Design of electric vehicle

Introduction:

An EV is a shortened acronym for an electric vehicle. EVs are vehicles that are either partially or fully powered on electric power. Electric vehicles have low running costs as they have less moving parts for maintaining and also very environmentally friendly as they use little or no fossil fuels (petrol or diesel). While some EVs used lead acid or nickel metal hydride batteries, the standard for modern battery electric vehicles is now considered to be lithium ion batteries as they have a greater longevity and are excellent at retaining energy, with a self discharge rate of just 5% per month. Despite this improved efficiency, there are still challenges with these batteries as they can experience thermal runaway, which have, for example, caused fires or explosions in the Tesla model S, although efforts have been made to improve the safety of these batteries.

Theory:

When pedal of the car is pressed, then:

• Controller takes and regulates electrical energy from batteries and inverters
• With the controller set, the inverter then sends a certain amount of electrical energy to the motor (according to the depth of pressure on the pedal)
• Electric motor converts electrical energy into mechanical energy (rotation)
• Rotation of the motor rotor rotates the transmission so the wheels turn and then the car moves.

There are 4 (four) types of electric cars, with the following outline:

• Battery Electric Vehicle (BEV)

Hybrid

• Hybrid Electric Vehicle (HEV)
• Plug-in Hybrid Electric Vehicle (PHEV)
• HybridHybrid Electric Vehicle (HEV)Plug-in Hybrid Electric Vehicle (PHEV)
• Fuel Cell Electric Vehicle (FCEV)

System level configuration:

The system level configuration deals with the control of the entire EV. The simple block diagram of a entire EV will consist of battery, Dc motor and its controlling unit, Transmission system and the wheels.

The block diagram is a simple layout of a EV. In the matlab model we are going to describe each and every blocks indivually.

The design of a EV is done with a required battery package and a desired dc motor in the simulink. The modelling of an EV is done using simulink to describe the complete working of the vehicle.

The above block diagram is used to stimulate a EV. The model consists of,

1.Vechicle body subsystem

2.Dc motor

3.Motor controller

4.Longitudinal driver

5.Drive cycle

6.Battery

7.SOC subsystem

Now we can see about the component level details of the block used in the modelling of the EV.

1.Vehicle body subsystem:

The subsytem consist of several blocks,

The subsystem consist of Wheel block, gear block and vehicle body block.

Tyre block:

Here we used the tyre block(magic wheel block) to make our wheel for the EV.The block has 4 ports namely A, N, S, H.

A port - It is Mechanical Rotational conserving port for wheel axle.The wheels in the same axle should be connected through this port.

N port - It is the normal reaction acting on it.This port is connected with the vechicle body block.

H port - It is Mechanical Translational conserving port for the wheel which produce the thrust developed in it.The 4 wheels are connected though this port and this port is also connected to the same translational port in the vechicle body port.

S port - It is the Slip output of the wheel.Here it is left unconected or it can be connected to a ps terminator which is used for no connection port.

The block is characterized by various block parameters,

• Peak longitudinal force is set to3000N
• The Slip is set to 10%
• The Tyre radius is given as 0.3m
• The wheel inertia is kept at 1Kgm^2
• The rolling coefficient is kept as 0.015

This tyre block is connected with the vechicle body simscape block

Vehicle Body:

The vechicle body block is used to connect the wheel to real world vechicle.It consist of NR, NF, beta, V, H, W block

Nf-Nr - They are output port for normal reaction connected to the front and rear wheels of the EV.

H port- It is the Mechanical translational conserving port connected to the H port of wheel.

V port - It is the actual output velocity of the vechicle which is send to scope to view the output graph after the simulation.It is also given as the feedback for the control of the vechicle.

beta - It is the road inclination angle.Here it is considered as constant 0 value.

W port- It is the head wind speed.Here it is considered as constant 0 value.

The parameters of the block are changed in order to control the aerodynamic drag force, body mass and road inclination distributed between the wheel axle.

• The gross weight is taken as 1200kg
• The geometric parameters are kept constant
• Drag coefficient is taken as 0.3
• The pitch dynamics for the vechicle are not considered.

Gear box:

Here simple gear box is used for the simulation.The B port is act as the input and the F port to be act as the output.The input is used to get the motor shaft into the gear box and the output port is used to transfer the motor shaft to the wheel axle.

The transfer of motor shaft depends on the gear ratio of the box.Here it is of the value 3.73.The shaft of the gear box is set to rotate in the same direction as that of the motor.

2.DC motor:

The dc motor is used for the simulation of the EV.It consist of both electrical terminal and mechanical terminals.

The + and - symbols denote the electrical input of the motor which is connected to the controlled power supply.

The R and C symbols denote the mechanical output of the motor.R port gives the torque and speed value which is given as input to the vechcile body subsystem.C port is connected to the mechanical referance.

The parameters of the motor are changed as,

• The field type is selected as permanent magnet
• No load speed is kept as 10000rpm
• Rated speed is 8000rpm
• Rated load is 60kw
• Rated dc supply is 330v

3.Motor controller:

If the dc motor is directly connected to the battery it will run at the desired voltage off the battery and we cannot control the speed of the motor.So it is advised to use a controller inbetween the motor and the battery.The controller consist of two main parts- H-bridge and PWM controlled voltage.

