Key test and development parameters for electric vehicle (EV) and hybrid vehicle (HEV) applications require

real life driving conditions to be simulated for a few hours. A conventional chassis dynamometer can be used

to simulate all the driving conditions for a specified driving pattern. However, it is impossible for a human driver

to apply the specified driving pattern for a few hours accurately. To overcome this difficulty, Tata Motors team

developed a driverless chassis dynamometer test set-up which can emulate specified driving patterns to an

EV/HEV powertrain for the desired amount of time.

DRIVERLESS CHASSIS DYNAMOMETER TESTING OF ELECTRIC VEHICLES

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COVER STORY ELECTRIC VEHICLES

EXPLANATIONS

With global warming and fuel crisis, the interest in hybrid vehicles and electric vehicles is at its lifetime peak. This leads to an explosion in EV development activi-ties by OEMs who otherwise have been focusing on conventional powertrain vehi-cles. Key test and development parameters for EV/HEV application comprise the range of distance a vehicle can travel in single charge, the torque/speed perform-ance of the powertrain, the accuracy of the state of charge estimation etc. Although specialised EV development and test equipment are available, they are costly and need special engineering skills to be mastered for meaningfully exploiting the potential. There are few patents awarded [3, 4] for such systems which are again costly and more complex. Hence every attempt should be made to adopt existing test equipment for development and test activities for EV subsystems as well as vehicle testing. [1] briefly explains this concept applied to the chassis dynamo-meter for EV test and development activi-ties. This article further explains the con-cept with a focus on control engineering aspects and explores manifold applications of this simple yet powerful concept for EV/HEV testing. First, the specific EV test-ing needs are explained while the follow-ing chapter dwells on strengths and limita-tions of chassis dynamometer testing as a test philosophy. The next section elabo-rates on the control engineering concepts underlying the implementation of a “driv-erless philosophy”. After this the impor-tance of open-loop modeling of the drive system for implementing the control algo-

rithm is highlighted. Additionally, various EV specific tests are described which can gainfully use this set-up without requiring huge investments in costly proprietary test equipment which is alternatively available. The last section provides concluding remarks, stressing a wider application of the concept for other types of vehicles.

SPECIAL TESTING NEEDS FOR EV/HEV DEVELOPMENT

An EV fundamentally differs from a con-ventional vehicle because of the electrical powertrain it uses, ➊. This changes the very DNA of the system architecture. Lim-ited onboard energy storage as well as fast “servo-grade” motion control capabilities are the two key distinguishing elements which set apart an EV powertrain from a conventional one. Limited onboard storage makes the range between two consecutive charges a crucial performance parameter. “Servo-grade” motion control brings into focus acceleration capabilities, as well as drivability due to much more agile motion response compared to a conventionally driven vehicle. Performance of liquid cool-ing of the motor as well as the motor con-trol power electronics – particularly when scaling a continuous grade – also becomes crucial. The DC-DC converter is another subsystem which is specific only to an EV architecture.

CHASSIS DYNAMOMETER TESTING: LIMITATIONS AND STRENGTHS

Traditionally, a chassis dynamometer has been used as a powerful performance evaluation tool for a conventional vehicle.

VISHWAS VAIDYA is Assistant General Manager, Electronics, at Tata Motors in

Pune, Maharashtra (India).

HARESH BHEREis Development Manager,

Advance Engineering at Tata Motors in Pune, Maharashtra (India).

AUTHORS

➊ Electric powertrain

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It is extensively used for assessing the engine emission performance of a vehicle. The driver needs to meticulously follow the driving profile displayed on a PC screen in the form of speed versus time at a stipulated gear position. Although this approach is fine for a conventionally driven vehicle which needs to be operated for a few minutes, an EV test might go for hours for assessing the battery perform-ance [2]. It would be next to impossible for any human driver to meticulously fol-low a driving cycle pattern for hours. This limitation can be addressed by using a computerised driver simulator as described in [1] and briefly explained in the subsequent sections of this article.

On the other hand, the strengths of a chassis dynamometer like electronic auto-matic load control which are used exten-sively for racing car performance evalua-tions as well as vehicle road performance simulations under diverse driving condi-tions can be gainfully exploited to simulate various electric current load patterns for the EV batteries as well as for assessing torque performance of the electrical drivetrain components like motor and controller.

CONTROL ENGINEERING PERSPECTIVE

Implementation of the driverless feature for the chassis dynamometer testing is based on emulation of the driver behavior using a computer. Generally, an electric drive system provided to control the trac-tion motor for an EV implements what is known as torque-loop. ➋ illustrates the concept. A torque set-point is provided via an electric voltage supplied by the throttle potentiometer. If the throttle position is maintained constantly by the driver, the vehicle experiences a continuous accelera-tion. However, the driver generally wishes to control speed of the vehicle depending on driving conditions as well as urgency to reach the destination. Hence the driver’s brain operates what is known as a speed-loop in control engineering parlance, ②. The driver’s eyes receive feedback regard-ing vehicle speed from the speedometer on the instrument cluster. The driver mentally compares the difference between the speed he wishes to reach and the actual speed. Based on the difference he applies appro-priate torque through the throttle so as to reduce the error between the two to almost

zero. The electric drivetrain needs to be applied with negative torque during decel-eration portion of the driving profile. This can be easily accomplished by regenerative braking of the drivetrain in which the trac-tion motor acts as a generator and pumps the electric power to charge the battery, thereby decelerating the vehicle. One needs to ensure that the battery charging current limits – as specified by the battery manu-facturer – are not violated. However, most batteries are capable of withstanding short-term electric transients, in the form of both charging as well as discharging currents.

