AN863 Datasheet, PDF (8/22 Pages) STMicroelectronics – Improved sensorless control with the ST62 MCU for universal motor

IMPROVED SENSORLESS CONTROL WITH THE ST62 MCU FOR UNIVERSAL MOTOR
3 IMPLEMENTATION
Figure 5 gives a schematic of the sensorless motor drive. The heart of the system is
the micro-controller, the ST6220, a low cost 8-bit general purpose micro. It is supplied
directly from the mains by the capacitive supply made of components R2/C1/C2/C3
and of the two diodes. This is made possible by its low current consumption, in the
range of 4 mA. This capacitive supply must be dimensioned to supply the micro, the
operational amplifier, and the triac gate during the firing. Resistor R1 is the current
sense resistor, it is only 0.22 ohm in order to keep the losses as low as possible (in
this example, the motor is a 500 watts type). For this reason, an operational amplifier
is needed in order to provide the micro-controller analog-to-digital converter input (pin
PB1) with a large enough voltage. As this analog-to-digital converter (ADC) is an 8 bit
type, its resolution (minimum voltage step discernible at its input) is around 20 mV
when the micro is supplied from 5 volts. The larger the voltage on the ADC input,
which is the op-amp output after some noise filtering by R6 and C5, the better the res-
olution on measured current i(t0), which will be used latter in the speed regulation al-
gorithm. A value around 1 volt is a reasonable compromise. The op-amp must be a
rail-to-rail type, at the output and inputs as well: for the input, the voltage measured
on the sense resistor is very low, and referred to the positive supply VDD. The output
should be able to swing from the micro-controller ground level to its VDD level, which
are the same as for the op-amp itself, in order to make full usage of the ADC dynamic
range.
Two noise filters (R10/C4 and R6/C5) are provided to remove brush noise from the
motor current signal, before it is measured by the internal ADC. Depending on the
motor, and with careful design, it is possible to do with only one filter.
Resistors R3, R4 and capacitor C6 are used to shape the mains voltage before
sending it to the micro input pin PB0. This digital signal, which shows an edge at each
zero-crossing of the mains (positive or negative edge depending on the half-cycle
sign), interrupts the micro-controller program to signal the zero-crossing. It is used to
synchronize the triac firing with the mains, and to trigger the ADC reading in order to
acquire the current value i(t0).
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