Transmitter (AT 400/3) & Receiver (AMR 400/3),
We have checked all hard wiring out okay, Checked for correct voltage okay and checked functionality of all attached proximities and lamps okay. Both transmitter and receiver have been lined up, and we have even replaced transmitter and receiver yet we still seem to get intermittent tripping. Can anybody help - we have been in touch with leuze and they say it could be another light source but affecting the receiver but we have none around the unit. Has anyone else had similar problems

 

I don't know your particular system but some generic possibilities are power quality, noise from other circuits, and improper grounding.

One troubleshooting method where you have checked out everything and still cannot find the problem is to draw an imaginary boundary around your system or subsystems and see what things cross this boundary(remember that there
are many paths into and out of a system, both normal and abnormal) and then eliminate them one by one as a cause of the problem. For example, you could start with your trip element as a subsystem and work your way out to the trip initiator.

This method and other troubleshooting methods are discussed in my book "Troubleshooting: A Technician's Guide," published by ISA (ISBN: 1-55617-705-4).

Bill Mostia
=====================================================
William(Bill) L. Mostia, Jr. P.E.
Partner
exida.com
Worldwide Excellence in Dependable Automation
[email protected] (b) [email protected] (h)
www.exida.com 281-334-3169
These opinions are my own and are offered on the basis of Caveat Emptor.

 

We have been using the Leuze Lumiflex light curtains on all new applications for several years without any problems and I am completely satisfied with them. They seem to be an excellent product.

However, we have used other brands in the past and have had certain problems with those. Most of the causes of the sort of problem you have described though are inherent to light curtains in general rather than a problem with a particular brands. Some problems I have seen are:

1) The receiver sees the transmitter of an adjacent light curtain. This may be fixed by swapping the receiver and transmitter of one of the light curtains so they point in opposite directions. If there are more than two light curtains in a row, you may need to install a (small) barrier or shield to prevent this interference.

2) A relay coil in the machine is injecting a voltage spike into the DC circuit when it switches off. This could be a problem even with a very small relay. The easiest solution is to ensure all relay (and valve) coils have diode or snubber suppression on them. If this is difficult to do, try putting the light curtain on its own power supply. Also check the routing of the cables between the light curtain and the safety relay to ensure they are not exposed to excessive electrical noise.

3) Excessive vibration on the machine is affecting the light curtains. Try isolating them from the vibration if this is the case.

4) Someone is actually tripping the light curtain. People may interrupt the light curtain without realising it. This is especially a problem when an operator has to turn as part of their work flow and may interrupt the light curtain with their elbow as they turn. The solution to this is better layout of the work station, and possibly an audible alarm if the light curtain is interrupted when it shouldn't be.

5) Some object (e.g. a cable) is interrupting the light curtain. Some additional examples of how this may occur include if production parts, gloves, etc. are placed near the light curtain and then move (due to vibration) and interrupt it.

If the above doesn't help, then if you would like to give more details about how you are applying the light curtain I may be able to give some additional advice.

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Michael Griffin
London, Ont. Canada
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My guess is these situations are more common than people think, and probably causes a lot of nuisance trips, and complaints about the light curtains that really are an application problem and not an equipment problem.

 

The worst light curtain application mistake that I saw was at a new plant that I visited a while ago. The equipment used light curtains as perimeter guards across the backs of the machines (instead of fixed guards). The light
curtains were being tripped accidentally so often by passers-by that they had to rope off the area near the machines to keep people away from them.

The light curtains had to be spaced out from the machines to clear all obstructions (to get a clear line of sight) and to meet the required stopping time. The rope barriers were then spaced more than arm's length from the light curtains. Anyone approaching the rope barriers received blood curdling threats from the operators to keep back.

Between all these factors, anyone trying to observe the production process to find out what the problems were needed a pair of binoculars if they wanted to have any hope of seeing what was going on inside the machines. Since observing was what we were there for this posed somewhat of a problem for us.

I saw several accidental trips over the course of a couple of days, all involving the machine builder's own personnel. The light curtains themselves worked flawlessly. The problem was they should never have been there in the first place.

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Michael Griffin
London, Ont. Canada
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