Version 1.0, August 31, 2001, Copyright, Hugh Jack 1993-2001

12.4.1 Optical (Photoelectric) Sensors

 

· Optical sensors can detect part presence using a light source and detector.

 

· Emitters generate light in visible and infrared light bands. These are usually LEDs or laser diodes.

 

· Detectors are designed to vary electrically as light intensity varies. The most common used is the phototransistor.

 

· Ambient light can interfere with a simple optical beam. As a result most sensors now use a modulated pulse with a frequency up to the low KHz range. This allows better detection at longer distances with lower power.

 

 

· The relative locations of the source and detectors, as well as surface conditions have a major impact on the selection of sensor types. These include,

  1. - distance to target
  2. - target characteristics (transparent, reflective, diffuse, etc.)
  3. - target size

 

· The simplest form uses a detector only with ambient or radiated light.

  1. - ambient light requires care in scene lighting
  2. - radiated light requires some sort of photometric phenomenon such as a hot part will radiate infrared light.

 

· Optic sensors can often be separated for space and other constraints.

  1. - fiberoptics allow the lens to be separated from the LED or phototransistor.
  2. - the phototransistors and LEDs can be separated from the other circuitry to fit the sensors into smaller parts.

 

· When the emitter and detector are separated and the beam is interrupted this is known as opposed mode.

 

 

· When the emitter and detector are in a single unit this is known as retroreflective.

 

 

· Polarized light can be generated using filters.

 

 

· Diffuse sensors are like the retroreflective type, except that the returning light does not need to be polarized.

 

 

· Alignment of the emitter is necessary, and can be a problem if the sensors are separated by a large distance and the beam intensity decreases.

 

 

· The beam of emitted light should generally be less than the width of the detected part.

 

 

· Separated sensors can detect reflective parts using specular reflection. This needs a reflective surface.

 

 

· By focussing emitters and detectors optics we can sense presence at a specific distance. This is known as convergent beams sensing.

 

 

· Fixed field sensors use a physical setting.

 

 

· Opposed beams can also be for a large range light curtains.

 

 

· Typical reflectivity values are given below [Banner Handbook of Photoelectric Sensing]

 

 

· Many sensors have sensitivity adjustments that will need to be adjusted to the materials.