What is a servo motor?
A servo motor is an engine that controls the operation of mechanical components in a servo system; it is a type of auxiliary motor with indirect speed change.
Servo motors enable highly accurate speed and position control, converting voltage signals into torque and speed to drive the controlled object. The rotor speed of a servo motor is controlled by the input signal and can respond quickly. In automatic control systems, they are used as actuators and possess characteristics such as a small electromechanical time constant, high linearity, and low starting voltage. They can convert received electrical signals into angular displacement or angular velocity output on the motor shaft. Servo motors are broadly classified into DC and AC servo motors. Their main characteristic is that they do not rotate when the signal voltage is zero, and their speed decreases uniformly as the torque increases.
Servo motor working principle
I. AC Servo Motor
The stator structure of an AC servo motor is basically similar to that of a capacitor-split-phase single-phase asynchronous motor . Its stator has two windings positioned 90° apart: one is the excitation winding Rf, which is always connected to the AC voltage Uf; the other is the control winding L, connected to the control signal voltage Uc. Therefore, an AC servo motor is also called a dual-servo motor. The rotor of an AC servo motor is usually made in a squirrel-cage configuration. However, to ensure a wide speed range, linear mechanical characteristics, no "self-rotation" phenomenon, and fast response performance, it should have higher rotor resistance and lower moment of inertia compared to ordinary motors.
Currently, there are two main types of rotor structures: one is a squirrel-cage rotor with high-resistivity conductors made of high-resistivity conductive material, which is made slender to reduce the rotor's moment of inertia; the other is a hollow cup-shaped rotor made of aluminum alloy with very thin walls, only 0.2-0.3 mm . To reduce the magnetic resistance of the magnetic circuit, a fixed inner stator is placed inside the hollow cup-shaped rotor . The hollow cup-shaped rotor has a very small moment of inertia, responds quickly, and runs smoothly, hence its widespread use. When there is no control voltage, the stator of an AC servo motor only has a pulsating magnetic field generated by the excitation winding, and the rotor remains stationary. When a control voltage is applied, a rotating magnetic field is generated in the stator, and the rotor rotates along the direction of the rotating magnetic field. Under constant load, the motor speed varies with the magnitude of the control voltage; when the phase of the control voltage is opposite, the servo motor will reverse.
Although the working principle of an AC servo motor is similar to that of a split-phase single-phase asynchronous motor, the rotor resistance of the former is much larger than that of the latter. Therefore, compared with a single-phase asynchronous motor, the servo motor has three significant characteristics: 1. High starting torque: Due to the high rotor resistance, its torque characteristic curve, as shown in curve 1 of Figure 3, is significantly different from the torque characteristic curve 2 of a common asynchronous motor. It can make the critical slip S0 > 1, which not only makes the torque characteristic (mechanical characteristic) closer to linear, but also has a larger starting torque. Therefore, when the stator has a control voltage, the rotor rotates immediately, which means it has the characteristics of fast starting and high sensitivity. 2. Wide operating range. 3. No self-rotation phenomenon: A normally operating servo motor will stop running immediately as soon as the control voltage is lost. When the servo motor loses the control voltage, it is in a single-phase operation state. Due to the high rotor resistance, the two torque characteristics (T1-S1, T2-S2 curves) and the combined torque characteristic (T-S curve) generated by the interaction of the two rotating magnetic fields rotating in opposite directions in the stator with the rotor are... The output power of an AC servo motor is generally 0.1-100W . When the power supply frequency is 50Hz, the voltage is 36V, 110V, 220V, or 380V; when the power supply frequency is 400Hz, the voltage is 20V, 26V, 36V, or 115V, among others.
AC servo motors operate smoothly and with low noise. However, their control characteristics are nonlinear, and due to high rotor resistance, losses are high and efficiency is low. Therefore, compared with DC servo motors of the same capacity, they are larger and heavier, and are only suitable for low-power control systems of 0.5-100W .
DC servo motor speed control principle diagram
Although the working principle of a servo motor and the principle of a servo motor are not much different in name, the difference in meaning is not far off!
