ATZ-Driverless Chassis Dynamo Meter Testing of Electric Vehicles

8/4/2019 ATZ-Driverless Chassis Dynamo Meter Testing of Electric Vehicles

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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 ChassisDynamometer testing of

eleCtriC vehiCles

12

Cover Story EET VEHE



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 beenfocusing 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,1. 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 anothersubsystem 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 (ndia).

HareSH BHere

is Development Manager,

Advance Engineering at Tata Motors

in Pune, Maharashtra (ndia).

ATH

1 Electric powertrain

112010 Volume 112 13



It is extensively used for assessing the

engine emission performance of a vehicle.

The driver needs to meticulously followthe 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 inthe 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. 2 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 wishesto 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. Theaccuracy 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

2 Electric vehicle control loop

3 ontrol system block diagram for electric vehicle testing

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14



map”.3 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 outerloop is now the “battery current loop”.

4 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 lifeusually 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 testset 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

hassis Dynamometer Testing For EV Power-

trains”, FTA 2010

[2] EE regulation 101

[3] Koji Yamamoto: Hybrid Electric Vehicle

Testing Method And ystems”, patent No:

6,457,351, 1999

[4] A.cott Keller: obotic ystem For

Testing f Electric Vehicle” Patent No:

5,430,645,1993

4 Proposed control system block diagram for

battery testing

112010 Volume 112 15

ATZ-Driverless Chassis Dynamo Meter Testing of Electric Vehicles

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