Analysis and Design of a Novel Three-Level LLCC Inverter ...

Analysis and Design of a Novel Three-Level
LLCC Inverter Supplying an Airborne Piezoelectric
Brake Actuator
Rongyuan Li, Norbert Fröhleke, Joachim Böcker
Institute of Power Electronics and Electrical Drives, Paderborn University
Warburger str.100, 33098 Paderborn, Germany
Abstract - This contribution is focused on the investigation of
a novel single-phase three-level PWM inverter in the kW power
range, feeding a high power multi-mass ultrasonic motor (MM-
USM) via a LLCC-type filter. In order to specify the power
supply requirements, the operating principle of the MM-USM
employed for a novel airborne brake actuator is briefed. The
control scheme is studied, and for attaining high dynamic
performance, a cascaded voltage and hysteresis current
controller is designed and implemented on a FPGA.
Measurement results indicate the progress towards high power
piezoelectric actuator power supply and tuning measures with
respect to modeling and simulations.
I. INTRODUCTION
The facilitation of a "More Electric" approach in future
aircrafts faces the bottleneck of low weight high power density
actuators, while "intelligence" for data processing is available
through microelectronics and software technologies. A
promising candidate as novel brake actuator is the multi-mass
ultrasonic motor (MM-USM) derived from the well known
travelling-wave type motor (TW-USM), which is the subject
of the European project PIBRAC [1] [2].
Electronic power supplies for ultrasonic piezoelectric
applications like actuators and sonotrodes are available in the
market, but the power range is limited to few tens of Watts,
which is far away from the kW range required for an multimass
piezoelectric aircraft brake actuator [3]. The major
challenge is to design an appropriate power supply to drive the
piezoelectric motor which is known for its distinct capacitive
behavior. On a closer inspection, the electrical behavior is
even more complicated. It even depends on the frequencydependent
interactions between actuator and load, i.e. the
mechanical subsystem of the brake. Previous works on
ultrasonic motors have shown that the quality factor as
measure of the system damping has a strong influence on the
converter topology to be chosen [3] [4] [5].
Over the last decade research and development of power
supplies which suit best to the needs of ultrasonic motors has
been conducted. A resonant inverter with LLCC-type output
filter presented in [4] [6] shows advanced characteristics and
best properties in respect to efficiency, stationary and dynamic
behaviour, control and commissioning. The drawbacks of
these resonant inverters are the large volume and heavy weight
of the resonant filter magnetic components [8].
In order to reduce the size and the weight of magnetic
components, a carrier-based PWM controlled inverter with LC
filter was investigated [7]. The fundamental frequency of
power inverter is determined by the mechanical resonant
frequency in a range of 20 to 40 kHz. This implies that the
inverter switching frequency is in the range of 140 to 300 kHz,
which produces consequently more switching losses and
eventually ends up with EMC issues. To overcome these
issues related to two-level PWM converters, either an optimal
pulse width modulation or a multi-level inverter technology,
are employed for the inverter enhancement [9] [10] [11] [12].
For the purpose of improving the performance of power
S1
D7
S5
u dc1
S2
U dc
i filter
i Cp
u dc2
D8
S3
S4
S6
u filter
Ls
Cs
Cable
Lp
Piezo ceramic
u Cp
Fig. 1 Single phase three levels PWM inverter plus LLCC-filter

