Regen Report

Modeling, Control, and Simulationof Regenerative Braking Systems

in Electric Vehicles

by

Michael Fisher, Youji Ma, and Ping Yu

December, 1999

ABSTRACT

Facing serious global environmental and energy challenges, the automotive industry

must develop new generations of low-emission, efficient automobiles. Regenerative

braking is already accepted as one of the best ways to improve the overall energy

efficiency of these vehicles.

A complete regenerative braking system for both EV’s and HEV’s is designed and

modeled, combining regenerative braking with supplemental hydraulic braking and ABS.

In designing the control strategy, tremendous efforts are made to recover maximum

kinematic energy, while still ensuring safe braking.

The 3-tier system, comprehensively covering a variety of input/output variables, is

constructed based on a half-car vehicle model using Simulink. Simulation results of

different driving scenarios show that the system is very efficient in recovering braking

energy, and is also very responsive to hazardous conditions such as panic braking and

slippery road surface. A final simulation demonstrates 86% brake energy recovery

efficiency in ECE-15 Fuel Economy testing cycles.

Table of Contents

Chapter 1. Introduction ……………………………………………………………1

1.1. Electric and Hybrid Electric Vehicles ……………………………………………1

1.2. Technical Challenges ……………………………………………………………2

1.3. Modern EV’s ……………………………………………………………………6

Chapter 2. Regenerative Braking System Design ……………………………………9

2.1. Regenerative Braking ……………………………………………………………9

2.2 General Principles ……………………………………………………………10

2.3 System Design Overview ……………………………………………………………12

2.4. Brake System ……………………………………………………………………14

2.5. Motor/Inverter Torque Characteristics during Regen Braking ……………………16

2.6. Priorities in Control Design ……………………………………………………18

2.7. Brake Torque Blending ……………………………………………………………19

Chapter 3. System Modeling ……………………………………………………………24

3.1. Tier 1 module ……………………………………………………………………24

3.2. Vehicle subsystem ……………………………………………………………27

3.3. Front and rear axle hydraulic pressure modulator subsystems ……………………30

3.4. Regen controller subsystem ……………………………………………………31

Chapter 4. Model Analysis ……………………………………………………………33

4.1. Scenario 1: From 55mph to 0mph with small braking force ……………………33

4.2. Scenario 2: From 55mph to 0mph with moderate braking force ……………35

4.3. Scenario 3: From 55mph to 0mph with large braking force ……………………37

4.4. Scenario 4: From 55mph to 0mph with large braking force on slippery road ……38

4.5. Scenario 5: From 35mph to 0mph with small braking force ……………………41

4.6 Scenario 6: 35mph coasting 15% downhill ……………………………………42

4.7. Scenario 7: ECE-15 fuel economy testing cycle ……………………………………44

Chapter 5. Conclusions ……………………………………………………………46

References ……………………………………………………………………………47

1

1. Introduction

1.1. Electric and Hybrid Electric Vehicles

The technology for electric vehicles (EV’s) has been available since the turn of

the century. In the early 1900’s EV’s were just as popular as their gasoline or steam-

powered counterparts. However, the abundance of petroleum soon allowed internal

combustion engines (ICE’s), which operate using fossil fuels such as gasoline and diesel,

to dominate the world’s automobile market. The noisiness and lower reliability of ICE

vehicles were soon outweighed by their lower cost and better range (32). Continued

development of the ICE engine over the last 80 years has made it so sophisticated that no

other options have posed real challenges to it.

However, as easily-recoverable petroleum deposits dwindle, automobile

populations soar, and cities become choked with combustion by-products, the ICE is

increasingly becoming the victim of its own success. In the next five decades the auto

industry will face a crucial test – how to meet the challenges of the exponentially

growing world population, the increasing wealth of developing countries with large

populations, the deteriorating global environment and the per capita energy being

consumed. It is estimated that the population will continue to explode, reaching 9 billion

in 2050. Combined with greater overall wealth, more energy will be demanded per

person, so we may expect far greater pollution due to automobile emissions unless we

stop abusing our precious energy in today’s manner. Automobiles must become cleaner

and more energy efficient.

Over the past three decades there has been resurgence in the development of

electric vehicle technology and the desire to once again produce electrically powered

vehicles for mass market. The renewed interest in EV’s is due largely to the realization

that our reserves of oil may soon be depleted, as well as a growing concern to make

automobile travel as environment-friendly as possible.

2

California, with a sizable share of the automobile market and the worst pollution

problem in the country, has been particularly effective at passing such mandates and is

responsible for driving much independent EV research (22). The California Air

Resources Board (CARB) had originally set a quota of 2% for all vehicles sold by 1998

to be zero-emission vehicles, which primarily includes EV’s (15). Although this mandate

was reconsidered, a quota of 10% by the year 2003 still stands, applying to the 7 biggest

car manufacturers in the state (Chrysler, Ford, GM, Honda, Mazda, Nissan, and Toyota)

(15, 22). Legislative activity is also being seen in Massachusetts and New York, though

not as aggressive as in California (22).

EV’s have only three primary components - the electric motor, motor controller,

and battery - making them much simpler than ICE vehicles which contain an engine,

transmission, exhaust, fuel-injection system, and muffler (18). With fewer moving parts

and without complicated fluid systems, EV’s today are more reliable than conventional

vehicles (8,18). They require no emission tests, oil changes, tune-ups, and less general

maintenance, meaning less time spent with a mechanic in the repair shop (8,32). In

addition, electric vehicles provide a more comfortable ride, with less noise and no gear

shifting (21,22). And likely the greatest advantage of EV’s is their energy flexibility and

potential independence of oil (18).

Yet along with the many advantages of electric vehicles come serious

disadvantages, which must be overcome if EV’s are to again own a sizeable portion of

the automobile market. The bottleneck is that the battery technology that has not offered

an inexpensive option with large storage capacity, greatly limiting the range of an

affordable EV and preventing them from competing on an equal level with ICE vehicles

(2,6).

