×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Overland Conveyor E-stop oull cords wired through PLCs

Overland Conveyor E-stop oull cords wired through PLCs

Overland Conveyor E-stop oull cords wired through PLCs

(OP)
We have a 3km overland conveyor project that requires emergency pull-cord stops. The customer wants to know which safety switches is pulled via a PLC/HMI.

Anyone have experience with this type of application?

Thanks

RE: Overland Conveyor E-stop oull cords wired through PLCs

Run a aux contact back from each e-stop to plc inputs and identify them on an HMI.

RE: Overland Conveyor E-stop oull cords wired through PLCs

If you are putting E-Stops into a PLC, it had better be a safety PLC or you will be setting yourself up for regulatory trouble (not to mention the risk to life and limb). If you then use a Safety PLC, it can tell you that information very easily because the inherent design issue of a Safety PLC is to know the location of each address and to know the difference between a working device and a failed one or a cut wire. Siemens S7-300 and 400 PLCs can be configured to have safety and regular I/O in the same system. All others I know of would need a separate PLC for safe and non-safe systems.

RE: Overland Conveyor E-stop oull cords wired through PLCs

If the conveyor has operator access or is interfacing some way with the conveyor then you need pull cords along the length of operator access.  I would think that if their is a catwalk or platform, or its on ground level that its needed according to osha.  see below  

But as far as a safety plc that is driven by different industries maybe that is a requirement in the robot or auto industry.  

As a general rule its always better to be too safe than not safe enough.  Also, if your in a court room, did you do your due dilegence in following regs that applied?

http://www.osha.gov/doc/outreachtraining/htmlfiles/mathan.html

this refers to access not if up in air by itself.

To reduce the severity of an injury, an emergency button or pull cord designed to stop the conveyor must be installed at the employee's work station. Continuously accessible conveyor belts should have an emergency stop cable that extends the entire length of the conveyor belt so that the cable can be accessed from any location along the belt. The emergency stop switch must be designed to be reset before the conveyor can be restarted. Before restarting a conveyor that has stopped due to an overload, appropriate personnel must inspect the conveyor and clear the stoppage before restarting.

Osha
see below if access by personnel
29 CFR 1910.212(a)(1), copy enclosed, requires that on any machine, nip points and hazards, such as the rollers and the edge of the moving conveyor, must be guarded. Since the moving belt must be guarded, a standard railing or some sort of an effective barrier guard must be provided on the inside of the platform between the employee and the belt edge. In any event, an emergency stop pull cord along the length of the conveyor is also required.

RE: Overland Conveyor E-stop oull cords wired through PLCs

(OP)
Thanks guys for the help.

I'm sorry, I'm in learning mode.
The way I believe these units work is that PULL CORD will have two contactors. One contact  will be hard wired, so to stop the conveyor. The other (contactor 2) serves as the I/O for the PLC.

1) If the 2nd contactor serves the I/O of the PLC only for signaling location purposes, why does it have to be a Safety PLC?

2) For the hardwire system, how to deal with voltage drop of a 3km hardwire signal?

Thanks.

RE: Overland Conveyor E-stop oull cords wired through PLCs

You do not require a safety PLC for aux contact input in to the PLC.  What is being assumed above is that you were running your e-stop string in to the plc, and in that scenariio yes it would require safety PLC.  But my understanding is you have a hard wired e-stop string and the e-stops have aux contacts on them to be used as inputs to a plc to identify which estop is tripped. No safety PLC needed in that scenario.

If it is feasible you could run a network (ethernet, devicenet, controlnet, profibus, or whatever) and have distributed input blocks of io near each estop on the machine and have the network xmit the signals back.

RE: Overland Conveyor E-stop oull cords wired through PLCs

You can also get pull cord switches with mechanical flags that drop when activated.  

You would bring back a separate pair of wires from each switch to the PLC in order to pinpoint which switch was pulled, unless you got very creative and maybe measured the resistance in the monitoring circuit, using the aux contacts to shunt out the rest of the loop.  This might require adding some additional resistors at the switches. This should be a dc control circuit, btw.

RE: Overland Conveyor E-stop oull cords wired through PLCs

We normally wire the input in parallel with a local lamp, therefore the HMI will alarm with the correct location and locally a light will be on at the lamp.

Some e-stops have a lamp built in.

RE: Overland Conveyor E-stop oull cords wired through PLCs

I would recommend to have the emergency swiches on AS-I bus. Each EM-swich would then have a unique adress. The rest is up to you and how you program the PLC and HMI-system. I have done this many times. if you are using simatic S7 series i can give you some more tips about programming this system

RE: Overland Conveyor E-stop oull cords wired through PLCs

(OP)
That's amazing, thanks guys. Over the weekend I read up on safety PLC and I understand how to do it using that method.

2nd round question is what if the conventional hardwired method is used. Not PLC, but strictly wire the pull switch contacts in series, yet how to account for a 3km Vdrop?

RE: Overland Conveyor E-stop oull cords wired through PLCs

You will probably have to use dc control to go 3 km.  For a 3 km conveyor, the circuit length may be 6 km.

