With the development of the world economy, the improvement of the science and technology level and the continuous improvement of the level of industrial automation in recent years, higher requirements have been needed on servo control technology. The application of servo motor control technology has many notable features, for example, it can reduce running time, improve the accurate calculation of the automatic control system and also improve the performance of the servo system to achieve the transition to a software servo control system.

Servo Control System

The servo system is a kind of automatic control system as well as a negative feedback control system, which can make the controlled object change automatically, continuously and accurately with the change of the given signal. The steady-state balance is dynamic and is used to control the displacement or rotation of the controlled object. From the principle of automatic control, in general, the servo control system mainly includes controlled objects, brakes, controllers, sensors, etc. Meanwhile, the brake is composed of a power amplifier and a motor. Servo control system consists of five parts, namely, controller, controlled object, actuator, detection and transmission, etc.

According to the different actuators, the servo system can be classified into two types: electrical servo system and electro-hydraulic servo system. Thereinto, the electrical servo system is easy to operate and maintain and comes with high reliability. While the electro-hydraulic servo system adopts electro-hydraulic pulse motor and electro-hydraulic servo motor as the servo drive device, which features fast response, good rigidity and small time constant. Additionally, it can generate high output torque and stable speed at low speed. However, it has some disadvantages, such as large size, high noise and oil leakage, which is caused by the hydraulic system requiring an oil supply system. In terms of feedback comparison mode, it not only includes pulse digital comparison servo system and amplitude comparison servo system, but also includes phase comparison servo system and full digital servo system. Compared with the phase comparison and amplitude comparison system, the pulse digital comparison system comes with simple structure, installation and easy maintenance as well as the relatively low requirements. Therefore, the amplitude comparison system has the least application and the pulse digital comparison servo system has a wider range than the phase comparison system as for the application range. Servo motor system is suitable for construction machinery, mining equipment, large agricultural equipment, fans, pumps, compressors and other industrial automation equipment.

AC Servo Motor Working Principle

How does an AC servo motor work? The AC servo motor working principle is basically the same as that of single phase induction motor. But AC servo motors solve such a problem, namely, if the control signal suddenly disappears while the AC servo motor is running, its rotation can stop immediately. However, during the operation of an ordinary induction motor, if the control signal disappears, the motor will generally continue to rotate. When the AC servo motor stops and if no control voltage is applied, only the excitation winding is energized to generate a pulsating magnetic field. Since the circular rotating magnetic fields generated by the two excitation windings have the same size and rotation speed, but the rotation directions are opposite, then the induced electromotive force and current generated by their cutting of the cage windings are the same in magnitude and opposite in direction and thus have the same magnitude and opposite direction as the resultant torque generated by their respective magnetic fields. If it is zero, the servo motor rotor will not run. Because the magnetic field inside the motor can be regarded as an elliptical rotating magnetic field synthesized by two circular rotating magnetic fields.

When the control winding receives the control voltage corresponding to the control signal, the two circular rotating magnetic fields will have unequal amplitudes, so the combined torque of the electromagnetic torque generated by the cutting rotor winding is not zero, so the servo motor will rotate in the direction of the forward rotating magnetic field with a larger torque. As the signal continues to increase, the forward rotating magnetic field and its torque continue to increase. Then as the resultant torque increases, the rotational speed of the rotor increases continuously. By changing the phase of the control voltage, the direction of the rotating magnetic field can be changed and the servo motor will reverse. When the control signal disappears, only the excitation winding is energized. Since the motor inside has the pulsating magnetic field at this time, the rotor can stop quickly.

Application of AC Servo Motor in Automatic Control System

1. Features for servo motor application in automatic control system

As we know, AC servo motor is a brushless motor. The small AC servo motor has two classifications, synchronous AC servo motor and asynchronous AC servo motor. The synchronous servo motor can achieve a large power in the stable operation at low speed and the power reaches the highest value at the highest rotation time and then it will also decrease as the rotation speed decreases. According to the characteristics of the servo motor, the use of the servo motor in the automatic control system has some obvious features, such as fast response in the absence of overshoot, larger torque even at low speed and in wider speed regulation range and high precision, etc.

2. The servo motor application improves the performance of the automatic control system

  • The control accuracy is improved.

Because the industrial automation has high requirements on the accuracy of production lines, the accuracy of AC servo system application in this field can reach the micron level and even the displacement accuracy of some advanced products is as high as 0.1 microns, which provides an effective guarantee for improving the accuracy of intelligent processing.

  • Larger torque output at low speed is achieved.

In general, automatic control systems need to operate at lower speeds. Under the condition of low speed, the torque output is a very important key to the normal operation of the normal system. If the torque output is small, it is not conducive to the smooth operation of the system. However, servo motor has a very advantageous feature, namely, its constant power output can be carried out. If the automatic control system uses a servo motor, which can ensure that the system outputs a large torque at a low speed and whether it is the longitudinal or the lateral driving torque performance, the best results can be achieved under such conditions. What's more, the use of servo motors can effectively shorten the transmission chain of the automatic control system, so that the friction of the transmission is reduced, thereby greatly improving the system efficiency.

  • Quick response capability is available.

Quick response is an important indicator of modern CNC machine tools. At present, the speed of CNC machine tools using the AC server system can be controlled within tens of milliseconds from zero to the maximum speed and no overshoot occurs when the speed changes. At the same time, when the load changes sharply, the system can recover in a very short time without oscillation.

  • The range of speed regulation can be expanded.

The automatic control system adopts a servo motor, its adjustable range of feed speed is 0-250m/min and it can be adjusted continuously. If the automatic control system does not use the servo motor, its adjustable range of feed speed is 0-23m/min. It can be seen that there are much difference between these two situations. Therefore, the use of the servo service motor expands the speed regulation range of the automatic control system.

Development Prospects of Servo Motor Control Technology

The motor control dedicated inheritance circuit is the most common form for enterprises to design the servo motor and the design software has the classification of complex programmable logic devices and field programmable logic arrays. The user and electronic system requirements are needed while designing the motor control integrated circuit. The circuit enables efficient scanning of operating boundaries and has the feature that user can control and program on the spot. The motor control ASIC comes with the features of short production and design time. Compared with general-purpose circuits, the products produced with integrated circuit electronic technology features light weight, low cost, small size, low power consumption as well as high quality. In a addition, the servo motor control technology is also reflected in terms of motor control MCU design, motor control DSP design, etc. The AC servo motor comes with the brushless structure (brushless AC servo motor), which has the features of high speed and low maintenance probability.

In the mid-1980s, servo motor control technology has been integrated with catalytic processing technology and will also gain great development in the future. Servo motor is the core component of automation system, it has AC servo motor and DC servo motor. Nowadays, AC servo motor gradually replaces DC servo motor with its superior performance, which has become the development trend of servo motor. With the rapid development of industrial automation, AC servo system has been widely used in industrial field. For example, servo motor drive has been widely used in industrial robots.