CLOSED LOOP GAPPING AT THE DLI CONVEYOR TO SHARP CHAIN INTERFACE USING WIS LOW INERTIA RELIANCE MOTOR AND ALLEN BRADLEY POWERFLEX 755 AC MOTOR DRIVES ON THE DLI AND LOG TURNER MOTORS
The transition point at the end of the DLI and the beginning of the sharp chain provides the last opportunity to measure and correct the log gap as the log approaches the sharp chain.
As the log gap approaches the end of the DLI it is measured every DLI chain tracking pulse. When the window of opportunity opens (no log overlap on the DLI and sharp chain) the gap is corrected by increasing or decreasing the speed of the DLI system proportionately to the amount of the error in the gap measurement as the log is bearing down on the sharp chain.
If the gap is measured to be too small the DLI system will slow down to correct the gap. If the gap is measured to be too big the DLI system will speed up to correct the gap. If there is no gap error the DLI speed automatically matches the sharp chain speed.
To allow the DLI system to run at a different speed than the sharp chain the PLC logic must detect that there is no log contacting the DLI system and the sharp chain with the same log simultaneously.
If the gap is too small the window of opportunity to correct the gap is very small so the DLI system must have the ability to decelerate very quickly.
If the gap is too large the window of opportunity to correct the gap is larger than when the gap is too small but the DLI system must have the ability to accelerate in very quickly to correct the gap before the window of opportunity closes.
The gap requirement at the DLI sharp chain interface varies with scanner solutions that dictate the chipping and sawing positions.
When the next scanner solution will need to move one or more of the side chipping heads inward the gap will be set wide enough to prevent the DLI from having to stop and wait for the side heads to clear and move inward. This will be the widest operating gap.
When the next set will cause the side chipping heads to move outward the gap target can be set to a target between the narrowest gap and the widest gap setting.
When the next set for the side heads is the same as the last set the side head chippers, they will not move for the approaching log so the gap target can go to a gap that the bandmill set lock zone can tolerate without causing the line to pause for the bandmill hydraulics to get into position. Typically the set lock zone at a twin bandsaw is only 24 inches so a gap of 5.5 or 6 feet can automatically be targeted when the line is running in high speed.
The narrowest operating gap will be automatically targeted if the side heads and the band mills do not need to move. This will be the smallest gap the downstream equipment will allow.
Therefore, if the mill sorts the logs by diameter before sending them to the canter line, the machine will automatically process logs with a gap of 4 or 5 feet (or less) as measured on the downstream side of the bandmills.
The ultimate goal is to prevent the machine from coming to a complete stop. The biggest reason the machine will need to stop is if the log must wait for set changes at the chipping and cutting sections. If the log gap is too small every log may need to stop and wait for set changes. If the ideal gap is set for each log the DLI system will change feed speeds until the gap target is achieved the machine may never come to a complete stop. The average speed will be as high as possible and the piece count will approach theoretical maximum capability.
The PLC logic also automatically “Speed Compensates” the gap target. If the line is running at 150 feet a minute it may only need a gap of the largest set lock zone plus 6 inches.
If the line is running at 600 feet per minute the gap may need to be the largest set lock zone plus 32 inches. Therefore with speed compensated gapping the gap at the outfeed of the machine will get smaller as the line speed drops.
For Closed Loop Log Gapping to work at it’s very best the DLI system needs to have excessive power and torque to provide sufficient deceleration and acceleration conditions to ensure the gap is on target before the log hits the sharp chain and the window of opportunity for gap correction closes.
With the WIS drive and motor package we have seen gap accuracy and repeatability of plus or minus 2 inches or better as the log hits the start of the sharp chain.
The gap accuracy is made possible by the acceleration and deceleration capability of the DLI chain drive system that can remove up to 4 feet or more gap error within the window of opportunity between logs at the DLI and sharp chain interface.
Production gains came from two different achievements.
The repeatability and accuracy of target gaps and the reduction of the average gap, as measured at the outfeed side of the bandmills, has allowed the machine to operate closer to the theoretical maximum production than ever before.
The machine also can now be programmed to avoid feedworks dead stops to wait for side head set changes. The new drive system provides the strength for more seamless “SET ZONE” feed speed programming (instead of dead stop programming) to anticipate setworks motion for the next log coming down the DLI.
In some severe cases a dead stop can still happen to wait for a hydraulics cylinder to complete it’s move (slow setworks, long cylinder moves). The new drive system allows the dead stop points to be moved farther down stream.
Dead stop points moved farther down stream can be used to filter out almost all feedworks pauses because the stopping torque is available when needed.
If the special high torque WIS motor wares out, a conventional 3-phase AC motor can be used as a replacement and the PowerFlex 755 can be quickly reprogrammed and the system is back running quickly.
We use Allen Bradley Ethernet IP to communicate to remote I/O and motor drives. The BOOTP utility (free) is used to setup network IP addresses, no special network software needs to be purchased and learned.
The objective has always been to process as many logs as possible with optimum recovery:
As other parts of the process increase in speed, there is Increased demand on the DLI drive to be as responsive as possible:
Why not increase the motor HP?:
Low inertia AC motor is 1/3 the inertia of a standard AC induction motor.
Comparable dynamic performance to a servo motor – same process payback.
Lower cost than servo motor.
Rugged, industrial design induction motor.
Will work with a standard Powerflex 700 drive.
Not limited in maximum speed.
Can be easily retrofitted into existing systems.