Analyze the basics of bipolar stepper motors
The bipolar stepping motor consists of two windings. In order to make the motor run smoothly, the two coils are continuously sinusoidal with a phase difference of 90 degrees, and the stepping motor starts to rotate.
Typically, stepper motors are not driven by analog linear amplifiers; instead, they are driven by PWM current regulation to convert a linear sine wave signal into discrete straight line signals. The sine wave can be divided into multiple segments, and as the number of segments increases, the waveform continues to approach the sine wave. In practical applications, the number of segments is from 4 to 2048 or more, and most stepper drive ICs use 4 to 64 segments. The whole step drive, only one phase is energized at each moment, the two-phase current alternates and the current direction switches, so that a total of four stepper motor mechanical states are generated. The half-step drive is relatively more complicated than the full-step drive mode. At the same time, both phases may need to be energized, as shown in Figure 1, which doubles the step resolution of the motor. Subdivided drive, the angle of the motor rotor step will decrease as the number of subdivisions increases, and the motor rotation will become more and more stable. For example, a 32-segment subdivision sequence is called an eighth-step drive mode.
The importance of current control accuracy
The position of the bipolar stepper motor rotor depends on the amount of current flowing through the two coil windings. In general, the main indicators for selecting a stepper motor are accurate mechanical positioning or precise mechanical system speed control. Therefore, the accuracy control of the winding current is very important for the smooth operation of the stepping motor.
In mechanical systems, there are two problems that can lead to inaccurate current control:
In the case of low speed operation or stepper motor for position control, the number of steps of each subsection motor is incorrect, resulting in incorrect positioning.
At high speeds, system nonlinearity can cause short-term motor speed changes, making the torque unstable and increasing motor noise and vibration.
PWM control and current decay mode (DecayMode)
Most stepper motor drive ICs rely on the inductance characteristics of the stepper motor windings to achieve PWM current regulation. Through the H-bridge circuit composed of the power MOSFETs corresponding to each winding, as the PWM control starts, the power supply voltage is applied to the motor windings, thereby generating a drive current. Once the current reaches the set value, the H-bridge switches the control state, causing the output current to decay. After a fixed time, a new PWM cycle will start again and the H-bridge will again generate the coil current.
This process is repeated to cause the winding current to rise and fall. Through current sampling and state control, the peak current value of each segment can be adjusted and controlled.
After the expected peak current is reached, there are two ways to control the current attenuation of the H-bridge drive winding:
Short-circuiting the winding (while turning on the MOSFET on the low or high side), the current decays slowly.
The H-bridge reverses conduction, or allows current to flow through the body diode of the MOSFET, and the current decays quickly.
