programming a Two Hand Tie Down


Thread Starter

Anibal Hernandez

Could anyone out there send me an example ladder logic for programming a Two Hand Tie Down (Switch 1 and Switch 2) using either GE, Koyo, Allen Bradley or Omron? Thanks Anibal Puerto Rico
I think you should be careful running the two hand tie down function through the PLC. As with any safety circuit you should not rely on the PLC. Use a good UL Listed control external to the PLC. Just my opinion! Joe Rizzolo
I don't know what your local regulations require but in Britain software control of two hand, anti tie down is not permitted. This must be hardwired. Regards, Bryan Weir Scotland, UK.

Michael Griffin

If you are planning on using "two hand control" (anti tie down) to protect an operator from serious injury, might I suggest using a "two hand control" relay in conjunction with your PLC? There has been a change over the last few years in what many people consider to be accepted practice in safety applications, and this includes moving certain safety functions out to dedicated hardware. You can find information on anti tie down relays (and other types of safety relays) and their application in the literature which the manufacturers provide. The largest manufacturers of these devices seem to be Jokab and Pilz, but a number of other companies are also in the business (Schmerzal, Banner, Siemens, etc.). ********************** Michael Griffin London, Ont. Canada [email protected] **********************
We have systems designed by Helm which do use the PLC for two hand control, but there are redundant SLC 500s used for the clutch/brake and for the two hand trip. The main point is to have a redundant interface or what is considered to be a fail safe design. This means that the trigger devices need to be checked on open and close and there needs to be more than one contact per hand to verify the operation. Then the redundant PLCs can make a determination of the correct action. I am not sure if I can share this info with you without checking with them first, they have been very difficult about program ownership. If you would like, I can do this or you can contact them directly to discuss proper implementation at (419) 893-4356. Dale

Anthony Kerstens

You people are giving me a chill up my spine. To make sure nothing unsafe gets implemented please read this complete message. -------------------------------------------- ANYBODY DOING WORK WITH MECHANICAL PRESS EQUIPMENT SHOULD GIVE THE PRESS CODES A GOOD READ. (and follow them to the letter) -------------------------------------------- ANSI B11.1-1988 4.10.1 Ladder diagrams. "A ladder diagram designed for electromechanical relays shall not be converted for use in a programmable logic controller or other programmable microprocessor logic system nor permitted to be used for the clutch/brake control or trip control. ....." CSA Z142-M90 5.2 Electronic Controls (to lengthy to type) -------------------------------------------- These do not outlaw PLC's, but say you must not just copy a circuit into logic. They impose requirements on reliability, self-checking, and external safety circuits (like brake monitors and anti-tie down monitors, or a redundant PLC as suggested by Dale Witman.) Just so we're clear. Trip control on a press is the two hand anti-tie down buttons being discussed here. You'll also find in the codes specifications about the size and spacing of these buttons. The easiest way to comply with the safety requirements is to buy equipment designed and certified as press controls. Or better yet, get someone WHO KNOWS WHAT THEY'RE DOING to do it for you. (Rockwell, ugh, has a group dedicated to press control and provides complete solutions.) THIS IS NOT SOMETHING TO JUST FOOL AROUND WITH. A press is a machine that can very quickly maim, amputate, or kill people. For your and your user's safety, please buy/borrow a copy of the press codes. And read them. Anthony Kerstens P.Eng.

Ralphsnyder, Grayg

Why would you want to assume the responsibility for designing an anti-tie down circuit? In the interests of liability and not re-inventing the wheel - I recommend - and have always used - someone else's anti-tie down relay. Most PLC manufacturer's will tell you not to use their product in a manner such that when it fails it could lead to a life threatening event. Eventually someone that doesn't know what they are doing is going to modify your program or make a wiring change that invalidates what you did. When your program or PLC fails then you are the bad guy (or girl). When their anti-tie down relay fails then a good bit of the burden is relieved from you. Of course, you have to apply it per the manufacturer's instructions. As stated by others, there is a whole bunch of anti-tie down relays available for your consideration. Many are around or under $100. Grayg Ralphsnyder Process Control

R A Peterson

Here also. The odd part is that I have seen it implemented by using a two-handed PB module with its output sent directly to the PLC and the PLC then initiating the desired action. The hard wired stuff makes a lot of sense for punch press type applications, but is really not applicable directly to a process controlled by a PLC. I used to write software for dial machines and the customers would put light curtains on the load/unload station so the operator could not start the machine cycle until his hand was out of the machine. They would also use 2-hand PB modules with the output sent to the PLC. The PLC would not allow a cycle start until the light curtain was clear AND the two hand PB module gave its signal. On a few machines that required a manual clampup, they would use one of the two hand PBs as the clamp signal. I.e. - if the light curtain was broken, and just one cycle start PB was pressed, that was a clamp signal. If the light curtain was free, and both PBs were pressed, that was a cycle start signal. I never liked this approach much, but the way the machine was mechanically designed I figured it was not worth arguing over as it was not a punch press, and was designed in such a way as getting ones hands caught in something was just about impossible. I generally lobbied for hardwiring the light curtain output into the table control so it could not move while the light curtain was broken, but alas the customer did not like this approach and had the PLC enable the table rotation. Eventually they gave up trying to make the load/unload stations safe "enough" and installed automatic loaders and unloaders. In my mind, this is the only "safe" way to implement loading parts into a dial type machine, and still have the machine practical. Of course this reduces the need for operators, which may or may not be good, depending on your POV.

