Designing the Power Train of TAMUQ Hybrid in Progress VehicleDone by: Mahmudul Alam, Project Lead, Jaber Al-Marri

Senior Year Design Project 2010, Electrical Engineering, Texas A&M University at Qatar

1. Introduction

Need Statement

Design, simulate, build, and test a power train of the HIPV that can propel the car both in forward and

reverse directions by means of a traction motor powered from its onboard batteries. The power train should

facilitate regeneration by braking. The whole system should meet the minimum performance requirements

and abide by all the rules and regulations stipulated in the Formula Hybrid 2010 Contest rule book.

Background and Conceptual Analysis

Any electric vehicle power train consists of three major elements: traction motor, battery and controller. A

conceptual illustration of the power train system is given in Fig.1.

Fig. 1 Conceptual Illustration of the power train

The motor converts electrical energy to

mechanical energy. It is part of the power

stage circuit, which also includes a

MOSFET bridge and filters. Switching

sequence of the MOSFETs determine the

mode of operation and the filter smoothes

the current and voltage ripple.

The controller co-ordinates among the

driver, the motor and the battery. Based

on the pulse width modulated (PWM)

signal generated by the micro- controller

, the gate drive circuit commands the

state of the semi- conductor switches,

which in turn regulates the power supply

from the battery to the motor. The duty ratio of the PWM signals depend on the brake and accelerator input

from the driver. The LCD notifies the driver about the driving mode and displays other necessary

information.

Battery supplies energy to both the power stage and the power electronic components of the control circuit.

2. Motor Sizing

0 5 10 150

10

20

30

40

50

60

70

80

90

100

time (s)

kilo

mete

rs/h

our

Car Weight 500 kg

Car Weight 300 kg

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1

9

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2 EqkWvmt

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restfromkphreachtoEnergyonAcceleratiforPower

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cruisevelocityconstforPowerP

tractive

C

Performance Requirement

Achieve 54 kph in less than 10 seconds.

Sizing

Chosen Motor

A permanent magnet DC motor (Mars E-TEK) was chosen. The chosen motor can provide power greater

than 6.525.

The main reason for choosing permanent magnet motor was it requires very simple controller. It does not

need any external field circuit and thus has less copper loss. Also it has reasonably high starting torque. The

drawbacks are low torque inducement due to weak magnetic field of permanent magnet stator and

demagnetization risk due to excessive current in the armature.

Simulation

The vehicle acceleration/speed was simulated as per Eq. 2.3. This differential equation takes care of all the

factors that may effect the speed of the vehicle. The simulation was performed for vehicle weight of 300 kg

and 500 kg. Fig. 2 says that lower weight vehicle can achieve speed faster.

Fig. 2 Vehicle speed vs. time for chosen motor

Here G = gear ratio, T0 = torque, = friction co-efficient, A =frontal area, Cd = aero dynamic drag co-

efficient, CW =wind speed co-efficient, = road angle, r = tyre radius, =air density, = mass factor, g =

gravitational constant, m = mass. Most factors were estimated.

rr

3. Converter Topology

In order to achieve motoring and regeneration both in forward and reverse directions, a full bridge converter

topology was chosen. During the motoring, the converter works as a buck converter and during regeneration, the

converter acts as boost converter. The switching sequence of the converter is illustrated in Fig. 3.

Fig. 3. Switching sequences to achieve four quadrants of operation (a) forward motoring,+v, +i; (b) forward

regeneration, +v, -i; (c) reverse motoring, -v, -i; (d) reverse regeneration, -v, +i.

(a) (b) (c) (d)

4. Control Circuit Design

Accelerator: Wheatstone bridge circuit produces 1.1-2.3 V for pedal travel from idle to WOT. An op-amp

amplifies this range up to 5-10 V. A differential amplifier next subtracts 5 V from this range to produce of 0-5 V.

Brake: A 5 k Ω resistor was used as break.

