Microcontrollers for the drive train in hybrid and electric vehicles
- Introduction
The electric
drive train is superior in many respects to the conventional drive train with
combustion engines. The electric motor has higher efficiency, as well as better
torque and performance. An electric drive train provides a much simpler
mechanical design while eliminating undesirable noise and pollution emissions.
Although today’s electrical energy storage systems limit the vehicle range
employing intelligent battery management and the hybrid concept (i.e., the
combination of electric motor and combustion engine), helps to compensate for
this. In addition, certain system faults can be dangerous to passengers and
damage the electrical components of the vehicle, so automotive safety
requirements, under the ISO 26262 standard, must be taken into account. This
article describes the types of e-machines used, the electronic controller for e-motor
control, and the resulting specific demands on the microcontroller. With the
Hercules™ TMS570LS safety microcontroller series, Texas Instruments offers
controllers that have the characteristics needed for use in the electric motor
controller.
Depending on the
type of vehicle, it can be powered by a single electric motor, one electric
motor per axle or one per wheel, as in wheel-hub motors. All of these systems
have been realized in electric vehicles or prototypes today. When braking in
recuperation mode, the kinetic energy is converted into electrical energy and
stored in the battery (regenerative braking). During this mode the electric
motors of the drive are used as generators.
- Power inverter and electric control unit
The
high-voltage battery in mild hybrids has a range of about 40–150V; in full
hybrids the voltage range is several hundred volts. The pulse inverters used
(DC/AC inverters) typically have a B6 bridge configuration with MOSFETs as
electronic circuit breakers for voltage ranges up to approximately 120V. At
higher voltages, insulated gate bipolar transistors (IGBTs) with the lowest
possible on-state resistance and low-switching losses are used. The motor
controller consists mainly of the digital microcontroller, components for
regulating and monitoring the motor and power electronics and modules for
processing sensor signals, communication and power supply
- Measuring the phase currents
To
regulate the torque of the motor, the microcontroller requires instantaneous
information on the phase currents of the motor in every control cycle. Phase
currents of several hundred amps can occur at high torques. For this reason, current
transformers with galvanic isolation are used between the primary (heavy
current circuit) and secondary circuit (electronic circuit). These converters
are based on the Hall Effect and typically deliver an output voltage on the
secondary side that is proportionate to the current to be measured. The
advantage of Hall Effect current converters is that they can be placed outside
the signal cable therefore they do not interfere with the signal
(contact-free). Using an alternative of serial measuring shunt resistors,
resistive losses and overheating would occur which is problematic for measuring
high currents, but these Hall Effect current converters do not exhibit these
effects and even resist very high currents in the primary cable
- The microcontroller
With
the Hercules™ TMS570LS safety microcontroller series, Texas Instruments offers
microcontrollers that are used today in the automotive sector in complex and
safety-critical systems. These microcontrollers aid in the development of
safety-critical applications since they were specifically designed to meet and
have been deemed suitable for use in safety integrity level 3 (SIL3) under the
IEC 61508 standard. Hercules TMS570LS safety microcontrollers will be
highlighted in greater detail below with respect to function and safety
characteristics for use in drive controls for electric vehicles. The
field-oriented principle for controlling rotating field e-machines is
state-of-the-art. Communication networks, online safety and diagnostic
functions, standardized software architectures (i.e., AUTOSAR), and the
field-oriented control routines can lead to high demands on the
microcontroller’s processing power and memory requirements. Faster
microcontrollers generally permit a higher function density and especially for
e-motor drives, better dynamics and control efficiency because shorter control
loop times can be achieved.
- Communication
Although
the trend indicates an integration of vehicle functions in a smaller number of
electronic controllers, the communication interfaces play an important role.
The integrated Ethernet, FlexRay™ and CAN modules of the Hercules™ TMS570LS
safety microcontroller can be used for local communication or connection to the
main vehicle network. Similar to the HTU, the FlexRay module includes a
transfer unit (FTU) to read out the data without CPU interaction. In addition,
sensors and application-specific integration circuits (ASICs) can be connected
to the controller via the SPI or LIN/SCI module. Many modules have their own
RAM in which the data to be sent or received can be buffered.
- Safety requirements
The
rotor magnet field of a PMSM motor energizes the stator coils as long as the
motor is turning. This is true even in case of an error that may short circuit
the inverter, or the stator windings of the motor. In such a case, the error
must be detected quickly and countermeasures must be taken in order to prevent
damage due to the high currents, but most important is to prevent a dangerous,
undesirable braking moment of the PMSM motor. The architecture of the electric
vehicle, which comes out very differently in wheel-hub motors and mild,
parallel or serial hybrids, is a deciding factor in the vehicle’s safety
considerations. For a mild hybrid, for example, lower safety demands are placed
on the electric drive than for pure electric driving. It is obvious that
braking processes are critical to safety and thus the safety consideration must
include the use of the electric motor in the generator operation as
regenerative or recuperation braking.
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