Proximity sensors detect objects without physical contact using various technologies like inductive, capacitive, ultrasonic and optical. Inductive sensors detect metallic objects using a coil and oscillator to create a magnetic field. Capacitive sensors detect metallic and nonmetallic objects by measuring capacitance changes between the sensor and object. Ultrasonic sensors use sound waves above human hearing range, while optical sensors use light beams reflected off objects. Key features of good sensors include precision, accuracy, response speed, operating range, reliability, easy calibration and low cost.

1. Photo voltaic cell
• Generation of potential difference when light
strikes a junction of two dissimilar metals is called
photovoltaic effect.
• Photovoltaic cell works on principle of
photovoltaic effect.
• Photovoltaic cell is an active transducer and need
not operate in vacuum or gas filled tube.
• Photodiode and photo transistor

2. Photo diode
• The reverse bias is applied against the p-n junction that
results in a very high resistance.
• The light ray is made to fall on n-layer.
• This results in decrease in resistance of the diode,
developing the reverse current due to sweep up of the
electron-hole pair.
• The reverse current is the measure of intensity of the
incident radiation.
• The response of the photo diode is quick and they can be
used as variable resistance devices.

3. Photo Transistor
• A base collector is arranged in parallel to the photo diode, a
bipolar transistor is formed.
• In this p-n junction the collector base is photo sensitive.
• The incidence radiation on the base results in reverse
current of photo diode, which is the base current of the
transistor.
• The base current is proportional to the intensity of light.
• If the base current is amplified by internal signal
processing, then the higher sensitivity of the photo
transistor can be achieved.

4. PROXIMITY SENSORS
-Vinayak Kulkarni

5. PROXIMITY SENSORS
• A proximity sensor detects an objects when the
object approaches within the detection range and
boundary of the sensor.
• Proximity sensors include all sensor that perform
non contact detection in comparison to sensors
such as limit switch, that detects the object by
physically contacting them.
• Proximity sensors are used in various facets of
manufacturing for detecting the approach of
metal and non metal objects.

6. Inductive proximity sensor

7. Inductive proximity sensors working principle
• Inductive proximity sensors detect the presence
of metallic objects.
• Their operating principle is based on a coil and
high frequency oscillator that creates a field in
the close surroundings of the sensing surface.
The presence of metal in the operating area
causes a change in the oscillation amplitude.
This change is identified by a threshold circuit,
which changes the output of the sensor. The
operating distance of the sensor depends on the
coil’s size as well as the target’s shape, size and
material.

8. Inductive proximity sensors
• An inductive sensor is an electronic proximity
sensor, which detects metallic objects without
touching them.
• Electric current generates a magnetic field,
which collapses generating a current that falls
asymptotically toward zero from its initial level
when the input electricity ceases.

9. Construction
• The proximity inductive sensor basically
consists of a wound coil located in front of a
permanent magnet.
• The permanent magnet is encased in a rugged
housing.
• The change in current in the coil is output
through the leads embedded in the resin.
• The leads connected to the display through a
connector gives signal for the presence of an
object in the vicinity.

10. Applications
• Common applications of inductive sensors
include metal detectors, car washes, and a
host of automated industrial processes.
Because the sensor does not require physical
contact it is particularly useful for applications
where access presents challenges or where
dirt is prevalent. The sensing range is rarely
greater than 6 cm, however, and it has no
directionality.

11. Capacitive proximity sensors

12. Capacitive proximity sensors working principle
• Capacitive sensors are used for non-contact
detection of metallic objects & nonmetallic
objects (liquid, plastic, wooden materials and so
on). Capacitive proximity sensors use the
variation of capacitance between the sensor and
the object being detected. When the object is at
a preset distance from the sensitive side of the
sensor, an electronic circuit inside the sensor
begins to oscillate. The rise or fall of such
oscillation is identified by a threshold circuit that
drives an amplifier for the operation of an
external load.

13. Applications
• Capacitive touch sensors are used in many
devices such as laptop track pads, digital audio
players, computer displays, mobile phones,
mobile devices and others.
• More and more design engineers are selecting
capacitive sensors for their versatility,
reliability and robustness and cost reduction
over mechanical switches.

14. Ultrasonic sensors
• Ultrasonic sensors are some times used in
place of optical sensors. Instead of using an
light beam, a high frequency sound wave is
used.
• This sound wave is above normal hearing
frequencies and are called ultrasonic.
Frequencies around 40 KHz are common.

15. What does an ultrasonic sensor do?
• It emits a short Ultrasonic Pulse ...
Sensor

16. What does an ultrasonic sensor do?
• ...and waits for the echo.
Sensor

17. What does an ultrasonic sensor do?
• It calculates the distance to the object
– waiting time
– speed of sound
Sensor
distance =
waiting time x speed of sound / 2

19. Optical sensor

20. Optical sensor
• The proximity of the object is detected by the action of the
travelling light move.
• The light emitted by the transmitter focuses on the object which
reflects to be received by the receiver photo diode.
• The light from the emitting diode is focused by the transmitter lens,
on to the object surface.
• The reflected waves travel back and received by the solid state
photo diode, through the receiver lens.
• The object within the range of the sensor can detect the presence.
• The focal length of the sensor lenses decide the range within which
the proximity of the object is detected.

21. DESIRABLE FEATURES FOR SENSORS AND TRANSDUCER
FEATURES FUNCTIONS
PRECISION Should be as high as possible
Deviation in measurement reading should be minimum.
ACCURACY Should be very high
Error between sensed and actual value should approach
zero.
SPEED OF RESPONSE Time taken to respond should be minimum
Response should be instantaneous.
OPERATING RANGE Wide operating range
Good accuracy over the range .
RELIABILITY Life to be high
Frequent failure are not acceptable.
CALIBRATION Drift should be minimum
Should be easy to calibrate
COST AND EASE The purchase cost should be low
The installation and operation should be easy.