Proceedings of the 2009 IEEE International Conference on Mechatronics. Malaga, Spain, April 2009.
MEMS and J2ME based Acceleration Real-time Measurement and Monitoring System for Fuel Cell City Bus Jianfeng Hua, Liangfei Xu, Xinfan Lin, Mingyin Hu, Jianqiu Li, Minggao Ouyang State Key Lab of Automobile Safety and Energy Dept. of Automotive Engineering, Tsinghua University Beijing 100084, China
[email protected] Abstract-Since Jul. 2008, Beijing started a clean public transportation plan for the 29th Olympic Games. As a part of this plan, three fuel cell buses will work in a fixed bus line for one year as green energy vehicle demonstration for public. This is the first time to demonstrate homemade fuel cell buses in Beijing. The buses really serve as an ordinary bus on the streets of Beijing. Some control strategies are not adequately mature yet during the complex transient city cycle and are still needed to be optimized with the updating experiment data collected with day-to-day operation. Due to the speciality of fuel cell engine and electrified power-train system, some particular measurement, monitoring and calibration devices are ensuring normal operation of the fuel cell bus in the demonstration. The 3-axial accelerations of the fuel cell buses are the significant parameters to evaluate dynamical performance and manipulation stability. Furthermore they also feature the characteristic of city cycle route in demonstration. In order to measure and monitor the 3-axial accelerations ofthe fuel cell bus rapidly and conveniently, a novel wireless portable online monitoring system based on Java 2nd Micro Edition (J2ME) and Micro-Electro Mechanical Systems (MEMS) technology is implemented. The monitoring terminal could be a cell phone, PDA or any other smart mobile device with Bluetooth interface. The data exchange between monitoring terminals and VCU (Vehicle Control Unit) is highly secure under the guarantee of the Bluetooth pairing and authentication mechanism.
attract many governments and companies to invest huge capitals to the research and development. As one of the first research institutions starting to develop the fuel cell bus in China, the state key lab of automobile safety and energy at Tsinghua University innovated the first fuel cell city bus in China in 2004 [1]. In July 2008, the city of Beijing started the clean transportation plan during the Olympics. 500 low emission and zero-emission vehicles, including three fuel cell city buses developed by Tsinghua University and its partner, served in the urban area and central Olympic area in Beijing. The three homemade fuel cell city buses will complete the demonstration in an ordinary bus line for a whole year. Fig. 1 shows one of the fuel cell city buses in Beijing clean transportation public demonstration 2008.
Keywords- Fuel cell city bus; Acceleration wireless measurement & nwnitoring; J2ME; Bluetooth; MEMS I.
INTRODUCTION
The increasingly conspicuous problems of energy crisis and environmental pollution are paid attention to by governments and people all over the world since they seriously affect the sustainable development of the human society. As the automotive industry is developing and the quantity of vehicles is dramatically increasing, the emission gas is discharging more and more pollutant into the environment and the traditional energy structure and its usage can not meet the demand of the human-beings any more. Confronted by the severe energy challenges, many countries are trying their best to search for alternative energy. The features of high efficiency, low emission and hydrogen-regenerability of the fuel cell
Figure I. A fuel cell city bus in Beijing public demonstration
For the first long-term demonstration of domestic fuel cell city bus, to monitor and analyze the operational data is very important to make it reliable and provides valuable data and experience for the development. A series of fast monitoring and measuring equipments based on the new technologies, have been successfully applied in the development of the fuel cell bus, which enables the rapid and accurate data monitoring and measuring of fuel cell city bus. Among various operation parameters, the each axial acceleration of the fuel cell city bus
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in operation is not only the significant indication for the manipulation stability and power performance, but also the reflection of the actual road condition of the demonstrating route, which is a key condition parameter in long-term driving optimization. This paper presents how the convenient Bluetooth on-line measurement of the three axial accelerations of the fuel cell city bus in operation is achieved by means of J2ME (Java 2nd Micro Edition) and MEMS (Micro-Electro Mechanical Systems) technologies. This system can be operated in most kinds of cell phones, PDAs or other portable mobile equipments which support JAVA virtual machine to wirelessly measure and monitor the acceleration through the Bluetooth technology. The process of measurement and data collection is very efficient, convenient and reliable and the data are directly stored in the record devices of the cell phone. The security of the data can be well guaranteed through the paring encryption mechanism of the Bluetooth technology. II.
