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CHAPTER 5
FORMULATION OF CONTROL STRATEGY AND
PROTOTYPE DEVELOPMENT
5.1 INTRODUCTION
Plug-in hybrid implementation in a two-wheeler is a good tradeoff
between an electric and IC engine power which ensure sufficient all-electric
range and minimum emissions as well. For heterogeneous India’s traffic
pattern, a single operating mode of the vehicle cannot satisfy the driving
pattern. In order to formulate the control strategy, all types of driving modes
need to be considered. The development of prototype vehicle involves the
design of control system with control strategy suitable for Indian city driving
conditions. This chapter discusses the formulation of control strategy and
development of control system followed by the conversion of selected base
two-wheeler into of plug-in hybrid electric two-wheeler.
5.2 FORMULATION OF CONTROL STRATEGY
The integration of conventional vehicle components with electric
propulsion components results in a vast number of potential hybrid electric
configurations. The series hybrid electric configuration is an interesting
solution for driving in urban areas with passenger cars, light duty vehicles as
well as with heavy-duty vehicles like city buses. On the other hand, parallel
hybrid electric powertrain configuration is more suitable for the family
or higher class vehicle segment, while driving on highway and long
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distances. In addition, a series-parallel hybrid powertrain system has a
complex transmission and increase in the number of components also makes
the integration more complicated (Montazeri-Gh et al 2006). As the
complexity of the vehicle configuration is increases, so do the demands for
control. As people may expect, there is no universal architecture that can be
considered superior in all practical aspects such as energy efficiency, vehicle
performance and range, driver comfort, manufacturing complexity, and
production cost. Therefore, in practice, automakers may choose different
architectures to achieve different goals and meet distinct transport segment
requirements.
Besides the powertrain configuration, a suitable power and energy
distribution system is also important. The control strategy plays a basic role.
A control strategy is an algorithm that manages the power split between the
IC engine and the electrical machine in order to reduce fuel consumption and
pollutant emissions. In a plug-in hybrid electric vehicle, the strategy will
attempt to use most of the energy from the battery pack. However, majority of
global two-wheelers population utilizes small displacement engines, generally
in the order of 50-150 cc. Hence, for two-wheelers of simple architecture with
low cost operation, there is a need to develop a simple powertrain design with
a simple control strategy which is less complex and easy for retro-fitment.
Electrification of kilometres through charge depleting operation in a
PHETW is expected to be a cost-effective way to continue to reducing fuel
consumption beyond HEV technology capabilities. The designed control
strategy does not necessarily provide maximum fuel savings over all driving
demand. This is because the national average daily travelled distance by
two-wheelers in India is close to 24 km/day. As per the survey, it is also
observed that about 61% of two-wheelers drive less than 25 kilometres per
day. Only 7% of two-wheelers travel more than 50 km per day and about 32%
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of two-wheelers travel in between 25 to 50 km per day. Therefore, choosing
the right electric range to handle the daily driving needs is essential. The
control system should utilize a strategy modified in real time depending on
the input from various sensors in the system.
Two control strategies can be applied to PHEV: the all-electric
strategy and the blended strategy (Markel 2007). In all-electric strategy, the
electric motor supplies all the power needed for the vehicle until the battery
reaches the predetermined minimum SOC level. In blended strategy, both
motor and engine work together to provide the power requirements. In this
work, both all-electric and blended strategies have been adopted to suit
two-wheelers in Indian cities to realize better driving performance and good
energy management. In all-electric strategy, it has been planned to cover
average daily travel distance with zero emissions. However, by selecting the
blended strategy at the beginning of the journey itself, the vehicle can travel
more than double the all-electric range with improved fuel economy and
minimum emissions. In both the strategies, the energy from the battery pack
has charge depleting mode. Therefore, three distinct modes were derived and
the switching logic was drafted for each mode of driving. The three modes of
operation are namely electric mode, hybrid mode and engine mode. The
electric mode uses all-electric strategy and hybrid mode uses blended
strategy, whereas engine mode is similar to conventional vehicle operation.
Figure 5.1 shows the simple flow chart of plug-in hybrid electric two-wheeler.
The rider can select the modes based on the driving range and battery pack
state-of-charge.
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Figure 5.1 Plug-in hybrid electric two-wheeler flow chart
5.2.1 Electric Mode
The electric mode of the prototype vehicle aims at providing an
eco-friendly transportation solution in urban driving. In this mode, the
converted plug-in hybrid electric two-wheeler utilizes power from the battery
alone with zero tail-pipe emissions. The charge-depleting all-electric strategy
emphasizes all electric vehicle operation over a desired distance in which
battery discharges to a minimum threshold. So, this mode has all the
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advantages of electric vehicle. The developed prototype vehicle has been
designed to provide an all electric range of about 25 km with further scope for
improvement, as range is a function of both battery size and amperage.
