By-passing safety light curtain

[Timbert]

The question I have is how are the operators protected when they are jogging the churn during unloading? It isn't so much how we can bypass the interlock (jumper wire anyone?) it is how to we mute the interlock and maintain a safe machine.

It sounds to me that there is no protection when they jog the churn other than the operator thinking "I'm standing next to an 8-foot machine with thousands of pounds of force I better be careful."

The correct way is to mute the light curtain when the hopper is in place, but you have to provide an alternative method of ensuring safety. One such method would be another light curtain mounted "outside" of the existing one in such a way as it would clear the hopper when the hopper is in the unload position.

Another possibility is to limit the rotation (range and/or speed) of the churn when in the unload position so as to ensure the hazard can be avoided.

As for your suggestions be careful implementing them as the mute function has to have the same safety integrity as the function it mutes. i.e. a simple switch can fail and be undetected causing a continuous mute.

 

[robcontec]

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.

if you do not want rotation of the churn while the hopper is in place, using the same safety relay for muting, the 2 safety out puts of the muting relay would energize 2 control relays. You would need 2 positively guide relays with 2 NO and 2 NC contacts. One NO contact from each control relay would mute the light screen, one NC contact from each relay would break your normal rotation enable circuit. One NC from each of these contacts will need to be tied back to the reset circuit of the muting safety relay. If one of the control relays fail, the muting relay will not be able to energize.

 

[milldrone]

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

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 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.

Leave the light curtain and door switches in place. Install two powered cable winches. These winches would not be connected to the current safeties. When it's time to dump, position the drum within a half turn. wind the winch cables with at least one half turn around the drum. Attach the cables to the drum. The cables will need to wrap in opposite directions. The object is when one winch is winding the other is unwinding, thus rotating and jogging the drum under the unloading operators control. When the drum is being controlled by the winches the brake will need to be released.

 

[rguimond]

My original description probably wasn't clear enough and I could have provided more information on the first sketch. This one should make things clear.

The operators must side on each side of the churn in order to open and lower the door. Once it's open, the churn has to be jogged slightly in both directions to "shake" the butter out into the hopper. Once dumped, the door is closed and it is jogged around a full rotation before repeating the process. Jogging speed is very slow - maybe 5 RPM.

I propose the magnetic RFID switch only to ensure the hopper is in position. When it's in place, there is no risk of anyone coming into contact with the door.

We can't use pressure mats to verify the operators are in position, as it would be a sanitation nightmare. We could, however, require operator #2 to press a second "jog" button within a second of the first operator before it would actually jog

 

[dlweber]

I have recently implemented a similar system,

I used two light curtains although you can do it with one if you buy the correct one
Two laser sensors capable of windowed detection

I have the laser sensors set up in specific positions and angles to hit at a certain spot on the hopper, far enough apart no person could trip them both at once.

The larger (bottom) light curtain mutes when both sensors are tripped, the upper light curtain is always on (learning curve for the fork truck drivers but they will figure it out)

EDIT: No person could reasonably hit both sensors, its a large hopper and the window to activate the laser sensors is about 4-6 inches

 

[dlweber]

Even if one person could trip both sensors at once they would not be able to maintain the sensors and make it through the light curtain

 

[rguimond]

I think I understand what your'e saying... Use sensors to detect movement of the hopper and use it to mute the light curtain. However, I want any interruption of the light curtain beam to cause the churn to stop. What I'm trying to accomplish is a higher degree of safety for the operators who unload the churn while inside the danger area.

 

[dlweber]

You said a few times that when the hopper is inserted into the area that the light curtains need to be bypassed (I prefer to use mute/blanked, bypassed to me has the feeling of something that isn't supposed to happen), now you say any interruption...

I think you mean what I am saying and the system described above does, any interruption of the light curtain EXCEPT for the proper insertion of the hopper should trip the system.

Its not so much movement the sensors are detecting, its placement. You could take the hopper and move it around, so long as it stays within the windows of detection, without tripping the system.

Also in a setup like this when the light curtains are in the muted state there should be an interlock to prevent external control from happening. Limiting speed of churn is also a good idea. Basically I am thinking of the churn as a big butter filled robot, and applying the same safety measures.

 

[dlweber]

On the idea of robot you could have physical key switches like a teach pendant has, for anything to break the light curtain the key switches must be turned then each operator will have some hand held device with three position deadman switches and the jog controls. I have seen hand held devices like this on the market.