By Loren Werneke, MoCo Engineering and Fabricating
The flagship 10-axis stickering stacker is our most sophisticated machine. Used on sawmill lines, the stickering stacker inserts wooden or aluminum spacers, called "stickers," on 1- or 2-ft centers between each course of cut-to-length lumber while simultaneously stacking the courses.
Typically, the motion on a stickering stacker was driven by hydraulics, variable-speed ac motors or a combination of both. MoCo had been considering an all-electric, servo-driven stacker designed to improve performance and reduce energy use. An experiment we conducted a few years ago proved that servos could operate well under sawmill conditions, but our company was unsure that the market would support such a machine.
Today, increased energy costs, decreased lumber prices and rising environmental concerns are impacting lumber mill profitability. To preserve profits, mills need greater productivity, less downtime, improved safety features, more energy efficiency and less potential for environmental cleanup issues. MoCo decided it was time to move forward with an all-electric, servo-driven machine that could provide lumber mills with an impressive return on investment. The company seized the opportunity by converting an existing 10-axis hydraulic stacker system to a line-regenerative, electric servo-driven synchronized stickering stacker (Figure 1). "The servo-electric concept helps us offer greater efficiency and long-term cost savings," says Allan Hahn, control specialist at MoCo. "The ability to reduce both energy use and environmental impact makes this stacker well-suited for today's challenging market."
Where the Drive Meets the Bus
To choose components that would make the servo-electric concept a reality, MoCo teamed with Northwest Motion (www.nwmotion.com), a local automation distributor that had been a supplier to MoCo since 2000. "The two suppliers we considered for this were Rockwell Automation and Rexroth," explains Hahn. "We went with Rexroth because I'd used their components at a previous company and was familiar with the ease of use with them. One of the biggest selling points was the follow-up help we had previously received working with Northwest Motion. They spent two weeks with us with the first startup, and then they helped with the fieldbus. I knew we'd get that level of support from them."
Based on its experience with servo designs, Northwest Motion recommended an IndraDrive system, IndraDyn servo motors and a common dc bus with regenerative capabilities so that excess power could be diverted from one axis to another or back onto a mill's main power grid. The final lineup included IndraDrive M digital intelligent servo drives with a CCD master communicating with a third-party PLC via DeviceNet, IndraDyn synchronous and asynchronous servo motors with IP65 protection class and the common dc bus with an HMV-R 65 kW regenerative power supply.
All motion functions and synchronization are handled by parameterizing drive firmware with no PLC code or programming required at the drive level. The drives are synchronized through high-speed Ethernet-based SERCOS III communications protocol and simultaneously control synchronization of the machine's fork lift, carriage, stick shuttle, stick placer, rake off, pull back and other motions.
"We put a Web port on the system, so we can get on the control system and troubleshoot while sitting at our desks via the Internet to diagnose issues such as servo sync or losing zero and to read the fault codes right from the drive," says Hahn.
The stacker has a 30–40% improvement in throughput over the hydraulic machine. Handling 8-ft-wide courses at up to 12 cycles/min and loads up to 40000 lb, the stacker receives lumber while delivering stickers into evenly spaced courses of wood. The lumber then is arranged and stacked automatically on the main hoist to be prepared for kiln delivery.
Each axis of motion follows its own onboard electronic motion profile while maintaining precise synchronization with other axes by simultaneously following a virtual master (VM). Each drive is capable of hosting a VM, and any axis in the system can be directed to follow any VM in the system. Free assignment and sharing of VMs for axis synchronization is a key to the system. Communication between drives is achieved using Rexroth's Cross Communication Drives (CCD) ring, with synchronization data traveling via SERCOS III communications protocol over Cat. 5e cable.
Operators can adjust stacker speed easily, matching throughput with production, or switch operating modes on-the-fly to allow independent or synchronized jogging or indexing of an axis to a specific position. In auto mode, axes remain electronically synchronized while reporting position and drive status back to the operator's display panel. No limit switches, timing gears or external encoders are needed to determine or control axis positioning (Figure 2). The PLC controller monitors the status of all drives in the system, allowing an orderly shutdown in case of an axis fault.