Abstract: A servo system is an electrical drive automatic control system that drives mechanical motion, uses a servo motor as the controlled object, a controller as the core, and a power electronic power conversion device as the actuator, all guided by automatic control theory. Modular machine tools typically consist of a combination of standard general-purpose components and machining-specific components. The power components are driven by electric motors or hydraulic systems, and the electrical system controls the automatic cycle of operation. They are typical mechatronic or mechatronic-hydraulic integrated automatic machining equipment. These systems control the torque, speed, and angle of the electric motor, converting electrical energy into mechanical energy to achieve the motion requirements of the moving machinery.
1. Introduction
Electrical servo technology is the most widely used, mainly because it is convenient and flexible to control, readily available drive energy, produces no pollution, and is relatively easy to maintain. Especially with the development of electronic and computer software technologies, it has provided a broad prospect for the development of electrical servo technology. As early as the 1970s, low-inertia servo DC motors were already in practical use. By the late 1970s, AC servo systems began to develop and gradually become practical, with AC servo motors becoming increasingly widely used and even trending towards replacing DC servo systems as the mainstream of electrical servo systems. Permanent magnet synchronous servo motors are even more widely used in AC servo systems due to the continuous improvement and decrease in price of permanent magnet materials, and their simpler control compared to asynchronous motors, making it easier to achieve high performance. Research on high-performance AC servo control technology with independent intellectual property rights, especially the most promising permanent magnet synchronous motor servo control technology, has significant theoretical and practical value.
With the development of high-efficiency inverters, digital signal controllers, high-performance servo motors, and control theory, AC servo systems are inevitably replacing DC servo systems. PMSM rotors have no excitation windings, resulting in high motor operating efficiency. The use of high-efficiency rare-earth permanent magnet materials for excitation effectively reduces the motor's size and weight, achieving high torque output and significantly reducing rotor inertia. Therefore, PMSMs are widely used in high-performance AC servo drive systems. Pulse width modulation (PWM) is an analog control method that modulates the bias of the transistor gate or base according to changes in the load, thereby changing the output transistor or the transistor's on-time in a switching power supply. This method allows the power supply's output voltage to remain constant under changing operating conditions and is a very effective technique for controlling analog circuits using the digital output of a microprocessor. PWM control technology, with its advantages of simple control, flexibility, and good dynamic response, has become the most widely used control method in power electronics technology and is a hot research topic. Since the boundaries between disciplines have blurred in today's scientific and technological development, combining modern control theory or implementing resonant-free soft-switching technology will become one of the main directions for the development of PWM control technology. In the 1980s, Dr. Broeck proposed a new pulse width modulation method—space vector PWM modulation—introducing space vectors into pulse width modulation. It has advantages such as a wide linear range, low high-order harmonics, and ease of digital implementation, and has been widely used in new types of drivers. This paper analyzes the principle of space vector PWM for three-phase AC motors and discusses the voltage output capability of a three-phase bridge voltage-source inverter using space vector PWM. A comparative analysis of SVPWM and carrier-based SPWM is conducted, highlighting the connection between SVPWM and SPWM with superimposed third harmonics. Different placement of the zero-sequence vector can lead to different SVPWM modulation methods; inserting only one zero-sequence vector per PWM cycle can reduce the number of switching operations by 1/3, thus achieving SVPWM modulation with minimal switching losses.
In recent years, servo motor control technology has been developing in three directions: AC, digital, and intelligent. As the actuator of CNC machine tools, the servo system integrates power electronic devices, control, drive, and protection. With the advancement of digital pulse width modulation technology, special motor material technology, microelectronics technology, and modern control technology, it has gone through a development process from stepper motors to DC motors, and then to AC motors.
2. Composition and Structure of AC Servo Motor
The structure of an AC servo motor can be mainly divided into two parts: the stator and the rotor. The stator structure is basically the same as that of a rotary transformer stator, with two-phase windings spaced 90° apart electrically within the stator core.
L1-L2 are called the excitation windings, and k1-k2 are called the control windings. Therefore, an AC servo motor is a two-phase AC motor.
Commonly used rotor structures include squirrel-cage rotors and non-magnetic cup-shaped rotors. The structure of a squirrel-cage AC servo motor consists of a rotor shaft, rotor core, and rotor windings. The rotor core is made of stacked silicon steel sheets, each stamped into a toothed and slotted shape, then stacked to press the shaft into the shaft hole. Each slot in the core contains a conductor bar, and all conductor bars are connected at both ends by two short-circuit rings, thus forming the squirrel-cage rotor windings.
