Types Of Proximity Sensors Used In Industrial Automation

When you write logic in a PLC that initiates motion, you want to be certain that the motion you are expecting actually occurs. This is where sensors come into play. Sensors provide indication to the PLC, robot, or other controller that some physical event has taken place. As examples, sensors may detect that a part is present, that a part is not present, that an actuator is in a certain position, that a lift is lowered or raised, that a door is open or closed, or that a spring-returned component is a certain distance away. Proximity sensors are a specific subset of sensors in general. In this article, we’ll look at some of the many different types of proximity sensors that are used in industrial automation.

Want to take the next step in your industrial automation career?

We want to help.


Are you looking for opportunities to advance your career and make more money?

At no cost to you, the Industrial Automation Connection can get you in touch with training and job opportunities in your area. Click one of the buttons below to get started!

Contents of this post:

What Is A Proximity Sensor?

An image of a "barrel prox", a type of proximity sensor that threads into a tapped hole. The sensor is triggered when an object of a certain material (depending on the sensor type) is placed or passes nearby the sensor. This is one type of proximity sensor utilized in industrial automation.
A “barrel prox“; a type of proximity sensor that threads into a tapped hole. Sensors of this type are triggered when an object of a certain material (depending on the sensor’s detection mechanism) is nearby the sensor. The sensor shown is an inductive barrel prox, which is a common type of proximity sensor utilized in industrial automation.

Proximity sensors – sometimes referred to as proximity switches – are sensors that are used in industrial automation and other applications. What distinguishes them from other sensors is that they can sense objects without having to touch them. Because they don’t have to physically interact with the objects they detect, proximity sensors often have no moving parts. Solid-state devices – devices that have no moving parts – often last much longer than devices which have to move to do their job. Wikipedia sums this up as follows:

A proximity sensor is a sensor able to detect the presence of nearby objects without any physical contact.

Proximity sensors can have a high reliability and long functional life because of the absence of mechanical parts and lack of physical contact between the sensor and the sensed object.

Wikipedia article on Proximity sensors

Advantages Of Solid-State Proximity Sensors

For the reasons above, proximity sensors are very popular in industrial automation. Other types of sensors, such as limit switches, require contact with the part.

Because they have to contact the part, limit switches require moving components. Because they have moving components, they may fail much sooner than a non-contact proximity sensor. For this reason, limit switches and other sensors that require internal motion are becoming less and less common.

An Allen Bradley limit switch. This switch is an example of a sensor that requires contact with the part and thus is subject to failure modes resulting from its internal motion.
An Allen Bradley limit switch. This switch is an example of a sensor that requires contact with the part and is thus subject to failure modes resulting from its own internal motion.

Proximity Sensor Types

The zoo of sensors on the market is quite diverse. Although there are other, more obscure sensors out there, I want to go over the types of proximity switches that are most commonly used in industrial automation:

Inductive Proximity Sensors

An inductive proximity sensor manufactured by Turck. This flat, rectangular form factor lends this sensor the moniker, "pancake prox."
An inductive proximity sensor manufactured by Turck. The flat, rectangular form factor lends this style the moniker: “pancake prox.” This is one example of an inductive proximity sensor. The barrel prox shown at the top of this article is another.

In industrial automation, inductive proximity sensors are one of the most common types of proxes. Inductive proximity sensors have a coil inside of them. The coil and body of the prox are designed to generate an electromagnetic field at the face of the prox. When a target is presented within the prox’s sensing range, the electromagnetic field is dampened. Once this dampening exceeds a certain threshold, the state of the output changes.

I felt a great disturbance in the force

Obi-Wan Kenobi, and inductive proximity sensors

The state of the output changes based on whether you’re using a “Normally Open” or “Normally Closed” sensor. Normally Open (or “NO”, or “N.O.”) sensors have outputs that are normally OFF (“open”). A NO sensor’s output turns ON when the sensor detects its target.

Similarly, Normally Closed sensor outputs are normally ON (“closed”). When an NC sensor sees a target, its output turns OFF. You can read much more about the concepts of NO and NC here.

Digital And Analog Proximity Sensors

The sensors above (NO and NC) are referred to as digital sensors. Digital, in this context, refers to the fact that the sensor’s output is either ON or OFF. In addition to digital sensors, there are analog sensors.

Analog sensors provide feedback as a variable voltage or current output. As the sensor’s target moves closer or further, the output signal increases or decreases. Using an analog sensor, you’re able to tell not just that the part is present, but also how far away the part is.

Click the following link for an in-depth look at the difference between analog and digital sensors.

Inductive Prox Applications

Because inductive proximity sensors utilize electromagnetic fields, they can only detect metallic objects. Within the domain of metallic objects, inductive proximity sensors respond differently to different metals. As sensors in this family have evolved, you can now purchase sensors that respond more sensitively to ferrous metals (such as iron and steel), nonferrous metals (such as aluminum), or sensors that respond to a variety of metals approximately equally.

