Post on 03-Mar-2016
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A differential drive is the most basic drive, which consists of two sets of wheels that can be driven independently.
To turn the vehicle (or robot) LEFT or RIGHT, wheels are rotated at "different" speeds or in different directions.
MOTION LEFT WHEEL RIGHT WHEEL
Our power supply consists of the rectifier circuit and
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.
The Rectifier Circuit
A line follower is basically a robot or a machine that takes external inputs (sensory feeds) and decides whether it is on the path or off it. Let us consider that we are trying to follow a certain white line on a black background.
How would the bot decide whether it is on the white line?.
LED - LDR
LED LDR sensors come into the picture here. For detecting a white line on a black surface the bot must be able to distinguish between them.
In an electric circuit distinguishing a difference in any field is achieved using difference in voltages. The sensors basically provide this voltage difference.
Here we take the example of a differential drive with a LED-LDR sensor array for line following.
LED-LDR sensor unit is used for detecting surfaces that reflect light at intensity different from the adjoining surfaces. **LED (Light Emitting Diode) : Small elongated bulb-like object that emits light. Available commonly in red, green, white etc. **LDR (Light Dependent Resistor) : Small resistor, resistance depending (non-linear) upon the intensity of incident light The resistance usually varies in the range of ~10k ohms. Hence a 10k ohm resistance should be used in series with a LDR.
LM339 Pin Diagram
Light Sensing Module using LED-LDR combination. As high intensity of light falls on the LDR, its resistance drops and the potential at -ve terminal of comparator increases. If this potential is less than that at the +ve terminal, a high signal is obtained as output.
Resistance of an LDR changes depending on the intensity of light that is incident on it.
Light (from LED) reflected from white surface -> brighter (i.e. more intense) Light reflected from the black surface ->Less intense
Two different voltages achieved, thus separating white from black.
Using these two voltages a mean voltage is decided which is used as the reference voltage. This is termed as calibration.
In comparator circuit,
Voltage higher than reference -> HIGH
Voltage lower than reference -> LOW
Circuit has 4 sensors
Each is connected to a comparator of the IC
Each should have its own potentiometer for thresholding
Brain of the autonomous robot
Has its own microprocessor, RAM and static flash storage
ATMEGA 16 [used here]: 16MHz processor, 1KB internal SRAM and 16KB of in-programmable memory
40 pins-8 are reserved for various functions; other 32 pins are general purpose pins which may be used for general input and output.
Pins operate in highs and lows. +5 volt is termed as high; ground potential is low.
Code designates pins as input pins or output pins.
Input Pins: Used to take input from surroundings, say, by hooking up with the output of a sensor
Output Pins: Used to control external actuators such as motors (via a motor driver).
ATMEGA 16 comes with several advanced features like timers, interrupts and analog to digital converters.
Code is burnt onto microcontroller using a programmer.
Motor Drivers:H Bridge
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. With an H-bridge, a microcontroller, logic chip, or remote control can electronically command the motor to go forward, reverse, brake, and coast.
Motor Drivers : H Bridge (Contd)
H-bridges are available as integrated circuits, or can be built from discrete components.
The term "H-bridge" 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).
S1 S2 S3 S4 Result 1 0 0 1 Motor rotates in one direction 0 1 1 0 Motor rotates in opposite direction 0 0 0 0 Motor free runs (coasts) 0 1 0 1 Motor brakes 1 0 1 0 Motor brakes
To power the motor, you turn on two switches that are diagonally opposed.
DC Motor Direction Control
H Bridge Circuit Diagram
M 1 2
H Bridge Working
Need of Motor Driver IC
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-bridge drivers. Hence, each IC can drive two motors
Note that a motor driver does not amplify the current; it only acts as a switch (An H bridge is nothing but 4 switches).
L293D : Motor Driver (Contd)
Drivers are enabled in pairs, with drivers 1 and 2 being enabled by the Enable pin. When an enable input is high (logic 1 or +5V), the associated drivers are enabled and their outputs are active and in phase with their inputs.
When the enable pin is low, the output is neither high nor low (disconnected), irrespective of the input.
Direction of the motor is controlled by asserting one of the inputs to motor to be high (logic 1) and the other to be low (logic 0).
To move the motor in opposite direction just interchange the logic applied to the two inputs of the motors.
L239D : Motor Driver (Contd)
Asserting both inputs to logic high or logic low will stop the motor.
Difference between L293NE and L293D: Output current per channel = 1A for L293 and 600mA for L293D.
Vs is used to power the motors while Vss powers the L293NE.
To control motor speed we can use pulse width modulation (PWM), applied to the enable pins of L293 driver.
PWM is the scheme in which the duty cycle of a square wave output from the microcontroller is varied to provide a varying average DC output.
What actually happens by applying a PWM pulse is that the motor is switched ON and OFF at a given frequency. In this way, the motor reacts to the time average of the power supply
An Example - Line Follower
A line follower is a robot capable of tracking a line drawn on a surface
Optical sensors capture the line position at the front end of the robot
The robot is steered to keep it always over the line
Block Layout of Line Follower
How many LED-LDR pairs are required to create an autonomous line following robot?
1 Sensor (The Edge Finder)
Only 1 sensor is needed to follow a line. It actually follows one edge of the line, continually sensing the transition from dark to light. Assuming a 2 motor drive system, one motor is activated when the line is seen, the other is activated when the line is not seen.
The Edge Finder
This works fine at slower speeds but becomes unusable as the bot goes faster -If the sensor crosses the other side of the line, it will may head back in the opposite direction or if