Increasing the energy capacity of the battery pack provides the ability to
extend the driving distance using electricity, but it would increase the
incremental cost of the battery.
5.2.2 Hybrid Mode
The effectiveness of fuel consumption in hybrid mode depends not
only on vehicle design, but also on the control strategy used. It defines how
and when power and energy will be provided or consumed by various
components of the vehicle (Markel and Wipke 2001). The charge-depleting
blended strategy of hybrid mode in plug-in hybrid electric two-wheeler aims at
providing the maximum utilization of the available energy - battery and IC
engine to run the vehicle. This mode is primarily meant for striking a balance
between emissions and the range of the vehicle.
In the hybrid mode, the control strategy is formulated in such a way
that the IC engine idling and low power modes could be eliminated to a great
extent. For the starting of the vehicle and at low speed-high torque region, the
battery pack supplies the power to the hub motor. The engine is OFF during
idling and low-load driving conditions where the engine efficiency would be
low. However, an electric motor provides high efficiency at low load when
compared to an engine. By adopting this control strategy, IC engine idling and
inefficient engine operation at low power modes are eliminated, which in turn
improves the overall efficiency and reduces the fuel consumption, thus reducing
emissions.
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SPEED >
SET SPEED
Figure 5.2 Hybrid mode flow chart
Figure 5.2 shows the logic flow chart of hybrid mode. The IC engine
takes over only when the speed of the vehicle exceeds the set-speed after a
delay of 5 seconds. The engine will be switched off when the speed of the
vehicle reaches below the set speed and remains in that state for about 5
seconds. However, the set-speed can be varied using the key pad built in the
control system. In the hybrid mode, the IC engine delivers power for high
speed driving and for hill climbing, while the electric wheel hub motor in the
front wheel is engaged for low speed driving. A unique feature of the control
strategy helps in eliminating the idling and low power operations of engine
for better fuel economy.
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5.2.3 Engine Mode
If the total distance to be travelled on a given particular day is more
than the daily average travel distance or if the battery packs SOC level drops
below the minimum threshold limit, the system will permit neither the electric
nor the hybrid mode. The conventional engine mode can be switched ON.
Hence, unlike the electric two-wheeler the range is not limited. This mode is
primarily meant for high speed driving and cruising wherein speed variations
are minimum. Better utilization of the IC engine is done at higher speeds and
the driver has the option of choosing the engine mode during off-peak traffic
hours etc. Since the electric and hybrid modes of a developed Plug-in hybrid
electric two-wheeler are designed for single rider with an average weight of
70 kg, whenever the pillion weight is added to the total vehicle weight the
engine mode will render support without losing the original performance of
the base vehicle.
5.3 DEVELOPMENT OF CONTROL SYSTEM
The control system is an important element in the development of
plug-in hybrid electric two-wheeler. It provides the path for flow of energy
between the various components when the vehicle is in motion. The main task
of the control system is to shift the power required by the vehicle between IC
engine and wheel hub motor. Figure 5.3 illustrates the block diagram of
control system with electronic accessories used respectively. The control
system utilizes a real time strategy depending on vehicle speed. Switching
from electric to hybrid mode and vice versa is facilitated by a microcontroller
which is provided with the above input. The control strategy is fed to the
controller in the form of a coded logic. So, based on the input signals, the
microcontroller decides the energizing of the corresponding relays so as to
actuate the respective relay. This microcontroller is programmed to work in
all the three modes of the control strategy.
Figure 5.3 Block diagram of control system
5.3.1 Microcontroller
The microcontroller is the heart of the control system that decides
the vehicle’s strategy and operation.
device, which integrates a number of the components of a microprocessor
system on a single chip. It has an inbuilt CPU (Central Processing Unit),
memory and peripherals to make it appear as a mini computer. The
microcontroller that has been used for this project is from the PIC (
Interface Controller) series. PIC microcontroller is the first RISC (Reduced
Input Set Computation)
(complementary metal oxide semiconductor).
separate bus for instruction and data, allowing simultaneous access of
program and data memory. EEPROM (Electrically Erasable Programmable
Read-only Memory), EPROM (Erasable Programmable Read
FLASH, etc., are some o
recently developed technology which is used in PIC 16F877. The data is
retained even when the power is switched off. Easy programming and erasing
are some of other features of PIC 16F877. Figure 5.4 shows the pin
of PIC 16F877.