1. Servo motors primarily rely on pulses for positioning. Essentially, a servo motor receives one pulse and rotates by the angle corresponding to that pulse, thus achieving displacement. Because servo motors themselves have the function of emitting pulses, they emit a corresponding number of pulses for each rotation. This creates a feedback loop, or closed loop, between the pulses received by the servo motor and the pulses emitted. In this way, the system knows how many pulses were sent to the servo motor and how many were received, allowing for very precise control of the motor's rotation and achieving accurate positioning down to 0.001mm .
DC servo motors are divided into brushed and brushless motors. Brushed motors are low-cost, simple in structure, have high starting torque, wide speed range, and are easy to control. They require maintenance, but maintenance is convenient (replacing carbon brushes). They generate electromagnetic interference and are subject to environmental requirements. Therefore, they can be used in cost-sensitive general industrial and civilian applications. Brushless motors are small in size, light in weight, have high output, fast response, high speed, low inertia, smooth rotation, and stable torque. They are complex to control but easy to implement intelligently. Their electronic commutation method is flexible, allowing for square wave or sine wave commutation. The motors are maintenance-free, highly efficient, operate at low temperatures, have very low electromagnetic radiation, and have a long lifespan, making them suitable for various environments. 2. AC servo motors are also brushless motors, divided into synchronous and asynchronous motors. Currently, synchronous motors are generally used in motion control because they have a wide power range and can achieve very high power. They have high inertia, a low maximum rotational speed, and their speed decreases rapidly with increasing power. Therefore, they are suitable for low-speed, stable operation applications. 3. The rotor inside a servo motor is a permanent magnet. The three-phase electricity (U/V/W) controlled by the driver creates an electromagnetic field, causing the rotor to rotate under the influence of this magnetic field. Simultaneously, the motor's built-in encoder feeds back signals to the driver. The driver compares the feedback value with the target value and adjusts the rotor's rotation angle accordingly. The accuracy of a servo motor depends on the accuracy (line count) of the encoder.
Working principle and function of servo motor :
The function of a servo motor is to drive the controlled object. The torque and speed of the controlled object are controlled by the signal voltage. When the magnitude and polarity of the signal voltage change, the rotation speed and direction of the motor also change accordingly.
Servo motor classification :
AC servo motors and DC servo motors.
AC servo motor :
The principle is the same as that of a two-phase AC asynchronous motor, with two windings on the stator—an excitation winding and a control winding.
The excitation winding and the control winding are spatially separated by 90°.
wiring:
Wiring of excitation winding and wiring of control winding
The purpose of connecting the capacitor C in series in the excitation winding is to generate a two-phase rotating magnetic field.
By appropriately selecting the size of the capacitor, the phase difference between the currents flowing through the two windings can be made close to 90°, thus generating a rotating magnetic field. Under the action of the rotating magnetic field, the rotor will rotate.
Example: By selecting a capacitor, the phasor relationship between voltage and current in the AC servo motor circuit can be made as shown in the figure.
1) When U2=0, the rotor stops.
At this point, although U2=0V, U1 still exists, seemingly indicating single-phase operation, but this differs from a single-phase asynchronous motor. If a single-phase motor starts running, it will continue to rotate even after the single-phase voltage is applied. Servo motors, however, cannot rotate under single-phase voltage.
Reason: The AC servo motor R2 is designed to be relatively large. Therefore, when U2=0, the T=f(s) curve of the AC servo motor is shown in the figure on the next page:
T=f(s) curve of AC servo motor (when U2=0)
When U2=0V, the combined torque T of the torques T' and T" generated by the positive and negative rotating magnetic fields of the pulsating magnetic field is different from that of a single-phase asynchronous motor. The direction of the combined torque is opposite to the direction of rotation, so the motor can stop immediately when U2=0V, which reflects the role of the control signal (rotation when there is control voltage, no rotation when there is no control voltage) to prevent loss of control.
(2) The AC servo motor R2 is designed to be large, so that Sm>1, Tst is large, the start-up is fast, and the stable operating range is large.
