Discrete Sensors 101

Discrete Sensors have existed in the automation & control landscape long before the advent of Programmable Logic Controllers, supplementing relay logic. The function of a discrete sensor is to send high/low, on/off or yes/no signals to the controller regarding the quantity of a physical parameter. The obvious benefit discrete sensors had over analog ones was the absence of deadband, detection speed, analog thresholds and other similar complexities.

Types

Robust plant operation is highly dependent on the right selection of sensors. The following are the most common types of sensors used in the field of automation:

Limit Switches

These sensors have a mechanical switch that turns on/off when it makes contact with a part. The switches are extensively used and available in various shapes and sizes. As solid-state technology makes headway, limit switches are being increasingly replaced, however, they are still present in several manufacturing plants.

Reed Switches

These are commonly used in pneumatics, featuring a mechanical switch that’s dependent on the position of a magnet. These are mounted on a cylinder where the piston has a magnet in it.

Proximity Switches

Proximity switches operate on the principle of induction, relying on a metal (usually iron) to function, even though non-ferrous materials are also used.

Photoelectric Sensors

These sensors are split into two parts: emitter and receiver, providing a cost-effective way for including tracking functionality in a system. They are commonly used in part detection or material handling.

Choosing a type of sensor

Once the mechanical characteristics of a sensor have been chosen, there are other considerations as well that need to be made:

PNP versus NPN

For all solid-state devices, choosing the polarity is requisite, determining the direction of current flow. PNP schemes are commonly found in the US, but if the equipment is coming from a different manufacturer, it is vital to know what kind of signal the PLC is expecting. “Sinking input” usually corresponds to a PNP while a “sourcing input” corresponds to NPN. Another way to look at it is: if the common terminal is 0VDC then it’s PNP, if its 24VDC then its NPN.

2-wire versus 3-wire

A mechanical contact follows 2-wire scheme while a solid-state one follows 3-wire.

Quick disconnect versus integrated cable

There are several sensors that offer plug-and-play functionalities, but have a higher cost compared to integrated cables. However, they are easy going in terms of maintenance and replacement.

There was a time when mechanical sensors provided truly discrete output, however, today most sensors use analog-to-digital conversion technologies through a microcontroller. On the other hand, advancements in PLC technologies (IO Link) have also allowed analog signals to be passed directly without conversion.

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