Figure 5.3 Block diagram of control system
Microcontroller
The microcontroller is the heart of the control system that decides
the vehicle’s strategy and operation. Microcontroller is a general purpose
device, which integrates a number of the components of a microprocessor
system on a single chip. It has an inbuilt CPU (Central Processing Unit),
memory and peripherals to make it appear as a mini computer. The
troller that has been used for this project is from the PIC (Peripheral
erface Controller) series. PIC microcontroller is the first RISC (Reduced
Set Computation) based microcontroller fabricated in CMOS
(complementary metal oxide semiconductor). This microcontroller uses
separate bus for instruction and data, allowing simultaneous access of
m and data memory. EEPROM (Electrically Erasable Programmable
y Memory), EPROM (Erasable Programmable Read-only Memory),
FLASH, etc., are some of the memories. Of these, FLASH is the most
recently developed technology which is used in PIC 16F877. The data is
retained even when the power is switched off. Easy programming and erasing
are some of other features of PIC 16F877. Figure 5.4 shows the pin diagram
of PIC 16F877.
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Figure 5.3 Block diagram of control system
The microcontroller is the heart of the control system that decides
Microcontroller is a general purpose
device, which integrates a number of the components of a microprocessor
system on a single chip. It has an inbuilt CPU (Central Processing Unit),
memory and peripherals to make it appear as a mini computer. The
troller that has been used for this project is from the PIC (Peripheral
series. PIC microcontroller is the first RISC (Reduced
fabricated in CMOS
This microcontroller uses
separate bus for instruction and data, allowing simultaneous access of
m and data memory. EEPROM (Electrically Erasable Programmable
y Memory), EPROM (Erasable Programmable Read-only Memory),
f the memories. Of these, FLASH is the most
recently developed technology which is used in PIC 16F877. The data is
retained even when the power is switched off. Easy programming and erasing
are some of other features of PIC 16F877. Figure 5.4 shows the pin diagram
5.3.2 Signal Conditioning Unit
Microcontrollers are widely used for control in power electronics.
They provide real time control by processing analog signals obtained from the
system. A suitable isolation interface needs to be designed for interaction
between the control circuit and hig
unit (SCU) provides the necessary interface between a high power grid
inverter and a low voltage controller unit. The signal conditioning unit accepts
input signals from the analog sensors and gives a conditioned o
DC corresponding to the entire range of each parameter.
accepts the digital sensor inputs and gives outputs in 10 bit binary with a
positive logic level of +5V.
Figure 5.4 Pin diagram of PIC 16F877
5.3.3 Relay
A relay is an electrically operated switch
the coil of the relay creates a magnetic field which attracts a lever and
Signal Conditioning Unit
Microcontrollers are widely used for control in power electronics.
They provide real time control by processing analog signals obtained from the
system. A suitable isolation interface needs to be designed for interaction
between the control circuit and high voltage hardware. A signal conditioning
t (SCU) provides the necessary interface between a high power grid
inverter and a low voltage controller unit. The signal conditioning unit accepts
input signals from the analog sensors and gives a conditioned output of 0
DC corresponding to the entire range of each parameter. This unit also
accepts the digital sensor inputs and gives outputs in 10 bit binary with a
itive logic level of +5V.
Figure 5.4 Pin diagram of PIC 16F877
Relay
A relay is an electrically operated switch. Current flowing through
the coil of the relay creates a magnetic field which attracts a lever and
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Microcontrollers are widely used for control in power electronics.
They provide real time control by processing analog signals obtained from the
system. A suitable isolation interface needs to be designed for interaction
h voltage hardware. A signal conditioning
t (SCU) provides the necessary interface between a high power grid
inverter and a low voltage controller unit. The signal conditioning unit accepts
input signals from the analog sensors and gives a conditioned output of 0-5V
DC corresponding to the entire range of each parameter. This unit also
accepts the digital sensor inputs and gives outputs in 10 bit binary with a
. Current flowing through
the coil of the relay creates a magnetic field which attracts a lever and
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changes the switch contacts. The coil current can be on or off so relays have
two switch positions and they are double throw (changeover) switches. Relays
allow one circuit to switch a second circuit which can be completely different
from the first. For example a low voltage battery circuit can use a relay to
switch a 230V AC mains circuit. There is no electrical connection inside the
relay between the two circuits-the link is magnetic and mechanical.
