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Considerations and comments to the safety engineering of machines

Safety engineering of machines - Multivac reports

Bernhard Langer, Head of Electronic Development Department in the sector Control Technology at Multivac analyses how safety engineering of machines can be realised cost-effectively. For that purpose solutions with sensors are not obvious. In fact, they need to be developed in comparison with magnetic swichtes, electronic safety switches and bus-capable sensors. So Bernahrd Langer outlines some problems and aspects for their solution.

The advantages of magnetic switches for safety purposes

March 23rd, 2011 - With safety sensors based on magnetic switches each with one NO contact and one NC contact, safety engineering of machines can be produced cost-effectively. This is true in particular if these switches can be interconnected in groups in order to minimise the number of necessary safety inputs.


However, on a higher performance level an evaluation of each of the two contacts of the magnetic switch has to take place. This causes a high wiring effort and for each magnetic switch 2 safety inputs are necessary.

Disadvantages enable the status "may be not ok"

Bernhard Langer is Head of the Department Electronic Development in the sector Control Technology of Multivac Sepp Haggenmüller GmbH & Co.KG, Wolfertschwenden, Germany

If it comes to a stop with motoring moved protection devices, it may be that the signal status of the two contacts of the magnetic switch delivers a non distinctive position.


This is caused by the design of the magnetic switch. The reed contacts cover position ranges, in which the two contacts can temporarily be in a status that do not fit to each other. The safety control tolerates this behaviour and treats it similarly as a contact bouncing. However, if this status is durably signalled by a stop of the machine, the safety control shuts down on fault. Now it must be looked for solutions that permit a travel operation of the drive although the drive was disconnected by the safety control (enabled driving with key-actuated switch in inching mode, setting of a safe direction of rotation etc.).


So the limits of this technology are achieved to some extent. I would like to put it this way: The magnetic switch is no longer a binary device delivering only the status ok. and not ok. In addition there is now the status „may be not ok“.

The pros and cons of electronic switches

Contrary to the magnetic switch the electronic safety switch is a straight binary device identifying the states ok and not ok. However, these switches are more expensive around the factor 10 than the magnetic switches regarded first. If these switches are also two-cannel connected to a safety control, the costs will be still higher than with the magnetic switches. More suitable are switches, which can be single-channel connected or bus-capable safety switches.

Fact finding of safety control systems

We investigated systems by 4 different manufacturers with the following result:

  • There are predominantly proprietary solutions suggesting more often than not buying sensors and safety control from the same supplier.
  • Mostly the sensors are to be wired star shaped to the safety control resulting in a considerable wiring effort already with 20 safety sensors.
  • Partly screened cables are necessary to connect the sensors.
  • The most cost saving system operated with bus-capable safety switches with AS-interface.

Our résumé in three points

1. A crucial aspect is the wiring effort. Optimal results can be achieved by bus-capable safety switches e.g. with AS-Interface.


2. Important requirement is that voltage supply and bus are in one cable.


3. There are a lot of manufacturers providing safety systems with AS-interface. Evaluation units and sensors can be combined producer-independently.


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