EE213 Term Project 2014 2015

Topics: Pulse-width modulation, Pulse wave, Square wave Pages: 5 (1566 words) Published: December 4, 2014
EE213 Electrical Circuits Laboratory Term Project
Two Speed Automatic Transmission System
1. Introduction
Transmission system is an assembly of gears,
shafts and electrical gadgets in an automobile. This system
forms a bridge between the engine and wheels to transmit
mechanical power required for movement. The complete
set up reduces the high rotational speed of engine to
lower wheel speed, since internal combustion engines in
automobiles operate between 600 and 7000 revolutions
per minute (rpm) whereas wheels rotate between 0 and
1800 rpm [1]. Moreover, transmission ensures the proper
amount of torque according to the speed of the car. For
example, greatest torque is required to move the car from
rest; however, one needs greater rpm for high speeds.
A transmission may have multiple gear ratios that
require switching as the speed varies. Switching can be
done manually (by the driver) or automatically. In any
case, there are usually a speedometer and a tachometer
in front of the operator that display the speed of the
automobile and rotation count of the engine, respectively.

Figure 1: Illustration of the manual transmission [2]

2. Term Project Description
In this project, you will simulate the tachometer of an automobile with automatic transmission. The simulation will be done for two different gear ratios, one of which represents low-speed/high-torque while the other stands for high-speed/low-torque. The variation of the DC level of a periodic voltage waveform will represent the variation of engine rpm. As soon as the DC level goes out of the predefined voltage range, your system should change the gear accordingly and engine rpm should be demonstrated by making use of the analog multimeters in the laboratory.

The system consists of four main blocks and the overall block diagram is given in Figure 2.

Variable Duty Cycle
Generator

DC Level Detector

Automatic Shifting
Mechanism

Figure 2 - System Block Diagram

Tachometer

2.1 Variable DC Voltage Generator
The variation of engine rpm will be modelled as DC level of a constant-frequency voltage waveform. For this purpose, you should first use a variable duty cycle generator, whose output will be used to generate variable DC voltage.

2.1.1 Variable Duty Cycle Generator
Suppose a rectangular waveform which only interchanges
between high and low voltage levels. A duty cycle is the percentage of one period in which a signal is high. Duty cycle, D, may be expressed as:

where T is the time the signal is high, and P is the period of the signal. Different duty cycles are exemplified in Figure 3.
Pulse Width Modulation (PWM) is a technique used to
encode information for transmission and refers to rectangular pulses of variable width. Moreover, PWM signals are also used to control the power supplied to electrical devices and highly favored due to power efficiency.

Figure 3 - Different Duty Cycle Examples

When the duty cycle of a rectangular wave is increased, e.g., the pulse is made wider, the average voltage also increases as illustrated in Figure 4.

Figure 4 - Pulse Width Modulated Waveform [3]

For this project, you will first design a variable duty cycle generator so that engine rpm will be represented by a DC voltage. Your design should include a variable resistor that will be adjusted by the user for the variation of duty cycle. The change in duty cycle must be continuous. 2.1.2 DC Level Detector

As the second step, you need to extract DC level information of the rectangular pulse train you generated at the first step. For this purpose, you will design a circuit that outputs a DC voltage which will indicate the average of PWM signal. Since the change in duty cycle is continuous, level of the DC voltage should also vary continuously.

2.2 Automatic Shifting Mechanism
Automatic Shifting Mechanism is the part that checks the DC voltage level and shifts the gear up if it exceeds the predefined voltage level. Shifting...

References: 1. http://en.wikipedia.org/wiki/Transmission_(mechanics)
2. http://repairpal.com/manual-transmission
3. http://www.electronics-tutorials.ws/blog/pulse-width-modulation.html
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