INST1015 - Photoelectric Sensors in Instrumentation

Photoelectric Sensors

• Use light to detect the presence or absence of an object
• Detection occurs if a light beam is interrupted or reflected by the object being sensed
• Consist of five basic components: light source, light detector, lenses, logic circuit, and output circuit

Light Source (LED)

• Light emitting diodes (LEDs) are the most commonly used light source
• Can be visible or non-visible (infrared)
• Operate due to electroluminescence – the generation of photons when electron-hole pairs are created by current flow through a PN junction
• Characteristics:
  • Very long lifespan (solid state devices)
  • Light intensity does not deteriorate quickly
  • Immune to shock and vibration
  • Produce a single wavelength
  • Popular for security applications (infrared)

Light Detectors

• A light sensor that detects the absence of an object is called a light sensor, detector, or receiver
• Most common light detectors:
  • Photoresistors or photocells
  • Photodiodes
  • Phototransistors

Photocell

• A passive resistive component that decreases resistance as exposure to light increases
• Typical dark resistance: 500kΩ to 20MΩ range

Photodiode

• A semiconductor device with a P-N junction that converts photons (light) into electrical current
• Two main modes of operation:
  • Photovoltaic or Unbiased Mode: illuminated photodiode generates a voltage which can be measured
  • Photoconductive or Reversed Biased Mode: reverse voltage is applied to the diode where the diode will not conduct without incident light

Phototransistor

• A semiconductor device that has a light-sensitive base region
• Operation is identical to a normal transistor except the base voltage is replaced by light intensity

Photoelectric Applications

• Nightlight
• Smoke Detector
• Coin Counter
• Irradiance Sensor

Photoelectric Sensors in Industry

• Can be categorized by:
  • Sensing method
  • Packaging style

Sensing Methods

• Opposed sensing / through beam:
  • Consists of two units, emitter and detector, positioned opposite each other
  • Object is detected when it breaks the light beam
  • Object must completely block the entire beam
  • Detection mode is light-to-dark
  • Most reliable sensing method
• Retro-reflective sensing:
  • Emitter and detector contained next to each other in the same housing
  • Light beam reflected back to the receiver by a prismatic reflector mounted opposite the sensor unit
  • Detection mode is light-to-dark
  • Not reliable for highly reflective objects, may go undetected
• Diffuse sensing:
  • Emitter and detector contained next to each other in the same housing
  • No need for reflector – relies on light being reflected by the surface of the object
  • Reflected light indicates the object is present
  • Detection mode is dark-to-light
  • Reflected light is diffused and not very strong, range is limited
• Convergent sensing:
  • Similar to diffuse sensing – dark-to-light detection mode
  • Emitter and detector contained next to each other in the same housing
  • Emitter and detector are set at the same angle from the vertical axis to capture reflected light
  • Sensor is designed to detect the object at one set distance – focal point
  • Objects closer or further from the focal point will not be detected
  • Typically used to detect parts in place or fill level
• Specular sensing:
  • Detects the difference between shiny and dull surfaces based on how the beam reflects (specular vs. diffuse)
  • Emitter and receiver contained in the same housing
  • The distance between the sensor and the object must remain constant