Design of a Multifunctional Data Acquisition and Telemetry System for Electric Vehicles - Application in a Solar Car

Transcription

1 Design of a Multifunctional Data Acquisition and Telemetry System for Electric Vehicles - Application in a Solar Car I. P. Tsoumas, E.D. Mitronikas, A.N. Safacas Abstract To fulfil the requirements of an experimental prototype electric vehicle, a system for data acquisition, telemetry and storage of all the important mechanical and electrical parameters should be integrated into the standard equipment. In the present paper, the structure and the implementation of a data acquisition and telemetry system which was designed to be incorporated in a prototype solar vehicle named Hermes is presented in detail with all its technical parameters. Hermes was constructed by the University of Patras Solar Car Team and has participated in the international Solar Car Race Phaethon Furthermore, practical results from the in situ operation of the data acquisition and telemetry system are presented and discussed. Keywords: data acquisition, communication, EV (electric vehicle), solar energy 1. Introduction For driving a test and development electric vehicle, especially in a demanding race, the knowledge of all the basic parameters of the vehicle operating conditions is required at every instant, in order to maintain high efficiency and reliability. It is also necessary to receive warnings about malfunctions that can imperil the safety of the driver and the vehicle. To fulfil the requirements of an experimental prototype vehicle, a system for data acquisition and storage of the important mechanical and electrical parameters during operation must be integrated into the standard equipment. This will allow the research team to study further the operational behavior of the vehicle and to extract useful information for the treatment of possible problems regarding the vehicle operational behavior as well as for further improvement of the vehicle design. It is very important that all the significant operation data are continuously transmitted via a wireless communication system to a host computer in the support vehicle during the race, so that all the critical operating parameters are under the supervision of the vehicle support team. By means of such a telemetry system the support team will be capable to give the appropriate guidance to the driver regarding the energy management during the race and about all actions that should be taken in critical situations. For the afore-mentioned reasons a data acquisition and telemetry system was designed to be incorporated in a prototype solar vehicle named Hermes. Hermes was constructed by the University of Patras Solar Car Team and has participated in the international Solar Car Race Phaethon 2004, which took place in Greece, May 2004, in the frame of the Cultural Olympiad. It should be noted that this system can be fitted in any similar application, including electric or hybrid vehicles. In both types of vehicles, energy management and interconnection with remote hosts is essential. For example, a telemetry system of bidirectional functionality, equipped with a GPRS (General Packet Radio Service) modem in order to extend its range, could give information about recharging infrastructures for electric vehicles. Even more, taking under consideration the energy reserves of the vehicle, it would give information about the recharging infrastructure that can be reached before the energy reserves are exhausted. For hybrid vehicles enhanced bidirectional features

2 can be applied in order to provide interaction with signaling systems to switch off the thermal engine in restricted areas, e.g. areas where gas emission is prohibited. 2. Basic considerations and outline of the telemetry system The constructed data acquisition and telemetry system consists of two subsystems: the data acquisition and management subsystem. the data telemetry, storage and optical representation subsystem. In the data acquisition and management subsystem the measured quantities (voltages, currents, temperatures, electric power and energy, etc.) of all the vehicle components are gathered and transmitted in predetermined time intervals to the data telemetry and optical representation subsystem through the CAN Bus. The basic advantages of the CAN Bus environment are the immunity to electromagnetic interference (mainly because of differential signals), the automatic error detection mechanisms, and the high bit rate which can reach up to 1MHz. The core of the subsystem is an industrial microcontroller equipped with the necessary flash program memory. The basic components of the data telemetry and optical representation subsystem are an industrial microcontroller, RF transceivers, a Liquid Crystal Display and a specially designed graphical user interface (GUI) in a host computer. The microcontroller is programmed to obtain the operation data from the data acquisition and management unit and transform them in the proper format for optical representation on the display. Furthermore, the data are transmitted to the support vehicle where they are received by a host computer, stored, and shown in a specially designed GUI. The data are categorized into three groups according to their significance for the vehicle driver: 1.data of which the driver must know the value, e.g. the vehicle speed. 2.data of which the driver doesn t concern about their value, but he must be aware if their limits are exceeded, e.g. the motor controller temperature. When the limits of this kind of parameters are exceeded, the system warns the driver with acoustical and optical signal. 3.data which do not concern the driver, e.g. the MPPTs voltage. These data are not shown the driver display unit and they are only transmitted to the host computer of the support team. Another important component of the telemetry system is the data storage, so that the team can further analyze them with more sophisticated software, summarize much of what happened during the day, e.g. energy balance, average speed etc., make comparisons against previous race days and correlate those differences to specific events. 3. Structure and technical characteristics - Instrumentation The presented data acquisition and telemetry system is based on the PIC 18F258F, a small yet powerful RISC architecture microprocessor. The particular model has been chosen, basically because of its useful peripherals, which include a fully configurable UART and a CAN unit. Another advantage of this model is that an automotive version with extended operating temperature range is available in the market. The chip also provides an A/D unit with five multiplexed inputs and digital I/O ports. The main goal designing the data acquisition system is to develop a flexible by means of hardware datalogger, which can be easily interconnected with peripherals and measuring devices using standard communication protocols. This way, the system becomes upgradeable and its functionality can be easily extended. For this purpose, the architecture which is shown in Fig.1 is implemented. As shown in Fig. 1, the dedicated devices connected to each bus can be divided into the following categories:

