Sensor Interfacing

Why Robots need Sensors

The first primitive of robotics is “Sense”.

A robot senses its environment by means of sensors and transducers

Different Types of Sensing

Touch Vision Listening Feel ( Pressure,Temperature, Humidity) Smell

Similar to human beings Robots Can sense all these with help of different types of sensors

Different Sensors

For vision a robot Uses 1. Camera2. Light and Infrared Sensors

Sound Sensors

The Common Sound Sensors are1. Microphone2. Ultrasound sensors

Temperature and Humidity

Thermistor Humidity Sensor

GAS SENSOR

Some More Sensors

Heartbeat Sensor

Passive Infrared Sensor

Sensor & Transducers

Transducer is a device which converts electrical signals to other kind of

energy and Vice-Versa.

Sensor is an assembly which contains some transducer and some

electronic processing circuitry

Resistive Transducers

Resistive Transducers are those, whose resistance varies with the change of the external signal.

Example – LDR, Thermistor

Commonly used Sensors in Robotics

Mainly Optical sensors are used in robotics in student level, because they are cheap and easily available

Commonly Used Optical sensors Are

1. Light Dependent Resistors2. Infrared Sensors

Light Dependent Resistor

An LDR has a resistance that varies according to the amount of visible light that falls on it. More the incident light, less the resistance. This is known as a negative resistance co-efficient.

How the Light Sensor works

A LDR Sensor array by THE ARELAB.

Here LDRs and High intensity LEDs are placed side by side to make them suitable for light sensing

Reflection type LDR sensor

LEDLDR

Why Light Sensors are Used

A Very common objective in the modern competition robots is to follow a white line on a black background

Making the Potential Divider

+

LD

R

RES

ISTIV

E

DIV

IDER

FIX

ED

RES

ISTA

NC

E

• The LDR gives change of the resistance with the variation of incident light. We have placed the LDR in one arm of a potential divider.

• Now, if the resistance of the LDR varies, the output voltage of the potential divider also changes

What to do next

It is clear that the output of the potential divider is an analog voltage. But Microcontroller does not accept the analog voltage. So we need to convert the analog voltage to digital before we feed it to the microcontroller.

For this conversion we can use1. Comparators2. ADCs

Comparators

-

COMPARATOR LM339

+

V+

V-

V0 V0 = High when V+ > V-V0 = Low when V+ < V-

Comparators

-

COMPARATOR LM339

+

V(LDR)

V( REF)

V0

V(LDR)= LDR output VoltageV( REF) = To be set by a potentiometer

Setting the V(Ref)

Rotating the potentiometer 22 times give the full swipe of the resistance range.

Connection of LDRs

FIX

ED

RES

ISTA

NC

E

RES

ISTIV

E

DIV

IDER

+ 5VPOTENTIOMETER

OUTPUT

V

Ω

+

LD

R-

COMPARATOR LM339+

5V

Checking the Resistance

On white Line – More light Reflects, so LDR resistance goes down.

On Black Background – Less light reflects, So LDR resistance goes up.

Function of the Comparator

V+

V-

V+ > V-

0 V

HIGH

-

+

Function of Comparator

V+

V-

V+ < V-

0 V

LOW

-

+

Function of Comparator

V+

V-

V+ > V-

5 V

HIGH

+ 5V

-

+

Drawbacks of LDR

Does not give precise measurements for position calculations.

Ambient interference degrades performance

Infrared Sensors

Infra-Red emitter sends IR pulses

Position calculation is done through intensity of reflected light received by the detector.

Ambient interference is negligible

How to position IR sensor

0.5cm

EMITTER

DETECTOR

IR connections

+ +

+

100 OHMS

10 K OHMS

IR SENSOR ARRAY

Black Wire – Output Voltage

Red Wire - +5V

Brown Wire - Gnd

Thank You