Turck’s uprox factor 1 sensor has been the standard in the automotive sector for twenty years. The same large switching distances for all metals, weld field immunity and a large degree of mounting flexibility are the key benefits of these inductive sensors without a ferrite core.

From “Smart Switches“, a Turck publication describing their family of inductive proxes that sense different metals equally

The fact that inductive sensors only sense metal objects can be of benefit in many applications. As non-metallic contaminants will be less likely to trigger an inductive sensor, sensors of this type are tolerant of dirt and moisture build-up. For this reason, they’re the go-to choice for detection of metallic components.

Learn More About Inductive Proximity Sensors

Capacitive Proximity Sensors

An image of a capacitive proximity sensor. This sensor's threads and face are blue, but the coloring is branding; it's not indicative of whether the prox is inductive or capacitive. You would need to look up the part number to know for sure.
A capacitive proximity sensor. Note that this prox has the same “barrel” form factor as the inductive sensor shown at the very top of this page. This capacitive sensor’s threads and face are blue, but the coloring is not indicative of whether the prox is inductive or capacitive. You would need to look up the sensor’s part number to determine its sensing type.

Outwardly, capacitive proxes can be quite similar to inductive proxes. The two types are often available in the same form factors. Where capacitive proximity sensors differ from inductive proxes is in the sensing mechanism.

Capacitive proxes work as capacitors. There is an energized metallic plate in the face of a capacitive prox. This plate serves as one side of the capacitor, with the prox’s target serving as the other side.

When a capacitive sensor is exposed to open air, the measured capacitance is low. As an object approaches the prox’s sensing area, the capacitance increases until a threshold is met and the output is set. Like inductive sensors, many capacitive sensors are wired as NO or NC digital sensors, or alternatively as analog sensors.

Capacitive Prox Applications

Whereas inductive sensors generate electromagnetic fields, capacitive sensors generate electrostatic fields. Capacitive sensors are an interesting type of proximity sensor used in manufacturing.

Because capacitive sensors detect changes in the capacitance of the field they generate, they have the special property of being able to detect non-conductive materials. Due to this attribute, capacitive proxes can detect plastic, glass, water or other liquids, biological materials, and more.

In fact, capacitive proxes are often used as liquid or solid level detection sensors. Because the sensitivity of many capacitive proxes is adjustable, these sensors can be set up to read the presence or absence of a material through the material’s container.

Learn More About Capacitive Proximity Sensors

Magnetic (Hall Effect) Proximity Sensors

An image of a magnetic (Hall Effect) proximity sensor. This sensor's face is blue, but the coloring is branding; it's not indicative of whether the prox is inductive, capacitive, or magnetic. You would need to look up the part number to know for sure.

A magnetic (Hall Effect) proximity sensor. Note that, like inductive and capacitive sensors, magnetic sensors are also available in the barrel form factor, and that the color of the face or threads is no indication of whether the prox is inductive, capacitive, or magnetic. You would need to look up the part number to determine the prox’s detection mechanism.

Magnetic, or “Hall Effect,” proximity sensors are triggered by magnets. As illustrated in the image above, magnetic sensors are available in the barrel form factor, among others. Hopefully you have seen by now that you cannot assume a sensor’s type by its form factor. When buying or “spec’ing” a prox for your company or application, you have to research the part number to be sure of what you’re buying.

Humanity has known of the Hall Effect since the 19th century. Hall Effect switches have been in use since at least the 1970’s. Modern Hall Effect sensors detect the presence and distance of a permanent magnet. With the right setup, magnetic sensors can also detect ferrous metals.

Like inductive and capacitive proxes, magnetic sensors can provide either digital or analog outputs. Digital outputs are either on or off, whereas analog outputs provide a variable voltage or current based on how far the part is from the sensor.

“Reed sensors” bear mentioning in this space. Reed sensors are in many ways equivalent to the solid-state Hall Effect sensor, except that reed sensors have tiny parts inside that move. Because reed switches function very similarly, but have moving parts, I’ve chosen to limit the conversation to Hall Effect sensors.

Magnetic Sensor Applications In Industrial Automation

Magnetic sensors are very common and have many applications across industries – from automotive to aerospace engineering. Within industrial automation specifically, there are several prominent applications for magnetic sensors.

An image of a magnetic cylinder sensor. This sensor would be mounted on the side of a pneumatic cylinder with T-Slots, and would be used to indicate the position of the cylinder. Generally, one sensor of this type would be located to indicate when the cylinder is in its retracted ("Home") position, and one sensor would be located to indicate when the cylinder is in its extended ("Work") position.
An image of a magnetic cylinder sensor. This sensor would be mounted in a slot in the side of a pneumatic cylinder, and would be used to indicate the position of the cylinder. Generally, one sensor of this type would be positioned to indicate when the cylinder is in its retracted (“Home”) position, and one sensor would be positioned to indicate when the cylinder is in its extended (“Work”) position.

