Final Project: Design of an Electric Vehicle

 

 Q. Create a MATLAB model of an electric car that uses a battery and a DC motor. Choose suitable blocks from the powertrain block set. prepare a report about your model including following 

Objectives

1. System-level configuration

2. Model parameters

3. results

$.conclusion

 

A:

The Electric Vehicle model has divided into four major parts which include the primary components of the system. They are the Drive source, Motor and power controller, Battery system, and vehicle body.

 

The prediction of performance and range of any vehicle is a very important concept for a designer which can be done easily by programming techniques using mathematics and spreadsheet programs such as MATLAB and EXCEL. These tools are an excellent basis for the simulation of an Electric vehicle and are much easier than other vehicles.

In order to determine the performance of an electric vehicle, the important parameter to be focused on is acceleration and top speed where the vehicles always have poor response. It is also mandatory for a designer to predict the range of a vehicle. Now we see the simulation of the electric vehicle model to calculate the distance traveled by the vehicle for a given drive cycle by changing the parameters in Simulink and simscape blocks that are modeled using algorithms and some governing mathematical equations.

Electric Vehicle Model :

 

 

The model has four subsystems which are explained in detail one by one, the first one is 

Vehicle Body SubSystem :

In the model, the vehicle body, simple gear, differential, and tires blocks are considered as a subsystem under the name Vehicle Body. If we open the subsystem Vehicle Body, we can see the connections between the blocks, and an explanation about each block connectivity and its function is given below.

 

 

 

 

 

 

 

 

 

 

 

  This block represents a two-axle vehicle body in longitudinal motion. The block accounts for body mass, aerodynamic drag, road inclination, and weight distribution between axles due to acceleration and road profile. 

  It has six terminals namely H, V, NF, NR, W, and Beta.

Connection "H': It is the mechanical translational conserving port associated with the horizontal motion of the vehicle body. the resulting traction motion developed by the tire should be connected to this port.

Connection 'V, NF, NR are physical signal output ports for vehicle velocity and front and rear wheel forces respectively.

Connection W and Beta are physical signal input ports corresponding to headwind speed and road inclination angle respectively.

Ih the model, constant load mass is considered. if the variable mass is taken into account, then center gravity, CG, and M ports come into play by accepting two-element vectors respectively x and y distance offsets from vehicle CG to additional mass load CG.

 

                                            

 

 

                                           

 

If we click on the vehicle body block, the above pop-up appears with default values. We need to change the values depending on the requirement of the model to achieve accurate results. The parameters changed for this model are

Mass -1000kg (default value is 1200kg)

Frontal Area-2m2(default value is 3m2)

Drag coefficient-0.15 (default value is 0.4)

 

Explanation:  The frontal area and the drag coefficient should be kept as low as possible so that the power requirements to overcome the aerodynamic forces can be reduced. The mass of the vehicle also has effects on the vehicle performance, range, and cost of an electrical vehicle. It mainly affects the rolling resistance, power, and energy. when the vehicle climbs a hill, there will be kinetic energy loss occurs when the vehicle is accelerating and decelerating. For these reasons, the vehicle mass, frontal area, and drag coefficient are taken low values.

 

 

 

   

 This block represents the longitudinal behavior of a highway tire characterized by the Tire magic formula. The block is built from tire-road interaction and simscape foundation library wheel and axle blocks. in this, we can also include the effects of inertia, stiffness, and damping.

 Four such tires are taken for this model and are connected to the vehicle body in the following way.

'H' terminal of the rear wheels and front wheels are connected to the H port of the vehicle body

N terminals of the rear wheels are connected to the NR port of the Vehicle body.

N terminals of the front wheels are connected to the NF port of the Vehicle body.

Terminals are axle ports of rear wheels that are connected to simple gear so that transmission is transformed to the vehicle body. through these ports.

S  terminals are connected to PS terminators so that they remain inactive.

 

                                    

 

 

 

 

For the tire block,  all default values are considered for this model. No changes have done.

 

 

           

This block represents fixed-ratio gear. It has two terminals B and F. B represents base and F represents the Follower of the gear and are mechanical rotational conserving ports. The F terminal is connected to the rear axle and the B terminal is connected to the DC motor.

