What’s The Difference Between Analog And Digital Sensors?

Sensors are the eyes and ears of your robot or Programmable Logic Controller. They allow your controller to determine the state of the objects and environment around your automated operations.

There are many differences between the wide variety of proximity sensors on the market. You can buy sensors that are inductive, magnetic, photoelectric, or ultrasonic. You can buy sensors that sense parts that are very close, or parts that are quite far away. Sensors can be big, or small. You can find sensors that detect a range of materials, detect only ferrous metals, detect only magnets, or even that detect materials inside of other containers.

Among all of these varieties, one of the most fundamental differences between proximity sensors is whether the sensor provides a digital or analog output. In this article, we’ll discuss the difference between analog and digital sensors, and why you would choose one or the other.


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Using Sensors To Tell A Controller What’s Out There

In the world of industrial automation (and in other settings, like home automation and Arduino development), a huge variety of sensors exists. Sensors provide feedback about the automated process to the PLC or other process controller. Imagine parts moving down an assembly line in a factory:

As each part moves down the line, the PLC must know the part’s location to know when it is time to perform each portion of the manufacturing process.

How can we tell a PLC where objects are in the real world? With proximity sensors, we can determine where objects are located and inform the PLC or other controller.

Additionally, we can validate that the correct object is located where we expect. For instance, you may have a station where two different – but similar – parts are loaded for further assembly. Sensors can confirm both that a part was loaded and that the right part was loaded. Take a look at the application shown below:

An image depicting several cans (3 tall, 1 short) proceeding down a conveyor belt. The closest can is present in front of a part present sensor station. There are two photoelectric sensors at that station. The lower sensor is triggered when either type of can (short or tall is present). The upper sensor is triggered only when a tall can is present. With these two sensors, the PLC or other controller is able to determine when the right can is present at this station. This image was used in a post detailing the difference between digital and analog sensors.
An example of Part Present Sensors used to check for the presence of a part, and to pick out which of two models is present. Click to view the full-size image.

Applications For Analog And Digital Sensors

In the case above, the controller only needs one piece of information from each sensor: is the sensor “made” or not?

What Does It Mean For A Sensor To Be “Made”?

What does someone mean when they say that a sensor is “made”? This means that the sensor “sees” whatever it is supposed to detect. In the scenario above, we’re using photoelectric sensors to detect the presence of the parts on the conveyor. Photoelectric sensors of this type emit a beam of laser light and wait to see if this light reflects back to the receiver.

In the case above, each sensor is “made” if its receiver detects a sufficient quantity of the reflected light. Different sensors have different detection mechanisms. There are surely dozens of detection methods, but around 5 common ones. For more information on that subject, check out this article on the different types of proximity sensors and their detection mechanisms.

When Are Digital Sensors Used?

Continuing with the previous thought, the controller only needs to know one thing from the sensors above: do they or do they not see a part?

More specifically, a PLC or other controller would only need one bit of information from each sensor. Is the sensor ON (made: a part is present), or is it OFF (not made: no part is present)?

We refer to sensors or other devices that only output one bit – meaning ON or OFF – as digital sensors. Digital sensors will have (at a minimum) one signal wire that will either be energized or not energized, based on whether the sensor is made. More on that below.

When Are Analog Sensors Used?

Let’s imagine a slightly different scenario. Let’s say that, at the sensor station on this conveyor, a robotic arm would come down and pick up each can as it passed by.

Because there is a little bit of variance in the left-to-right position of each pallet on the conveyor, the cans won’t be in exactly the same place every time. As a result, the robotic arm may have a hard time picking up each can.

One way to solve this problem would be to use a sensor that could tell the controller how far away the cans are from the sensor station. This would provide the can’s location along the horizontal axis of the conveyor. With this information, the controller could provide more specific positional data to the robotic arm. This “offset” to the robot would allow it to pick up the cans reliably.

An image depicting 4 cans proceeding down a conveyor belt. Each of the cans is on a pallet, and the horizontal position of each pallet  on the conveyor varies slightly. The closest can is present in front of a part present sensor station. There are two photoelectric sensors at that station. The lower sensor is a digital sensor that indicates to the controller whether or not a can is present at all. The upper sensor is an analog sensor, sometimes referred to as a "time of flight" sensor. Where the lower, digital sensor provides only "on or off" feedback to the controller, the upper, analog sensor provides specific feedback that represents how far away the part is from the sensor. With these two sensors, the PLC or other controller is able to determine both that a can is present in front of the sensor station, and where the can is located horizontally on the conveyor.
Due to variance in the horizontal position of the pallets on the conveyor, the precise location of each can is unknown. An analog sensor can provide feedback to the controller with the distance between the sensor and the can. Click to view the full-size image.