The structure of a non-magnetic cup-shaped rotor AC servo motor is identical to that of a squirrel-cage rotor servo motor. The inner stator is made of stacked annular steel sheets and typically does not contain windings; it simply replaces the core of the squirrel-cage rotor as part of the motor's magnetic circuit. Between the inner and outer stators, a slender hollow rotor is mounted on a shaft. This hollow rotor is cup-shaped, hence the name "hollow cup-shaped rotor." The hollow cup is made of non-magnetic materials such as aluminum or copper, and its walls are extremely thin, typically around 0.3mm. The cup-shaped rotor fits around the inner stator core and can rotate freely in the air gap between the inner and outer stators via the shaft, while the inner and outer stators remain stationary. Compared to a squirrel-cage rotor, the non-magnetic cup-shaped rotor has lower inertia and lower bearing friction torque. Because its rotor has no teeth or slots, there is no tooth-slot adhesion between the stator and rotor, and the torque does not change with different rotor positions. During constant-speed rotation, the rotor generally does not vibrate and operates smoothly. However, due to its larger air gap between the inner and outer stators (the cup wall thickness plus the air gaps on both sides of the cup wall), the excitation current is larger, reducing the utilization rate of the motor. Therefore, within a certain power range and for the same volume and weight, the starting torque and output power of the cup-shaped rotor servo motor are smaller than those of the squirrel-cage rotor servo motor. In addition, the structure and manufacturing process of the cup-shaped rotor servo motor are more complex. Therefore, the squirrel-cage rotor servo motor is currently widely used, and non-magnetic cup-shaped rotor servo motors are only used in certain special applications requiring very smooth operation (such as integrating circuits).
3. Working principle of AC servo motor
The working principle of an AC servo motor is not fundamentally different from that of a single-phase induction motor. However, an AC servo motor must possess one crucial characteristic: it must overcome the so-called "self-rotation" phenomenon. This means it should not rotate without a control signal, and especially if it is already rotating, it should stop immediately if the control signal disappears. In contrast, a regular induction motor, once started, often continues to rotate even after the control signal is lost.
When the motor is initially stationary, if no control voltage is applied to the control winding, only the excitation winding is energized, generating a pulsating magnetic field. This pulsating magnetic field can be considered as two circular rotating magnetic fields. These two circular rotating magnetic fields rotate in opposite directions with the same magnitude and speed. The resulting forward and reverse rotating magnetic fields cut the cage winding (or cup-shaped wall) and induce electromotive forces and currents (or eddy currents) of the same magnitude but opposite phases. The torques generated by these currents interacting with their respective magnetic fields are also equal in magnitude and opposite in direction, resulting in a net torque of zero, preventing the servo motor rotor from rotating. Once the control system receives a deviation signal, the control winding must receive a corresponding control voltage. Under normal circumstances, the magnetic field generated inside the motor is an elliptical rotating magnetic field. An elliptical rotating magnetic field can be considered as the resultant of two circular rotating magnetic fields. These two circular rotating magnetic fields have unequal amplitudes (the forward rotating magnetic field, rotating in the same direction as the original elliptical rotating magnetic field, is larger, while the reverse rotating magnetic field, rotating in the opposite direction, is smaller), but they rotate in opposite directions at the same speed. The electromotive force and current induced by the cutting of the rotor windings, as well as the resulting electromagnetic torque, are in opposite directions and of unequal magnitude (larger for forward rotation, smaller for reverse rotation). The resultant torque is not zero, so the servo motor rotates in the direction of the forward magnetic field. As the signal strengthens, the magnetic field approaches a circle. At this point, the forward magnetic field and its torque increase, while the reverse magnetic field and its torque decrease, resulting in a larger resultant torque. If the load torque remains constant, the rotor speed increases. If the phase of the control voltage is changed, i.e., shifted by 180°, the direction of the rotating magnetic field reverses, and therefore the direction of the resulting torque also reverses, causing the servo motor to reverse. If the control signal disappears, and only current flows through the excitation winding, the magnetic field generated by the servo motor will be a pulsating magnetic field, and the rotor will quickly stop. The squirrel-cage rotor (or a non-magnetic cup-shaped rotor) rotates because of a rotating magnetic field in space. The rotating magnetic field cuts the rotor bars, inducing electromotive force and current in the rotor bars. The current in the rotor bars then interacts with the rotating magnetic field to produce force and torque. The direction of the torque is the same as the direction of the rotating magnetic field, so the rotor rotates in the same direction as the rotating magnetic field.
With the continuous development and advancement of hydraulic technology and the expanding application areas and scope, the requirements for system flexibility and various performance aspects are becoming increasingly stringent. Traditional system designs that focus on completing predetermined actuator cycles and are limited by static system performance are far from meeting these demands. Therefore, it is essential for modern hydraulic system design researchers to study the dynamic characteristics of systems, understand and master the dynamic working characteristics and parameter changes of hydraulic systems, in order to improve the system's response characteristics, control accuracy, and operational reliability.
