What are the different types of outputs for sensors?

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What are the different types of outputs for sensors?

All sensors have one or more (types of) outputs. An output generally sends the currently known measured value of the sensor. What that signal looks like depends on the output type. What is the difference between all these outputs and when is which output type used? This article explains the most common forms.

Sensors can have many different types of outputs. The possible outputs of a sensors detemine in which applications the sensor can be used. Vice versa, the same can be said: the type of application detemines which sensor you need.

In this article we discuss the different possible outputs. how these work and for which use cases these can be applied. 

Digital vs. Analog

First, we make a distinction between two types of outputs: an analog and a digital output. A sensor with a digital output signals a logical value. In other words: Yes or No, 0 or 1, True or False, Valid or Invalid . A digital output is very well-suited to indicate the presence of an object (at a certain distance) or detecting whether a set limit value has been reached. Does the sensor "see" the object or not? Is the value reached or not? During a detection or non-detection the logical value changes from a 0 to a 1, or vice versa! Examples of digital (switching) outputs are PNP/NPN, relay, solid state relay and PushPull.

A sensor with an analog output is capable of giving a signal that is continuously partallel to the measured value. An analog signal is a signal that can register values without intervals. Think of a constantly fluctuating temperature in an outdoor location, such as the conveyor belts in the production of steel beams: the analog output changes parallel en mostly linear with the change of the measurement of the sensor. Another example is the change of a distance from 0 to 1.000 cm or a temperature drops from 200°C to 20°C. Examples of analog outputs are 0-10 Vdc, 4-20 mA, 0-5 Vdc or 0-20 mA.

Types of digital outputs: PNP or NPN

Sensors with a PNP or NPN switching contact make use of a transistor output. The type of transistor output determines whether the sensor switches PNP or NPN. Sensors with a PNP or NPN switching output are equipped with at least three wires; A " + " (Pin 1 / brown wire), a " – " (Pin 3 / blue wire) and a switching wire (Pin 4 / black wire).

PNP switching output

The load is switching between the switching wire (4) and the – (3) within a sensor with a PNP switching output.

NPN switching output

The load is switching between the switching wire (4) and the + (1) within a sensor with a NPN switching output.

PushPull switching output

A PushPull output means that the switching component of a sensor consists of two transistors. This is a type of output in which it is possible to alternately switch PNP as well as NPN. The circuit is designed in such a way that any voltage between a certain limit will make the sensor switch NPN, while a lower voltage provides a PNP output. Sensors with a PushPull output are versatile to use in applications that require a PNP and NPN output. The advantage is also that there is no need for developing the same sensor but with an NPN or PNP output.

Solid State Relay (SSR) output

A solid state relay (SSR) is also known as an optocoupler relay or semiconductor relay. It is a type of relay without a mechanical switch, contrary to a more conventional relay. Conventional mechanical relays have the advantage of being able to switch higher power rates, but because of moving parts are susceptible to wear and tear. A solid state relay switches by use of a light-sensitive diode and is, because of this, free from wear and tear. In addition it is also capable of higher switching frequencies.

Normally Open, Normally Closed or Antivalent output

All sensors with a digital switching contact, whether it is a PNP/NPN, solid state or PushPull contact have the ability to switch Normally Open (NO) or Normally Closed (NC). In some cases the sensors only switch NO or NC. If a sensor is capable of switching NO as well as NC it is called antivalent.

Normally Open (NO) switching output

A Normally Open (NO) contact switches the signal "high" at the moment that the sensor detects an object. When the sensor does not detect anything, the signal will be "low", so the output is open in neutral. At that moment there is no current over the switching wire because the circuit is interrupted.

Normally Closed (NC) switching output

A Normally Closed (NC) contact switches exactly the other way around. The signal is "high" when in neutral. When the sensor detects something the signal will drop and the switch will open. The big advantage of an NC switching contact is that when a sensor or switch malfunctions, the signal remains open which makes the application "fail safe".

Antivalent switching output

Sensors with an antivalent output have 2 outputs which will always switch opposite of each other. One output switches NO and the other switches NC. Both signals have to be provided separately to the PLC or processing unit. An antivalent output provides extra reliability and thus safety, because the switches can never be in the same state simultaneously. In that case, there is a false detection or a malfunction. So, a sensor with an antivalent output is completely different from a sensor with 2 PNP or NPN switching outputs.
 

4-20 mA current output

A 4-20 mA output on a sensor is an electrical connection (current loop), fed by a constant voltage (the suppyl voltage of the sensor, like 24 Vdc). This is connected to a converter that converts a measurable unit into a direct current between 4 and 20 mA. This is a standard for industrial sensors and communication in which 4 mA represents 0% of the physical unit and 20 mA is 100%. Everything in between is divided linearly. In a sensor for a distance measurement an example would be 4 mA = 0 mm and 20 mA = 1000 mm.

The signal starts at 4 mA instead of 0 mA because, contrary to a 0-10 Vdc output, to be able to make a distinction between a 0 signal and a malfunctioning. Another advantage of using 4-20 mA is that the signal is unaffected by voltage losses in the wiring and is insusceptible to electrical noise.

Advantages:
Fail safe by starting at 4 mA
Insusceptible to voltage losses
Unaffected by electrical noise

Disadvantages:
Sometimes pricier

0-10 V voltage output

A 0-10 V voltage output is the most common 'straight forward' analog output found on the most analog sensors, controllers and PLCs worldwide. Just like with a 4-20 mA output, the starting point and end point of the measuring value is distributed linearly between 0 and 10 V.

Types of sensor outputs that work with voltage are more sensitive for electrical noise and are not well-fit for long distance transfers. Because the signal starts at 0V it is harder to detect a malfunction then with a current output. A voltage output is in some cases cheaper to produce, which explains the higher price for an analog sensor with a 4-20 mA output instead of a 0-10V one.

Advantages:
Available on most sensors, controllers and PLCs
Cheap to produce
‘Straight forward’ signal processing

Disadvantages:
Sensitive to electrical noise
Affected by voltage losses
Not fail safe, starts at 0 V

 

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