1. Introduction
In the last decade, electric vehicles (EVs) have experienced a continuous and noteworthy improvement in their performance. Nowadays, electric automobiles have the potential to become a feasible alternative to conventional internal combustion vehicles, especially in urban environments.
As EV technology develops, better performance is expected, and therefore more specific power electronics converters and more sophisticated control strategies are imposed on these systems. Since this technological demand must be fulfilled within the educational community, it is the responsibility of niversities to teach and train future engineers on this matter, although it is also very desirable that the industry collaborates with this process for mutual benefit.
Mainly due to the difficulty in accessing this kind of drive in a real vehicle, studies on electric traction technologies are typically based on theoretical lessons, while practical application of the fundamental concepts is required for a realistic active learning experience [
1]. In most educational centers, this practical experience has been carried out only by means of computer simulation models [
2,
3], without the benefits brought by the actual machines and by the performance of field tests in the laboratory [
4].
To overcome the limitations of the abovementioned methodologies, a laboratory-based platform is proposed in this paper. Similar works have already been published for other engineering applications, such as programmable and configurable embedded systems [
5], electrical drives control [
6,
7] and wind generators control [
8,
9]. The work presented in [
5] shows how these kind of project-based learning methods may improve teaching in multidisciplinary systems, while [
8,
9] are examples of teaching methodologies combining simulation and a laboratory test bench.
This paper describes a complete system dedicated to the teaching and training of engineering students on the control of electric traction drives. The work has been developed within the Polytechnic University of Madrid in collaboration with CIEMAT, a public research center focused on the transfer of knowledge between researchers and industry.
2. Teaching Objectives and General Approach
The main objectives of the proposed educational project are to complement the theoretical lessons regarding traction drives and their control, to motivate the students’ interest in topics covered in those lectures and to introduce them to the design process of control strategies for EVs. In terms of technology, the project exposes students to devices currently used in EVs, such as batteries, power converters and traction machines, which they will meet in the industry. Besides, the course also covers some design tools for new control strategies.
The educational project targets Master’s degree students in the field of mechanical, electrical or electronic engineering. It was designed as part of an already existing 40-h course in EVs. The proposed teaching methodology comprises two different learning levels:
- (1)
A basic level, consisting in a computer simulator based on electrical models of the different components of the drive: energy storage system, power electronics converters, electrical machine and the vehicle itself. A first learning stage includes a series of exercises designed to introduce the students to the system. In a second stage, the students develop their own control strategies based on simulations.
- (2)
An advanced level, consisting in a reduced scale emulator of the powertrain of an electric vehicle. This emulator is a laboratory workbench that includes a DC voltage source, two power electronics inverters, and two electric motors, one to emulate the inertia and motion resistances of the vehicle and the other the traction machine itself. Using this equipment, and always assisted by a supervisor, the students can test the control strategies developed in the previous stage.
During the development of this system and its associated training methodology, especial focus was put on three remarkable and differentiating aspects of EV teaching:
- (1)
Electric vehicles are a multidisciplinary subject. Therefore, students with different backgrounds (from mechanical to electronic engineering) had to be taken into account.
- (2)
A smooth learning curve was desirable, which meant carefully planning the progression of the different exercises within the course.
- (3)
The course had to be as practical as possible. In this sense, it was not enough to show the students a laboratory platform. Higher interaction with the test bench was desired.
Despite its educational focus, the proposed computer and laboratory platforms were designed for both research and teaching purposes. In this sense, the system is also suitable for testing new control strategies in electrical traction drive research.
3. Technology Overview: Powertrain of a Battery Electric Vehicle
The powertrain of a vehicle includes all the necessary components used to transform stored energy into kinetic energy for propulsion purposes. In a rechargeable electric automobile, this always includes the on-board energy storage system (usually batteries, but sometimes a combination of batteries and ultracapacitors [
10]), one or more bidirectional power electronics converters (usually two, one DC/DC converter and one inverter or DC/AC converter), and one or more rotatory electrical machines [
11,
12]. Other elements such as differentials or additional gears are also present depending on the powertrain topology.
