Sensors detect the presence or absence of a target and send an electrical signal based on that change.  In addition to devices like limit switches (which have to physically make contact with an object to detect it), there are 5 major categories of sensors that are regularly used in industrial automation.

Proximity Sensors

Proximity sensors detect their targets by creating an electrical field in front of the sensor and measuring any changes to the field.  There are three styles of proximity sensor - inductive, capacitive, and magnetic - of which inductive is by far the most common.

Photoeye Sensors

Photoeyes detect their targets by sensing how much light is either reflected off or blocked by their targets.  There are 4 main configurations of photoeye - diffuse reflective, retro reflective, thru beam, and limited reflective.  Depending on which configuration they have very different detection properties, making them an extremely flexible type of sensor.

Distance Sensors

Distance sensors also utilize light for detection, but unlike other photoeyes, they use the light to determine how far a target is.  This allows them to detect a wide variety of targets such as curved or shiny pieces.  There are two main types of distance sensor, triangulation-based and time-of-flight.

Fiber Sensors

Fiber optic sensors detect targets exactly as photoeyes do, but use a fiber optic cable to transmit the light far away from the main sensor.  The fiber optic cable does not have any electronics inside of it, so these sensors can have specialized features such as small size, high temperature, high-speed detection, and more.

Ultrasonic Sensors

A sensor is said to be operating in a Normally Open mode if the sensor turns its output on when it detects its target and turns its output off when there is no target present.

Wiring Configurations

There are several different ways a sensor can receive its power and send its output.  The two most common configurations are 3-wire and 2-wire configurations.

3 Wire

A 3-wire sensor will have at least (but sometimes more than) 3 wires.  2 wires will act as the sensors power supply (+/- DC power) and a 3rd wire will carry the output from the sensors.  The output is most commonly a transistor output that will give a + PNP signal or a - NPN signal, but may also be an analog output.

2 Wire

A 2-wire sensor will have 2 wires that will simultaneously act as the sensor's output and power supply.  The wires will pass a low-voltage current through the sensor under normal conditions, and will pass a higher voltage current when the sensor detects its target.

NPN/PNP Transistor Outputs 

NPN and PNP are two types of transistor outputs.  Transistor outputs utilize a semiconductor transistor to send the sensors output and have a number of advantages over other output types like relay outputs, as they have no moving parts.  This means that they are smaller, faster, and last longer than relay outputs.

NPN

NPN is the output type that provides (-) DC power.  To connect this output type to a relay you would attach the output to the negative side of the coil and connect (+) power to the positive side of the coil.  If connecting to a controller, the manufacturer will typically specify whether the sensor needs to have NPN or PNP outputs.

PNP

PNP is the output type that provides (+) DC power.  To connect this output type to a relay you would attach the output to the positive side of the coil and connect (-) power to the negative side of the coil.  If connecting to a controller, the manufacturer will typically specify whether the sensor needs to have NPN or PNP outputs.

Normally Open/Normally Closed 

The logic of a sensor determines whether the output turns on or off when the sensor detects its target.  The terminology of normally open and normally closed originate with relay outputs (when the contact would literally either be normally open or normally closed) but applies to 2 wire and 3 wire transistor outputs as well.

Normally Open

The logic of a sensor determines whether the output turns on or off when the sensor detects its target.  The terminology of normally open and normally closed originate with relay outputs (when the contact would literally either be normally open or normally closed) but applies to 2 wire and 3 wire transistor outputs as well.

Normally Closed

A sensor is said to be operating in a Normally Closed mode if the sensor turns its output on when there is no target present and turns its output off when it detects its target.

Output Types and Logic

The logic of a sensor determines whether the output turns on or off when the sensor detects its target.  The terminology of normally open and normally closed originate with relay outputs (when the contact would literally either be normally open or normally closed) but applies to 2 wire and 3 wire transistor outputs as well.

Mutual Interference

Mutual interference is a problem that occurs when another sensor picks up the signal from one sensor unintentionally.  An example of this is with photoeyes, where light reflected from a target might hit another sensor and cause it to fall trip.  While this is not a common problem, it is frequently only discovered late in the machine design/assembly process.  Omron has a number of sensors that have internal features that stop sensors from interfering with each other.

Timing Functions

While timing adjustment is usually done in the controller, a number of our sensors also have on board timing functions so that the controller does not need to be reprogrammed to accommodate changes to sensors or the machine.

On-delay

An On-delay will prevent the sensor output from turning on until the sensor has detected the object for a given amount of time.  This is frequently used to suppress false trips from problems like vibration which tend to last a very short amount of time.

Off-delay

An Off-delay will hold the sensor output on for a given amount of time after the sensor stops detecting a target.  This is frequently used to ignore features in the target.  A hole in a target might normally turn an output off temporarily but an Off-delay would hold the output on until the rest of the target came into view of the sensor.

One-shot

A One-shot will send the sensor output for a specified period of time, and then turn it off regardless of whether it sees the target or not.