• Skip to primary navigation
  • Skip to main content
  • Skip to primary sidebar
  • Skip to footer

Motion Control Tips

Automation • Motion Control • Power Transmission

  • News
    • Industry News
    • Editor Blogs
    • Video
  • Controls
    • HMIs
    • PC-Based Controllers
    • PLCs + PACs
    • Stand-Alone Controllers
    • Software
  • Drives
    • Servo Drives
    • Stepper Drives
  • Encoders
    • Absolute Encoders
    • Incremental Encoders
    • Rotary Encoders
  • Mechanical
    • Bearings
    • Brakes + Clutches
    • Belt + chain
    • Couplings
    • Gears + Gearing
    • Lubrication
    • Shock + Vibration Mitigation
    • Springs + Rings + Seals
  • Linear
    • Actuators
    • Linear Motors
    • Linear Encoders
  • Motors
    • AC Motors
    • DC Motors
    • Brushless Motors
    • Gearmotors
    • Piezo Motors
    • Servo Motors
    • Stepper Motors
  • Systems
    • Conveyors + linear transport systems
    • Gantries + Stages
    • Rotary Tables
    • Grippers + End Effectors
    • Robotics
  • Networks
    • Connections + Sliprings
    • Fieldbuses
    • I/O
    • Sensors + Vision
  • FAQs
    • Motion Casebook
    • Motion Selection Guides
  • Suppliers
You are here: Home / FAQs + basics / What is external device monitoring and how does it relate to machine safety?

What is external device monitoring and how does it relate to machine safety?

April 1, 2022 By Danielle Collins Leave a Comment

According to the Machinery Directive 2006/42/EC, technical protective measures should be used to reduce risks that cannot be avoided through safe design measures. These technical protective measures include safety devices such as door locks, covers, light curtains, and other equipment that monitors the machine movements. When the safety device controls and external device, such as a contactor or relay, to switch off power and stop a machine or prevent a hazardous function, the safety system must also implement measures for external device monitoring.


External device monitoring is typically required to achieve the highest machine safety ratings — PLe Category 3 or 4 per ISO 13849-1, or SIL 3 per IEC 62061.


External device monitoring (EDM) is simply a means by which a safety or protective device, such as a light curtain or safety relay, monitors the state of external devices, such as contactors or machine primary control elements (MPCEs), that are controlled by the safety device. If the external device is found to have an unsafe state, such as fused contacts or problems with wiring, the safety device will lockout.

Simply put, the safety device checks to ensure the external device (a contactor, for example) is de-energized when the safety device is triggered. It does this by using two normally closed (NC) contacts to monitor the status of the normally open (NO) external device contacts (such as machine power contacts). The NC monitoring contacts are mechanically linked to and positively guided by the NO contacts. In this positively-guided configuration, the NC and NO contacts operate together and cannot be closed simultaneously.

positively guided contacts
Mechanically-linked, positively guided contacts cannot be closed at the same time.
Image credit: Honeywell

When the safety device output is off, the NC monitoring contacts are closed, the contactor’s NO contacts are open, and 24 V is applied to the EDM input. However, if one of the NO contacting elements malfunctions and is closed — for example, by being fused — the monitoring NC contact cannot close, and the required voltage cannot be delivered to the EDM input.

Alternatively, when the safety device output is on, the NC monitoring contacts are open and the contactor’s NO contacts are closed. But if one of the NC monitoring contacts malfunctions (fused closed), the NO contacts cannot close. The safety device recognizes either of these abnormal conditions and prevents the machine or operation from re-starting.

External device monitoring can be implemented as either one-channel monitoring or two-channel monitoring. One-channel monitoring uses contacts that are wired in series, with feedback provided to the start condition of the safety device. Two-channel monitoring requires that the contacts be wired independently, and both NC contacts must be closed before the safety device can be reset. While one-channel monitoring can detect a malfunction in the operation of the contacts, two-channel monitoring can also detect a malfunction in the safety circuit wiring.

one-channel external device monitoring
With one-channel monitoring, the contactors (K3 and K4) are connected in series.
Images credit: Schneider Electric
two-channel external device monitoring
With two-channel monitoring, the contactors (K3 and K4) are connected independently, allowing wiring faults to be detected.

You may also like:

  • machine functional safety
    What machine safety functions address speed, and when are they…
  • servo drive
    Safety, increased connectivity top drive trends

  • The difference between contactors and motor starters (and reduced-voltage starters)
  • integrated safety
    What does integrated safety in motion control mean?
  • safe motion monitoring
    What is safe motion monitoring?

Filed Under: AC Motors, Drives + Supplies, FAQs + basics, Featured, Industrial Automation, Servo Drives, Servo Motors

Reader Interactions

Leave a Reply

You must be logged in to post a comment.

Primary Sidebar

POWER TRANSMISSION REFERENCE GUIDE

DESIGN GUIDE LIBRARY

“motion
Subscribe Today

RSS Featured White Papers

  • Specifying electric rodless actuators: Ten tips for maximizing actuator life and system performance
  • The truth about actuator life: Screw drive survival
  • Top Ten Tips: How to specify electric rod-style actuators for optimal performance, reliability and efficiency

Footer

Motion Control Tips

DESIGN WORLD NETWORK

Design World Online
The Robot Report
Coupling Tips
Linear Motion Tips
Bearing Tips
Fastener Engineering.

MOTION CONTROL TIPS

Subscribe to our newsletter
Advertise with us
Contact us
About us
Follow us on TwitterAdd us on FacebookAdd us on LinkedInAdd us on YouTubeAdd us on Instagram

Copyright © 2022 · WTWH Media LLC and its licensors. All rights reserved.
The material on this site may not be reproduced, distributed, transmitted, cached or otherwise used, except with the prior written permission of WTWH Media.

Privacy Policy | RSS