Written by: Khaled Magdy & Taher Madany
What are Proximity Sensors?
We can say that proximity sensor is a device which detects objects nearby without any physical contact up to nominal range or sensor’s vicinity. In brief we can also say that Sensors which convert information on the movement or presence of an object into an electrical signal are called proximity sensors.
How Proximity Sensors Work?
When anything comes in the range of Proximity sensor it flash out infrared beam and monitors reflections. When sensor senses reflections it confirms that there’s an object nearby.
Working:
An Inductive Proximity Sensor consists of an oscillator, a ferrite core with coil, a detector circuit, an output circuit, housing, and a cable or
connector. The oscillator generates a sine wave of a fixed frequency. This signal is used to drive the coil. The coil in conjunction with ferrite core induces a electromagnetic field. When the field lines are interrupted by a metal object, the oscillator voltage is reduced, proportional to the size and distance of the object from the coil. The reduction in the oscillator voltage is caused by eddy currents induced in the metal interrupting the field lines. This reduction in voltage of the oscillator is detected by the detecting circuit.
Wiring The Proximity Sensor
Types of Proximity Sensors
There are several types of proximity sensor which are used according to the need, material detection and many other things. To classify them here are its types:
Inductive Proximity Sensors
Device which generates output signal or electrical signal when metal objects are either inside or entering into its sensing area from any direction. The metal objects above includes iron, aluminum, brass, copper, etc with varied sensing distances.
Other factors which affect Proximity sensors: flat targets are preferable, targets larger than the sensing face may increase the sensing distance. First inductive proximity sensor was introduced in the mid 60’s.
Capacitive Proximity Sensors
It can also detect metals but along with it can also detect resins, liquids, powders, etc. This sensor working can vary accordingly covering material, cable longness, noise senstivity. Its sensing distance also vary according to factors such as the temperature, the sensing object, surrounding objects, and the mounting distance between Sensors. Its maximum range of sensing is 25 mm.
Shielded and Unshielded
Question
Why is there a difference in the effect of surrounding metal between Shielded Proximity Sensors and Unshielded Proximity Sensors?
Answer
As shown in Figure 1, the surface of the detection coil on Shielded Proximity Sensors is covered with metal, so the flux is concentrated at the front of Sensors, which reduces the influence of surrounding metal.
As shown in Figure 2, the surface of the sensing coil on Unshielded Proximity Sensors is not covered with metal, so flux is also generated from the surface, which makes Sensors easily influenced by surrounding metal.
Line Follower
What is a line follower?
Line follower is a machine that can follow a path. The path can be visible like a black
line on a white surface (or vice-versa) or it can be invisible like a magnetic field.
Why build a line follower?
Sensing a line and maneuvering the robot to stay on course, while constantly correcting
wrong moves using feedback mechanism forms a simple yet effective closed loop
system. As a programmer you get an opportunity to ‘teach’ the robot how to follow the
line thus giving it a human-like property of responding to stimuli.
Practical applications of a line follower : Automated cars running on roads with
embedded magnets; guidance system for industrial robots moving on shop floor etc.
A Closer Look at the QTI
The QTI module is designed for close proximity infrared (IR) detection.
Take a look at the small square black box just above the QTI label. It’s
nested below the capacitor and between the two resistors. That’s a QRD1114 reflective object sensor.
There’s an infrared diode behind its clear window and an infrared transistor behind its black window.
When the infrared emitted by the diode reflects off a surface and returns to the black window, it
strikes the infrared transistor’s base, causing it to conduct current.
The more infrared incident on the transistor’s base, the more current it
conducts.
Wiring The QTI
When used as an analog sensor, the
QTI can detect shades of gray on
paper and distances over a short
range if the light in the room
remains constant. With this circuit,
you can set P3 high and then test it
with RCTIME to measure how long it
takes the capacitor to discharge
through the IR transistor. Since the
IR transistor conducts more or less
current depending on how much IR it
receives, the RCTIME measurement
can give you an indication of
distance or shade of gray.
If all you want to know is whether a line is black or white, the QTI can be converted to a
digital sensor by adding a 10 kΩ resistor across its W and R terminals. After doing so, the
QTI behaves similarly to the circuit on the right. When W is connected to Vdd and B is
connected to Vss, the R terminal’s voltage will drop below 1.4 V when the IR transistor sees
infrared reflected from the IR LED. When the IR LED’s signal is mostly absorbed by a black
surface, the voltage at R goes above 1.4 V. Since the BASIC Stamp interprets any voltage
above 1.4 V as 1 and any voltage below 1.4 V as 0, this circuit gives us a quick and easy
way to detect a black line on a white background.
Building the Sensing Circuits
If you apply 5 V to a QTI's W pin, its R pin will rise above 1.4 V if it detects a black surface, or fall below
1.4 V if it detects a white surface. In other words, the QTI sends a binary-1 if it does not see its IR
reflection or a binary-0 if it does. Only one QTI should be turned on at any given time to make sure that
one QTI doesn't see the reflection of another QTI's IR signal. With this rule in mind, P5, P6, and P7 each
connect to a QTI's W pin. P5 connects to the right QTI, P6 to the center QTI, and P7 to the left QTI. All
the B pins are tied to Vss. All the R pins are tied together and connected to P3. We'll turn each QTI on,
one at a time, read P3, and then turn that QTI off again. If the QTI that receives 5 V sees it reflection, it
will drive the voltage at P3 low; otherwise, it will be pulled high by the 10 kΩ resistor.
