SENSORS

introduction to sensors

Sensors are devices that measure the quantity of a physical change and generate an electrical signal. Sensors are used in varies machines as inputs. Most of the time they are connected to automated controller systems. The systems can be PLC, PLR, microcontroller or it can be relay connected systems.

There two types of sensors as

                                                                                                I.  Active sensors.

                                                                                              II.  Passive sensors.

Active sensors: -   Active sensors must connect to a power source get the output signal. The sensor       must have an external power to make it work.  

Passive sensors: -  This sensor does not need any external power to get the output. It produced it own power to generate a signal. Most of the time the signal must be amplified to get a proper output.

PROXIMITY SENSORS

                                  Proximity sensor is a sensor that sense and detect the presence of a near by object without any physical contact.Proximity sensor use the method of emits an electromagnetic or an electro static or a beam of electromagnetic radiation and get the feed back for changes in the field of return signal.Capacitive proximity and photoelectric sensors is suitable for plastic or non metallic substance sensing. Inductive proximity sensors are suitable for sensing metal objects.

inductive sensors

inductive sensors working theory

     Inductive sensors are type of sensors that can detect metallic object without touching the metal. It consists
with an induction loop electric current generates a current that falls asymptotically towards zero from its initial
level when the input electricity ceases.
Inductive sensors are type of sensors that can detect metallic object without touching the metal. It consists
with an induction loop electric current generates a current that falls asymptotically towards zero from its initial
level when the input electricity ceases. 

The filed generates by resonate circuit of the oscillator energy is absorbed when a metal piece brought into
front of the radio frequency field near sensing surface causing the oscillation to stop. Hence the demodulate
voltage trigger disappears. For DC switches the output transistor, which can be either NPN or PNP to drive
an electronic circuit or relay                            

inductive sensors 

inductive sensors 

inductive sensors 

Properties of inductive sensors

        v  Inductive sensors recognize all materials with good electrical conductivity.Their function is confined neither magnetize able materials nor metals. (e.g. They also recognize graphite).

        v  Objects can be detected either moving of stationary.

        v  Objects with large surface areas recognized more readily than objects, which are small compared to the sensor area.

                                v  Most of  times the switching distance is equal or below than 15mm

Use of inductive sensors

        v  Measuring the speed and the direction of the metallic gear.

        v  Detecting the position of metallic objects.

        v Using as came controls.

   capacitive sensors

CAPACITIVE SENSORS working theory

 Capacitive sensors are sensors that can detect metallic or nonmetallic objects without touch. It measures the
change in capacitance in the electrical field of a capacitor caused by the approach of an object as a barrier.
The sensor consists of RC oscillating circuit. An electrostatic field built up in the space between active
electrode and earth electrode. If an object introduced to this stray field, the capacitance of the capacitor
increases, thus detecting not only highly conductive materials, but also all insulators, which possess a high
dielectric constant. Materials such as plastics, glass, ceramics, liquids, and wood are the examples of detect
by the capacitive sensors                                 .

CAPACITIVE SENSORS working theory

Use of capacitive sensors

v  Level control of non-conductive liquids Ex:- oil, alcohol, fuel

v  Level control granular substance  Ex:- flour , wheat, sugar

v  Sensing substances through a protective layer Ex:- glass

CAPACITIVE SENSORS 

 photoelectric sensors (Opto-electronic sensors)

                                  

Photoelectric sensors

                                 Photoelectric sensors offer non-contact sensing of almost any substance or object up to a range of 10 meters. Photoelectric sensors consist of a light source (usually an LED, light emitting diode, in either infrared or visible light spectrum) and a detector (photodiode). Due to the high intensity infra-red energy beam, these sensors have major advantages over other opto-electronic systems when employed in dusty environment. With their focused beam and long range, opto-electronic sensors are increasingly used in applications where other sensing techniques are lacking in sensing distance or accuracy. 

Photoelectric sensors are available in a variety of modes including

        infra-red proximity (diffuse Reflective)

infra-red proximity working theory

                                












Proximity type photoelectric sensors detect the light reflected by the target itself. Proximity photoelectric
sensors are preferable for general purpose sensing applications, particularly where the detected object is only
accessible from one direction. 

 TRANSMITTED BEAM (THROUGH-BEAM)
     Transmitted beam photoelectric sensors use separate infrared transmitters and receivers. Objects passing 
between the two parts interrupt the infrared beam, causing the receiver to output a signal. 

transmitted beam SENSORS working theory

    

retroreflective (Reflex)

Retroreflective photoelectric sensors operate by sensing the light beam that is reflected back from a target

reflector. As with through beam models, objects which interrupt the beam activate an electronic output. 

retroreflective SENSORS working theory

  Polarized retroreflective (Polarized Reflex)

Polarized retroreflective sensors work like normal retroreflective sensors but use a polarizing filter in front of

the transmitter and receiver optics. These filters are designed so that shiny objects are reliably detected. 

Polarized retroreflective SENSORS working theory

  fiber optic

Fiber optic sensors use fiber optic cable to conduct light from the LED to the sensing area, and another cable

to return light from the sensing area to the receiver. By using fiber optic cables, the electronics can be

protected from hostile environments such as temperature extremes and harsh chemicals. Fiber optics also

allows sensing in extremely confined spaces. 

fiber optic SENSORS 

BACKGROUND REJECTION

Background rejection sensors use a special arrangement of two sensing zones. The near-field zone is

where objects can be detected. The far-field zone is where objects cannot be detected. There is an

extremely sharp cut-off between these zones. The cut-off range is adjustable. These sensors are ideal for

applications where background objects need to be ignored.

Background Rejection SENSORS working theory

      ultrasonic sensors

Ultrasonic sensors can be used as a proximity switch where an object to be detected As a reflect barrier

using a reflector so that the ultrasonic waves can be interrupted thereby pro­ducing an output signal

(Applicable only to rectangular proximity switches.) The objects to detect can be solid, liquid, or in powder

form with certainty at a switching distance of 6m. 

Ultrasonic sensors working theory

Advantages of ultrasonic sensors

v  Relatively the detecting area is wide.

v  Object detection irrespective of color and material.

v  It detects transparent objects also.

v  Outdoor applications are possible.

Disadvantages of ultrasonic sensors

v  Object surface should be at right angles.

v  Relatively the reaction time is high.

v  It is expensive than optical sensors.

v  Cannot detect sound absorbing materials.