Project e-smart: an e-mobility research platform for students made by students – part 2
Part 1 of this two-part article described the project goal and the overall characteristics of the car to be designed. Part 2 focuses on vehicle control systems as well as on the design and simulation methodology. In addition, it provides a project outlook
3.2 Vehicle Control System
The vehicle control system obtains information from various installed sensors. So the vehicle’s dynamics in terms of acceleration, velocity, driven distance in longitudinal and lateral direction as well as yaw rate and GPS position is measured. Sensors for the accelerator and brake pedal as well as steering angle and steering angle rate indicate the driver’s request.
The vehicle battery, power train and on-board electrical system are also equipped with many sensors to assess voltages and currents used for monitoring the systems operation and to calculate power flows for energy consumption analysis. Mechanical variables like wheels speeds, motor speed or motor torque are obtained via can bus from the related control units.
Vehicle Control System hardware
The E-Smart control system design is a hierarchic layout. A central rapid control prototyping unit takes responsibility for controlling the whole vehicle operation in all modes. The main control functions as power train control, charging control as well as monitoring and diagnosis are implemented on the central control unit. Using different type of digital, analogue, can and serial interfaces it can operate all devices. For this reason it is connected via two introduced main can buses to all relevant other control units and to diverse gateways which enable data transfer to further can buses and so further control units. The two main can buses are divided to an energy can and a drive can.
The energy can connects the central control unit with a further rapid control prototyping unit used for battery cell monitoring and as a gateway to the on-board charger control unit, the commercial battery management control unit, several converter modules measuring energetic variables and a Bluetooth gateway which is used to transmit can messages with the charging station. The drive can is mainly used to control the power train, so it connects the inverter control unit, the optical longitudinal dynamics measurement unit, several other sensors and the former Smart can bus for the reason of a rest bus simulation of the removed diesel engine control unit and receiving information from the Smart control units over a gateway.
Vehicle Control System schematic (top) and HMI concept (bottom)
In addition to the central control unit an embedded pc was installed in the vehicle. Booth devices are connected via Ethernet and can communicate with each other. This is especially used to assess control and measured variables from the central control unit and to store them on the hard disk drive of the embedded PC. By means of an integrated HMI connected to the embedded pc it is also possible to monitor control and measured variables in real time and to change parameter variables in the central control unit. Using the HMI the vehicle driver has full access to all important variables (e.g. the installed regenerative breaking rate) and can change parameters and modes comfortably.
The battery pack for automotive applications implies more design considerations; such as imbalance between different cells in the same battery pack. Therefore, additional active balancing system is designed and optimized to improve the battery pack capacity and the performance. The advantages of the active balancing systems are prevention of single cells overcharge, shuttling the excessive charge to less charged cells and increase the battery pack efficiency, capacity and life. However, these advantages are marred by the additional components, cost and complex control. Active balancing using shuttling capacitors system is intended to be used to balance the cells during charging, discharging and ideal states. Mathematical and simulation models were built to formulate of the limitations in this balancing system; such as, long balancing time and associated losses. Numerical models helps to find and apply the optimal configuration and shuttling sequence control to the balancing system by deriving the relationship between the cells capacity, the shuttling frequency, the shuttling capacitor size and the shuttling sequence. Real experimental prototypes were developed to verify the simulation results for the proposed active cell balancing method under different design and operating conditions.
Testing the balancing circuit
The electric vehicle (EV) and its infrastructures are one of the strongest candidates to employ PV systems; where the major barrier for the applicability of EV is the scarce capacity of the conventional electrical energy storage systems. The PV power source can be used to charge the main battery of the EV or to feed the electronic equipment on board. When a PV cell is subjected to sunlight, a part of the energy of the incident photons is converted to electricity and the leftover energy is converted to waste heat energy; which lowers the PV conversion efficiency.
The conversion efficiency of a PV module used in an EV can be improved by hybridizing it with thermoelectric generator (TEG) in order to have a PV-TEG Hybrid Power Source (HPS). The hybridization limitations and constraints are defined to have the best hybridized configuration.
The proposed PV-HPS system has many benefits for the EV and the environment. The power capacity of the solar power augmented EV can be higher than that of the normal EV. The energy storage system terminal voltage can be kept relatively more constant and balanced, where this contributed to a very stable and low temperature operation of the battery, which benefited the life of the battery. Also, using another mean of the power source than a completely independent power source from the power grid to charge the battery will reduce the demand from the grid and increase the merge of the EV in automotive market. The reduction in the EVs dependency on the power coming from the grid will lead to less harmful emissions and thus more environmental friendly vehicles.
3.3 Vehicle Measurements, Modeling and Simulation
Several measurements could be carried out with the E-Smart in different stages of the conversion process. So in advance of vehicle conversion the power train was installed on a power train test bench and the mechanical and electrical variables were gauged under several static and dynamic loading conditions. Efficiency maps, maximum load characteristics as well as dynamic response of the of inverter and AC induction motor were recorded and evaluated.
Electric drive at the test bench (top) and measured motor efficiency map (bottom)
After conversion, the vehicle was tested on the Bosch test track at Boxberg. Driving several maneuvers under reproducible conditions on special track sections the interaction of electrical variables and such describing the vehicle’s dynamics (e.g. acceleration, dynamic wheel diameter, maximum speed, etc.) was measured and evaluated. These data was especially helpful to determine driving resistances and to gather the driving dynamics limits of the electric propelled vehicle.
