Overland Conveyor E-stop oull cords wired through PLCs

[majesus]

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

 

[jde777]

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

 

[jraef]

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.

 

[controlsdude]

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.

 

[majesus]

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.

 

[jde777]

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.

 

[dpc]

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.

 

[CanadianTaff]

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.

 

[oyvindb]

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

 

[majesus]

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?

 

[dpc]

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.

 

[itsmoked]

majesus;

Check this recent thread:
thread240-208641




Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[controlsdude]

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?

 

[dpc]

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.

 

[majesus]

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 Control/Lighting Contactors/M-379F.pdf

Cheers,
Maj

 

[roydm]

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

 

[chilejimenez]

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.

 

[OregonSaluki]

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.

 

[roydm]

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

 

[OhioAviator]

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.