1.2 Technical Challenges

The auto industry and governments worldwide must commit to supporting the

development of next generation tools for mass and personal transportation. The technical

3

challenges are achievable. Current technology has already nurtured the options of pure

electric vehicles, hybrid electric vehicles (HEV’s including serial and parallel) and fuel

cell hybrid vehicles. Figure 1 shows an approximate comparison of CO2 emissions of

these options versus a conventional gasoline car and a compressed natural gas (CNG)

vehicle. It should be noted that CO2 emissions can roughly represent the levels of energy

consumed to propel a car without external electric charging port. In the case of EV’s

which generate zero emissions, emissions are instead produced at the power plant, though

non-mobile emissions are normally easier to control than mobile emissions.

Projected CO2 Emissions

0 0.2 0.4 0.6 0.8 1

Gasoline (26MPG)

Parallel Hybrid

CNG Vehicle

Series Hybrid

Fuel Cell Hybrid

EV (Power Plant)

Figure 1. Projected CO2 emissions

It is easy to show the projected scenario through a series of improvements from a

series of innovations. For example, in most centralized metropolises, city driving is one

of the largest energy wasting sectors. Assuming we have a car with traditional gasoline

engine, drive line and aerodynamics, of 100 units of total energy input to an engine, only

15 are eventually converted to driving wheels (Figure 2-a).

4

Engine Losses58.0

Accessories2.0

Engine

100

21.0

15.0

KineticEnergy

9.0

RollingFriction

3.5

Aero2.5

Standby19.0

Transmission

Drivetrain Losses6.0

Figure 2-a. Baseline fuel economy

By adding a 50%-efficient regenerative braking system with a small motor

(generator) and small energy reservoir (battery), we can recall 3.7 units of energy which

will result in nearly a 20% saving at the input end (Figure 2-b).

Engine Losses43.6

Accessories2.0

Engine80.4 15.8

Fuel Economy = +24.4%

Electric Motor

11.3

3.7*15.0

KineticEnergy

9.0

Rollingfriction

3.5

Aero2.5

Braking0

No brake heat

Rollingfriction

1.0

Aero0.5

Transmission


Standby19.0 Battery

*Assume 50%efficient

7.5 -- Through RegenerativeBraking

Figure 2-b. Energy flow with regenerative braking

We can improve powertrain and drive-line as well. We can add idle fuel cut-off,

improve transmission efficiency by removing torque converter, and make the car into a

5

semi-parallel HEV. With this configuration we will need 62% of the energy to do the

same job (Figure 2-c).

Engine Losses37.3

Accessories2.0

Engine61.9 13.5


E. Motor

11.3

3.715.0

KineticEnergy9.0

Rollingfriction3.5

Aero2.5

Braking0

Rollingfriction1.0

Aero0.5

Transmission


Standby9.0 Battery

7.5

Figure 2-c. Energy flow of a parallel HEV

Next, convert the car to a Full-Series HEV with no idle waste but higher fuel

efficiency, load leveling, regenerative braking and electric accessories. This requires

only 55.7% of the original energy (Figure 2-d) and the fuel economy has 79.5%

improvement.

Engine Losses

37.6

Accessories2.0

Engine55.7

16.1

Generator


E. Motor

11.3

3.715.0

KineticEnergy9.0

Rollingfriction3.5

Aero2.5

Braking0

RollingF riction

1.0

Aero0.5

Battery

Figure 2-d. Energy flow of a serial HEV

6

Finally, we convert the car to a pure electric vehicle by replacing the heat engine by

a power plant and a inductive charger. There is loss of power transmission through the

network-charger-battery. We will still have the same energy efficiency but greatly save

the cost of the car – over 75% of improvement on fuel efficiency (Figure 2-e).


15.0

Accessories2.0

19.4

E. Motor11.3

3.7

KineticE nergy9.0

R olling3.5

Aero

2.5

B raking0

R olling1.0

Aero

0.5

Battery

57.0

Charger

Figure 2-e. Overall energy consumption of a pure EV

This saving is not achieved by sacrificing the comfort (size, weight) of the car, but

by changing the energy management. Of greater saving is the further reduction of

vehicle weight and improvement of aerodynamics which will reduce the energy

requirement at the output end. Indeed, there is huge potential for us to improve the

overall energy efficiency of automotive transportation, especially in those areas with low-

speed, stop-go traffic conditions.

1.3 Modern EV’s

Nearly every car manufacturer today offers an electric vehicle in their product

line, though sales are generally limited to specific regions of the country and to specific

markets. Every model that we studied, described in further detail below, features

7

regenerative braking. Indeed this shows how the industry values regenerative braking,

and how integral such a feature has become on all electric vehicles.

One of the first available for general sale was GM’s EV1, which was part of a

$350 million development project by that company (35). Research for the EV-1 began in

1988, and it was brought to market nine years later, as the most energy efficient

automobile ever (11). Each member of this project group test drove the EV-1, and

evaluated its regenerative braking system. In most cases, there is no noticeable

difference from conventional braking; there is an audible click when regenerative braking

is engaged but the effect cannot be felt otherwise. Unfortunately, the EV-1 can only be

rented or purchased in Arizona or southern California because cold weather elsewhere

adversely affects its lead-acid battery (35). Today there are 34 retailers in Los Angeles,

Orange County, San Diego, the San Francisco Bay area, Sacramento, Phoenix and

Tucson, who lease and service the EV-1 (28).

About two years ago Chevrolet came public with the S-10 electric pick-up truck,

which also uses a lead-acid battery (24). In 1997, Honda’s EV PLUS became the first

EV on the market to use NiMH batteries, giving it a relatively high range of 125 miles

(29). Honda also advertises its regenerative braking system, claiming it makes stop-and-

go rush hour traffic smooth and effortless, and recovers energy spent climbing up hills on

the way down (29).

Ford’s production line also now offers two electric vehicles. The Ranger EV is

another pick-up truck and the Ecostar is a two-passenger van intended for use in delivery

service (26,27). Both feature power-assisted hydraulic braking and regenerative braking.