The dynamic characteristics of a hydraulic system refer to its properties as it moves from a previous equilibrium state to a new one. These characteristics are primarily caused by changes in the transmission and control systems, as well as external disturbances. During this process, the performance of each system parameter over time determines the quality of the system's dynamic characteristics. The dynamic characteristics mainly manifest as stability (the instantaneous peak and fluctuation of pressure in the system) and transient response quality (the response quality and speed of the actuators and control mechanisms).
The main research methods for the dynamic characteristics of hydraulic systems include transfer function analysis, simulation, experimental research, and digital simulation. Digital simulation utilizes computer technology to study the dynamic characteristics of hydraulic systems. First, a digital model of the hydraulic system's dynamic process—the state equations—is established. Then, the time-domain solutions of the main variables in the system during the dynamic process are obtained on a computer. This method is applicable to both linear and nonlinear systems, and can simulate the changes in various system parameters under the action of input functions. This provides a direct and comprehensive understanding of the system's dynamic process, allowing researchers to predict the dynamic performance of the hydraulic system during the design phase. This enables timely verification and improvement of the design results, ensuring the system's performance and reliability. It offers advantages such as accuracy, adaptability, short cycle time, and low cost.
4PWM modulation technology and dead-time compensation technology
IGBT (Insulated Gate Bipolar Transistor) is a composite, fully controllable, voltage-driven power semiconductor device composed of a BJT (Bipolar Junction Transistor) and a MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor). It combines the advantages of both MOSFETs (high input impedance) and GTRs (low on-state voltage drop). GTRs have a low saturation voltage and high current density, but require a large drive current; MOSFETs have very low drive power and fast switching speed, but a large on-state voltage drop and low current density. IGBTs combine the advantages of both devices, offering low drive power and a low saturation voltage. They are ideally suited for applications in DC power conversion systems of 600V and above, such as AC motors, frequency converters , switching power supplies, lighting circuits, and traction drives.
In the SVPWM modulation control of induction motors, stator current is predicted, the impact of dead zone is calculated, and a predictive compensation algorithm is proposed. The characteristics of the inverter dead zone are analyzed through simulation, and a mathematical model of the dead zone and a nonlinear model of the entire system are established. An adaptive variable structure control strategy is adopted to eliminate the influence of the inverter dead zone. This approach does not require measurement of dead zone parameters, exhibits strong robustness, and enables global system stability and accurate position tracking.
5. Sensorless control technology
Removing position sensors in permanent magnet synchronous motor (PMSM) drive systems is more challenging because the motor's three phases are always energized, there is no back EMF signal available, and the required position information is not limited to the six commutation points of a brushless DC motor. This necessitates designing more complex observers to estimate accurate position information using measured phase voltages and currents. A flux linkage observer was designed by establishing flux linkage equations, utilizing the position information contained in harmonic reactive power. Salient-pole PMSMs have an advantage over non-salient-pole PMSMs in utilizing sensorless technology because the inductance of a salient-pole motor varies sinusoidally with rotor rotation, a characteristic that can be used to detect rotor position at low speeds. Similarly, reducing the number of current sensors in PMSM drive systems is also a focus for cost reduction. This paper presents a method that, with appropriate techniques, requires only one current sensor to detect the bus current, instead of using three separate current sensors to detect the three-phase current.
6PMSM Robust Control
Various robust control methods applied to permanent magnet synchronous motors (PMSMs) have also attracted considerable interest from researchers. This is because traditional PID control is likely to deteriorate the dynamic characteristics of the control system when the motor load or motor parameters change. Such changes in motor load or motor parameters are unavoidable. Therefore, it is necessary to design a robust controller to suppress the impact of parameter variations on control performance. To meet this need, a sliding mode variable structure control scheme has been proposed, and an adaptive control strategy has been proposed to design the position and speed controllers for PMSMs. Fuzzy control strategies, as an optimistic alternative to PID control, have also been introduced into PMSM controllers to improve the robustness of PMSMs in the face of load torque variations.
Furthermore, a relatively complex current control strategy was proposed using space vector modulation technology for the current control of permanent magnet synchronous motors. These advanced current controllers incorporate predictive control methods and present a fully digital control scheme to improve the characteristics of the current loop.
7 Conclusions
Modern AC servo drives possess parameter memory, fault self-diagnosis, and analysis functions. Most imported drives feature load inertia measurement and automatic gain adjustment; some can automatically identify motor parameters and measure encoder zero position, while others can automatically suppress vibration. However, the servo object exhibits significant uncertainty and nonlinearity, and the system is also subject to varying degrees of interference during operation. Therefore, conventional control strategies are insufficient to meet the control requirements of high-performance permanent magnet synchronous motor (PMSM) servo systems. Thus, introducing advanced "composite control strategies" based on new developments in control theory to improve the performance of the controller—a core component of the PMSM servo system—and compensate for the "hard constraints" inherent in the system, should be a major breakthrough in the development of high-performance PMSM servo systems.