A dc interposing relay (fail safe) should not draw too much current - you just need to get the coil data and check the voltage drop.  You may need to increase the wire size beyond what you would normally use, but I think it is possible using dc relay coil.  AC will be a problem.  

RE: Overland Conveyor E-stop oull cords wired through PLCs

Your estop circuit is local to its control panel.  How would your estop circuit be 3 miles long?  Why would your pull cord be 3 miles long?  

Why would it be necessary to interlock the panels as your suggesting?  If one control panel stops on estop would not the other just stop on regular control?  Is it necessary to interlock the estop zones between panels?  

If this is a long belt it might be better to ramp down the belt then pull estop on the power on the motor.  Are you sure your not going to damage the belt on estop?

RE: Overland Conveyor E-stop oull cords wired through PLCs

controlsdude:

http://www.aeec.com/Conveyor/Pull_Cord_Switches/

Something like these will be located all along the length of the conveyor.  When you get your arm stuck in the belt sometimes it's a long reach to the control panel. smile

RE: Overland Conveyor E-stop oull cords wired through PLCs

(OP)
DPC and Itsmoked, thank you guys for the valuable help.

Exactly what I was looking for. Interesting info, and simple solution really. Thanks for teaching it to me.

Through the other thread I was recommended to read
"Distant Control of AC Relays, Contactors, and Starters" from square D.  I found the article here:
http://ecatalog.squared.com/pubs/Motor%20Control/Lighting%20Contactors/M-379F.pdf



Cheers,
Maj :)

RE: Overland Conveyor E-stop oull cords wired through PLCs

I did somthing similar years ago on a similar conveyor using a resistor chain, 100 Ohms in each switch connected to the N/O contacts. When a switch was pulled an Ohmmeter (100, 200,300, , ,) indicated the first activated switch, when reset indicated next etc.

Roy

RE: Overland Conveyor E-stop oull cords wired through PLCs

I have experience with the
http://www.aeec.com/Conveyor/Pull_Cord_Switches/
I've use them twice recently. To get tech support from the manufacturer is really hard (they are in south africa), I strongly recommend to take a look at http://www.ringway.com.au/
and their Ring Line product. I have a few other long overland conveyor projects and I'm thinking to use this for the emergency stop system.
Another advantage using these systems is that you offload the voltage drop calculations and concerns to the system supplier.

RE: Overland Conveyor E-stop oull cords wired through PLCs

If your system is small, a safety PLC is a bit overkill. Kinda like killing a fly with a sledgehammer, IMO.

Omron, Pilz, Allen-Bradley have some good safety relay products which give you Cat-3 safety in conjunction with a regular PLC.  

RE: Overland Conveyor E-stop oull cords wired through PLCs

Majesus.
         I have attached a sketch showing switch indication circuit. I used 100 Ohm resistors as this minimized the effect of wire resistance or dirty contact.
The Ohmeter had the 2 least significant digits blanked out so it read in units of 100 Ohms.
It wasn't perfect, occasionaly you would get a switch half pulled.
Most important with pull cords to have them quite loose otherwise cold weather can be a problem.
Roy

RE: Overland Conveyor E-stop oull cords wired through PLCs

The conveyor stop system you are needing is quite common throughout the mining industry. The Ringline system that Chilejimenez recommended is an excellent product. A very similar product is also available from another Australian company called Austdac. They're both a derivative of the Dupline system from Carlo Gavazzi.

I don't recommend trying to roll your own system such as has been suggested in previous posts. After more than 25 years in the mining industry around the world, the personal liability in doing such a thing just isn't worth it.

RE: Overland Conveyor E-stop oull cords wired through PLCs

Some of the posters suggest a Safety PLC, I don't see that.
No-one is going to be working on the conveyor with just the pullcord pulled. It must be locked out!
The problem with long conveyors is finding the switch that has beem pulled. I don't think running each switch back to a common point is an option. My suggestion uses 1 4c cable to pick up all the switches in series.
Roy

RE: Overland Conveyor E-stop oull cords wired through PLCs

(OP)
The solution used was a Pull Stop with two  NC contacts.
One switch was wired in series along the conveyor using a DC source then hard wired directly to the conveyor's power source using an interposing relay.

The second switch were grouped per 200ft, and then wired into a remote I/O port, they were then radio back to the main PLC for "pulled switch" detection.

Maj

RE: Overland Conveyor E-stop oull cords wired through PLCs

Sounds like a good solution
Thanks for the feedback
Roy

RE: Overland Conveyor E-stop oull cords wired through PLCs

(OP)
Thank you guys as well.

Love it. :)

RE: Overland Conveyor E-stop oull cords wired through PLCs

The Austdac stuff is wicked expensive. plus their system is a 3 wire system. Ringline is a copy of dupline. I use the new Dupline safe system. all of the safety components are SIL3 cat4. It is possible to do a 3km run using #12 AWG depending on your switch spacing.

regards.
CE

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources