AN2281 datasheet(25/38 Pages) STMICROELECTRONICS

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AN2281

Conclusion and results

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Conclusion and results

A low-voltage three-phase AC permanent magnet motor (PMAC or BLAC) control system

has been developed using ST7FLITE35. Some concluding remarks concerning the CPU

load and the code memory size are given below. This is followed by a series of oscilloscope

captures illustrating the proper behavior of the system.

7.1

Motor control related CPU load

The CPU load computation has been performed when using closed loop driving mode, with

Ph/f linear relationship enabled and V/f limitation disabled. The system was driving a 4 pole-

pair motor, running at 10,000 rpm.

The most important contributors to the CPU load were these motor control tasks:

●

Sine wave generation. As discussed in Section 3.1, the sine wave update is

performed in the 12-bit autoreload timer overflow interrupt service routine

(LART_OVF1_IT_Routine). When the three PWM duty cycles are not computed, the

execution time of this interrupt service routine is about 4 µsec. while, when the three

duty cycles are updated, the execution time is around 27.5 µsec. Considering then, that

a interrupt is generated every 64 µsec., the contribution of this task to the CPU load is

equivalent to:

●

Hall sensor signal semi-period measurement. As earlier discussed, this task is

performed in the Lite Timer Input Capture interrupt service routine. Considering a 4

pole-pair motor running at 10,000 rpm, the incoming Hall sensor signal frequency is

666.7Hz. One LTIC interrupt is therefore generated every 750µsec. Since the execution

time of this routine, is around 26.5 µsec., the contribution of this task to the CPU load,

under the described conditions, is equivalent to 3.5%.

●

PI regulation. Assuming a sampling time of 25msecs and considering that the

execution time of this routine is, in worst case conditions, around 150µsec, the

contribution of PI regulation to the CPU load is less than 1% and therefore negligible.

●

Period to frequency conversion. In order to guarantee a high level of

synchronization, sine wave frequency should be computed, starting from the measured

semi-periods, at least once per Hall sensor period.

Considering that, for a motor with 4 pole pairs running at 10,000 rpm, the incoming Hall

sensor signal frequency is 666.7Hz and that the average time required for executing the

time to frequency conversion is around 222µsec, the contribution of this task to the CPU

load is given by

Under the described conditions, the overall CPU load is then

4+27.5

2*64

*100=24.6%

222*10-4*666.7=14.8%

CPU load = 24.6 + 3.5 + 14.8 = 42.9%

(7.1)

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