A very common application for magnetic proximity sensors is cylinder indication. Pneumatic cylinders are a common means of linear actuation. A linear actuator is a device that moves something from one position to another in a straight line. Pneumatic cylinders that are designed to be used with magnetic sensors have pistons with ring magnets.

An image depicting a pneumatic cylinder with a ring magnet inside. The purpose of the ring magnet is to provide a target for magnetic sensors mounted on the exterior of the cylinder. The image shows two sensors mounted on the cylinders. One sensor indicates the "Home" position of the cylinder, while the other sensor indicates the "Work" position.
An image from Wikipedia of a pneumatic cylinder with a ring magnet mounted on its piston. The purpose of the ring magnet is to provide a target for magnetic sensors installed on the exterior of the cylinder. In the application shown, one sensor provides indication when the cylinder is in its “Home”, or “Retracted” position, while the other sensor indicates the “Work”, or “Advanced” position.

When the piston moves within sensing range, the ring magnet’s magnetic field triggers the sensor. This provides indication back to the controller as to whether your requested actuation actually occurred.

A pneumatic cylinder with two magnetic cylinder sensors installed. The cylinder sensors are triggered when the ring magnet attached to the cylinder's piston is in a certain position.
A pneumatic cylinder with two magnetic sensors installed (the two slim rectangular devices facing each other in the upper slot).

Magnetic Position Sensors With Analog Outputs

A pneumatic cylinder with an analog magnetic sensor from the Sick MPS family of sensors. These sensors provide allow you to observe the position of the cylinder very precisely.
A pneumatic cylinder with an analog magnetic sensor from the Sick MPS family of sensors. These sensors provide very precise feedback on the position of the cylinder’s piston.

Magnetic sensors with analog outputs (like this Sick MPS-T) can indicate precise positions of pneumatic actuators. Sensors of this type act like LVDT’s, providing high-resolution feedback on the position of the magnet within the sensor’s sensing range. With many of these sensors, you can observe the position of a cylinder within tenths of a millimeter. That type of resolution allows very precise control of industrial processes.

Another Application For Magnetic Proximity Sensors

There is another application in which you might find this type of proximity sensor used in industrial automation. In certain situations that are difficult for inductive sensors, magnetic proximity sensors can thrive.

The sensing range for inductive sensors is dependent on the size of the sensor. The smaller the face of an inductive proximity sensor, the smaller its sensing range. With small inductive proxes, sensing ranges are in the single digit millimeter and sub-millimeter distances.

Even small magnetic proximity sensors often have sensing ranges much greater than many inductive proximity sensors. For this reason, magnetic sensors are sometimes used where a small sensor is needed, but the target is not very close. If this is the case, a magnet may need to be mounted on the target.

Learn More About Magnetic Proximity Sensors

Photoelectric Proximity Sensors

Photoelectric sensors, or “photo eyes,” are triggered by the presence or absence of electromagnetic radiation. Most commonly, this electromagnetic radiation would be infrared or visible light.

Depending on the type of sensor and application, photoelectric sensors may require some setup. This is due to the many ways in which photoelectric sensors can be utilized and the versatility offered by many sensors in when the sensor will turn on or off its output.

When discussing photo eyes, the concepts of Normally Open and Normally Closed often translate to Dark On and Light On. Depending on how your sensor is configured, Light On generally means that the sensor’s output will only be ON when the emitter’s beam is detected at the receiver. Dark On is the opposite condition.

There are three broad types of photoelectric sensors:

Through-Beam Photoelectric Sensors

Through-beam photo eyes consist of two separate electronic components: an emitter and a receiver. The emitter shines a beam of infrared or visible light. The receiver detects this light and the sensor’s output changes when a certain level of light is seen.

An illustration of a through-beam photoelectric sensor application. The image shows a can on a pallet on a conveyor. On either side of the conveyor are the two through-beam photo eye modules: one emitter and one receiver. The "effective beam" of light is shown transmitting from the emitter to the receiver.
A through-beam photoelectric sensor application. When the effective beam between the emitter and receiver is blocked, the sensor’s output state will change. Click the image to view full size.

Opposed (through-beam) photo eyes are often used to detect small parts. As long as the part is big enough to interrupt the effective beam, the sensor will change its output state.

Sensors of this type can look across a conveyor or feed track to count parts or to trigger an event when a part is seen. Through-beam sensors are very accurate and have a long sensing range. One of their chief drawbacks is that they require the purchase and installation of two costly components: an emitter and a receiver.

An example of a through-beam photoelectric sensor that you’re likely to be familiar with is your garage door safety sensor. If the beam between the two photo eye components at the bottom of your garage door is broken, your garage door will not close. This type of sensor setup – meaning a sensor with the two separate emitter and receiver photo eye components – is the type of through-beam photo eye most commonly seen in industrial automation.

A Second Style Of Through-Beam Photo Eyes

When using opposed photoelectric sensors, aligning the emitter and receiver can be a potentially painful element of the setup process. To combat this, sensor manufacturers have developed products that eliminate the need to align the emitter and receiver for certain applications.

