Accurate length measurement

[BigAl]

Maybe you guys can help me with this. I have a customer who purchased a cutoff system and is now setting it up at his facility. The system has two cut off saws, one that is fixed the other that can be moved by a hydraulic cylinder to cut different lengths.

The system is designed so that a piece of lumber would come down a conveyor with steel rollers. A stop is set up after the first saw, so that the lumber would stop at a specific length, maybe 6 feet. The operator would then move the second blade to cut the remainder to a useable length. For example if the lumber was 13. 5 feet the operator would move the blade to cut off an 8 foot length. Both cuts occur at the same time, after the piece is clamped.

The customer would like us to automate this system using a PLC. I have seen systems like this, and know that it is possible. The only challenge we have is to get an accurate measurement of the length of the lumber. The thickness will vary between 1 and 8 inches thick. We thought about using an encoder on the drive sprocket, but from what I see, the piece will slip, as the steel rollers start to rotate. The other idea was to use a rotary encoder on a wheel, which would ride on top of the wood. Our customer has had experience with these and says that they always get smashed when a piece jams.
I have been told that there are measuring devices available that ride on a linear shaft, driven by a linear servo. A sensor is mounted on the end of this and when it reaches the end of the wood, the counte from the servo encoder are used to calculate the length. While I am sure that this will work, I am also sure that it is pretty expensive.

Maybe someone here has some experince with this, and can recommend some device to measure the length. Keep in mind that the first cut will always be at least 6 foot, and the longest piece will have a total length of 17 foot.

Any help would be greatly appreciated.

 

[Pierre]

Are you cutting them pieces in length of 6 feet?

If so, you only have to know if it's more than 6 and more than 12. That would be easy with only photoeyes but I suspect there is more to it.

Vision system could be a choice. The lighting is always the key factor in this type of system. Have a look at what is available now in the market. Every day a new vision with more capabilities come on the market. Cheaper, easyer, faster.

Describe to what choices of length the second and third pieces are cut.

 

[Norml]

I've done the same thing with a metal cutoff line but didn't interface it with a PLC. I used a digital readout (DRO) with a sensor that moved over a fixed scale that looks like a tape measure. We went up to 40' with it and was within .020" accuracy.

 

[nonuke]

Take a look in www.micro-epsilon.com. they have a laser-gauging and distance sensors up to 250m. Model optoNCDTILR

 

[mjisaacs]

...Not sure if I am understanding your needs, bot you can also try MTS Sensors.

 

[Clay B.]

Can the second blade move to the end of the piece of lumber? I might be visualizing this wrong.
But....

If the moving blade can move past the end of the peice of lumber it is simple.

Your going to have to have some type of positioning feedback from the moving saw to position it.
Use this coupled with some type of sensor that can "see the end of the board"
When the sensor "sees" the end of the board you can use the blade position to tell you how long the board is then you can calculate your cut. The second saw has to be repositioned anyway so the only step you would really be adding would be a home position for the second saw blade. This can be set in motion as the cut wood ejects so it should not effect cycle time. If you offset the sensor from the blade. Example your "usuable" lumber is in 6 inch increments, then offset the sensor 6" forward of the blade movement from home. So when it gets to the end of the board and it see the board is lets say 12' 5" then the saw would only move foward 1".

Like I said it is contengent on the second saw blade and how it works.

 

[TConnolly]

Are you trying to measure the wood, or the distance between the two saw blades?

 

[curlyandshemp]

Maybe you guys can help me with this. I have a customer who purchased a cutoff system and is now setting it up at his facility. The system has two cut off saws, one that is fixed the other that can be moved by a hydraulic cylinder to cut different lengths.

The system is designed so that a piece of lumber would come down a conveyor with steel rollers. A stop is set up after the first saw, so that the lumber would stop at a specific length, maybe 6 feet. The operator would then move the second blade to cut the remainder to a useable length. For example if the lumber was 13. 5 feet the operator would move the blade to cut off an 8 foot length. Both cuts occur at the same time, after the piece is clamped.

The customer would like us to automate this system using a PLC. I have seen systems like this, and know that it is possible. The only challenge we have is to get an accurate measurement of the length of the lumber. The thickness will vary between 1 and 8 inches thick. We thought about using an encoder on the drive sprocket, but from what I see, the piece will slip, as the steel rollers start to rotate. The other idea was to use a rotary encoder on a wheel, which would ride on top of the wood. Our customer has had experience with these and says that they always get smashed when a piece jams.
I have been told that there are measuring devices available that ride on a linear shaft, driven by a linear servo. A sensor is mounted on the end of this and when it reaches the end of the wood, the counte from the servo encoder are used to calculate the length. While I am sure that this will work, I am also sure that it is pretty expensive.

Maybe someone here has some experince with this, and can recommend some device to measure the length. Keep in mind that the first cut will always be at least 6 foot, and the longest piece will have a total length of 17 foot.

Any help would be greatly appreciated.

What we have done in the past to measure length of product in motion is to use a photo-eye and a proximity sensor.

