proximity sensors selection guideProximity sensors are non-contact devices which detect the presence of nearby objects. They may employ one of a variety of sensing technologies.

 

Sensing Basics

 

Proximity sensing involves detection of objects within a finite field. Proximity sensors are designed to output a value (typically a simple ON signal) when a prescribed object passes within their sensing range. The definitions and specifications below can be used to describe the sensing environment and a sensor's capabilities.

 

The object (sometimes referred to as a standard sensing object or target) refers to the part or entity a sensor is designed to detect. Because different sensor types are capable of detecting different objects, the object's material, size, surface characteristics, and rate of speed must be carefully considered.

 

Sensing Range and Frequency

 proximity sensors selection guide

Ranges are the most important proximity sensor specifications to consider. A device's sensing distance refers to the distance in which an object triggers an ON signal. It may be considered a sensor's maximum possible sensing range. Set distance is defined as the maximum stable detection range when temperature and voltage factors are considered, and is typically 70-80% of the sensing distance. These specifications are described in the image at right.

 

Response time and frequency specifications describe a sensor's speed. Response time (t1) refers to the time between an object moving into a sensor's range and the point when the sensor outputs an ON signal. Conversely, t2 describes the time between an object moving out of a sensor's range and the point when the sensor registers an OFF signal.

 

Frequency (f) describes the number of possible sensing repetitions per second and is measured in hertz (Hz). The image below shows a sensor application's frequency and formula, which involves both response specifications.

 

proximity sensors selection guide

Image credit: Omron Industrial Automation

   

Applications

 

Proximity sensors are useful in any application involving detection of an object within a defined range, including:

 

  • Inspection: detection of broken tool bits or surface defects; quality assurance
  • Positioning: detection of tool position
  • Process control: control of conveyors and processes based on part or level detection; stack height control
  • Transportation: tollbooth control; garage door control
  • Level detection: silos; food processing; bulk material or liquid level sensing and control

Types

 

Proximity sensors are typically classified by the sensing technology they employ. These types are described in the table below.

 

Type

Description

Range

Target

Speed

Applications

Image

Capacitive

Constructed with two parallel plates as an open capacitor; targets induce changes in capacitance.

3-60 mm

Non-ferrous materials

Relatively slow

Close-range, non-ferrous sensing; tank liquid level detection, sight glass monitoring.

proximity sensors selection guide

Eddy Current

Similar to inductive sensors; could be considered high-end inductive types.

Relatively short

Ferrous materials

Average

Precision, high-resolution sensing in contaminated environments.

 proximity sensors selection guide

Hall Effect / Magnetic

Measures the presence or absence of object based on an external magnetic field.

4-40 mm

Ferromagnetic

High

Measurement of fast rotational velocity.

 proximity sensors selection guide

Inductive

Involve a wound iron core; coil inductance changes with presence of object within sensing range.

4-40 mm

Ferrous materials

Average

Close-range detection of ferrous materials; hazardous environments.

 proximity sensors selection guide

Photoelectric

Use laser emitters and reflectors or receivers; targets cut off or reflect emissions.

1-60 mm

Many types

Average

Long-range detection of small or large objects; automatic faucets, color-dependent sensing.

 proximity sensors selection guide

Ultrasonic

Similar to photoelectric types, but use sound waves instead of visible emissions.

Up to ~400 mm

Many types

Relatively high

Long-range detection of multi-colored objects with varying surface properties; machine automation, continuous level control.

 proximity sensors selection guide

 

Table image credits: Protocontrol | efunda | ElectronicsTutorial | Protocontrol

     

Standards

 

The standards listed below all pertain to proximity sensors. Devices may be manufactured, employed, and tested according to these published documents.

 

  • IEC 60947-5-2 Control circuit devices and switching elements in low-voltage switchgear: DC switching interface for proximity sensors and switching amplifiers
  • FORD ES2 Proximity sensors
  • MIL-S-52868 Capacitance proximity sensors

References

 

MachineDesign - Proximity sensors: The difference between the four main types

 

Omron Industrial Automation - Proximity sensing (basic guide)

 

Image credits:

elobau sensor technology, inc. | Omron Industrial Automation