Photoelectric “fork” sensors integrate the emitter and receiver on opposing posts on the same frame. The fork sensor’s design eliminates the need to install and align separate emitter and receiver modules. For certain applications, this design eliminates many of the drawbacks of traditional through-beam photo eyes.

Retro-Reflective Photoelectric Sensors

Retro-reflective photo eyes contain both the emitter and receiver in the same module. They rely on a reflector to bounce the emitter’s beam back to the receiver. Because they require a reflector, they share one of the drawbacks of through-beam photo eyes (installation and alignment of two separate components), but do improve in another area (reduced cost as they don’t require a separate receiver module).

An illustration of a retro-reflective photoelectric sensor application. The image shows a can on a pallet on a conveyor. On either side of the conveyor are the retro-reflector and the combined emitter/receiver photo eye module. The "effective beam" of light is shown transmitting from the emitter, bouncing off the retro-reflector, and then traveling back to the receiver.
A retro-reflective photoelectric sensor application. The effective beam is bounced off of the reflector in order to return to the receiver. Click the image to view full size.

Photo eyes of this variety are subject to one distinct weakness: if the light emitted by the emitter bounces off a shiny object and enters the receiver (referred to as “proxing”), it can fool the receiver into thinking that the beam is unbroken. Retro-reflective photo eyes with polarized filters are able to defeat this issue.

An illustration of a non-polarized retro-reflective photoelectric sensor application. The image shows a can on a pallet on a conveyor. On either side of the conveyor are the retro-reflector and the combined emitter/receiver photo eye module. The "effective beam" of light is shown transmitting from the emitter, bouncing off the can, and then traveling back to the receiver. Non-polarized photo eyes of this type can sometimes be fooled by objects with reflective surfaces.
A non-polarized retro-reflective photoelectric sensor application. Non-polarized sensors have a longer sensing range than polarized versions. One issue with non-polarized sensors, however, is “proxing.” The sensor can be fooled by objects with highly reflective surfaces. Click the image to view full size.
Polarized Retro-Reflective Photo Eyes

Some retro-reflective photoelectric sensors use polarized filters. When combined with specialized reflectors, polarization allows the retro-reflective sensor to differentiate between light that bounced off of the reflector and light that bounced off of a shiny object. This can be particularly helpful when the parts that you are trying to sense are made of a shiny metal (or otherwise have a highly reflective surface).

Polarized retro-reflective photo eyes have a polarizing filter at the output of the emitter. This filter allows only light that is oscillating on a certain axis to pass through. The receiver, which is mounted next to the emitter in the same module, also has a polarizing filter, which only permits light oscillating on the axis perpendicular to that of the emitter.

An illustration of a polarized retro-reflective photoelectric sensor application. The image shows a can on a pallet on a conveyor. On either side of the conveyor are the retro-reflector and the combined emitter/receiver photo eye module. The "effective beam" of light is shown transmitting from the emitter, bouncing off the reflector, and then traveling back to the receiver. A polarizing filter on the emitter only permits light oscillating in a certain direction to be emitted. The retro-reflector causes the polarization of the beam to be rotated by 90° before bouncing back towards the receiver. The receiver also has a polarized filter which only permits light oscillating opposite to the emitter's filter. This ensures that the light received bounced off of the retro-reflector.
A polarized retro-reflective photoelectric sensor application. Polarized sensors use polarizing filters to limit the orientation of light that is emitted and that can trigger the receiver. The reflector causes the orientation of the light to rotate by 90° before bouncing back towards the sensor’s receiver. The polarization properties of this type of setup allow the sensor to determine whether the light received bounced off of the retro-reflector (and not some other reflective surface). Click the image to view full size.

With these polarizing filters, light only enters the receiver if its polarization axis was rotated 90° from what the emitter initially put out. This change in polarization indicates to the sensor that the light it’s seeing has bounced off of the retro-reflector, and not off of some other shiny object.

An illustration of a polarized retro-reflective photoelectric sensor application. The image shows a can on a pallet on a conveyor. On either side of the conveyor are the retro-reflector and the combined emitter/receiver photo eye module. The "effective beam" of light is shown transmitting from the emitter, bouncing off the can, and then traveling back to the receiver. A polarizing filter on the emitter only permits light oscillating in a certain direction to be emitted. Because the light received did not bounce off of the retro-reflector, it was not rotated 90°. The receiver's polarized filter blocks any light that is oscillating in the same orientation as the emitter's filter. This ensures that the sensor is not fooled by the light that reflected off of a shiny surface other than the retro-reflector.
When an object with a reflective surface breaks the beam, light is reflected back to the receiver. The receiver’s polarizing filter blocks this light because it is the same orientation as the light emitted. This ensures that the sensor will only be triggered by light that bounced off of the reflector. Click the image to view full size.

