Proximity 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

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.

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.

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