Anti -Tie Down - Non Repeat - No Timer

[raydog26]

hrm yes it seems I was way off with my thinking then. thanks for clarifying that. If im not mistaken what i read for proper safety requirements you have to use a timer relay ? if that term is accurate. So basically if both buttons are not pressed within a certain window of time like 1 second it will not allow the machine to cycle and both buttons would need to be let go for it go again.

 

[L D[AR2P#0.0]]

EDIT: Because the LogixPro Simulator program does not have any provision for pressing two pushbuttons at the same time, I set up the first two rungs of the program as an optional "button presser" routine. If PB3 is enabled and pressed, it causes PB1 and PB2 to be pressed at the exact same time (which is almost impossible for a human operator to do).

Siemens PLCSIM has an option to run one scan at a time allowing you to "press" as many inputs as you like simultaneously. Is this an option for you?

 

[Lancie1]

LD,

That's a wise thought. I just tried LogixPro in single-scan mode. The problem was that (although the PLC only did one scan), the LogixPro pushbutton panel (I/O simulator) only allows "pressing" a button using the mouse. I only have one mouse connected. Maybe I could connect two and use one to press each simulated pushbutton....

Well, I connected a second mouse, but still LogixPro only gives me one "hand" to press one button. I guess I am stuck with simulating the pressing of two buttons with one additional button.

 

[Clay B.]

hrm yes it seems I was way off with my thinking then. thanks for clarifying that. If im not mistaken what i read for proper safety requirements you have to use a timer relay ? if that term is accurate. So basically if both buttons are not pressed within a certain window of time like 1 second it will not allow the machine to cycle and both buttons would need to be let go for it go again.

Actually in the "real" world you would be required to have redundant circuitry know as a "Safety Circuit" as well as have the realisstic expectation that both buttons could not be pressed at exactly the same time. "Exactly the same time" is when both inputs go high or low in the same scan.

In the real world you have "Bubba". Bubba is an idiot who is determind to hurt himself on the machine you design. You job is to prevent Bubba from doing this. So anti tie down, redundant circuits are required.

Paul manged to hint around about his age with his program. What he did was create a "timer" out of the scan. Before PLC's had accurate freee running clocks this was needed to preform time functions.

While your teachers program would never be used in the real world without a lawsuit it is not bad in that it shows how you get internal bits to interact with I/O

Side note: Most stamping machines and presses would also have a light curtain or some oher form of guarding around the press to prevent Bubba from being Bubba.

 

[TurpoUrpo]

Actually in the "real" world you would be required to have redundant circuitry know as a "Safety Circuit" as well as have the realisstic expectation that both buttons could not be pressed at exactly the same time. "Exactly the same time" is when both inputs go high or low in the same scan.

In the real world you have "Bubba". Bubba is an idiot who is determind to hurt himself on the machine you design. You job is to prevent Bubba from doing this. So anti tie down, redundant circuits are required.

Paul manged to hint around about his age with his program. What he did was create a "timer" out of the scan. Before PLC's had accurate freee running clocks this was needed to preform time functions.

While your teachers program would never be used in the real world without a lawsuit it is not bad in that it shows how you get internal bits to interact with I/O

Side note: Most stamping machines and presses would also have a light curtain or some oher form of guarding around the press to prevent Bubba from being Bubba.

I would like to add, that even when appearance of Bubba is that Bubba is stupid, in reality Bubba is very clever in finding all possibilities of hurting himself or worse someone else with machine he's using.

 

[Lancie1]

So basically if both buttons are not pressed within a certain window of time like 1 second it will not allow the machine to cycle and both buttons would need to be let go for it go again.

Raydog, Yes, that is the basic logic (which can be demonstrated with a PLC program), but the safety rules for real presses require that special safety-rated redundant-circuit pushbuttons and safety relays be used. The safety relays have self-checking circuits that detect a failure on any circuit.

 

[Lancie1]

Raydog,

Look at the attached picture and see how simple the logic is when timers are allowed to be used.

 

[kamenges]

!!!!! SYAMNTICS ALERT !!!!!

We have been jumbling together all sorts of concepts that have their own definitions and lumping them under "anti-tiedown". This is because the systems we tends to work on contain all of them so we think of them as a unit. They are not.

Anti-tiedown just means that an action cannot be retriggered until all concurrent operator have been released if any concurrent operator is released. It says nothing about when the operators are actuated relative to each other. That brings in the concept of simultaneaty (or is that simultaneousness). In addition, anti-tiedown doesn't automatically require non-retriggerability. These are all separate items and concepts that combine for effective two-hand control safety. But if someone asks for anti-tiedown capability it doesn't automatically require the other items.

Keith

 

[Lancie1]

Keith,
You are correct, we are lumping a lot of other control methods in with the phrase "anti-tiedown". Just like we misuse the term "flip-flop" to mean a type of logic program where one single-pole pushbutton alternates a device from On to Off. It is all about semantics, but good luck with that campaign here!

 

[pal]

Paul manged to hint around about his age with his program. What he did was create a "timer" out of the scan. Before PLC's had accurate freee running clocks this was needed to preform time functions.

Clay B.
Although I do have a few years under the belt, and I started using the technique for delays less than the 1/10 sec. from those earlier timers, the main use I have is for rs232 ascii messages. Some of them required considerable decoding and encoding , so I spread the computations over 2 or 3 scans to avoid big changes in scan times.
regards, Paul

 

[iant]

hrm yes it seems I was way off with my thinking then. thanks for clarifying that. If im not mistaken what i read for proper safety requirements you have to use a timer relay ? if that term is accurate. So basically if both buttons are not pressed within a certain window of time like 1 second it will not allow the machine to cycle and both buttons would need to be let go for it go again.

There are many requirements for safety systems - ALL
relate back to 'Risk Analsys'
the easiest thing to remember is a PLC is Primarily NOT a safety device.
there are exceptions.
if you want to look at some basic safety sites http://www.pilz.com/
http://www.sick.com/

there are many more - it has become a specialised field over the last 15 ~ 20 years