Sensors: Proximity

Proximity - Spatial Presence

Proximity Sensors

Turck Proximity Sensors

1. Inductive Proximity Sensors

   Inductive proximity sensors are widely used in the modern high speed process control environment for the detection, positioning and counting of ferrous and non-ferrous metal objects. Due to the method of construction and superior performance of inductive sensors, they are increasingly used to replace the traditional limit switch, thus upgrading speed and reliability of existing machinery.

   Principle of Operation

   Inductive proximity sensors respond to ferrous and non - ferrous metal objects. They will also detect metal through a layer of non - metal material. An inductive sensor consists of an oscillator circuit (ie. the sensing part) and an output circuit including a switching device (eg. transistor or thyristor), all housed in a resin encapsulated body. An essential part of the oscillator circuit is the inductance coil creating a magnetic field in front of the sensing face. When the magnetic field is disturbed, the output circuit responds by either closing the output switch (normally open version type NO) or by opening the output switch (normally closed version type NC).

2. Capacitive Sensors

    

   Capacitive sensors are often successfully used in applications which cannot be solved with other sensing techniques. Capacitive sensors respond to a change in the dielectric medium surrounding the active face and can thus be tuned to sense almost any substance. Capacitive sensors can, also, sense a substance through a layer of glass, plastic or thin carton.

   Some typical applications for capacitive sensors are:

   1. Level control of non-conductive liquids (oil, alcohol, fuel).
   2. Level control of granular substances (flour, wheat, sugar).
   3. Sensing substances through a protective layer (eg. glass).

   The fact that capacitive sensors respond to most substances, necessitates some care during the installation, adjustment and long term operation of the sensor. The sensitivity of capacitive sensors is affected by the moisture content and the density of the substance to be sensed. Deposits of excessive dust and dirt on or around the sensing face of the sensor, cause erratic response and hence the sensor may require periodic cleaning if used in a polluting environment.

   Principle of Operation

   Capacitive sensors respond to any substance with a high dielectric constant (water, oil, fuel, sugar, paper) without necessarily making physical contact. They are less suitable for polystyrene and similar low density substances. Operation is based on an internal oscillator with two capacitive plate-electrodes, tuned to respond when a substance approaches the sensing face. When the target is sensed, the output switch will either close to activate a load for a normally open option or the switch will open to de-activate the load for a normally closed option. The LED will illuminate when the output switch closes.

3. Photoelectric or Opto-electronic 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 enviroments. 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:

   • Infrared Proximity (Diffuse Reflective)

     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 (Thru-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.

   • Retroreflective (Reflex)

     Retroreflective photoelectric sensors operate by sensing the light beam that is reflected back from a target reflector. As with thru beam models, objects which interrupt the beam activate an electronic output.

   • 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.

   • 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 allow sensing in extremely confined spaces.

   • Background Rejection

     STI's 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.

4. Ultrasonic sensors

   Ultrasonic sensor utilize the reflection of high frequency (20KHz) sound waves to detect parts or distances to the parts. The two basic ultrasonic sensor types are:

   1. Electrostatic - Uses capacitive effects for longer range sensing and wider bandwidth with more sensitivity.

   2. Piezoelectric - These rugged and inexpensive sensors operate by a charge displacement during the strain in crystal lattices.

   In general, ultrasonic sensors are the best choice for transparent targets. They can detect a sheet of transparent plastic film as easily as a wooden pallet.