By-passing safety light curtain

rguimond · Jul 25, 2015

Just looking for opinions here...

To prevent employees from coming into contact with a large, rotating butter churn, a light curtain was installed that stops the churn and applies a brake if the beam is interrupted. However, the beam is also interrupted by a hopper that is rolled into position near the churn so the butter can be dumped. In order to position the churn and to get all the butter out, the churn must be jogged. The light curtain is in series with several E-stops, a two churn door safety switches and a stop button. If any of the safety devices are tripped, the MCR unlatches and the system must be reset (armed) by pressing a start/reset button.

I have no choice but to by-pass the light curtain while the hopper is in place.

Here are some of the things I've been pondering:

1. A momentary push-button in parallel with the light curtain NC contact. The button would have to be pressed continuously while re-arming the system and while jogging. If the button were to be released, the MCR would unlatch. If nothing else, this would be very inconvenient and cumbersome for the operator.

2. Could I use something like an RFID safety switch (the chip on the hopper and the reader attached to the churn base) to by-pass the light curtain? The reader would be attached permanently to a swing-arm that the operator would move towards the chip once the hopper is in place.

3. A cheaper and simpler solution may be to use a key attached to the hopper that will only reach the control panel when the hopper is in the "dump" position. The key switch would by-pass the light curtain.

I would appreciate any feedback and suggestions

bernie_carlton · Jul 25, 2015

Can the process of rotation of the churn be altered with a function to stop in the correct position? If so this operation could be triggered from outside the safety curtain. It's always better to modify the process to make it safer rather than trying to find a 'relatively safe' way to bypass the guarding.

Highland Controls · Jul 25, 2015

Can you blank sections of the light curtain? If the position of the churn isn't the same as a person walking into it, that might be possible.

rguimond · Jul 25, 2015

Thanks for the responses.

Unfortunatel, there is no real way of stopping the churn in the correct position automatically. When the churn is jogged to the right position, the door (which has hinges on the circumference of the drum) is opened manually. This takes two employees. Butter is dumped and pushed into the hopper. Sometimes, the churn is jogged a bit forward and reverse to "shake" the butter out.

I have considered trying to control where the churn stops, but it's a real challenge. There may be up to 3000 Lbs of butter in a clump on one side of the barrel, which makes it really off-balance. The barrel is 8' in diameter. We're looking at thousands of foot-pounds of torque.

There's no possibility of a person walking into the churn while the hopper is in place. An operator stands on either side of the hopper while the dumping takes place. Only one operator handles the controls. All he has available at the dumping station are "reset", "E-stop", "jog fwd", "jog rev" and "door-switch by-pass". He has to jug the churn with the door open to "shake" the butter out. The control station in the "safe" area has "run fwd" and "run rev" buttons, but no "door-switch by-pass"

The second operator has an E-stop at his position.

I guess I could add a "dead-man" switch for the second operator to increase the safety factor. He would have to press it before the main operator could jog it.

GlenGineer · Jul 25, 2015

Do you actually need the light curtain? Would another type of safety device work as well but not detect the churn. Maybe pressure plates or a safety mat?

Phil Buchanan · Jul 25, 2015

Sounds like you just need to mute the portion of the light curtain that is blocked by the hopper for the time that the hopper is in place. Have a button to say hopper insert or something like that and that will mute the light curtain and if a smaller hopper or a person enters it will still trip because they will trip beams other than what are muted.

adisharr · Jul 26, 2015

rguimond said:
Just looking for opinions here...

To prevent employees from coming into contact with a large, rotating butter churn, a light curtain was installed that stops the churn and applies a brake if the beam is interrupted. However, the beam is also interrupted by a hopper that is rolled into position near the churn so the butter can be dumped. In order to position the churn and to get all the butter out, the churn must be jogged. The light curtain is in series with several E-stops, a two churn door safety switches and a stop button. If any of the safety devices are tripped, the MCR unlatches and the system must be reset (armed) by pressing a start/reset button.

I have no choice but to by-pass the light curtain while the hopper is in place.

Here are some of the things I've been pondering:

1. A momentary push-button in parallel with the light curtain NC contact. The button would have to be pressed continuously while re-arming the system and while jogging. If the button were to be released, the MCR would unlatch. If nothing else, this would be very inconvenient and cumbersome for the operator.

2. Could I use something like an RFID safety switch (the chip on the hopper and the reader attached to the churn base) to by-pass the light curtain? The reader would be attached permanently to a swing-arm that the operator would move towards the chip once the hopper is in place.

