1. How do laser through beam sensors work?

Laser through beam proximity sensors (also known as proximity switches) are used to detect the presence or absence of a target object. They consist of a laser emitter (normally a photo diode) and a detector (normally a photo electric sensor). The laser diode emits a laser beam, which is detected by the photo electric sensor. When a target enters the region between the emitter and detector, partially or completely blocking the laser, a reduction in light intensity is recorded by the detector. If a preset threshold light intensity is crossed, the sensors output will switch states to indicate the presence of the target. Laser through beam proximity sensors are not capable of measuring distance but rather only the presence or absence of a target. Due to the need to align the emitter and detector, they are more difficult to setup then many other proximity sensors and switches.

2. General characteristics of laser through beam sensors

Laser through beam proximity sensors are low cost, highly reliable sensors capable of detecting the presence of targets (but not measuring their distance) at distances of 0 to 200 meters. They provide a Boolean switching output (i.e. logic 0 or logic 1). Laser through beam proximity sensors are non-contact and solid state meaning that they do not experience mechanical wear and have an extremely long lifetime. They operate at high sampling rates of 1-10 kHz and can accurately detect targets travelling at extremely high velocities. They have excellent environmental protection (IP67) and their readings are largely unaffected by contamination of the target with dirt, debris or liquid. However, they are sensitive to high intensity background light. The emitter and detector must in most cases be separately mounted and aligned. However, fork configuration sensors are available for short sensing ranges, in which the emitter and detector are permanently fixed onto a common frame.

Balluff fork optical through beam sensor.jpg

3. Input and output signals

Laser through beam proximity sensors operate on a 10-30 Vdc supply voltage. With the exception of fork type sensors, the emitter and detector are separately wired to the voltage supply. The emitter may include additional inputs such as a beam enable/disable. The emitted laser beam is detected by the photo electric sensor which outputs a Boolean signal of 0 V for a logic 0 or the supply voltage (less the voltage drop across the transistor) for a logic 1, in accordance to whether the threshold light intensity has been crossed (indicating a disrupted laser beam). The output may be either normally closed or normally open, which is often referred to as light switching or dark switching respectively. Many sensors have both normally closed and normally open outputs or programmable outputs.

4. Applications of laser through beam sensors

Laser through beam proximity sensors are used predominately for counting parts on production lines because of their high reliability and ability to detect small objects. Furthermore, they are used for fill level detection through transparent bottles and containers. Laser through beam proximity sensors are also used extensively in safety applications such as detecting the presence of a person between automatic doors or within a restricted work area.

5. Typical specification

CostLow
Measurement range0-200 m
VelocityVery high
Sample rate1-10 kHz
Spot size0.1-500 mm
LifetimeVery high
Ambient temperature-20 to 80 °C
Supply voltage10-30 Vdc
Output voltageBoolean NO/NC
Ingress protectionIP67
Passive / activeActive
Contact / non-contactNon-contact

6. Purchasing tips

  • Pilot light: It is possible to purchase a laser through beam proximity sensor with a pilot light on the emitter, which projects onto the detector, aiding in alignment by providing visual feedback during installation. These are particularly useful for aligning long range proximity sensors.
  • Adjustable threshold: Many laser through beam proximity sensors include adjustable threshold intensities. These are particularly useful for sensing object smaller than the laser spot diameter or for sensing an object within a translucent container.
  • Spot diameter: The laser spot diameter affects the minimum size of target that can be detected since small targets may not block a sufficient proportion of the laser. Depending on the sensor model, the spot diameter is typically 0.1-10 mm a short distance from the emitter. Due to divergence of the beam, the spot diameter increases with distance from the emitter and may reach 500 mm with long range sensors. It is possible to reduce the spot diameter by placing a slit mask over the emitter, though this will also decrease the maximum sensing distance.
  • PNP/NPN: The transistor used to switch output states can be of either PNP or NPN type (also known as current sourcing and current sinking types respectively). When interfacing the sensor to a PLC, it is vital that the sensor is of the opposite type to the PLC port e.g. a current sourcing sensor will only work with a current sinking PLC port. Many Laser through beam proximity sensors include both a PNP and a NPN output.

7. Advantages of laser through beam sensors

Laser through beam proximity sensors:

  • Are low cost and highly reliable proximity sensors/switches.
  • Have a wide measurement range, with models available with ranges from 2 mm to over 200 meters.
  • Have high a high degree of environmental resistance, with a typical ingress protection rating of IP67.
  • Can be used to measure fill level through transparent containers (this requires an adjustable threshold).

8. Disadvantages of laser through beam sensors

Laser through beam proximity sensors:

  • Must be installed at two points of a system (emitter and detector), both of which require wiring.
  • Can be difficult to install (with the exception of fork type proximity sensors) because of the need for alignment of the emitter and detector.
  • Are sensitive to background light, with many not capable of operating in direct sunlight.

9. Application tips

  • Slit masks: A mask may be placed over the emitter to decrease the effective beam diameter. Doing so enables smaller objects to be detected (as small as several millimeters) but results in a reduced measurement range because less light reaches the detector.
  • Multiple sensors: Multiple proximity switches can be connected in series or parallel to a single process controller input port to achieve logic functions. Two sensors connected in series act as an AND gate. Two sensors connected in parallel act as an OR gate. The circuits used to implement parallel and series connections can be found here.
  • Mutual interference: When connecting multiple detectors in close proximity, each incoming laser beams may interfere with adjacent detectors. Proximity sensors are available with an interference prevention function which enables mounting in close proximity.
  • High temperature: The emitter and detector and limited to maximum operating temperatures of 80 °C. However, there are laser through beam proximity sensors in which fiber optic cables are connected to the emitter output and detector input. The fiber optic cables, capable of operating at over 300 °C, enable removal of the emitter and detector from the high temperature region.
  • Reflections: A laser beam traveling close to a reflective surface may reflect off the surface due to the beam’s divergence, potentially disrupting the detector. It is possible to prevent reflected light from reaching the detector by placing opaque barriers perpendicular to the surface, tall enough to block the reflection.

Ultrasonic through beam proximity sensors