Has somebody already done this?

I am looking at an application that I hope someone in this forum has seen before. A customer that I am working with in the lumber industry, has a conveyor which transports pieces of wood which are of various lengths down to a sorting mechanism. Currently there are 4 photo eyes set up at various positions, to try and establish the length of the wood. Depending on the inputs from the eyes the PLC will sort the wood accordingly, and dump it to the correct bin.
The owner of this piece of machinery would like me to make it more accurate. His major complaint is that the majority of the pieces that come down the conveyor, do not sit perfectly straight on the belt. This causes the eyes, to read a short piece, and thus some wood is wasted. The owner has already tried several mechanical fixes to try and straighten the pieces directly before the eyes, but because of the nature of the wood, there are still some pieces becoming skewed.
The first fix that we looked at was a vision system. Because of the width of the conveyor, we found that we would require at least two cameras. There would also be some pretty intense math that would have to be executed within the PLC to determine the length of each piece. Currently the I am a little wary of this fix. Not only is it expensive, but when I look at the environment that the cameras will be in, I am not sure how long they will last.
I have talked to a few manufacturers reps, regarding different sensors, but I am very cynical. These guys always seem to wear clean white shirts, with a pair of Dockers. When I tell them it's a dirty environment, I am not sure they know what I mean. If any of you have some experience using a vision system, or other sensor, in an application similar to this, I would appreciate your thoughts, and/or advice.

Thank you

Big Al

Hi,

From your discriptions, I think a vision system would be a better option which gives out 4 digital outputs for 4 different sizes. Advantage would be, the wood do not necessarily have to be very much orientated to do the inspection, thus reducing flaws in measurement.

Environment wise, for this application the camera would be in a quite a distance looking down the conveyor. If the distance don't give any advantage, then pack it up in a box with a see through glass window.

There are many vision systems around, DVT, PPT, Cognex etc. Give a call to any one of them and get a better understanding.

Good Luck

I know there is a huge push in the "vision" industry. We have a loaner from a local rep to try before we buy....Perhaps you could work out a situation like that?


Greg

Well, mechanical solution was my first suggestion. Vision system was my next...

Are these boards the same cross section and same type of wood? Could you use a scale under the conveyor belt to weigh them? Your machine probably wouldn't allow for that, right?

Can you tell us about your specifications? Are your "standard" sizes several feet, inches, or microns apart?

AK

What type of conveyer??

My first impression would be to run the rough lumber,(presumably logs), through a series of rollers supporting and driving the logs. the rollers would be underneath the logs, and offset 45 deg. from center. This might cause the log to self center and go streight, making it much easer to measure accuratly with the photo eyes.

I have used a Cognix vision system before, with a contractor setting it up and me maintaining it.

It was set up with two camera feeding into a PC card with an industrial PC doing all the hard maths (Which industrial PCs are good at).
This was then run through an IO card and relays to a PLC. The PLC sent a signal to the PC to tell it when to take the picture, the PC replied half a second later with the results via 8 output bits.
The card we used was able to connect to up to 4 cameras.

The main problems we had was lighting.
We had to carefully setup the area with the correct lighting, and, more importantly, the correct shadows. At 6pm on a summer's day, the sun would also shine in the wrong direction and the system would not work, requireing us to add some shutters to the windows.
We also operated in a dusty environment, but that didn't affect these cameras. They were looking down onto the conveyor.
We installed a third one looking sideways, and this would get a puff of dust each operation. After a few days, it would not be able to operate. Having the operators clean it at the beginning of each shift fixed the problem.

Hi Big Al,

This is a common process in wood/lumber industries. You have to feed a shift register with length/type/quality... data and track the parts all along the sorting conveyor, until they are properly ejected. Can you provide some information about the conveying sections (side by side only, in line somewhere?) If there is an in line section, a sensor + pulse generator will be required. Mechanical legs with or without encoders (depending on the conveying direction) are also common and reliable.

Regarding the vision systems in wood processing, it works. Yes, they deserve to be very carefully installed due to the harsh environment, temperature constraints, light sources... Check the prices, ask your "men in white" for industrial references and call. If you want to try one, also consider some additional laser projectors. The laser line(s) may be easier to track or count than the wood itself (changing colors and weak areas producing inaccurate readings). As usual your actual sizes may not require to get the whole bed dimensions covered. By the way, is there a reference side in this measurement application? If not you should create one.

Laurent

K.I.S.S.

No question in my mind, the pieces of wood must be centralised. It sounds as if it doesn't matter if they are not perfectly parallel to the conveyor but they must be reasonably straightened. That way you can measure them with anything you want, even the magic visiojn system the "men-in-white" want to sell you. If they are angled the vision system s/ware will, as you rightly say, have to do some intense number crunching, It seems a little OTT for this kind of app. Straighten the pieces up and stick an encoder and 1 sensor on the conveyor.

