Difference Between Inductive And Capacitive Proximity Sensors

Have you recently found yourself staring at two seemingly identical proximity switches? Have you wondered, what the heck is the difference between these two? Or maybe you’re getting ready to buy some sensors. You want to make sure you pick up the right device for the right application. Below, we’ll illustrate the differences between capacitive and inductive proximity sensors. We’ll look at how inductive proximity sensors work, how capacitive proximity sensors work, and compare these two types of proximity sensors.

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Inductive Proximity Sensor Working Principle

An inductive proximity sensor available for purchase on Amazon. The sensor has a chrome finish with a green face.
An inductive prox in a “barrel” form factor

Inductive proximity sensors are perhaps the most common type of prox switch in industrial automation. These sensors generate electromagnetic fields to detect their target. This field is strongest when no target is present in front of the sensor.

An inductive proximity sensor emitting its electromagnetic field at full strength, because no target is present in front of the sensor. This image is being used to illustrate the difference between capacitive and inductive proximity switches.
Inductive proximity sensor with no part present

When a target passes in front of the sensor, eddy currents form in the electromagnetic field, lowering the amplitude of the field as measured at the sensor.

When a metallic target is presented to an inductive prox sensor, the amplitude of the sensor's electromagnetic field is dampened.
When sufficient dampening takes place, the sensor's output changes state.
Inductive proximity sensor with a part present

Applications For Inductive Prox Switches

Because inductive proxes use electromagnetism, they are only able to detect metallic objects. Some inductive prox switches have a harder time detecting non-ferrous metals. Non-ferrous metal are metals that don’t contain very much iron. If you want to detect a metallic object, there are a variety of inductive proxes on the market. Depending on the iron content of your target, you can find inductive sensors to fit any of the following applications:

  • Inductive proximity sensors that primarily respond to ferrous metals (such as steel and iron)
  • Inductive proximity sensors that primarily respond to non-ferrous metals (such as aluminum)
  • Also, inductive proximity sensors that respond to varying metals equally

Because inductive proxes only sense metal objects, they have some advantages for certain applications:

  • Inductive proxes can detect metal objects through plastic (or other non-metallic) containers
  • Because they only sense metals, inductive proximity sensors are tolerant to dust build-up on the face of the prox

Inductive proxes can come with either Normally Open or Normally Closed outputs, or as analog sensors. You can hook up the sensor’s output to a PLC, robot, or other controller to detect machine motion.

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Capacitive Proximity Sensor Working Principle

A capacitive proximity sensor available for purchase on Amazon. The sensor has a chrome finish with a green face. I deliberately selected a prox that looked the same in its capacitive and inductive forms.
A capacitive proximity sensor. Look familiar?

Capacitive proximity sensors are another type of prox switch that are commonly used in industrial automation. Compare the picture of this capacitive prox to the inductive prox in the section above. Note that you wouldn’t be able to tell which sensor was inductive or capacitive just by looking at them. Always check your prox’s part number to determine its specs.

You saw above that inductive proxes generate electromagnetic fields. Capacitive proxes, on the other hand, generate electrostatic fields. Capacitive proxes house a capacitive plate behind the face of the prox. This plate generates an electrostatic field in front of the prox. Whereas inductive prox fields are highest with no target present, capacitive sensors detect very low capacitance when no target is present.

A capacitive proximity sensor with no part present in front of it. With no part present, the capacitance measured by the sensor is low.
A capacitive prox switch with no part present

When a target passes in front of a capacitive prox, the target acts as a second capacitive plate. For this reason, the presence of a target increases the capacitance measured by the prox. When a certain threshold of capacitance is met, the sensor’s output state will change.

A capacitive proximity sensor being made by its target. The difference between capacitive and inductive proximity sensors is the presence of an electrostatic versus an electromagnetic detection field, respectively.
A capacitive proximity sensor being “made” by its target

Unlike inductive sensors, many capacitive sensors are adjustable. This allows you to set the sensitivity of the prox to ensure accurate detection. With adjustable sensitivity, capacitive proxes have a few tricks up their sleeve. Adjustable capacitive proxes can differentiate between material thickness in certain cases. They can even detect liquids inside of containers.

