1.
Increased Ripple-Free Torque or Speed in BLDC Motors
Operating under Current or Voltage Saturation
Technology Summary
The CSA invention presents a novel electronic commutator of BLDC motors with any backEMF waveform such that the commutator minimizes power dissipation, subject to current
and voltage limits of the motor's drivers. When one or more phases reach their voltage
and/or current saturation levels, the controller optimally reshapes the stator currents of
the remaining phases for continuing accurate, ripple-free torque production -- the
controller automatically reshapes the excitation currents in such a way that the motor
continues to generate the torque that is requested. A closed-form solution for the optimal
phase currents at given angular position, velocity and desired torque, renders the control
algorithm suitable for real-time implementation.
This innovation allows the motor to operate above the rated speed and torque that would
be achieved without current reshaping. This optimal management of the motor's excitation
currents can significantly increase the rated speed or torque of the motor in the face of
voltage and current limits of the drivers. Experiments show that the maximum torque
capability is boosted by 20% when the phase saturation is considered in the phase current
shape function.
In addition, the torque controller can be used as a remedial strategy to compensate for a
phase failure by optimally reshaping the currents of the remaining healthy phases for
accurate torque production. The control algorithm allows the motor to optimally generate
precise torque, even when operating under a phase failure.
Market opportunities for the CSA technology are in applications that require BLDC motors
with high efficiency, and those that require BLDC motors with reliable (i.e., fault-tolerant)
performance.
Of particular commercial interest is the technology's ability to provide ripple-free torque in
a BLDC motor that is operating, intentionally, with one or more phases in voltage and/or
current saturation. This feature of the technology provides system designers with an
opportunity to substantially increase the speed and/or torque from a given BLDC motor.
2.
Increased Ripple-Free Torque or Speed in BLDC Motors
Page 2
Background to the Technology
Brushless DC (BLDC) motors are commonly used as the drives of servo systems in a wide
range of industrial applications from robotics and automation to aerospace and military.
Accurate and ripple-free torque control of BLDC motors is essential for precision control of
such servo systems.
In BLDC motors, the electric power is distributed by an electronically controlled
commutation system. The conventional electronic commutator incorporates a feedback
from the rotor angular position into a control system, which excites the stator coils of the
motor in a specific order, in order to rotate the magnetic field generated by the coils to be
followed along by the rotor. These conventional drivers of BLDC motors produce
sinusoidal (or, alternatively, trapezoidal) current waveforms for smooth motor operation.
In practice, however, non-ideal motors do not have a perfect sinusoidal distribution of
magneto-motive force, and hence the sinusoidal commutation can result in torque ripple.
Suppressing the torque ripple of the motor drive of the servo system can significantly
improve system performance by reducing speed fluctuations.
The conventional control approach is to assume that the phase currents can be controlled
accurately and instantaneously, and therefore the currents can be treated as the control
inputs. The waveforms of the motor phase currents are then adequately pre-shaped, so
that the generated torque is equal to the requested torque. The limitation of this approach
is that the motor's drivers have fixed rated current and voltage limits, and some of them
may not be able to deliver the current inputs dictated by the electronic commutator that
may occur when the motor operates at high torque or speed. Consequently, the
performance of the torque production may significantly deteriorate as a result of the phase
current distortions caused by the voltage or current saturation of the amplifiers.
When there is a fixed inverter voltage and current, flux weakening allows a BLDC motor to
operate above the base speed in constant-power, high-speed regions. Below the rated
speed, all of the stator currents can be used to produce torque. Above the rated speed, a
part of the stator current must be used to oppose the permanent magnet flux while the
remaining portion is used to produce torque. These techniques are adequate only for a
BLDC motor with a perfectly shaped, sinusoidal back-EMF waveform, and when, in
addition, phase current limits are not taken into account.
General Description of the CSA Technology
This CSA innovation provides a closed-form solution for optimal excitation currents for
accurate torque control of brushless motors with any waveform that minimizes power
dissipation subject to current and voltage limits of the motor's drivers. When the motor
terminal voltages and/or phase currents reach their saturation levels, the controller
automatically reshapes the excitation currents in such a way that the motor generates
torque as requested. This optimal management of the motor's excitation currents can
significantly increase the rated speed or torque of the motor in the face of the voltage and
current limits of the drivers.
3.
Increased Ripple-Free Torque or Speed in BLDC Motors
Page 3
Another aspect of the innovation is that the torque controller can optimally compensate for
the failure of a phase and reshape the currents of the healthy phases for accurate torque
production. In the event that an open-circuit or short-circuit of a winding occurs, the
torque controller can isolate the faulty phase in order to generate the torque that is
requested, given the voltage and current constraints of the healthy phases.
A novel aspect of this CSA technology is the optimal non-linear feedback from the rotor's
angular position and angular rate (speed) that makes accurate torque production possible
when the voltage or current of one or more phases reach their saturation level, or when
phase failure occurs.
The CSA control algorithm permits torque sharing among phases when some phases
saturate. This results in a considerable increase in the attainable maximum motor torque
because the torque controller automatically increases the torque contribution of the
unsaturated phases when one phase saturates. Experiments show that the maximum
torque capability is boosted by 20% when the phase saturation is considered in the phase
current shape function.
Experimental results, described below, illustrate the capability of the controller to achieve
precise torque production during voltage/current saturation of the motor's drivers or a
phase failure.
Validation of the Technology
The performance of this innovative controller has been demonstrated using simulation and
experimental data. Experimental results obtained from a brushless servomotor under the
proposed torque controller demonstrated accurate torque production under
voltage/current saturation of the motor's drivers or failure of one phase.
In order to evaluate the performance of the optimal torque controller, experiments were
conducted on a three-phase synchronous motor with nine pole pairs. Two tests were set
up as follows:
Requested torque 10 Nm, speed 21 rad/s. In this test, phase voltages reached their
limits, but the phase currents were far from the current limit.
Requested torque 25 Nm, speed 2 rad/s. In this test, the phase currents reached
their limit, while the terminal voltages were not saturated.
These experiments demonstrated that the optimal torque controller can deliver accurate
torque production in the face of current and voltage saturation.
As described above, the optimal torque controller can produce accurate torque even under
operation of a single phase failure. In this experiment, the current circuit of the motor's
first phase was virtually broken by sending zero signal to the enable port of the
4.
Increased Ripple-Free Torque or Speed in BLDC Motors
Page 4
corresponding power amplifier. The objective was to produce the same torque, 10 Nm, as
the three phases by using only the remaining two phases. Experimental results show that
the motor still produces the desired constant torque when the torque controller is
designed based on the two healthy phases.
Patent Protection
Patent applications have been filed in Canada and USA.
Publication
The CSA technology is described in the following publication, available from CSA upon
request:
Aghili, F., "Optimal and Fault-Tolerant Torque Control of Servo Motors Subject to
Voltage and Current Limits", accepted for inclusion in a future issue of IEEE
Transactions on Control System Technology.
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