WORKING OF IR SENSORS

  IR SENSORS

An IR (Infrared) sensor is used to detect obstacles in front of the robot. Whenever an obstacle is detected it gives out a signal.

                                        Applications of the IR sensor:

IR sensors are mostly used in robotic applications for the following:

1.       Proximity sensing: Detecting whether an obstacle is present within a pre-specified distance or not.

2.       Color sensing: The main application is the line follower. It is used to differentiate between black and white surfaces.

3.       Mapping and localization : Making a map of the surrounding and finding the position of the robot with respect to the surroundings.

                     WORKING OF THE IR SENSORS:

The IR sensors basically work using infrared rays. Infrared rays are a part of light spectrum that human can’t see but can be detected using special electronic gadgets. All objects in this universe emit infrared radiation including humans. The radiations emitted by different objects differ significantly. The IR sensor detects changes in the IR radiation when different objects fall on it.

The IR sensor is essentially composed of two parts:

1. Emitter

2. The detector

 The emitter generates infrared rays of a particular type. The detector continuously monitors the environment for any traces of IR light of the same type as the emitter. Whenever an obstacle comes in front of the IR sensor, the IR light emitted by the emitter gets reflected to the detector. The detector detects this light and gives out a signal.

The picture shown is very simple black box model of the IR sensor. The sensor emits IR light and gives a signal when it detects the reflected light.

Now suppose we want to use our IR sensor outdoors in sunlight. Sunlight also has an IR component. This causes our sensors to wrongly assume that there is an obstacle in front of it so how do we get around this? We use an essential property of sunlight – it is continuous. So instead of sending out the ir signal continuously from the emitter, we send it pulses. This type of IR sensor is called a TSOP- thin small outline package.

The IR light is emitted in pulses. The frequency is the inverse of Time period.

                               CHOOSING THE CORRECT IR SENSOR:

Choosing an IR SENSOR with TSOP for indoor purposes is redundant. Hence it is important to use the right sensor, keeping in mind the following:

Usage conditions: whether the sensor will be used indoors or outdoors.

Cost: TSOPs are immune to sunlight but it is not the right choice for indoor applications simply because it is costly.

                              Advanced reading – IR sensor:

An IR sensor consists of an emitter, detector and associated circuitry. The emitter is simply an IR led and the detector is simply an IR photodiode which is sensitive to IR light of the same wavelength as that emitted by the IR led .whenever light falls on the photodiode, its resistance decreases and this allows more current to flow through it. Now if a resistance is connected in series with the photo diode, the increase in current through it causes the voltage across the resistance to increase. This is because

                                                                                 V= IR

  Where,

V= voltage across the resistor

I= current flowing through the resistor

R=resistance of the resistor

  As it apparent from the above relation, the voltage increases with increase in current. Now if we compare this voltage with a preset voltage, we can know when IR light is falling on the detector, i.e., when there is an obstacle in front of the robot. The current that does this is called an operation amplifier.

It has three terminals: a +ve terminal, a –ve terminal and an output terminal. If the voltage at the +ve terminal is more than the voltage at the –ve terminal the output is a HIGH (+5V) and if the voltage at the –ve terminal is more than the voltage at the +ve terminal the output is low (0) volt. Now if we connect the –ve terminal to a preset voltage and the +ve terminal to the photo diode resistor junction, we can get  an output indicating whether an obstacle is present or not.

HERE HOW IT HAPPENS:

1. When the IR emitter is powered it begins to emit IR light .this cannot be seen with the naked eye. However it can be seen with naked eyes .However it can be seen through the lens of a camera.

2. The resistance and hence the output voltage of the IR receiver keeps changing as the intensity of the ir light incident on it is changed. When less IR light is incident (when there is no obstacle), the output voltage and vice versa.

3. This output voltage is connected to the non inverting pin of the op amp.

4. The output of the potentiometer is connected to the inverting voltage pin of the op amp.

5. Recall that the non inverting voltage of the op amp is greater than the inverting voltage, the output of the op amp is HIGH (5V) and when the non inverting voltage of the op amp is greater than the inverting voltage, the output of the op amp is HIGH (5V).

6. An LED is usually connected to the output of the op amp.

7 . When there is no obstacle in front of the sensor, the output voltage of the receiver is higher than the output of the potentiometer.

8. Hence, non inverting voltage of the op amp is greater than its inverting voltage and correspondingly the output of the op amp id HIGH and the LED glows.

9. When there is an obstacle in front of the sensor, the output voltage of the receiver decreases and goes below that of the potentiometer.

10. Hence the non inverting voltage of the op amp is lesser than its inverting voltage and its output becomes LOW (0V). Hence the LED stops glowing

                                                        CALIBRATING AN IR SENSOR

By changing the resistance of the potentiometer, the reference or the inverting voltage of the op amp is changed. This is very useful in order to make the IR sensor work in different lightning conditions. Doing this changes the range of the IR sensor.

Sunlight and some artificial lights have a significant components of IR light .This IR light adds to and in some cases replaces the light of the IR emitter. Hence the receiver continuously receives IR light irrespective of the presence and absence of an obstacle. Therefore a conventional IR sensor does not work in presence of sunlight or alight with a significant component of IR light.