Proximity Sensor Basics


  • Locate proximity sensors within the larger electronic sensor family.
  • Describe the purpose of a proximity sensor.
  • Recognize common proximity sensor technologies and their schematic symbols.

All In the Family

Proximity sensors are a subgroup of position sensors. Unlike many position sensors, proximity sensors do not need to touch their target to function.

For example, in a bottling plant, beverage bottles used to trip a mechanical limit switch during counting. Now a proximity switch can provide the same information without using a mechanical device that will eventually wear out.

As these bottles move along the conveyor belt they break a light beam. The signal from the sensor allows a PLC to count the bottles as they move by.

What's the Purpose of a Proximity Sensor?

A proximity sensor is used to detect the physical presence or absence of a target object within their nominal range.

Nominal Range: The maximum distance at which a proximity sensor can reliably detect the presence of its target material. Some manufacturers also refer to this as operating distance.

Because they don't require touch, and usually have no moving parts, proximity sensors should have long lifespans without intrinsic reliability issues. This makes them very popular for all kinds of automation. You might find a proximity sensor

  • In a car manufacturing plant, detecting whether a workpiece has moved into position for the next weld.
  • In your smartphone, turning the screen off automatically when you bring your phone up to your ear.
  • On your vehicle bumper, alerting you if you are about to back into something.
  • Under the asphalt at a busy intersection, controlling the traffic light as it senses cars.

Common Proximity Sensor Technologies

  • Inductive
  • Capacitive
  • Photoelectric
  • Ultrasonic

As we examine these broad categories of sensors, we'll also look at some vulnerabilities of each sensor type. In many cases, sensor manufacturers have come up with clever methods to reduce these vulnerabilities, often by using additional components and specific setups tailored to the exact sensing job at hand. So when learning about generalized flaws in each sensor type, remember that these issues can sometimes be mitigated.

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