By INGO SKURAS 
Senior Specialist, iMotion Product Marketing 
Infineon Technologies
www.infineon.com

 

 

Designers of motion control applications are
under pressure to optimize performance and efficiency while keeping component
count and space to an absolute minimum. The ideal situation is sensorless control
of the permanent magnet synchronous motors (PMSMs) used in these applications,
but such control is complex and has traditionally required extensive design and
coding skills to develop the appropriate algorithms. 

 

In recent years digital IC technologies have
been introduced that reduce the coding burden on the designer. With the latest evolution
of advanced semiconductor controllers and support tools it’s now possible to
implement sophisticated sensorless control schemes without the need to program
any code at all.

 

In general, PMSMs offer several advantages
over other motor types. The lack of a commutator makes the PMSM more reliable
than a DC Motor. And by generating the rotor magnetic flux with permanent
magnets, the PMSM is more efficient than an AC induction motor that requires
both magnetization and stator current. As a result, PMSMs can be found in the
home, in industry, in automotive and in aerospace applications. Typical
household applications would include white goods such as refrigerators,
dishwashers or washing machines. In industry, the PMSM can be found in a variety
of applications including robotics, fans, pumps, hoists, power tools, textile
machines and hand tools. PMSMs can also be found in electric and hybrid cars, and
the combination of reliability and efficiency makes them well-suited to the
latest genre of vehicle – drones.

 

Benefits of
sensorless field-oriented control (FOC)

 

FOC is a mathematical vector control
technique for controlling both AC and brushless DC motors. A large advantage
when compared to alternative open loop control is that FOC allows measurement
and control of torque. FOC deals with flux and torque vectors as two loops
running in parallel. Thus when used in an AC induction motor, especially at lower speeds, FOC
can improve efficiency as the decoupling of the magnetizing flux and torque
producing current allows for more precise control of both elements.

 

FOC is particularly useful for PMSM systems as it provides
an efficient way to control a synchronous motor in a variable speed drive or
other applications that have very dynamic loads. It also is possible to
eliminate the sensor (typically an encoder) and provide closed loop control without
adversely increasing system cost. The removal of the sensor also eliminates the
associated wiring, weight and energy usage resulting in a more efficient, compact
and reliable solution.

 

Compared to AC
induction, engineers start with greater efficiency in a permanent magnet
system, such as a brushless DC motor because no current is needed to generate
rotor magnetic flux. The precise linear control provided by FOC better supports
variable speed operation and reduced torque ripple translates to quieter
operation of fans due to smoother motor performance.

 

Challenges
of implementing FOC control schemes

 

With all of the associated equations, FOC is
more complex to understand than commutation – at least initially. However,
peeling back the mystery reveals a simple control loop and four principle steps
to implementing an FOC-based motor control system; i) the current flowing in
the motor is measured ii) this is compared to the control input (desired
current) iii) the difference is amplified and used as a control voltage signal
iv) the control signal is applied to the motor via a modulation process.

 

The major challenge with implementing FOC is
managing the trade-offs between required system performance and cost. A simple
shunt in each phase of the motor or a single shunt on the DC link path may
suffice for a basic motor control drive.

 

The overall system precision is a function of
the resolution of the system ADC (10 bits is typical although 12 bits can be
used in high-precision applications) and also the speed of the control loop
(10kHz works for many applications and 20kHz is commonly used for more
precision). However, higher bit count and higher operating frequencies require
greater processing power, thus entailing more cost.

 

Platform approach
simplifies integration

 

With the advent of integrated design
platforms, engineers now have an easy path to implementation of a sensorless
FOC system for any variable-speed three-phase motor application. All necessary
control algorithms can be embedded in the control IC. A configuration utility
enables users to quickly get the motor running and move to application testing
and hardware design more rapidly than with traditional design approaches.

 

An example of this approach is the IRMCK099 (Fig. 1), a low-cost, high-performance
OTP memory based motion control IC designed primarily for sensorless motor
control applications. Unlike earlier motor control ICs that required some
programming of the MCU element, this IC features a Motor Control Engine (MCE)
with control algorithms (based on standard library blocks) included in firmware
along with a hardware accelerator. This MCE implements sensorless FOC in both
interior and surface PMSMs using single or leg shunt current feedback through a
combination of hardware and firmware elements.

 

fapo_Infineon01_IRMCK099_MotorControl_dec2016

 

Fig. 1:
Block diagram – IRMCK099 motion control IC

 

 

The control IC incorporates all major system
elements in a 5mm x 5mm, 32pin QFN package that operates from a single 3.3V
supply. A built-in ADC offers 12bit resolution and a rapid 2μS conversion time,
making it ideal for precision applications. Alongside the advanced ADC is a 100
MHz internal oscillator that removes the need for an external clock. A Sigma
Delta DAC provides a two-channel analog output and all analog inputs are
factory calibrated. Along with 16-kB of on-board OTP memory with a CRC memory
check, the device features advanced communication and interface subsystems including
a 57.6kBps UART, 8 digital GPIO ports, 6 PWM outputs and an I2C
interface. Four register-selectable control inputs enhance design flexibility
by supporting motor speed and direction control via UART, analog voltage,
frequency or duty cycle. This ensures simplified interfaces within a wide
variety of applications.

 

The single shunt reconstruction feature
allows the accurate current measurement needed for FOC control to be achieved
with a single external shunt, thus minimizing external analog and digital
circuitry. Phase shift PWM eliminates the minimum pulse limitation, improving
motor start at low speeds and reducing acoustic noise in operation.

 

Other built in safety and protection features
include rotor lock protection and catch spin to detect any rotation of the
motor before a control input is applied.

 

“No Code” implementation

 

Compared to MCU or DSP-based FOC designs, the
dedicated IC approach makes it possible to realize a motion control design
without programming.

 

Parameter calculations can be performed using
a GUI-based drive configuration and design tool (Fig. 2). It starts with a simple form-based dialog box that captures
all motor parameters and application information such as speed and acceleration
in an easy-to-understand, engineer-friendly format – along with text and
pictures where needed. Once complete, the parameters are automatically exported
to a design tool. This tool facilitates drive testing and parameter tuning. The
engineer can start and stop the motor as well as achieving best performance by
optimizing drive parameters under load allowing meaningful editing of data in
individual control registers. A powerful parametric trace tool allows the user
to trace and plot internal control variables, thus rapidly debugging and
improving the drive performance.

 

fapo_Infineon02_IRMCK099_MotorControl_dec2016

 

 

Fig. 2: iMOTION provides a simplified design process
for Field Oriented Control of motors

 

 

fapo_Infineon03_IRMCK099_MotorControl_dec2016

 

Fig. 3: The
iMOTION reference design board allows easy migration from design to production

 

 

Rapid development of motor control systems is
further supported by a range of development and support tools, including
reference design boards (Fig. 3).
The RDB on the left is a motor control system incorporating the control IC and an
intelligent power module (IPM) along with all necessary components and the RDB
on the right features a smart µIPM.