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Proximity Sensor Basics

Objectives

  • 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.

Inductive Proximity Sensors

Inductive proximity sensors require ferrous metal in their targets. (Ferrous metals contain iron.) Not sure which metals are ferrous? Here are some common ores and alloys.

Non-FerrousFerrous
  • Copper
  • Bronze
  • Aluminum
  • Lead
  • Zinc
  • Tin
  • Iron (Surprise!)
  • Steel
    • Carbon Steel
    • Alloy Steel
    • Stainless Steel

How It Works

An inductive proximity sensor generates an electromagnetic field in front of the active face.

The active face of a sensor is the plane where the sensing takes place. In the case of our inductive proximity sensor, it's the face in front of the coil.
Active Face

If a ferrous item enters the sensor's nominal range, the field shrinks. This triggers a response from the sensor that the target has been detected. The black wire carries the sensor signal — voltage if the sensor is Normally Open (NO), or no voltage if the sensor is Normally Closed (NC).

The sensor relies on a circuit called a Schmitt trigger. The exact configuration of the Schmitt trigger is outside of the scope of this session, so for our purposes, you only need to know that it creates a threshold for AC signal amplitude. If the incoming AC wave shrinks below the threshold, the Schmitt trigger provides a voltage signal out. An amplifier cranks up that output to a stronger signal.

Try It Out!

The Takeaway: The electromagnetic field changes relative to the proximity of a target material. That change is what is sensed and reported on by the inductive proximity sensor. The Schmitt trigger controls the sensitivity of the sensor.

(Don't worry about the details in the transparent state, unless, of course, you want to!)

Schematic Symbol

The schematic symbol for a proximity sensor uses the standard sensor square envelope, packed with very specific information. Let's look at what each of the 4 symbols inside are telling us.

NPN NO Version

PNP NC Version

This symbol indicates a proximity sensor.

This symbol tells us whether the sensor is PNP or NPN.

The elemental symbol for iron indicates that this is an inductive sensor.

This symbol tells us whether the sensor is Normally Open or Normally Closed.

Not sure about wiring PNP and NPN sensors?
Check out our lesson on Sensor Wiring!

Pick Me!

Advantages

  • Unaffected by many types of environmental contamination, like dust and mud.
  • Very fast sensing rate.

Limitations

  • Nominal range is very short.
  • Target must contain iron.
  • Vulnerable to electromagnetic interference.
  • Heavy, bulky, and expensive relative to some other proximity sensors.

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