Polarizing filters exclude some of the light that would otherwise enter the receiver. For this reason, polarized retro-reflective photo eyes have a shorter range than non-polarized versions.

Corner-Cube Retro-Reflectors

Some of the magic taking place with polarization is in the retro-reflector. Innocent looking (but secretly really cool) corner-cube reflectors are used with retro-reflective photoelectric sensors to cause the shift in polarization. Corner-cube reflectors accomplish two things to assist in retro-reflective photoelectric sensing:

  • The polarization (“phase”) of the light is shifted by 90°
  • Unlike a mirror, which reflects light away from its source as a function of the angle of incidence, the geometry of corner-cube reflectors causes light to be reflected back to its origin

With these special optical properties, corner-cube retro-reflectors are an ideal solution for ensuring that the light reaching the receiver actually bounced off of the reflector, and not some other shiny surface. Due to their high reflectivity and the fact that they direct light back to its source, corner-cube reflectors are the same types of reflectors that we trust for bicycle and highway safety. In fact, these devices are the same reflectors that we left on the moon to be able to measure its distance from the earth.

Diffuse Photoelectric Sensors

The third common type of photoelectric proximity sensor employed in industrial automation is the “diffuse” photo eye. Diffuse photo eyes only require the installation of one module in one location. For this reason, of the three types of photoelectric sensors at which we’ve looked, diffuse sensors are the easiest, cheapest, and quickest to install.

Like retro-reflective sensors, diffuse photoelectric sensors house both the emitter and receiver in the same module. While through-beam and retro-reflective sensors maintain sight of the beam until it is broken, diffuse sensors can’t see their beam until the sensor’s target is present.

Click the image to view full size.
A diffuse photoelectric sensor application. By default, a diffuse sensor’s emitted beam shines off to infinity, with no light reflecting back to the sensor’s receiver. The emitted light only reflects back to the receiver when the sensor’s target blocks the emitted beam. Click the image to view full size.

With diffuse sensors, the sensor’s target actually acts as the reflector. When the emitter’s light bounces off of the target, the light “diffuses” in many directions, with some portion of the emitted light striking the receiver.

An illustration of a diffuse photoelectric sensor application. The image shows a can on a pallet on a conveyor. The diffuse photo eye (combined emitter and receiver module) is mounted on one side of the conveyor and its beam can be seen shining and bouncing off of the can, which is positioned on the conveyor in front of the sensor. Because the target is present, the emitted beam scatters off of the surface of the target. Some of this light will bounce back towards the sensor and enter the receiver.
A diffuse photoelectric sensor application. When a part is present in front of the sensor, the emitted beam reflects off of the part’s surface. Some of this light will reflect back towards the sensor and enter the receiver, causing the sensor to change its output state. Click the image to view full size.

Because diffuse sensors rely on the detected object to scatter the emitted light, these photo eyes are more sensitive to:

  • The distance between the sensor and the target
  • The reflectivity of the target
  • The curvature and other topological properties of the target’s surface
Learn More About Photoelectric Proximity Sensors

Ultrasonic Proximity Sensors

A final common category of proximity sensor utilized in manufacturing is the ultrasonic sensor. Ultrasonic proximity sensors are similar in many ways to photoelectric sensors, except that they use high frequency sound instead of light. The use of sound can be an advantage in certain cases, as the sensor’s performance will be unaffected by smoke, dust, lighting, or the reflectivity of the target’s surface.

Like photo eyes, ultrasonic sensors can be purchased in an either an “opposed” (through-beam) configuration, or in a “diffuse” (reflective) configuration. The comparison with photo eyes is directly analogous.

Through-Beam Ultrasonic Proximity Sensors

By that, I mean that through-beam ultrasonic proxes use a separate transmitter and receiver module, just as through-beam photo eyes do. Through-beam ultrasonic modules would be mounted across a conveyor or other monitored area.

The “beam” of high frequency sound that is output from the transmitter would be detected continuously by the receiver. The sensor’s output changes state when the signal between transmitter and receiver is blocked by the sensor’s target.

Diffuse Ultrasonic Proximity Sensors

Similarly, diffuse ultrasonic proxes house the receiver and transmitter in the same module. Just like diffuse photoelectric sensors, the sensor’s target reflects sound waves back to the receiver as it passes in front of the sensor. With diffuse sensors, the signal is not detected at the receiver by default. The sensor’s output changes when a target moves in front of the sensor, reflecting the signal back towards the receiver. In my experience, this is the more common ultrasonic sensor.

Diffuse ultrasonic sensors utilize “time of flight” to determine the presence of an object. Because the speed of sound in air is known, the sensor is able to determine how far away an object is. It does so by monitoring the time that it takes for an ultrasonic signal to bounce back to the receiver after being emitted.

Sensors of this type can be purchased with analog or digital outputs. Diffuse ultrasonic proximity sensors with digital outputs indicate to the controller only that an object is present or not present. Ultrasonic proxes with analog outputs typically provide distance feedback. In other words, they can tell you not only that a part is present, but how far away that part is.