Analog Sensors Provide Precise Feedback

For this type of application you would need an analog sensor. Analog sensors provide specific feedback along some scale. This feedback could correlate with distance (as in the case above), pressure, temperature, or other data. In each case, though, an analog sensor would provide a specific value back to the controller, instead of just an on or off condition.

In the case shown above, an analog sensor could provide feedback to the controller not only in terms of whether a part is present, but also in terms of how far away the part is. This type of sensor (a photoelectric or ultrasonic sensor that reports the distance to an object) is referred to as a “time of flight” sensor.

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Digital Sensors

As mentioned above, a sensor is digital if its output only has two possible states: on or off. The most basic of this type of sensor will typically have 3 wires coming into it:

  1. A power wire (typically +24VDC in modern industrial automation)
  2. A neutral wire (0VDC)
  3. A signal wire (provides feedback to a PLC or other controller)

A digital sensor will output one of two possible states on its signal wire:

  • On (the sensor energizes the signal wire with voltage and a small amount of current)
  • Off (no voltage on the signal wire)

The sensor changes its output state when it detects a part. The controller monitors the sensor’s output state via the signal wire. When do you think the sensor will energize the signal wire? If it sees a part, or if it does not see a part?

Normally Open Prox Sensors

To me, the intuitive answer to the question above is that a “prox” sensor’s signal wire will be energized when the sensor is made. The sensor sees a part (or senses whatever it was that it was designed to sense), and then it outputs a signal back to the PLC. “Hey, I see something!”

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.
Normally Open “prox” behavior. By default, the sensor’s output is off. Only when the sensor is made does it turn its output on. This graph was borrowed from our post on Normally Open, Normally Closed, and other proximity sensor basics. Click to view the full-size image.

This type of output scheme is referred to as Normally Open, or “N.O.”.

  • Normal: The typical state of the sensor, which is to say, when the sensor is not made
  • Open: The sensor’s output circuit is “open”, meaning that the sensor has de-energized its signal wire (no voltage present)

The most basic (and common) type of proximity sensor you will see is a Normally Open inductive proximity sensor with a single output.

Normally Closed Prox Sensors

It is not uncommon, however, to see sensors with an inverted output scheme to the one above. Sensors of this type are referred to as “Normally Closed” sensors. To many people, this type of proximity sensor works “backwards” to what’s intuitive. Normally Closed sensor output is on by default and only turns off when the sensor is triggered.

Normally Closed sensors have many applications. For instance, many safety circuits are Normally Closed. As mentioned above, “N.C.” sensors provide output voltage constantly, until the sensor detects something.

When you perform electrical troubleshooting, you soon figure out that opens (breaks in a wire) are more common than shorts (when an unintended source energizes a wire). NC sensors and circuits are good for situations where you want to know that something is working at all times, instead of finding out only when it’s time to detect something that the sensor or its wiring has failed.

Normally Closed Proximity Sensor Applications

For instance, consider fire detection. Let’s go for fire detection around an aircraft engine. If I’m designing a safety circuit for an aircraft engine bay, I really want to know if there’s a fire in there. Typically, there won’t be a fire in there. So, typically, my fire detection sensor is sitting around idly – it is not reporting that there is a fire.

Well, what happens if there is a fire? Is my sensor working properly? Is there damage to its cables? Has a wiring harness that passes its signal back to the cockpit come loose?

If I only report a signal when I detect something, as is the case with Normally Open sensors, then I only know if there’s a problem when there is a fire, but no alarm!

How Do Normally Closed Sensors Function?

Normally Closed, sensors, on the other hand, provide a signal back to the controller all the time. They only de-energize the signal if they’re made. Because NC sensors provide constant voltage back to the controller, I will immediately know if an NC sensor has failed due to damage or wiring issues.

This is because a loss of signal, which is the more common electrical failure, looks the same as a Normally Closed sensor that is operating normally and made.

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.
Normally Closed sensor outputs work opposite to the behavior of Normally Open sensors. Their functionality is inverse. In other words, if you had an NC and an NO sensor both sensing the same target, their outputs would always be opposite. Click to view the full-size image.

How To Conceptualize Normally Closed Sensors

To summarize, NC sensors energize their outputs normally, and only de-energize their outputs when they are made. For this reason, one way to think of the behavior of NC sensors as follows: “I’m safe, I’m safe, I’m safe, hey – I see something! <drops the output signal>”

To learn more on this topic, check out this in-depth post on the concepts of Normally Open and Normally Closed.

