AN2502 Datasheet, PDF (1/9 Pages) STMicroelectronics – Enhanced sensorless startup control of BLDC motors using ST7FMC

AN2502
Application note
Enhanced sensorless startup control of BLDC motors
using ST7FMC
Introduction
The ST7FMC microcontroller is capable of controlling 3 phase brushless DC (BLDC) motors
in position sensorless mode because of its powerful motor control macro cell. It is capable of
monitoring zero crossings of all 3 back EMF (BEMF) voltages using a direct BEMF detection
feature, where the entire motor terminal voltage is directly applied to an onchip zero
crossing detector during the zero crossing period without any attenuation. Hence it is able to
track the BEMF zero crossings even at very low motor speeds.
This direct BEMF detection feature helps to start the motor in auto commutated mode right
from zero speed. In traditional methods, where the motor voltage signals are scaled down at
the comparator input, it is necessary to step the rotor through a few rotations initially to let
the comparator see sufficient levels of BEMF signals to detect zero crossings. The rate at
which the rotor can be stepped through these initial rotations is highly influenced by load
inertia, and hence a step timing table has to be created on the basis of load inertia. This can
be a cumbersome task if the load inertia is not constant. With ST7FMC, because of the
direct BEMF detection feature, this procedure can be avoided completely, thereby saving on
development time as well as code memory space.
In any case, an alignment of the rotor to a known position is mandatory. At the end of
alignment, if a commutation is performed by switching the next set of windings into
conduction, the rotor is forced to turn in the direction of new step. If the winding current is
set close to rated motor current at the end of alignment, then this step change will apply a
rated torque on the rotor. Generally, this torque should be sufficient to kick the rotor to turn in
the right direction at a reasonable rate (or speed), inducing a reasonable BEMF in the
floating winding good enough for the zero crossing detector to identify a zero crossing as the
full BEMF is available at the comparator input without any attenuation. Hence from the very
first step, the device can be set in auto commutated mode like in a Hall position sensor
based control.
March 2007
Rev 1
1/9
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