Light Curtain Distance Calculation

kku

Member
Join Date
May 2013
Location
Toronto
Posts
39
I am trying to calculate the safe distance for a light curtain. What's a good way to determine the maximum stopping time of a hazard? In my case it's a rotary dial table. The light curtain is supposed to stop the rotation whenever the light curtain is tripped.

I found a formula on Omron / STi's website. I attached it for reference. I am planning on using a light curtain capable of hand detection. The minimum detection distance is 20 mm.
 
what we used to do is get a stick and see if we could break the light curtain while the dial is moving and not damage the stick. the stick representing one's fingers. you don't even want to come close to damaging the stick.

then again, our turntables and dials weren't that fast.
for punch presses, i heard someone say throw a volley ball. if it gets smashed or caught, back the light curtain out.

regards,
james
 
It is easiest to calculate the stopping time by looking at what contributes to this time. I usually see it broken down in to sensor, control, motion reaction times. i.e. what is the reaction time of the light curtain, how long does it take for the control power to be removed, how long does deceleration of the axis take. You have to look at the maximum time for each of these.

It can be measured empirically as well.
 
the 'K' distance of 2 meters per second is 2.5 Mtrs/sec here
you need to check that.
As far as 20mm sensing the closer your guard senses the closer your guard can be fitted. (14mm) is better.

this is the PITCH between sensors

the 'T' constant is the TOTAL time the system takes to stop the machine.
this also needs to be checked during the process.
What turns the table - how do you Know that has stopped.

there are a lot of traps with light curtains
Remember that a Standard PLC is NOT a Safety device
so don't intend to us it as the GUARDED STOP for the process.
You can stop the program BUT the protection needs to be seperate from the PLC
 
All guarding should be triple redundant. ie a dry contact in the device in the e-stop circuit;a separate dry contact in the e-stop RELAY circuit; and of course in the PLC to indicate the fault.
 
Hi and welcome cougar1559
most light curtain controllers are (Cat 4 in Aus) sil3 or higher
you are 100% correct with your statement.
however it is very much dependent on the actual application and trip points
triple redundancy is the highest level - Only a risk analysis can determine if the highest level is needed.

the more common question is "What checks the checker?"

my concern is oddly enough not the installation of the light curtain.
because the controller will do it's job.
But the actual machine stopping device

how do you confirm the valve or brake has worked in time and what happens if it fails. Can the machine still be stopped.
Normally if a catastrophic failure occurs with the stopping device
Cat. 3 - means that the machine should not allow the next cycle.
Cat. 4 - Means the machine should lock out prior to any further movement

There are other posts on this site so do a search please
 
I did mean to say that triple redundancy is risk analysis dependent.
as with any safety guarding.
It may not be needed to that level in some applications.
 
This thread stared out with a press application. Presses in all forms are highest risk. As far as checking the checker, we use the PLC to do that. Just put use an input from every change in the stop circuit and program the logic.

Hugs and handshakes,
David
 
This thread stared out with a press application. Presses in all forms are highest risk. As far as checking the checker, we use the PLC to do that. Just put use an input from every change in the stop circuit and program the logic.

Hugs and handshakes,
David

I agree with you David

Understand that I was responsible for the Toyota Altona Press safety for 16 years
so I am well aware of the safety requirements
Car body sized presses are a big risk.

how easy is it for someone to 'force' a PLC contact of change the program
Alternative is it a Programmable safety relay or a safety PLC. which are you using.
there is no way I would ever trust a non safety PLC with any safety circuit.
 
Now hooooold on there Babalouie! :rolleyes:

cougar1559 said:
As far as checking the checker, we use the PLC to do that. Just put use an input from every change in the stop circuit and program the logic...

In the Safety World, a redundant "checker", and subsequent redundant "checkers", must be at least at the same Safety Integrity Level (SIL) as the first "checker". Auxiliary contacts on Safety devices, wired back to standard PLC inputs, should only be used for status indication, or bringing the program cycle, not the hazard, to a desired state before a re-start. These auxiliary contact signals must not perform any part of the Safety Function i.e. bringing the system to a Safe State.

...All guarding should be triple redundant...

As Ian pointed out, not necessarily.

Guarding may use single, or dual redundancy, but rarely would use triple redundancy, even on presses.
Again, redundancy being of an equal Safety Level as the first, and second "checkers".

But more importantly...

1. ie a dry contact in the device in the e-stop circuit;
2. a separate dry contact in the e-stop RELAY circuit;
3. and of course in the PLC to indicate the fault...

...again these are auxiliary contacts and must not act as part of a Safety Function. This is not triple redundancy, but merely auxiliary status monitoring. The thinking here would be to enable a fault status if any one of those inputs were to change.

But, either way, this is not how the principle of triple redundancy works.

Triple redundancy uses a voting system that decides at what failure point will the Safety Function be triggered. The failure point is determined by a pre-selected voting architecture, such as 1-out-of-three (1oo3), or two-out-of-three (2oo3).

For example:

1oo3 Architecture

If one of three Safety Channels, i.e. one-out-of-three, on a guard interlock switch, were to fail open, the Safety Function must trip.

2oo3 Architecture

If one of three Safety Channels were to fail open, the Safety Function must not be triggered, as the other two Channels are voting to maintain the Safety Function. However, if a second Channel then failed open, two-out-of-three have now failed and the Safety Function must be tripped.

...Hugs and handshakes,
David

If you're doing what I think you're doing, I pray you don't lose those arms for hugging and hands for shaking! 🤞🏻

kku,

I haven't time right now (yaaawwnn :sleep:) to add to the light curtain setup, but I'll throw my 2 cents in when I get a chance.

G.
 
Last edited:
you all realize that we are literally writing what is already written in the Safety standards of ALL our countries that have standards.
even kku has copied the standard from OMRON
 
Yes Ian, but there can be subtle, to gaping differences in our interpretations of the standards. I'm always curious to know how others interpret things here, especially with regard to Safety matters.

Quoting verbatim often does little good.

I always try to write what I know in a hopefully more easier to understand, and interesting format than the reams of regulations and standards that the OPs have either not bothered to read, or didn't understand.

Sometimes Google can overload an individual with too many option, so they come to the likes of here for a more concise answer.

The OP in this case found that OMRON info, and I'm sure others, but obviously doesn't fully understand how to calculate certain elements of the equation. You have kindly explained those elements. Two key questions were asked and I intend to answer them as only I know how...at length! :rolleyes:

This is why I rarely reply with short answers when I feel detail is required.

Ok sleep this time...ngt!

G.
 

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