SYSTEM CONFIGURATION
A.
Configuration of Power Train The hybrid power system structure of the fuel cell city bus is shown in Fig. 2, consisting of the fuel cell engine, DC-DC, power battery, generator controller (inverter) and three-phase asynchronous induction motor. PEM Fuel Cell
Inverter DC-AC
DC-DC
Ni-MH Power Battery
Figure 2. Power train configuration of the fuel cell city bus
The bus adopts the electrified chassis with two rated 40kW proton exchange membrane fuel cell (PEMFC) engines connected in parallel. The out-put power of the PEMFC engines, which is the factual power source, is distributed properly into the power battery and the driving motor through the modulation and management of the DC-DC according to the global optimized control strategy. When the driving power is small, the fuel cell will charge the power battery. When the power demand of the bus increases, the power battery and the fuel cell can simultaneously provide energy to drive the motor. The proper adjustment to the output power by the vehicle management system keeps the state of charge (SoC) of the power battery in balance. The distributed control system is adopted for the complex structure of the dynamic system of the fuel cell city bus, as shown in Fig. 3. Each part has independent controller. The vehicle controller is in charge of the energy management for the whole system and the coordination of each part. Each controller of the distributed control system communicates through the CAN bus in-vehicle networks to meet the
requirements of the system with large quantity of reliable data exchange in real time. veu: Vehicle Control Unit MS: Management System CANH
Figure 3. Power train distributed control system of the fuel cell city bus
B.
Bluetooth Monitoring and Measuring System Among many short-distance wireless communication technologies, Bluetooth is the outstanding one and used widely. The globally uniform working band of Bluetooth technology is 2.4GHz ISM. It adopts the fast frequency-hopping spread spectrum technology with 1600 million per second which is strongly anti-jamming. The standard effective distance of Bluetooth Class 2 is two meters and of Bluetooth Class 1 is 100 meters. The elaborate protocol of Bluetooth technology standard has a layered structure. The structure of the whole protocol is simple with the forward Golay code and the automatic retransmission mechanism to ensure the reliability of the links. Equipments which follow the Bluetooth protocol will be able to adopt wireless communication links instead of complex wires in the traditional network and realize the fast, agile, safe, cheap and low power-consumed data communication very conveniently. All kinds of applications which follow the Bluetooth technology will be surely easy to install and manipulate with high security mechanism and complete interoperability to realize the communication at any time in any place. The Bluetooth technology will promptly develop in all kinds of fields such as automobile industry, information household electrical appliances, and medical equipments and so on. The protocol Bluetooth 2.0 issued recently and its supplement EDR really open the gate of Bluetooth application in the automobile industry. With the help of the protocol, the communication rate is up to 3Mbps for the firs time, surpassing the communication standard CAN bus which has been widely applied in the automobile industry. That is to say, the bottleneck of the Bluetooth application in the in-vehicle networks of the vehicle control system and its communication technology has been solved in Protocol 2.0. It symbolized that Bluetooth technology is not only in good application of the in-car entertainment system, but also has a promising prospect of the vehicle control system. This technology had its first application in the real time acceleration monitoring and measuring of the fuel cell city bus. The structure of the Bluetooth acceleration monitoring and measuring system based on the sensor MEMS and the technology J2ME is shown in Fig. 4. The signals from the accelerometer based on the MEMS technology are measured by the vehicle controller of the fuel cell city bus. Through
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filtering, sampling and analog-to-digital conversion (ADC), three axial accelerations which are from forward, lateral and vertical orientations during the operation of the bus can be derived. The vehicle controller connects to the outside Bluetooth monitor through the Bluetooth wireless module which here is EK-l1 made by Bluegiga in Finland. This module accords for the Bluetooth 2.0 + EDR Class 1 standard. Its largest communication rate can reach 3Mbps and the longest transmitting distance is 100 meters ideally. The Bluetooth signals can be sent and received through the outside or inner antenna. The vehicle controller connects to each subnode controller of the power train through the CAN bus invehicle networks. LAN
Network Server
On-line Database
.--_ _M_o_n_it...,.ri~ T"m'"I-C,lIp"." MEMS Accelerometer
•
't
Cellular Networks
~
Vehicle Control Unit (VCU)
A.
