Understanding Proximity Switches: How They Detect and Control Motion

Getting Close to a Proximity Switch

What’s in a name? In the case of a proximity switch, everything! A proximity switch is exactly what it sounds like. It’s something that switches on (or off) when it comes close to something. Proximity switch. Pretty straightforward!

Proximity switches are also nearly everywhere. They’re remarkably simple, incredibly useful devices that play important roles in the safe and efficient running of countless devices, and once you know what to look for, you’ll spot them everywhere!

Of course, in this article we’re primarily considering proximity switches in the machining and manufacturing fields. On most machine tools, proximity switches work as safety features.

Varieties of Proximity Switches

How does a proximity switch work? It works by completing an electrical circuit. In other words, when you bring a proximity switch within a certain range of an object, or in direct contact with it, you complete an electrical circuit, triggering the switch. Proximity switches come in four primary kinds:

Acoustic

Think of a dolphin or whale, or perhaps a bat: these animals find their prey and navigate through echolocation, using sound and echoes to determine a target’s location. An acoustic proximity switch operates the same way.

An inaudible pulse of sound is sent from the switch, and then a transducer measures how long it takes the sound to bounce back. That measurement tells the receiver how far away, or how close, the target is, allowing the switch to turn on within a certain range. 

Infrared

Same principles as the acoustic switch – only here, the pulse of sound is replaced by an infrared light beam. On more modern or sophisticated switches the light will be at a very particular wavelength, so the sensor isn’t confused by background lighting.

Capacitive

The first two categories are fairly simple, but capacitive switches are a little more complex. They measure the changes in radio frequency that occur when two objects move closer to each other.

Those changes are strongly influenced by an object’s ability to conduct electricity, so a capacitive switch varies in sensitivity depending on the conductivity of the material.

Inductive

Like capacitive switches, inductive switches measure changes in a field or frequency. In this case, inductive switches detect changes in magnetic fields. Metal detectors work along the same principle, telling you when a metal object is within a set range of the switch.

However, inductive switches do have a fairly large weakness – like their metal detector counterparts, an inductive switch can only be triggered by magnetic objects. No metal, no magnet, no switch.

Proximity Switch Uses

Acoustic, infrared, capacitive and inductive switches are the four major categories of proximity switches. But what are proximity switches for?

There are a number of different applications for proximity switches; we’ll cover the main ones below.

• To shut something off

This might be the most common use. When a particular part of a machine comes too close to another part, a proximity switch is tripped and the machine shuts off. It’s a safety feature using a proximity switch.

• To turn something on

The same idea – a safety feature – but done in reverse. Some machines, particularly CNC lathes or other enclosed cutting or machining areas, won’t operate unless a particular door is closed or a lid sealed. Why? There’s usually a proximity switch in the door or lid; unless it is closed, the circuit won’t be closed and the machine won’t start.  

• To send an alert or warning

Think of the reverse warning on a car, letting you know with a beep or tone if your bumper’s getting too close to the curb. Proximity switches don’t always require physical contact before they sound an alert, so they can be used to warn you of impending dangers, not just present ones. 

• To indicate position

On CNC machine tools, proximity switches are also used to tell the computer where the tool is at a given point. CNC machines use g-code to execute a series of instructions and proximity switches to locate the tool. As an example, if g-code is the planned route for a hike, the proximity switches are the GPS coordinates. 

Proximity switches perform a deceptively simple function, but that simplicity makes them incredibly versatile and useful components to modern machine tools.