Power Supply for Control Circuit: All power electronic components need 5 V or 12 V supply. Since the 48 V

battery is the only power source, a switch mode regulator is used to get 12 V from 48 V and a linear regulator is

used to get 5 V from 12 V.

Battery/Capacitor voltage feedback: If Vbatt > 48 V or < 24 V, the system automatically shuts down to protect it

from over/undervoltage. Regeneration is allowed only if Vbatt < 75% of nominal voltage to prevent over voltage.

Pre-charge capacitor feedback is needed to determine if it is charged. Voltage divider is used to provide voltage

feedbacks and it was designed such that it gives 5 V for 48 V and 0 V for 0 V.

Gate-Drive: Two bootstrap gate drive ICs provide the gate signals for the MOSFETs.

Current Feedback: A bi-directional hall sensor is used for current feedback. Current feedback is necessary to

prevent MOSFET damage from over-current and to prevent demagnetization in permanent magnet motor.

Speed Feedback: To measure speed and prevent over speeding, an incremental encoder measures speed and

provides square wave output. A frequency-voltage converter then converts it to an appropriate voltage signal.

Pre-Charge/Protection Logic: Pre-charge circuit prevents filter capacitor from inrush current stress by

charging it through controlled current. Protection logic prevents shoot-through between MOSFETs.

Fig 4. Schematic of the power train.

5. Program AlgorithmThe program for the power train control circuit was developed using a big

loop and two sub-routines. The loop runs infinitely and calls the shut down

and direction check sub-routine when necessary. No interrupt was used.

After the system is powered up, a welcome message is shown. The micro-

controller then initializes the PWM, sets data direction registers and

configure ADC. The system then charges the filter capacitor. If there is no

fault with the pre-charge, it asks the driver to enable the power stage.

After power stage is enabled, direction is checked. Then depending on the

brake and accelerator reading, system operating mode is determined. The

brake overrides the accelerator.

6. Firmware Development, System Simulation and

Implementation

In each mode, voltage,

current and speed are

checked to see if

required MOSFETS be

turned on and if speed

and mot/regen current

can be increased as per

brake or acceleration

input. If any parameter is

> max set limit or < min

set limit, power stage

signals are disabled and

the loop starts again.

Fig. 5 Direction check

subroutine

Legend: Brk = brake, Acc= Accelerator, Th= Threshold , FWD= Forward, REV= Reverse,

Bat= Battery, Spd = Speed, Mot= Motoring, Rgn = Regeneration. Sw= switch, Drct=Direct.

Fig. 6 The main loop.

7. Acknowledgement

We are thankful to Dr. Shehab Ahmed & Dr. Mazen Saghir for all their guidance and support. We are also deeply

thankful to Mr. Abdallah Mardawi who helped us throughout the project. We also thank our parents whose

psychological support and inspiring words kept our work spirit enlivened at times when get going was tough.

Firmware is written based on the algorithm that has been explained above. The firmware is

written in BASIC language. The program was compiled by Mikrobasic, a compiler developed

by Mikroelektronica.

The whole power train circuit shown in Fig. 4 is simulated using a software called

Proteus ISIS. This is a software that facilitates the user to download the compiled

hex file in its virtual micro-controller and simulate. The chosen micro-controller

PIC16F877A is available in Proteus ISIS library. The simulation required numerous

stages of code re-writing, code debugging and circuit modification.

Fig 9. System Implementation.

Fig 7. Microbasic

Fig 8. Proteus ISIS

The system built is shown in Figure 9. Unfortunately

only part of the system works. The working subsystems

are current sensor, accelerator interface with micro-

controller, speed encoder feedback, brake, and LCD.

The reason the whole system cannot be tested is the

failure to load the program in the micro-controller. For a

successful micro-controller programming, the code

needs to be downloaded in chunks in it and tested along

side code writing. When the firmware was being

developed, the micro-controller was not available. And

with a non in-circuit programmer like PICStartPlus, it

was not possible to download the already developed and

simulated code in the PIC given the time students had

after the arrival of micro-controller.