Process control technology of acceleration and deceleration of stepping motor
Because of the wide application of stepping motor, there are more and more researches on the control of stepping motor. If the step pulse changes too fast when starting or accelerating, the rotor can not follow the change of electric signal due to inertia, resulting in locked rotor or out of step. If it stops or decelerates due to the same reason, it may lead to overstep. In order to prevent locked rotor, out of step and over step, and to improve the working frequency, it is necessary to control the speed of stepping motor.
The speed of stepping motor depends on pulse frequency, number of rotor teeth and beat. Its angular velocity is directly proportional to the pulse frequency, and is synchronized with the pulse in time. Therefore, when the number of rotor teeth and running beat is fixed, the desired speed can be obtained by controlling the pulse frequency. Because the stepping motor is started with its synchronous torque, in order not to lose step, the starting frequency is not high. Especially with the increase of power, the diameter of rotor increases, the inertia increases, and the difference between the starting frequency and the maximum operating frequency may be ten times.
The starting frequency characteristic of the stepping motor makes it impossible to reach the running frequency directly when it starts, but there must be a starting process, that is, from a low speed to the running speed gradually. The running frequency can't be reduced to zero immediately when it is stopped, but there should be a process of gradually reducing the speed to zero at high speed.
The output torque of the stepping motor decreases with the rise of the pulse frequency. The higher the starting frequency is, the smaller the starting torque is, and the worse the ability to drive the load is. When the stepping motor is started, it will cause out of step and overshoot when it is stopped. In order to make the stepper motor reach the required speed quickly without losing step or overshoot, the key is to make the torque required by acceleration fully utilize the torque provided by the stepper motor under various operating frequencies and not exceed the torque. Therefore, the operation of stepper motor generally needs to go through three stages of acceleration, uniform speed and deceleration, which requires the acceleration and deceleration process time to be as short as possible and the constant speed time to be as long as possible. Especially in the work requiring quick response, the time from the starting point to the end point is the shortest, which requires the shortest process of acceleration and deceleration, while the speed at constant speed is the highest.
Many researchers at home and abroad have done a lot of research on the speed control technology of stepping motor. They have established a variety of mathematical models of acceleration and deceleration control, such as exponential model, linear model, etc. on this basis, they have designed and developed a variety of control circuits, improved the motion characteristics of stepping motor, extended the application range of stepping motor, exponential acceleration and deceleration considered the inherent torque frequency characteristics of stepping motor, It can not only ensure that the stepping motor does not lose step in motion, but also give full play to the inherent characteristics of the motor and shorten the time of speed up and down. However, due to the change of the load of the motor, it is difficult to realize the linear acceleration and deceleration only consider that the angular speed of the motor in the load capacity range is proportional to the pulse, and not change due to the fluctuation of the power supply voltage and load environment. The acceleration of this method is constant The disadvantage is that the output torque of the stepping motor does not fully consider the characteristics of the change with the speed, and the stepping motor will lose step at high speed.