Figure 1 shows one of the most common powertrain topologies found in battery EVs, which is the one studied in this work.
Figure 1.
Powertrain topology of a battery electric vehicle used in this paper.
Figure 1.
Powertrain topology of a battery electric vehicle used in this paper.
Given the above topology, the control strategy is as follows:
- (1)
The DC/AC converter works as a voltage source inverter. By means of this converter, both machine flux and machine torque are controlled, either directly or indirectly. The power needed by the motor is instantaneously drawn from the DC link, unless the drive is performing a regenerative braking, in which case power will be delivered to the DC link.
- (2)
The DC/DC converter works as a conventional buck-boost converter. It dynamically adapts the battery voltage to the DC link voltage while supplying the power needed by the inverter. In traction drives, the DC voltage is usually higher than the battery voltage. The DC voltage may be constant or not, depending on the switching strategy selected for the inverter.
In this work, this same topology was chosen for the educational platform because it is one of the most extensively used traction technologies and because it is highly suitable for the implementation of advanced control features.
5. Laboratory-Based Teaching Platform
5.1. Test Bench Description and Operation
The second training tool developed within this educational project is a laboratory test bench. This platform contains the same elements as the simulation model: batteries, two power converters and the electric motor (the power scheme, topology and modulation techniques are the same as well). The vehicle itself is replaced by another electric machine—induction machine (IM)—that emulates both the inertia and the resistive torque of the real vehicle.
Figure 8 shows a picture of the test bench, driving pedals included.
Figure 8.
Laboratory test bench: (a) Electric cabinet containing three power electronic converters, bench with a PMSM (traction machine) and a IM (emulator machine), and driving pedals; (b) safety room for batteries and ultracapacitors.
Figure 8.
Laboratory test bench: (a) Electric cabinet containing three power electronic converters, bench with a PMSM (traction machine) and a IM (emulator machine), and driving pedals; (b) safety room for batteries and ultracapacitors.
Three power electronic converters are needed in the test bench because of the inclusion of the vehicle emulator, which consists of the IM driven by a second inverter. Both inverters feed from the same DC link, so that power is recirculated when any of the machines is in generator mode, minimizing energy consumption from the batteries.
Table 1 contains some rated values for the main devices comprising the test bench.
Table 1.
Teaching Laboratory Platform: Nominal Values.
Table 1.
Teaching Laboratory Platform: Nominal Values.
Traction machine | Load machine (emulator) |
Type | Interior PMSM | Type | Squirrel cage IM |
Power | 3.3 kW | Power | 2.2 kW |
Speed | 1500 rpm | Speed | 1420 rpm |
Pole pairs | 2 | Pole pairs | 2 |
Voltage | 400 VRMS | Voltage | 400 VRMS |
Stator current | 4.84 ARMS | Stator current | 4.84 ARMS |
Inertia | 0.0068 kg·m2 | Inertia | 0.0075 kg·m2 |
Power converters | Batteries |
Type | IGBT module | Type | NiMH c |
DC voltage | 380 VDC a | Voltage (total) | 120 V (10 × 12) |
Current | 60 ARMS | Capacity @C/3 | 100 Ah |
Switch. freq. | 20 kHz (max) | | |
Emulated EV | Aerodynamics and rolling |
Type | Fictitious car | Air density ρ
| 1.2 kg/m3 |
Total mass M | 1200 kg | AF | 1.89 m2 |
Max speed b | 51.3 km/h | CD | 0.37 |
ERR | 0.25 m | µroll,0 | 0.0135 |
iGEAR | 17.544 | µroll,S | 0.0055 |
µGEAR | 95% | | |
Similarly to the PMSM inverter, the modulation strategy for the IM inverter is also a conventional hysteresis-band current control with single fixed band. The control scheme has two levels as well. The lower level, the inverter controller, is responsible for the modulation technique. The upper level is the torque controller, which determines a current reference [I
sd*, I
sq*] in dq coordinates (rotor magnetizing current reference frame for the IM [
20]) by means of the following expressions:
where T
load (Nm) is given by Equation (14) and K
T [Nm/A] is the torque constant of the IM (supposing linearity;
i.e., torque is directly proportional to q-axis current for a given d-axis current). Magnetizing current I
sd* is set to its rated value, so the torque provided by the IM is controlled by means of I
sq exclusively. T
inertia (Nm) is a fictitious torque that emulates the inertia of the vehicle, as expressed in:
where J
emu is the part of the inertia to be emulated, J
vehicle is the total inertia of the vehicle (as seen by the traction machine) and J
mot is the actual inertia of the test bench (PMSM rotor + IM rotor + shaft).