Applications For Capacitive Prox Switches

Nearly any material presented to a capacitive prox will increase the capacitance that the sensor measures. For this reason, capacitive proxes are able to detect both metallic and non-metallic targets. Some applications for which capacitive prox sensors are well-suited include:

  • Sensing of non-metallic materials. This can include plastic, glass, liquids, biological matter, and more
  • Detection of a certain quantity or thickness of material
  • Liquid detection from the outside of the container

Like inductive sensors, capacitive sensor outputs can be NO, NC, or analog.

Comparison Between Inductive and Capacitive Proximity Sensors

While there are clear differences between the two types of sensors, there are similarities, as well. To start with what’s similar, let’s talk about the internals of inductive and capacitive prox switches:

Similarities Between Capacitive And Inductive Proxes

Many of the internal working principles of capacitive and inductive proxes are similar. Capacitive and inductive proxes are both typically fed by DC power. Because they need AC power for the sensor circuit, both types of proxes typically have “oscillators”. Oscillators are electronic circuits that generate AC power from a DC input. You can read more about oscillators in this write-up on Wikipedia.

Additionally, both types of sensors typically have a trigger circuit. Depending on the type of sensor, this circuit activates when the sensed signal passes either above or below a certain threshold.

Lastly, both types of sensors have an output circuit. This circuit switches the sensor’s output when the signal threshold is met. If the sensor is Normally Open, the sensor’s output will turn ON when it sees a part.

For Normally Closed sensors, the opposite is true. In other words, the output for Normally Closed sensors is on by default. When the prox sees a target, the output is turned off.

Other sensors on the market have analog outputs. Analog sensors provide a varying voltage or current output. For example, a sensor may provide an output ranging from 2 to 10 volts, DC. The output varies with how much of the sensor’s field is disturbed. In other words, the more the sensor detects the part, the greater its output value will be.

Form Factors

Another manner in which inductive and capacitive proxes are similar is the form factors in which they’re available. By “form factor”, I mean the physical shape and characteristics of the sensor. Among the sensors I see in use, the three most common form factors are shown below. (Images from Turck website):

  • “Barrel”:   A "barrel" prox that can be threaded into a tapped hole. Both inductive and capacitive sensors come in this and other form factors.
  • “Pancake”:   A "pancake" proximity sensor.
  • “Ice Cube”:   An "ice cube" prox form factor.

Among these, the “barrel” form factor is extremely common. In most cases, it won’t be possible to tell whether a sensor is inductive or capacitive just by looking at it. To determine which type of sensor you’re looking at, you will typically need to find the sensor’s part number and look it up online.

Difference Between Capacitive And Inductive Proximity Switches

The primary difference between inductive and capacitive proximity sensors is the sensing method:

  • Inductive sensors use a coil to generate an electromagnetic field
    • With no target present, the field is at its strongest amplitude
    • When a target is present, the electromagnetic field weakens. This weakening is caused by Eddy currents induced in the target
  • Capacitive sensors use a capacitive plate to generate an electrostatic field
    • With no target present, the field is at its lowest capacitance
    • When a target is present, the target acts as a second capacitive plate. The sensor will measure a higher capacitance when a target is present

Summary Of Capacitive And Inductive Prox Sensor Characteristics

For quick reference, here’s everything above, summed up in one place:

Inductive SensorsCapacitive Sensors
Sensing FieldElectromagneticElectrostatic
Sensing MechanismCoilPlate
Materials DetectedMetalsAlmost Any
(Commonly) Adjustable?NoYes
Resilient To Contaminants?YesNo
Sense Through Materials?Metallic Through Non-MetallicMany Through Non-Metallic
Liquid Level-SensingNoYes

More Information

For additional information, here are some further resources. These resources cover inductive, capacitive, and other proximity sensors:

Thank you for reading!

Thanks for reading this! I hope it helped you to gain a bit of an understanding on the difference between inductive and capacitive proxes. If you have any questions, feel free to ask in the comments at the bottom of this page. Is there anything in the industrial automation world that you’re struggling to understand? Something you’d just like a little bit more info on?

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