DocID023635 Rev 2

19/34

AN4158

Peak current control with PWM and constant off-time

34

3.3

Microstepping

As described in Section 1.2 on page 9, by driving the stepper motor phases with sinusoidal

currents the step resolution and the motion smoothness can be improved (microstepping).

In order to obtain a microstepping operation using the peak current control with fixed off-

respective microstep current pairs in the proper sequence (see Equation 2 on page 10 and

Table 4 on page 10).

During the positive slope of the sinewave, the reference current value is increased at each

microstep change and the control algorithm reaches the new target value in a single control

cycle, as shown in Figure 14. During the negative slope of the sinewave, the current

reference is progressively reduced down to the zero and the control algorithm may fail to

properly follow the variations (Figure 14).

Figure 14. Peak current control in microstepping (slow decay only)

The key point is that, except for particular implementations, the phase current can be

sensed during the on-time only. As a consequence, the reference increase can be easily

managed extending the on-time until the new threshold is reached, but the decrease must

be performed as a sequence of off-time which slowly decreases the phase current until the

attainment of the threshold.

This issue is particularly critical when the slow decay is used because of the limited current

decay during the off-time. On the other hand, when the fast decay is used, the large current

drop may limit the microstepping effectiveness causing a current ripple bigger than the

minimum microstep-to-microstep variation.

Another limitation of the microstepping implementation through the peak current control

algorithm is the impossibility of effectively controlling the average phase current. Whereas

the actual average position of the stator magnetic field is related to the average phase

current values (see Figure 15), the peak current control introduces an error in its positioning

and then in microstepping implementation.

Poor performance in correspondance

with the zero crossing of the current sinewave

AM15116v1