You ain't going to believe this but.....

You ain't going to believe this but,,,,, yesterday I was asked to do something I thought only existed in "HNC/Student" query's and never thought would end up in the "Real world" PLC programming,,,,,,,,

Outline : We are machine builders.

We quote for small assembly machine
We build small assembly machine
Machine has "Safety Light Barrier" installed.
One "Start pushbutton" to initiate cycle.

Customer requests second "Start pushbutton to be added" on other side of small machine.

Why: Because the operators are used to "TWo-Hand" control and would find 1 x Start pushbutton different ! bonkhead

(I'm loading the gun) Customer: "Can you program a two-hand control into the software ????? even though it has absolutly nothing to do with safety ????

I do, I programmed a NON safe Two-Hand control sequence into a machine that dosen't need it as the Light barrier covers all safety.......... banghead

I'm waiting for the next phonecall from the customer to ask if I can program the machine light tower (Indications) like a goddamn traffic light utoh

anyone else being asked to do anything like this ?? I've heard of customers being always right but this is f*#kin ridiculous


anyway,

had to share,,

Tell that customer that two-hand control increases the cycle time = lowers the production rate = throws money out the window.
Maybe that can turn him around.

You could change the two buttons into something meaningful:
Normally the sequence continues when the light barrier signals OK.
Button 1: Interrupt sequence.
Button 2. Continue sequence.
The opeator then has a means as to interrupt the automatic sequence when something goes wrong, or he isnt ready.

I agree that in your case, it seems to serve no purpose, but I HAVE done the two-hand control in software. Similar to your application, the machines we built had a light curtain that was 'enabled' when the cycle started. If the light curtain was broken during cycle, the machine would E-Stop.

I used the dual button system to force BOTH of the operator's hands out of the light curtain. If it only had one button, the operator may get ahead of themselves, and push the button before their other hand was clear of the light curtain, which of course resulted in the machine shutting off.

IOW, the light curtain provided the safety, the two-hand control simply prevented unnecessary machine stoppage. Worked well!...

beerchug

-Eric

I agree with Eric. We do that way all the time. Some customers want it, some don't. At least on our machines, it doesn't affect cycle time.

On our machines we have introduced light curtains for three reasons:

1. It reduces the cycle time. This because the program is set up to continue immediately when the operator is clear of the machine. For short cycle times every millisecond counts.

2. It is safer, all things considered. (only pitfall is that the time to stop moving parts has to be considered against how far into the machine an operator can reach within the same time).

3. It reduces the strain on the operator because he has to do fever steps in each cycle (keypresses). This is a major advantage with the repetitive nature of these jobs. With a cycle time of 10 sec. you have saved him of almost 3000 presses on the buttons for each shift. (I am pussled that some operators really want to keep this feature. Could it be because they dont really believe the safety of the light curtain ?)

Jesper,

you hit the nail on the head

"3. It reduces the strain on the operator because he has to do fever steps in each cycle (keypresses). This is a major advantage with the repetitive nature of these jobs. With a cycle time of 10 sec. you have saved him of almost 3000 presses on the buttons for each shift. (I am pussled that some operators really want to keep this feature. Could it be because they dont really believe the safety of the light curtain ?)"

these days of Reptitive strain injury and all sorts of wonderful Ergonomics on machinery I cannot understand this, ok Eric I see what you mean, but is that not a bit childish in content, as it is one thing to design for a foolproof scheme but to design for this seems a bit OTT, whats next footpedals so they can't kick the machine when it dosen't work, sorry thats getting too sarcastic ! but c'mon, The customer has spent money on an semi-automatic machine, yet still wants the operator busy with the particular assembly, why didn't they simply specify a Two-Hand control safety circuit as this is a tiny machine, I'll try to get a picture later.....

Charge Double.

Give them a Murphy Switch for the second button.

Goddammit,

don't drag that one up again.............

Two Words

Be Aware

You can add Two-Hand controls on any machine that doesn't require it under OSHA guidelines.


BUT

Any Two-Hand control must meet all OSHA operational standards!

This includes redundant wiring and Safety PLC (if PLC based.)

If you just add a second Start PB, and add some logic to a standard PLC for timing, the machinery can (& probably will) be shut down during an OSHA inspection until it is retrofitted to current specs. Been There Done That.

Also, if the OSHA inspection is due to a "lost time accident" the Electrical Controls Engineer(s) that designed, wired &/or programmed the control can be individually cited & held responsible, along with their employer(s.)

CYA        CYA       CYA         CYA         CYA        CYA  

Interesting Greg, I did not know that. Any idea where you read that? I don't see it in my copy of CFR 29.

Sorry Mylo, but I have to retract my earlier statement. I thought I had done the two hand control in the PLC, but I just checked, and it seems I ended up using a dedicated module. I see from the schematic that I had run out of inputs. Here's a picture. The two-hand control is the second from the left in the top row.

[attachment]

By the way, those Banner two-hand controls were recalled. How THAT for safe?... lolis

I still don't see why a non-safety application MUST use a two-hand control, but you know the government...

If Greg's information is correct, then I guess I was lucky I ran out of inputs!...

beerchug

-Eric

OSHA takes the fun out of everything.

If I have an extra switch and light, I like to hook them up together to see if anyone will ask dumb quesstions.

NEED THREE SWITCHES??? set one up on a latch circuit in the PLC, so the machine has to be shutdown to clear it. Oh yes, add an alarm horn to the latched output for that switch, and a nameplate that says "DO NOT PUSH".

OSHA? We don't need no stinking OSHA, just ask 4-finger Tommy or One-Eyed Mike, or "Lefty".

regards.....casey

The only mention of two-hand controls in CFR 29 that I've found is buried in the "Mechanical Power Presses" section. Not every machine that should have a two-hand control is a "Mechanical Power Press", you know...

CFR 1910.217 Mechanical Power Presses.

(5) Excluded machines. Press brakes, hydraulic and pneumatic power presses, bulldozers, hot bending and hot metal presses, forging presses and hammers, riveting machines and similar types of fastener appicators are excluded from the requirements of this section.

Does that mean I DON'T need a two-hand control on these types of machines?...

Oh, and why the heck is BULLDOZERS in the list????... lolis

beerchug

-Eric

Eric

Nice Looking Panel!

Would like to see a thread start up with panel pictures, to see how others do their panels. Might spot something new and innovative.

Regards to all.....casey

Thanks, Casey. We're always in such a rush that I usually forget to take pictures before a machine ships, so I don't have many pictures. There's another one of my panels 'on display' at Ron's site (HERE).

Care to start a pictures thread?...

beerchug

-Eric