Figure 5.5 Relay circuit
Figure 5.5 shows the typical relay circuit. The relay circuit is
designed to control the load. The load may either be a motor or any other
load. The load is turned ON and OFF through relay. The relay ON and OFF
is controlled by the pair of switching transistors (BC 547) and is connected in
the Q2 transistor collector terminal. The relay common pin is connected to a
supply voltage. The normally open (NO) pin is connected to load. When high
pulse signal is given to base of the Q1 transistors, the transistor conducts and
shorts the collector and emitter terminal, thus giving zero signals to the base
of the Q2 transistor. So the relay is in the OFF state. When low pulse is given
to base of transistor Q1 transistor, the transistor is turned OFF. At this point,
12 V is given to base of the Q2 transistor so that the transistor acts as the
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conductor and relay is turned ON. Hence the common terminal and NO
terminal of relay are shorted. At this point, load gets the supply voltage
through relay. Table 5.1 gives the switching operation of the relays.
In order to meet the switching functions, the control system has two
relays namely the start relay and stop relay. The control current and voltage of
relay is around 100 mA and 12 V respectively.
Table 5.1 Switching operation of relays
SpeedStart Relay
Position
Stop Relay
PositionOperation
Above 30 km/h NO NOPower flow from battery
to starter motor
Below 30 km/h NC
NC for certain
period and then
NO
Ignition switch is
switched OFF for a
certain period and then
switched ON
5.3.4 Inductive Proximity Sensor
Inductive proximity sensors generate an electromagnetic field and
detect the eddy current losses induced when the metal target enters the field.
The field is generated by a coil wrapped round a ferrite core, which is used by
a transistorized circuit to produce oscillations. The target, while entering the
electromagnetic field produced by the coil, will decrease the oscillations due
to eddy currents developed in the target. If the target approaches the sensor
within the so-called sensing range, the oscillations cannot be produced. Then,
the detector circuit generates an output signal, controlling a relay or a
switch. Figure 5.6 shows the lay-out of inductive proximity sensor. The
proximity sensor use reed switch-based technology, which offers reliability of
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up to 5 million cycles. The standard operating temperature range is
from 40°C to 125°C.
Figure 5.6 Lay-out of inductive proximity sensor
In this work, the sensor is mounted in the front wheel pointing it
towards a metal piece in the wheel and three hexagonal bolts on the wheel
disc are used as metal targets to sense the vehicle speed. Microcontroller
programmed to count as one revolution as three times bolt is sensed. This
speed is given as an input to the microcontroller in order to facilitate the
switching process. An LCD display is also connected to the circuitry that
displays the count value proportional to the wheel speed. Figure 5.7 shows the
inductive proximity sensor fitted with metal targets on the front wheel.
The energy management strategy is fed to the controller in the form
of a coded logic. Based on the input signals, the microcontroller decides the
energizing of the corresponding relays so as to actuate the respective relay.
This microcontroller is programmed to work in all the three modes of the
energy management strategy. Figures 5.8 and 5.9 show the logical circuit and
circuit board with accessories of control system.
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Figure 5.7 Front wheel fitted with proximity sensor
Figure 5.8 Circuit board and accessories of control system
Figure 5.9 Logical circuit of the control systemFigure 5.9 Logical circuit of the control systemFigure 5.9 Logical circuit of the control system
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5.4 PROTOTYPE DEVELOPMENT
Base vehicle platform used for the prototype development was a
commercially available 98cc, 2-stroke petrol vehicle. Figure 5.10 shows the
simple lay-out of a plug-in hybrid electric two-wheeler.
Figure 5.10 Lay-out and energy flow in a converted plug-in hybrid
electric two-wheeler
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The selected base two-wheeler for this work is modified into a
plug-in hybrid electric two-wheeler by retrofitting with wheel hub motor in
the front wheel and the battery pack placed at the foot rest area. The reason
for mounting the hub motor in the front wheel is due to the constraints in
modifying the existing two wheeler design with transmission set-up. The hub
motor drives the front wheel, whereas the IC engine drives the rear wheel
through continuously variable transmission (CVT), as in the existing vehicle.
A 48 Vt battery pack is a set of 4 batteries. The four batteries in the pack are
connected in series so that the output voltage is 48V and capacity is 20Ah.
The battery pack is placed at the foot rest. However, it can be placed below
the seat by adopting some modifications during design and manufacturing of
the vehicle. The inductive proximity sensor fitted with metal targets on the
front wheel gives speed of the vehicle to the microcontroller in order to
facilitate the switching process. A plug is provided for charging the battery
pack using a standard home power outlet through converters when the vehicle
is at rest. Figure 5.11 shows the converted plug-in hybrid electric
two-wheeler. Table 5.2 gives the specifications of the converted plug-in
hybrid electric two-wheeler.
