representing the amount of position change) or absolute angular position signals. The first derivative of this signal with respect to time is velocity, and the second derivative is acceleration. Therefore, rotary encoders are the best choice for feedback sensors in servo systems.
Servo motor:
Electric motors are the most commonly used motion actuators. Motor drivers that directly provide closed-loop control of position, speed, and torque are called "servo motors." Commonly used servo motors are AC permanent magnet synchronous motors, and sometimes AC permanent magnet synchronous motors are simply called servo motors.
AC permanent magnet synchronous motor:
The rotor is made of permanent magnet material, so as it rotates, its speed changes in response to the changing magnetic field of the stator. Furthermore, the rotor speed equals the speed driven by the stator's magnetic force, hence the term "synchronous." AC permanent magnet synchronous motors are equipped with encoders to meet their synchronous driving requirements. These encoders provide not only angular position signals (such as incremental pulse signals or absolute digital signals) but also rotor position commutation signals (such as UVW or single-turn sine/cosine C/D signals). The angular position signal serves as closed-loop feedback for position and speed, while the rotor commutation signal is used for closed-loop feedback of the motor's current loop and torque input to achieve synchronous rotor rotation. Therefore, AC permanent magnet synchronous motors, by directly installing encoders, obtain position, speed, and torque feedback information and closed-loop control, naturally possessing "servo" characteristics.
In fact, it is not only AC permanent magnet synchronous motors that can have servo characteristics. AC asynchronous motors, through their controllers (frequency converters) and sensor feedback (such as encoders), can also achieve closed-loop control and follow-up response of position, speed, and even output torque by controller commands, and thus can also achieve "servo" system characteristics.
Whether something can be called a "servo" depends on whether its follow-up response and control accuracy in terms of position, speed, and output force can meet the usage requirements, not on what kind of motor actuator is used.
Based on advancements in frequency conversion technology, servo drive controllers employ more precise control techniques and algorithms in their internal current, speed, and position loops compared to conventional frequency converters. Functionally, they are significantly more powerful than traditional frequency converters, primarily enabling precise position control. Speed and position are controlled via commands sent from a host controller (although some frequency converter-servo controllers integrate control units or directly set position and speed parameters within the drive via bus communication, also known as PG cards). The drive's internal algorithms, faster and more accurate calculations, and superior electronic components make it superior to frequency converters.
The frequency converter and the motor form an open-loop control for speed variation, while the stepper motor and the driver form an open-loop control for position (step) variation. If a sensor (such as an encoder) is added to the frequency converter motor system or the stepper motor system, an external command controller (such as a PLC or a control card integrated in the motor driver) can also achieve a dual closed loop for position and speed, and at the same time guarantee the response to the motor output force and stop positioning, thus realizing a "servo" control system.
Servo control systems encompass not only the motion actuator motors but also the mechanical transmission systems, such as gearboxes, lead screws, and gear drives. These mechanical transmission systems are subject to machining and assembly errors, as well as the influence of temperature changes, wear, and other environmental factors. To prevent these errors from affecting control accuracy, sensors are sometimes added to the motion endpoint to provide position and speed feedback to the servo control system and correct these errors. This control method is called "full closed-loop" control, such as adding a linear encoder or rotary encoder. To ensure long-term accuracy of position control, a zero-point position sensor or an absolute encoder is needed in the control-execution system. Absolute encoders, due to their internal unique encoding of each mechanical position, are not affected by external interference or loss of position information after power outages.
Whether it's an AC permanent magnet synchronous motor (also known as a servo motor), or a variable frequency motor, stepper motor, or other mechanical actuators, a complete "servo" control system requires feedback correction from a controller, mechanical transmission system, and terminal sensors. The control accuracy (position accuracy and follow-up time response) of a servo system is jointly determined by the actuator motor, motor driver, mechanical transmission, and system controller. AC permanent magnet synchronous motors and their drivers, due to their inherent "synchronization" design, offer the highest servo control accuracy. However, to ensure the control accuracy and reliability of the motion execution terminal, it's also necessary to consider the accuracy of the mechanical transmission system and the accuracy and reliability of the terminal position sensors (such as absolute encoders).
For example, in the closed-loop control of elevator car lifting, an encoder (such as the ERN1387 from Heidenhain, Germany) is already installed on the elevator lifting host. This encoder provides incremental A and B sine and cosine signals, with 2048 pulse cycles per week, and C and D sine and cosine signals for one cycle per turn. The C and D sine and cosine signals for one turn per week, after coarse position segmentation, can provide commutation information for the motor's UVW phase. The sine and cosine signals for 2048 cycles per week, after further subdivision, can obtain high-resolution position changes. This high-resolution position change information is mainly used for acceleration calculation over a very short time. Because accurate acceleration feedback is required when the time variable is very small, a large amount of position change information is needed. This requires the encoder to have very high resolution and accurate position accuracy to ensure accurate acceleration feedback for controlling the motor input current.
However, due to mechanical errors in the elevator's mechanical system, the elevator still needs feedback from external leveling sensors when it stops at each floor to obtain accurate positioning. For example, a leveling photoelectric switch or a leveling absolute multi-turn encoder can be used to form a closed-loop servo system with accurate positioning.
In practice, a servo system may require two encoders (or only one). One is located at the high-speed end of the motor, providing feedback on commutation and acceleration. This feedback enters the motor driver, determining the commutation and magnitude of the motor's control current (torque loop). The other is used for accurate positioning at the low-speed end of the position terminal. The encoder at the motor end needs high resolution, commonly using a high-resolution incremental encoder to obtain fine changes in acceleration. The encoder at the motion terminal needs accurate and reliable positioning, commonly using an absolute multi-turn encoder (linear encoders are also used).
If only one encoder is used (e.g., only the motor-side encoder), then high precision in the mechanical transmission part is required for positioning. Currently, high-precision mechanical transmission parts are almost entirely controlled by Japanese and German manufacturers, who hold a monopoly on this technology. Adding a sensor (encoder) to the terminal is one way to circumvent this monopoly.
In a frequency converter control system, since no motor commutation signal is required, the encoder can be directly installed on the motion end, also known as the low-speed end.
We have two concepts: a servo system and a servo motor. These are not the same thing. A servo motor is a special kind of actuator. Its motor drive design is based on closed-loop control of position, speed, and torque from the very beginning. However, as a part of the actuator, the motor does not represent the entire servo system.
Can all closed-loop control be called a servo system? No, it refers to achieving fast control and ensuring accuracy in both spatial (positional) precision and time response. However, the "fast" speed and positional accuracy of a servo system are relative; there will always be some deviation. This is also one of the characteristics of a servo system. The real work of servo systems is to eliminate the impact of this deviation on the control result.