In cases where a test profile demands additional braking torque beyond the charging constraints of the battery, the same may be emulated by a friction brak-ing mechanism whose friction torque could be controlled by electric means to preserve the “driverless” nature of the test. However, in the real-life experiments con-ducted by the authors, such constraints were not faced and “one-foot” EV opera-tion as described in [1] could be easily implemented based on regenerative brak-ing. To the best of our knowledge most of the driving cycles used for vehicle testing do not present aggressive deceleration making total regenerative braking as a viable option for this application.

OPEN-LOOP MODELING FOR FEED-FORWARD MAP AND CONTROL STRATEGY TUNING

The most critical part of designing this type of test set-up is to arrive at an accurate control strategy to close the speed-loop. Here again a study of driver’s behavior proves useful. An experienced driver oper-ates the throttle to a value close enough to the final torque set-point through his or her experience and waits for a while to gauge the vehicle response to the same. The accuracy of this initial set-point decides how close the actual vehicle speed moves to the driver’s wish. Based on the error, the driver now fine-tunes the throttle position so as to attain the desired speed set-point in his or her mind. In control engineering parlance this approach is known as “feed-forward strategy”. We can conduct open-loop experiments on the chassis dynamo-meter to determine the closest initial torque set-points for attaining various values of final vehicle speeds and prepare a look-up table known as “feed-forward-

➋ Electric vehicle control loop

➌ Control system block diagram for electric vehicle testing

COVER STORY ELECTRIC VEHICLES

14

map”. ➌ shows the complete control sys-tem block diagram.

EV SPECIFIC TESTS ON THE DRIVERLESS CHASSIS DYNAMOMETER

The driverless chassis dynamometer can be used for several battery related tests. Apart from assessing the range perform-ance, a pre-defined load current profile could be emulated using this set-up. A battery current versus time profile needs to be implemented. Hence the battery cur-rent value becomes the set-point in place of vehicle speed now. Thus, the outer loop is now the “battery current loop”.

➍ shows the control engineering sys-tem diagram. Such a test generates valua-ble information needed to model the battery.

Furthermore, life cycle tests can be con-ducted on the driverless chassis dynamom-eter. EV batteries have a limited service life depending on the number of charge/dis-charge cycles they undergo during their life time. In fact, the battery life can be the sin-gle most factor behind the commercial suc-cess of a vehicle, since a short battery life usually means escalated life cycle costs for the customers. Hence this set-up can be employed to perform charge/discharge cycling of batteries or modules to obtain charge and discharge capacity, energy, DC internal resistance etc.

Another battery related test is the par-tial discharge test. In real life applications, EVs would be put on charge even before their battery is partially discharged and the dynamometer will be used to study the effect of partial discharge on battery capacity.

Additionally, dynamic power charge/discharge using the dynamic stress test (DST) procedures set by the US Advanced Battery Consortium (USABC) can be con-ducted. The charging can be effected by means of a negative torque set-point. However, it may require access to the internal sub-system parameters of the electric drive which need to be operated into fourth quadrant for charging the bat-tery as per stipulated pattern.

Next to battery related tests, the driver-less chassis dynamometer is also used for tests on vehicle level, for example the sustained hill climb test. Regarding start-ing torque and hence initial gradability, EVs are better than conventional vehicles. However, when it comes to scale sus-tained slopes like long flyovers or steep hills, the power semiconductors used in the drive electronics may heat and trip the drive stranding the vehicle in the traffic.

This test can be safely carried out on the dynamometer by using the “Electronic Automatic Load Control“ feature provided by most dynamometers. A constant torque load corresponding to the stipulated grade could be exerted by the dynamometer. The speed loop can attain a lower speed set-point which is applicable as per the vehicle specifications during hill climb.

Another kind of test on vehicle level are the adverse temperature tests. Sub-zero temperatures may adversely affect battery capacity while high temperatures encoun-tered in hot climates may restrict torque capacity of the electric drivetrain and hence drivability. No driver will enjoy physically entering the temperature cham-ber to drive the vehicle. Hence driverless testing definitely is a blessing in such situation.

CONCLUDING REMARKS AND FURTHER ROADMAP

Although this article describes a powerful technique for EV testing, the concept could also be extended to conventionally driven vehicles with automatic transmis-sions or automated manual transmissions, provided the friction braking is also auto-mated using electro-pneumatic or electro-hydraulic control systems.

This mechanism could also be employed for refining control algorithms by employ-ing some “auto-learning techniques”, for example artificial neural networks etc., in the computer software controlling the test set-up. The article undoubtedly presents a powerful tool with immense potential lim-ited only by the user’s imagination.

REFERENCES [1] Vaidya Vishawas, Bhere Haresh: Driverless Chassis Dynamometer Testing For EV Power-trains”, FISITA 2010[2] ECE regulation 101[3] Koji Yamamoto: Hybrid Electric Vehicle Testing Method And Systems”, US patent No: US 6,457,351, 1999[4] A.Scott Keller: Robotic System For Testing Of Electric Vehicle” US Patent No: US 5,430,645,1993

➍ Proposed control system block diagram for battery testing

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