Learning by Doing

[Eric Nelson]

I wonder if I can exchange it?

Don't bother. What you bought is more appropriate for your application, because:

You wount be able to operate coils large relays or solinoids from the DC outputs, but you can operate almost anything from the Relay outputs.

Except a pulse train to a stepper motor. At least not for very long...

Instead of calling A-D to exchange you PLC, call them to order one of their stepper systems. Yes, they sell these now...

It seems that designers are always getting goaded into using them by their relative simplicity and low cost, and then paying the price in the form of missed steps, lack of torque at speed, and a host of other idiosyncrasies associated with an open-loop stepper system

I'm not a fan of steppers mostly for those reasons, but your budget kinda rules out a servo. Since you're not moving all that fast, you may want to try it first without an encoder. If you find it isn't repeatable, then adding an encoder will allow the PLC to make up for the missed steps, slippage, etc.

Basically, an open loop system simply instructs the stepper to move 'n' steps to move the wire the correct distance. Without the feedback of an encoder, the PLC doesn't know if the wire actually moved the correct distance, for whatever reason.

Open-Loop:
PLC: "Give me 60 inches of wire"
Response: "I'll try, but I won't make any promises"

Closed-Loop:
PLC: "Give me 60 inches of wire"
Response: "I gave you exactly 'x' inches of wire"

beerchug

-Eric

 

[Eric Nelson]

I just read your latest response, and I think you'll do just fine!... :nodi:

beerchug

-Eric

 

[Bitmore]

Hello pstephens,
My opinion may or may not hold much validity, especially among many "viewers" here, but, I have noticed one thing about you..
You have done what most other questioners have not,,, your homework and investigation! It would be a joy to me to have other users/viewers with the same amount of enthusiasm as you!
You have already recieved many good ideas and advice, quite frankly, I have nothing more to add to your project. So now I'm out of my nature (hihi). You came to the right place, presented the pertinant data, demonstrated the thought process along with a plan, and outlined your goals!
My next thought is... Are you looking for a job? I can certainly use a person like you!
Good luck with all of your ventures (heck it's just good engineering), and may God bless!

bitmore

 

[Gerry M]

Pstepehns,

Back to the pinch roller and encoder discussion.

It worries me a little if you mount an encoder on the pinch roller, or decide not to use one at all. Because the wire may slip in the pinch roller if say the wire gets a little tangled on the roll or if there is any slip on acceleration.

I would reccommend using an idler roller with an encoder on it as a means of measuring length.

 

[JesperMP]

pstephens,
as no-one else has mentioned it I think that I will:
You also have to consider safety aspects.

Can someone get his hand/finger into that shear cutter or the clamp ?
Are there anywhere else on the design that someone can get into trouble ?

The best strategy is to have completely passive safety, meaning that the dangerous parts are constantly covered over - and with warning signs to inform that there is something dangerous inside.
You may want to add a E-stop button with a dedicated safety relay to stop all moving parts.
To design a machine means that you must make a socalled risk assesment.

All this because the design is now controlled automatically and not by a person.

 

[elevmike]

Laurent is right about the stepping motor. You will need DC/solid state outputs to drive the setepper drive. You can add a relay to activate the sheer. So if your going the stepper motor route, keep the D0-06DD1 PLC.

Reguarding the display, Looing at the operator panel you designed, and now better understanding the setup you want to acheive, stick with the EZ display. Good choice.

To be honest, my only expierance with steppers was some time ago, and was only intended to learn about them. My 1st impression was to use a small 1/2 HP AC, or DC motor and Drive, with an encoder for feedback. I like feedback, but a great bulk of my expierance is in elevators where it's very necessary in most cases. A stepper may be more complicated, but better suited to the project. AD has a few examples of pulse output in some of their manuals that dont look too daunting. Personally I would think that the project could be done successfully either way. A cost analaysis on the feed drive components may be in order to help you decide. If it were totally up to me I wouldnt use a stepper, only because of my lack of expierance with them and the need for feedback, and torque may negate the issue.

A small electical mech brake on the support shaft of the reel migh be in order. That way you wount have to pull through the friction device, lowering the demand on the motor, and lowering the possability for slip between the drive rollers, and wire. You also wont need as much pressure on the rollers which may lessen the impression left on the wire.

The rollers: You can make up a roller setup then get them treated with a friction surface like these people do here: http://www.surfalloy.com/ They have many grades and textures available, and I'm gonna guess that the cost would be about a few hundred bucks, based on my experience. With the right texture/grade surface, you wont leave much of an impression, if any at all, on the wire. This will also help to lower the power requirements of the drive, and wear on the bearings etc..