supply, a novel scheme was developed by combining a singlephase
three-level PWM inverter and a LLCC filter shown in
Fig. 1. It is designed in a way to reduce the total harmonic
distortion (THD) of the motor voltage, and to compensate
locally for the reactive power of the motor, since the motor is
supplied via a long cable due to its displacement from the
power supply.
metallic blocks connected with eight piezo-ceramic multilayer
stacks with alternating polarization. The structure is excited by
the piezo stacks at its mechanical eigenfrequency which is
designed for 35 kHz (called tangential mode) so that each
metallic block would oscillate in the plane of the ring.
The structure is able to oscillate also orthogonally to the
surface of the stator rings and rotor disk (normal mode),
Tangential direction
CsT
L sT
L mT
C mT
R pT
C pT
L pT
u CpT
R mT
Inverter filter motor
Tangential
piezoelement
Tangential Mode
Normal
direction
Normal
piezoelement
Rotor disk
Metallic
block
L sN
L mN
C mN
CsN
Normal Mode
R pN
C pN
L pN
u CpN
R mN
Inverter filter motor
By combining the three-level PWM inverter and
aforementioned resonant filter, the driving voltage of the
multi-mass piezoelectric motor can even be varied in a
suitable frequency range for characterizing the built actuators;
though the output filter shows an optimized filter performance
at minimized volume and weight, compared to the classical
resonant inverters.
Voltage and current cascade controls were designed to act
as inner control loops of the whole piezoelectric brake actuator
control system. A hysteresis current controller [13] [14] is
employed in the inner control loop to improve the dynamic
behaviour.
Summarily, the proposed single-phase three-level LLCC
inverter is studied in this contribution, and the employed
cascaded digital voltage and hysteresis current controller are
discussed in detail. Simulations as well as experimental results
are presented.
Fig. 2 Operating Principle of a Multi-Masse Ultrasonic Motor
excited by normal piezo stacks at the same frequency as the
tangential mode, but with appropriate phase shift. Thus,
elliptical movements of the metallic blocks will result that
generate thrust by temporary clamping of the disks [1] [5].
The operating peak voltage of the fundamental for both
modes is 270V, and the maximum power consumption is
1.5kW for tangential mode and 60W for normal mode.
II. PRINCIPLE OF MOTOR OPERATION AND DRIVING SCHEME
The multi-mass ultrasonic motor (MM-USM) consists of
two pairs of stator rings and two rotor discs connected to the
shaft. One rotor disc is sandwiched between a pair of stator
rings, as shown in Fig. 2. Each stator ring houses eight
Fig. 3 Frequency response u Cp /u filter

Fig. 4 Cascaded voltage and hysteresis current control scheme
III. THREE-LEVEL INVERTER WITH LLCC-FILTER
The topology shown in Fig. 1 consists of a hybrid threelevel
PWM inverter followed by a LLCC-type filter with an
isolating transformer and the respective capacitances and
inductances of the wiring cables being integrated as part of the
filter. The left leg of the PWM inverter is composed of four
MOSFETs (S1–S4) used to generate high frequency PWM
output voltage. S5 and S6, forming the right leg of the PWM
inverter, are employed to alternate the polarity of the exciting
waveform at the fundamental frequency.
The frequency response of LLCC-filter is shown in Fig. 3.
By its inspection the advantages of LLCC-filters, such as
robustness to parameter variation e.g. of the piezo capacitance
and simple controllability compared with LC-filters become
obvious. The second resonant frequency of the filter can be set
at a higher frequency compared with the inverter with resonant
modulation, since the important 3rd and 5th harmonics are
largely reduced by the PWM strategy so there is no need to
suppress these frequencies by the filter. As a result, smaller
and lighter filter components can be used, and the dynamic
response is improved.
Moreover, the bode diagrams of Fig. 3 show that the
operating frequency of designed three-level PWM controlled
LLCC-type filter can be varied within a range of 20-60kHz
without any need to adopt the filter components, compared to
narrow band 30-40kHz with the design of the resonant
controlled inverter. This advantageous property can be utilized,
if an actuator is to be characterized or driven versus a large
bandwidth.
IV. CONTROL SCHEME
The power supply control loop acts as inner control loop of
the whole piezoelectric brake actuator control system.
Therefore the task of the inner loop is to control the voltage
amplitude and operating frequency of the tangential and
normal modes of the motor along with the phase angle
between these two modes.
A cascade voltage and current control scheme was designed
to satisfy the brake system requirements and provide the
flexibility for commissioning. As shown in Fig. 4, the
reference variables are voltage u Cp * (u Cp,s *, u Cp,c *), frequency
f p * and phase angle Dj, the feedback signals are current i filter
voltage u Cp and voltage u Pi of the piezoelectric element.
By means of a demodulation, the feedback signals are
decomposed into sine and cosine components, yielding the
Fourier coefficients at fundamental frequency. In this way, the
requirements of signal processing are largely reduced, because
the amplitudes of these signals are only slowly varying
compared with the fundamental oscillation.
Fig. 5 Idealized switching network of three-level inverter

