Requirements and concepts of HEV systems
In particular, PHEV and FHEV, depending on the respective system concept, have a higher complexity from the perspective of cooperative control strategies (ICE and electric drive), and in addition, they are more sensitive to the space constraints of application components due to the combination/addition of ICE and electric drive functionalities. This applies not only to mechatronic components, but also to electronics such as digital chipsets, analog and power components.
The complexity of the system described above comes from the following upper-level functions: When the vehicle is decelerating, kinetic energy is converted into electrical energy by the electric motor and stored in the battery. During acceleration, the electrical energy from the battery is used to assist the ICE, thus saving fuel consumption. The FHEV with a high-power electric motor means a high generator capacity, so more kinetic energy can be recovered (or restored) during deceleration, thus improving fuel efficiency by several tens of percent.
HEV Control: The Concept of Complexity
Figure 3: HEV classification
There are several hybrid architectures for HEVs, of which Figure 3 describes the broad categories:
The simplest is
the parallel hybrid system
. The motor is placed in parallel with the ICE. The motor/generator assists acceleration by utilizing electrical energy from the battery, and during deceleration the battery is charged by using the motor as a generator. The benefits of this system are lower cost and less control complexity.
In
the case of
a Series Hybrid system
, the kinetic energy generated by the ICE is converted into electrical energy by a generator, which is then used again by another electric motor to generate kinetic energy. This may seem to be a waste of cost and energy. However, the advantage of this approach is that it allows the ICE to be operated in the most fuel-efficient speed/torque range. This is because the ICE is fuel-inefficient at low speeds (e.g. <1500rpm), high speeds (e.g. >4000rpm), and low torque ranges.
The Series/Parallel hybrid system
is the most complex system. When the ICE is operating in a fuel-efficient speed/torque range, the ICE output is delivered directly to the wheels through the clutch and transmission. If torque assistance is needed, the electric motor can assist with acceleration, and the ICE can save fuel like a parallel hybrid system. When the vehicle speed is very slow, the clutch is released, and the system acts like a series hybrid system to avoid operating the ICE in a low fuel efficiency range.
In the case of series and series/parallel hybrid powertrain configurations, a combination of two motor/generator arrangements that are tightly and interdependently controlled are generally required.
HEV Control: Key Challenges and Solutions
From the traction motor system concept presented earlier, it is clear that the respective control and synchronization efforts tend to be complex due to the heavy communication load between the two entities and the increased diagnostic effort required to maintain the target safety level (ASIL-level), especially in the case of series/parallel hybrid systems.
An obvious solution to optimize these efforts is to integrate the two inverter control systems into one ECU, operated by a highly specialized microcontroller (MCU). By using this concept, synchronization between the two inverter control loops can be achieved within one microcontroller, resulting in high communication bandwidth and reduced latency. In addition, by selecting target devices that comply with safety level ASIL, diagnostics and functional safety concepts will become simpler and more straightforward. Another benefit of the integrated solution is of course a highly optimized bill of materials (BOM) with reduced part space requirements, which is a very beneficial effect for the entire system concept.
Solution: MCUs with integrated xEV support
A key to HEV-specific MCUs is to offload the vector math calculations of the motor control algorithm to dedicated processing IP. By using this approach, the MCU can be equipped with a smaller number of CPU cores while taking on the other software tasks mentioned above.
Enhanced Motor Control Unit (EMU3)
The embedded "Enhanced Motor Control Unit" (EMU Gen3) is a set of individual motor control accelerator modules that calculate three-phase PWM comparison values using a vector control algorithm and generate a rectangular wave pattern based on the motor current value measured by the A/D converter. In addition, the angle value of the motor is obtained by the integrated "Resolver to Digital Converter" (RDC3A) that performs the position sensor interface function. The three-phase motor timer TSG3 uses the calculation results of the EMU3 to output PWM and rectangular waves.
Figure 4: Enhanced Motor Control Unit (EMU3)
The EMU3 IP can be used to implement motor control functions in combination with its specific function blocks and user-specific software intervention. Thus, flexible control concepts combining hardware acceleration and individual user software can be implemented.
Figure 5: Flexible control of the motor in combination with user-specific software intervention
Dual Motor/Generator Control
The key solution to achieve dual motor/generator control capability is based on the previously introduced motor control IP (“EMU3”) and how the embedded position sensor interface is integrated into the microcontroller system.
The following diagram shows the actual method of controlling two motors (see the appendix for abbreviation definitions):
CPU2 and CPU3 each control one motor. By using EMU3, the processing of performance-intensive motor control algorithms (such as Park/Clark transformations used to generate PWM patterns) has been moved from the CPU to EMU3. This allows other important software tasks (such as diagnostic processing) to be performed by the CPU.
CPU1 can also be used for other functions: for example, DC/DC converter control can be implemented as an optionally integrated additional function to optimize the overall HEV system layout.
The RDC3A is an MCU-integrated (equivalent to the Tamagawa AU6805) dual resolver-to-digital converter interface, or more generally a motor position sensor interface, capable of connecting to analog resolver or inductive position sensor signals.
Figure 6: Example of a system controlling dual motor/generators
京公网安备 11010802033920号