Ultrasonic Sensor Advantages and Disadvantages

As mentioned above, the use of sound can give ultrasonic sensors an advantage over photoelectric sensors in certain cases. Ambient lighting and the presence of smoke or dust have the potential to affect the performance of photo eyes. These factors are largely irrelevant to ultrasonic sensors. Ultrasonic sensors are also able to detect irregularly shaped objects, such as meshes and springs, that may be difficult for photo eyes to detect reliably.

While ultrasonic proximity sensors are very reliable in many cases, there are some factors that can affect performance. For instance, certain soft targets absorb sound waves and so may be difficult for ultrasonic sensors to detect. Temperature changes can also affect reliability. To combat this, many modern ultrasonic sensors have integrated temperature detection and calibration.

Learn More About Ultrasonic Proximity Sensors

Comparing Proximity Sensor Types

There are many different types of proximity sensors on the market. Depending on your application, there may be only one prox that is appropriate, or you may have a choice. Let’s take a last look at the different types of proximity sensors commonly implemented in factory automation:

InductiveCapacitiveMagneticPhotoelectricUltrasonic
Install ComplexitySimpleSimpleCan be trickyCan be trickyCan be tricky
Setup ComplexitySimpleSensitivity adjustableSimpleCan be trickyCan be tricky
Max RangeVery Short (a few CM)Very ShortShort (several CM)Very Long (many meters)Long (10-20 meters)
Materials DetectedMetalsAlmost anyMagnets, ferrous metalsAlmost any, shiny can be troubleAlmost any, soft can be trouble
Sense Through Container?Metal inside of non-metalSometimes possible with adjustmentMagnet/ferrous inside of non-ferrousCertain sensors, certain conditionsCertain sensors, certain conditions
Affected by Lighting/Color?NoNoNoYesNo
Close-Range Detection?YesYesYesYesDead zone
Liquid Level Sensing?NoYesNoYesYes

I hope that this post has been helpful for you in sorting out the various types of proximity sensors used in manufacturing. If you would like to see more content like this, enter your email for more in-depth industrial automation articles:

Thanks for reading, and make sure to leave a comment below with any thoughts!

NO and NC (Normally Open and Normally Closed) Proximity Sensor Basics

When I was learning PLC programming, I remember scratching my head about some of the concepts surrounding proximity sensors. Digital or analog, Normally Open (NO or N.O.), or Normally Closed (NC or N.C.)?  What exactly does it mean for a sensor to be NO or NC? What effect will it have when I’m checking the state of the sensor at the PLC or other controller?

Proximity sensors set up on an automation line.
Common “barrel proxes” (pronounced “prawksez”) set up to detect parts or features of parts as they move down a conveyor. When objects made of certain materials (depending on the sensor type) pass in front of a proximity sensor (sometimes referred to as a proximity switch), it is “made,” changing the state of its output signal.

Want to take the next step in your industrial automation career?

We want to help.


Are you looking for opportunities to advance your career and make more money?

At no cost to you, the Industrial Automation Connection can get you in touch with training and job opportunities in your area. Click one of the buttons below to get started!

NO and NC, and Other Proximity Sensor Basics

NO or NC refers to the way that a sensor is wired and in what state its output signal will be when the sensor is “made.” A sensor is “made” when an object is present that the sensor has been set up to detect. The characteristics of the sensor determine whether or not an object will detected. These characteristics can include its detection type (inductive, capacitive, ultrasonic, photoelectric, etc.), sensing range (how far away the part can be from the sensor), and other factors.  The point of a proximity sensor, or “prox,” is to know that an object is there or not there. When a sensor detects an object, its output state changes.

Digital sensors

Someone might refer to the types of proximity sensors described above as “digital proxes.”  In this context, digital has a somewhat different denotation than the typical use of the word outside of industrial automation. If a sensor is “digital,” it only has two possible output states: on or off.

There are a multitude of different sensors on the market. There are small sensors, large sensors, laser sensors, sensors like the barrel proxes above which have no configuration whatsoever, sensors that require quite a bit of set up, and everything in between. If a sensor’s sole purpose is to detect whether or not an object is present somewhere, its output is typically digital (either on or off). For this reason, people sometimes refer to sensors of this type as “switches”. Like a light switch in your home, they either turn an output on or off.

In this regard, you can think of the behavior of a prox switch or other digital output as being just like that of the paperclip switch that turned on a small lightbulb in your 2nd grade science class. The prox sensor is the paperclip, and the target passing in front of the prox is your hand pushing down on the paperclip to change the switch’s output state.

Analog sensors

Aside from digital outputs, there are devices with “analog” outputs. Analog sensors output a specific value within a range (anywhere from 2V to 10V, for instance). As one example, sensors with analog outputs can be used to tell a machine how far away something is.