Digital Sensors With Both Normally Open And Normally Closed Outputs

Alright, we’ve covered the layups above, now let’s go for the 3-pointer:

How do you know if a Normally Open sensor has failed, or if its cabling is damaged?

Well, its output is normally off, so you only know if an NO sensor has failed when it is supposed to be indicating that it is made, but it’s not indicating.

But what if you had a sensor that provided one indication when it was made, and a different indication when it was not made?

The most robust digital sensors, in my opinion, are sensors which have both NO and NC signal outputs. Sensors of this type have “complementary outputs”. Let’s dive into that just a bit:

When wiring sensors for industrial automation, you’ll often find the following color code used:

PinWire ColorPurpose
1BrownPower (+24VDC)
2WhiteSignal 2
3BlueCommon (0VDC)
4BlackSignal 1

Prox Sensors With Complementary Outputs

With a basic NO or NC prox, only the Signal 1 wire provides feedback to the PLC. When you’re using a prox with complementary outputs, however, there will always be a signal back from the sensor to the PLC. This is because one signal wire is N.O., and the other signal wire is N.C. When Signal 1 is energized, Signal 2 is de-energized, and vice-versa.

For this reason, with a prox with complementary outputs, there will always be a signal output from the sensor – something to say “hey, if nothing else, you know I’m powered on”.

While other failures are possible, this at least covers an open circuit, which is the most common type of electrical defect. Complementary outputs ensure that the PLC will at least know that the sensor is communicating.

With all of this said, proxes with just one output (NO or NC) are much more commonly used in industrial automation than proxes with complementary outputs.

Digital Sensor Applications

What Do Digital Sensors Detect, Conceptually?

When will you use a digital sensor? Well, most of the time, is the answer. Sensors with digital outputs are the typical type of sensor used in manufacturing and automation.

Digital sensors have a simplistic, on/off output circuit. For this reason, we use digital sensors where binary conditions exist:

  • Material is either present or not present
  • Temperature or pressure is either higher than a certain value, or not
  • A tank is full, or not
  • Parts are aligned or are out of alignment
  • A cylinder is in a certain position, or not

The most common sensor used in manufacturing, in my experience, is an inductive proximity sensor with a Normally Open digital output. It’s about as basic as a sensor could be. This is meant both in terms of the sensor itself and in terms of wiring it to the PLC.

Real-Life Applications For Digital Sensors

With this in mind, however, note that even though a digital output circuit is simple, the sensor’s processing circuit can be complex.

For instance, some vision systems can provide a digital output if the camera detects a certain object in its image. Many pressure sensors, time-of-flight sensors, and other devices trigger their digital outputs at a certain threshold value for pressure or distance. The user can often program this threshold. So, the idea here is that although a digital sensor’s output is always straightforward (either ON or OFF), the means of determining if the output should turn on can be complex.

In keeping with the above, here are some digital sensor applications:

  • Part present sensing
  • Error-prevention (keeping someone from loading the wrong part) by validating that a certain feature of the part is there or not there
  • Tank full or tank empty
  • Cylinder extended, cylinder retracted
  • A particular side of a rotating assembly is in position
  • Pressure or temperature above or below a specific value
  • Button pressed or released
  • Door open or closed
  • Mechanical overtravel (a prox placed just beyond the normal motion of a machine to detect when something has gone awry)
  • Personnel safety (light screen clear, safety mat clear, etc.)

Analog Sensors

Now that that’s out of the way, let’s get back to the differences between digital and analog sensors. Above, you read that digital sensors are the more common type of proximity sensor. Digital sensors are, often, very straightforward and to-the-point to implement in your process.

One difference between digital and analog sensors is that analog sensors often require more programming to integrate with your PLC, robot, or controller. In exchange for this, they offer precision feedback that digital sensors are unable to provide.

Analog Outputs

A sensor with digital outputs only conveys a “0” (off) or a “1” (on) to the PLC. The primary manner in which analog sensors differ from digital sensors is their output scheme.

Unlike digital sensor outputs, which are either on or off, analog outputs convey a range of values. This range will typically correlate on a linear scale to the range of pressures, temperatures, or distances that the sensor can detect. Let me break that down:

An Example Of An Analog Sensor

Take a look at the following analog pressure sensor:

Telemecanique WMLP100PD230 Pressure Sensor

This pressure sensor is a device that senses the pressure in some hose or pipe. Note the following details from the product’s specifications:

  • Gauge Range: 0-100 PSI
  • Analog Output: 4-20 mA

Analog Sensor Specifications

The numbers above are pretty critical bits of data about the operation of this analog sensor. What these specifications are telling you is that the sensor is able to report back specifically how much pressure it detects. The limits to its pressure sensing range are, obviously, 0 and 100 Pounds per Square Inch.

