Final Project: Design of an Electric Vehicle

AIM : To design an Electric Vehicle with a DC motor

 

INTRODUCTION : 

An electric vehicle (EV) is one that operates on an electric motor, instead of an internal-combustion engine that generates      power by burning a mix of fuel and gases. 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.

ELECTRIC VEHICLE :

The following is the basic block diagram of an Electric vehicle

The following is the brief explanation of each part of the Electric vehicle:

 Battery :

A battery is used to power the electric motors of a  Battery Electric Vehicle (BEV) or Hybrid Electric Vehicle (HEV) etc. These batteries are usually rechargeable batteries. These batteries are specifically designed for a high ampere-hour (or kilowatt-hour) capacity.

Electric-vehicle batteries differ from starting, lighting, ignition (SLI) batteries as they are designed to give power over sustained periods of time and are deep cycle batteries. Batteries for electric vehicles are characterized by their relatively high power to weight ratio, specific energy, energy density. 

The batteries with small and light weight are desirable because they reduce the weight of the vehicle and therefore improve its performance. Types of batteries used for an Electric Vehicle are Lithium-Ion batteries, Nickel-metal Hydride batteries, Lead-Acid batteries, Ultracapacitors, Recycling Batteries. Mostly Lithium-Ion batteries are used because of its high performance characteristics.

The matlab model for the battery is as below:

We can input the values in battery pack as required for the simulation. 

 

 Power Converter :

The power conversion systems can be classified according to type of input and output power, which can be used to convert the forms and levels of electrical energy. Converters can convert AC-DC voltage, frequency, or both combined. Power converters are of three types:

a. AC to DC also known as Rectifier

b. DC to DC also known as Chopper

c. DC to AC also known as Invertor

AC to DC - Rectifiers:

In electric vehicles, the converter is built inside the car. This type of arrangement is known as Onboard Charger. It converts power from AC to DC then feeds the power into car's battery. 

DC to DC - Convertors:

Mostly used in Electric Vehicles to get the required voltage. For this reason these are called Choppers. These are used for:

i. To increase the input voltage i.e Booster converter

ii. To reduce the voltage i.e Buck converter

iii. Faster charging

DC to AC - Invertors:

 Frequency variations can be done in this as DC is converted to AC

DC to AC inverter is placed between Battery and Motor because that inverter should control the power, torque, performance parameters by adjusting the AC supply. Generally for PluginHybrid Electric Vehicles uses this.

 

So the type of converter we use in this problem is H bridge power converter.

An H-bridge is an electronic circuit that switches the polarity of a voltage applied to a load. This allow DC motors to run forwards or backwards as we need Regenerative braking for the motor.

 

DC motor :

It converts direct current electrical energy into mechanical energy. When a current-carrying conductor is placed in a magnetic field it experiences a torque and has a tendency to move. In other words, when a magnetic field and an electric field interact, a mechanical force is produced.This is the working principle of DC motor. 

 

 

So To design an Electric vehicle the following are the different parameters, blocks used in matlab simulation:

Let,

Total weight of vehicle = 1500 kg

The model designed in matlab simulink is as below:

 

 

The system level configurations are explained clearly:

i. Passenger Car: 

 The passenger car has various components to describe like tyre, vehicle body, sensors etc

 Tire (Magic formula) : 

 

 

Tire represents the longitudinal behavior of a highway tire characterized by the tire Magic Formula. The block is built from Tire-Road Interaction (Magic Formula) and Simscape Foundation Library Wheel and Axle blocks. Optionally, the effects of tire inertia, stiffness, and damping can be included.

Connection A is the mechanical rotational conserving port for the wheel axle. Connection H is the mechanical translational conserving port for the wheel hub through which the thrust developed by the tire is applied to the vehicle. Connection N is a physical signal input port that applies the normal force acting on the tire. The force is considered positive if it acts downwards. Connection S is a physical signal output port that reports the tire slip. Optionally expose physical signal port M by setting Parameterize by to Physical signal Magic Formula coefficients. Physical signal port M accepts a four element vector corresponding to the B, C, D, and E Magic Formula coefficients.

The following are the different parameter values we can fix as we need for tire block

 

Vehicle Body :

Represents a two-axle vehicle body in longitudinal motion. The block accounts for body mass, aerodynamic drag, road incline, and weight distribution between axles due to acceleration and road profile. The vehicle can have the same or a different number of wheels on each axle. Optionally include pitch and suspension dynamics or additional variable mass and inertia. The vehicle does not move vertically relative to the ground.

Connection H is the mechanical translational conserving port associated with the horizontal motion of the vehicle body. The resulting traction motion developed by tires should be connected to this port. Connections V, NF, and NR are physical signal output ports for vehicle velocity and front and rear normal wheel forces, respectively. Wheel forces are considered positive if acting downwards. Connections W and beta are physical signal input ports corresponding to headwind speed and road inclination angle, respectively. If variable mass is modeled, the physical signal input ports CG and M are exposed. CG accepts a two- element vector representing the x and y distance offsets from vehicle CG to additional load mass CG. M represents the additional mass. If both variable mass and pitch dynamics are included, the physical signal port J accepts the inertia of the additional mass about its own CG.

