How do sensors work?

Proximity Sensor Types and Operation

Types of Proximity Sensors

Proximity Sensors detect objects in the proximity of the sensor without physical contact with the object. Common types of proximity sensors are inductive proximity sensors, capacative proximity sensors, and photoelectric sensors. Other sensors, like physical limit switches, are not considered proximity sensors because they require physical contact with the object.

How Does an Inductive Proximity Sensor Work?

An inductive proximity sensor can detect metal objects by sensing how the metal object changes a high-frequency, oscillating electromagnetic field emitted by the sensor. When a target metal passes through the electromagnetic field, the electromagnetic field causes Eddy currents in the target metal, which change the electromagnetic field. The inductive proximity sensor senses the change in the electromagnetic field and changes its output to signal that it has detected the target object.

What are the Common Applications of Inductive Proximity Sensors?

Because dirt and oil are non-metallic and do not affect the electromagnetic field, an inductive proximity sensor will continue to operate properly in dirty industrial locations. Traffic lights, factory automation equipment, and metal detectors are common examples of inductive proximity sensors.

Sensing Range (Operating Distance) of an Inductive Proximity Sensor

The exact sensing range at which a target will be detected by an inductive proximity sensor is influenced by the type of target metal, the surface area of the target, the size of the sensor, sensor style, voltage and temperature. The maximum distance at which the inductive proximity sensor will detect the target object is also called the Operating Distance (Sn). Some inductive proximity sensors expect the target to approach head on (axial approach), while many other inductive proximity sensors expect the target to approach from the side (radial or lateral approach). The radial sensing range of inductive proximity sensors can vary from less than a mm to 100mm. For exact details for your sensor, see the sensor's specification sheet. The Sensing Range Chart below shows some example sensing ranges for standard size targets made of 400 series steel for typical sensors.

Approximate Sensing Range (Operating Distance) Chart for Inductive Proximity Sensors
Tubular Sensor
Inductive proximity sensors
Sensing Style Size of the Sensor Approx. Max Sensing Range
Shielded Tubular 4mm 0.8mm - 1mm
Shielded Tubular 30mm 10mm - 15mm
Unshielded Tubular 6.5mm 2mm
Unshielded Tubular 30mm 22mm - 29mm
Analog Tubular 12mm 8mm
Analog Tubular 30mm 25mm
Limit Switch
inductive proximity limit switch
Sensing Style Size of the Sensor Approx. Max Sensing Range
Shielded Limit Switch 118x40x40mm 13mm
Unshielded Limit Switch 118x40x40mm 24mm
Pancake Sensor
inductive proximity pancake sensor
Sensing Style Size of the Sensor Approx. Max Sensing Range
Shielded Long Range Pancake 79x79X39mm 40mm
Unshielded Long Range Pancake 79x79X39mm 100mm

What is an Extended Range (Long Range) Inductive Proximity Sensor?

An extended range sensor can detect metal objects two to three times farther away than a standard range sensor.

How Does the Target Material Affect the Sensing Range?

Because metals have different inductive properties, the type of metal will affect the distance at which an object is detected. See your specifications sheet for your specific sensor to get the correction factor for your target metal and sensor size. Multiply the sensor range by the correction factor to get the range for detecting that target metal. Steel targets can be detected from farther away than other metals. Copper targets affect the electromagnetic field less than other metals, so copper targets must be close to the sensor to be detected. The chart below shows some typical values for correction or reduction factors for common target metals.

Correction/Reduction Factors for Typical Target Metals
Target Material Corrective Factor
Construction Steel (400 series) 0.90 - 1.00
Stainless Steel (300 Series) 0.65 - 0.85
Brass 0.35 - 0.50
Aluminum 0.35 - 0.47
Copper 0.30 - 0.40

How Does the Target Size Affect the Sensing Range of an Inductive Proximity Sensor?

Sensing Range or Rating Operating Distance specifications are given for a Standard Size Target, which is typically 1mm thick with a surface area equal to the larger of the size of the sensor face or 3 times the minimum sensing range. Targets smaller than the standard target size will have a smaller sensing range.

Mounting Restrictions for Inductive Proximity Sensors

Inductive Proximity Sensors are offered in different sensor styles: shielded, semi-shielded, and unshielded (or non-embeddable). Shielded proximity sensors can be mounted with the mounting metal touching the sensor. Semi-Shielded Inductive Proximity Sensors must be mounted with a metal free zone around the sensor. Unshielded sensors or non-embeddable sensors must mounted with an even larger area around the sensor that has no metal.

Operation of a Limit Switch

What is a Limit Switch?

