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The open loop numerical control system composed of stepping motor and drive circuit has been widely used in economical numerical control machine tools because of its simple structure, low price and reliable performance, and it occupies an important position in the numerical control process of China's machine tool industry. status. Stepper motors are often used for precise positioning, so it is important to ensure that the motor is not out of step.
Out of step and its harm
When the stepping motor is in normal operation, it moves one step angle each time it receives a control pulse, that is, it moves forward. If the control pulse is continuously input, the motor continuously rotates accordingly. The loss of stepping motor includes losing and stepping. When losing a step, the number of steps the rotor advances is smaller than the number of pulses; when the step is exceeded, the number of steps the rotor advances is more than the number of pulses. The number of step-loss and step-over steps at a time is equal to an integral multiple of the number of beats. When the loss of step is serious, the rotor will stay in one position or vibrate around one position. If the step is serious, the machine tool will overshoot. Stepper motors are an important part of the open-loop feed system, and their performance directly affects the performance of the CNC system. The out-of-step of the motor will affect the stability and control accuracy of the CNC system, resulting in a decrease in the machining accuracy of CNC machine tools.
Out of step causes and solutions
1. The acceleration of the rotor is slow. The rotating magnetic field of the sub-stepping motor is slower than the rotating magnetic field of the stepping motor. That is, if the speed is lower than the commutating speed, the stepping motor will lose its synchronism. This is because the electric energy input to the motor is insufficient, and the synchronous torque generated in the stepping motor cannot make the rotor speed follow the rotation speed of the stator magnetic field, thereby causing loss of synchronization. Since the dynamic output torque of the stepping motor decreases as the continuous operating frequency increases, any operation frequency higher than this frequency will cause lost motion. This out-of-step description of the stepper motor has insufficient torque and insufficient drag capability. Solution: 1 Increase the electromagnetic torque generated by the stepper motor itself. For this reason, the drive current can be appropriately increased in the rated current range; when the torque is insufficient in the high-frequency range, the drive voltage of the drive circuit can be appropriately increased; a stepper motor with large torque can be used instead. 2 Reduce the torque that the stepper motor needs to overcome. For this reason, the operating frequency of the motor can be appropriately reduced in order to increase the output torque of the motor; a longer acceleration time is set so that the rotor can obtain sufficient energy.
2. The average speed of the rotor is higher than the average rotation speed of the stator magnetic field. The average speed of the rotor is higher than the average rotation speed of the stator magnetic field. At this time, the excitation time of the stator is longer and greater than the time required for the rotor step, the rotor is in the stepping process. Excessive energy is obtained in the motor, so that the output torque generated by the stepping motor is increased, so that the motor is stepped. When a stepping motor is used to drive mechanisms that act to move the load up and down, stepping is more likely to occur because the required torque of the motor decreases when the load moves downward. Solution: Reduce the drive current of the stepper motor to reduce the output torque of the stepper motor.
3. The inertia of the stepping motor and the loaded load The inertia of the stepping motor itself and the loaded load make it impossible for the motor to start and stop immediately during operation. Instead, the lost motion occurs at the start and the step occurs at the stop. Solution: Through an acceleration and deceleration process, which starts at a lower speed, then gradually accelerates to a certain speed, and then gradually decelerates until it stops. Performing reasonable and smooth acceleration and deceleration control is the key to ensure reliable, efficient and accurate operation of the stepping drive system.
4. Stepping motor resonance resonance is also a cause of out of step. When the stepper motor is in continuous operation, resonance will occur if the frequency of the control pulse is equal to the natural frequency of the stepper motor. During a control pulse period, the vibration is not sufficiently attenuated and the next pulse comes. Therefore, the dynamic error in the vicinity of the resonance frequency is maximized and the stepping motor is desynchronized. Solution: properly reduce the stepper motor drive current; use subdivision drive method; use the damping method, including the mechanical damping method. The above method can effectively eliminate the motor oscillation and avoid the out-of-step phenomenon.
Stepping motor out of step causes analysis and solution
A comprehensive analysis of the causes of the stepping motor's out-of-step in the process of economical numerically controlled ship is made, and corresponding solutions are proposed.