Final Project: Design of an Electric Vehicle

OBJECTIVES:To prepare a technical report about the EV model including

1.System level configurations

2.Model parameters

3.Results

4.Conclusion

Theory:

An electric vehicle(EV) is a vehicle that uses one or more electric motors or traction motors for propulsion.An electric vehicle may be powered through a collector system by electricity from 0ff-vehicle sources, or may be self-contained with abattery, solar panels, fuel cells or an electric generator to convert fuel to electricty.EVs include, but are not limited to, road and rail vehicles, surface and underwater vessels, electric aircraft and electric spacecraft.

Components of an EV:

• EVs have no need for the fuel engine and gear transmission, the two of the most crucial components for internal combustion vehicles.
• An electric powered car has three primary components-battery, motor controller, and the electric motor.

1.Battery:

The battery of an electric car can be charged through the use of ordinary grid electricity at a specialized power station.But aside from the conventional lithium-ion battery tecgnologies, there are also other major battery technologies which can be used for electric cars.

• lithium-ion batteries:This battery technology gives extra performance and range.However, it also carries the highest price tag.Lithium-ion batteries are lighter than lead acid and nickel metal.These are also the batteries used in digital cameras and smartphones.
• lead acid batteries:This battery technology is the most popular.It is also the cheapest among the battery technologies.What's good about it is it's 97 percentage recyclable.
• nickel metal hydride batteries:This battery technology provide higher output and better performance but it costs much more than lead-acid batteries.

2.Motor controller:

The motor controller of an electric car administries its complete operation and the distribution of its power at any given moment.It acts as a floodgate between the motor and batteries.It helps monitor and regulates all key performance indications such as the vehicle's operator, motor, battery, and accelerator pedal.It has a microprocessor which can limit or redirect current.it is used to either improve the mechanical performance of the car or suit the operator's driving style.There are also more refined controllers which are capable of greater accurecy and thus, higher efficiency.

3.Electric motor:

Unlike a gasoline engine with lots of moving parts, a motor only has one moving part.This makes it a very reliable source of motive power.Choosing an lectric motor depends on the car's system voltage.They can be structured to use either AC or DC current.AC motors are less expensive and lighter compared to DC motors.They are also common and they tend to suffer from less mechanical wear and tear.However, AC technology requires a more refined or sophisticated motor controller.

PROCEDURE:

(1).Vehicle body subsystem:

• The vehicle is assumed to be a front axle driven.
• The wheels are modelled using simple tire(magic formula).
• The 'N' port corresponds to the mechanical rotational conserving port for the wheel axle.Thus both wheels on the same axle should from a connection through their respective A ports.Connection 'H' corresponds to the mechanical translational conserving port for the wheel hub through which the thrust developed by the tire is applied to the vehicle.Thus, all the 4 wheels from a connection through their respective H ports and this is in turn connected to the translational hub of the vehicle body.The connection 'S' represents the output port for slip of the tire.
• The tires are parameterized by peak longitudinal force and corresponding slip.
• The other block parameters such as - rated vertical load, peak longitudinal force at rated load and slip at peak force at rated load are kept their default values.
• The tire radius is given as 0.3m.
• The tire inertia is kept as 1 kg-m^2.Also, the rolling resistance is given with a constant coefficient of 0.015.
• This 2 axle (4 wheel) assebly is now connected to velocity body simscape block.

• This block is basically represents a two-axle vehicle body in longitudinal motion.The block accounts for- body mass, aerodynamic drag, road incline, weight distribution between axles due to acceleration road profile.
• Here connection 'H' is the mechanical translational conserving port hub.
• 'NF' and 'NR' correspond to the output ports for normal reaction forces on front axle and rear axle wheels respectively.Connection 'V' represents the actual output translational velocity of the vehicle.
• 'beta' is the road inclination angle and 'W' corresponds to the headwind speed.
• The gross weight is given to be 1200 kg.
• The complete subsystem is shown below:

• The input for this subsystem is taken as the rotational speed of the motor.This input is fed to thr front axle through a simple gear representing the final drive ratio of 3.73.

(2)Dc motor:

• Simulink provides an inbulit model block for DC motor which converts electrical input into mechanical rotational output.
• Here, the motor field type is kept as permanent magnet and the model parameterization is done by rated load and speed.The armature inductance and the mechanical parameters are given their default values.
• The connection 'R' represents the rotor while the connection 'C' is for casing.Thus, the R port is connected to the input of the vehicle subsystem created earlier.The C port is connected to a mechanical rotational reference.
• The + and - terminals of the motor are connected to the motor controller.If these terminals are directly connected to a battery, the rquired DC motor will run on the rated capacity of the battery.This will eventually result into no control over the DC motor and the vehicle will run at top speed throughout the simulation.Thus, a motor controller is very much needed.