The Toyota RAV4 EV is the only electric sport-utility vehicle in today’s

automobile market, currently on sale in Japan for about $45,000 (33). The RAV4 EV

uses disc brakes on the front wheels, drum brakes on the rear, with additional

regenerative function on the rear wheels (25). Nissan’s Altra EV makes use of original

electric vacuum-pump assisted regenerative braking combined with ABS (31).

8

Regenerative braking and ABS are also standard on the Chrysler Electric Powered

Interurban Commuter (EPIC) Electric Minivan (30).

Anticipating no major breakthrough in battery technology in the near future, the

solution has to be HEV’s, which contain both an electric motor and small internal

combustion engine (23). Toyota Prius is the first commercially available HEV with

features a semi-parallel configuration, and Honda will soon market its first HEV, the

Insight, in the United States. The Big Three in the US are also expected to debut their

first generation of HEV’s in 2000 Detroit Auto Show.

9

2. Regenerative Braking System Design

2.1. Regenerative Braking

To improve performance of EV and HEV’s, researchers are active in developing

batteries that store more energy, and in reducing energy consumption though advanced

aerodynamics and weight minimization. Successful attempts have also been made in

recovering energy that the vehicle loses during braking, and this continues to be a major

area of research. Figure 3.a shows the regenerative braking system layout of GM’s EV1.

Figure 3.a. Brake system of GM’s EV1

Regenerative braking seeks to harness energy that would otherwise be dissipated

as heat when a vehicle decelerates. Conventional braking utilizes friction to effectively

decelerate a vehicle, but wastes the large amount of kinetic energy that a rapidly moving

automobile may possess. For example, a 1500kg vehicle traveling at 70km/h has 300kJ

10

of kinetic energy, which drops to zero as the vehicle comes to rest. If properly harnessed,

this is enough to propel that same vehicle for 1.8 km at 70km/h (7). With regenerative

braking, the motor is made to function as a generator when a vehicle decelerates,

funneling energy back into the battery (1). The effect is significant energy recovery

leading to increased vehicle range.

Regenerative braking is also meaningful for ICE vehicles. If adequate additional

energy storage measures can be employed, such as pressure energy storage or high-

voltage (42V) accessory power battery, a hydraulic or an electric motor can serve as the

regenerative braking generator.

2.2 General Principles

An important ratio in evaluating regenerative braking performance is E2/E1 which

shows the percentage of energy that is returned to the vehicle by using regenerative

braking (34). E1 is the energy total energy lost (kinematic and potential) during braking

process, while E2 is the energy recovered and returned to the battery. Braking energy can

be calculated by integrating the braking power:

Eb = Pbdt∫ (1)

E2/E1 is typically expressed for an entire driving cycle, or can be studied during braking

only (we use the latter). Also important is the equation for braking power:

−−−= 2

2

1)cos()sin(

1000VACfmgmg

dt

dVm

VP fDarb ρθθ (kW) (2)

where m is mass in kg, V is speed in m/s, fr is rolling resistance, ρa is air density, θ is the

grade angle (uphill positive), CD is aerodynamic drag, and Af is frontal area. Braking

power is extremely important in regenerative braking controller design, as regenerative

braking should only be applied within a certain power limit as well as depending on the

11

battery state of charge (7,34). High power values present a danger to the battery because

recharging too fast lowers a battery’s life.

In all cases that we studied, electrical braking was combined with hydraulic,

friction braking. This is because regenerative braking is not adequate in all situations,

especially when rapid deceleration is necessary. Design of sophisticated braking

controllers that combine regenerative braking with hydraulic braking is thus necessary to

maintain safety, provide the driver with adequate feel, and recover as much energy as

possible. The strategy for a controller is generally to first achieve desired deceleration

without locking either wheels, especially the rear wheels (7). Second comes the goal of

maximizing energy recovery. In the past 10 years patents have been awarded to several

automobile manufacturers including Ford, General Motors, and Mitsubishi, for

regenerative braking controllers (5, 9, 12, 13, 16, 17).

During emergency braking, with deceleration near 1g, a very high energy

recovery rate is theoretically possible with regenerative braking. For example, in slowing

down a 1500kg vehicle from 70km/h at 0.8g, braking power can reach 250kW (7). Such

high rates are potentially harmful to a battery, and all energy in excess of 20kW should

be handled by other means. Although it may be diverted to a flywheel storage device

(also for regeneration), in today’s EV’s excess energy is typically dissipated by friction

brakes (34). Hydraulic braking is thus used in conjunction with electric braking for rapid

deceleration, past the point of maximum energy recovery (1). Fortunately, everyday

driving seldom involves emergency braking; in normal driving cycles, regenerative

braking is responsible for most of the vehicle’s deceleration.

In front wheel driven EV’s energy can only be recovered at the front axle, with

the rear axle using friction braking. There exists an ideal curve for the distribution of

braking force on the front and rear wheel axles (to avoid rear-wheel lockup) that a

braking controller must strive to achieve (7). Typically, this means that about 35% of the

energy that is dissipated at the rear wheels is unavailable for regeneration (1).

12

Another goal of braking controllers is to provide braking feel similar to that of

conventional hydraulic brakes, as this is what most drivers are familiar with. Today’s

electric vehicles have such advanced braking controllers which strive to simulate such

braking feel that in these author’s test drives of GM’s EV-1, no difference in braking was

noticeable. Indeed there have been many patents awarded for controllers that make

efforts to ensure adequate feel by, for example, varying the time lag between depression

of the brake pedal and application of regenerative braking (10, 14).

Recently, regenerative braking has been integrated with anti-lock braking to

provide electric vehicles with superior deceleration performance (1). Anti-skid control

systems require that regenerative braking be momentarily deactivated when slip is

detected, so that the wheels are allowed to unlock (3,10). In this way safety is not

compromised, and some energy can still be recovered.