H-bridge:

This block represent the H-bridge motor drive.It has $ inputs and two output terminals.

PWM port - It is the input port to get the voltage sampled to a PWM signal.This bridge can operate in two mode.In PWM mode the bridge will run if the given pwm is greatr than the threshold value.In averaged mode, the pwm voltage value is divided by the pwm signal to determine the ratio of on time of the bridge.

REF port - This is a referance voltage combined with the pwm port to give the smooth operation of the motor.It is usually grounded.

REV port - This determines the reverse motion of the motor.Here it is grounded.

BRk port - It is for the braking of the vechicle.It is connected with the longitutional drive block through a controlled voltage source.

The other parameters of the block includes,

• The simulation mode selected is averaged mode as it saves our time for simulation.
• The regenerative braking is enabled, so wwhen the vechicle starts deceleration the battery gets charged.
• The load current is set to get smoothed value
• The output amplitute value is set as 330v same as that of the battery voltage.
• The input threshold value is kept as constant parameters.

Controlled PWM voltage:

Simulink provides an inbuilt controlled PWM voltage block.It is required for providing pulses to the H-bridge circuit.

It consist of two referance voltage inputs which are used to provide the necessary voltage to the motor.The input is provided by the driver and the block generates accordingly the pulse for acceleration and for brakes applied.

It has two output port, PWM port is connected with the PWM signal of the H- bridge and the ref is also connected to the ref of the H-bridge for controlling it.

The parameters of block includes,

• The PWM frequency is set to 1000Hz.
• The simulation mode is averaged.
• The scaling of voltage is done as 0V for 0% duty cycle and 5V for 100% duty cycle.

4.Longitudinal driver:

It is a inbuilt block provided by the powertrainblockset.It is used to give driver commands like acceleration and braking commands through the referance and feedback values.

Velref - It is used to get referance input value of velocity from the predefined drive cycle data.

VelFdbk - This port gets the value of feedback velocity from the vehicle body block and it is compared with the referance velocity to achieve the desired acceleration and deceleration

Grade - It is used to define the gradibility for the vechicle.Here we have assumed it be be 0.

Accelcmd - It is a output port used to provide the acceleration command to the motor.It is connected with the positive of controlled pwm voltage through the controlled voltage source.

Decelcmd - It is also a output port used to provide the decceleration command to the motor.It is connected with the brk port of the H-bridge through the controlled voltage source.

Info - It is signal for the various bus signal.Here it is left unconnected.

The block is controlled through a PI controller method with tracking windup and feed forward gains.

5.Drive cycle:

The drive cycle source is the block used to provide referance speed for simulation.Here we have selected the FTP75 standard drive cycle.It contains data points of 2474 seconds.The refspd port id connected to the ref of longitudinal driver.

The ref speed of the drive cycle is also given to a scope along with the velocity in the vehicle body block to check with the coresponding velocities.The output velocity is also given to a integrator block and given to a scope to measure the distance travelled by the vehicle.

6.Battery:

The simple battery model is used for the simulation.The battery is connected through a controlled current source and a current sensor is connected with it and the motor.The output of the controlled current source is given to the soc subsystems.

The parameters of the battery is to be,

• The charging capacity is of finite.
• The nominal voltage of the battery is 400V
• The ampere hour rating is to be 50

7.SOC subsystem:

The SOC subsystem is created to calculate the state of charge of the battery.This system takes the battery current as the input and gives the state of charge as the output.

The current from the battery is integrated to get the charge value and the charge value gets converted into the ampere hour value.This converted value is then compared with the standard battery charging capacity.Here it is assumed to be 50Ah.

The compared value is then converted into the percentage value and it will denote the state of charge of the battery.The output is also given to a scope to view it graphically.

Simulation Results:

Now after completing all the above connections we need to run the model.The model is to be run for 2474s as the drive cycle contains 2474 points.If we choose other drive cycle options like wide open throttle we can decide the total number of time the vechicle has to run.

The result from our simulation are as follows,

The above graph shows the ref speed and the vechicle actual speed.The yellow line denotes the ref speed and the blue line denotes the actual speed of the vechicle.We can see that the actual speed is not following the ref speed.This is due to the parameters of the motor, if we increase the rating of the motor we can get a very precise output.

The above graph shows the distance covered by the vechicel during the entire time period.The graph tends to increase as there is no negative velocity in the drive cycle.The vechicel have covered a total of 20000m in 2474s of the drive cycle data.We can also see the relation between the velocity and distance, when the velocity tends to be 0 distance remains the same.

The above graph shows the State of charge of the battery.The capacity of the battery is decreasing as the vechicel is running.After running the entire cycle the state of charge of the battery falls to a value of 35%.

Conclusion:

The model of a EV with simple battery and a dc motor is created and the model is simulated to obtain the above results. The model shows the performance of a EV. It can be improved by designing the battery and the dc motor accordingly to the reqiured parameters. This model will be very much useful in deciding the actual parameters of a EV. This model gives as a basic understanding of how a EV will actually work