June 2014
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M
OTOR
C
ONTROL
critical to E: Non Flight critical. DO178B sets
development and compliance standards for
software used in avionic applications.
Analog controllers are also commonly used in
space applications to minimize the cost of ra-
diation-proof components such as processors,
ASICs or FPGAs. Radiation test and character-
ization is still required. Total Dose Testing pre-
dicts the life of the electronics and Single
Event Testing predicts reaction of events such
as solar flares. Manufacturers of these devices
such as Data Device Corporation (DDC) design
to meet common radiation requirements and
perform testing to verify radiation tolerance
performance. Additionally, hybrid microcir-
cuits save space and weight.
Digital motor controllers offer several per-
formance and efficiency advantages that make
them the controller selection of choice for
many applications. The versatility of digital
controllers has advanced as the evolution of
DSP (digital signal processing) and ASIC (ap-
plication-specific integrated circuit) based
processors now enable designers to create flex-
ible products and improve time to market.
The most versatile of these designs are based
on DSP architectures which allow integration
from simpler sensorless systems to complex
multi-axis position control systems. The pro-
cessing power of the DSP, along with associated
graphical user interfaces (GUI), takes the com-
plicated math out of the user design, requiring
only basic knowledge and support to meet the
expected motor system performance goals.
The torque, speed and position loops are often
calculated for the designer based upon motor
and system parameter entry. Many controllers
offer multiple control options.
The embedded control and control logic in the
DSP can contain complex mathematical calcu-
lations and algorithms that are required to
gain the efficiencies of the field-oriented (FOC)
sinusoidal motor commutation technique. This
technique delivers power to the motor by
means of a sinusoidal (sine) waveform. The si-
nusoidal
signal
provides
maximum
voltage/speed in relation to the DC bus voltage
and reduces noise by over 30% relative to a
trapezoidal (trap) drive. The trap drive com-
mutates the motor with a trapezoidal AC signal.
The system losses in a sine drive are in the
motor, while the trapezoidal drive losses are in
the controller. Additionally, a sine wound motor
will improve motor efficiencies as well. The
torque ripple on a sinusoidal motor can be as
low as 1%, while the ripple for a trapezoidal
motor is over 13-14%. The sinusoidal system
also reduces noise, which is essential to meeting
EMI requirements. The trap drive system EMI
signature and current ripple are higher due to
the sharp edges and flatness of a trapezoidal
signal. These signals are modulated by the
PWM frequency in the motor controller.
The processing power of DSPs also enables
flexible motor control that can be utilized in a
wide range of applications, from a sensorless
motor system, such as a fan or pump, to a
complex multi-axis design, such as those that
are used in turrets and robotics. The speed
controller uses internal sensing and algorithms
that are required for speed regulation and also
sets the control loop parameters for torque.
Torque is proportional to current and speed is
proportional to voltage. The bandwidth for
the current/torque loop is generally greater
than that of the speed loop.
Torque controllers are used in applications
that require holding torque and changes in di-
rection, since these controllers maintain smooth
transitions in torque through zero speed. This
is known as a four quadrant controller. Con-
trolling current/ torque to the motor will allow
for precision speed control. Torque controllers
utilize a position sensor on the motor to deter-
mine the position of the shaft, in order to en-
ergize the appropriate winding for precision
control. This is most commonly a Hall Effect
device, but can alternatively be resolvers, en-
coders etc. A position controller utilizes an in-
terface with position sensors on the motor
and at the load. The position loop is the outer
control loop in this system. The speed and
torque loops must be tuned as well. All three
control loops must be tuned based on the
motor and system parameters.
Programmable motor control devices include
a GUI that will aid and perform these calcula-
tions based upon the motor used and system
requirements. As a system is implemented in
the lab or fielded, system parameters often
change and may require tuning to attain the
desired performance. The GUI is the perfect
tool to minimize the time impact of additional
tuning. Another benefit of tunable controllers
is that a motor can be swapped out and its re-
placement made operational with the simple
change of parameters in a short time. Multiple
motors can be used with the same controller.
This will reduce the cost of ownership which is
a key consideration for motor control systems.
DSP-based solutions also allow for interfacing
with host processor controlled systems that
communicate on serial networks such as CAN,
RS-485, RS-422 etc. Alternatively, speed and/or
torque can be set by means of an analog
voltage input when advanced features are not
required. Also, on-board or system processors
can coordinate 2-axis movement as required
in satellite base stations, radars, turrets or ro-
botic systems. Motor control suppliers such as
Data Device Corporation offer products that
incorporate all control algorithms and sensor
interfaces, as well as provide advanced protec-
tion, such as overtemperature, overcurrent,
Sensorless motor control system
Dual-axis motor control
