Lect 25

Question 1

What are proximity sensors?

Answer 1

An emmiter(LED) transmits a light with a defined wavelenght. Light it eventually reflected back from an object

Question 2

What is infrared sensor?

Answer 2

It is a proximity sensor. Used to measure the proximity of an object. A light source(LED) emitting a ray beam, which is then captured by a receiver (photo-transistor) after reflected by an object.
It is an active and exteroceptive sensor because it s measuring something outside the robot.
Because it is emitting(throwing) light to the environment, and the receiver is capturing the light reflected by an object.

** If there is an external source of energy, this energy will also be captured by the receiver and it is not possible to discriminate if it is led or ambient light. This is a problem of cross sensitivity.

One solution for this is to use a very narrow bandwidth to avoid for example the sun. If the receiver is too far away it wont be able to detect the object.

They are not very reliable because amount of light coming back does not depend on only the distance but also the interaction of the infrared with the obstacle. If the obstacle is very dark, it wont be able to reflect a light and it will absorb it therefore the receiver wont be able to capture this object. Darker objects may appear further compared to the lighter objects.
Infrared is near infrared (televizon kumandası gibi)

Question 3

Thermal infrared?

Answer 3

It works like infrared but with longer wavelenghts compared to the infrared. Thermal or PIR sensors measuring the infrared. Since thermal infrared is far infrared, we can perceive infrared coming from a hot body.

Question 4

Advatnages and disadvantages of infrared light?

Answer 4

Usually they are active sensors and exteroceptive sensors.
 Emits particular wavelengths and the reflected energy coming from an object is captured by the receiver.
Advantages:
Cheap
Can detect dark/light
Disadvantages:
Sensitivity to light conditions
Short range so we cant go fast

Question 5

What are 3d cameras?

Answer 5

Infrared sensors not only giving us the distance of an obstacle in 1D but also can take an image with a distance in every direction which is 3D

Question 6

What is triangulation ranging?

Answer 6

Geometrical properties of images to estabilish distance measurements.

Question 7

What is Laser Triangulation? 1D

Answer 7

Triangulation means that the distance measurement by angle calculation.
“"”Position sensitive device or linear camera””” , lens to enlarge the energy coming from the object.
We need colimated source of energy otherwise we will lose energy. The reflection of the known pattern is captured by a receiver and together with known geometric values, the system can use simple triangulation to establish the range measurements. IF the receiver measures the position of reflection along 1 single axis then it is called as 1D

Question 8

What is Structured Light? 2D and 3D

Answer 8

Can be used to f,nd the distance in a surface or a line. It again uses triangulation active ranging, which means that the sensor uses geometric properties to establish distance. This time the difference between laser triangulation is, the receiver measures the position of the reflection along two orthogonal axes. If one replaces the linear camera or PSD with a CCD or CMOS then we can recover distance to a large set of points instead of from only one point. Worsk as the same as light stripper but only no camera just a laser. The minimum distance you can sense is depending on your pixels of the camera.

Question 9

What is specular and difused reflection?

Answer 9

The light reflected from all the directions is called a specular reflection.
If the light is coming from only 1 direction it is called as diffused reflection.

Every object has diffused and specular reflection.

Question 10

What is colimated source of energy?

Answer 10

Energy staying on a small surface rather than spreading.

Question 11

What are kinect sensors?

Answer 11

Structured light. 2 cameras 1 standard RGB camera and infrared camera

Question 12

What are the ways to use in order to estimate the distance?

Answer 12

1) Measuring the amount of energy that is coming back by knowing the geometrical structure of the sensor we can reconstruct the distance.
(laser triangulation(1D) or structured light(in 2 or 3D)

2) Doppler effect:
The apparent change in the frequency of the waves caused by relative motion between the object and the receiver. Therefore the object is moving far away from the receiver, therefore it will receive energy at lower frequencies. If it is approaching to the receiver, then the receiver will receive higher frequencies.

BY MEASURING THESE WE CAN ESTIMATE THE DIRECTION AND THE SPEED OF THE MOTION OF AN OBJECT

Question 13

What are the different range (distance) sensors used?

Proximity ve range birbirinden biraz farklı. Range aynı zamanda distance sensor olarakta geçiyor.

Answer 13

Sonar
Laser range finder
Time of flight camera
Structured light

Question 14

Doppler effect sensing?

Answer 14

Laser or sound

Question 15

The difference between the proximity and range sensors?

Answer 15

Proxmitiy sensors sense if an object is withing the sensing area, but it does not indicate the distance between the sensor and the object of interest.
Distance sesnros sense the distance from the object and measuring device through an output current

Question 16

What is “"”time of flight””” in range sensors?

Answer 16

We emmit the energy, we measure the time the energy takes to arrive to the receiver

Question 17

What is structured light?

Answer 17

Project certain shape of light onto the object, look at the shape that laser pattern assuming the image, knowing the pattern and trigonometry, we can calculate 3D shape and measure the object with the laser.
DOES NOT USE TIME OF FLIGHT. USES TRIANGULATION TO MEASURE 3RD DIMENSION

Question 18

Sonars and laser range finders?

Answer 18

For sonars and laser range finders, they are absed on the time of flight.
For laser range finders instead of using time of flight, we use something related with the time which is PHASE SHIFT OF LIGHT WAVE

Question 19

What are ultrasonic range sensors? (sonars)

Answer 19

they emmit the sound and measure the time of flight. They ar cheap but highly unreliable. They work with ultrasonic sound waves. They are highly unreliable because they suffer from cross talk. Soft surfaces absorb most of the sound energy. and specular reflection is a problem of sound energy

To solve this:
more than one sonar, one sonar is active emmits the energy and when it finishes the other one is active.

Question 20

Why there are sources of errors from the sensors?

Answer 20

Opening angle
Cross Talk
Specular reflection (energy going in 1 direction the object is reflecting the other directions but nı energy coming back to the sensor)

Infinite reading: no obstacle nothing because sometimes sound waves are affected with the specular reflection (no energy coming back, infinite reading)

Question 21

What are occupancy grids?

Answer 21

Built by the ultrasounds or laser range finders. We can plot the space infront of the robot from the sensor readings. We can put a grid everytime we have a measurement. First we create a local occupancy grid by wiritng free occupied or unknown. Then we integrate the occupancy grid while the robot is still moving anf we can create a gloabal occupancy grid.

Question 22

What are laser range finders?

Answer 22

We have a source of laser light, we have a receiver of laser light, but we are exploiting a different principle. We are not exploiting triangulation as before, we are exploiting the time of flight like sonars. But this time instead of measuring the time we measure sımething related with time which is phase shift. It is measuring the phase between the two signals. The two signals are having the same frquency byt the phase shift among them is different and the phase shif arttıkça dstance artıyor demek.
By measuring the phase shift between the transmitted and reflected beams, we can calculate the range/distance.
If the phase shift is bigger than one period the sensor wont measure it because it can only measure up to some limit.
Since phase shift is really small, you can end up getting a wrong reading. They ned precise calibration

Question 23

Laser range sensors?

Answer 23

Typical range image of 2D laser range sensor with rotating mirror. Not a 3D scanning, we have just 1Dot(laser) 1 line of laser and we are moving the line of the laser in every direction we have one dot and the mirror is rotating therefore we can measure every direction of the environment. The ones close to the sensor is better,the ones further have very noisy measurements
Occlusion is hard for this devices.