(3) When the magnitude of the control voltage U2 changes, the rotor speed changes accordingly, and the speed is proportional to the voltage U2. When the polarity of U2 changes, the direction of rotation of the rotor changes.
application
AC servo motors typically have an output power ranging from 0.1 to 100W, and their power supply frequencies include 50Hz and 400Hz. They have a wide range of applications, such as in automatic control and automatic temperature recording systems.
DC servo motor
Structure: Basically the same as a DC motor. It is made thinner and longer to reduce rotational inertia.
Working principle: Same as DC motor.
Power supply method: separately excited. The field winding and armature are powered by two independent power sources:
U1 is the excitation voltage, and U2 is the armature voltage.
Based on mechanical properties, we can know that:
(1) When U1 (i.e. magnetic flux ￠) remains constant, under a certain load, U2↑ , n↑.
(2) When U2=0, the motor stops immediately.
Reverse rotation: The polarity of the armature voltage changes, causing the motor to reverse.
application:
DC servo motors are stiffer than AC servo motors. They are often used in systems with slightly higher power, and their output power is typically 1-600W. They have many applications, such as position control in servo systems.
Application of servo motors in industrial robots
The robotics industry is booming and flourishing, with numerous machine tool manufacturers, servo manufacturers, and other qualified companies turning to the robotics market. Why are machine tool manufacturers and servo manufacturers so actively transforming and developing robots ? Industrial robots have four main components: the robot body, servo motor, reducer, and controller.
Stepper motors are used to drive the joints of robots and are required to have the highest power-to-weight ratio and torque-to-inertia ratio, high starting torque, low inertia, and a wide and smooth speed range.
The growth of the robotics industry requires breakthroughs in areas such as servo systems and integrated control. Currently, my country is still in the stage of waiting for breakthroughs in areas such as servo systems, which has an adverse impact on the domestic robotics industry.
The general structure of an industrial robot's electric servo system consists of three closed-loop control loops: current loop, speed loop, and position loop. Typically, for AC servo drives, multiple functions such as position control, speed control, and torque control can be achieved by manually setting their internal functional parameters.
The continuous advancement of industrial automation has led to persistently high demand for automation software and hardware. Among these, the domestic market for industrial robots has been steadily growing, and my country is expected to become the world's largest market for such products in 2015.
At the same time, this directly drives market demand for servo systems. The Mingzhi stepper servo motor system supplied by Meilake perfectly integrates servo control technology into an integrated motor, featuring high precision, good stability, and high speed.
Currently, AC and DC servo motors with high starting torque, large torque, and low inertia are widely used in industrial robots. Other motors, such as AC servo motors and stepper motors, are also used in industrial robots depending on different application requirements.
Future Trends in the Servo Motor Industry
Modern AC servo systems, having transitioned from analog to digital, now feature ubiquitous internal digital control loops, including commutation, current, speed, and position control. This is primarily achieved through new power semiconductor devices, such as high-performance DSPs combined with FPGAs, and even dedicated servo modules are not uncommon. Furthermore, new power devices or modules are updated every 2-2.5 years, and new software algorithms are constantly evolving. International manufacturers' servo products are also updated approximately every 5 years—in short, product lifecycles are becoming shorter and the pace of change is accelerating. Summarizing the technical and product roadmaps of domestic and international servo manufacturers, and considering changes in market demand, we can see the following latest development trends in servo motor systems:
Increase efficiency
While increasing efficiency has always been a crucial development topic for servo systems, further improvements are still needed. This primarily includes increasing the efficiency of the motor itself: for example, improving the performance of permanent magnet materials and designing better magnet mounting structures; and also increasing the efficiency of the drive system: including optimizing the inverter drive circuit, optimizing acceleration and deceleration, regenerative braking and energy feedback, and improving cooling methods.
direct drive
Direct drive includes turntable servo drives using disc motors and linear servo drives using linear motors. By eliminating transmission errors from intermediate mechanical transmission devices (such as gearboxes), it achieves high speed and high positioning accuracy. Furthermore, the ability of linear motors to easily change shape allows for the miniaturization and weight reduction of various devices employing linear mechanisms.
High speed, high precision, high performance
By employing higher precision encoders, higher sampling accuracy and data bit depth, faster DSPs, high-performance rotary and linear motors without cogging effect, and various modern control strategies such as adaptive and artificial intelligence, the fundamental indicators (control speed and control accuracy) of servo systems are continuously improved.
