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System Topology
The flux synchronization is derived from the position information coming from sensors, or from sensorless
techniques. From the position, the controller determines the appropriate pair of transistors (Q1 to Q6) that
must be driven. The regulation of the current to a fixed 60° reference can be realized in either of the two
different modes:
• The Pulse Width Modulation (PWM) Mode:
The supply voltage is chopped at a fixed frequency with a duty cycle depending on the current error.
Therefore, both the current and the rate of change of current can be controlled. The two phase supply
duration is limited by the two phase commutation angles. The main advantage of the PWM strategy is
that the chopping frequency is a fixed parameter; hence, acoustic and electromagnetic noises are
relatively easy to filter.
There are also two ways of handling the drive current switching: hard chopping and soft chopping. In
the hard chopping technique, both phase transistors are driven by the same pulsed signal: the two
transistors are switched-on and switched-off at the same time. The power electronics board is then
easier to design and is also cheaper as it handles only three pulsed signals. A disadvantage of the
hard chopping operation is that it increases the current ripple by a large factor in comparison with the
soft chopping approach.
The soft chopping approach allows not only a control of the current and of the rate of change of the
current but a minimization of the current ripple as well. In this soft chopping mode, the low side
transistor is left ON during the phase supply and the high side transistor switches according to the
pulsed signal. In this case, the power electronics board has to handle six PWM signals.
• The Hysteresis Mode:
In the hysteresis-type current regulator, the power transistors are switched off and on according to
whether the current is greater or less than a reference current. The error is used directly to control the
states of the power transistors. The hysteresis controller is used to limit the phase current within a
preset hysteresis band. As the supply voltage is fixed, the result is that the switching frequency varies
as the current error varies. Therefore, the current chopping operation is not a fixed chopping frequency
PWM technique. This method is more commonly implemented in drives where motor speed and load
do not vary too much, so that the variation in switching frequency is small. Here again, both hard and
soft chopping schemes are possible. Since the width of the tolerance band is a design parameter, this
mode allows current control to be as precise as desired, but acoustic and electromagnetic noise are
difficult to filter because of the varying switching frequency.
4.2 Shaft Position Sensors
The position information is used to generate precise firing commands for the power converter, ensuring
drive stability and fast dynamic response. In servo applications position feedback is also used in the
position feedback loop. Velocity feedback can be derived from the position data, eliminating a separate
velocity transducer for the speed control loop.
Three common types of position sensors are used: the incremental sensors, the three Hall Effect sensor,
and the resolver.
• The incremental sensors use optically coded disks with either single track or quadrature resolution to
produce a series of square wave pulses. The position is determined by counting the number of pulses
from a known reference position. Quadrature encoders are direction sensitive and do not produce false
data due to any vibration when the shaft begins rotation. The Quadrature encoder pulse unit of the
F280x handles encoders’ output lines and can provide 1, 2 or 4 times the encoder resolution. Speed
information is available by counting the number of pulses within a fix time period.
• The three Hall Effect sensors provide three overlapping signals giving a 60° wide position range. The
three signals can be wired to the F280x input capture/GPIO pins, therefore, speed information is
available by measuring the time interval between two input captures. The time interval is automatically
stored by the 280x into a specific register at each input capture. From speed information, it is
numerically possible to get the precise position information needed for sharp firing commands.
• The resolver is made up of three windings (different from the motor’s windings): one linked to the rotor
and supplied with a sinusoidal source and two other orthogonal coils linked to the stator. A back EMF
is induced by the rotating coil in each of the two stator resolver windings. By decoding these two
signals, it is possible to get cos(q) and sin(q) where q is the rotor position. The resolver resolution
depends only on the AD conversion.
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SPRABQ6–July 2013 Trapezoidal Control of BLDC Motors Using Hall Effect Sensors
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