SENSORLESS FIELD ORIENTED CONTROL OF BRUSHLESS ...

SVM Implementation
The block diagram of the general SVM inverter is shown in Figure 4.40. The commanded voltage
SV is passed to the SVM control unit as its α- and β- components.
Figure 4.40 – SVM inverter.
The switching scheme is a fixed set of rules that determines which active and null base SVs
should be used to synthesize the commanded SV. There are many variations but it seems that
most implementations have rules based on sextants similar to the previous discussion. Therefore,
the first task of the SVM algorithm is to identify the sextant in which the commanded SV lies;
this is fairly simple. After the sextant is identified the controller knows which base SVs it must
modulate based on the switching rules. The second task of the SVM controller is therefore to
determine the duration of each base SV (Figure 4.37); this is more difficult. The final task is to
issue the leg control commands. Fortunately, this can be accomplished using the standard centeraligned
PWM unit available in many microcontrollers [83, p.261] but there are other methods.
This method is shown in Figure 4.41. The durations are computed as duty cycles which are
converted to timer comparison values; these are updated every SVM period, although some PWM
units are capable of half-cycle reloading. The change in each time-compare value at the update
points has been greatly exaggerated.
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