A limit switch is a sensor that is designed to change the state of a switch when an object moves past its limit. Limit switches are often used to count the number of objects in industrial or commercial manufacturing or as safety interlock switches for doors.

How Does a Limit Switch Work?

There are two general categories of limit switches: a physical limit switch and an inductive proximity limit switch.

Physical Limit Switch

Physical limit switch with roller
A Physical Limit Switch may sense an object using physical contact when the object pushes the sensor actuator (whisker, lever, roller plunger) passed a "limit" which changes the state of the switch.

Inductive Proximity Limit Switch

inductive proximity limit switch
An Inductive Proximity Limit Switch senses an object without contact using electromagnetic fields as described above for inductive proximity sensors.

How does a Capacitive Proximity Sensor Work?

capacitive proximity sensor
A capacitive proximity sensor can sense both metals and non-metals, including paper, plastic and liquids like oil, water or paint. The capacitive proximity sensor works without physically contacting the object by creating a field around the sensor using the dielectric principles of capacitance. When the object reaches an operate point, the capacitive proximity sensor detects the object by sensing changes in capacitance between the object and the sensor. Ambient temperature changes and other nearby objects can interfere with detection using capacitive proximity sensors. Capacitive proximity sensors can be used to detect product position in manufacturing plants, to measure brake disk deformation, and even to detect changes in fluid levels for monitoring toner levels in printers.

How does a Photoelectric Sensor Work?

A photoelectric sensor uses light to detect the presence or absence of an object without actually touching the object. Three common types of photoelectric sensors are diffuse reflective sensors, retro reflective sensors, and thru beam sensors.

Diffuse Reflective Photoelectric Sensor

A diffuse reflective photoelectric sensor detects the presence of an object by emitting a light beam and measuring the light reflecting off the object's surface. A diffuse reflective sensor has the shortest sensing range of all photoelectric sensors.

Diffuse-Reflective Photoelectric Sensor

Retro Reflective Photoelectric Sensor

A retro reflective photoelectric sensor, or reflex sensor, utilizes a special reflector that returns the light beam back to its sensor. An object is detected when it interrupts the reflected light from reaching the retro reflective sensor. A retro reflective sensor has a medium sensing range.

Retro-Reflective Photoelectric Sensor

Thru Beam Photoelectric Sensor

A thru beam photoelectric sensor has a separate emitter and receiver that detect an object when it interrupts the continuous light beam from the emitter to the receiver. A thru beam sensor provides a maximum detection range and is ideal for sensing opaque objects. Maintaining the calibration of a thru beam sensor is critical to ensure that the light beam transmitted by the thru beam light emitter reaches the thru beam receiver when not interrupted by an object.

Thru-Beam Photoelectric Sensor

Wiring for Sensors

2 Wire Sensor

A 2 wire sensor is intended to be connected in series with the controlled load. Because these sensors derive the power to energize their internal electronics through the load they control, a minimum current is drawn through the load when the sensor is in the open state. This current is so small that it can be ignored and will not turn on electromechanical devices such as relays and solenoids. However, this current could be enough to operate an electronic load. Cutler-Hammer's 2 wire sensors have the lowest leakage current in the industry and are suitable for many electronic loads.

2 Wire AC NO (Normally Open) Sensor
AC 2 Wire Normally Open (NO) Sensor

3 Wire Sensor

A 3 wire sensor connects in parallel with the load they control, drawing power from a separate source for their internal electronics. This prevents substantial current flow through the load when the device is open.

AC NO(Normally Open)/NC(Normally Closed) or DC PNP(Switched Positive)
AC 3-Wire NO/NC or DC PNP Sensor

Operation of Logic Modules

A proximity sensor detects the presence or absence of an object in its vicinity and communicates this information to logic modules, which use it to make decisions or trigger specific actions in automated systems.

On Delay

Adjustable delay between the time an object is sensed and when the switch function occurs.

Off Delay

Adjustable delay between the time an object leaves the sensing field and when the switch transfers back to its non-sensing state.

On and Off

A combination of on delay and off delay adjustments.

Delayed Single Shot

Adjusts the length of time a switch remains in the "ON" cycle after an object is sensed, regardless of how long the object stays within the sensing field. The "ON" cycle can also be delayed after the object is sensed.

Inductive Proximity

Inductive proximity sensors

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Limit Switch Inductive Proximity Sensor

Limit Switch Inductive Proximity Sensor

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Capacitive Proximity

Capacitive Proximity Sensor

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Quick disconnect connect proximity photoelectric sensor

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Logic Controllers

programmable logic controller

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Warning: When using this information to perform electrical work, call a licensed electrician and consult the NEC® for safety. All licensed electricians have passed examinations covering the National Electric Code®, know state and local building codes, and may carry insurance to cover damages.