Final Project: Design of an Electric Vehicle

Aim: Design an electric vehicle and run the vehicle using different drive cycles.

Design of Electric vehicle

Aim: Design an electric vehicle and runthe vehicle using different drive cycles.

Theory: An electric vehicle (EV) is defined as a vehicle that uses an electric motor or a traction motor for propulsion as opposed to an internal combustion (IC) engine used by conventional vehicles. Electric vehicles use electricity for power instead of fossil fuels as conventional vehicles would and because of this difference, electric vehicles do not produce any exhaust gases or pollution. The power needed for the operation of an EV can come from a variety of sources such as batteries, generators, or through a collector system. Batteries fitted in to the body of an EV are capable of high energy densities in order for them to meet the power demands and range requirements. Generators are machines which generate electrical energy from an input of mechanical work and are a part of the electric motor, and they operate during braking to produce the phenomenon of 'regenerative braking'. Collector systems are off-vehicle systems which allow for the charging of an EV from homes or public charging stations. The electricity used for charging in these systems are generated from solar panels which can be used by the grid or by homes of EV owners so that they can charge their vehicles more conveniently.

EVs are increasing in demand in recent years due to the need to reduce vehicle emissions which are damaging to the environment. Government incentives and private companies are working on schemes to quicken the adoption of electric vehicles. Since EVs do not produce any emissions, they are more environmentally friendly than conventional vehicles are. The generation of the electricty used to charge the vehicles also comes from solar energy (a renewable energy source) and not fossil fuels which makes them sustainable. EVs can meet the same performance standards of conventional vehicles and even surpass that of theirs and this has been seen with the automotive company Tesla. The Tesla Model 3 is the best selling EV in the world and is capable of a range of approximately 350km on single charge. It also has an acceleration of 0 - 60 mph in 5.6 seconds and a top speed of 130 mph.

EVs offer several advantages over conventional vehicles besides the lack of pollution and lack of need to rely on fossil fuels. One is that, EVs are more efficient meaning that they offer more MPGe (miles per gallon gasoline equivalent), meaning that you will spend lesser on recharging an EV than you would to refuel a conventional vehicle. This because the more efficient the EV is, the more distance you travel with the available energy. In the US, Tesla owners can charge their EVs for free at their private charging stations for life which results in significant savings in fuel. Regenerative braking in EVs adds to their efficiency as the unused energy in braking gets reused purposefully. Another benefit is that EVs have fewer moving/rotating parts and this means lesser maintenance costs than you would have with IC engine vehicles e.g. changing oil, transmission fluid. This, in the long term would give better savings than you would get with conventional vehicles. Lesser moving parts means there is lesser friction between moving parts and hence quieter operation.

In spite of these feats, it will be difficult for a worldwide adoption of EVs for several reasons. One reason is convenience. In order for EVs to dominate the automotive market, their needs to be more convenient ways for consumers to be able to charge their vehicles. Their needs to be several charging stations on the highways and roads and this can be supported if everyone's homes were generating solar power. It needs to be as easy to charge an EV as it is to fill an IC engine vehicle with fuel at a petrol station. Another reason is the cost of the EVs. EVs are still a relatively new technology and only account for 2-3% of the worldwide market. This is because they cost considerable more than conventional vehicles due to the high cost of batteries and electric components. But, the cost of batteries is getting cheaper over time and manufacturers are developing newer and more efficient batteries. The goal is to produce a 'million mile' battery which, if made will be a revolution in the automotive sector. Another disadvantage is that the components in EVs operate at high voltages and currents and this can be a hazard if the components explode or don't work as they should.

A compromise between IC engine vehicles and EVs exists as Hybrid Electric Vehicles (HEVs). HEVs combine an IC engine with an electric propulsion system to produce a 'hybrid' between the two different classes of vehicles. The purpose of this is to use the power available form the electric powertrain to improve the fuel economy of the vehicle so that it does not rely solely on the petrol/diesel it uses as fuel. This in turn will also improve the overall performance of the HEV. The IC engine can be used to power the electric motor to charge the battery or use directly in the electric drive motor with a combination of a motor and generator in one. HEVs also implement regenerative braking to recharge their batteries among other features used in EVs. Compared to EVs, HEVs represent more of the automotive market with a 19% market share on all automobiles in the world. This is because HEVs still run on fossil fuels which is still more convenient as a fuel source in most parts of the world. The all time best selling HEV to date is the Toyota Prius.

