View Full Version : anti tie-down circuit

December 16th, 2002, 05:47 PM
I'm taking a course and I'm trying to figure out how to draw an anti-tie-down circuit. Does anyone know how?

Mike Williams
December 16th, 2002, 06:08 PM
I think you are talking about a circuit which requires an operator to have both hands on start switches in order to begin a machine cycle. This minimizes the possibility of an operator having a hand in the work area when a machine cycle is started. If one or both switches are "tied-down", the control will not allow a new machine cycle to begin.

The circuit is not complicated, what it does is important. This should be enough info for you to give it a shot. Post what you try and you will get help.

john paley
December 16th, 2002, 06:24 PM
Yes. I can tell you what it should do, but you figure out how to code it, OK?

The idea is to have two push buttons, usually palm buttons on a machine that operates in a specific cycle. These buttons should be physically located so that the operator must use both hands to operate both buttons. The operator must operate both buttons simultaneously to start the cycle, and hold them both until the cycle progresses to some "safe point"--i.e. where the operator cannot physicaly get his hand(s) into the pinch point. He then may release the buttons and the cycle will complete. For a punch press, this "safe point" would be when the die closes at the bottom of the stroke. For some machines this "safe point" may well be the end of the cycle.

Now for the anti tie down / anti repeat. If the operator holds the buttons throughout the cycle, the machine must stop at completion of the cycle. Before the cycle may be started again, BOTH buttons must be fully released and re applied. This prevents the operator from running the machine in a "continuous" cycle. It also prevents him from "tying" down one of the two buttons and operating the machne one-handed.

Easy to do in PLC ladder logic, although not legal, I think, for lots of machinery, like presses, which require a lot of redundant circuitry.

December 16th, 2002, 08:05 PM
Thanks Big Cheese and Mike Williams for your responses. Yes, that is what was meant by anti tie down. I've already completed it in relay logic, it's just that we have to convert it to plc's and we don't have a lot of resources at our disposal to figure it out. I'll try to work it out, though, and I thank you again for taking the time.

Eric Nelson
December 16th, 2002, 08:12 PM
Originally posted by john paley
The operator must operate both buttons simultaneously to start the cycle...

Just to clarify this for Darren. As far as anti tie-downs go, simultaneously means "within a 1/2 second of each other".

I doubt you'll find an operator that can operate both buttons at EXACTLY the same time. I've seen quite a few operators that can't even do it in the 1/2 second window... :p

By the way, that's also a hint for Darren that you'll need to use timers!... :D



December 16th, 2002, 08:18 PM
You might also check out [URL=http://www.bannerengineering.com] and look at their two hand switch set-ups...might give you some ideas..OPTO-Touch.....

December 16th, 2002, 08:44 PM
FYI, IMHO, for anti-tiedown, I'd never program it
where Safety, liability, etc. were involved.

There's a sub-$200.00 module made by NOLATRON (and
maybe others, I don't know) that does it all,
including the latching or unlatching feature.

I will say that a NOLATRON uses the N.O. contact and the
N.C. contact of the pushbutton.

See where that leads you.

December 16th, 2002, 09:06 PM
This was actually quite harder than it appeared. Will this work? Can someone right it better?

December 16th, 2002, 10:01 PM
As John mentioned, that and other issues with safety are not permitted to be done VIA PLC or control. Emergency stops, Anti Tiedowns and safety Lock-outs just to name a few. Although they can be diplicated in the PLC for status.

December 16th, 2002, 11:50 PM
That isnt totally correct, there are a few PLC's designed to work with safety. AB has the GuardPLC's (which they bought from another company that is still being sold thru others as are some of their light curtains etc).

Safety issues look at redundant factors and a plc can provide those BUT when using a plc for safety issues it is not to do process work...ie it can only do the things involved with safety.

I have had to attend an abundance of seminars/training recently because of corporation issues on safety. At the same time I have to deal with engineers that do not seem to care (I suspect budget issues) about code/osha issues on what is suppose to be done to conform to the rules/guidelines.

There are many tools out there to use, it takes alot of time/energy to determine what to use.

December 17th, 2002, 12:29 AM
I don't know how it is with AB GuardPLC, but I've worked with Pilz, there you have to use a Programme-block which i is Certified.
Using a Saftety PLC is no guarantee for that the code is right !!!!



