AN863 Datasheet, PDF (17/22 Pages) STMicroelectronics – Improved sensorless control with the ST62 MCU for universal motor

IMPROVED SENSORLESS CONTROL WITH THE ST62 MCU FOR UNIVERSAL MOTOR
ANNEX 2 CREATING THE CURRENT COMPENSATION TABLE
A second assembly program is provided, namedcaract.asm. It must be used in place
of “regul.asm”. Its purpose is to allow characterization of the motor in order to create
the table. Contrary to “regul.asm”, it does not perform any regulation, but instead fires
the triac at a constant delay td which can be adjusted through the whole range
(VITMAX to VITMIN) by means of the two push buttons. At the same time, the RS232
connection allows to measure the present values for td (firing delay) and it0 (meas-
ured current at zero crossing), as explained in annex 3. The procedure to follow is de-
scribed below:
– select the desired speed for which the table must be optimized. Figure 12 gives an
example (600 rpm) of the shape that current it0 has while td is varied. This curve is
in fact one of the curves family in Figure 12 of annex 1. This speed is obtained with
no load by adjusting td with the push-buttons. The RS232 connection allows to
measure and record td and it0 when the desired speed is obtained.
– add a small constant load on the motor shaft. The speed naturally decreases. Then
decrease the firing delay td with the push buttons until the speed is back to 600 rpm.
At this time, record td and it0.
– increase slightly the load. Decrease td to come back to 600 rpm. Record again td
and it0. Keep increasing the load until td reaches the minimum allowed, and record
a few couples (td,it0).
– If you make a graph of the couples (td,it0) with td in abscissa, you will obtain a curve
similar to the one in Figure 12. Subtracting the current value for small td, approxi-
mately 80 in Figure 12, will give the values for the look-up table: the table coeffi-
cients are zero for td = 0 to 4mS, then increase regularly up to approximately 22 for
td = 8mS. (note that, for the vertical axis, the values given are not amperes, but reg-
ister values output by the ADC converter. They can be related to the actual current
by taking account that the ADC value changes by one unit for 20 mV voltage change
at its input PB1: when the input is maximum, 5 volts, the ADC value is 255). In the
example of Figure 12, the table will look similar to the following:
td (mS)
0 1 2 3 4 5 5.5 6 6.5 7 7.5 8
coefficient 0 0 0 0 0 3 4 7 10 15 18 22
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