In this report, I will be modelling an electric vehicle with a DC motor and battery system using the SIMULINK software. The components I will need for my model exist as blocks in the SIMULINK block library. I will be modelling this EV for a drive cycle and measure the velocity characteristics and the current and SOC characteristics of its battery system.

Components:

The main components of an EV are:

· batteries

· power converters

· motor controllers

· motor/generator

· transmission system

· wheels/tires

Batteries store the electrical energy needed to power the EV in all of its systems and needs. Power converters convert the power coming from the battery to a usable form for the motor.

This conversion can be from DC to AC or vice versa and can even be between 2 different magnitudes of DC to DC or AC to AC. Motor controllers are electronics which control the amount of power delivered to the motor via control signals so that the motor is running as commanded. The motor is used for propulsion in order to move the vehicle in forward or reverse motoring. The generator is a part of the motor and just works in the opposite method to the motor. When braking, the vehicle can generate electricity and send this power back to the battery, charging it (regenerative braking). The transmission system is needed for the vehicle to change speeds as it accelerates/decelerates. The wheels are needed for traction on the roads and depending on the powertrain, the EV may be an FWD or RWD (forward or rear wheel drive). If a vehicle is an FWD, only the front axle wheels produce propulsion i.e.. rotate from motor effect and if it is an RWD, the back axle wheels produce it instead. All wheel drives (AWDs) also exist where the front and rear axle wheels work together to produce propulsion.

System Level Configuration:

To model an EV in SIMULINK, we will need to model the aforementioned components separately and then have them work together. All the main components are available in the library of SIMULINK blocks and we will be adding each component and configuring its parameters to fit the requirements of the model. First, I have modelled the vehicle main body.

Vehicle Body

To model a vehicle's body, we need to first add the vehicle body block and this is shown below:

This block contains 6 ports which can be used to model the various aspects of the vehicle body. The ports are mentioned and elaborated below:

· H - This stands for 'Hub' and is used to connect the hubs of the wheels to the main body of the vehicle.

· V - This is a physical signal output port which stands for 'Velocity' and is used to output the speed/velocity at which the vehicle is moving at.

· W - This is a physical signal input port and allows us to model a 'headwind' acting on the vehicle by giving a speed for it.

· NR - This is a physical signal output port to model the rear wheel normal forces.

· NF - This is a physical signal output to model the front wheel normal forces.

· beta - This is a physical signal input port and allows us to model the vehicle on an inclination angle to create the effect of a hill climbing force.

As you can see, we are able to modify various parameters of the vehicle body e.g. mass, number of wheels per axle, distance of the center of gravity to the front and rear axles. These are the main parameters and have been left with the default values assigned when the block was added to the workspace.

Tires

The tires used in this EV model are the 'magic formula' tires. There are several tire blocks that can be used for modelling a vehicle but I have used the magic formula ones because it allows us to model the longitudinal behavior of vehicle tires on highways. They also allow us to model extra effects such as tire inertia, stiffness which allow for greater accuracy of real world conditions. The magic formula tire block is shown below:

We can see that it has 4 ports and they are:

· N - This port stands for 'Normal' and is the physical signal input port for the normal force acting on the tire and is considered positive in the downwards direction.

· S - This port stands for 'Slip' and is the physical signal output port is that reports the slip acting on the tire.

· A - This port stands for 'Axle' and is the mechanical rotational conserving port for the wheel axle.

· H - This port stands for 'Hub' and is the mechanical translational conserving port for the wheel hub through which the thrust developed by the tires is applied to the vehicle body.

As we can see, we are parameterizing the tires with 'Load dependent Magic Formula coefficients' and the remaining parameters that follow with it are default.

Vehicle Body Subsystem:

As seen in the diagram above, the various blocks are connected to model the vehicle body. 4 magic formula tire blocks are used as the vehicle will have 4 tires, 2 on each axle. The tires are LR, LF, RR and RF (left rear, left front, right rear, and right front). The normal ports of each tire are connected to the appropriate NR and NF ports on the vehicle body block. The hubs of each tire are connected together and an extra connection is made from them to the vehicle body block. The 'S' ports of the LR and LF tires have been modelled with a slip using a small subsystem. The simple gear and inertia blocks are connected to the axle port of the tires. The wind velocity and road incline are provided with constant physical signals of 5m/s and 0 degrees respectively. The velocity port has been connected to the velocity subsystem and the connection from the subsystem is open to the 'v' port. The base port of the simple gear has been left open to the 'R' port to take in a rotational signal. If all these connections are made, all of these components can be selected and made in to a subsystem shown below:

This subsystem is created so that less space is taken up in the workspace and now, we only need the open ended connections to be modified.