Regenerative braking can recover 25-30% of the kinetic energy lost during

braking. Depending on the particular driving cycle, this can result in as much as a 20%

recovery of total energy, effectively increasing the vehicle range by the same amount. In

electric vehicles, where range is the primary limitation, a 20% increase is extremely

valuable. In the following pages we present our own model of a regenerative braking

system, and discuss implementation and performance of various braking controllers.

2.3 System Design Overview

In this class project, our model is built based on a configuration that is most popular

among current commercialized OEM systems: Regen (a popular abbreviation in the

industry for regenerative braking) in the braking system with supplemental hydraulic

braking and ABS. Toyota’s RAV-4, Electric SUV, GM’s EV1 and Ford’s Ranger EV all

use this configuration. The GM EV1 also employs an additional rear electric braking to

reduce the total size and weight of braking system but the brake torque controls strategy

is the same. This type of system can achieve brake torque management by optimized

13

blending between Regen and friction braking as well as optimized torque distribution

between front and rear, so it is widely applied to the current design of EV and HEV’s.

In order to ensure brake safety, the system is constructed based on a “X” dual-

circuit hydraulic system that groups the 4-wheel disc brakes in to LF-RR and RF-LR

lines. The car is front wheel drive so the motor only receives regenerative braking torque

from the front axle. The Braking Torque Control Module (BTCM) manages the braking

torque blending between the two torque sources. It also implements ABS when the

wheels are locked up.

To simplify the system a bicycle model is applied in this class project but the

strategy of torque blending between hydraulic and Regen, and that of torque split

between front and rear, are equal to the full-car model. The design specifications of the

EV or HEV are shown in Table 1.

14

Table 1. Vehicle Specifications

Testing Mass (with one driver) 1510 kg (Curb 1450kg, Driver 60kg)

Length 4,300 mm.

Width 1,760 mm.

Height 1,300 mm.

Height of Center of Gravity (CG) 600 mm

Wheelbase 2,500 mm.

Front Axle to CG (a) 1,100 mm

Rear Axle to CG (b) 1,400 mm

Frontal area 1.89 m2.

Coefficient of Aerodynamics 0.19

Top Speed 128Km/Hr (80 Mile/Hr.), electronically

regulated

Tire 175/65R14 High Pressure, Low

Resistance, Rolling Radius 0.31m

Reduction Gear Ratio 10:1

Top Motor Speed 11,000rpm @ 80MPH

2.4. Brake System

As shown in Figure 3b, the heart of the system is the Brake Torque Control Module

that manages brake torque distribution and blending. Based on a variety of inputs, the

BTCM calculates the demanded vehicle deceleration and total braking torque, and

divides them into the Regen demand and hydraulic torque. The hydraulic torque is again

split for the front and rear axles.

15

RegenOn/OffSwitch

Front AxleHydraulic

Pressure Modulator

Regen Controller ACMotor

ReductionGear

Battery Pack

PowerInverterAC/DC

Battery

SOC

VehicleSpeed

FrontWheelSpeed

Rear HydroBrake Rr. Whl.

Cylinder

Frt. Whl.Cylinder

Mstr BrkCylinder

BrakePadel

Rear AxleHydraulic

Pressure Modulator

ProportionalPressureValve

LinePressureTransducer

RegenTorqueCommand

RearWheelSpeed

Front HydroBrake

BTCM

Figure 3b. Brake system layout

Inputs to the BTCM include:

• Master Cylinder Line Pressure -- The brake line pressure transducers sense line

pressure at the outlet of the master cylinder, and send the pressure signal to the

BTCM.

• Regenerative Braking On/Off – the customer has a choice to enable or disable

regenerative braking.

• Battery State of Charge – the Battery State of Charge is read via the serial link

between battery control module and BTCM. BTCM uses SOC signal to calculate the

torque limit of Regen braking.

16

• Vehicle Speed – Vehicle speed is read from the output shaft of motor. Since there is

a fixed reduction ratio between motor and wheel the output shaft speed is

proportional to vehicle speed.

• Wheel speed – the wheel speed sensors feed wheel speed signal to the BTCM for

judgement of tire slip ratio and lock-up.

Outputs from the BTCM include:

• Pressure reduction command to the Front Axle Hydraulic Pressure Modulator –

reduces hydraulic braking torque on the front axle to maximize Regen braking portion

and to avoid wheel lock-up.

• Pressure reduction command to the Front Axle Hydraulic Pressure Modulator –

serves solely for ABS pressure release to avoid lock-up at the rear axle.

• Regen Torque command – tells the motor/inverter assembly how much regenerative

braking torque to apply.

2.5. Motor/Inverter Torque Characteristics during Regen Braking

The braking torque characteristics of an HEV/EV electric motor is like a flipped

driving torque curve. However, it not only depends on the speed of the motor but also

depends on the State Of Charge (SOC) of the battery.

17

0 10 20 30 40 50 60 70 800

200

400

600

800

1000

1200

1400

1600

1800SOC=0%

SOC=20%

SOC=40%

SOC=60%

SOC=80%

SOC=100%, Torque=0

Figure 4. Motor/Inverter torque characteristics

Figure 4 shows the brake torque characteristics of GM’s EV1 motor, which is

representative of an electric vehicle motor in braking. We have borrowed this model in

our project. Clearly the brake torque shows different relations to the motor speed in three

speed bands -- low speed, mid speed and high speed, with linear saturation by SOC.

1. Low Speed Band (0-10mph)

At very low speed, from 0 to 10mph, the braking torque increases with motor speed.

T = 170 * V*(1-SOC), 0<V<=10mph

T – Torque, Nm

V – Vehicle Speed, mile/hour

SOC – Battery State of Charge, 100% is full and 0% is empty

2. Mid Speed Band (Torque Saturation):

The medium speed band is where the motor can fully utilize its torque capacity but

the torque is saturated by the inverter’s capacity. Therefore the torque is a constant

but at a high level of output.

