These two current attenuation modes are called slow attenuation and fast attenuation.
Since the motor windings are inductive, the rate of change of current depends on the applied voltage and the sense of the coil. For a stepper motor to run quickly, the ideal situation is to be able to control the drive current to change in a short period of time. Unfortunately, a motor generates a voltage in the opposite direction of the applied voltage, which tends to change the current, called "back EMF." Therefore, the faster the motor speed is, the larger the back electromotive force is. Under the action of the motor, the phase current decreases as the speed increases, resulting in a smaller torque. To alleviate these problems, either increase the drive voltage or reduce the motor winding inductance. Reducing the inductance means that with fewer turns, a higher current is needed to achieve the same magnetic field strength and torque.
Traditional peak current control problem
Conventional stepper motor peak current control usually only detects the peak current through the coil. When the expected peak current is reached, the H-bridge switches to the on-state, causing the output current to decay (fast decay, slow decay, or a combination of both) for a fixed amount of time, or to wait for the end of a PWM period. When the current is attenuated, the driver IC cannot detect the output current, causing some problems.
In general, it is best to use slow decay to get a smaller current ripple, and the average current can more accurately track the peak current. However, as the step rate increases, slow decay does not reduce the winding current in time, and accurate current regulation cannot be guaranteed.
To prevent sampling current spikes, at the beginning of each PWM cycle, there is a very short time (blankingtime) that does not sample the winding current, then the current is uncontrolled. This can cause severe current waveform distortion and unstable motor operation
After the sine wave reaches its peak value, the current begins to decay and then increases until the H-bridge operates in a high-impedance state, and the current continues to decay to zero.
To avoid this, many stepper motor drive chips use a slow decay mode when the current amplitude increases and a fast decay or mixed attenuation (in combination with fast decay and slow decay) modes when the current amplitude decreases. However, the average current of these two attenuation modes is completely different because the current ripple in the fast decay mode is relatively large. As a result, the average current values in the two modes differ greatly, resulting in unstable motor operation.
