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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).