 

                              

 

Explanation :

Here the Gear Ratio is changed to 7 to increase 7times the torque produced by the motor. This torque is provided to the rear axle of the vehicle to increase the propulsion. Electrical vehicles transmission is much simpler as the motor provides motion from zero speed upwardsi.e.by just connecting a simple gear ratio provided with differential. since the differential causes power losses, many vehicles connect the wheels directly to the motor via a gearbox. 

 

 

 

 

4. Differential Block :

 

 

                                                    

 

Default values of the block parameters are taken for simulation. The differential provides the tractive force equally to both rear and front wheels. But due to the space and power loss constraints, these differentials are eliminated. In this model, the differential is placed providing meshing, vicious and inertia losses are all zero. If the differential is removed, then a powerful electronic controller is needed to provide equalized torque to all the wheels.

 

 

Explanation : 

PS – Simulink converter is the block used to connect the vehicle body which is a simscape signal block to the display block which is a physical signal block. Since both of them are from different libraries we need to use converters to make the connection.  The output value is the velocity and its units is in m/s. since the reference input velocity from drive cycle is taken in km/hr, the output velocity value is also to be in km/hr. So to change the units from m/s to km/hr, a small subsystem has been created and its output value is displayed using display block.

 

                           

 

 

 

 

 

 

Explanation :

Every single value of output velocity is multiplied by 3600/1000 to convert the velocity units from m/s to km/hr. This value output is again provided as velocity feedback input value for each cycle to Longitudinal driver to provide acceleration command for every second.

This completes the subsystem, Vehicle Body.

 

 

 

 

 

Power controller Subsystem :

when the power controller subsystem is clicked the below circuit diagram will appear.

 

The main components in this subsystem are DC motor, H-Bridge converter, and controlled PWM voltage source.

 

 

1. DC Motor Block:

This block represents the electrical and torque characteristics of a DC motor. while using this block we assume that no electromagnetic energy is lost and hence the black emf and torque constants have same numerical values.

Motor parameters can be either be specified directly or derived from no-load speed and torque. when a positive current flows from the electrical + to - port, 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. since the motor rotates the casing of the motor need to be kept stable, for that we connect the casing terminal C to the mechanical rotational reference block. the R port is connected to the simple gear B terminal port.

 

                                                  

 

The parameters changed in this block are

Model Parameterization is By rated power, rated speed and no-load speed.

No-load speed=6000rpm

Rated Speed at rated load is 4000rpm

Rated load(mechanical power) is 5000W

The Rated DC supply voltage is 1.5 V

The remaining all value is kept constant.

 

 

 

2. H- bridge converter Block:

This block represents an H- bridge motor drive. This block can be driven by the controlled PWM voltage block in PWM or average mode. In PWM mode, the motor is powered if the PWM port voltage is above the enable threshold voltage.

 

 

 

Parameters changed here from default are

Simulation mode is Averaged

Input Voltage for 100% duty cycle is 1

Output voltage amplitude is 1

The remaining all are kept constant.

 

Explanation :

Controlled PWM device receives constant voltage from the controlled voltage source and converts into pulse signals with two switching actions ON and OFF and sends to the power converter. Here the power converter taken is an H-bridge power circuit that receives the pulse width modulations and provides the required amount of voltage to the DC motor. Now the Dc motor will be under the control of this power converter and receive the exact amount of current to produce the required amount of flux. This is because the flux quantity generated through the coils depends on the power supply. If more current is available means more flux generates and the rotor rotates at high speed due to electro magnetic induction. So then the rotating motor transmits the required amount of torque to the shaft of the wheels via gearbox.

The average output voltage amplitude is one because the amplitude set here are 0 and 1. The pulse signal modulates between 0 and 1.0 means OFF state and 1 means ON state.

So with the increase in voltage and frequency, the power supply will also increase. So the power is being supplied to the motor depending on the voltage and frequency ratio and the demand of traction power at wheels. If more power is required the H-bridge circuit draws from the battery of the vehicle.

 

 

3. Battery Subsystem :

This subsystem included a Battery of lithium-on, a controlled current source, Bus selector. The port1 is connected to the current sensor in the subsystem Power controller.

