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Introduction of stepper motor
Stepping motor is also called pulse motor. Based on the most basic principle of electromagnet, it is a kind of electromagnet that can rotate freely. Its action principle is to generate electromagnetic torque by the change of air gap permeability. Its original model originated from 1830 to 1860. Around 1870, it began to try to control and was applied to the electrode delivery mechanism of hydrogen arc lamps. This is considered to be the original stepping motor. In 1923, James weir French invented three-phase variable reluctance, which is the predecessor of stepping motor. At the beginning of the 20th century, stepping motors were widely used in automatic telephone exchanges. As the western capitalist powers scrambled for colonies, stepping motors were widely used in independent systems such as ships and aircraft lacking AC power. In the late 1950s, the invention of transistors was also gradually applied to stepping motors, which made digital control easier. After the 1980s, because of the appearance of low-cost microcomputers in a multi-functional manner, the control methods of stepping motors were more flexible and diverse. [2]
The biggest difference between stepper motor and other control motors is that it receives digital control signals (electric pulse signals) and converts them into corresponding angular displacement or linear displacement. It is an actuator that completes digital mode conversion. Moreover, it can open-loop position control, and input a pulse signal to obtain a specified position increment. Compared with the traditional DC control system, the so-called incremental position control system has significantly reduced cost and almost no system adjustment. The angular displacement of the stepping motor is strictly proportional to the number of input pulses, and is synchronized with the pulses in time. Therefore, as long as the number and frequency of the pulses and the phase sequence of the motor winding are controlled, the required rotation angle, speed and direction can be obtained. [2]
The stepping motor in China started in the early 1970s. From the mid-1970s to the mid-1980s, it was the development stage of finished products. New varieties and high-performance motors were continuously developed. At present, with the development of science and technology, especially the development of permanent magnet materials, semiconductor technology and computer technology, the stepping motor has been widely used in many fields. [2]
Control technology and development of stepping motor
As a special motor for control, the stepper motor cannot be directly connected to the DC or AC power supply to work, and a special drive power supply (stepper motor driver) must be used. Before the development of microelectronic technology, especially computer technology, the controller (pulse signal generator) was completely realized by hardware. The control system used separate components or integrated circuits to form the control loop. Not only was the debugging and installation complicated, but also a large number of components were consumed. Once the control scheme was finalized, the circuit must be redesigned. This makes it necessary to develop different drivers for different motors. The development difficulty and development cost are high, and the control difficulty is large, which limits the promotion of stepping motors. [2]
As the stepping motor is a device that converts electrical pulses into discrete mechanical motion and has good data control characteristics, the computer has become the ideal driving source of the stepping motor. With the development of microelectronics and computer technology, the control mode of combining software and hardware has become the mainstream, that is, the control pulse is generated by the program to drive the hardware circuit. The single-chip microcomputer controls the stepping motor through software, which better exploits the potential of the motor. Therefore, it has become an inevitable trend to control the stepping motor with single-chip microcomputer, and it is also in line with the trend of the digital era.
The main classification：
There are many structural forms and classification methods of stepping motor, which are generally divided into reluctance type, permanent magnet type and mixed magnetic type according to the excitation mode. According to the number of phases, it can be divided into single phase, two phase, three phase and many equal forms.
In the stepping motor used in our country, the reactant stepping motor is mainly used. The operation performance of stepper motor is closely related to the control mode. The stepper motor control system can be divided into the following three categories: open-loop control system, closed-loop control system and semi-closed-loop control system. Semi-closed loop control systems are generally classified as open loop or closed loop systems in practice.
The main structure：
The structure diagram of the three-phase reluctance stepping motor model is shown in the overview diagram. Its stator and rotor cores are stacked by silicon steel sheets. There are six magnetic poles on the stator, and each of the two opposite magnetic poles is wound around the same phase winding. The three-phase winding is connected into a star shape as a control winding. There are no windings on the rotor core, only four teeth, the width of the teeth is equal to the width of the stator pole boot. [1]
Step motor acceleration and deceleration process control technology
Because of the wide application of stepper motor, there are more and more studies on the control of stepper motor. When starting or accelerating, if the step pulse changes too fast, the rotor can not follow the change of electrical signal due to inertia, resulting in blocked or out of step when stopping or decelerating, for the same reason, it may produce super step. In order to prevent blocking, out of step and over step, and improve the working frequency, it is necessary to control the speed of stepping motor. [2]
The speed of stepper motor depends on pulse frequency, rotor teeth and beat number. Its angular velocity is proportional to the pulse frequency and synchronized in time with the pulse. Therefore, when the number of rotor teeth and the number of running beats is fixed, the desired speed can be obtained by controlling the pulse frequency. Because the stepper motor is started with the help of its synchronous torque, the starting frequency is not high in order not to lose step. Especially with the increase of power, the rotor diameter increases, the inertia increases, the starting frequency and the highest operating frequency may differ by as much as ten times.
The starting frequency characteristics of the stepper motor make the stepper motor cannot directly reach the operating frequency when it starts, but there should be a starting process, that is, gradually increasing speed from a low speed to the running speed. When stopping, the operating frequency should not be reduced to zero immediately, but there should be a high speed gradual deceleration to zero.
The output torque of stepper motor decreases with the rise of pulse frequency, the higher the starting frequency, the smaller the starting torque, the worse the ability to drive the load, the out-of-step will be caused when starting, and the overshoot will occur when stopping. In order to make the stepper motor quickly reach the required speed without losing step or overshoot, the key is to make the acceleration in the process of acceleration, the torque required by the acceleration can not only make full use of the torque provided by the stepper motor under each operating frequency, but also can not exceed this torque. Therefore, the operation of stepper motor generally goes through three stages of acceleration, uniform speed and deceleration, which requires the acceleration and deceleration process to be as short as possible and the constant speed time to be as long as possible. Especially in the work requiring fast response, the running time from the start to the end is required to be the shortest, which must require the shortest process of acceleration and deceleration, and the speed at constant speed is the highest.
Scientific and technological workers at home and abroad have done a lot of research on the speed control technology of stepper motor, and established a variety of acceleration and deceleration control mathematical models, such as exponential model, linear model, and on this basis, design and develop a variety of control circuits, improve the motion characteristics of stepper motor. The application scope of the stepper motor extended index considering the stepper motor acceleration and deceleration torque inherent frequency characteristics of stepping motor can be ensured in sports do not break step, and give full play to the inherent characteristic of the motor, shortened the time speed, but due to the change of the motor load, it is difficult to achieve and linear deceleration motor is only considered in load capacity range of the angular velocity is proportional to the pulse of this relationship, Not due to the fluctuation of power supply voltage and load environment, the acceleration of this speed up method is constant. Its disadvantage is that it does not fully consider the characteristics of the stepper motor output torque changing with speed, and the stepper motor will lose step at high speed.
Schematic diagram of model structure of three-phase reluctance stepping motor