18

T = 1700(1-SOC), 10<V<=30mph

3. High Speed Band (Power Saturation):

In the high speed band the torque is limited by the limit of power output of the

motor/inverter system. It initially decreases rapidly, but the decreasing rate slows

down. The EV has a maximum speed of 80m/h corresponding to the red line speed of

11,000 rpm of motor, which is also a cutoff line of Regen torque. The polynomial

approximation in this band is:

T = -0.0119V3+4.3578V2–317.4946V + 7840, 30<V≤80mph

2.6. Priorities in Control Design

1. Avoid wheel lock-up under any circumstances and ensure full ABS function in

emergency – wheel lock-up is one of the most hazardous conditions for the occupants

in a vehicle. In Regen braking system most of the wheel lock-up happens with high

brake torque demand, which requires assistance of friction braking. However, in the

case of very low road traction, such as road coefficient of friction as low as 0.1, only

motor braking torque will be high enough to lock up front wheels. The real wheels

will not provide any brake force so that the vehicle will lose steering and stop in an

excessively large distance. Therefore, the controller has to disable regenerative

braking and brake entirely by friction. This allows the sophisticated ABS brake

system to maintain stability and maneuverability of the vehicle and also provide the

maximum braking force.

2. Meet driver’s brake demand, to the extent of road surface traction.

3. Achieve maximum regeneration gain in all conditions -- as mentioned before, the

efficiency of regenerative braking should be targeted as high as possible. However,

there are constraints from the motor brake torque characteristics and the desired

deceleration.

19

4. Protect battery from overcharge or spike-charge and ensure safety – continuous over

charging, or transient charging spikes are undesirable since they will damage the

battery. Three measures have to be applied: 1) The regenerative brake torque has to

abide the SOC limit of battery, by not exceeding the torque limit of certain SOC. 2)

During ABS activation, Regen has to be disconnected to protect the battery and the

circuit from the abrupt voltage change. 3) At very low speeds the current output from

the AC motor will oscillate at low frequency so the DC output will become spiky.

Additional charging output may be lower than the battery voltage so the vehicle will

be reversed. Regen has to be disabled to avoid the above scenarios.

2.7. Brake Torque Blending

Table 2. Regen Control Logic

Regen Front Hydraulic Rear Hydraulic

(1) ABS activation Cut-off BTCM Torque

Command

BTCM Torque

Command

(2) Regen capacity

greater than the sum of

demanded front and

rear, with exception of

(1)

Activated

TRegen=Tdemand

Energy recall efficiency high

Deactivated Deactivated

(3) Regen Capacity

greater than front

demanded torque, with

exception of (1)

Activated

Tdemand= TRegen+THydro, rear

Energy Recall Efficiency

Lower than (2)

Deactivated Activated

(4) Regen Capacity

greater than less than

front demanded torque,

with exception of (1)

Activated

Tdemand= TRegen+ THydro, front

+ THydro, rear

Energy Recall Efficiency

Lowest

Activated Activated

Under normal brake conditions without wheel lock-up, the brake torque is

distributed to the front and rear at a constant ratio of 2:1, which approximately

20

corresponds to a conventional brake design, with 0.6 road surface coefficient of friction.

In implementing Regen braking, the brake torque is blended according to the logic shown

in Table 2.

Two scenarios are shown in the following discussion explain the torque

management of the system.

(1). Brake Torque Demand can be lower than the Regen Capacity Limit. As shown in

Figure 5, assuming a battery SOC at 40%, with speed of 52MPH, the BTCM sensed a

brake line pressure that corresponds to a constant brake torque request of 1200N-m.

At this speed the Regen torque upper limit is as low as only 200N-m, so most of the

torque has to be generated by the hydraulic system to obtain an overall front/rear

brake torque ratio of 2:1. As the vehicle slows down the Regen torque continue to

increase following the polynomial curve and the hydraulic front brake torque

decreases at the same rate to maintain the torque demand. At 37MPH the Regen not

only has taken over the duty of front hydraulic but also partially the rear. At around

34MPH where the Regen torque meets the total torque demand, all the brake duty is

taken over by Regen and the hydraulic brakes of both front and rear are deactivated).

This situation will last until the Regen torque decrease again in the low speed band

and the capacity limit again becomes lower than the torque demand. During the time

when all brake is Regen, the vehicle recalls energy from braking at the highest

efficiency because no friction braking is used.

21

0 10 20 30 40 50 60 70 800

200

400

600

800

1000

1200

1400

1600

1800Motor&Inverter Brake Torque Characteristics

Vehicle Speed, Mile/Hour

Bra

ke T

orqu

e, N

m SOC=40%

Torque Demand = 1200Nm

Regen Torque = 200 Nm @ 52 MPH

Rr Hydro = 200Nm

Regen Torque = Torque Demand @ 15MPH Frt Hydro = Rr Hydro=0

Vehicle Slows Down

Front Hydro =333Nm

Figure 5. Brake torque blending in scenario one

(2). In the case of a fairly high brake torque demand which always exceeds the Regen

braking capacity limit, such as 1200N-m at 40% SOC, although the Regen is kept at

the highest output, the hydraulic brakes have to be applied to assist attaining brake

torque demand. In this case, as shown in Figure 6, from initial speed of 37 MPH to

the end speed of 15MPH, the Regen brake torque varies with its capacity limit and the

remaining is obtained from front and rear hydraulic brakes.

22

0 10 20 30 40 50 60 70 800

200

400

600

800

1000

1200

1400

1600

1800Motor&Inverter Brake Torque Characteristics

Vehicle Speed, Mile/Hour

Bra

ke T

orqu

e, N

m

SOC=40%

Torque Demand = 1200Nm

Regen Torque = 200Nm Nm @ 52 MPH

Rr Hydro = 400Nm

Regen Torque = 1020Nm

Vehicle Slows Down ......