 

PS constant

 The values for wind and beta are given as below:

 

ii. Motor Gear system:

 The matlab model for this is as below

 

DC Motor

This block represents the electrical and torque characteristics of a DC motor. The block assumes that no electromagnetic energy is lost, and hence the back-emf and torque constants have the same numerical value when in SI units. Motor parameters can either be specified directly, or derived from no-load speed and stall torque. If no information is available on armature inductance, this parameter can be set to some small non-zero value. When a positive current flows from the electrical + to - ports, a positive torque acts from the mechanical C to R ports. Motor torque direction can be changed by altering the sign of the back-emf or torque constants.

 

Ideal Rotational Sensor:

The block represents an ideal mechanical rotational motion sensor, that is, a device that converts an across variable measured between two mechanical rotational nodes into a control signal proportional to angular velocity or angle. The sensor is ideal since it does not account for inertia, friction, delays, energy consumption, and so on.

Connections R and C are mechanical rotational conserving ports and connections W and A are physical signal output ports for velocity and angular displacement, respectively.

 

Inertia:

As the motor is rotating part we have to represent the inertia for rotating shaft. The block represents an ideal mechanical rotational inertia. The block has one mechanical rotational conserving port. The block positive direction is from its port to the reference point. This means that the inertia torque is positive if the inertia is accelerated in the positive direction.

Gear Box:

The block represents an ideal, non-planetary, fixed gear ratio gear box. The gear box is characterized by its only parameter, Gear ratio, which can be positive or negative. Connections S and O are mechanical rotational conserving ports associated with the box input and output shaft, respectively. The gear ratio is determined as the ratio of the input shaft angular velocity to that of the output shaft.

 

The block generates torque in positive direction if a positive torque is applied to the input shaft and the ratio is assigned a positive value.

 

 

 

 

iii. Controller :

 The simulink model for the controller is as below:

H-Bridge:

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 Enable 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. Using this ratio and assumptions about the load, the block applies an average voltage to the load that achieves the correct average load current. The Simulation mode parameter value must be the same for the Controlled PWM Voltage and H-Bridge blocks.

If the REV port voltage is greater than the Reverse threshold voltage, then the output voltage polarity is reversed. If the BRK port voltage is greater than the Braking threshold voltage, then the output terminals are short circuited via one bridge arm in series with the parallel combination of a second bridge arm and a freewheeling diode. Voltages at ports PWM, REV and BRK are defined relative to the REF port.

 

Controlled Voltage (PWM):

This block creates a Pulse-Width Modulated (PWM) voltage across the PWM and REF ports. The output voltage is zero when the pulse is low, and is equal to the Output voltage amplitude parameter when high. Duty cycle is set by the input value. Right-click the block and select Simscape->Block choices to switch between electrical +ref/-ref ports and PS input u to specify the input value.

 

At time zero, the pulse is initialized as high unless the duty cycle is set to zero or the Pulse delay time is greater than zero. The Simulation mode can be set to PWM or Averaged. In PWM mode, the output is a PWM signal. In Averaged mode, the output is constant with value equal to the averaged PWM signal.

 

 

Controlled voltage source and Current sensor:

 

Controlled Voltage Source block represents an ideal voltage source that is powerful enough to maintain the specified voltage at its output regardless of the current passing through it. The output voltage is V = Vs, where Vs is the numerical value presented at the physical signal port.

The Current Sensor block represents an ideal current sensor, that is, a device that converts current measured in any electrical branch into a physical signal proportional to the current.

 

 

iv. Battery Pack :

 

 

Battery:

This block models a battery. If you select Infinite for the Battery charge capacity parameter, the block models the battery as a series internal resistance and a constant voltage source. If you select Finite for the Battery charge capacity parameter, the block models the battery as a series internal resistance plus a charge-dependent voltage source defined by:

V = Vnom*SOC/(1-beta*(1-SOC))

where SOC is the state of charge and Vnom is the nominal voltage. Coefficient beta is calculated to satisfy a user-defined data point [AH1,V1]

 

v. SOC: 

 SOC calculation of battery is performed to trace the battery reliability and range of the vehicle model. It is calculated by reducing the usage from nominal SOC value on timely basis.

 

 

vi. Longitudinal Driver: 

 

A parametric longitudinal speed tracking controller for generating normalized acceleration and braking commands based on reference and feedback velocities

For calculation of distance travelled

 

 

 

vii. Drive cycle : 

A driving cycle is a series of data points representing the speed of a vehicle versus time. Driving cycles are produced by different countries and organizations to assess the performance of vehicles in various ways, as for instance fuel consumption, electric vehicle autonomy and polluting emissions.

Another use for driving cycles is in vehicle simulations. More specifically, they are used in propulsion system simulations to predict performance of internal combustion engines, transmissions, electric drive systems, batteries, fuel cell systems, and similar components.

Drive cycle block generates the standard longitudinal drive cycle which can be used for simulation. It is used to predict engine torque and fuel consumption that a vehicle requires to achieve desired speed and acceleration for a given gear shift reference. It gives braking commands for vehicle models. 

There are several duty cycles used as per the requirement. We can change the drive cycle input

 

Different drive cycles are considered in this to simulate in all cycles.

 

 

SIMULATION :

 

 The following are the different simulation results obtained for different drive cycles:

 For FTP75 drive cycle 2474 sec

i. At starting the SOC of battery is very high i.e at 100 but after some time it gradually reduced.

ii. Output

From the plot we can see both drive cycle plot (yellow) and vehicle speed plot (blue) are represented

As vehicle speed plot follows the drive cycle plot, we can say that the model is designed correctly.

 

 

CONCLUSION :

 Electric Vehicle is designed by using different Simulink blocks and their outputs are plotted. Driving cycle is used as reference speed. Battery SOC is also calculated.