Integration and consolidation
The vertical integration of motors, feedback, control, drives, and communication has become a development trend in low-power servo systems. Sometimes we call motors that integrate drives and communication intelligent motors, and sometimes we call drives that integrate motion control and communication intelligent servo drives. The integration of motors, drives, and controls allows for a closer integration of these three aspects from design and manufacturing to operation and maintenance. However, this approach faces greater technical challenges and challenges in meeting engineers' usage habits, making it difficult to become mainstream and remaining a small, distinctive segment within the overall servo market.
generalization
The general-purpose driver features numerous parameters and rich menu functions, allowing users to easily configure it into five operating modes without altering the hardware: V/F control, sensorless open-loop vector control, closed-loop flux vector control, permanent magnet brushless AC servo motor control, and regenerative unit control. Suitable for various applications, it can drive different types of motors, such as asynchronous motors, permanent magnet synchronous motors, brushless DC motors, and stepper motors. It can also adapt to different sensor types, even those without position sensors. A semi-closed-loop control system can be constructed using the motor's built-in feedback, or a high-precision fully closed-loop control system can be formed by connecting to external position, speed, or torque sensors via an interface.
Intelligent
Modern AC servo drives all possess parameter memory, fault self-diagnosis, and analysis functions. The vast majority of imported drives feature load inertia measurement and automatic gain adjustment. Some can automatically identify motor parameters and automatically measure encoder zero position, while others can automatically suppress vibration. Integrating electronic gears, electronic cams, synchronous tracking, interpolation motion, and other control functions with the drive provides a superior experience for servo users.
Networking and modularization
Integrating fieldbus, industrial Ethernet, and even wireless network technologies into servo drives has become common practice for European and American manufacturers. A key direction in the development of modern industrial local area networks (LANs) and a focal point of competition among various bus standards is how to meet the real-time, reliable, and synchronous requirements of data transmission in high-performance motion control. With the increasing domestic demand for large-scale distributed control devices and the successful development of high-end CNC systems, the development of networked digital servos has become an urgent priority. Modularization refers not only to the combination of servo drive modules, power supply modules, regenerative braking modules, and communication modules, but also to the modularity and reusability of the servo drive's internal software and hardware.
From fault diagnosis to predictive maintenance
With the continuous development of machine safety standards, traditional fault diagnosis and protection technologies have become outdated. The latest products incorporate predictive maintenance technology, enabling people to understand the dynamic trends of important technical parameters in a timely manner via the Internet and take preventive measures. For example, monitoring current increases, assessing peak currents when the load changes, monitoring temperature sensors when the casing or core temperature rises, and being vigilant for any distortions in the current waveform.
Specialization and diversification
While general-purpose servo product lines exist in the market, servo systems specifically designed and manufactured for particular applications are ubiquitous. The emergence of motors utilizing different magnetic material properties, shapes, surface bonding structures, and embedded permanent magnet rotor structures, along with the use of segmented core structure technology in Japan, has enabled high-efficiency, high-volume, and automated production of permanent magnet brushless servo motors, prompting research from domestic manufacturers.
Miniaturization and large-scale
Both permanent magnet brushless servo motors and stepper motors are actively developing towards smaller sizes, such as 20, 28, and 35mm outer diameters; at the same time, they are also developing models with higher power and larger sizes, with 500KW permanent magnet servo motors already appearing. This reflects a trend towards polarization.
Experimental methods are also improving.
Unlike traditional motor testing, the performance of servo motors is mainly reflected in control speed and control precision. This raises a problem: traditional motor testing methods only apply to the motor itself and cannot analyze the control characteristics of the servo system.
In response to this situation, the MPT hybrid motor testing system can continuously and dynamically apply loads to the motor under test through free loading engine technology, thereby simulating the working conditions of the motor under test in a real environment. This enables various tests such as dynamic response control of the motor and its control system, actual working condition simulation, and aging, bringing motor testing into the dynamic era and meeting the current testing needs of the servo motion system industry for motion control-related projects.
