Sensors, Actuators

Picking the right discrete sensor for a machine application

There are many considerations for picking a discrete sensor for a machine application. These include if the sensor is being used for position detection or part detection or if the application is hazardous. See four factors for industrial sensor specification.

By Frank Lamb, Automation Consulting LLC November 30, 2017

When designing machinery, one of the first tasks for an engineer is to select the types of sensors, the right discrete sensors for the application. Generally, discrete sensors for factory automation can be divided into two categories: those that detect the positions of mechanisms and tooling (parts of the machine) and those that detect the parts being processed.  

Types of discrete sensors: Hall-effect sensors, proximity sensor, photoelectric, limit switches 

When choosing types of sensors, it’s easy for designers to select Hall-effect sensors for pneumatic cylinders since these often are presented as accessories. Hall-effect sensors are designed to detect the magnetic piston inside of a cylinder body and determine whether it is extended or retracted. This works fine as long as the cylinder shaft can’t become unattached from the tooling; detection of tooling using an inductive proximity switch is the only foolproof way to determine that the tooling is in the correct position. Photoelectric sensors are useful for applications when the tooling is not made of metal.

Photoelectrics and inductive proximity switches often are used for part detection. Photoeyes are useful for designers when selecting types of sensors since they can be located farther from the target. This opens up several sensing possibilities such as through-beam (separate emitter and receiver); retroreflective (using a corner-cube bicycle type reflector or reflective tape); or diffuse (using light reflected off of the target). Form factor, such as fiber optics that can reach into tight spaces or larger self-contained factory automation sensors that may have a longer range, also should be considered.

Visible beam photoelectrics are often easier to align and infrared sensors also have a longer range when choosing among types of sensors. Laser sensors, while more expensive, can be used to detect smaller parts and also extend the sensing range. The other important consideration that needs to be made is the voltage and output type of the sensor. With the exception of some large conveyor systems and process control facilities, discrete sensors are almost always 24 V dc. Positive-negative-positive (PNP) sourcing sensors are used most commonly in the U.S., but factory automation equipment from other countries may use negative-positive-negative (NPN) sensors, or sinking types.

Sensors with contact closure type outputs such as limit switches also are used occasionally, but like ac sensors, they are becoming less common. Contact closures are best for applications where the sensor does not switch often. Physical contacts wear and often have a specified lifetime number of operations.

Four factors for industrial sensor specification 

For factory automation, sensor specification depends on a lot of factors, to choose types of sensors, such as:

  1. How much space is available for mounting
  2. What type of target is being detected
  3. With what type of control system it needs to be interfaced
  4. Sensor cost and availability of replacements.

Discrete sensors for harsh conditions

In some cases, resistance to chemicals and water are also a consideration for industrial sensors for factory applications. Coolant in metal cutting applications and caustic washdown liquids in food processing are some common substances that may need to be evaluated. This also may contribute to the type of electrical connection the sensor may require; quick disconnect or hard conduit.

There are many resources online to check when selecting discrete sensors for a machine application. Keeping these considerations in mind when making a decision will make the process of selecting types of sensors for factory automation much simpler.

Frank Lamb, Automation Consulting LLC, is a Control Engineering Editorial Advisory Board member. Automation Consulting LLC is a CFE Media content partner. Edited by Chris Vavra, production editor, Control Engineering, CFE Media, cvavra@cfemedia.com.

More answers

www.controleng.com keywords: discrete sensors, types of sensors, factory automation, sensor selection

Key Concepts

  • Discrete sensors can be divided into two categories: those that detect the positions of mechanisms and tooling and those that detect the parts being processed.
  • Photoelectric and hall-effect sensors commonly are used for discrete applications.
  • Depending on the application, resistance to chemicals and water also should be considered when picking a discrete sensor among types of sensors.

Consider this

When looking at types of sensors, what other considerations should be made when selecting a discrete sensor?

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