Tracking offset of lugs on 2 conveyors?

[Millwright]

I have a system that has 2 chains with 3 lugs on each chain on a common shaft. The lugs have to be within 1mm of each other as they push a panel through a saw or the panels are crooked. Currently we have to measure everything manually on each set of lugs to ensure it is square.


I want to track the offset of the 2 lugs opposite each other at a set location. Contrologix 1756-L71 processor, Powerflex 700 VFD on the chains, photoeye can be whatever is required. If the offset is more than 1mm I would like to alarm it on the HMI. Chain speed is 250 ft/min.


I am told by my programmers that the scan time is too long to be able to do this with even with the fastest P/E and PLC Card available from AB. My questions are:
1. Is it possible with AB equipment?
2. If not should I try Beckhoff because they have faster scan times?


Any assistance appreciated.

 

[chavak]

Looks like a vision system application, rather than sensors

 

[chelton]

Since you are able to manually measure it the chain conveyor must have a rest position?
Could you set up a laser distance sensor that measures the difference while stationary?, or do you require a live measurement?

 

[Millwright]

Looks like a vision system application, rather than sensors

Yes that's what we were thinking we may have to try, but cost might be prohibitive.

 

[Millwright]

Since you are able to manually measure it the chain conveyor must have a rest position?
Could you set up a laser distance sensor that measures the difference while stationary?, or do you require a live measurement?

No, the chain stops and starts but coasts so it isn't accurate.

 

[PLC]

I thought AB does gearing with MAG instructions?
And offsets
I can’t believe that cpu can’t do that motion application

 

[Ken Roach]

Take a step back, folks, and read carefully. This is a lug-driven panel saw with a common headshaft.

The variation in the lug positions is a function of the stretch and wear on the chain and lugs and sprocket. They *cannot* be adjusted by speeding up or slowing down a motor. They're typically adjusted with a screw and jam nut, and get the everloving bejeezus beaten out of them all day every day.

This sounds like an misalignment detection application, and it's a difficult one because of the practical mechanics of the system. There is sawdust and grease and wood flakes everywhere, and the distance across the saw is several feet, and you're dealing with a chain that is half slack and is placed under tension every cycle.

Let's assume you have a very fast photoeye, and have mounted it in a way that the lugs will pass through individual sensors (like a fork sensor with a transmitter and receiver) where they won't be damaged and are under a similar load and tension.

250 feet per minute is 1270 mm per second, so 1 mm goes by in 0.787 milliseconds.

That's very fast for a high quality photoeye, and too fast for most general-purpose PLC I/O.

If I was going to try to do this measurement at full speed, I would wire up two sensors to a PLC that had EtherNet/IP and dedicated high-speed interrupts, like a Micro 820 or MicroLogix 1100. Those could timestamp the inputs with their own high-resolution clock and measure the offset between them.

Or, I would wire the best photoeyes I could to the same ControlLogix fast input module in the same chassis as the CPU, and run them as change-of-state interrupts to grab the microsecond timestamp of the ControlLogix, then do the measurement during a "dry cycle" of the saw at a fraction of full speed.

 

[PLC]

Aaahhh
Thanks Ken

 

[GaryS]

Interesting
I would do it a little differently
If you can find laser measuring sensor with the resolution to read the fine.
From the description what you are trying to do is measure the wear in the chain
That makes things challenging.
Whit out actually being on site myself I can only offer thoughts.
I would install 3 to 5 lasers pointing from the discharge end to the panels coming down to the saw
The outside sensor’s looking at the lugs and the others looking at the panels as check on the
It would take some careful setup to get everything in line correctly. The lug sensors would give you difference in the position at it travels down the table.
Te bigger question what are you going to do when the lugs are out of position, with a common shaft there is no way to adjust the alignment. If the you adjust the tension on one chain the other 2 lugs will move even more out of line the only way to deal with it with a common shaft would be to replace both chains with new.
One way would be to use roller guides to square up the panels before they enter the saw. If the panels start out square they will always pass through the saw square.
The best way to handle this would be to split the 2 chains using 2 vector drives and set up a control to read the position difference as they are traveling down the line and correct the alignment it using the drives this way you would be correcting each set of lugs and any wear in the chains would be corrected on the fly.
Even using a controlled clutch on one or both drive sprockets would not do what you need because you can’t control them to precision necessary.
I would be interested in hearing about your solution

 

[Ken Roach]

I'm interested to hear some context about the machine system.

Are you cutting MDF or plywood or OSB right out of a press ? Or is this a complex saw system for making furniture or cabinets or doors ?

Is this a "second pass" saw, at a right angle to a saw that trims two edges of the panel ? With just three chains, I would think it has to be an edges-only or a three-blade saw.

How do you align the lugs now ?

What do you think you can improve about the process by detecting mis-alignment of the lugs ?

 

[Jim3846]

Can you mount an encoder on the idler shaft or drive shaft? what if you mounted proximity switches at where the lugs go into the area to make sure the panel is square?
Is there some way to install a take up sprocket on each side and use some mechanism to adjust the chain to where the lugs can become square? Maybe a linier shaft to adjust chain tension?

I am not a mechanical genius I think these are the items you need to control this chain.
Items needed:
Hollow shaft Encoder that is a PPI on shaft (0.5 mm resolution) Probably going to need a high speed counter card on the plc side?
Proximity switches, dc preferred, more sensitive, to look for the lugs for squareness
Take up sprocket mechanism to adjust takeup slack on each side.

Do you have a VFD on the chain motor?