Mobile Platform for Testing Electric Traction Motor

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the hardware and software related to the electric drive system and the monitoring system. Key-Words: - electric vehicle, traction motors, electric .... Instruments TMS320LF2407 Digital Signal .... [6] Katsuhiko Ogata, Modern Control Engineering,.

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Mobile Platform for Testing Electric Traction Motor Prototypes in Real Road Conditions STEFAN GHEORGHE Politehnica University of Bucharest ROMANIA [email protected] MIRCEA COVRIG Politehnica University of Bucharest ROMANIA [email protected] GRIGORE DANCIU Politehnica University of Bucharest ROMANIA COSTIN CEPISCA Politehnica University of Bucharest ROMANIA [email protected] TUDOR URSU Military Technical Academy Bucharest ROMANIA [email protected] SORIUN DAN GRIGORESCU Politehnica University of Bucharest ROMANIA [email protected] SANDA VICTORINNE PATURCA Politehnica University of Bucharest ROMANIA [email protected] NICOLAE JULA Military Technical Academy Bucharest ROMANIA [email protected] DORIN OPREA Politehnica University of Bucharest ROMANIA DANIEL SERBAN Politehnica University of Bucharest ROMANIA Abstract - This paper presents a mobile platform designed and built by the authors for testing the electric traction motors in real operating conditions. There are presented the solutions chosen for the mechanical part, the hardware and software related to the electric drive system and the monitoring system. Key-Words: - electric vehicle, traction motors, electric drives

1 Introduction

which are fed by power inverters connected to direct current sources.

Modern drive systems of the electric vehicles are based on permanent magnet synchronous motors [1]

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The electric motor design, as well as the optimization of the motor-inverter system performance, is related to the type of wheel drive system: independent drive, with or without reductor, or differential based drive. The numeric simulation of the electrical drive and vehicle basic cinematic may offer good results regarding the evaluation of the whole system behavior and performance, but it becomes very complex when the real road conditions, along with the mechanical elements, are to be considered, especially in the case of an independent motorwheel drive. For this reason, a research team from the Department of Electrical Machines and Drives, Faculty of Electrical Engineering, Polytechnic University of Bucharest, has designed and built a mobile platform for testing the electric motor prototypes in real road conditions. The project was financially supported by the CEEX National Research Program.

PT M1

PM 2

M2

Fig.1 The propulsion system of an independent driving wheels electric vehicle  The motors torque command is taken from a potentiometer linked to the vehicle acceleration cable.  Each motor is separately controlled by its own power module, PM1 and PM2, as shown in figure 1.  For the system monitoring it was made a dedicated software application, which is running on a laptop PC. The vehicle was chosen by taking into account that its dimensions influence on the constraints imposed to the propulsion system parameters:  The maximum motors dimensions are given by the distance between wheels, considering also the bear axles length;  The maximum string batteries voltage is given by the dimensions and number of the batteries – limited by the available space in the back half of the car and the maximum supported mass. The above mentioned limitations are summarized in table1.

2 Technical requirements assigned to the platform and the resulted solutions The following platform requirements were established on purpose to satisfy the testing conditions of the electric motors and motor-inverter system:  safely movement in various road conditions;  to make possible the easy attach / detach the the motors;  to have a direct (no reductor) and independent two-wheel drive system;  each axle must have its own real-time motor control function implemented at the corresponding power modules level;  real-time monitoring system for the characteristic quantities. To accomplish these requirements, the following solutions were found:  It was chosen a small classic vehicle, from which the engine, gearbox, differential and the cooling and exhausting systems were detached.  Each wheel is driven by its own electric motor using the classic vehicle bear axles  Both electric motors are fixed on a special built structure which is immovable relative to the chassis The propulsion system is shown in figure 1.

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PM 1

Table 1.

3 Platform description For platform implementation it was chosen a small classic vehicle, which for using ints chassis, mechanical brakes, steering and lighting. Based on the requirements and the research team experience, there were established the electro-

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mechanical characteristics of the platform and the power modules specifications.