December 17th, 2002, 04:31 AM
I must not be up to date on my safety. I thought O.S.H.A. insisted that emergency stops & lockouts were to be hardwired. If they have strayed from that I think it is a mistake.


Alan Case
December 17th, 2002, 04:40 AM
Roger. PLC control of a safety circuit is possible. BUT it is not done with any normal PLC. The Pilz and the AB safety PLCs are programmed with function blocks that are certified by a relevant authority, the controllers themselves use at least 2 processors that must match on the decision and the inputs are as far as I can remember are duplicated. Regards Alan Case

December 17th, 2002, 04:43 AM
Thanks Allen, Just wasn't aware, which isn't an unusual position for me these days.

Have a good one or two.

Rick Densing
December 17th, 2002, 08:08 AM
I know I have posted this before, but when I went back, my post was gone.

Allen Nelson
December 17th, 2002, 08:57 AM

Your code can be cleaned up quite a bit.

On your rungs 2 & #, you have one-shots of the PBs and the PBs themselves, the EN of the timer sealing in the one-shot of the PB, and the not DN of the timer (which would have the timer start over again when it times out, but the one-shot and EN would prevent that anyway - especially condsidering that you are only interested in the TT bit from the timer anyway).

Furthermore, having the -|/|- of the "Both have been pressed" on the timers will drop out the timers on the scan after you have the "Both" bit. Since the timers drop out, their TT do to, causeing the "Both" bit to drop out. Therefore the "Both" bit is essentially a one-shot already, so there's no need to add a one-shot to it to drive the "Cycle start"

Whenever you find yourself using multiple references to the same bit (or bits that the same bit drives), you know you've done something wrong. Here's a cleaned up version.


And just to reiterate what everyone else has, home-grown ladder logic is no substitute for Real safety switches or certified safety logic in safety PLCs.

Rick Densing
December 17th, 2002, 09:12 AM

Shouldn't your contacts for the buttons in the seal branch be NO?

Allen Nelson
December 17th, 2002, 09:28 AM
Rick - you are absolutely correct.

And this illustrates exactly WHY you don't rely on home-grown logic for safety - it's too easy to get it wrong, or to change it (accidentally or deliberately).

December 17th, 2002, 11:49 AM
Rick Densing

Good ladder, Rick, short and straight to the point, no mess.

I had drawn it the same except I used the timer DN --]/[-- bit instead of the --] [-- TT bit. Is there any reason for not using the DN bit? for my future ref.


December 17th, 2002, 04:59 PM
All right, I'm a little confused, somebody please help me.
I use the following code on all of my programs for a two PB cycle start on a machine. BTW, machines where I can legally go through a plc. Anyways, here's my code and you will notice I'm not using any timers. It will not allow a tie-down or a repeat. I don't know maybe I'm a little confused on the difference. One thing it does not do is require you to press both buttons within a required time. Is this ok? I know it should be done if your not going through a plc, but I just don't see the need for timers in the plc code. Is this a requirement or something?
Please help!!!!!!!!


X0 X1 M1
M0 |
---| |----------|
Start OFF

X0 X1
---| |---| |------------------------------(M1)----

M0 M1 M100
---| |-------| |----|------|/|------------(M20)----
M20 |
----| |-------------|

BTW, the two push buttons are hardwired N/O.

Rick Densing
December 17th, 2002, 05:13 PM
Roger- I don't see any reason.

Tim- By not having a timer you can tie down for one cycle. Say I take a C-clamp and actuate one button, then I lean into the machine and press the other with my hand, leaving one free.

I don't know if it matters as there are ways to defeat the timers, too.

December 17th, 2002, 05:27 PM
Thanks Rick, I couldn't see any either but maybe my eyes are bad.

Both ladders, with timer/without timer, can be defeated in the same manner. If I lay a brick on one button I can still use the other hand to cut off my free hand, neat huh? The only difference is that with the timer I must be very fast (depending on the preset) but without the timer I can take my time and savor the moment.

AS I sees it

December 17th, 2002, 05:31 PM
Tim, your circuit is flawed. As Rick has said, there is nothing to stop you wedging PB X0 in then using just one hand to control the machine through one cycle. The other hand could be inside the machine.

December 17th, 2002, 05:35 PM

I see, so what I have is just an anti-repeat, but not a fully functional anti-tiedown.

