2-Hand Anti Tie Down

[kcor 1]

Have a project that will involve using AB Micrologix 1200 plc's to control a press type machine? On the prototype, I used a Allen Bradley light curtain connected to a AB safety relay that would give a signal to the PLC to indicate that a operator has broken the light curtain and loaded a product into the Press unit, I then used Two Hand Anti Tie Down Switches (Banner) that utilize a small control board that both switches connect into, that if both switches are made within .5 seconds of each other, you get a input to the PLC to start the cycle, and if the product was loaded, but now there is nothing breaking the light curtain, then it is okay to start the cycle, but if the light curtain gets broken again before the cycle is completed, e-stop the machine. Am looking at building over 70 of the units, and have been asked to save as much money as possible due to the amount of units and the budget that has been approved. What I am not so certain about is this - I can save quite a bit of money, when multiplied by over 70 units, if I just build the two hand anti tie down control into the AB PLc code, and just use two palm buttons connected to two seperate inputs into the plc. Since this is a press type operation that I will be using the AB PLC's on, I am not quite certain if there are any legal issues according to anytype of OSHA requirements that I am not aware of, as far as safety issues. I have also asked our internal Safety folks, and they have said that they are not sure if it is okay to do this, since it is a press operation, but they are not sure. I need to order the parts ( while the money is there), and was wondering if anyone out there who may have a answer, or could direct me to a site where I may be able to find a answer. Thanks I have searched the OSHA website that was posted on a earlier post that I made, and have been unable to locate a answer. Has anyone else

[panic mode 246]

don't build your own safety circuits, buy proper two hand safety controllers and apply it according to docmentation.

[Spedley 0]

I wouldn't if I were you. I don't know about the rest of the world but in the EU the machine needs a double safety circuit which isolates the electrics and air etc where there is any risk of danger. This means it must be hard wired and that anything which could still be moving after an emergency stop (or breaking the light curtain) must have a physical guard. You would not get a safety cirtifcate here and the machine would literally have to be isolated and cordoned off until it was corrected.

[TConnolly 13]

Is this a clutch type punch press? If so then stop right now. The Ml1200 is unsuited for this type of press. AB has a redundant clutch press control/monitoring package based on redundant ML1500's that is fully tested and certified. If this is a slower press then there are still several issues you have to deal with. First one I see right away is wiring the light curtain into the PLC. You may monitor the light curtain status with the PLC and use that status in the course of your program to operate IO, however, the light curtain input to the PLC CANNOT be a primary control element. You must wire your machine such that if the light curtain is blocked then power is deprived from all acutators with a safety concern, even if the PLC is faulted or even if a PLC output is fused on. I generally do this with a redundant safety relay wired to provide power to the PLC card to which safety related IO is connected. The safety relays must be a self monitoring guided contact type safety rated relay. See attached PDF for a sample wiring diagram. A two hand anti-tie down is easy to implement in the ladder. HOWEVER you cannot do this as a safety circuit. Therefore its only permissible if a two hand-anti tie down would not really be required by OSHA anyways. BST XIC PB1 NXB XIC PB2 BND XIO K TON AntiTieDown .001 500 0 XIC PB1 XIC PB2 XIO AntiTieDown/DN OTE K But there are some questions to ask yourself: Can the machine be stopped should the light curatin be breached before a fast moving operator can reach the work area? Is there any way to reach the work area besides through the light curatin? Is there any removable interlocked or non-interlociked safety shield which would allow the opeartor to circumvent the light curatin and reach theh work area? Is there any possible PLC failure, including welded output contacts or shorted outputs that would allow any actuator to function at the wrong time that would not be covered by a hard-wired safety system? About the ML1200: Its a very nice little PLC. Don't push it on the output contact loading however. Be very conservative. ----------------------------------------------------------------- While we are on the subject: One of our local vendors called me several years back and asked if I could provide some independent evaluation of an accident. A woman who worked for them had gotten several fingers broken in a machine. The machine was a compact 2 ton hydraulic open C-frame press equipped with two hand anti-tie down controls and soft touch buttons. All control components appeared to be in good shape and were functioning correctly. There was no explanation for the unexpected machine actuation. When I spoke with the woman she less than candid and embarrassed about the accident, but otherwise very friendly. She was a pretty young woman and was obviously “enhanced” with a sizable quantity of silicone. I was at a loss to explain the accident until the next day when it hit me what had happened: She had leaned forward to reach into the work area and had actuated the soft touch buttons with her breasts, which were right at the button height for someone her height. She also knew that was what had happened and that was the reason for her embarrassment at explaining how the accident happened. As she was the only woman in the shop and I know how "us guys" can be, I understand why she was emberassed. I called the plant manager and related my suspicions. He called back later to tell me that she had confirmed that was indeed what had happened. The ring guard on the soft touch buttons was proven to be in-adequate. I suggested that they relocate the buttons to be on opposite planes, one facing right and one facing left and inform the press manufacturer and soft button manufacturer to re-evalute the design. All in all, everyone was lucky the woman was not a litigious individual. I tell this story not because she accidentally started the machine with her breasts (it’s not funny), but to illustrate how even the most carefully thought out safety system can over look something. wd07.pdf Edited 12 Sep 2005 by Alaric