∧
∧
∧
∧
∧
*
*
*
− u

3-level inverter
FPGA
Controller borad
Equivalent load
LLCC filter
Fig. 8 Power supply prototype
V. IMPLEMENTATION
An experimental prototype of 1.5kW shown in Fig. 8 was
built to verify the operation principle of the proposed inverter.
The rated output amplitude is 270V at a frequency of 35 kHz,
the input DC-link voltage of 270V is supplied from the aircraft
power grid. The components and parameters are listed in
Tables 2 and 3.
Table 2 Component data
S1 – S6 CoolMOS SPP24N60C3
D7 – D10 SiC Schottky IDT12S60C
Piezoelectric capacitance C p 137 nF
Parallel inductor L p 151 μH
Series inductor L s 24 μH
Series capacitor C s 860 nF
The proposed current controller is implemented by means
of a field programmable gate array (FPGA) device. A very fast
ADC device is applied to sample the inverter current i filter .
Due to the fact that the target motor is still under
construction, an equivalent capacitive / resistive load was built
and used to evaluate the power supply prototype. The
experimental waveforms of the tangential mode are shown in
Fig. 9, Fig.11 and Fig.12, showing that the voltage of piezo
elements u Cp are nicely sinusoidal, only the phase between i filter
and u filter is slightly different, which implies that the power
factor of the power supply is nearly one.
Table 3 Parameters of Three-Level PWM inverter
with LLCC Filter
Apparent power
Power factor 0.95
Frequency
RMS current of i filter
Current of switch S1
RMS
average
1542 VA
35kHz
7.5 A
4.2 A
1.8 A
The measurement results show some deviations from
simulation. The reasons for this are due to the switching
frequency limitation of MOSFET drivers, the delay time of the
current measurement and signal processing.
Because of these reasons mentioned above the allowed
current tolerance band Δi * is increased during a first test.
Consequently, the switching frequency is lower than that
attained by simulation, and some low frequency harmonic
distortion is observed in the output voltage shown in Fig. 12.
A new simulation (Fig. 10) is made to compare the results.
If respective delay and processing times are accounted in the
simulation model, the congruency between simulated and
measured results improves significantly.

u filter
i filter
33.0kHz
66.0kHz
99.0kHz
Ch3: input voltage of filter u filter (50V/div), Ch2: Input current of filter (i filter , 5A/div)
Fig. 9 Experimental waveforms of tangential mode
Mathe1: Fast Fourier Transform (FFT) of piezo element voltage u Cp
Fig. 12 Experimental load voltage harmonic spectrum
[V]
[A]
t [s]
u filter and i filter : voltage and current of inverter output
Fig. 10 Simulation waveforms of tangential mode
VI. CONCLUSION
The three-level PWM inverter is more complex, but yields
less THD, smaller and lighter filter components and larger
bandwidth compared with resonant inverter.
LLCC filter exhibits the robustness in respect to parameter
variations and reduces filter components and cable volume due
to local reactive power compensation.
An FPGA is employed as controller by reason of its
flexibility, fast and parallel calculation property.
The control scheme comprising a hysteresis current control
is analysed in detail. The operation of the power supply
system is verified by simulation and measurement, but the
physical performance is restricted by the delay of current
measurement.
Until now the current measurement and current error
calculation are implemented fully digital. For the next step, a
fast hybrid version is being developed by using of analog
current error comparison in order to improve the dynamic and
reduce THD.
u Cp
i filter
Ch1: voltage of piezo element u Cp (50V/div), Ch2: Input current of filter (i filter , 5A/div)
Fig. 11 Experimental waveforms of tangential mode
ACKNOWLEDGEMENT
Thanks belong to the Europe Community for funding the
PIBRAC project under AST4-CT-2005-516111 as well as our
project partners
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