In order to provide feedback to the PLC, the analog sensor will output a specific current that corresponds with the amount of pressure it senses. The analog output range tells you the current range that will correspond to the pressure sensed. In this case, the sensor will output between 4 and 20 milliamps of current.

The current output will correlate to the pressure sensed. In many cases, the smallest value of voltage or current feedback will correspond with the smallest limit to the sensing range. This is not always the case, however, and so you must check the datasheet for an analog sensor to determine its functionality.

Analog Sensor Datasheets

To figure out how our analog sensor works, let’s take a look at its datasheet:

On Page 1, you’ll find the “Analog output function” and “Pressure setting range” values that correspond to the numbers you saw above.

On Page 4, you’ll see how to wire the sensor to the PLC or other controller. Page 4 shows the sensor’s “pin-out”. There are pins on the sensor’s connector that connect to outside wiring for power and feedback. The pin-out tells you which of the connector’s pins connect to outside wiring. Additionally, it tells you what the outside wiring needs to connect to.

Now, take a look at Page 5. On this page, you’ll see a graph of the sensor’s output value as it corresponds to various input (pressure) values. Note that the graph is a straight line. This tells you (assuming the unit scales are linear, anyway) that there is a linear correlation between pressure and current output.

With this in mind, you can easily calculate the pressure in the field based on the sensor’s analog output. Which is key, as this is precisely what you would need to do to develop logic for this sensor in your PLC program.

A graph showing the correlation between the pressure detected by an analog sensor and that sensor's analog output. This graph is used in an article detailing the differences between digital and analog sensors.
The Telemecanique pressure sensor’s analog output value as it corresponds to the pressure detected by the sensor.

Analog I/O Considerations

There’s one additional consideration when using sensors with analog outputs. That consideration is the availability of analog inputs to which your analog sensors must be connected.

Virtually every PLC will come with digital inputs. In other words, it is virtually guaranteed that you will be able to hook up some number of digital sensors to your PLC or controller. Typically, you will be able to do this for digital sensors “out of the box”. By this, I mean that digital sensors can be “hooked right up”. The PLC can access the sensors’ inputs, and monitor signals right away, without having to do anything “extra”.

Analog sensors provide analog outputs. An analog output from your sensor has to hook up to an analog input to talk to your PLC.

Most input ports will accept either digital or analog inputs, but not both. As mentioned above, most PLC’s accept digital inputs out of the box. Not all PLC’s accept analog inputs natively.

So, to connect analog sensors to a PLC or other controller, you must have:

  • A PLC with analog inputs built in, or
  • An add-on analog input card that you can connect to your PLC, or
  • A separate “input block” that connects to your sensor, which then talks to the PLC

In most scenarios, it won’t be difficult to find a solution for hooking up analog inputs to your PLC. With that said, it is something to consider when adding analog sensors to an automated process.

Analog Sensor Applications

So, when would we use analog sensors?

Analog sensors can be more difficult to program and integrate than digital sensors. However, they provide feedback to the PLC that digital sensors cannot provide.

Analog sensors give you precise feedback. Use an analog sensor:

  • When it’s necessary to know not just that a part is there, but how far away the part is
  • If you need to know not just whether a tank is full or empty, but how full or empty it is
  • If you want to monitor specific temperature or pressure

With those concepts in mind, here are some applications for analog sensors:

  • Alignment check (verifying a part is in a specific position) with a photoelectric time-of-flight sensor
  • Predictive maintenance: monitoring for rises in temperature or vibration
  • Motor speed control based on the amount of liquid in a tank
  • Distance-sensing via an ultrasonic time-of-flight sensor to allow a robot to approach a part that is an unknown distance away
  • Light (brightness) sensors
  • Cylinder extension detection (measuring how far a cylinder is extended) using analog magnetic sensors

Additional Information

I hope this sheds some light on the differences between digital and analog sensors. Selecting, setting up, wiring, and configuring sensors is a key skill-set for Industrial Automation Engineers and Technicians. Similarly, understanding the difference between analog and digital sensors and outputs is part of the foundation of knowledge that will help you to succeed in automation.

For additional information on the differences between digital and analog sensors, and on proximity sensors in general, check out the following resources:

Do you have any lingering questions about proximity sensors, or other topics related to industrial automation? Head down to the bottom of the page and leave a comment. If you’ve got a great question, we may just write an article in response!

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.

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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:

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.

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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.

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