Front Hydro =600Nm

Regen Torque = 600 Nm @ 37 MPH

Frt Hydro=200Nm

Rr Hydro = 400Nm

Rr Hydro=180NmFrt Hydro= 0

Figure 6 Brake torque blending in scenario two

Regenerative Braking System for Electric VehicleME568-Vehicle Control Systems Term Project

by Youji Ma, Ping Yu, and Michael Fisher

December, 1999

rear axledisc brake

-K-

propotionalvalve

motor/generator

gear

front axledisc brake

Vehicle finalspeed (mph)

Vehicle

Vehicle Initial speed (mph)

time

Slippery road factor

Double click to plotf, r, regen torques

Run the model first

Double click to plotregen power & energy

Run the model first

Road % slop

Regencontroller

RegenOn/Offswitch

Rear axlehydraulicpressure

modulatorMastercylinder

Double click formore information

Front axlehydraulicpressure

modulator

Double clickto run model

Brake pedal force (N)

+ -

Battery state of charge

Double click to plotvehicle speed

Run the model first

Double click to plotwheel speed & slip

Run the model first

rearpressure

pressure reduction

pressure reduction

demanded torque

speed

speed

pressure

pressure

motor torque

frontpressure

electric power

rear axle torque

torque

axle toque

24

3. System Modeling

3.1. Tier 1 module

Matlab and Simulink were used to model the regenerative braking system. The

whole system includes 7 files:

Rbs.mdl Main Simulink model file

Runrbs.m M file that is called by rbs.mdl, can also run directly in Matlab.

Rbsdata.m M file that specify system parameters, is called by rbs.mdl for initialization

Plot_vs.m M file that plot vehicle speed, is called by rbs.mdl

Plot_t.m M file that plot braking torques, is called by rbs.mdl

Plot_e.m M file that plot regenerative braking power and energy, is called by rbs.mdl

Plot_s.m M file that plot wheel speed and slip, is called by rbs.mdl

The Simulink model of the system is shown in Figure 7 (previous page). All the

masks that have green background are model inputs or parameters that can be adjusted to

simulate different driving scenarios. When they are double clicked, the user will be

prompted to input values. All the input values are real numbers except “Regen on/off

switch”, which is a check box. The icons of the input masks and their meanings are listed

as follows:

Battery state of charge. 1 means full and 0 means empty. Its Simulink

model is shown in Figure 8.

Regen on/off switch, toggle on to activate regenerative braking, toggle off

to turn it off.

Driver braking force. Under normal road condition and 0 inclination angle,

the braking force/ deceleration ratio is roughly 38 N/ 0.1g.

25

Vehicle initial speed before braking.(mph)

Vehicle final speed after braking.(mph)

Slippery road factor. 1 means ideal dry road.

Percentage road grade. 0 is flat road, positive means uphill and negative

means downhill.

Figure 8. Simulink model of battery state of charge subsystem

All the masks that have red drop shadows are executable. Their icons and meanings

are listed as follows.

Double click to run the model

Double click to plot vehicle speed (must run the model first)

Double click to plot front hydraulic, rear hydraulic, and

regenerative braking torques (must run the model first)

26

Double click to plot regenerative braking power and energy vs.

vehicle kinematic and potential energy, or instantaneous and

overall efficiency (must run the model first)

Double click to plot front and rear wheel speed and slip (must

run the model first)

Double click to show help information and acknowledgements

There are other masked components in the model including:

The master cylinder is a proportional gain which transfer the driver braking

force into hydraulic braking pressure.

The disc brake is also a proportional gain which transfer hydraulic braking

pressure into braking torque.

The gear connects the motor to the front axle. In electric vehicle the

transmission usually is a constant ratio gear set. In the model, the gear is also

a proportional gain that transfer motor torque to front axle torque.

The motor/generator generate braking torque and electric power during

regenerative braking, its Simulink model is shown in fig.

Figure 9. Simulink model of motor/generator subsystem

27

The model is created such that the depth of subsystem hierarchy is controlled within

three layers. The major four subsystems of the model are vehicle subsystem, front axle

hydraulic pressure modulator subsystem, rear axle hydraulic pressure modulator

subsystem, and Regen controller subsystem.

3.2. Vehicle subsystem

The Simulink model of vehicle subsystem is shown in Figure 10

(separate page). Bicycle model is used. The function of the vehicle

subsystem is to model the dynamics of front axle, rear axle, and

vehicle. It also includes modules that calculate vehicle kinematic and

potential energy and their rate of change so that the efficiency of

regenerative braking can be evaluated. It can also evaluate the program

termination criteria (from current speed and vehicle final speed) for

stopping model execution.

The major subsystems within vehicle subsystem include:

Front and rear slip calculation subsystem. Its Simulink model is shown

in Figure 11.

Figure 11. Simulink model of front and rear slip calculation subsystem

This block is used to calculate the dynamics of vehicle and

front and rear axles

1

speed

f(u)

wind resistance

u(3)

vehiclespeed

rw

rw1

rw

rw

s

1

rear wheelspeed

1.6*

mph to m/s1

1.6*

mph to m/s

m*g*f

m*g*f

m*g

m*g

lower limit2

lower limit1

lower limit

f(u)

kinematic energylost rate

s

1

front wheelspeed

s

1

Vehicle speed

cos

sin

p_power

p_energy

k_power

k_energy

distance

speed

atan(u[1]/100)

Theta

SwitchSTOP

Mu-sllipfriction curve

Fxf&

Fxr

Fxf & Fxr

s

1

s

1

s

1

0

1

Calculatefront & rear slip

Calculate front & rear slip

1/rw

1/rw2

1/rw

1/rw1

1/rw

-K-

1/2/Jwr

-K-

1/2/Jwf

-1/m

-1/m

6

Road % slop

5

slippery road factor

4

rear torque

3

front torque

2

vehicle finalspeed (mph)

1

vehicle initialspeed (mph)

front & rear mu

fxr

fxf

Rx

m/s

front & rear slip

m/s

29

Mu-slip calculation subsystem, which use a polynomial curve fitting to

calculate the friction coefficients of front and rear axles. Its Simulink

model is shown in Figure 12.

Figure 12. Simulink model of mu-slip calculation subsystem

Fxf & Fxr calculation subsystem. Its Simulink model is shown in

Figure 13.