Curt Wuollet

I can send you C code if you want. Of course it's for educational purposes only. These guys can be pretty uptight. Regards cww
If I am not wrong, Pilz has PLC program for two hand operation which is certified by certain organization (TUV?). That implies, safety operation controlled by software is allowed, but needs to be certified. Mark

Michael Griffin

At 11:50 26/01/01 -0500, Anthony Kerstens P.Eng. wrote: >You people are giving me a chill up my spine. >To make sure nothing unsafe gets implemented >please read this complete message. > >-------------------------------------------- >ANYBODY DOING WORK WITH MECHANICAL PRESS EQUIPMENT >SHOULD GIVE THE PRESS CODES A GOOD READ. >(and follow them to the letter) >-------------------------------------------- I'm not sure the originator of this thread was talking about a stamping press. There are a lot of anti-tie-down applications with machines that are much more benign. Many might give you a bruise or a scrape at the most. However, if we are talking about two hand control, then the originator probably has some reason to be concerned about the safety of the machine. I too am reluctant to give what might be bad advice. At the very least then, an anti-tie-down relay should be considered. This may be considered for machines which are only slightly or moderately dangerous. An application specialist from a safety relay company may be able to give proper advice on the particular application. Certainly nobody here can, as we have no idea what the application is. If a stamping press was the application though, then an anti-tie-down relay by itself is completely inadequate. A proper press control package is required. I have a bit of experience working around stamping presses (some years ago), but I would never even consider designing my own press control. These machines are simply too dangerous for amateurs. There was at least one response on this subject that seemed to imply that some of the other replies were over-reacting a bit. I consider these replies on this subject entirely reasonable, if a bit strongly put. You can't work long in the metal stamping business without coming to the realisation that bad things can and do happen to other people if someone is careless with safety systems. I didn't work anywhere that had an accident, but I've heard the stories. I believe that Mr. Kerstens used to work for AG Simpson (who have a lot of presses), so I am sure he would agree with me on this. There are a lot of stamping presses in southern Ontario, and in the past not all of the control systems were built by people who knew what they were doing. There are laws now requiring that machine designs be reviewed and approved for safety (PSR under section 7 of the Ontario Health and Safety Act), and the reason for these laws is because of past accidents. I appologise if anyone finds this letter boring, but I am reminded that this is a forum in which there are people with various levels of experience and where some people are learning new things. I wanted to be completely clear that this is not one of these subjects where reasonable people are currently debating both sides of the issue. Any difference of opinion has most likely been due to different assumptions as to what the application was. Some machines are more dangerous than others, and some are in a league of their own. ********************** Michael Griffin London, Ont. Canada [email protected] **********************

Anthony Kerstens

I won't apologize if my previous comment seemed a little strong. I would rather live with scorn than live with a bad conscience. That said, if the application is not for a mechanical press, then look at CSA Z432-94 "Safeguarding of Machinery" (or its ANSI equivalent). This code says very little about two hand controls, but it is filled with reams of useful information about controls, estops, guarding and more. Anthony Kerstens P.Eng.

Hakan Ozevin

Two hand control can only be done with certified relays (e.g. Pilz, Tesch, Siemens, Schneider) or a certified PLC (e.g. Siemens 95 F/P). The rest of the solutions are not real 2 hand control, and should not be done. To pay an additional 200-300 USD for this function might seem too expensive, but it is nothing compared to human life or injury. Hakan Ozevin
im not sure if im too late.... please read whole post, conditions/systems vary greatly and its the programmers responsibility of any ill effects: button1 AND NOT button 2 OR button 2 AND NOT button 1 OR ondelaytimer1->start ondelaytimer1(15 ms?) -----reset ondelaytimer if not button one and not button 2. button1 AND button2 AND NOT ondelaytimer1(bit which says timer has reached preset time and turned on) AND NOT clamp_sealed->close_enabled the press (in this case) should not open if user lets go!! it should hold(off the push up pressure, but not the other one) . a light which goes off when clamp_sealed... or a bell that goes off would let user know they can let go.
We are an Industrial Distributor of Precision Hand Tools, and have a customer requesting info
on "Schmerzal". I am unable to find any information, or maybe you can cross over part
numbers of theirs, either way, I need to find out about this. Please respond ASAP.

Thank you, Laurie/Sales
[email protected]

To clear this up: If the two hand control function is used as a safety function (which in 99% of cases this is true) programmable electronic devices (eg. a PLC) should not be used. The two hand control must be implemented by specific safety component (as recognised in Europe under the machnery directive), i.e a Two Hand Control Safety Relay. The only exception to the above is to use a safety programmable device such as the micro safe PLC, e.g provided by Pilz (PNOZmulti). If you require any further claification please let me know.