Click the following link if you’d like to learn more about the differences between digital and analog sensors. For now, let’s take a look at how Normally Open and Normally Closed sensors differ in their behavior:

Normally Open Devices

A graph depicting an example of sensor output behavior for a Normally Open sensor. By default, the sensor's output is off, or "low." When the sensor detects an object in its sensing range, the output is switched on. When the object then leaves the sensor's range, the output returns to its default state of low.
This graph shows the behavior of a simple, Normally Open proximity sensor as an object passes in front of the sensor and then passes out of its sensing range. When an NO proximity sensor detects its target, its output signal is turned on (energized with voltage). When the object is no longer detected by the sensor, the output state changes back to the original state (no voltage on the signal wire). Click the image to view full size.

As mentioned above, the purpose of a proximity sensor is to tell a machine when something is present in front of the sensor.  So, what actually happens when the sensor detects an object?  Well, the sensor’s output changes state. This means that the sensor either energizes an output signal wire with a small amount of electricity, or not.

Like a light switch at your house that is off, an NO sensor will not, by default, put out a voltage to its output wire. Returning to the paperclip circuit analogy, an NO sensor’s default state is similar to the paperclip lifted off the thumbtack. The switch breaks the output circuit by default; hence, the output circuit is “normally open”. Referring to the graph above, when an NO sensor is in its default state (does not detect a target), the sensor’s output is off.

What happens when the sensor is made?

When an appropriate object passes within the sensor’s sensing range, the sensor outputs a voltage through its signal wire. This signal can indicate to a controller that the target has “made” the sensor. So long as the target remains within sensing range, the prox will continue to provide voltage on its output signal.

What’s the point of this? This is how the sensor “tells” the controller: “hey, I’m energizing my output as a signal to you that there is something in front of me right now.”

As you can see in the graph above, once the object passes out of the range of the sensor, the sensor will turn off its output. A controller would now see that the sensor is in its normal, “off” state.

As a brief aside, there are quite a few ways to refer to something as being “on” or “off”.  Below are some other ways you might hear someone refer to a signal as being on or off.  In my opinion, all of these are more or less equivalent:

OnOff
HighLow
EnergizedExtinguished
LitOut
MadeNot Made

Normally Closed Devices

NC sensors and other devices behave exactly opposite to NO devices in regards to their outputs. NC devices are, as indicated by their name, normally closed, meaning that their output is on by default.  Only when an object makes the sensor does the signal actually turn off.  Here’s a simplified graph of the signal behavior for an NC sensor:

A graph depicting an example of sensor output behavior for a Normally Closed sensor. By default, the sensor's output is high. When the sensor detects an object in its sensing range, the output is switched off. When the object then leaves the sensor's range, the output returns to its default state of being energized.
Here you can see that the behavior of a Normally Closed sensor is directly opposite that of an NO sensor; they are the negation of each other.
When an NC prox is made, the signal is actually “brought low.” Click the image to view full size.

If you understood the behavior of Normally Open sensors, then you also understand the behavior of Normally Closed sensors; one is simply the inverse of the other.  If an NO and NC sensor were set up to detect the same object, the NO sensor’s output would be on when the NC sensor was off, and vice-versa.

Default Output StateOutput State When Sensor Is Made
NO SensorsOffOn
NC SensorsOnOff

Why choose an NO or an NC sensor?

Due to these differences in output behavior, Normally Open and Normally Closed sensors are better or worse for certain applications.

All cables and electrical components will eventually fail.  To get an idea of why you might choose one sensor or another, let’s first talk about how we want our systems to behave when a cable or sensor is damaged, and we no longer get the signals we’re relying on to control machine motion.

The two most common types of electrical failures are “opens” and “shorts,” with opens being the most common.  An open is an unwanted break in a circuit. Cuts, crushing, or other damage to the cable can cause an open.

An example of a Normally Closed application: Emergency Stop

Modern factories are populated throughout with “E-Stop buttons”. Emergency Stop buttons can be used by anyone in the facility if an unsafe condition is observed. Slap an E-Stop, and all machine motion will come to a halt as quickly as possible.

A red emergency stop button that would be present throughout a factory for use in an emergency to stop the factory.
A typical E-Stop button.

Remember that you can think of a prox sensor as just another type of switch. What we traditionally think of as a switch is usually switched by mechanical action. Proxes are typically solid-state devices with internal electronics that turn outputs on and off. An E-Stop is an example of a true mechanical switch. When someone presses an E-Stop, metal contacts inside of the device open or close its output circuits.

NC or NO?

Let’s consider whether the E-Stop should be a Normally Open or Normally Closed device. With a Normally Open E-Stop, the button’s outputs will be off (open) when the button is in its default (not pressed) state.

In an emergency, someone hits the E-Stop.  The mechanical action of pressing the button causes the normally open contacts to close, energizing the button’s outputs. Now the controller can detect those outputs, and we can use this status in our logic to halt machine motion. Cool.

Except… let’s return to the concept of an unwanted break in our circuit. What happens if the cable that connects the E-Stop button to the controller has been damaged?