 

 

1.Battery Block :

 

using this battery block, the state of charge (SOC) , current and voltage across the battery can also be calculated

                                          

 

The parameters taken for this model are

Nominal voltage 48V

Rated Capacity-50Ah

The initial state of charge is 90%

The remaining values are kept constant or default values.

 

Explanation :

In this model after simulation, the SOC of the battery remained 100% even though the initial state of discharge given was 90%. It means less battery power is drawn for the Motor by the circuit and the battery would have gained energy during regeneration. Battery Current utilized is 7A by the DC motor for the propulsion of the vehicle. Most of the internal energy provided by the power controller is used for the vehicle propulsion for the entire drive cycle.

Longitudinal Driver: Longitudinal driver block takes the responsibility of calculating acceleration by taking two inputs one from the drive cycle velocity input and the other one from vehicle traction velocity at each one second of time interval and is repeatedly provides the input to the voltage source till the end of the drive cycle.

 

 

 

    

 

                                           

 

 

 All the default data provided in the driver block is considered as input to the simulation. If needed the data can be varied and observe the simulation of the vehicle.

 

 

 

      

The range of electric vehicles is the major problem faced by the automobile industry because storing electrical energy efficiently is a very tough task for engineers.  This is a critical issue in the design of any electrical vehicle. For that what we can do is test the vehicle by driving in reality or in simulation through a profile of ever-changing speeds. These test drive cycles have been developed to provide a realistic and practical test for the emissions of vehicles. Now for this model, the FTP75 drive cycle is chosen and with this cycle input reference, simulation of the model is done. With this simulation, the distance traveled by the vehicle at the end of this cycle is calculated using MATLAB software. In this drive cycle, the condition or the velocity of the propelling vehicle is noted at each second. The important data that needs to maintain as a record is the amount of charge removed from the battery, the depth of discharge of the battery, and the distance traveled. By taking these velocities as input and is fed to the longitudinal driver block. It calculates acceleration and thus the tractive effort, and thus the motor power, torque, and speed. Using this data motor efficiency can be found and also the electrical power going into the motor. This calculation is repeated every one second till the end of the cycle.

                                         

 

 

  Controlled Voltage Source: Provides constant voltage to the PWM voltage source

Electrical ground: It is needed to discharge the excess charges flowing through the devices. so all the negative terminals of the devices are connected to the ground.

 

PS Terminator: This block is connected to the unconnected terminals which are inactive in the model.

PS Constant:  This block represents a constant and is taken from the simscape library.

Controlled current source: It is used to provide current to the battery and is a physical signal.

 

Bus Selector:  This block  accepts bus as input which can be created from Bus creator and outputs using a bus object.

Solver configuration Block:  Each physical network represented by a connected Simscape block diagram requires solver settings information for simulation

 

     

 

 

 

Calculate the distance traveled by the vehicle using the velocity data available at the end of each second.

  

Subsystem for the distance calculation :

 

 

 

 

 

 

 

Result: So at the end of the FTP75 drive cycle, the total distance traveled by the vehicle is 16.22Km

 

 

 

Plot: Velocity Drive Cycle: Most of the time vehicle velocity-time characteristics follows the FTP75drive cycle velocity inputs for the given power supply and drive. But at few areas, it is mismatching with the data. The range of the cycle is closely 90km/hr. The distance travelled during this drive cycle is 16.7km and the vehicle is propelling till the end of 2474 seconds. After 1500 seconds, the vehicle comes to rest for some time which shows zero velocity in the graph. During this period regeneration takes place that means the current will flow back to the storage device.

 

 

Plot: Current taken during simulation is almost constant at high speeds and high amounts of current are taken by the motor.

 

 plot: SOC%:Battery discharge energy is less and absorbed energy for the first few seconds and remained constant .

 

 

 

 

 

 

 

Battery Voltage Plot: High voltages exist during most of the cycle period. Need to focus on reducing high power transient conditions.

 

 

 

 

 

 

 

Using programming techniques of MATLAB, simulations can be easily performed and can predict the vehicle performance and range of the vehicle which is cost and time-effective.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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