3. A cheaper and simpler solution may be to use a key attached to the hopper that will only reach the control panel when the hopper is in the "dump" position. The key switch would by-pass the light curtain.

I would appreciate any feedback and suggestions

I would lean towards #2 or #3. I would also give the other operator a dead man switch.

Ideally, I would require the operators to put the system into an additional mode which could only be active for a specific time before needing to be reset. An indicator would flash to indicate this mode was active. Only in this mode would the system be allowed to jog. That, in addition to the dead man and RFID / interlock switch changes should be pretty solid.

I also tend to use a lot of programmable safety controllers which facilitate these types of changes fairly easily.

mk42 · Jul 27, 2015

adisharr said:
I also tend to use a lot of programmable safety controllers which facilitate these types of changes fairly easily.

This sounds like a situation where a programmable safety controller would be a good solution. When you have that many devices in a safety chain, it can make it really hard to modify any of them later. Most safety controllers have light curtain muting code built in, and this could easily be combined with the rest of the system.

shooter · Jul 27, 2015

Move the curtain till the hopper is inside the space of the churn, that means the curtain is safe even when the hopper is in place.
It means you can not yog when there is also a person in the controlled space which is fine.
Make the jog function it resets the safety auto, and with low speed.
Operators tend to be too close to moving machines, to guide the butter, however as the weight is very high please tell them it is too dangerous to handle manual.

Aabeck · Jul 27, 2015

What about rotating the light curtain to horizontal, above the level of the hopper, but where a person entering would break it & stop the machine?

amberman · Jul 27, 2015

Fairly standard light curtain application - it needs muting with sensors inside the guarded area which are broken by the hopper before it breaks the light curtain itself. If the existing light curtain does not have a muting function I'd recommend replacing it with one that does rather than overriding the safety function with any modifications yourself.

James Mcquade · Jul 27, 2015

rguimond,

I have seen a similar situation and this is what they did.

There is a keyed switch in the main control room that must be turned off and removed placing the system in manual.
The operator takes the key with him to the manual station and inserts the key and turns to the manual operation and disables the light curtain. They then jog the parts into place.
When the operator is finished, he then turns the key back to auto mode and re-enables the light curtain.
The operator then goes to the main control panel and inserts the key and turns the system back to auto and restarts the unit.

You will need to do a risk assessment involving safety, engineering, and maintenance to see what must be done and verify that you meet all your federal / state / local codes.

regards,
james

rguimond · Jul 27, 2015

I attached a .pdf to give everyone a better idea of the configuration. Access to the space to the top of the butter hopper in the "normal" position is necessary.

During "loading", the light curtain has to be taken out of the safety circuit. I'm hoping the use of a magnetic RFID key, like http://www.automationdirect.com/adc/Shopping/Catalog/Safety/Safety_Switches/IDEM_Non-Contact_Magnetic_Locking_RFID_Switches will be sufficient, especially if only "JOG" function is available in this situation

robcontec · Jul 27, 2015

You could use 2 switches to mute the light curtian. If the hopper moves in to the curtain, limit switches can be set up that make just prior to the hopper interupting the light curtain.

generally the 2 switches need to be set up so that they can not be made by some one.

the switches should be connected to a safety relay, forcing both switches to be made at the same time. the out put of the safety relay is used to mute the light curtain

rguimond · Jul 27, 2015

robcontec said:
You could use 2 switches to mute the light curtian. If the hopper moves in to the curtain, limit switches can be set up that make just prior to the hopper interupting the light curtain.

generally the 2 switches need to be set up so that they can not be made by some one.

the switches should be connected to a safety relay, forcing both switches to be made at the same time. the out put of the safety relay is used to mute the light curtain

I can see the advantage of that in some situations, but in this situation, I never want the churn to rotate after anything (hopper included) breaks the light curtain beam.

Once the hopper is in place (which is confirmed by the magnetic RFID switch), the risks change considerably. It is no longer possible for someone to get struck by, or entangled in, the door.

By-passing safety light curtain

rguimond

Lifetime Supporting Member

bernie_carlton

Lifetime Supporting Member + Moderator

Highland Controls

Member

rguimond

Lifetime Supporting Member

GlenGineer

Member

Phil Buchanan

Lifetime Supporting Member

adisharr

Member

mk42

Lifetime Supporting Member

shooter

Member

Aabeck

Member

amberman

Member

James Mcquade

Member

rguimond

Lifetime Supporting Member

robcontec

Member

rguimond

Lifetime Supporting Member

Similar Topics