Part 1 described the basic concept and conversion of the Smart ForTwo into the e-smart.
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 are measured. Sensors for the accelerator and brake pedal as well as steering angle and steering angle rate indicate driver input.
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 wheel speeds, motor speed, or motor torque are obtained via CAN bus from the related control units.
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 also implemented on the central control unit. Using different types of digital, analog, can and serial interfaces it can operate all devices. For this reason the central control unit is connected via two main CAN buses to all relevant other control units and to diverse gateways. The latter enable data transfer to further CAN buses and thus 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 which handles a "simulation" of the removed diesel engine control unit and receiving information from the Smart control units.
In addition to the central control unit an embedded PC was installed in the vehicle. Both devices are connected via Ethernet and can communicate with each other. This is 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 increasing 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 is intended to be used to balance the cells during charging, discharging, and "idle" states.
Mathematical and simulation models were built to formulate the limitations in this balancing system, such as long balancing time and associated losses. Numerical models help 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.
To read the complete article, which also details photovoltaic charging, vehicle modeling and simulation, and future use of this vehicle platform, click here, courtesy of EE Times Europe Automotive.
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