ST AN2502 Application note

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

www.st.com

Contents AN2502

Contents

1 Implementation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

2 Experimental results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

3 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Appendix A Associated software . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

4 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

2/9

AN2502 Implementation

1 Implementation

Alignment is done by passing current through motor windings corresponding to one of 6
steps of inverter. Let us call it STEP 0. At the end of alignment, the rotor comes to a parking
position that will be a magnetically neutral position on the floating winding.

During alignment, bits DAC(MCRA) and SWA(MCRA) are set to zero and MCOMP is also
set to zero. This has two implications. If DAC bit is zero, then writing to MPHST will only
update the preload registers and become active only during a commutation (or C) event.
With SWA bit being zero, a zero value on MCOMP will retain MTIM at zero and will create a
C event for every clock effectively transferring associated preload registers to active
registers with one clock latency. Hence, at the end of alignment, if the MPHST register is
written with the next step (STEP 1), the inverter switching states will change to the new step
(STEP1) during the next clock cycle and the rotor will turn in the direction of new step. The
CPB(MCRB) bit must also be set appropriately corresponding to the new step to identify
zero crossing for the new step when it happens. To ensure a good starting torque, the
dutycycle may be selected to ensure rated current in the motor windings during this time.

At this instant, the SWA(MCRA) bit should be set HIGH to put the device into auto­commutated mode. In this mode, commutation instances are estimated by hardware and its
associated time count is loaded into MCOMP at the time of zero crossing (Z) events, based
on zero crossing periods MZREG/ MZPRV and a coefficient MWGHT. Before setting SWA,
since MCOMP was zero, MTIM was forced to remain at zero generating C events for every
clock. Once SWA is set, the hardware watches out for a D event and the MTIM timer starts
counting from zero in pursuit of D and Z events. After setting up SWA, the CIM(MIMR) bit
should be set to enable C interrupts. At this point, a C interrupt takes place immediately
because its flag was previously set repeatedly when SWA was zero, as explained above.

Inside the C interrupt routine, the MPHST and MCRB registers are updated for the next step
(STEP2). These are only written into preload registers and are not active yet. If simulated
demag option (SDM(MCRB)=1) is chosen, then MDREG must be updated inside the C
interrupt routine. After the generation of a D event, the comparator hardware is set up to
identify zero crossing of the floating winding. The active step on the inverter is STEP 1 yet,
and if the motor current is good enough to produce a good starting torque to turn the rotor at
a good rate, it will generate a reasonable BEMF on the floating winding that will pass
through a natural zero crossing during this inverter step. This event is captured by the
hardware and a Z event is generated. This event buffers MZREG into MZPRV and captures
MTIM into MZREG and also loads MCOMP with a new time value, based on
MZREG/MZPRV and MWGHT, to setup the next commutation event. When a match
between MTIM and MCOMP happens, a C event is generated transferring contents from
preload MPHST to active state setting up next inverter step (STEP 2). Updating of all
relevant registers inside the C routine is repeated as before, and this C event is followed by
D event and subsequently by Z event and thus the cycle repeats continuously. The
sequence of events during starting is captured in Figure 1.

3/9