3.1 Electro-mechanical characteristics As a solution of the motors coupling structure, it was built a motor supporting structure that was fixed to the chassis – a convenient solution regarding the amount of modification that are to be done, considering the initial vehicle configuration. 3.1.1 Propulsion Two brushless DC motors, previously designed by the research team and being subjected to test, were fixed on the built structure and coupled to the wheels using the vehicle bear axles, as it can be seen in figure 2.

(a)

(b) Fig.3 One of the designed brushless DC motors (a) and its characteristics (b) Fig. 2. – The brushless DC motors for test, fixed on the platform

This offer the possibility of implementing an advanced traction system, including the following:  The mechanical differential is no more needed – its function is performed by the motor control system  ABS (Antilock Braking System) function  ASR (anti-Slip Regulation) function

The designed brushless DC motors specifications are presented in table 2, and the motors mechanical characteristics, from laboratory tests, are shown in figure 3 Table 2 Nominal current

20 A

Maximal current:

109 A

Maximal dc voltage

110 V

Nominal torque:

30 Nm

Peak starting torque:

210 Nm

Nominal speed

250 RPM

Maximal speed

450 RPM

3.1.2 Sources Each electric motor is fed by a power module. The torque command is transmitted through a pedal position transducer which is linked to the acceleration cable, as it can be seen in figure 2. The scheme of the electric drive system is presented in figure 4. The power circuit, working at 120V voltage, includes the battery string, the circuit breaker CB1, contactor C1 and the power modules. For the contactor command it was used an additional battery, B2, the relay CR1 and the vehicle key switch.

One advantage of the independently driven wheel propulsion system is that it can fully exploit the superior dynamic performance of the electric motors, as compared to the combustion engines.

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Traction batteries

F1

Circuit breaker CB 1

Contactor C1

PT

3.2 Intelligent power modules controlling the motor axes The basic structure of the command and control system is illustrated in figure 7. Both motor axes are independently driven, using one power modules for each motor PM1 and PM2, as shown in the same figure.

Fwd/Rew

2x 16mm

2

PM1

M1

F2 Tn1 D1

F4

B2

0,75 mm 2

Contact key

Relay CR1 4x 16mm 2

Lc

PM2 B1

F3

M2

16mm 2

Tn2

Fig. 4 The scheme of the electric drive system The power modules are placed onto to the battery package, consisting in ten Lead-acid Batteries – GB 12-100 type, BSB Power, 12 V and 100 Ah capacity, connected in series. The battery package is put in two rows, in place of the vehicle back seats, as shown in figure 5.

Fig. 7 The schematic of the independent wheel drive system Each power module, shown in figure 8, consists in two main parts: - the power part – basically a three-phase bridge inverter, with pulse-width-modulation (PWM) driven IGBTs; - the control part, which is implemented on a dedicated motion control board - Technosoft MSK2407, built around the fixed point Texas Instruments TMS320LF2407 Digital Signal Controller.

Fig. 5 The battery package and power modules The commutation and protection devices – the fuses, circuit breaker and contactor were placed so that to obtain a minimum wiring length [2]. The contactor was placed near the batteries and the circuit breaker was disposed between the front seats, to be accessible to the driver. The position of these elements is shown in figure 6.

