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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.