Motor:

For the EV model, I have selected to use a DC motor block as the motor. This is because DC motors are easier to model and have fewer parameters to modify for a particular application. For this model, a DC motor will do just fine. The DC motor block is shown below:

As we can see, there are 2 sides to the block: the mechanical side on the right and the electrical side on the left. The mechanical side has 2 ports, C and R. The C refers to the casing and the R for the rotation of the rotor which produces a torque. The electrical side has a positive and a negative terminal for the voltages to be applied. If a positive voltage is applied to the DC motor block from the positive to the negative terminal, a positive torque will act from the C port to the R port. The direction of the torque produced by the DC motor can be changed by changing the sign of the back emf or the torque constant. The C port of the mechanical side needs to be connected to a mechanical rotational reference because it will be stationary. The R port will be connected to the open connection of the simple gear from the vehicle body block. The positive terminal on the electrical side is connected to the positive terminal of a power converter which in my case will be an 'H bridge'.

The mechanical rotational reference block is shown below:

Double clicking the DC motor block takes us to its parameters. The electrical torque parameters are first considered. Here, I have chosen to parameterize the block 'By rated power, rated speed & no-load speed'. This lets us modify parameters such as DC supply voltage, rated load. I have kept all values default except for the no load speed, rated speed, rated load and the DC supply voltage which were changed to the values shown in the image below:

Once the DC motor block has been parameterized, the following connections need to be made:

Power Converter:

The power converter is needed to provide power of the correct form and magnitude from one component to another. In the case of EVs, a power converter is needed to provide a DC voltage from the battery of the EV to the DC motor so that it can operate properly with the required parameters. As mentioned earlier, I have used an H-Bridge motor drive as my power converter. Its block is shown belo

The H Bridge has 2 sides to it: the left side is for the controlling of the voltage signals and the right side is for the terminals to provide the controlled voltage to the DC motor. The left side ports of the H-Bridge block are:

· PWM - This stands for Pulse Width Modulation which will provide the voltage pulses to the H-Bridge either in the PWM or average mode.

· REF - This stands for Reference.

· REV - This stands for Reverse.

· BRK - This stands for Brake.

The last parameters for the H-Bride pertain to its bridge parameters. Only the first parameter of 'Output voltage amplitude' was modified. This is because the DC motor has been parameterized to run on a DC supply voltage of 330V. So this same value is used for the output voltage amplitude.

Once these parameters have been applied, close the parameter settings. The connections for this block will be made later on.

Controlled PWM Voltage:

Next, I have used a 'Controlled PWM Voltage' block to provide a controlled pulse width modulated voltage to the H-Bridge block. The block will output the PWM voltage through its 'PWM' and 'REF' ports which will feed in to the same ports found on the H-Bridge block. If the pulse is low the output voltage is considered 0V whereas if the pulse is high, the output voltage will be equal to the output voltage amplitude which was set to 330V in the H-Bridge block. The pulses of voltage created by this block are dependent on the duty cycle which is the ratio of the time that the switch is ON to the time taken for 1 cycle (which is the time for which the switch is ON and OFF). The duty cycle is set by the input of the controlled PWM voltage block.

The control PWM voltage block:

The block parameters are shown in the image below:

The main settings pertain to the PWM parameters. Here all the parameters are maintained at their default values. I have chosen the 'Averaged' simulation mode so that the average value of the voltage pulses are generated and fed in to the H-Bridge. This will allow for quicker simulation times, just like for the H-Bridge block parameters. The remaining parameters of 'Input Scaling' and 'Output Voltage' are unchanged.

If these parameters have been applied, close the block parameter window. The connections for this block will be made later on.