3 Measuring units: The main purpose of these units is to collect the data and transfer useful information to the CAN bus. Data can be inserted as a digital signal (digital input) or an analog signal ranging from 0 to 5 volts. There is also a serial interface for connecting RS232 compatible measuring devices. Fig. 1. Architecture of the proposed interconnection. Display units: A display unit using LCD has been designed for displaying data transferred through the CAN bus. This unit receives specific information collected from the measuring units. Transceiver units: These units transmit and receive information though a RF transceiver, which is operating in half-duplex mode. In our case, bidirectional communication is an advantage, as handshaking signal can be exchanged and commands can be sent to the data logger. Interface units: This unit collects all information from the CAN bus and sends it though an RS232 interface to the host computer. Except from the above, standard, non-dedicated devices can be connected to the bus and transmit data. An example is the build-in measuring unit of the MPPTs. To take advantage of the microprocessor features resulting to a flexible data acquisition module, special care has been taken in developing a simple and effective communication protocol. This protocol should take advantage of the CAN architecture and should also be capable for transferring data between different points within the vehicle and between the vehicle and the host computer. According the developed protocol, data transferred through the bus is organized into packets. Each packet starts with a start byte and a byte containing the length of the packet (up to 255). Packets contain information about the sender and the recipient of the data and also about the type of data (i.e. measured data, or command to the data logger). The proposed addressing divides the whole

4 net into domains, each of which has its unique domain address. Therefore the source and destination addresses have two parts: domain and address. This is in short the header of the packet, which is followed by data. For each measured quantity four bytes are used, containing the quantity ID, the value and the measuring unit. In table 1, the structure of each packet is shown Start byte Packet length Destination domain Cmd Destination address Decimal Point position Source domain Source address Data0: ID high byte Data0: ID low byte Data0: value high byte Data0: value low byte Cmd Data0: measuring unit ID... CRC end of packet Table 1. Packet structure of the proposed communication protocol. It should be noted here that with this protocol, packets of up to 255 bytes length can be transferred. As these packets are traveling through the CAN bus, they break into smaller packets and recombined after transmission at the destination device.

5 Fig. 2. Structure of the measurement system used in the solar vehicle. Using the proposed structure, a measuring system has been designed for use in a solar vehicle. The structure of the specific system is shown in Fig. 2. In this application, the following modules are used: A measuring unit, which collects information about the motor armature voltage (permanent magnet DC motor), the motor current, the battery temperature and the motor temperature through A/D inputs. Battery cells voltages and temperatures are measured at the battery safety unit, provided by the battery manufacturer and transferred to the data logger measuring unit via the RS232 interface. A display unit at the cockpit. A transceiver unit, located at the solar car. The MPPT units transfer their voltage and current values directly to the CAN bus. A second transceiver unit, located at the support vehicle, is transferring the data in order to monitor the vehicle condition, log data and improve energy policy. A host interface (figure 3) for collecting information has been also developed. The operating quantities (voltage, current, power, energy, temperature, distance) are on-line represented in a graphical user Fig. 3. Graphical user interface for data acquisition and representation. interface that has been developed in Visual Basic. The serial port of the host computer is connected to the RF transceiver through RS-232 interface and the program reads the data from the serial port with the help of a Visual Basic Active-X component. This way, the support team can analyze the system

6 performance and diagnose any fault conditions during the race. The measured data are simultaneously stored in ASCII-files in the hard disk, so that a post collection analysis is possible in Matlab or other sophisticated software. 4. Practical experience Testbench and in situ telemetry system operation The constructed system was tested at the testbench before its incorporation in the solar vehicle, in order to ensure proper operation of all the subsystems. At first, the microprocessor unit (figure 4) and especially the CAN-Bus and RS-232 interfaces were tested with the help of small test programs written in assembly language. After that, the unit was interconnected with the measuring sensors and the assembly code was completed and optimized. The final tests concerned proper operation of the RF transceivers (figure 5) and the graphical user interface for the data representation and storage to a host computer (figure 6). The testbench was followed by the incorporation of the modules into the solar car Hermes (figures 7 and 8) and the in situ tests. Fig. 4. Microcontroller box. Fig. 5. Transceiver box.

7 Fig. 6. Testing of the transceiver unit and the data acquisition GUI. The testing was made within the University Campus of Patras. In order to reduce the noise in the measured signals, the data acquisition modules were supplied by a separate 12V battery and appropriate analog filters have been added to the analog inputs of the microcontroller. In figure 9 some on-line measurements of the motor s voltage and current are shown. Fig. 7. Solar car Hermes during the race. Fig. 8. Solar car Hermes at the finish line.

