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.

11/30/2017


Frank Lamb, Automation Consulting LLC, is a Control Engineering Editorial Advisory Board member. Courtesy: Control EngineeringWhen designing machinery, one of the first tasks for an engineer is to select discrete sensors for the application. Generally, discrete sensors 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

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 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 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. 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 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.

Sensor specification depends on a lot of factors, such as: 

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

Sensors for harsh conditions

In some cases, resistance to chemicals and water are also a consideration. 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 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

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. 

Consider this

What other considerations should be made when selecting a discrete sensor?

ONLINE extra

See related sensor articles from Frank Lamb linked below.



The Engineers' Choice Awards highlight some of the best new control, instrumentation and automation products as chosen by Control Engineering subscribers. Vote now (if qualified)!
The System Integrator Giants program lists the top 100 system integrators among companies listed in CFE Media's Global System Integrator Database.
Each year, a panel of Control Engineering and Plant Engineering editors and industry expert judges select the System Integrator of the Year Award winners in three categories.
This eGuide illustrates solutions, applications and benefits of machine vision systems.
Learn how to increase device reliability in harsh environments and decrease unplanned system downtime.
This eGuide contains a series of articles and videos that considers theoretical and practical; immediate needs and a look into the future.
Maximize ROI with integrated control system approach; Microcontrollers vs. PLCs; Power quality; Accelerate and rewire IIoT; Traits for excellent engineers
HMI effectiveness; Distributed I/O; Engineers' Choice Award finalists; System Integrator advice; Inside Machines
Women in engineering; Engineering Leaders Under 40; PID benefits and drawbacks; Ladder logic; Cloud computing
Programmable logic controllers (PLCs) represent the logic (decision) part of the control loop of sense, decide, and actuate. As we know, PLCs aren’t the only option for making decisions in a control loop, but they are likely why you’re here.
This digital report explains how plant engineers and subject matter experts (SME) need support for time series data and its many challenges.
This article collection contains several articles on how advancements in vision system designs, computing power, algorithms, optics, and communications are making machine vision more cost effective than ever before.

Find and connect with the most suitable service provider for your unique application. Start searching the Global System Integrator Database Now!

Control room technology innovation; Practical approaches to corrosion protection; Pipeline regulator revises quality programs
Cloud, mobility, and remote operations; SCADA and contextual mobility; Custom UPS empowering a secure pipeline
Infrastructure for natural gas expansion; Artificial lift methods; Disruptive technology and fugitive gas emissions
Automation Engineer; Wood Group
System Integrator; Cross Integrated Systems Group
Jose S. Vasquez, Jr.
Fire & Life Safety Engineer; Technip USA Inc.
This course focuses on climate analysis, appropriateness of cooling system selection, and combining cooling systems.
This course will help identify and reveal electrical hazards and identify the solutions to implementing and maintaining a safe work environment.
This course explains how maintaining power and communication systems through emergency power-generation systems is critical.
The Engineers' Choice Awards highlight some of the best new control, instrumentation and automation products as chosen by Control Engineering subscribers. Vote now (if qualified)!
The System Integrator Giants program lists the top 100 system integrators among companies listed in CFE Media's Global System Integrator Database.
Each year, a panel of Control Engineering and Plant Engineering editors and industry expert judges select the System Integrator of the Year Award winners in three categories.
This eGuide illustrates solutions, applications and benefits of machine vision systems.
Learn how to increase device reliability in harsh environments and decrease unplanned system downtime.
This eGuide contains a series of articles and videos that considers theoretical and practical; immediate needs and a look into the future.
Maximize ROI with integrated control system approach; Microcontrollers vs. PLCs; Power quality; Accelerate and rewire IIoT; Traits for excellent engineers
HMI effectiveness; Distributed I/O; Engineers' Choice Award finalists; System Integrator advice; Inside Machines
Women in engineering; Engineering Leaders Under 40; PID benefits and drawbacks; Ladder logic; Cloud computing
Programmable logic controllers (PLCs) represent the logic (decision) part of the control loop of sense, decide, and actuate. As we know, PLCs aren’t the only option for making decisions in a control loop, but they are likely why you’re here.
This digital report explains how plant engineers and subject matter experts (SME) need support for time series data and its many challenges.
This article collection contains several articles on how advancements in vision system designs, computing power, algorithms, optics, and communications are making machine vision more cost effective than ever before.

Find and connect with the most suitable service provider for your unique application. Start searching the Global System Integrator Database Now!

Control room technology innovation; Practical approaches to corrosion protection; Pipeline regulator revises quality programs
Cloud, mobility, and remote operations; SCADA and contextual mobility; Custom UPS empowering a secure pipeline
Infrastructure for natural gas expansion; Artificial lift methods; Disruptive technology and fugitive gas emissions
Automation Engineer; Wood Group
System Integrator; Cross Integrated Systems Group
Jose S. Vasquez, Jr.
Fire & Life Safety Engineer; Technip USA Inc.
This course focuses on climate analysis, appropriateness of cooling system selection, and combining cooling systems.
This course will help identify and reveal electrical hazards and identify the solutions to implementing and maintaining a safe work environment.
This course explains how maintaining power and communication systems through emergency power-generation systems is critical.
The Engineers' Choice Awards highlight some of the best new control, instrumentation and automation products as chosen by Control Engineering subscribers. Vote now (if qualified)!
The System Integrator Giants program lists the top 100 system integrators among companies listed in CFE Media's Global System Integrator Database.
Each year, a panel of Control Engineering and Plant Engineering editors and industry expert judges select the System Integrator of the Year Award winners in three categories.
This eGuide illustrates solutions, applications and benefits of machine vision systems.
Learn how to increase device reliability in harsh environments and decrease unplanned system downtime.
This eGuide contains a series of articles and videos that considers theoretical and practical; immediate needs and a look into the future.
Maximize ROI with integrated control system approach; Microcontrollers vs. PLCs; Power quality; Accelerate and rewire IIoT; Traits for excellent engineers
HMI effectiveness; Distributed I/O; Engineers' Choice Award finalists; System Integrator advice; Inside Machines
Women in engineering; Engineering Leaders Under 40; PID benefits and drawbacks; Ladder logic; Cloud computing
Programmable logic controllers (PLCs) represent the logic (decision) part of the control loop of sense, decide, and actuate. As we know, PLCs aren’t the only option for making decisions in a control loop, but they are likely why you’re here.
This digital report explains how plant engineers and subject matter experts (SME) need support for time series data and its many challenges.
This article collection contains several articles on how advancements in vision system designs, computing power, algorithms, optics, and communications are making machine vision more cost effective than ever before.

Find and connect with the most suitable service provider for your unique application. Start searching the Global System Integrator Database Now!

Control room technology innovation; Practical approaches to corrosion protection; Pipeline regulator revises quality programs
Cloud, mobility, and remote operations; SCADA and contextual mobility; Custom UPS empowering a secure pipeline
Infrastructure for natural gas expansion; Artificial lift methods; Disruptive technology and fugitive gas emissions
Automation Engineer; Wood Group
System Integrator; Cross Integrated Systems Group
Jose S. Vasquez, Jr.
Fire & Life Safety Engineer; Technip USA Inc.
This course focuses on climate analysis, appropriateness of cooling system selection, and combining cooling systems.
This course will help identify and reveal electrical hazards and identify the solutions to implementing and maintaining a safe work environment.
This course explains how maintaining power and communication systems through emergency power-generation systems is critical.
click me