Figure 13. Simulink model of Fxf & Fxr calculation subsystem

30

3.3. Front and rear axle hydraulic pressure modulator subsystems

The models of front axle hydraulic pressure modulator and rear axle

hydraulic pressure modulator are very similar. The Simulink model of

front modulator is shown in Figure 14. Its functions include pressure

modulation for anti-lock purpose and pressure modulation for

regenerative braking purpose. For the anti-lock part, it is basically a

rule-based controller which includes low speed cutoff.

Figure 14. Simulink model of front axle hydraulic pressure modulator subsystem

31

3.4. Regen controller subsystem

The model of Regen controller is shown in Figure 15. The function of

this subsystem is to determine the distribution of braking torques

among Regen braking, front axle hydraulic braking, rear axle hydraulic

braking. Its inputs are driver demand (through a pressure transducer on

the output port of the master cylinder) and Regen torque capability

(through a subsystem). It is basically a rule based controller that applies

different control strategies for three different scenarios:

1. Regen braking capability is larger than the sum of front and rear

axle demand torque

2. Regen braking capability is larger than the demand torque of front

axle but less than the sum of front and rear axles

3. Regen braking capability is less than the demand torque of front

axle

Figure 15. Simulink model of Regen controller subsystem

The major subsystems within Regen controller subsystem include:

32

The Simulink model of Regen on/off controller subsystem is shown in

Figure 16. Its function is to determine whether the regenerative braking

system should be turned on based on driver switch and wheel slip. If

lock happens to either front or rear wheel, the regenerative braking is

automatically turned off so that the braking pressure modulator systems

can execute anti-lock control.

Figure 16. Simulink model of Regen on/off controller subsystem

The Simulink model of Regen torque capability subsystem is shown in

Figure 17. Its function is to determine the torque capability of

regenerative braking based on battery state of charge and motor speed

(vehicle speed since the gear ratio is constant) through three sections of

polynomial curve fitting.

Figure 17. Simulink model of Regen torque capability subsystem

33

4. Model Analysis

Different braking scenarios are run on the model. Unless otherwise specified, the

parameters used in the model are:

Battery state of charge is 0.5

Regen is on

Slippery road factor is 1

Percentage road grade is 0

4.1. Scenario 1: From 55mph to 0mph with small braking force

The braking force is 50 N. Figure18 shows the brake distribution among the Regen

braking, front and rear hydraulic braking. The Regen braking takes over all the hydraulic

braking over a period of time.

0 2 4 6 8 10 12 14 16 18 200

100

200

300

400

500

600Brake torques

Tor

que(

Nm

)

Time(secs)

regen front hydraulicrear hydraulic

Figure 18. Torque distribution in scenario 1

Figure 19 shows the Regen power and vehicle energy losing rate. The instantaneous

efficiency of regenerative braking can be seen. The effect of the Regen torque capability

on the two ends of vehicle speed can be clearly seen.

34

0 2 4 6 8 10 12 14 16 18 200

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5x 10

4 regen power vs. vehicle energy lost rate (Kinematic & potential)

pow

er (

W)

Time(secs)

regen vehicle

Figure 19. Regen power in scenario 1

Figure 20 shows the Regen energy and vehicle lost energy. The overall efficiency

of regenerative braking is about 66.1%.

0 2 4 6 8 10 12 14 16 18 200

0.5

1

1.5

2

2.5

3

3.5

4

4.5x 10

5 regen energy vs. vehicle lost energy (Kinematic & potential)

ener

gy (

J)

Time(secs)

regen vehicle

Figure 20. Regen energy in scenario 1

Due to there is torque transfer between the front and rear axles. The wheel slips will

also change. Figure 21 shows the wheel slips of front and rear axles.

35

0 5 10 15 20-0.025

-0.02

-0.015

-0.01

-0.005

0Front wheel slip

Slip

Time(secs)0 5 10 15 20

-0.01

-0.008

-0.006

-0.004

-0.002

0Rear wheel slip

Slip

Time(secs)

Figure 21. Front and real wheel slips in scenario 1

4.2. Scenario 2: From 55mph to 0mph with moderate braking force

The braking force is 100 N. Figure 22 shows the brake distribution among the

Regen braking, front and rear hydraulic braking. The Regen braking takes over all the

front hydraulic braking over a period of time but never takes over all the rear hydraulic

braking.

0 1 2 3 4 5 6 7 8 9 100

100

200

300

400

500

600

700

800

900Brake torques

Tor

que(

Nm

)

Time(secs)



Figure 23 shows the Regen power and vehicle energy lost rate.

36

0 1 2 3 4 5 6 7 8 9 100

1

2

3

4

5

6

7

8

9x 10


pow

er (

W)

Time(secs)

regen vehicle




0 1 2 3 4 5 6 7 8 9 100

0.5

1

1.5

2

2.5

3

3.5

4

4.5x 10


ener

gy (

J)

Time(secs)

regen vehicle


Due to there is torque transfer between the front and rear axles. The wheel slips will

also change (figures not shown).

37

4.3. Scenario 3: From 55mph to 0mph with large braking force

The braking force is 150 N. Figure 25 shows the brake distribution among the

Regen braking, front and rear hydraulic braking. The Regen braking never takes over the

front hydraulic braking and the rear hydraulic braking torque never changes.

0 1 2 3 4 5 6 70

200

400

600

800

1000

1200Brake torques

Tor

que(

Nm

)

Time(secs)




0 1 2 3 4 5 6 70

2

4

6

8

10

12

14x 10


pow

er (

W)

Time(secs)

regen vehicle




38

0 1 2 3 4 5 6 70

0.5

1

1.5

2

2.5

3

3.5

4

4.5x 10


ener

gy (

J)

Time(secs)

regen vehicle


Since there is no torque transfer between front and rear axles, the wheel slips

remain unchanged.

