Image Credit: Panasonic Electric Works | Carlton-Bates Company | Banner Engineering Corp.

 

Fiber optic proximity sensors are used to detect the proximity of target objects using light. Light is supplied and returned via fiber optic cables. Fiber optic cables can fit in small spaces, are not susceptible to electrical noise, and exhibit no danger of sparking or shorting.

  

 

Fiber Optics Physics Demonstration

Video Credit: bcitphysics "BCIT" British Columbia Institute of Technology 

 

 

 

Selection Criteria

 

 When searching for fiber optic proximity sensors, sensing performance and optical configuration are the most important parameters to consider. Other considerations include cable material, emitted beam, modes of operation, body type and various features.  

 

Sensing Performance

 

 Specifications for sensing performance include detecting range, position measurement window, minimum detectable object, and response time.

  • Detecting range is the range of sensor detection. For presence sensors, this goes up to the maximum distance for which the signal is stable.
  • Position measurement window is the width of linear region for the sensor. For example, if the sensor could measure between 14 and 24 cm, this window would be 10 cm.
  • Minimum detectable object is the smallest sized object detectable by the sensor.
  • Response time is the time from target object entering detection zone to the production of the detection signal.

Optical Configuration

 

 Choices for optical configuration for fiber optic proximity sensors include through beam, retroreflective, polarized retroreflective, diffuse, divergent, convergent, fixed field, and adjustable field. 

  • Through beam (or opposed mode) sensors incorporate a transmitter and a receiver on opposite sides of the target and evaluate absence or presence based on transmittance or blockage of the beam respectively. 
  • Retroreflective sensors incorporate the emitter and detector in one body with parallel beams; a reflector opposite the target path is used to reflect the emitted beam back to detector; presence is detected when reflected beam is interrupted by an opaque object. 
  • Polarized retroreflective sensors are a variant of the retroreflective in which the emitter and receiver have polarizing filters 90° apart, thereby making the reflector the only recognizable source. 
  • Diffuse sensors detect presence when any portion of the diffuse reflected signal bounces back from the detected object.
  • Divergent beam sensors are short-range diffuse-type sensors without any collimating lenses.
  • Convergent sensors, fixed focus sensors, or fixed distance optics (such as lenses) are used to focus the emitter beam at a fixed distance from the sensor. 
  • Fixed-field sensors are designed to have a distance limit beyond which they will not detect objects, no matter how reflective. 
  • Adjustable field sensors utilize a cutoff distance beyond which a target will not be detected, even if it is more reflective than the target. Some photoelectric sensors can be set for multiple different optical sensing modes. 

 

Cable Material

 

 The material of the fiber optic cable is an important part of the fiber optic sensor. 

  • Glass fiber exhibits very good optical qualities and typically carries high temperature ratings. 
  • Plastic fiber can be cut to length in the field and can be flexible enough to accommodate various routing configurations.

 

 

Fiber Optic Cable. Image Credit: ElectronicsWeekly.com

Emitted Beam

 

Fiber optic proximity sensors use a light source, such as an LED or Laser, to emit either a visible or infrared beam.

 

Visible - The beam of the sensor is in the visible light spectrum range (380 - 750nm wavelength)
Infrared - The beam of the sensor is in the infrared light spectrum range (750 - 1000nm wavelength)

 

 

Mode of Operation

 

 The mode of operation describes how an event is triggered. The device can be energized when light is received (referred to as Light-on mode), when light is not received (referred to as Dark-on mode) or may be programmable to operate in either mode of operation.

 

Light-on - Output is "high" or on as long as light is detected.
Dark-on - Output is "high" or on as long as no light is detected.
Light/dark programmable - Light or dark activation is programmable or switch selectable.  

 

 

 

Body Type

 

 Fiber optic proximity sensors are typically housed in either a barrel or rectangular housing.

 

 

 

 

Threaded barrel and rectangular housing. Image Credit: SENSOPART | Banner Engineering Corp.

 

  • A barrel body style is cylindrical in shape. The barrel of the sensor is typically threaded so that with the help of two lock-nuts the sensor can easily be adjusted closer or further from the target as necessary.
  • A rectangular or block body style is a one piece rectangular or block shaped sensor. 

Features

 

 There are a number of features that may be important to consider when selecting fiber optic proximity sensors.

  • Field adjustable sensors are required when adjustments need to be made while the proximity sensor is in use. Potentiometers are commonly used to control the detection range.
  • Intrinsically safe (IS) sensors are incapable of releasing sufficient electrical or thermal energy under normal or abnormal conditions to cause ignition of a specific hazardous atmospheric mixture in its most ignited concentration.
  • Self-teaching proximity sensors can be auto-calibrated to trigger the switch at a specific target location. 
  • Materials of construction and enclosure ratings must be considered when the sensor will be located outdoors or exposed to extreme temperatures, or under the influence of wet, humid, dusty, dirty or corrosive process conditions.
Enclosure ratings are specified by "National Electrical Manufacturers Association (NEMA)" or "Ingress Protection (IP)" ratings. 

 

 


Resources

 

 

Engineer's Handbook - Proximity Sensors

OMRON Industrial Automation - Photoelectric Sensors

ElectronicsWeekly.com - Only Connect: A Guide to fiber optics