5.2. Teaching Methodology and Case Study
In order to familiarize the students with the platform, a complete user guide for the system operation is provided to them in advance. This guide contains the description and technical characteristics of all the system components, so that they become familiar with the test bench even before going to the laboratory. Once in the practical session, and always under supervision, the students are asked to perform the following tasks:
- (1)
First, the students take turns to drive the “virtual electric vehicle”. They must follow a certain urban driving cycle by controlling the drive speed via the accelerator and brake pedals. This urban cycle lasts for one minute and consists in a few accelerations and brakings. In order to increase motivation, each student is given a score that values their performance as drivers once the driving cycle is finished (see example in
Figure 9).
Figure 9.
First exercise: 60-s driving cycle with pedals (only the first 23 s are shown). (a) Good driver (score: 8.8/10); (b) Not so good a driver (score: 6.1/10).
Figure 9.
First exercise: 60-s driving cycle with pedals (only the first 23 s are shown). (a) Good driver (score: 8.8/10); (b) Not so good a driver (score: 6.1/10).
The exact cycle that each student must follow is chosen randomly from four pre-defined cycles, thus preventing them from memorizing the cycle. Each student drives the cycle two times: the first one with the MTPA trajectory disabled (Id* = 0) and the second one with the MTPA enabled (Id* < 0). This way, they can feel the difference between the two control strategies, as the second one is livelier (i.e., the pedals need to be pressed more intensely in the first driving cycle). Besides, since the students already modified the simulation to include the MTPA trajectory, this serves as an example of how the simulation model may be used to develop new control strategies, which is precisely what they will do in the last exercise described later in this section.
The PC controlling the test bench has two monitors. The first one displays all the information needed by the driver (e.g., speed reference and actual speed, pedals statuses and score), while the second one shows electrical magnitudes of interest (e.g., phase currents and dq currents of both machines, motor torque and emulator torque). While one of the students is driving, the rest can watch the evolution of these electrical magnitudes in the second screen and become familiar with the system operation.
- (2)
Next, each team implements their chosen control improvement in the test bench assisted by the teacher. Since the control hardware is based on a dSPACE development board (MicroAutoBox II) [
32], almost no extra programming needs to be done to implement what the students already developed in Simulink
®. The control interface, programmed in dSPACE ControlDesk
® Next Generation, is also highly reusable, further shortening the implementation time.
- (3)
Finally, each team drives the urban cycle of the first exercise again, but with the control modifications implemented in the previous point. Before driving, each team explains to the rest what they have done and what improvement they expect to achieve. This way, all the students learn from what each team has done, regardless of their success.
As an example of the platform capabilities, a case study is presented next.
Figure 10 shows laboratory results for the same vehicle and urban cycle that were implemented in the simulation presented in previous section (
Figure 7). Speed
Figure 10a and PMSM currents
Figure 10b are very close to those obtained in the simulation. IM currents, responsible for generating the load torque, are also depicted in
Figure 10c.
Figure 10.
ARTEMIS driving cycle laboratory test (only the first 52 s are shown). (a) Mechanical speed; (b) PMSM dq currents; (c) IM dq currents.
Figure 10.
ARTEMIS driving cycle laboratory test (only the first 52 s are shown). (a) Mechanical speed; (b) PMSM dq currents; (c) IM dq currents.
6. Teaching Results
6.1. Course Assessment
The proposed learning project have been applied to the EVs course (which is assessed annually) during the last three years. The number of students was always very low, from 8 to 12. This was done on purpose to better follow the course development and the students’ progression during the first years after the implementation of the proposed activity. Overall, the teaching platform may be considered successful, given that both the teachers and the students think that they have a better understanding of an EV powertrain and its control since this educational project was included within the EVs course.