Beyond this, the E-Smart participated among four other electric vehicles in a huge FKFS test person study in which it was driven about 50 times by a statistic verified test person collective on a 60 km driving cycle on public roads in Stuttgart area. Aim of this study was to determine statistic relevant variables of the energy consumption in electric vehicles and influencing variables. With the data selected in this real-life driving cycles the statistic energy consumption of the E-Smart could be evaluated as well as influences like driving style, road type or environmental conditions analyzed.
With all the obtained measurement data the students were able to build component models and to accurately parameterize them. These models constitute the basis for developing an overall vehicle model and designing many software control function using common software and model in the loop techniques. In this way, software control functions for driving and regenerative breaking strategy, cruise control and charging control were designed and tested before they were successfully implemented in the vehicle’s central control unit and applied in the vehicle.
Overall vehicle model
4 Summary and Outlook
A commercial compact car vehicle with combustion engine was converted to an electric vehicle by students of the University of Stuttgart at and the vehicle’s operational capability was proved by several tests on test benches, test tracks and on public roads. It was first time presented to public at the 16th Euroforum congress for electronic vehicle systems in Munich 2012.
Regarding the open and easily extendable E/E-architecture and control software the converted electric vehicle offers a platform for manifold issues concerning E-Mobility research. In this way, it is also planned to be involved as platform to implement and test new technologies within certain public funded projects by the Ministry for Science, Research and Arts of Baden-Württemberg and the Federal Ministry for Education and Research of Germany.
The current vehicle constitutes a very convenient starting point for further developments and allows the students to implement and to investigate their ideas in a practical manner in a real existing vehicle that generates real measurement data. Currently targeted research issues directly originate from the three main research topics mentioned at the beginning of this report. So one big research area is the reliable prediction and the optimization of the vehicle’s driving range. To achieve this aims, the vehicle is upgraded with further sensors like a wide range radar system that enables for an advanced cruise control and an electronic horizon providing predictive route information that can be used to predict the remaining driving range in a more accurate way but also to implement more intelligent driving and regenerative breaking strategies in order to optimize power train’s energy consumption. Also an interface to the internet is considered to gather useful information from the web and to use it directly as input to the central control unit but also to upload important vehicle’s variables to access them from anywhere.
Besides the driving operation the charging issue plays a major role in further developments. The fast DC current charging is currently approved, there are still some challenges to master in the charging control and the battery cell monitoring regarding the handling of the high power transfer rates. Besides this, the inductive charging hardware is integrated into the vehicle, first application of this technique will follow soon.
Basis and premise of all research, developments and tests with the E-Smart is safety dealing with the electric vehicle. Much effort is spend to analyze safety-critical weak spots in the high voltage system, to derive requirements and solutions to remedy them as well as to implement the relevant hardware measures and diagnosis functions. Safety requirements in electric vehicles are an important general issue, the insights and solutions made with the E-Smart can be transferred to other electric vehicles.
About the Authors:
Dipl.-Ing. Andreas Freuer, FKFS, Stuttgart
Andreas Freuer studied Mechanical Engineering with thematic priority in control and systems theory at the University of Stuttgart and received the Dipl.-Ing. degree in 2010. Since April 2010 he works as research assistant at the department of automotive mechatronics at the Institute of Automotive Engineering and Vehicle Engines Stuttgart (FKFS). In his Ph.D. thesis he is concerned with functions and control software predicting and reducing energy consumption in electric vehicles.
M.Sc. Omar Abu Mohareb, FKFS, Stuttgart
Omar Abu Mohareb received the B.Sc. and M.Sc. degrees in Mechatronics Engineering from Al-Balqa Applied University in Jordan in 2004 and 2007; respectively. He is currently a researcher in FKFS and a Ph.D. candidate in the Institute of Internal Combustion Engines and Automotive Engineering (IVK) at University of Stuttgart. His area of research focus is in the field of power converters and power electronics for automotive applications.
Dr.-Ing. Michael Grimm, FKFS, Stuttgart
Michael Grimm received the Dipl.-Ing. and the Ph.D. degrees in Mechanical Engineering from the University of Stuttgart, Germany, in 2000 and 2007, respectively. From 2000 to 2007 he was with the Institute for Internal Combustion Engines and Automotive Engineering at the University of Stuttgart, as an Assistant in the field of a tamper-proof method which is capable for every day life for vehicle evaluation. Since 2007 he is the head of department Mechatronic / Elektronics of the Research Institute of Automotive Engineering and Vehicle Engines Stuttgart (FKFS).
Prof. Dr.-Ing. Hans-Christian Reuss, IVK/FKFS, University of Stuttgart
Hans-Christian Reuss received the Dipl.-Ing. degree in Electrical Engineering in 1984 and the Ph.D. degree in 1989 from Technical University Berlin. Hans-Christian Reuss is member of the managing board of the Research Institute of Automotive Engineering and Vehicle Engines Stuttgart (FKFS) and the Institute of Internal Combustion Engines and Automotive Engineering (IVK) at University of Stuttgart where he helds the chair for automotive mechatronics since 2004.
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