Hi,

Perhaps this company has some sollution for your problem

New UV Sensor Detects the Invisible
Link

Rudi

Use curtain of light (not a guard safety curtain) made by a Company such as Banner. Such curtains are commonly used in spray paint
and powder coating applications to evaluate the size of the part.
Position transmitter at the bottom under the conveyor and receiver at the top above the conveyor. The curtain should span the conveyor width. Banner has variaty of sizes and if necessary use two or more.
As the log moves inside the curtain you will be able to determine
the left most point and the right most point. Install an encoder
on the conveyor if you have not done so already.
A combination of number of pulses and knowledge of the leftmost and the right most positions are enough to calculate the real length based on some simple trigonometry. Just count the pulses while
any photoeye of the light curtain array is covered.
Make sure that you have an encoder giving you enough pulses to achieve desired precision.
This looks like a very nice project.

I never worked in wood processing industry but I would
agree with kennyb and mike...

Unless logs are badly bent, encoder and photoeye
should do the job witt no problem (or just few existing
photoeyes).

I don't know what kind of volume is this supposed to
handle (are the logs masured in batches?) and at what
rate but if you have to singulate the logs for sorting,
there must be a way keep them in position to take the
measurement (or actually keep them moving to straighten
them up like with a V shaped conveyor section if inline
or revolving mechanism if parallel...).

Well, BigAl...

The first step to finding a solution for any problem is to carefully consider then define the problem as completely and as accurately as possible. My favorite phrase for this kind of situation is... "Know everything you can about your Enemy. Only then can you beat him!".

You are more familiar with the problem because you can see it, or at least, you have seen it. This doesn't mean that you "know" the problem. And we certainly don't "know" the real problem.

We don' know nuttin' no how... 'cause you ain't tol' us much 'cept you got a problem.

So... how about a little info?

What kind of conveyor are you using? Rollers? Belt? Some sort of chain with dogs of some kind?

What kind of speeds are we talking about here?

Once a board begins moving past an eye, is the speed of the board absolutely consistent? If so, how so? If not, why not?

What is the nature of the wood? Are they logs? Are they rough-cuts? Are they finished?

You say, "...because of the nature of the wood..." it is hard to straighten the wood as it moves past the eyes. What is it about the nature of the wood that makes it hard to straighten?

What kind of accuracy are you looking for? +/- a foot or two? +/- an inch or two? Less? More?

This is a really important point... is there ALWAYS a gap between the pieces? Is that gap guaranteed? It doesn't have to be a consistent gap, but it always has to be greater than some minimum. The gap must be enough to ensure that the separation between pieces is detectable.

The more you help us help you... well, the more we can help you... duh?

My first inclination is to say... Blow off the "vision system".
It's too expensive, too complicated and too environmentally sensitive... three-strikes, yer out!

Stick with a system that is mechanically simple. Use something that forces the pieces up against a straight-edge; something the ensures gaps between pieces; something that forces the pieces to move at a really consistent speed.

This should be something that the night-shift mechanic can work on.

Of course, there is always the concern that the owner doesn't want to rebuild his conveyor system to force those conditions.

Some of the conditions I indicated are required... others we might be able to slide on... but, only if you give us a little more info!

So, how 'bout it Al... give us half a chance to help you!

I agree with Terry

The vision system is overkill. I wrote code and installed lumber sorters as my first job out of the navy. I have never seen this problem, but back then we didn't didn't have curve or shaping sawing that purposely cuts curved boards. I would consider hold down shoes to keep the lumber back agains the lugs. I would also consider more photocells. You may need more photo cells by the 0 inch or lumber line where the fence is incase the lumber is not even ended. Third, you can consider the phase between when these photo cells are broken and unbroken. This will give you an idea about the angle of the lumber in the lug. This should be unecessary if you can be sure the lumber is pushed back against the lugs. You really are making a crude scanner.

BigAl,

Yes, it has been done before. Yes, I have worked closely with Lumber Sorters. Yes, measuring boards and sorting can be done with simple switches.

The key is the TYPE OF CONVEYOR. You need a chain or belt conveyor with equally spaced lugs. These lugs are metal prongs that are spaced across the width of the conveyor to catch and pull the board along, with the length of the board perpendicular to the direction of travel of the conveyor. The conveyor must have an encoder to track each set of lugs. Each board must be "straightened", as you described it, to the extent that when it is placed on the conveyor, the next set of lugs pushes up against it and forces it to be perpendicular to the movement of the belt. In other words, the board must not be skewed so that it is resting partially against two different sets of lugs. There are automatic Unstackers, machines that can place boards on a conveyor in this manner.

From there, you simply pass the board over a line of switches, either photoswitches or even mechanical limit switches. The most common method places a switch every 2 feet. The last tripped switch in the line gives you the usable length of the board. Any board that does not get measured for some reason, is sent to the end of the line, where a Reject bin is located.