[kcor 1]

Thanks for all of the inputs. Very Helpful

[brisutterfield 0]

Wouldn't the two hand anti tiedown ckt look like this... XIC PB1 XIC PB2 XIO K TON ANTITIEDOWN .001 500 0

[kcor 1]

Once again thanks for info. To answer some questions. The machine can only be loaded through the light curtain, there are guards on all sides, and top that can only be removed by the use of Allen Wrench tools. Since the operator has to reach through the curtain to load, then after looking over the post, then it appears that if the light curtain is broken at anytime, then as was shown in the pdf that was posted, then break the commons to the plc outputs through the light curtain safety relay. Once the operator removes hands from the load area, then any advice on how to reset the light curtain, so that once the two anti tie down pushbuttons are pressed, that are outside the load area, the machine will cycle. The circuit that I saw in the AB safety catalog, shows that the dual channel monitoring safety relay has a reset input that in the catalog is connected through a push button switch. Uncertain as to how to automatically reset the safety relay once your hands are not breaking the curtain anymore, so the outputs of the plc can operate once the anti tie down switches are pressed. In other words, is there a way to connect a light curtain through a safety relay, so that if you reach through the curtain, the safety relay outputs will open( which would break the commons to the outputs as shown in the posted pdf) , but once you remove your hands from the light curtain, the safety relay will reset and allow the voltage to be connected to commons for the outputs of the plc. Can this be done form the light curtain itself?, and if so, any advice would be aprreciated. I will be meeting with my local AB rep on Thursday, who is also the local AB safety expert, so hopefully between what I learn from the post on this site, and from him on Thursday, will be able to complete the design by Friday. Thanks alot

[Snerkel 0]

If you can get double pole 2-hand tie downs then you could use the operation of these for both the intended operation and for reseting the light barrier.

[gravitar 3]

Here's the little I know on the subject: An OSHA-recognized 2-hand anti-tiedown circuit must be implemented with circuitry recognized as "Control Reliable". I believe this means "Always fails into a safe condition". A standard PLC is *NOT* considered control reliable. There are ways in which a PLC can fail into a potenitally unsafe condition. There are PLCs that ARE control reliable and can be used for this purpose. I got to play around with A-B's safety PLCs and they actually have an anti-tiedown instruction, spcifically for this purpose. Rather than a safety PLC though, most of us use a dedicated anti-tiedown circuit to monitor the start buttons. It sounds like you have one of these. That should satisfy the need for control reliability, if wired in properly. Other people have already posted ideas as to how to wire it. As for the light curtain, they usually have a self-resetting mode of operation. the light curtain's contacts open or close in real-time, based on its current clear/blocked status. I don't see a problem with using them that way, if you have a machine that can quickly cease all motion. As for HOW quickly it stops, there is an equation that should be included in your light curtain documentation that will tell you how far away the light curtains must be mounted from the moving equipment. Like everyone else has said, the e-stop/light curtain circuit must operate independently of the PLC, not THROUGH the PLC. It is fine if the PLC monitors the state of the safety circuit, but as a general rule of thumb, you should be able to stop the machine and deactivate all sources of energy with the e-stop or light curtain, even if all the outputs of the PLC were forced on. As for hard-guards.. The OSHA guy will tell you that you must not be able to reach "over, under, around or through" the guards and reach a pinch point. If you have a hinged or otherwise removable gate for maintenance, it must have a safety gate switch tied into the E-stop string, even if you have an allen-bolt to hold it shut. There are probably specific safety requirements over annd above what we've discussed that OSHA spells out for your particular type of machinery. For example, we have a lot of robotic workcells here and I just found out that there is a OSHA-required maximum distance from the floor to the bottom of the light curtain for robot cells. Many of our robots that were just delivered to us from a "large, reputable, well-respected" integrator did NOT meet these requirements. Since you're making 70 of these, it would be prudent to show your design to someone licensed to perform a safety analysis. I know A-B GTS will do this, and there's probably several other companies in your area that would be qualified for this. It sure would be a shame to have to go back and modify 70 machines after the fact, if you made a wrong assumption!

[kcor 1]

thanks alot have used a couple of light curtains only for guarding only. If they where broken, after a safety cover was removed, estop the machine, but first time trying to use it to indicate if a product is loaded before the cycle can be started. Hopefully with the info from here, will have it all worked out by the end of the week.

[gravitar 3]

You can use a pair of light curtains for controlling the cycle of a machine, as opposed to a safety-related function. That's fine. Just make sure you are addressing safety somewhere else! We have experimented with using TWO pairs of light curtains on 4-station dial tables. One for the operator load station, and the other for the three other positions where all the "action" happens. That way the operator can load a part and not worry about the table turning when their hands are on it, yet you can still spear a nut in, weld, and unload without the operator interrupting the cycle. That's whats cool about being in controls, you can do just about anything if you're creative!

