Back to Basics: Limit switches for conveyors

Process monitoring and control improves with proper application of limit switches in conveyor applications. See diagram.

10/24/2011


An industrial conveyor belt usually consists of two or more pulleys, with a continuous loop of material (belt) that rotates around them. One or both pulleys are powered, moving the belt and the material on the belt forward. There are two main industrial classes of belt conveyors: general material handling (moving boxes along inside a factory) and bulk material handling (transporting industrial/agricultural materials).

Limit switches are used to provide conveyor system monitoring and control as well as safety in case of a problem. A limit switch is a precision snap-action switch that has been encased to protect it from external forces, such as hazardous chemicals, water, oil and dirt, and is used to detect presence or absence in areas where physical contact is allowed.

Discrete sensors, properly applied, can improve usefulness and lower risk of a conveyor application. Courtesy: Honeywell Sensing & Control

In the conveyor belt application shown, a variety of limit switches can be used.

1. Often used on conveyors designed for use in harsh indoor and outdoor environments, heavy-duty limit switches can reliably indicate position for system controls. In this configuration, the limit switch could be used to count the items flowing through it. It could also be used to ensure that materials are correctly positioned and will sound an alarm or stop the belt if there is a problem.

2. Safety switches: Cable-pull limit switches are emergency stop switches that control access around the conveyor perimeter or provide emergency-stop cable-pulls along the conveyor. These kinds of limit switches provide a highly reliable, highly visible, safe-to-use rope-pull device to protect operators working near conveying systems.

3. Hazardous location switches are very similar to switch 1 but are housed in sealed enclosures to prevent a spark or electrical discharge from setting off an explosion. They are often used for door or diverter position detection in outdoor, above-ground, potentially explosive environments, such as grain handling and oil and gas applications.

4. Safety and hazardous location switches have also been designed with an explosion-proof housing. Often used in outdoor, above-ground, potentially explosive environments, such as grain handling conveyors, or oil and gas applications, hazardous location switches can quickly and reliably stop system operation when the switch is triggered by the operator.

Limit switches can also be used in other ways on conveyor belts. For example, a limit switch alongside the belt could ensure that packages and materials are correctly positioned on the conveyor belt. The limit switch signal connects to a controller that will stop the belt if there is a problem.

To prevent jamming the belt’s discharge, limit switches could measure the number or height of the products on the conveyor belt. If too much product is going down the belt, the system will shut down before damage or spillage occurs.

- Richard Staiert is Honeywell applications engineer. Edited by Mark Hoske, CFE Media, Control Engineering, www.controleng.com.

http://sensing.honeywell.com 

Honeywell Sensing & Control – Micro Switch limit switches information

http://sensing.honeywell.com/index.cfm?Ne=3025&ci_id=154339&N=3555&la_id=1 

Sensors Channel

http://www.controleng.com/new-products/sensors.html 

Control Engineering tutorials

http://www.controleng.com/channels/tutorials.html 



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.
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
Robotic integration and cloud connections; SCADA and cybersecurity; Motor efficiency standards; Open- and closed-loop control; Augmented reality
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 motion control advances and solutions can help with machine control, automated control on assembly lines, integration of robotics and automation, and machine safety.
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