Fig. 6 Commutation and protection elements placement

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Fig. 8 Inside view of a power module

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The board is software programmable using a highlevel motion language and the IPM Motion Studio software package, both created by Technosoft. This package running on a PC, allows connection between PC and MCSK II, its setup configuration, the controller tuning etc. The program is downloaded from PC through RS-232 interface. The primal software component of the control part is the real time motor control kernel, as part of the firmware already supplied with the board. There are several motor control techniques which can be easy chosen and configured using the graphical user interface: sensored (encoder) and sensorless vector control for sinusoidal permanent magnet motors (PMSM), or trapezoidal control for Brushless DC Motors using Hall sensors. The software is especially designed for multiple axis control - the power modules are connected in a network using CAN (Controller Area Network) bus for communication. In the actual configuration of the vehicle drive system, one power module the master receives the torque command through an analog input, and transmits it to the slave power module by the CAN bus. The signal schematic is briefly shown in figure 7. The distinctive feature of the entire drive system control is the modality chosen for the supervisor implementation. This is not a separate entity, but it is included in the master power module (PM1), at software level. The solution is economical and

practical, as the controller board has sufficient processing and memory resources so that additional higher level tasks are supported: battery management, critical events counting and signaling.

4 The platform monitoring system The specific motor and drives quantities can be acquired and viewed during the tests using a monitoring system, created for this purpose. The system main components are the transducers, the signal conditioning module, a PCMCIA data acquisition card – National Instruments DAQCard6062E – connected to a laptop and software application created in LabVIEW environment. The analog signals from the voltage and current transducer are sampled with 20 kS/s. In actual configuration, the vehicle speed is calculated as the mean value of the individual motor velocities, which are determined using the pulses from the brushless DC motor Hall sensors. The software application allows for on-line monitoring the measured quantities (motor and batteries currents, voltages and speed) and the calculated quantities (motor torque, power) in a continuous running time window of up to 60 seconds width. In figure 9 is shown an extras from the monitoring application front panel, where there can be seen the variation of the vehicle speed and the total current and power delivered by the batteries.

Fig. 9 Sample view of the monitoring application front panel showing the variation of the vehicle speed (up), and of the current (middle) and power (down) delivered by the batteries during vehicle start

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Fig. 10 The variation of the motor’s phase current and line voltage

References: [1] Navrapescu,V, Covrig,M, Todos,P, Comanda numerica a vitezei masinii asincrone (available only in romanian), ICPE Publishing, Bucharest, 1998 [2] Danciu,G, Echipament electric si electronic auto (available only in romanian), Matrix Rom, 2004 [3] Bose, B. K. Modern Power Electronics and AC Drives. Prentice Hall PTR, Prentice-Hall, Inc., U.S.A., 2002. [4] Leonhard W. Control of electrical drives, Springer Verlag, Berlin, Germany, 1990 [5] Peter Vas, Sensorless Vector and Direct Torque Control, Oxford University Press; Oxford, U.K.,1998 [6] Katsuhiko Ogata, Modern Control Engineering, Prentice-Hall, Inc., U.S.A., 1997. [7] Andrei,H, Spinei,F, Cepisca,C, Chicco,G, Grigorescu, S,Dascalescu,S, Minimum disipated power for linear and nonlinear electric circuits,WSEAS Transactions on Circuits and Systems, Issue 11, volume 5, november 2006, pp. 1620-1626, ISSN 1109-2734 [8] Cepisca,C, Covrig,M, Grigoresc,S.D, Jula,N, The efficiency of real time sequences FFT Computing, Proceedings of the 2nd European Computing Conference (ECC08)-New Aspects on Computers Research, Malta, September, 11-13,2008

The acquired quantities can also be displayed in detail, as shown in figure 10 – where it is represented the variation of the phase current and line voltage of one motor. The time axis of two or more graphs can be synchronously scrolled, when offline, in order to analyze and correlate quantities variations during the test.

5 Conclusion This paper summarized the solutions adopted by the authors concerning the design and development of a mobile platform for electric traction motors test. The main feature of this platform is the independent driving wheels system, which was chosen to be implemented due to its advantages regarding the precise control of each driving wheel and the much simpler mechanic part. The use of a specific motion control boards along with their software package permit easy set-up and tuning of several motor control techniques, so that not only the motors, but the whole electric drive system can be tested in different regimes encountered in real operating conditions. A dedicated monitoring system was built to acquire various motor and drives quantities during tests, which can be used for refining and/or validating the prototype motors.

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