Controlled Voltage Source:

The input ports of the controlled PWM voltage block are still unconnected and that is because they require a controlled voltage source to provide the voltages to produce the voltage pulses to be fed in to the H-Bridge. For this, I have used the controlled voltage source block. This block is useful because it will provide the necessary voltage that is powerful enough to maintain the voltage at the output regardless of the current that passes through it. This controlled voltage source block is shown below:

In this model, 2 controlled voltage sources have been used. One block is to provide the voltage for the acceleration of the vehicle and the other is to provide the voltage for the deceleration of the vehicle.

Electrical Reference:

The electrical reference block is used to create a grounding effect for electrical circuits. Any model that uses electrical or electronics components will need to have at least one electrical reference block in it. The electrical reference block is shown below:

In my EV model, we are using this block for 2 occasions. The first is for to create an electrical ground between the controlled voltage source, controlled PWM voltage and the H-Bridge blocks. The second is for the battery system which will be modelled afterwards.

Solver Configuration:

The solver configuration block is used to solve any necessary equations in a model and in my case, I have used them in the connection to the 'REV' port of the H-Bridge block.

All blocks connected to each other.

Battery System:

Battery:

The battery will be the energy store of the EV and will contain the electrical energy in DC form and provide this energy to the DC motor for it to operate. The battery block used is a simple one and considers only the basic parameters. The battery block is shown below:

The battery block parameters are shown below:

As we can see, the battery block parameters include the nominal voltage, the internal resistance and the battery charge capacity. I have used a nominal voltage of 400V, an internal resistance of 0.2 Ohms and an infinite battery charge capacity. The nominal voltage is selected so that it is more than sufficient to supply the voltage to the DC motor and the H-Bridge blocks.

Controlled Current Source:

The controlled current source block is a part of the battery system used in the EV model. Similar to the controlled voltage source, this block will represent an ideal current source that is powerful enough to maintain the specified current regardless of the voltage across it. The controlled current source block is shown below:

Current Sensor:

The current sensor block represents an ideal current sensor and is used to convert the current measured in an electrical circuit to a physical signal that is proportional to the current. This block is used to measure the current between the battery circuit (controlled current source) and the DC motor. The current sensor block is shown below:

The positive and negative terminals represent the terminals across which the current sensor is inserted in to the circuit. The third port outputs the physical signal and in my model, it will send the signal to the controlled current source.

From the image above, we can see that the current sensor is embedded in between the electrical side of the DC motor and the H-Bridge. The current sensor can be connected from its physical signal port to the remaining components in the battery system including an electrical reference block. This is shown in the image below:

SOC & Current:

SOC stands for state of charge and it is a measurement of the available charge in a battery. When a battery is fully charged, it is considered to have a 100% charge and conversely 0% charge if fully discharged. In this EV model, I want to measure the current and the SOC of the battery as the simulation is run. For this, I have used a scope block which shows the output signal we wish to measure as a plot against time. 2 scopes have been used, one for the current and the other for the SOC. In order for this to be measured, we need to use a 'PS - Simulink converter' block. This is because the current signal is a physical one and we are measuring it as a logic signal in the SIMULINK workspace.

The PS-Simulink converter block is shown below:

The scope block is shown below:

SOC Subsystem:

To measure the state of charge of the battery, a subsystem was created of various blocks. The first block is the 'Rate Transition' block and it is used to handle data transfer between different rates and tasks. Next, a 'Gain' block is used in order to multiply or divide the signal coming from the rate transition block. In this case, the gain block will divide the current signal with the ampere rating of the battery which has been taken as 100Ah multiplied by 3600 (seconds in an hour). This is shown in the gain block parameters below:

Then, a 'Discrete - Time Integrator' block is used to perform a discrete integration of the current signal with respect to the time. A constant block is used to represent the initial SOC of the battery. Assuming that the battery is always fully charged when performing a simulation, I have altered the constant block with a value of 1 representing 100%. Next, a 'sum' block is used to perform addition and subtraction operations. The signal from the 'discrete time integrator' block will be subtracted from the constant block representing 100% charge. These components are connected as shown below:

These blocks are then made in to a subsystem called 'SOC Estimate' and linked with the battery circuit using the PS-Simulink converter block. A scope block is used at the junction of the PS-Simulink converter block to plot the output signal of the current from the battery. Another scope is used to plot the output signal of the SOC from the SOC subsystem. This is shown in the image below:

Speed Measurement:

In order to measure the speed of the EV model, I needed to use the scope block once again. I modified the scope block to have 2 input ports where one port takes the output velocity signal from the vehicle subsystem block and the other port takes the output signal from the drive cycle to the reference velocity port on the longitudinal driver block. This has been done so that the scope can show us the plots of the reference velocity and the actual velocity of the vehicle to see how they compare with each other.

