Machine & Operator Safety
Key considerations for using and selecting non-contact switches
By Tina Hull
By Tina Hull
Oct. 24, 2014 – My husband and I painted the walls and installed new wood trim in our home. Everything was great until the day our friend fell against an open door and it hit the wall behind it, damaging the drywall. Yes, we forgot to replace the doorstop. Unfortunately, another weekend of summertime activities was replaced with spackling and repainting.
In some ways, the doors for safeguarding machinery are similar to the doors in a house — if installed improperly or used incorrectly, they can both cause or be damaged. So how can we prevent damage when using non-contact switches — interlocking devices designed to protect people and machines — on our doors? It’s simple: use a doorstop. Non-contact switches and doorstops should go hand in hand.
In applications where the installations are close together, check the manual for minimum distance between multiple sets of non-contact switches. It is usually between 25 and 50 mm, depending on the switch and its orientation. Also check the user manual for minimum gap, which is typically around two millimetres.
But first, you need to make sure the application is appropriate for non-contact switches — and doorstops — and there are multiple factors to consider when making the decision.
The ideal applications have small, light-weight doors that lift up, swing or slide open. You’ll want to avoid non-contact switches on heavy doors since the door can droop over time and cause false trips due to unanticipated misalignment. Another suitable use is for applications traditionally using limit switches. Limit switches are difficult to use and are easy to bypass with a piece of gum and a nickel since they detect ferrous metal objects.
Non-contact switches are an option for compact machines. For industries requiring cleaner work environments, sealed non-contact switches are a solution since they don’t generate particles or cause contamination. They can handle higher ambient operating temperatures and many are rated for IP67.
Some smaller machines that do not allow full body access use Plexiglass as a barrier. What happens if too much force is applied to Plexiglass? It breaks. Therefore, using non-contact switches with doorstops will reduce the probability of broken Plexiglass compared to using a mechanical switch, which is usually associated with high force when closing.
There are several types of non-contact switches, so which one should you use?
One widely available is the magnetic type that uses a universal reed switch. Metal reeds are sealed inside a glass tube and the contacts are actuated when the magnetic field is present. They are excellent for harsh environments, such as flammable gas and corrosion, since they are sealed and have a low resistance when closed. They are available in coded and non-coded versions. The non-coded versions have just one set of north and south poles, which makes it easy for an operator to carry a spare magnet in their pocket to bypass the switch when no one is looking. The coded versions come in different combinations of north and south poles. It is a lot like keys, each having its own pattern. Trying to find another one to bypass the system is far more difficult. Remember how the switch is sealed in a glass tube? Any excessive force causing the two ends to collide oftentimes damages the switch since it shatters the glass tube. These are difficult to troubleshoot since the switch is sealed inside the housing.
Here are some other issues with reed switches:
1. When placed in series, you can’t determine which one caused the shutdown.
2. If a door is closed too slowly, the magnets may not make contact within the specified time and may not allow the system to restart. (You can overcome this by opening and closing the door again.)
3. Most companies mount these on ferrous material (steel frames). Ferrous material sucks out the magnetism and can decrease the gap by up to 90 per cent. (You can fix this by installing a non-metal spacer. Check your user manual, but many only need a five-millimetre spacer.)
4. Machines subject to vibration and shock can cause false tripping, especially on systems with tight tolerances.
5. Magnetic particles may stick to it.
If these are problems for you, a solution might be non-contact switches using electromagnetic induction method, also known as hall effect. An external magnetic field turns on the contacts. The transducer converts the signal from one form of energy to another, which then varies the output voltage in response to the magnetic field. If you are subject to false tripping due to vibration or shock or operators who open the door too slowly, this may be a solution since these switches use a sequence of contacts so they do not suffer from contact bounce. Some models available meet a category 4/PLe.
Sometimes it isn’t the non-contact switches causing the problem, but the installation or environment. If used in areas around electromagnetic equipment — such as RFID, proximity sensors, motors, inverters and switching power supplies — a surge absorber may be needed, especially for versions that have an inrush current rating less than 0.5 mA.
For equipment where the switches intermittently work correctly, especially when wiring has been double-checked and is confirmed to be correct, check the type of power supply and the power consumption. Safety devices should use a power supply certified for SELV (Safety Extra Low Voltage).
Use a safety-rated controller for monitoring, since non-contact switches do not have direct opening action.
Finally, finish the application by using tamper-resistant screws or fill in the head with a tamper-proof varnish or compound that will not have a detrimental effect on the plastic switch case.
Selecting the correct application, technology, installation and monitoring for your machinery plays a huge part in keeping your machines running safely and increasing their lifespan. And if a non-contact switch is the right technology for your application, make sure you use a doorstop.
Tina Hull is a product engineer with Omron Automation and Safety.
This column originally appeared in the October 2014 issue of Manufacturing AUTOMATION.