MEMS MEASUREMENT PRINCIPLE
Measurement Principle ofMEMS Sensor
The vehicle motion state monitoring is the basis to evaluate the manipulation stability and active safety of vehicles. The conventional sensor is considerably limited in automotive application due to their disadvantages of large, heavy and high cost. The acceleration measuring system mentioned before adopts the micro-electro- mechanical system (MEMS) technology. The MEMS stems from the integrated circuit technology, which has been gradually applied into the automotive engineering field [5].The sensors based on MEMS technology is relatively small, light sensitive, rapid responsive and easy to produce. In general, MEMS has several features as below:
i Wi
GPRS
Connection
III.
Blnetooth Connection
In-Vehicle CAN Networks Figure 4. MEMS & J2ME based B1uetooth monitoring and measurement system The vehicle controller forms the Bluetooth connection as the Bluetooth slave. The monitor terminal is platform equipment with Bluetooth communication function, such as cell phone, PDA or computer which forms a Bluetooth connection as the Bluetooth master and exchange data with the vehicle controller via Bluetooth in the application program developed on the J2ME platform. The connection of Bluetooth needs pair encryption. The process is that one side provides the code and the other side gets the pair secret key by inputting the correct password. Cell phones after pairing can monitor the vehicle controller without frequent pairing, which not only ensures the security and confidentiality of the data links but also simplifies the process of entering the password. The vehicle controller connects to the in-vehicle network through the CAN bus interface equipment, so the system can monitor and measure accelerations of the running fuel cell bus from each orientation and also can monitor the whole CAN network in real time. The monitoring data can be stored into the inner Micro-SD card in the cell phone or uploaded into the on-line server through the GPRS network, and the data is shared with authorized computers to provide researcher a reference database for control strategy optimization.
(1). Miniaturization: MEMS devices normally feature small form, lightweight, low power consumption, light inertia, high resonant frequency and short response time; (2). Mass productability: thousands of micro mechanical parts or a complete MEMS device can be made from one silicon by micro processing technique, which reduces the cost noticeably; (3). MEMS sensors based on the silicon technology feature well electric performance; (4). Integration: several sensors and actuators can be integrated into a MEMS sensor with high reliability and stability. As an important branch of MEMS, micro inertia sensor develops quickly in recent years. Because of its advantages of small volume and high integration, micro inertia sensor is replacing the traditional electrical and mechanical sensor in the automotive application gradually. According to the methods of measurement, micro accelerometer can be classified into several different categories, such as capacitive type, piezoresistive type, tunnel type, resonant type and thermalelectric type. The capacitive accelerometer has been primarily applied in low-g acceleration measurement due to its high sensitivity, low noise and simple structure. [6]. ADXL330, a three-axial MEMS accelerometer with the measuring range of ±2g, has been integrated into the system. The chip size is only 4mm x 4mm x 1.5mm. Three axial accelerations of movement can be measured simultaneously. Signals generated by the MEMS sensitive unit are processed through AC amplifiers, filters and output amplifier integrated in the chip before outputting the analog voltage signals. The sensor parameters are shown in Tab. 1. TABLE I.
ADXL330 MEMS ACCELEROMETER PARAMETERS
Measurement Range Sensitivity
Min. ±3g
Typical ±3.6g
N/A
270mV/g
300mV/g
330mV/g
Output in Equilibrium
1.2V
1.5V
1.8V
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Max.
B.
Acceleration Signal Processing Circuit The electromagnetic environment of the fuel cell city bus is comparatively complicated. In case that the signals are interfered, the third-order Butterworth active low-pass filter is adopted to filter the signals after measuring by the MEMS accelerometer in order to effectively eliminate the influence caused by the noise.
