E Stop

[ptonsparky]

E stop to me means shut the power off, all of it. How should it affect the pneumatics? Are they to be returned to a safe position before power is removed?

 

[boater bill]

My experience is to have the pneumatics maintain the safe position on an e-stop. Whether it is to turn off the solenoid or cycle off. I have had to do either on machines. It really depnds on the operation of the machine or process.

Maybe some more ideas about what the e-stop is stopping would help.

 

[480sparky]

An e-stop may be more than just a disconnect. It may trigger an order of events. It depends on the situation you have. We need more specifics than just 'pneumatics.'

 

[cschmid]

E-stop means it is an emergency and everything stops. all electrical power is to be disconnected. This does poss some very interesting situations concerning hyd., Pneumatics, and other stored energy items. we created points that we classified as safe points for stored energy items and automatically locked them in that position. This is an area you must work with local OSHA representative to create plan. Yet there are some rules in this area. I do not have my OSHA manual in front of me and do not have that memorized so will not make any rule quotes.

 

[petersonra]

An e-stop may be more than just a disconnect. It may trigger an order of events. It depends on the situation you have. We need more specifics than just 'pneumatics.'

I do not believe an e-stop qualifies as a disconnecting means.

In many cases, you can just drop power to everything and the design of the equipment causes it to go to a safe condition. Sometimes you do have to keep power on some thing(s) to bring them to a safe condition prior to removing power.

Normally, you would want to remove whatever sources of energy might exist to stop any possible movement. That could mean having center open valves that relieve pressure on both sides of an air cylinder for example.

 

[cschmid]

I must correct that part of my comment and E-stop is not a disconnecting means for the power. poor choice of words.. It controls the emergency stopping sequence of the area it is involved with.

 

[ptonsparky]

It is a cut off saw for plastic sheets or rolls 1/16 to 5/8 thick, 60" wide and up to 300 feet long. Every so many feet the sheet is cut. A PLC controls the air solenoids etc. For instance at 8.02567' the plastic sheet is clamped (air), table now moves at same speed as sheet assisted by air, saw starts, vacum starts, saw lowers(air), DC motor moves saw across sheet. After cut saw raises(air), clamp is released(air), DC motor reverses to home positon, table returns to home position(air).

Owner would like clamp to raise, and saw to raise when E-stop is pushed, the rest can remain in place. He has defined this as a safe position. Resonable request.

Normally I would use the E-stop to remove power to input, outputs and any control relays involved. Never was involved with the air solenoids before.

 

[cschmid]

I believe it is a reasonable request. Does the saw blade stop as well? I believe he is hoping to save damage to the saw blade and maybe any mishap from debris created by a stationery saw running. what is the default position for the saw? is it a two way power pneumatic cylinders?

 

[FNCnca]

Check out NFPA 79 emergency stops.

 

[cschmid]

NFPA 79: 9.2.5.3.3 Stop
Where required, provisions to connect protective devices and interlocks shall be provided. Where applicable, the stop function shall signal the logic of the control system that such a condition exists. The reset of the stop function shall not initiate any hazardous conditions. In addition to the requirements for stop, the emergency stop shall have the following requirements:
1. It shall override all other functions and operations in all modes.
2. Power to the machine actuators, which causes a hazardous condition(s), shall be removed as quickly as possible without creating other hazards (e.g., by the provision of mechanical means of stopping requiring no external power, by reverse current braking for a Category 1 stop).
3. Reset of an emergency stop circuit shall not initiate a restart.

 

[boater bill]

OK sparky,

Here's a thought, hard wire an estop relay with multiple contacts. Normal operation energizes the estop relay coil. Wire a NO contact between the PLC output and the solenoids in question to de-energize them if they are spring return 3-way valves. Clamp and saw rise if it is a spring return 3-way. If it is a 4-way or non-spring return will require a little more thought, but posssible. I would also use a estop relay contact as in input to the plc to indicate a warning, but the saw blade and other operations should also be hardwired between the plc output and device.

This is most important:
NEVER use the PLC as the e-stop shut down device only! Hard wire is the only way to do it and keep someone from getting hurt. Logic will sometimes fail and if that's my hand caught in the saw, I would want it hard wired.

Hope that helps.

