AN863 Datasheet, PDF (12/22 Pages) STMicroelectronics – Improved sensorless control with the ST62 MCU for universal motor

IMPROVED SENSORLESS CONTROL WITH THE ST62 MCU FOR UNIVERSAL MOTOR
Figure 6. The program is a loop synchronized on the mains zero crossing (z.c.).
It is identical on both half-cycles, except for the i(t0) current
measurement, regulation subroutine and speed demand subroutine
The adaptation of the program to a specific motor type can be done by adjusting the
coefficients KI and KP, and the contents of compensation table table(td). The table is
necessary only for the largest firing delays (slow speed). The slower the motor should
be able to run, the more attention should be given to the elaboration of this table.
The proportional coefficient KP adjusts how strongly the system reacts to a speed
error: the larger this coefficient, the faster the reaction to load change, but if this term
becomes too large, some instability may occur. The integral coefficient KI adjusts the
transient settling time when the load changes: the larger KI, the faster the settling
time, but again, instability may occur if this coefficient becomes too large. Trial and
error is the best way to find the best compromises.
The latest detail which must be mentioned in the program is the operational amplifier
gain adjust. At high speed, current it0 becomes very small; if we want to measure it
with a reasonable accuracy, we must amplify it even more than in case of low speed.
This is why the op-amp is fitted with a dual gain scheme, with help of resistor R5, con-
nected to VDD or left open by the micro pins PB2/PB3/PB4. The op-amp gain is set
to 10 for the low speed range, 40 for the high speed range. This gain setting is man-
aged in subroutine “speed”, which also checks for user speed demand.
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