Step-motor accurate but physical movement not
Step-motor accurate but physical movement not
(OP)
I have a fabricating machine which uses a stepper-motor-chain-feed combination to move 2x4 wood up to 24' along its axis. The stepper is pulsed for the calibrated/required movement but the actual movement accuracy is lacking.
I am looking for a measurement method/device which will give the woods location within 1/32" tolerance.
A rotating/pulse signal wheel that rolls along the top with movement doesnt work because of wood characterisics (sap/nonhomogeneous/etc).
Any suggestions?
I am looking for a measurement method/device which will give the woods location within 1/32" tolerance.
A rotating/pulse signal wheel that rolls along the top with movement doesnt work because of wood characterisics (sap/nonhomogeneous/etc).
Any suggestions?





RE: Step-motor accurate but physical movement not
Are you saying that the wood slips on the driving rollers? I.e. the motor and drive roller turns the required number of turns, but the wood is sliding on whatever is supposed to move it? Essentially, my suggestion would be to then somehow clamp the wood to the conveyor/chain, or use a pinch roller to force the wood to make good friction contact to the drive mechanism.
RE: Step-motor accurate but physical movement not
RE: Step-motor accurate but physical movement not
RE: Step-motor accurate but physical movement not
RE: Step-motor accurate but physical movement not
Or do you have enough power in the stepper to let you load up the chain drive by friction or counterspring to keep the chain under tension and eliminate the slop?
Or can you operate the stepper at a slower speed over the last 1" or so to sneak up on the right position?
Or can the clamping mechanism pull backwards as it operates before cutting, so thatthe slop is pulled out of the chain?
Or can the chain drive be replaced with a ball screw? A toothed belt?
Is 2x4 the only size of timber being processed? 24 ft of 2x4 is not very heavy, is the chain drive unnecessarily heavy for the duty?
Maybe the same technology that is used in optical mouses might be adapted to measuring the timber going past.
Jeff
RE: Step-motor accurate but physical movement not
The reel idea would work, but require manual labor to place the cable after each cut, and thereby slow the process still.
The optical approach sounds interesting:
Can you get 1/32" tolerance in 24' though?
RE: Step-motor accurate but physical movement not
That chain is obviously too expensive to replace. It can't have a tensioning idler fitted to it?
Jeff
RE: Step-motor accurate but physical movement not
The rotary encoder on a cable reel is probably your best bet, just automate a simple process by which an "L" shaped bracket sticks up and catches the leading edge of the 2x4 and stays on it, then attach the cable sensor to that bracket. I've seen that done on what are called 'Flying Cut Off Saws" in the lumber industry. If I can remember the mane of the cable pull encoders I'll post it.
"Venditori de oleum-vipera non vigere excordis populi"
RE: Step-motor accurate but physical movement not
RE: Step-motor accurate but physical movement not
Can a beam be shot up to 24' at the lumber end and reflected back to a measuring device?
It seems like the end cut of wood would be too variable a surface and, the 1/32" tolerance is still desireable.
RE: Step-motor accurate but physical movement not
RE: Step-motor accurate but physical movement not
I like the sound of that laser idea (no physical contact) better though. Anybody know if it's possible?
RE: Step-motor accurate but physical movement not
(Think of the cheap light-beam circuit such as on electric garage door interlocks, department store "entry" buzzers, etc.) Same idea. You just need a higher precision version.
Naturally, the end of the lumber may not be perfectly flat. So, where is the reference point? You might end up using 3 or 4 laser proxs and have a circuit such as: "when 3 of the 4 lasers sense lumber = trigger the cut-off" to avoid triggering too soon due to a wood fiber protrusion.
RE: Step-motor accurate but physical movement not
I am trying to monitor the position constantly as it moves. It may stop & start 30 times in 24'. The whole idea is to know where it is all the time so that the requested movement can be made.
RE: Step-motor accurate but physical movement not
RE: Step-motor accurate but physical movement not
RE: Step-motor accurate but physical movement not
If it does not continously feed in the same direction, then I would suggest you add an external linear scale for position feedback. I have seen some people on continous converyor systems put an encoder on the distal end to compensante for chain/belt slop.
You start out saying stepper, then swtich to servo, which is it and what brand/mfg?
Is the gripper a positive gripper - I mean does it actually clamp and hole the wood? Or does it just push the wood and not actually clamp on it? If it doesn't have a positive grip, that can cause problems.
RE: Step-motor accurate but physical movement not
The 6' chain travels through about 2.5' of flat slotted track then loops around an 8" diameter toothed gear and always runs the same direction.
RE: Step-motor accurate but physical movement not
When you first start out, does it make accurate moves? And drifts over time - tolerance gets worse and worse? Or is it always sloppy?
For example, say you move 2ft of the wood and process it (your cutting, drilling?). That, lets just use hole, that hole is in tolerance (+/-0.0156" or +/- 1/64). Then you make another move and you accumulate a little more error, but still in your tolerance. Over time you keep accumulating error (repeatable error) and when you hit 20 feet your off by 0.5 inch?
If this is the case, there could be a problem with your scaling in the motion controller. Maybe you do not have your conversion factor right from rotary to linear. If it is correct and you notice a repeatable accumulation in the error, adjust your scaling to compensate for the repeatable error.
>The chain has cogs on it that grip the wood.
The 6' chain travels through about 2.5' of flat slotted track then loops around an 8" diameter toothed gear and always runs the same direction.
After you advance, the gripper comes back around to grip the wood again and advance it. Is there anything holding the wood when the gripper releases? When you grab or release the 2x4, it could advance a little on you..thus maybe that is why your looking for someway to sense it.
Are you feeding multiple piecies at a time? $200K chain sound pretty expensive. Could of purchase a 30 foot actuator less than that and controller to get your desired accuracy. Must be all the wood/sap and other stuff that collects on the system that led you to a chain drive? If only feeding (1) piece at a time and your accuracy problems are in the current griper set up, maybe you could redesign the gripper assembly. Have a parallel jaw gripper that clamps to the side of the 2x4. You feed your distance, then another clamp holds the 2x4 in place, you release the actuator gripper, move back you 2.5 feet, reclamp on the 2x4 then release your other holding gripper.
Maybe more detail on your process and your actuator setup if you can share?