Bipolar stepper motor contains two windings, in order to make the motor running smoothly, and constantly to the two coils to phase difference of 90 degrees sine wave, the stepper motor began to turn up.
In general, the stepper motor is not driven by an analog linear amplifier; instead, it is driven by a PWM current regulator to convert a linear sine wave signal into a discrete straight line signal. Sine wave can be divided into many segments, with the increase of the number of segments, the waveform is close to sine wave. In practical applications, the number of segments from 4 to 2048 or more, most of the stepper drive IC using the 4 to 64 segments.
The driver is driven by a whole step, and only one electric power is connected at each moment, and the alternating current and the current direction are switched, so that the mechanical state of the four stepping motors is generated. Half step drive, compared to the whole step drive is relatively complex, at the same time, the two may need to be energized, as shown in Figure 1, so that the stepper motor resolution has doubled. Subdivision drive, motor rotor step angle will be increased with the increase of fine fraction, motor rotation is also more and more stable, for example, a 32 segment sequence called the 1/8 step drive mode (see Figure 1).
Figure 1: subdivided drive current waveforms.
Importance of current control accuracy
The position of the rotor of a bipolar stepper motor depends on the magnitude of the current flowing through the two coil windings. In general, the main indicators of the stepper motor selection, accurate positioning of the mechanical or precise speed control of mechanical systems. Therefore, the precision control of winding current is very important for the smooth operation of the stepper motor.
In mechanical systems, there are two problems that can lead to inaccurate current control:
In the case of low speed operation or the use of stepper motor for position control, the number of steps of each fine segment motor is wrong, resulting in the wrong location.
Under high speed operation, the nonlinear system will cause the change of the speed of the motor, so that the torque is unstable.
PWM control and current decay mode (Decay Mode)
Most of the stepper motor driver IC, the stepper motor windings rely on the characteristics of PWM current regulation. The power supply voltage is applied to the winding of the motor by the H bridge circuit which is composed of the power MOSFET corresponding to each winding, and the driving current is generated with the PWM control. Once the current reaches the set value, the H bridge switches the control state, causing the output current to decay. After a certain period of time, a new PWM cycle will start again, and the H bridge will produce the coil current again.
Repeat this process to cause the winding current to rise and fall. The peak current value of each segment can be adjusted by current sampling and state control.
After the expected peak current is reached, there are two ways to control the current attenuation of the H bridge drive winding:
Winding short circuit (while opening the low side or high side of the MOSFET), slow current decay.
• H bridge reverse conduction, or allow current to flow through the MOSFET body diode, current decay fast.
These two types of current decay are called slow fading and fast fading (see Figure 2).
Figure 2:H bridge working state.
Since the motor winding is inductive, the rate of current change depends on the applied voltage and the coil inductance. Stepper motor to run quickly, the ideal situation is to be able to control the drive current in a very short period of time. Unfortunately, the motor will produce a voltage, the direction of the voltage and the opposite direction, the trend of the change of the resistance current, known as the back emf".
Therefore, the faster the motor speed, the greater the reverse electromotive force, under the action of the motor with the increase in speed and phase current decreases, resulting in smaller torque. To alleviate these problems, either to improve the driving voltage, or to reduce the motor winding inductance. Reducing inductance means that with fewer turns, a higher current is required to achieve the same magnetic field strength and torque.
Traditional peak current control
Conventional stepper motor peak current control, usually only detected by the coil peak current. When the desired peak current is reached, the H bridge switches on the on state, causing the output current to decay (fast decay, slow decay, or a combination of both), for a fixed period of time, or for the end of a PWM cycle. Current decay, the drive IC can not detect the output current, leading to some problems.
In general, it is best to use slow decay, you can get a smaller current ripple, the average current can be more accurate tracking peak current. However, with the increase of the step rate, the slow decay can not reduce the winding current in time, which can not guarantee the accurate current regulation.
In order to prevent sampling to the switching current spike, at the beginning of each PWM cycle, there is a very short time (blanking time) is not sampling the winding current, then the current is out of control. This can lead to severe distortion of the current waveform and instability of the motor (see Figure 3).
Figure 3: current distortion in slow decay mode.
After the sine wave reaches the peak, the current begins to decay and then increases until the H bridge operates in a high impedance state, and the current continues to zero.
To avoid this, many stepper motor driver chips, when the current increase in the amplitude of the slow decay mode, fast decay or mixed use reduction in current amplitude attenuation hours (with fast decay and slow decay mode). However, the average currents of the two decay modes are quite different because the current ripple is relatively large when the fast decay mode is large. As a result, the average value of the current in the two modes varies greatly, resulting in the operation of the motor is not stable (see Figure 4).
Figure 4: the waveform of the traditional peak current control
Figure 4 waveform