Go for feedback, (highly reccomended, Thank you Eric) it's really not expensive or complicated, mount the encoder on the ideler/pressure rollers to eliminate the possability of measuring slip and cutting short. In the case of elevators we place a tach/encoder on the motor to feedback to the drive, but positioning is done from the car top due to the slip between the driver and the hoist cables. Same concept here.

Question: are you going to use the existing sheer? It seems that you could, thus lowering the cost of the project, and allowing for more money for other stuff.

Mike.

 

[Doug-P]

I'm wondering, once this gizmo is 'automated', what's the operator's part?

 

[elevmike]

I'm wondering, once this gizmo is 'automated', what's the operator's part?

What operator??

JesperMP,

It seems that this project could be very ealily guarded once it's compleated. The sheer looks like it would be very diffucult to stick your finger into it anyway.

 

[JesperMP]

mike,
I dont know how long the movement of the shear cutter is, but I can see several places where a finger can get in.
Guessing by the length of the slot the shear cutter comes out of the movement can be quite much, but maybe it is shorter than the slot allows for.

 

[elevmike]

JesperMP,

Take a look a the shear. It seems that the only moving part here is the upper jaw. It also seems that it have little travel. Your right about bring up the safety issue. My preception is that it may be not very diffucult to address.

 

[pstephens]

Yes, that's correct, Mike. The blade closest to you in the picture is the one that moves. It pivots downward just enough to shear off the 1/8 dia wire -- which is not very much. As I said, the fact that I could incorporate this shear unit in my design was one of the reasons that I elected to take this job. Designing devices that have the capability of maiming is not one of my favorite tasks, but one that I take very seriously (and charge appropriately for). One nice thing is that most people don't have any appendages as small as 1/8" diameter There is a possible finger-pinch point between the top of the movable shear blade and the guard housing, but like you say, I think it's easily addressed.

BTW, I really appreciate all of the feedback you guys have given me so far! I've got an appointment this morning, but I want to respond to some of your ideas afterwards. Thanks again.

 

[elevmike]

More on the encoder.


"This is something I would like to consider in more detail. My first reaction to the idea of a closed-loop drive system was that this project could not bear the cost, and that it would greatly complicate the control system (further adding to the cost). But that may be just my own fear and ignorance. I'd like to learn more about how an encoder would be integrated with the control system."

(6" x 3.14)/.125" = 150.72 miniumum ppr. So go out and buy a 200 or 360 ppr quadature encoder. The D0-06DD1 you bought will handle the encoder with out the need to buy a high speed counter. So the added cost is about $150.00. The manual has plenty of examples. You want to be able to go in both directions in the event of overshoot (plan for this). So you will be setting up the program with an up/down counter (mode 20 I think). I can provide you with some ready made code that you can play with. Basicly you want to set up a menu of cut lenths in your NV Ram, (V7000+) and the operator will choose which lenth from the menu. The value for lets say a 24" cut may be 500 counts. So you store the value of 500 in V7000. If the operator chooses a 24" cut run then the program will use a comparative contact that will compare the value of V7000 to the encoder count.

You can also provide for speed control. Lets say you accellerate then you want to slow down as you come up to lets say 400 counts, then creep to 500, stop and cut. You can also use counter presets, but I'm not into that because my applications often require more then 16 or so presets. Also the operator can adjust the lenght of cut throug the EZ by addressing the Vmem and chaning the value.

 

[Ron Beaufort]

quick sideline ...

Greetings pstephens,

I’m in the same boat as Bitmore in post #18 ... not much personal experience in this particular line ... but I would like to second Bitmore’s compliments ...

I can’t make you a job offer, but I can guarantee that you could easily work in the technical writing field ... you write EXTREMELY well! ...

ok ... back to work ...

 

[Rube]

pstephens, welcome to PLC heaven, even if there are some devils around.

I used to work with a similar machine and our wire was fed thru some straightening wheels before it got measured and sheared. The straightening wheels were driven nonstop and the wire was sheared on the fly. The wire triggered a limit switch on a spring-loaded, "get out of the way" plate at the end of the machine's tray. When the plate moved due to being pushed by the advancing wire, the shear cut it, and the plate sprung back home. The Limit Switch was eventually replaced by a photoeye.

I don't know if this is a viable solution in your case but your photos tend to make me think it is. This would save installing a stepper motor and the required controls. Of course this all depends on how fast the shear is as well.

 

[DesertDog]

Re: Re: The Existing System

1st of all bear with me; Im a terrible speller and their's no spell check on the posting page of the forum....

I use iespell found at http://www.iespell.com/download.php It gives you spell check anywhere there is input on internet explorer. It does require word as that's where it gets it dictionary from.