Fig. 5 is the circuit schematics of the third-order Butterworth active low-pass filter. The left is the signal input and the right is the filter output. The cut-off frequency can be derived from the R5B, C7B, R7B, RllB, C3B and C9B RC-array. CIB, RIB, C4B and R3B are used to prevent the autonomous oscillation of the rail-to-rail operational amplifier. A zener diode limits the maximum input voltage to protect the analog-digital converter of the microcontroller. JIB LIB
AlNl
"""" DIB
lLC
3
U01C LTl679CS
R5B 3
VA-12 RlSB
""""
Figure 5. 3rd-order Butterworth low-pass filter
According to the sampling frequency and noise disturbance in experiments, the target cut-off frequency is fixed at 200 Hz. The normalization computation form of the 3rd-order Butterworth filter is shown in Tab. 2. Values of R5B, C7B, R7B, R11B, C3B and C9B can be derived according to fn and Qn in the table. The calculation process is as below:
Select the close standard value according to the RC array: RSB
= 36kQ ,
~B
=RllB
=
71.5kQ , C9B =5.6nF
(6)
According to the experience, the resistance should be 130
KQ and the capacitor should be 0.01 uF for the anti-oscillatory
RC values. TABLE II.
3RD-ORDERBUTTERWORTH LOW-PASS FILTER NORMALIZATION CALCULATION FORM
In 3'd-order
Qn
1;
1.0
QI
0.5
fz
1.0
Q2
1.000
C. Accelerometer Calibration To improve the measurement accuracy of the acceleration, three-axial acceleration signals need to be calibrated. Here Z axial acceleration is taken as an example. Calibration process consists of five test, including 19, 0.5g (incline at an angle of 60), 0, -0.5g (incline at an angle of -60) and -lg calibration. 1000 samples were measured in each time. The Z-axial acceleration measurement diagram with -0.5g condition is shown in Fig. 6. The range of the analog-digital conversion value is O~ 1023. Z Axial Measurement Curve (-O.5g)
(1). Take C7B=C3B=22nF according to El2 series of the capacitance standard. Since Q1 is 0.5, so it can be regarded as a first-order filter.
1)
_
HoB -
1 27iXfcXC7B
1 27i x 200Hz x 22nF
(2)
36.l9kQ
(2). According to C3B=22nF, Q2=1.000, it can be derived:
C =~= 22nF =llnF 1
2xQz
(3)
2xl
100
200
300
400
500
600
700
800
900
1000
Sampling Counter
Figure 6. Z-axial acceleration measurement diagram with -0.5g condition
Tab. 3 shows the calibration results of the Z-axial acceleration.
(4) TABLE III.
C C9B = __ 1_ = 5.5nF 2xQz
(5)
ADXL330 MEMS ACCELEROMETER Z-AXIAL CALIBRATION
Acceleration (g) 1 0.5 0 -0.5 -1
Mean 406.7790 372.8710 338.89 302.9970 270.5070
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Variance 2.2004 1.7861 1.4834 1.5265 1.2212
Voltage (V) 1.986 1.821 1.655 1.480 1.321
As shown in Tab. 4, Z axial resolution is calculated according to the calibration value. TABLE IV.
ADXL330 MEMS ACCELEROMETER Z-AXIAL RESOLUTION CALIBRATION DATA
1- cosr g = 0.015% g
0)
Being placed at an angle of 60 degree, the deviation of the system is 1 degree and the error in the result is:
development environment (IDE) Netbean or Eclipse, the J2ME development environment can be easily built up. Special development platform is also called manufacturer development tool which is the special kit developed by each cell phone manufacturer according to its cell phone types. Most cell phone manufacturers provide their special development tool to make convenience for development on their own cell phone. Corresponding simulators are also provided to perform simulation platform in the computers to improve the developing efficiency and quality. Special development platform is provided by almost all reputed companies, such as NOKIA, Motorola and SonyEricsson. Through API system functions provided by the development platform, the monitoring terminal and the realtime acceleration measurement of the fuel cell bus can be quickly built up. Fig. 7 shows the software structure of the portable monitoring terminal based on J2ME CLDC 1.01MIDP 2.0 API platforms and file system development kit, which is developed and compiled in the Eclipse environment. ---I
max{lo.5
COS61:gl,lo.5 COS59: g l}=3.02% cos60 g cos60 g
(8)
Under the condition of the 300Mv/g resolution, the deviation is around 4.5mV, which is acceptable to be ignored in the application of the fuel cell city bus. The output drifts of the sensor influenced the temperature are compensated by the software to improve the precision. IV.
