How Does a Proximity Switch Work

[questionator]

How Does a Proximity Switch Work? That's the question of the day for me. I'm looking for a simple explanation of how and why they work, nothing too technical! I guess a detailed explanation of a  proximity switch wouldn't hurt but I'm looking for simplicity when it comes to the circuitry.

Thanks in advance!

[maintech]

How Does a Proximity Switch Work? That's the question of the day for me. I'm looking for a simple explanation of how and why they work, nothing too technical!

Definition/Explanation of Proximity Switch: A proximity switch is a switch that is electrically activated without mechanical movement of contacts typically activated by the presence of a ferrous metal (or non-ferrous material depending on the type of proximity switch).

I've got some good technical info on how these switches really work, I'll dig this stuff up and post later.

[maintech]

I had an old Allen Bradley pamphlet here and I knew it would come in handy someday. It has an explanation of inductive presence sensing. Some dry material and some interesting stuff too. But if you want to know how a proximity switch works, this should provide you every detail you need.

• The resonant circuit oscillator stops when metal is sensed.
• The demodulator converts the oscillator signal to a DC voltage level.
• The trigger changes condition when the oscillator stops.
• The output amplifier will drive the load.

   

[maintech]

Here's detailed info on shielded versus unshielded proximity switches. You'll often hear people in the field discussing when to use a shielded prox switch and when you can use unshielded.

Flush Mounting in metal would required a shielded proximity switch. These switches are threaded to the top of the switch, the full length of the switch basically.

Non-flush mounting for switches where metal housing does not extend to sensing face of switch, in other words, these switches have the plastic material extending out of the face of the switch.

[maintech]

Next subject of interest for how a proximity switch works would be sensing range and its relation to the switch diameter. The larger diameter prox switches are capable of sensing at greater distances as depicted in the Allen Bradley diagram.

For Shielded Prox Switches:

• 12mm Proximity Switch can sense 2mm
• 18mm Proximity Switch can sense 5mm
• 30mm Proximity Switch can sense 10mm

For Un-shielded Prox Switches:

• 12mm Proximity Switch can sense 4mm
• 18mm Proximity Switch can sense 8mm
• 30mm Proximity Switch can sense 15mm

[maintech]

Next on the agenda for the explanation of a proximity switch is the sensing range versus the target material. The sensing material really dictates the overall range of the switch.

From the AB Brochure: The actual effective operating distance will vary from the Rated Operating Distance because of manufacturing tolerances and the effects of ambient temperature and supply voltage variations.

The effective operating distance could vary up to +/- 20% due to the above mentioned conditions.
It should also be noted that the Rated Operating Distance will be affected by the type of material being sensed and its shape.

Typical conversion factors for common materials are:

• Mild Steel: Approximately 1.0 x Rated Operating Distance
• Stainless Steel:Approximately 0.9 x Rated Operating Distance
• Brass: Approximately 0.5 x Rated Operating Distance
• Aluminum: Approximately 0.45 x Rated Operating Distance
• Copper: Approximately 0.4 x Rated Operating Distance

[maintech]

The next discussion of how a proximity switch works would be the standard target size and how to calculate sensing distance. This topic is related more to engineering on the highest level.

From the AB Brochure: The active face of a proximity switch is the surface where a high-frequency electro-magnetic field emerges (however, no direct magnetic field occurs).

The target consists of steel, 1 mm thick, square form with side lengths equal to the diameter of the circle of the sensing surface, or 3 x the nominal switching distance, if this is greater than the di­ameter of the sensing surface circle.



The switching distance should be cal­culated with the target parallel to the sensing surface and is dependent on the material, size, and thickness of the target.

[maintech]

Next topic for how a proximity switch works would be hysteresis. Proximity switch hysteresis enable these switches to provide a more stable switching experience when there is real-world instability like vibration.

From the AB brochure: Hysteresis is the difference (distance) between the switching points when the target is approaching (switch-on) and leaving (switch-off) the active face of the proximity switch.

The value is expressed as a percentage of the switching distance. Without hysteresis a switch will hunt should there be vibration of the target.

[maintech]

Here's how ripple pertains to proximity switches, basically ripple voltage can be no more than 10% for DC proximity switches.

From the AB Brochure: Ripple is the alternating voltage superimposed on the DC voltage (peak  to peak) in %.

For the operation of DC voltage switches, a filtered DC voltage with a ripple of 10% maximum is required.

[maintech]

Proximity Switching Frequency is another important aspect in the world of prox. switches... In other words, how many times can the switch operate per second. You can usually find the rated switching frequency in literature the accompanies the switch when you purchase, it is always a documented spec.

Explanation from the AB Brochure: The switching frequency is the maximum possible number of impulse repetitions per second. The test basis of switching frequency is a pulse/pause ratio of 1:2.

[maintech]

Proximity switch temperature drift is the change in sensitivity based on the ambient temperature. The picture indicates the change from -13 degrees F to 158 degrees F. Change is not dramatic, but can be a factor for certain applications if there are dramatic temperature changes.

[maintech]

And finally, proximity switch response time... This is the time it takes to open and close a proximity switch. The way I understand this is that the manufacturer specs usually depict average conditions like temperature and sensing material.

From the AB brochure: It is expressed in V/microsecond for closing and opening of the proximity switch between 10% and 90% of the total voltage change. Technical data indicates the least favorable value, tr is rise time and tf is fall time.