The photo eye is used to detect presence of product, and the proximity sensor counts sprockets on a gear or wheel that is connected to the belt moving the product.

When the photo eye senses the leading edge of the product, a counter is reset, and then the pulses from the proximity sensor are accumulated. Accuracy is the amount of distance the belt travels per sprocket on the gear.

Ian

 

[CCatterton]

I used to work in a veneer mill, and we had a system with a similar setup. It had a fixed saw on a conveyor. The saw was activated by a joystick, in and out. We incorporated 2 laser sensors, mounted in two different locations infront of the saw. because the saw and lasers were in fixed locations on the conveyor, we were able to triangulate the distance using the math functions. We would start the calculation with the lower laser and the head cut, move the log down the conveyor where the operator wanted to make a tail cut and the upper laser would store a value then the PLC would calulate the difference between the 2 lasers. The PLC was a SLC 5/03. I do not remember the brand of the sensor, and I apoligize if this is confusing, I have be up for about 20+ hours and wanted to put in my 2 cents before I forgot about your post.

Pic of conveyor: (side view)
xxxxxxxxxS l
xxxxxxxxxS------- l
oooooooooSooooooooo
xxxxxxxxxS
xxxxxxxxxS
Legend: l = laser
S = saw blade
o = conveyor
- = log
x = ignore the x's, tried to put spaces, but the post took them out


Another view: (top view)

xxxx|-----|
xxxx|-----|
xxxx|-----|
xxxx|-----|
xxxx|-----|
xxxx|-----|
xxxx|-----|
xxxsssssssssssss
xxxx|--1--|
cccc|--1--|
cccc|--1--|laser (elevated) roughly 1.5' - 2' above conveyor
cccc|--1--|
cccc|--1--|
cccc|--1--| note: lasers are mounted in the center of conveyor
cccc|--1--|
cccc|--1--|
cccc|--1--|
cccc|--1--|
laser (mounted at end of conveyor and roughly 1/2 foot
about conveyor)

Legend: |-----| = conveyor
s = saw
1 = log
c = loading conveyor
x = ignore the x's, tried to put spaces, but the post took them out



As stated earlier, I apoligize if this is confusing.

Clayton


Edit: Ok...tried to fix the pix, but they went back to original


Edit again: I drew a couple of paint pix top and side view...hopefully it helps

Clayton

 

[milldrone]

BigAl,
More details are needed to give a good response. How many cycles per minute? Accuracy of cut +- .020" or +- .500"? Does the hydraulic cylinder that moves the second saw have a positioning system already in place? And the total travel of the cylinder? Can a hold down or press roll be used to "secure" the wood to the rolls? Is this logs or lumber? You said "The thickness will vary between 1 and 8 inches thick". What about the width? Is the lumber green or dried? You said "The operator would then move the second blade to cut the remainder to a useable length". Does this length change from day to day or is it 2 foot nominal increments? You said "The other idea was to use a rotary encoder on a wheel, which would ride on top of the wood. Our customer has had experience with these and says that they always get smashed when a piece jams". Does the customer want something "sawmill tough and simple"?

 

[wborst]

Why not replace one of the steel rollers with a spring loaded rubber roller that applies pressure from the bottom to keep the roller in contact with the board. Then connect an incremental encoder to this roller. If less accuracy is required, then a sprocket and inductive prox switch could be used to track the motion of the rubber roller and give a length measurement. A photo switch would be required to detect the leading and trailing edge of the board as it passes over the rubber roller in order to know when to start and stop counting pulses.

 

[OkiePC]

Put an LVDT on the hydraulically movable axis...robust and out of the way...Oh yeah, the cylinder needs a proportional directional control valve. (servo valve)

In the PLC/HMI add a user adjustable offset value for variations in each blade position...You should only have to adjust these following blade maintenance, and after that, the system should be as repeatable as your hydraulic controller error in LVDT counts * the resolution of the LVDT.
You can close the loop for the hydraulic positioner in the PLC or add a motion controller. So now the operator enters the desired lenngth, and the PLC (or motion controller) ramps the analog signal to the servo valve to obtain the correct LVDT value...If you want super high speed control, use a servo motion controller. If the movable saw can take a little extra time getting into position, use the PLC, (which will still be much faster than the human operator would be able to do it.)

JM2cents

 

[parky]

Hold on you say a stop at 6ft from stationary blade then the other blade must be at the left (assume log goes from left to right) so detect end of log measure it then using another encoder on travelling saw to cut close to end i.e. 8.5ft

I once did a travelling cut off saw at a steel tube forming mill this would measure the tube as it was extruded, match the speed of the travelling tube & cut it on the fly then return back before doing it again this was complicated as the tube was continually on the move so the measured tube length could vary before the saw got to home position i.e. saw matches speed at cut off point, chops length in the mean time saw is still travelling foward, then back, length extruded now = 4 metres, required is 6 metres wait until 6 meters match speed & chop we also had to calculate the rate the blade cut the tube based on 3 points going through tube, size of blade * number of teeth to get max wear out of blade we called the calculation the rubiks tube it was so complex.

so your app should be pretty simple.

good luck