 

[justdavemamm]

Being a Controls Engineer I'll agree with Bill and add the most people are starting to use Positive Force Guided Safety Relays for E-Stop chains. One project I recently did had 2x300 amp contactors wired in series that were controled by the E-stop/Safety relay.

Also, I will disconnect power from the whole plc output card vs haveing a contact in series for each output.

 

[Mporter4]

I Have Done A Good Bit Of Machine Guarding / E-stop Circuits And There Are Alot Of Different Approaches Taken. For Smaller Machines We Have Mechnical Guards With Locking Type Solenoid Saftey Switches.also A Zero Speed Switch Is Interlocked To The Solenoids Of The Doors. And An Air Dump For The Pnuematic Equiptment.
Several Plants We Work In Have Asked Us To Wire Equiptment With A Catagory 3 Rating. Which Means Redundant Wiring, Saftey Relays, Redundant Contactors, Monitoring Contacts,etc. Checkout Sti, And Banner For Some Helpful Devices.
also Look Into Light Curtains.

 

[dbuckley]

... most people are starting to use Positive Force Guided Safety Relays for E-Stop chains.

Are you not mandated to use safety relays and other controls in a e-stop system?

 

[justdavemamm]

Are you not mandated to use safety relays and other controls in a e-stop system?

What 'mandate' are you refering to ? Please provide actual text.

 

[cschmid]

All E-stops are required to be hard wired. Yes this involves E-stop relays that are controlled by only E-stop power. I would also drop my PLC output power. It is so important to know what type of pneumatics you have. You might need to purchase some new gear and change some logic in the program to compensate. spring return means it might require more pressure to push it down in order to achieve the same down time and then a stop and hold function to maintain down pressure at a certain level. You don't want to affect line speed due to much pressure on finish line belt. This logic might already exsist as I do not know.

 

[cschmid]

The only reason not to drop PLC output power is if it controls something outside of the E-stop area.

 

[ELA]

Amazing as it seems there are now safety PLCs.
EMO circuits can now be wired using these PLCs designed specifically for EMO circuits. These PLCs have redunant microprocessors and I/O. They are certified specifically for safety use.
There are many standards that dictate how EMO circuits must behave depending upon the machinery type and where it is going to be installed.

A risk assessment must be performed to determine the level or catagory of the safety circuit required. The catagory then determines the level of protection required, including use of force guided relays, redundancy, cross checking wiring type etc.

This type of safety is usually built into the machine from the start. If you are engineering your own safety add-on I would exercise caution.

We usually used air solenoids that defaulted to the position we wanted the equipment to be in when power was removed (normally open or normally closed valves).

 

[dbuckley]

What 'mandate' are you refering to ? Please provide actual text.

Ok, in Europe to bring something (almost anything) "to market" (which effectively means to "sell something") that thing is required to be CE marked. To bring something to market that isn't CE marked is an offense, go directly to jail.

What the CE mark means is that the thing complies with the relevant EU directives, and in the context of estops, that directive is the Machinery Directive. (1) The Machinery Directive incorporates a number of standards, one of which is EN954 - "Safety of machinery—Safety-related parts of control systems…". This defines how things like estops can work.

Now having done a bit of a google, it seems that

In the U.S., the Occupational Safety & Health Administration (OSHA) states: "OSHA encourages employers to abide by the more current industry consensus standards [such as the CE Machinery Directive] since those standards are more likely to be abreast of the state of the art than an applicable OSHA standard may be."

So it seems that you are not mandated to use safety relays, but encouraged to.

If "safety relay" is a term unfamiliar, start at the Pilz website. As noted above, it's not just about safety relays, there are PLCs that can be used in safety critical situations, some of which use multiple processor elements and vote on the control status, and if the elements disagree, an emergency stop is initiated. Of course, with a PLC, for any significant size program, the issue is more likely to be bad programming than PLC failure, and thus you don't program these sorts of systems freezing one's ass off sitting on the factory floor...



1: If the thing is electrical it'll also need to comply with other directives, including the Low Voltage Directive, which is about electrical safety, and the EMC directive, to ensure it doesn't interfere with other electrical stuff, and be RoHS compliant, and have a disposal strategy and a bunch of other stuff.

 

[coulter]

All E-stops are required to be hard wired. ...

Could you give a source for this "requirement"? I don't know of any regulatory law on this subject.

Edited to add, "for the US."

carl