A stepper motor is an open-loop control motor that converts electrical pulse signals into angular displacement or linear displacement. In the case of no step loss, the speed and stop position of the motor only depend on the frequency and number of pulses of the pulse signal and are not affected by the load change. When the stepper motor receives a pulse signal, the motor will follow the set value. The direction turns a fixed angle (step angle or step angle), and the motor angular displacement and motor speed are controlled by controlling the number of pulses and pulse frequency to achieve precise open-loop control. In addition, there is always a certain error between the angle rotated by the stepper motor and the theoretical step distance, and there is always a certain error from a certain step to any step. However, the number of steps per revolution of the stepper motor In the same way, the step error will not accumulate for a long time without losing step.
The above mentioned are all in the case of not losing step, how can we avoid losing step? First we need to know the factors that cause the motor to lose step:
- Improper selection of the stepper motor, insufficient motor torque or the inertia of the object movement exceeds the self-locking force of the motor, resulting in lost steps or lost steps.
- Improper selection of the driver, the current of the matching driver is too small, which affects the normal operation of the motor. Nowadays, many drivers with virtual current on the market use the peak current as the rated current to fool consumers. The rated current of the driver selection should be greater than the rated current of the stepper motor. 1.2-1.5 times the current.
- Improper selection of the supporting power supply, the supporting power supply should be 1.5-2 times the rated power supply of the drive, and the virtual standard of the power supply is more serious than that of the drive.
- The control part should eliminate interference, keep away from the inverter, and prevent static electricity. The equipment should be grounded to prevent induced electricity, the signal line should be shielded, the appropriate starting frequency should be set, and acceleration and deceleration should be done well.
When the rotor acceleration is slower than the rotating magnetic field of the stepper motor, that is, when the rotor speed is lower than the commutation speed, the motor will lose step. This is because the electric energy input to the motor is insufficient, and the generated torque cannot make the rotor speed keep up with the rotation speed of the stator magnetic field, resulting in loss of synchronization. The average speed of the rotor is higher than the average rotation speed of the stator magnetic field, which means that the stator is energized and excited for a long time, which is longer than the time required for stepping. The rotor obtains too much energy during the stepping process, resulting in excessive torque generated by the motor. Cause the motor to overstep.
The above reasons for the stepper motor out of step are in essence caused by the improper selection of the stepper motor driver. Only by selecting the correct and suitable stepper driver can the stepper motor exert its precise control advantages. To choose a suitable driver, it is necessary to use a driver with a current greater than or equal to this current according to the current of the motor. If low vibration or high precision is required, a subdivision driver can be used. For high-torque motors, use high-voltage drives as much as possible to obtain good high-speed performance. At the same time, for the driving power supply, many people directly use the switching power supply as the driving power supply. However, it is generally best not to use the switching power supply, especially the high-torque motor, unless the switching power supply with more than twice the required power is used. Because the motor is a large inductive load when it is working, it will form an instantaneous high voltage on the power supply end. The overload performance of the switching power supply is not good, it will protect the shutdown, and its precise voltage regulation performance is not required, which may sometimes cause damage to the switching power supply and the driver. For the drive power of the stepper motor, a conventional toroidal or R-type transformer transformed DC power supply can be used.
How to solve the problem of stepper motor out of step?
Microstep current curve
In a large number of micro-step drive systems, combined with the influence of the non-uniformity of the step angle on the stepper motor out-of-step, it is ideal to use the subdivision method of uniform rotation of the current vector with constant amplitude. The two-phase hybrid stepper motor is used as the For example, by adding sinusoidal current to the two-phase windings of the motor to achieve “constant amplitude and uniformity” of the stator current composite vector, the mathematical model of the two-phase winding current in micro-step rotation is expressed as: the micro-step parameters can be precalculated, with The form of the table is stored in the EPROM, the parameter value is obtained by looking up the table, and the frequency is changed by micro-stepping. Because it does not need to go through complex calculations, it will not take up too much CPU time. At the same time, this method can also be used. Fitting a frequency change that is closer to the ideal change curve to achieve smooth control of the acceleration and deceleration of the stepper motor. In addition, this subdivision method also solves the problem of the unevenness of the micro-step angle to a large extent. It will be widely used in the control system of stepper motor that does not require particularly high precision.
Selection of driving circuit
Another measure to avoid out-of-step and reduce vibration lies in the design of the drive system. There are many drive methods for stepper motors, including single and dual voltage drive, high and low voltage drive, 0 bridge drive, boost drive, chopper Constant current drive and pulse width modulation (PWM) constant current drive, etc., because microstep drive needs to control the size of phase winding current, so only single voltage string resistance drive, chopper constant current drive and PWM constant current drive are suitable for microstep drive Control, the single-voltage series resistance drive mode reduces the circuit time constant due to the series resistance, and the continuous return time constant decreases greatly at the cut-off time, thereby accelerating the current discharge, which is conducive to improving the high-frequency response of the stepper motor, and also because the loop The increased damping is beneficial to reduce the resonance of the motor, but its main disadvantage is that the loss is large and the efficiency is low; when the chopper constant current drive is used, the driving voltage is high, and the current rises quickly. When the required value is reached, due to the sampling resistance feedback Control function, the winding current can be constant at a certain value, and does not change with the speed of the motor, so as to ensure that the motor can output a constant torque in a wide frequency range, and another advantage of using chopper constant current drive It reduces the occurrence of motor resonance. The basic reason for motor resonance is excess energy, and the energy input by the chopper constant current drive is automatically adjusted with the winding current. When the energy is excess, the freewheeling time is prolonged and the power supply time is reduced. , so the accumulation of energy can be reduced. PWM constant current drive uses digital pulses to directly control the duty cycle of the current waveform, which is simpler than the chopping constant current drive circuit and more suitable for single-chip microcomputers to directly use digital signal control. Therefore, PWM is selected. Constant current driven driver.
Selection of acceleration curve
In terms of controlling the step-out of the stepper motor, the acceleration curve of the stepper motor is also very important. There are generally two acceleration laws, one is to increase the speed according to the linear law, and the other is to increase the speed according to the exponential law! The acceleration is constant when accelerating according to the linear law, so the torque generated by the stepper motor is required to be constant. From the characteristics of the motor itself, the output torque can be basically considered constant within the range of the speed is not very high! But in fact, when the motor speed increases, the winding current will gradually decrease due to the effect of the back electromotive force and the winding inductance, so the output torque will decrease. The law of change, the microcomputer can use discrete methods to approach the ideal lifting curve in the process of controlling the acceleration of the stepper motor.
Measures to reduce electromagnetic interference
The following measures are mainly taken for the electromagnetic interference of the single-chip microcomputer to the control system of the stepper motor: adding an optoelectronic isolation circuit to the driving circuit of the single-chip microcomputer and the stepper motor can effectively suppress the electromagnetic interference and improve the stability of the system; The conduction speed of the main transformer can reduce the intensity of electromagnetic interference, and an absorption circuit can also be added to suppress the generation of surges, and the core structure of the main transformer can be reasonably selected to reduce the magnetic leakage intensity; another important aspect is the electronic circuit. Reasonable layout, control the distance and relative direction of the interference source and the interfered component, keep the sensitive component away from the interference source, separate the connection lines for different purposes, do not run parallel lines, and the wiring of a loop crosses each other in the middle position and the left and right loops of the loop. Half of the area should be roughly equal to reduce the induced potential, the wire should be shielded and a reasonable grounding design, etc., taking these measures will effectively reduce the impact of electromagnetic interference on the single-chip control system.