4.4. Scenario 4: From 55mph to 0mph with large braking force on slippery road

(enough to lock the wheels)

The inputs are the same as the ones in scenario 3 except the road slippery factor is

0.5. Figure 28 shows the brake distribution among the Regen braking, front and rear

hydraulic braking. Since lock-up happens first to the front wheel, the Regen system is

shut off and ABS takes over the control of front and rear hydraulic pressure and then,

braking torques.

39

0 1 2 3 4 5 6 7 80

200

400

600

800

1000

1200Brake torques

Tor

que(

Nm

)

Time(secs)



Figure 29 shows the Regen power and vehicle energy lost rate. Regen is shut off for

most of the time.

0 1 2 3 4 5 6 7 80

2

4

6

8

10

12x 10


pow

er (

W)

Time(secs)

regen vehicle



of regenerative braking is about 0.2% because the top priority is safety rather than

efficiency under the conditions.

40

0 1 2 3 4 5 6 7 80

0.5

1

1.5

2

2.5

3

3.5

4

4.5x 10


ener

gy (

J)

Time(secs)

regen vehicle


The front and rear wheel speeds are shown in figure 31(a). The front and real wheel

slips are shown in figure 31(b).

0 2 4 6 80

10

20

30

40

50

60

70

80Vehicle speed and front wheel speed

Spe

ed(r

ad/s

ec)

Time(secs)

vehicle speed front axle speed

0 2 4 6 80

10

20

30

40

50

60

70

80Vehicle speed and rear wheel speed

Spe

ed(r

ad/s

ec)

Time(secs)

vehicle speed rear axle speed

Figure 31(a). Front and rear wheel speed in scenario 4

0 2 4 6 8-1

-0.8

-0.6

-0.4

-0.2

0Front wheel slip

Slip

Time(secs)0 2 4 6 8

-0.25

-0.2

-0.15

-0.1

-0.05

0Rear wheel slip

Slip

Time(secs)

Figure 31(b). Front and rear wheel speed in scenario 4

41

4.5. Scenario 5: From 35mph to 0mph with small braking force

The braking force is 50 N. Figure 32 shows the brake distribution among the Regen

braking, front and rear hydraulic braking. The Regen braking takes over all the hydraulic

braking almost all the time.

0 2 4 6 8 10 120

100

200

300

400

500

600Brake torques

Tor

que(

Nm

)

Time(secs)




0 2 4 6 8 10 120

0.5

1

1.5

2

2.5

3x 10


pow

er (

W)

Time(secs)

regen vehicle




42

0 2 4 6 8 10 120

2

4

6

8

10

12

14

16

18x 10


ener

gy (

J)

Time(secs)

regen vehicle


4.6. Scenario 6: 35mph coasting 15% downhill

The braking force is 53.7N. Figure 35 shows the speed of the vehicle. Figure 36

shows the brake torque distribution among the Regen braking, front and rear hydraulic

braking. The Regen braking takes over all the braking all the time.

0 1 2 3 4 5 6 7 8 9 10

5

10

15

20

25

30

35

40

45

50Vehicle speed

Spe

ed(m

ph)

Time(secs)

Figure 35. Vehicle speed in scenario 6

43

0 1 2 3 4 5 6 7 8 9 100

100

200

300

400

500

600

700

800

900

1000Brake torques

Tor

que(

Nm

)

Time(secs)




0 1 2 3 4 5 6 7 8 9 100

0.5

1

1.5

2

2.5

3

3.5x 10


pow

er (

W)

Time(secs)

regen vehicle




44

0 1 2 3 4 5 6 7 8 9 100

0.5

1

1.5

2

2.5

3

3.5x 10


ener

gy (

J)

Time(secs)

regen vehicle


4.7. Scenario 7: ECE-15 fuel economy testing cycle

ECE-15 (ECE R84) fuel economy testing cycle has 15 working conditions as shown

in Fig. Four of the fifteen working conditions in the cycle are decelerations that are

highlighted in the Figure 39.

ECE-15 Fuel Economy Testing Cycle

0

10

20

30

40

50

60

0 20 40 60 80 100 120 140 160 180

time(second)

spee

d (

km/h

)

Figure 39 ECE fuel economy testing cycle

45

The specifications for the four decelerating conditions are:

Vehicle speed Time

1 15km/h - 0km/m (9.375mph - 0mph) 5 seconds

2 32km/h - 0km/m (20mph - 0mph) 11 seconds

3 55km/h - 35km/m (34.375mph – 21.875mph) 8 seconds

4 35km/h - 0km/m (21.875mph – 0mph) 12 seconds

The performances of regenerative braking in each deceleration condition and its

overall performance are listed in following table.

Force Recovered Energy Vehicle lost energy Efficiency

1 29.5 N 1.0266×104 J 1.2239×104 J 83.88%

2 28.5 N 4.8725×104 J 5.5703×104 J 87.47%

3 23.9 N 8.4344×104 J 9.7491×104 J 86.51%

4 28.7 N 5.8433×104 J 6.6637×104 J 87.69%

Overall 2.0177×105 J 2.3207×105 J 86.94%

The efficiency of regenerative braking is very high in the ECE-15 fuel economy

testing cycle. This is due to that the speed in the cycle is moderate. In all the four

decelerating conditions the regenerative braking provides all the braking torque for most

of the time.

46

5. Conclusions

Our system model shows that performance of regenerative braking varies

depending on various driving conditions.

• The energy recovery efficiency is highest within a speed band of 10 to 40 mph.

• The energy recovery efficiency is highest in cases of relatively low battery state of

charge.

• The energy recovery efficiency is highest in cases of relatively low deceleration,

where friction braking or ABS are unnecessary.

In all cases, our system was able to provide adequate brake force, combining

regenerative braking with friction braking and ABS, where appropriate. Safety was not

compromised, but high recovery rates were still attained.

Our recommendations for future work include addition of feedback for the battery

state of charge, which changes considerably during a lengthy driving cycle. Effort could

be invested in obtaining a more comprehensive model of the motor/inverter assembly

and, for more complete accuracy, a full car model should be considered. Also, sensor

data to monitor any disturbances and inherent noise could be incorporated.

47

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48

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50

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Regen Report

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