(3) Motor controller:

• H-bridge circuit:

• This block represents an H-bridge motor drive.The block can be driven by the controlled 'PWM' voltage block in PWM or averaged mode.
• In PWM mode, the motor is powered if the PWM port voltage is above the enabled threshold voltage.
• In averaged mode, the PWM port voltage divided by the PWM signal amplitude parameter defines the ratio of the on-time to the PWM period.
• Connection 'REF' is for thr reference input.This combined with the 'PWM' connected will form the pulse input for the H-bridge.Hre, the REF input is connected to the electrical reference(ground).
• Connection 'REV' corresponds to the reverse motion of the motor which essentially means, the backward motion of the vehicle.Here, the backward motion of the vehicle is not accounted for and so, the port is connected to the electrical refernce.
• Connection 'BRK' is for braking of the vehicle.
• Since, the PWM mode results in a huge amount of simulation time.Averaged mode is selected for this simulation.
• Load current characterstics are considered to be smoothed here.
• Also, the regenerative braking is enabled for the simulation.This means, when the vehicle starts decelerating, corresponding amount of charge will be fed back to the batter.
• The input threshold parameters are left with their default values.
• Simulink provides an inbuilt "Controlled PWM voltage" block.This block is used for providing the proper pulse inputs to the H-

• The PWM and REF connections are for the corresponding ports on the H-bridge circuit.the 2 reference inputs correspond to the throttle inputs given by the driver.The block generated corresponding pulse width as per the accelerations and brakes applied by the driver himself.
• Here, the PWM frequency is 1000 Hz. also, the simulation mode is averaged as that in the H-bridge circuit.
• The input scaling parameters are: 0v for 0% duty cycle and 5v for 100% duty cycle.Acoordingly, the output voltage amplitude becomes 5v.
• The H-bridge circuit along with the PWM input, together from the motor controller circuit.

(4)Longitudinal driver:

• longitudinal driver is an inbuilt block provided by the powertrain blockset.

• It is a parametric longitudinal speed tracking controller for generating normalized acceleration and braking commands based on reference and feedback velocities.
• The 'velref' port corresponds to the feedback velocity.The actual velocity output given by the vehicle body is connected here.By comparing the actual velocity with the reference velocity, the driver block generated acceleration and braking signals in order to minimise the error between the two concerned velocitites.
• 'Grade' corresponds to the grade angle. for this simulation, no inclination is considered and hence, a constant block with value 0 is connected.
• 'info' gives the output for thr bus signal for different block calculations like difference in reference vehicle speed and vehicle speed, etc.
• 'accelcmd' and 'decelcmd' correspond to the acceleration and deceleration commands generated respectively and are connected to the corresponding ports of the controlled PWM voltage block.
• Here, the selected control type is PI.Accordingly, the block implements proportional-integral(PI) control with tracking windup and feed-forward gains.

(5).Battery pack:

• To model the battery pack, a simple battery block is used. this is because, if the generic battery blockmis used instead, the simulation will run for a much longer time.
• here, the selected option for battery charge capacity is finite.Accordingly, the block models the battery as a series internal resistance plus a charge-dependent voltage source.if the chosen option had been infinite, the voltage source would have been constant.
• The battery nominal voltage is given as 400v.The ampere-hour rating is assumed to be 50.
• To ensure the battery supplies current as per the requirement of the motor controller and not the rated current, a current sensor and controlled current source pair is connected

(6)Reference velocity(drive cycle):

• The drive cycle source block is used to provide the reference speed for the simulation.
• It generates a standard or user-specified longitudinal drive cycle.

• Here, the selected drive cycle is the standard FTP75.

(7)Battery state of charge:

• A small subsystem is created to calculate the state of charge of battery.
• this subsystem takes the battery current as the input.
• The essential process to calculate the SOC is by subtracting the charge from the rated capacity of the battery.
• Accordingly, the current is integrated w.r.t time to get the charge in coulums.Now this coulumb value is modified into ampere hours.This value is now subtracted from the charging capacity of the battery
• Finally, the remaning charge is now converted into percentage value of the charging capacity of the battery.

MODEL:

Simulation:

• The system is ran for simulation for 2474s which is the total time for which the input drive cycle is defined.
• A solver configuration is added to ensure proper solving of the mathematical equations by the model.

• scopes are connected to analyze different outputs such as the SOC of the bttery, the reference velocity and the actual velocity of the vehicle and the distance covered by the vehicle during the total run.
• The 2 velocities are first converted from m/s to kmph values. for this, simple gain blocks are used.
• the distance is calculated by simply time-integration of the vehicle velocity.

OUTPUTS:

(1)Ref. speed v/s actual vehicle speed:

(2).Distance covered:

(3).Battery state of charge:

Conclusion:

The model was succefully ran for simulation under the standard FTP75 drive cycle.The results have been properly analyzed.Also, learnt a quite a lot of things like-

• How to model an EV and carry out a real time simulation.
• How a motor controller works and what role it plays in the control of the electric motor.
• How to calculate the SOC of the battery.
• Different powertrian blocks provided by simulink which can be used to simulate a vehicle model.