8 Fig. 9. On-line measurements of the motor s voltage and current. According to the specifications, the RF transceivers should at least have a range of 500 m, which is an average distance between the support vehicle and the solar car. The transmission was tested in a distance of 1000 m and operated without problems. 5. Conclusions Future development The presented data acquisition and telemetry system has been used for the measurement of the important operating quantities of a solar vehicle and for the transmission of the data through RF transceivers to a host computer within an area of 1 km. The practical experience confirmed its satisfactory operation. It constitutes a substantial tool for the improvement of the energy policy and the optimization of the vehicle s performance. An important feature is the simple and effective communication protocol which was developed, so that the system can easily fit into other similar data acquisition and telemetry applications, such as wind parks telemetry applications. It is the intention of the authors that the telemetry system will be expanded in the future into an intelligent diagnostic and maintenance system with additional sensors (e.g. mechanical accelerometers) and enhanced capabilities, like expert systems for early problem diagnosis. It will also have bidirectional operation, interacting dynamically with the driver and the car. Bidirectional telemetry will be used for sending information to the driver, or transmitting control sequences to the car during operation.

9 References [1]. T. Gage, " Living with an EV: 30,000 miles in one year", EVS 13 Symposium Proceedings, Osaka, Japan, October 13-16, [2]. J. Folchert, D. Naunin, "Modular CAN-Bus-System for Small Vehicles", EVS 18 Symposium Proceedings, Berlin, Germany, October 20-24, [3]. N. Jinrui, W. Jianqun, S. Fengchun, F. Liding, "The Implementation Method for CAN Communication in Electric Vehicle Control System", EVS 19 Symposium Proceedings, Bexco, Korea, October 19-23, 2002, pp [4]. D. Kang, G. Goodarzi, "Diagnostics and Maintenance Architecture for Future Transportation", EVS 19 Symposium Proceedings, Bexco, Korea, October 19-23, 2002, pp [5]. Z. Shimizu, Y. Komatsu, M. Torii, M. Takamuro, "Solar Car Cruising strategy and its Supporting System", JSAE Review (19), 1998, pp [6]. K. Müller-Glaser, G. Frick, E. Sax, M. Kühl, "Multiparadigm Modelling in Embedded Systems Design", IEEE Transactions on Control Systems Technology, Volume 12, No 2, pp , March Authors Ioannis P. Tsoumas, PhD Student University of Patras Department of Electrical and Computer Engineering Electromechanical Energy Conversion Laboratory Rion-Patras, GREECE tel: , fax: ytsoumas@ee.upatras.gr Yannis Tsoumas was born in Athens, Greece, in November In October 1994 he was accepted after national entrance examinations at the Electrical and Computer Engineering Department of Patras University (Greece). In July 2000 he received his diploma in Electrical and Computer Engineering. Currently he is PhD student at the Electromechanical Energy Conversion Laboratory, University of Patras, in the field of Electrical Motion Systems. Domains of his scientific interest are Electrical Motion Systems, Power Electronics - Industrial Electronics, Renewable Energy Sources, Electrical Machines Monitoring and Diagnosis. Mr. Tsoumas is a member of the Technical Chamber of Greece, the Hellenic Institute of Electric Vehicles (HELIEV) and the Association of German Electrical Engineers (VDE). Epaminondas D. Mitronikas, Lecturer University of Patras Electrical and Computer engineering Department Laboratory of Electromechanical energy Conversion Rion-Patras, GREECE Tel , Fax e.mitronikas@ee.upatras.gr Epaminondas D. Mitronikas was born in Agrinio, Greece, in March He received the Dipl.-Eng. degree in electrical and computer engineering from the Department of Electrical and Computer Engineering, University of Patras, Rio-Patras, Greece, in In January 2002 he received the Ph.D. degree in Electrical Engineering and from December 2003 he is a Lecturer at the Department of Electrical and Computer Engineering, University of Patras. His research interests include power electronics, electrical machines, simulation and control of electric motor drive systems for industrial and automotive applications. Mr. Mitronikas is member of the Technical Chamber of Greece and the Hellenic Institute of Electric Vehicles (HELIEV).

10 Athanassios N. Safacas, Professor University of Patras Electrical and Computer engineering Department Laboratory of Electromechanical energy Conversion Rion-Patras, GREECE Tel , Fax Athanasios N. Safacas was born in Amphilohia, Greece, in January He received the Dipl.-Ing. and the Dr.- Ing. degrees in electrical engineering from Karlsruhe University, Karlsruhe, Germany, in 1967 and 1970, respectively. During 1971, he worked with Siemens, Karlsruhe. Since 1975 he has been a Professor with the Department of Electrical and Computer Engineering, University of Patras, Rio-Patras, Greece, and Director of the Electromechanical Energy Conversion Laboratory. His teaching and research activities are in electrical machines, power electronics and electric motor drive systems. Prof. Safacas is member of the IEEE, VDE, CIGRE, EPE Association, AVERE, IASTED, and the Technical Chamber of Greece. Acknowledgement This work has been partially supported by the project Design, Study and Construction of a Prototype Solar Car of High Performance to Take Part in the Solar Race PHAETHON 2004 which was established in the University of Patras Research Committee from contributions from private and company donations.