AN1627 Freescale Semiconductor / Motorola, AN1627 Datasheet - Page 12

• Manufacturer Part Number: AN1627
• Description: Low Cost High Efficiency Sensorless Drive for Brushless dc Motor Using MC68HC (7)05MC4
• Manufacturer: Freescale Semiconductor / Motorola

5
5.1
Two methods were tried. The first idea was to calculate the commutation events from the obtained zero
crossing times. This method was shown to be not robust enough. Any disturbance to the feedback signal
(due to noise or motor asymmetry) caused high torque ripples and motor stoppage.
Because of these problems, the second method was developed. Here, the motor is running as
a synchronous motor. The operating value of the voltage is created in such a way, that the Back-EMF
is aligned with the commutation. The phase Back-EMF zero crossing point is kept within a time window
(90°<
close to a simple vector control method. This does not require calculation of the next commutation event,
directly based on the Back-EMF sensing. Therefore this is a more stable algorithm in case of feedback
signal disturbance. No motor stoppage can occur. When using this algorithm the motor speed variation
is very low.
The control flow consist of five phases: Alignment stage, Ramp-up stage, Stabilizing stage, PLL (Phase
Locked Loop) Acquisition stage and Running (PLL Locked) stage.
5.1.1 Alignment
Before the motor starts, there is a short time (which depends on the motors electrical time constant)
when two phases are supplied by power. The Current Controller keeps the current within predefined
limits. This stage is necessary in order to create a high start-up torque (see (EQ 2-3.)).
5.1.2 Ramp-up
Here the motor is starting and ramped up (an “S” curve ramp creates a smooth transient) until it reaches
the working speed. The Current Controller keeps current at the maximum limit in order to ensure that
the rotor will not become locked. The Back-EMF sensing technique enables a sensorless detection of
the rotor position, however the drive must be started without this feedback. It is caused by the fact that
the amplitude of the induced voltage is proportional to the motor speed. Hence, the Back-EMF cannot
be sensed at a very low speed and a special start-up algorithm must be performed.
5.1.3 Stabilizing
The motor is now running for a short time at constant speed. The motor speed is stabilized before the
synchronization with the Back-EMF feedback takes place.
5.1.4 PLL Acquisition
The current controller is switched off. Only the over-current detection is left on. The motor is supplied
from a voltage source. This transition must be done very carefully. The actual PWM duty cycle (phase
voltage) is decreased until several zero crossing points can be sensed within the target time window
(90°<
create oscillation of the whole drive.
5.1.5 Running (PLL Locked)
The following conditions have to be met to enter this stage:
Then the PLL controller keeps the phase shift
motor is now running with a good efficiency. Current measurement, over-current detection and motor
stall detection is still continuously applied.
MOTOROLA
12


< 180° see Figure 6-2.; this can be changed with respect to type of application). This principle is
< 180° see Figure 5-2.). The rate of decrease must be tuned for the application and must not
1.
2.
SOFTWARE
Control Algorithm
Zero crossing events have to be sensed within a time window (90°<
several times (e.g. 6
Current peaks during one commutation period must be below a certain limit
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at the right value by controlling the phase voltage. The

< 180°)
REV 0.2
AN1627