robotics-a beginner's guide

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A powerpoint presentation for beginners in robotics

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    The word robotics is used to collectively define a field in engineering that covers the mimicking of various human characteristics

    Sound concepts in many engineering disciplines is needed for working in this particular field

    It finds its uses in all aspects of our life

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    An automatic industrial machine replacing the human in hazardous work

    An automatic mobile sweeper machine at a modern home

    An automatic toy car for a child to play with

    A machine removing mines in a war field all by itself and many more

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    Here we are aiming at design and prototyping of mobile robots capable of controlled locomotion.

    It may be via human or automatic control.

    Here we aim to make a bot capable of traversing a white line on a black surface automatically.

    It must be able to perform certain tasks we set for it.

    The task must be achieved within some given limitations.

    Keeping this as the basic idea we will be able to further widen our applications

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    Locomotion system

    Power supply system

    Actuators

    Sensory devices for feedback

    Sensor Data processing unit

    Control system

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    As the name suggests a mobile robot must have a system to make it move

    This system gives our machine the ability to move forward, backward and take turns

    It may also provide for climbing up and down

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    A simple equation: Power is the product of Torque and Angular velocity

    P = X

    This implies that if we want more torque (pulling capacity) from the same motor we may have to sacrifice speed and vice versa

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    The dc motors (tape motors) available have very high speed of rotation which is generally not needed. But what they lack is torque output

    For reduction in speed and increase in pulling capacity we use pulley or gear systems

    These are governed by: 1 X r1 = 2 X r2 The above concept will be discussed shortly

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    They convert the electrical energy into meaningful mechanical work

    Mechanical output can be rotational or linear (straight line)

    Motors provide rotational motion

    Electromagnets provide linear motion

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    AC Motors : Not used much in robotics

    DC Motors : Finds extensive general use

    Stepper Motors : For controlled rotation

    Servo Motors : For controlled rotation

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    As the name suggests, a motor which uses a DC (Direct Current) power

    Can run in both directions

    Speed Controllable

    Direction of rotation controlled by polarity of current / voltage

    Speed of rotation controlled by average energy (power) fed to the motor

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    The motors we use in making differential drive are simple DC motors.

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    Direction of rotation controlled by polarity of current / voltage

    Speed of rotation controlled by average energy (power) fed to the motor

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    Operating Voltage : Recommended voltage for powering the motor

    Operating Current : Current drawn at a certain load on the shaft

    Stall Current : Maximum current drawn, when motor not allowed to rotate

    Stall Torque : Rotation force needed to hold the motor in stall condition

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    Free running torque & current are ideally zero

    Increased load implies, increased torque, current drawn & power consumption

    Power supplied by a motor is the product of output shafts rotational velocity & torque

    DC Motors are highspeed, low-torque devices

    Using gears, the high speed of the motor is traded off into torque

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    Zero speed at maximum load (stall torque)

    Highest speed while free running (zero load)

    Highest power at half speed & half load

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    This is the car type drive and the most common in real world but not in robot world as in automobiles.

    The translation and rotation are independent of each other. But translation and rotation are interlinked hence this system faces severe path planning problem

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    A close relative of the differential drive system

    It is mostly used in tracked machines e.g. tanks. Also finds application in some four / six wheeled robots

    The left and right wheels are driven independently

    Steering is accomplished by actuating each side at a different rate or in a different direction, causing the wheels or tracks to slip, or skid, on the ground

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    In this mechanism the machine chassis (body) is deformed to achieve rotation in contrast to the steering wheels in car type drive

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    It is made up of a system of motors. One set of which drive the wheels and the other set turns the wheels in a synchronous fashion

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    The most unique type of Locomotion system

    It is composed of a four wheeled chassis and a platform that can be raised or lowered

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    A differential drive is the most basic drive, which consists of two sets of wheels that can be driven independently.

    By using different movements of

    the right and left wheels it is possible to make the robot

    move forward, backwards,

    right, left and even for zero degree turns.

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    Simplicity and ease of use makes it the most preferred system by beginners

    Independent drives makes it difficult for straight line motion. The differences in motors and frictional profile of the two wheels cause them to move with slight turning effect

    The above drawback must be countered with appropriate feedback system. Suitable for human controlled remote robots

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    Our power supply consists of the rectifier circuit and the transformer.

    The transformer is used to step down voltage from

    220 volt AC to 12 volt AC( or any other desired value).

    Further the rectifier converts 12v AC to 12v DC supply.

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    A transformer is a device that transfers electrical

    energy from one circuit to another through inductively coupled conductors.

    To make a simple differential drive, a simple

    transformer with following specifications can be used:

    1 Amp, C.T. 12 V.( it steps down a 220V AC to 12 V AC.

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    220 V AC supply

    12V AC

    -12V AC

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    A rectifier is an electrical device that converts alternating current (AC), current that periodically reverses direction, to direct current (DC), current that flows in only one direction.

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    Electrolytic Capacitor

    Integrated circuit (IC 78XX)

    p side

    n side

    1N4001 pn junction diode

    +

    -

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    Most electrolytic capacitors are polarized, i.e., they have a positive and negative terminal. Connecting them to wrong terminals of power supply can destroy it. Adding to this, these capacitors are used only for DC voltages.

    Fig: A blown capacitor

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    These ICs are voltage regulator integrated circuits. The voltage regulator IC maintains the output voltage at a constant value. The xx in 78xx indicates the fixed output voltage it is designed to provide. 7805 provides +5V regulated power supply. Similarly there are several other ICs available in this category like 7806, 7809, 7812, 7815.

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    Another fact to note about this series of ICs is that they have a voltage range in which they work. For any voltage above that range the IC goes into a thermal shutdown and will give you a very low arbitrary voltage value. For example, the 7805 IC has a range between 5V and 16V.

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    These are current amplifying circuits

    A low current control signal is converted into a proportionally higher current signal that can drive the motor

    Power Transistors can switch high currents. The ICmax is usually of the order of Amps as compared to small signal transistors which have ICmax in mA

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    H Bridge Circuit Diagram

    S1

    S3

    S2

    S4

    M1 2

    VCC

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    L293D Pin Diagram

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    It is an electronic circuit which enables a voltage to be applied across a load in either direction.

    It allows a circuit full control over a standard electric DC motor. That is, with an H-bridge, a microcontroller, logic chip, or remote control can electronically command the motor to go forward, reverse, brake, and coast.

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    H-bridges are available as integrated circuits, or can be built from discrete components.

    The term "H-bridge" is derived from the typical graphical representation of such a circuit, which is built with four switches, either solid-state (eg, L293/ L298) or mechanical (eg, relays).

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    The current provided by the MCU is of the order of 5mA and that required by a motor is ~500mA. Hence, motor cant be controlled directly by MCU and we need an interface between the MCU and the motor.

    A Motor Driver IC like L293D or L298 is used for this purpose which has two H-b