However, over these three years, some difficulties of practical nature have been observed. First, the course cannot support a large number of students, given that it is very time-consuming for the teachers, who need to dedicate a few hours to supervise each team of students. Secondly, the students find it very difficult to think of possible control improvements beyond those suggested by the teachers, despite what they had studied in more elementary courses. A lack of previous formation justifies this in the case of mechanical engineering students. Thirdly, the number of hours the students need to dedicate is higher than originally expected. This is because the level of understanding needed to improve the control strategy is higher than the level of understanding needed just to analyze a certain control strategy.
6.2. Students’ Feedback
Students were asked to provide informal feedback at the conclusion of the practical class. Their answers indicated that the activity was interesting, very powerful from the learning point of view but probably too time-consuming. The most common issue reported by the students is the high workload.
Besides, the students were asked to answer an anonymous questionnaire (yes/no answer) with four simple questions:
- (Q1)
Did this course help you understand electric traction drives and their control?
- (Q2)
Do you think that you have learned more after the inclusion of this practical session (when compared to a conventional course)?
- (Q3)
Are you satisfied with the simulation platform?
- (Q4)
Are you satisfied with the laboratory platform?
Note that Q2 asks the students to compare their current experience (the proposed activity) with an experience they have not had (such as the previous version of the course). Unfortunately, there is not enough data regarding the previous version of the course to assess how the proposed activity improves the students’ experience by comparing both feedbacks, so Q2 is used instead.
Results for this questionnaire are shown in
Figure 11. The students’ response to the first two questions is clearly positive, a minority of them preferring more conventional practical sessions.
Figure 11.
Student survey results.
Figure 11.
Student survey results.
Regarding Q3, many students complained the first year about the time required for the simulation model to run (up to 30 min in an average computer, which was too much for the students to test as many control strategies as possible). The model was improved in this sense after the first year, thanks to the elimination of the energy storage part of the model (see
Figure 2), thus halving the time needed to simulate and improving the students’ feedback in years 2 and 3.
Another flaw pointed out by the students is the number of laboratory platforms available (just one), which is the reason why Q4 gets the worse feedback overall, especially when the number of students is large. Some of them suggested that at least a second platform would be needed in order to optimize the time spent in the laboratory. While this is true, replicating the original platform is costly and may require the attendance of a second teacher to the laboratory sessions.
7. Conclusions
A two-level (simulation + laboratory) educational platform has been developed in order to improve the learning process on electrical traction drives and their control. The teaching methodology focus on the students’ interaction with the platform, both the simulation model and the laboratory test bench. This educational tool is intended to complement theoretical lessons.
In the simulation stage, the students develop their own control strategies/switching techniques. The most promising results are later implemented in the test bench during the laboratory stage. The students have the chance of comparing these control strategies not only by testing them, but also by driving a virtual EV by means of an accelerator and a braking pedal. This methodology has been tested in a Master’s course during three years. Both teachers and students feel that this platform improves the students’ understanding in electrical traction drives and their control. Its main drawback is the limitation in the number of students per class.
Acknowledgments
The authors would like to thank the students Pablo Vélez, Miguel Hoyos and David Cid for their contribution to this work. This work was supported in part by the TECMUSA Project (“Plan Nacional de Investigación Científica 2009–2012”, Spain), and by the SEGVAUTO Project (“Convocatoria de ayudas para la realización de programas de actividades de I+D entre grupos de investigación de la CAM en tecnologías”, ORDEN 679/2009, 19 February Ref: S2009/DPI-1509).
Author Contributions
Pablo Moreno-Torres and Jaime R. Arribas designed the educational project and its associated teaching methodology. Pablo Moreno-Torres established the simulation model and ran the simulations. All authors contributed to the laboratory platform design and construction. Jaime R. Arribas was responsible for the students’ feedback, while all authors participated in the course assessment. The paper was written by Pablo Moreno-Torres and reviewed by all the co-authors.
Conflicts of Interest
The authors declare no conflict of interest.
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