Device Search
Service Search SfP Cormection
1 1 1 1 1 1 1
J2ME BLUETOOTH MONITORING AND MEASURING SYSTEM
J2ME is a part of Java2, which is a highly advanced operation environment the same as J2SE and J2EE, focusing on consumer electronic equipments like the car navigation system. This technology is the key to realize the portable monitoring terminal in the system mentioned before. The intelligent cell phone becomes the latest trend gradually as the electronic technique is developing. Since the portable working feature of cell phones, cell phones could be applied as the new monitoring terminal which is more convenient compared with computer monitoring terminal. Not only are modem cell phone and PDAs equipped with high-quality screen and input device, mass storage devices and Bluetooth RF, but also provide an open Java application platform for J2ME development. J2ME is one of the most popular cell phone platform for humanmachine interface and programming development, which is quite compatible with the cell phone or PDA' platform monitoring terminal. Normally J2ME platform is classified into that of general development and special development. All J2ME applications in cell phones can be developed and operated on the general development platform. It mainly used to develop applications which are not involved with the screen size and applications without special development tools. General development tools are mainly J2ME Wireless Toolkit produced by Sun Corporation. With famous integrated
Figure 7. Acceleration measurement & monitoring terminal for fuel cell bus
This terminal software consists of the terminal display and the data exchange. The monitoring terminal displays the acceleration curve dynamically on the cell phone screen based on the Canvas development of J2ME graphic display. The data exchange consists of data management, documents connection, GPRS connection and Bluetooth connection. The data management is mainly setting all kinds of parameters of the terminal software. Users can get a perfect monitoring effect by choosing appropriated parameters; the documents connection is to manage data recorded in the cell phone storage equipment. Users can easily access to SDIMMC card inside the cell phone and store the monitoring data into the card through the documents connection development kit provided by MIDP; The real-time measuring data can be uploaded to the network database via the GPRS connection for the further analysis; the Bluetooth connection is the most important part of data communication which is the interface of the monitoring terminal and the vehicle controller of the fuel cell bus. By the Bluetooth protocol stack through the standard Bluetooth communication API JSR82, the monitoring terminal can
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connect with the vehicle controller of the fuel cell bus through the SPP (Serial Port Profile) interface so that they can exchange data according to the preconcerted communication protocol. The process of connecting to the vehicle controller needs the pairing password. Only being paired can the equipment visit the SPP interface service provided by the vehicle controller. The monitoring data security is guaranteed under the Bluetooth encryption mechanism. Fig. 8 shows the test of the monitoring terminal in a NOKIA cell phone. By connecting with the ECU of the fuel cell bus via the Bluetooth connection, the actual measurement can be displayed on the screen of the cell phone and stored in the SD/MMC card inside the cell phone. Those data can be sent to the network server via GPRS connection.
(4). Mass storage cards such as SD/MMC card are adopted as the storage devices, which is small and portable. Compared with the hard disk of the computer, it is more reliable working in the vibration and gradient environment. Furthermore, the monitoring and measuring data can be uploaded dynamically to the network server via GPRS connection of the cell phone. After a long time of demonstration operation, an expert system based on the network server can be set up to provide a good platform for the tele-diagnosis. This case continues to move on as the demonstration operation of the fuel cell bus is ongoing. ACKNOWLEDGMENT
The authors appreciate the fmancial support offered by the National 863 Hi-tech Research Program during the II th fiveyear plan of China. REFERENCES [1]
[2]
Figure 8. Portable monitoring terminal on a NOKIA cell phone
V.
[3]
CONCLUSION
During the demonstration of the fuel cell city bus, the data measurement and monitoring of the fuel cell bus is significant for safe operation and control strategy optimization. This article narrates how to take advantage of the advanced Bluetooth communication technology, combined with the Micro-Electro Mechanical Systems (MEMS) and the J2ME software platform, to realize the wireless acceleration monitor and measurement in each axial direction of the fuel cell bus. Compared with the traditional monitoring system using notebook or PC, this new measurement method has following advantages; (I). Bluetooth 2.0 + EDR Classl protocol has a fast transmission speed, good capacity of fault tolerance, high confidentiality and enhanced capacity of anti-interference. The monitoring and measuring system based on this technology has a high stability. (2). It can be used in cell phones, PDAs, computers and many other monitoring terminals without supply of 220V alternating current. It is small, portable and suitable for experiment and measurement in vehicles.
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(3). It is wireless so the monitoring and measurement will not be limited by the wires. The simple structure provides more security for the system.
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