Last question, should I start using an anti-tiedown on my plc programs? The reason I'm asking is all our machines that come from Japan only have the anti-repeat for the start push buttons.

Thank You,

December 17th, 2002, 05:41 PM

Ricks ladder, which I also drew, can be defeated the same as yours but by setting the preset very low it limits the chances.

December 17th, 2002, 05:52 PM
Talk it over with your safety coordinator, then decide.

December 17th, 2002, 06:13 PM
FYI: STI has a decent site for information on safety devices and standards involved. Some standards you have to purchase but they offer OSHA and overviews of other standards.

STI did offer a plc but if I am not mistaken the PILZ safety PLC is the company that Allen Bradley purchased. They also bought a company that made safety light curtains for many manufacturers, I recently purchased some from Sick that are the same as the AB line.


They also offer a two hand ie anti-tie down device

December 17th, 2002, 06:44 PM
Speaking of STI,


December 17th, 2002, 07:24 PM
Hi Darren:

Anti-tie down is not normally done through normal PLC. It has to be hard wire. This can be done by using a couple timers and relay or You can buy an anti tie -down timer/relay in a 11 pins plug in timer. If you need a manufacture of this device, please let me know and I will furnish it to you.

another wat to do this is to used a capicitor to fire a one shot, this is not true nall the time.

December 18th, 2002, 12:51 AM
Jeez, after looking at your code, and looking at mine, it is obvious that my code is far superior. Just kidding. Nicely written Allen. I kept going back and adding logic to try and fix the code that I origionally started with instead of just re-doing. Probably explains the jury-rig affect of mine. Thanks for taking the time to critique. Please continue to do so in the future. Definetely need the help. Shows, me at least, how "user friendly" code is harder to write than complicated looking code.

December 18th, 2002, 04:42 AM
I use the following code on all of my programs for a two PB cycle start on a machine. BTW, machines where I can legally go through a plc.

Just curious Tim, I was under the impression that now you have to have a safety relay or safety plc for an anti-tie down. Is there certain machinery where it is not required?

Rick Densing
December 18th, 2002, 07:24 AM
In my case, I usually have a light curtain or other safety device between the operator and the machine. The two hand setup is just extra and helps stop accidental starting of the machine. With the safety devices, no one should get hurt, but you could break the machine.

Allen Nelson
December 18th, 2002, 08:17 AM

All I was doing was simplifying your code, removing double references to the pushbutons (the PB one-shot along with the PB on your timer rung), etc.

The code can be simplified further. First off, take the last rung (written correctly):

---+----| |--------|/|--------| |--------|/|----+----( )
| |
+----| |--------| |--------| |---------------+

Since -| |- BUTTON_A and -| |- BUTTON_B appear on both branches, the rung can be reduced to:

-------| |---------| |----+---|/|--------|/|----+----( )
| |
+----| |--------------+


And then you ask yourself "Why do I need two timers?", and so you remove them, and you are left with this:


which, gee, is the logic that Rick posted (only using the NOT /DN bit instead of the /TT bit).

But you wrote it, not me. I just eliminated the one-shots, since the timers were only going to run while the PB was pushed anyway, and removed the logic that was making the OK TO START bit a one-shot.

I did cheat and add the seal branch, since when I was done, I could see that the OK TO START bit was only going to be ON for the duration of the shortest timer, making it neither a one-shot nor a sustained bit (but plenty good enough to initate a start cycle in the way that you intended it to be used).

ganutenator (Mike is it?), you said:
I kept going back and adding logic to try and fix the code that I origionally started instead of just re-doing.

One rule of thumb in debugging I use is that I know I'm doing it right when I'm removing code. Adding code is usually just a bandage on top of a wound. You have to remove existing bandages, and clean out the wound for it to heal right. That isn't the same as re-writing it

Greg Gauper
December 18th, 2002, 08:26 AM
Something else to consider....

I have worked on a number of machines in which the required number of operators could vary from 1 to 4 people, depending on how big the part was that was being made by the machine (bigger parts = more people). This could change from day to day depending on the production run requirements.