A simplified schematic depiction of an E-Stop circuit. A power supply on the left feeds power to an E-Stop switch which feeds an input to a controller on the right. There is a break in the connection between the E-Stop and controller.
A simplified depiction of an E-Stop circuit. The E-Stop is shown as an NO switch for the purpose of illustrating the concept; in reality, E-Stops are typically NC. If the E-Stop were NO, a break in the wire would prevent the stop signal from reaching the controller in an emergency. Click image to view full size.

Safety first

If the E-Stop is a Normally Open device, and its cable becomes damaged, then when we go to activate the E-Stop, we will never get a signal back to our controller telling it to halt production. To the controller, a damaged electrical system and the default output of a Normally Open switch look exactly the same. In either case, there would be no incoming voltage to the controller’s input.

If the E-Stop in this example were Normally Open, you would only check for its output signal when you needed it to stop the line. As a result, you have no way of knowing whether the button or cable is damaged until it’s too late. A Normally Open switch wouldn’t just be a bad choice for this application, it would be dangerous. In an emergency, an ineffective E-Stop could contribute to someone being severely injured or killed.

Making the right choice for the right application

For this reason, E-Stops and most safety devices are Normally Closed. When a Normally Closed E-Stop is in its default position, the contacts close the circuit and return a signal to the controller indicating that the system is safe. Because the E-Stop returns a signal constantly, any condition that causes the E-Stop signal to go low will be detected. Aside from someone actually pressing the button, some other possible causes for losing the E-Stop safe signal might include loss of power to the system, failure of the E-Stop’s cable, or failure of the E-Stop button itself.

Now, since our Normally Closed E-Stop is always sending a signal back to the controller when it’s in the safe position, we set our logic up so that we must constantly see the signal from the E-Stop to allow the factory to run.  You could think of this type of Normally Closed signal as a constant “thumbs-up” to the controller that the system is safe.  In the controller logic, machine motion would only be permitted when the expected signals from all safety devices are present.

A view of a pilot in the cockpit of an American military jet. The pilot is giving the thumbs up sign with his left hand.
Who’s got one thumb and flies a jet?

Along this same line of thought, other sensors that detect unsafe conditions, such as tank overfill, are typically Normally Closed. Because NC sensors return a signal by default, any loss of that signal will immediately indicate that the system is not safe.

Are you beginning or advancing your career in the field of industrial automation?

Let us help you take the next step.


There are organizations out there looking for people like yourself.

At no cost to you, IAC will work to connect you with opportunities in your area. Click one of the buttons below to get started!

An example of a Normally Open application: Part Present

For less safety-critical applications, Normally Open sensors work just fine and in fact are found more commonly in industrial automation than NC sensors.  In certain cases, use of an NO sensor would actually be preferable, and many people find it easier to interpret the behavior of NO sensors when it comes time to debug an electrical or programming issue.

“Part Present” applications, for instance, often use NO sensors.  Let’s say that you want a robot to pick up a part and move it to another location. When the robot moves to the “pick position,” you want to be able to verify that the part is positioned in the robot’s “end effector” before allowing the robot to attempt to move the part. An end effector is a fixture bolted to the robot arm that is custom-built for picking up a particular part.

Normally Open sensors are ideal for this type of Part Present detection, as they only send the signal that the part has been picked up if they actively sense material. If a cord or sensor is damaged in this type of application, the sensor will simply never output its signal. Because the robot won’t see the necessary signal, robot motion will halt until the problem can be corrected.

Two yellow Fanuc robots are moving pieces of metal in an automation cell.
Two Fanuc robots performing material handling operations in an automation cell.  Their end effectors are the orange fixtures attached to the ends of the robotic arms.  The end effectors likely use Normally Open “part present” sensors to verify that the part is properly loaded before moving away from the pickup positions.

NC and NO Sensors

There’s a common thread in both the Normally Closed and the Normally Open applications described above. With either NO or NC, you want positive indication before you allow the system to move. By positive indication, I mean that you want the PLC to see the signal from the sensor go high.

In the E-Stop application, you want to be able to move the system by default. You only want to disable motion if a certain condition is met (someone slaps the E-Stop). Hence, you want the signal to be on by default (Normally Closed). You only want the signal to go low if your system isn’t safe.

In the Part Present application, you want the robot to stop at the pickup position by default. You only want to enable motion under certain conditions (the part positioned properly in the end effector). Hence, you want the signal to be off by default (Normally Open). You only want the signal to go high if your part is properly loaded.


Hopefully, this has shed a bit of light on some of the basics of proximity sensors, including the concepts of Normally Open and Normally Closed. There is a lot to be said about the many sensors on the market and their functionality. Click the following link for an in-depth look at the various types of sensors and how they work.

Anything you wish we would add to this article? Send us an email or let us know in the comments section!

If you found value in this content, let IAC keep you posted whenever we have something new for you! Sign up below; it only takes a second. 🙂