Final Project: Design of an Electric Vehicle

Ans. An Electric Vehicle (EV) is a vehicle that uses one or more electric motors or traction motors for propulsion. 

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.

Block diagram of an EV:

1. Drive Cycle: The term "driving cycle" refers to a test that measures vehicle speed over time. It is used to evaluate a car's fuel consumption and pollutants emissions in a standardized way to compare similar vehicles. The driving cycle is carried out on a chassis dynamometer, where tailpipes automobile emissions are collected and analyzed to assess emission. Here we use FTP-75 transient cycles this involve many speed variations, typical of on-road driving conditions. 

2.Drive Controller: It tracks the actual speed of the vehicle which will match with the given input.

3.Power Converter : Converts the power from AC to DC and provides a certain level of power according to the vehicle application. H-Bridge circuit helps in regeneration of power at the time of retardation.

4.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 technologies, 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. It also has highest price tag. 
• Lead Acid Batteries : These battery are most popular. It is also the cheapest among the batterytechnologies. 
• Nickel Metal Hydride Batteries : This battery technology provides higher output and better performance but it costs much more than lead-acid batteries.

5.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 electric 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 more common and they tend to suffer from less mechanical wear and tear. However, AC technology requires a more refined or sophisticated motor controller.

 PROCEDURE : We start the modeling from Design of type, vehicle body.

(1) Vehicle Body Subsystem :

• The vehicle is assumed to be a front axle driven (4 wheels - 2 on each axle).
• The wheels are modelled using simple Tire (Magic formula) simulink blocks

Tire: 

The N port of the tire corresponds to the Normal Reaction acting on it.
The A port corresponds to the Mechanical Rotational conserving port for the wheel axle. Thus both the wheels on the same axle should form 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 form 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.

Tire parameters: 

• 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 with their default values.
• The tire radius is given as 0.3 m.
• The tire inertia is kept as 1 kg-m^2. Also, the rolling resistance is given with a constant coefficient of 0.015.

Vehicle Body: This 2 axle (4 wheel) assebly is now connected to a Velocity Body simscape block.

• This block 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, the connection H is the mechanical translational conserving port hub.
  NF & NR correspond to the output ports for nomral 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 & W corresponds to the headwind speed.

Parameters of Vehicle Body:

• The gross weight is given to be 1200 kg.
• Drag coefficient:0.4                    Air Density:1.18
• The pitch dynamics for the vehicle are not considered for this simulation.

Simple Gear: The input for this subsystem is taken as the Rotational speed of the motor. This input is fed to the front axle through a Simple Gear representing the Final Drive Ratio of 2
Here, the rotational direction of the output shaft is kept the same as that of the input shaft. Also, no meshing losses are considered for the simulation.

Parameters:

(2) DC Motor :

    

• Simulink provides an inbuilt bodel 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 connection R represents the Rotor (rotational output) while the connection C is for the Casing (stationary). 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 required 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.

Parameters of Motor:

The armature inductance and the mechanical parameters are given their default values:

(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 the reference input. This combined with the PWM connected will form the pulse input for the H-Bridge.
  Here, 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 reference.
• Connection BRK is for the 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 characteristics 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 battery.
  Accordingly, the BRK input is connected to a Controlled Voltage Source. This will generate emf corresponding to the intensity of brakes applied and fed the power back to the battery.

Parameters For H-bridge:

• The Input Threshold parameters are left with their default values.
• The Output Voltage Amplitude of the H-Bridge circuit is given the same value as the rated DC supply voltage of the DC motor.

(4) Controlled PWM Voltage 

This block is used for providing the proper pulse inputs to the H-Brdige circuit.  

• The PWM & 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. This block generates corresponding pulse width as per the accelerations and brakes apllied by the driver.

Parameters:

Here, the PWM frequency is 1000 Hz. Also, the simulation mode is Averaged as that in the H-Bridge circuit.

(5) Longitudinal Driver :From powertrain blockset

• It is a parametric longitudinal speed tracking controller for generating normalized Acceleration and Braking commands based on reference and feedback velocities.
• VelRef port is the reference velocity port where the input drive cycle data is fed.
• VelFdbk port corresponds to the feedback velocity. The actual velocity output given by the vehicle body is connected here. By comparing the actual (feedback) velocity with the reference velocity, the driver block generates acceleration and braking signals in order to minimise the error between the two concerned velocities.
• Grade is the hill climbing angle. For this simulation, no inclination is considered and hence, a constant block with value 0 is connected.
• Info gives calculations like difference in reference vehicle speed and vehicle speed.
• AccelCmd & DecelCmd correspond to the acceleration and deceleration commands generated respectively and connected to the corresponding ports of the Controlled PWM Voltage block.

Parameters: Here, the selected control type is PI. Acordingly, the block implements proportional-integral (PI) control with tracking windup and feed-forward gains.

(6) Battery Pack :To model the Battery pack, a generic battery block is used.

This battery(Type:LI) is the most popular battery types. Temperature and aging (due to cycling) effects can be specified for Lithium-Ion battery type.Also, the temperature and the battery aging are disabled.

Parameters:

• The Battery Nominal Voltage is given as 400V.
• The Ampere-hour rating is assumed to be 100.
• Initial SOC= 100;  Battery response time=20 sec.

(7) 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.

Parameters:

Model: The system is ran for simulation for 2474 sec which is the total time for which the input drive cycle is defined.

So, the model RAn for 2474 sec as required time for Drive cycle. 

Outputs: 

SOC=11.5;        Voltage=418.2

• The State-of-Charge of the battery at the end of simulation run is nearly about 11 %.
• This means that the battery is not completely drained at the end and the vehicle can run on the same drive cycle for a few more times before the battery SOC becomes 0.
• Also, the thing to be noted here is, since we had enabled the regenerative braking for the simulation, the battery SOC increases slightly whenever the vehicle undergoes heavy deceleration.
• and voltage is also fluctuating as vehicle is going through acceleration and deceleration throughout the drive cycle.

Distance= 6.258

• It may be noted that the distance is increasing throughout.
• The total distance covered by the vehicle is about 6.258 km.

Speed Graph:

• As can be seen from the above picture, the actual velocity of the vehicle differs from the reference drive cycle velocity.
• This is because, the chosen battery capacity and motors are not able to propel the speed upto the required extent.
• If a battery of higher capacity is chosen, the actual velocity will tend to match with the reference velocity.

Conclusion:

The model was succesfully ran for simulation under the standard FTP75 drive cycle. The results have been properly analyzed.

learnt things like -how to model an EV and carry out a real time simulation and how a motor controller works according to the drivecycle and what role it plays in the control of the electric motor.