[TConnolly 13]

An anti tie down is a device that requires both buttons to be pushed simultaneously. If both are not pushed then the anti-tie down has to lock out until both buttons are released. Because its difficult to push the buttons at exactly the same time, a half second delay between buttons is usually allowed. If either the left hand or the right hand button is pushed then the 1/2 second timer starts. When the other button is pressed then the timer is locked out and reset. However, if the timer times out first, then the start bit cannot be made and then both buttons have to be released. This is to preven an operator from wrapping a button with tape to hold it down, or jamming something in it to hold it down. If he does, the timer times out and the system cannot operate. There is actually an example on this site at: http://www.mrplc.com/kb/index.php?page=ind...x_v2&id=13&c=12 But I have to emphasize again, that the PLC absolutely cannot be the primary safety control element. If you have a light curtain and it can function as your control reliable system then you can do the anti-tie-down in the PLC, but you had better be dang sure you have it right - the PLC is a solid state device that runs software. It is fallible. Edited 13 Sep 2005 by Alaric

[kcor 1]

Thanks alot Will actually be using a anti tie down controller instead of the plc, will be using the light curtain to monitor if anything enters once cycle has started, if so drop out the MCR relay, which will kill the 24vdc to the outputs of the plc, as well as the main air valve. Info on post has been helpful.

[TConnolly 13]

That aiv valve is also a safety component. Once it is de-energized it should vent all downstream pressure so that there is no stored energy in the form of compressed air trapped in the system. An air cylinder can keep moving and applying force if the downstream pressure is not vented. Make sure all valves with a potential safety issue are selected to vent when unpowered. It doesnt do any good to vent the master valve if the cylinder control valve has a three position closed center spool - the cylinder would still be applying force and have energy stored in it.

[jstolaruk 9]

I don't know the size of your ram but be aware you may (probably) need a check valve AND/OR rod-lock in order to keep the ram from dropping once the air supply has been dumped. Edited 13 Sep 2005 by jstolaruk

[Guest rock]

thanks using a double acting valve, with a center off, has to be energized to come down, and then energized to go back up. If you loose power, will stay in position, not move up or down. required by the safety folks where i work.

[panic mode 246]

center closed 3-position valve is all fine and dandy but it is not safety device. you can check rosco cross monitoring safety valves (or festo). bottom line is this: press is not a toy or general machinery and that's why it has dedicated safety standard just like robots. in Canada (also applies to USA) it is called Z142-02. This has to be followed in addition to Z460-05 and Z432-04.

[Spedley 0]

I once wrote a program for a machine like that. There was a table that went fron horizontal to vertical to allow for operations on the part but if you pressed the e-stop, all air was vented and the table would slowly drop down. The problem was fixed with a 3 way valve but why it was designed with a fundamental flaw I don't know?

[TConnolly 13]

It won't necessarily stay in place with pneumatics nor will it necessarily render the machine in a safe state. Pneumatics is different from hydraulics because air is a compressible/expandable medium. Assuming this is a vertical downstroke press..... What happens when the rod is travelling downwards and the Emergency Stop is activated? The valve will center. Air will not flow into the cyclinder, but air also cannot flow out of the cylinder - the air trapped in the cylinder will expand/compress until a force equilibrium is reached - during which the piston will move or apply force. At the instant before the valve closes, the rod end will have been vented and air pressure is being applied to the piston. When the valve closes, air pressure will be trapped on the top side of the piston. This air will continue to move the piston, expanding until it meets resistance. That resistance could come from compressing the air trapped in the rod end of the cylinder until rod pressure rises to a level to counterbalance the force on the piston. The piston will continue to downstroke in order to compress the rod end air The resistance might come from the workpiece. The resistance might also be an operator's hand which he just inserted into the work area. Now not only is the operator injured, he is stuck, and cannot be easily freed without turning the machine back on so the rescuer can operate that closed center valve to relieve the force trapping the operators hand. In this case the emergency stop just made a bad situation worse because it left the machine in a stored energy condition without a way to safely release it. OSHA requries that stored energy in the form of compressed AIR be safely dealt with. You are also required to safely release or contain any mechanical stored energy, such as a spring, or any stored energy in the form of gravitational potential energy (falling objects). I don't know the details of your machine, but I'm just trying to help you brainstorm through the possibilities. Edited 14 Sep 2005 by Alaric

[dellae 0]

When using interposing relays/contactors in safety circuits, they may ONLY be of the 'guided contact' type, such as from Schneider (d, K & F range), Rockwell and SOME others. For Robot cells and other Death & Distruction machines the safety circuits must be to Category 4 / SIL3. That is dual monitering circuits, dual shut down circuits, cross circuit monitering and Reset circuit feedback monitering from the final energy switching devices. There are a number of regulations in most countrys that cover this, eg Europe=EN954-1 and many others. There are many pitfalls for the unweary - I make a nice living rebuilding machines companys import and then discover they are not aloud to use until they have been certified to the relavent standards. For a 2 hand controller, if you need to use one then you are not aloude to use a standard plc. There are dedicated Safety controllers for this, Schneider, Rockwell, Schmesial are some of the main producers. Omron makes a very nice compact Safety PLC (NE1A) (beats Rockwell and PILZ)