Classification Sensors in Automation
In automation, there exists a variety of different sensors. This is only an attempt to have an overview of the different sensor types:
Active and passive sensors
Active sensors generate a voltage themselves and do not require any extra supply voltage to function. Examples: Tachogenerator, photocell, thermocouple
Passive sensors require a supply voltage. Examples: Measuring bridges, etc.
NPN and PNP sensors
NPN sensors have a NPN transistor in their switching stage. Here the load is connected with the switching output and the usually 24V supply voltage.
PNP sensors have a PNP transistor in their switching stage. Here the load is connected to the switching output and the common ground (-). Therefore they are also called positive switching.
Analog sensors produce an analog output signal proportional to the measurand. Examples are: Temperature sensors, pressure sensors, tachometers, etc. They can be classified by their output signals:
Analog sensor with voltage output:
Analog sensors with a voltage output usually provide a signal voltage of 0 to 10 V. They are usually less expensive than analog sensors with a current output, but sending a voltage signal over long distances results in voltage losses due to wire and contact resistances. Also voltage signals are sensitive to electromagnetic noise pickup. To minimize this drawback, shielded wires can be used, but with long distances they get costly.
Analog sensor with current output:
The 4...20 mA output is a very robust and reliable signal as the current flows through all components and the current can be maintained due to an internal sensor control. Faults caused by an open circuit or a lost feed can be detected by reading „0 mA“, while reading a signal between 4 and 20 mA would indicate a properly working circuit.