The main switching circuit was located near the battery. This circuit
was made to operate on a separate 12V power source to ensure isolation of the
electrical supply of the control system from the main drive electrical system.
This is more advantageous as the control system is more fail-safe and can be
programmed to perform various other actions in case of a failure in the
electrical system of the main drive or an accident. Individual components of
the system operate at different currents and hence suitable wires need to be
used. The wiring has to be fool-proof to ensure that no short-circuiting occurs
under any circumstances. Similarly, it is important to note that there are no
open ends and the vehicle must be electrically safe for a person to handle.
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Figure 5.11 Plug-in hybrid electric two-wheeler prototype
Table 5.2 Specifications of a converted plug-in hybrid electric
two-wheeler
Specifications Converted Plug-in Hybrid Electric Two-Wheeler
Engine 2-stroke (SI)
Engine displacement 98cc
Ignition Electronic
Engine max. power 7.7 bhp @ 5500 rpm
Engine max. torque 1 kgm @4500 rpm
Transmission Automatic (CVT)
Kerb weight 132kg
Maximum speed 95 km/h
Electric motor800 Watt, 48 V hub drive BLDC traction motor,
rated torque 33 Nm @ 150 rpm
Battery 20 Ah, 12 V VRLA traction battery – 4 Nos.
Battery charging
time8 hrs
Wheel base 1,215 mm
Fuel tank capacity 6 litres
Tyre size 3.50x10 4 PR
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5.4.1 Operation of PHETW
There are two keys in the vehicle - one for the electric powertrain
system and another for IC engine powertrain system. In electric mode, only
electric powertrain key is turned ON, whereas in hybrid mode, both the keys
are turned ON. The speed of vehicle is detected by the speed sensor
(proximity sensor) and the signals are sent to the signal conditioning unit. The
signal conditioning unit converts the pulses from the sensor into an equivalent
(0 to 5V) range.
In the electric mode, the electric powertrain key is turned ON and
the IC engine powertrain key is turned OFF. Even as the vehicle accelerates
beyond the set speed, there is no trigger to crank the engine. Hence, the
vehicle continues to drive in the electric mode.
In the hybrid mode, both the electric powertrain key and the IC
engine powertrain key are turned ON. The vehicle starts initially using the
electric motor power and then accelerates up to the set-speed. As soon as the
vehicle crosses the set speed the IC engine is cranked using a starter motor
and drives the vehicle. The IC engine takes over only when the speed of the
vehicle exceeds the set-speed after a delay of 5 seconds. As the engine spools
up, the EMF from the alternator is greater than 12 V, which is fed to the stop
pin of the motor controller. This ensures cut-off of power supply to the motor.
The engine will be switched off when the speed of the vehicle reaches below
the set speed and remains consistent in that state for 5 seconds. However, the
set-speed can be varied by using key pad built in the control system.
In the hybrid mode, the microcontroller keeps the ignition relay
switched ON throughout the mode. The relay operating the starter motor is
switched OFF. When the speed of the vehicle crosses 30 km/h, after a delay
of 5 seconds, the relay operating the starter motor is switched ON for
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5 seconds only to avoid excess cranking. The engine spins and gains speed to
generate sufficient EMF (12-14 V). This is fed to the stop pin of motor
controller, and then power supply to the hub motor is cut-off. As the speed of
the vehicle goes below 30 km/h, the microcontroller keeps the ignition relay
switched OFF for 5 seconds (this is to turn OFF the engine) and is again
turned ON. The relay operating the starter motor is switched OFF and the
generated EMF from alternator comes below 12 V which makes the motor
controller to resume back its power supply to the motor.
In engine mode, electric powertrain key is turned OFF and the
IC engine powertrain key is turned ON. The vehicle is mobilised from the
beginning using IC engine. It can be cranked either by using the starter motor
or by kick start pedal. Alternator gives generated current signal to stop pin of
hub motor controller, hence there is no power supply to it.
5.5 CONCLUDING REMARKS
Based on the driving conditions in Indian cities, the following control
strategies were derived.
Two control strategies namely all-electric strategy and
blended strategy were used.
The control system developed can use three modes: electric
mode, hybrid mode and engine mode. The user can select a
particular mode based on the driving condition and battery
charge condition.
The next chapter presents the testing and performance of powertrain
elements and converted plug-in hybrid electric two-wheeler in detail. This
chapter also discusses the comparison of simulation results with road test
results for simulation validation.