Distance Measurement:

I have used the scope block and mathematical operator blocks to measure the distance travelled by the vehicle according to the drive cycle data. The area under a velocity time graph tells us the distance travelled by an object and so in my model. This means that the integral of the velocity with respect to time, gives us the distance travelled. So, I have implemented the integrator block to integrate the velocity signal that is output from the vehicle subsystem.

The integrator block is shown below:

From the integrator block, I needed to use the 'Divide' block along with a 'Constant' block having a value of 3600 in order to convert the divide the integrated speed signal which is in kilometres per hour by 3600. This will give us the distance travelled in kilometres.

The constant block with a value of 3600:

The 'Divide' block:

Finally, a 'Display' block is used to show the distance travelled as the simulation is run. The display

block is shown below:

Longitudinal Driver:

The longitudinal driver block is used for giving normalized acceleration and deceleration commands to a controller based on 2 sets of data. The first data set is the reference velocity and the second data set is the feedback velocity. The longitudinal driver block is shown below:

The longitudinal driver block has the following ports:

· VelRef - This stands for the reference velocity and it is the velocity data provided from a drive cycle source.

· VelFdbk - This stands for the feedback velocity and it is the velocity data provided from the vehicle body's velocity port which outputs the velocity data.

· Grade - This port provides a grade or inclination angle for the surface on which the vehicle is travelling in degrees.

· Info - This port is for the bus signals which consist of block calculations e.g. acceleration/deceleration command, difference between reference and vehicle speed.

· AccelCmd - This port provides the acceleration command to a controlled voltage source.

· DecelCmd - This port provides the deceleration command to another controlled voltage source.

The working of the longitudinal driver block is simple. The reference velocity data coming from a drive cycle source is compared to the feedback velocity data coming from the vehicle body block. If the reference velocity is greater than the feedback velocity, an acceleration command will be given as an output to the controlled voltage source. If the reference velocity is lesser than the feedback velocity, a deceleration command is given as the output to another controlled voltage source instead. For the acceleration and deceleration commands to be given as output signals, I have used a Simulink - PS converter block. This block is needed whenever we wish to connect a Simulink input signal to a physical signal. In this case, the input signal is velocity data and this is logic data which we want to convert to a physical signal i.e.. voltages to the controlled voltage source.

The Simulink - PS converter block is shown below:

The longitudinal driver block parameters are shown in the image below:

As we can see, I am using a PI controller for this block and I have set the reference and feedback velocities to be in m/s units. All the other parameters are unchanged.

Drive Cycle:

A drive cycle is a data set that contains velocity and time data for the purpose of modelling the motion of a vehicle. The drive cycle will be a plot of the velocity against the time and it is made to resemble typical driving conditions that a consumer may experience themselves in. For instance, they can model highway driving situations, driving in urban areas, or driving with full power etc. Drive cycles are used by automotive companies for various testing procedures such as mileage, pollution tests, and acceleration and top speed tests. There are various different drive cycles that are industry standard e.g. Urban Dynamometer Driving Schedule (UDDS), FTP 75.

In my EV model, a drive cycle is used as the reference velocity data for the longitudinal driver block to send acceleration or deceleration commands according to the drive cycle. There are 3 ways of implementing a drive cycle. One method is to create or own drive cycle with a spreadsheet of velocity data in one column and time data in another colum.

Drive cycle parameters:

Once the drive cycle parameters have been set as shown above, the output port of the drive cycle can be connected to the reference speed port of the longitudinal driver block. If all the parameters have been set as discussed, the connections can be made as shown in the image below:

Result:

I RUN the us60 drive cycle at default drivecycle time 600sec and see the scope and analyse the Diffrence between drive cycle and vehicle graph.

Current scope:

SOC scope:

Accel Scope:

Main scope:

The distance cover by vehicle is 3.782m/s.

Conclusion:

Design the electric vehicle by using simulink library blocks and powertrain blocks and analyse the vehicle data.

US06 drive cylcle data we use and result shown in above image.

Distance cover by Ev vehicle is 3.782m/s.

Note: slx file and all important screenshots uploaded.