These machines all had operator T-stands with dual palm buttons, connected to 'Joy' plugs for quick set up. On each T-stand all of the palm buttons had a N.O. contact and a N.C. contact. On each pair of palm buttons the N.C. contact were wired in series to a single input thru the 'Joy' plug connector and all of these inputs were programmed in series to trigger a 'time delay off' timer for the anti-tie down circuit. So once any single pushbutton had been pressed, the series circuit lost power and started the timer. Each individual pushbutton had its N.O. contact wired to a discrete input. All of the active pushbuttons on each T-stand had to be pressed within the anti-tie down time period as sensed by all of the N.O. contacts.

The thing that made this flexible was that each T-stand 'Joy' plug could be replaced with a 'Dummy' plug that had the inputs jumpered out for any T-stand not being used. The anti-tie down logic didn't have to be reprogrammed when they changed the number of operators. And no, the machine was not permitted to run with all 4 dummy plugs installed. The machine also had hard wired light curtains and safety mats for primary safety, the anti-tie down circuit was for redundancy.

December 18th, 2002, 09:10 AM
Just another flavor
HOLEY SMOKE RICK, things sure are easy once you know, huh?
By the way I used your ladder, just rearranged things in PAINT.


Rick Densing
December 18th, 2002, 09:27 AM
Roger- if you make it a .jpg, it will display

December 18th, 2002, 09:28 AM
From experience I would not let a PLC control the anti-tie down circuit. Whether it is OSHA approved or not. I once ran a supervisory
bit through the PLC to maintain fluid level and then sound the alarm.

The MITSU puked and hence no level control, thing overtemped, bled all of its refrigerant into the air(10 55gal drum$)$$$$$$, and did not sound the PLC controled alarm. Embedded intelligence only works if its scanning.(I love the scare crow in the Wizard-of-Oz, if I only had a brain)

Another story: The DH+ networking all MMIs puked lost all control, no stop/starts, no VFD% control (already running), machine was on its own with no way to input any commands. This stuff happens. !!ESTOP!! ...then crunch...then Alot of lost time. (This is the reason why cars have airbags and houses have smoke detectors, and Murphies law thrives on imperfection).

Please design for failure. (Watchdogs, Hardwired safetys, un defeatable lockouts,ect.)

"Its hard to idiot proof when idiots are so ingenious"

December 18th, 2002, 03:16 PM

We have safety lights on all our machines. I'm wondering if this is why this code came that way from Japan. You have the saftey lights, but I would still rather start using the proper anti-tiedown like Rick D.
I don't know Japans standards, but I've learned most of my programming from the Japanese Engineers.

You'll have to look up the safety requirements on your particular machine. I'm not talking about a press, its just what Rick is doing adding a ___________ feature, "I'm hesatent to say ..safety, because the plc's I'm going through are no way, control reliable". To my knowledge you can program an anti-tiedown, but this cannot be considered in no way apart of your control reliable circuit,except on some plcs as other have pointed out.

Have a good Day,

December 18th, 2002, 11:08 PM
Allen, once again I am impressed. Yes, it is Mike

Have been experimenting around with the ole Boolean Algebra thingy.

lets let

Button_A = A
Button A Timer = B
Button_B = C
Button B Timer = D
OK to start = E


---+----| |--------|/|--------| |--------|/|----+----( )
| |
+----| |--------| |--------| |---------------+

could be written as

(A*B'*C*D') + (E*A*C)

could be reduced to


and further reduced to


which is the same as

-------| |---------| |----+---|/|--------|/|----+----( )
| |
+----| |--------------+

or at least I think.

Been messing with this stuff just a little. Has anyone used Boolean algebra to simplify code? How do you deal with oneshots, timers, scan, etc.?

Short painful story:
Had this boss once that would draw these Boolean Logic Diagrams (at least that is what he called them) which were drawn up as logic gates. He would hand them to the PLC programmers and they would go to town programming the PLC code. Nice thought, but it would make for some disaster code because the code wouldn't appear to follow any logical order. Everything would be dumped in one program file, no organization. A coil that would be used to fire a sequence in rung 4 might not appear until rung 400. I reduced a page of Logic gates down to only one N.O. contact once using Boolean Algebra. He was red. Contributed to me finally getting laid off. hence the www.prousucks.com domain that I purchased for $11.00. Now, I am being paid by his ex customers to fix the logic. Can't seem to escape it.

So I defintely need some pointers in reducing code more efficiently. I am going to study the above posts some more before I turn in. I wish I could reduce code that quick.