How Delta fits in the Motion Control WorldPress Roll Case Study
Coastal Machinery Develops Automatic Press Roll for use with Planer Machines
Coastal Machinery Company, of Portland, OR is a leading manufacturer of planing mill equipment for the forest products industry. With a history of "firsts" - including the first 1,500 fpm planer, the first 2,000 fpm planer, and the first 3,000 fpm capability planer - the company has now developed the first automatic press roll control system for use with planer machines. Key to the high performance of Coastal Machinery's new press rolls is use of an advanced hydraulic motion controller supplied by Delta Computer Systems, Inc. of Vancouver, WA.
"We call them 'intelligent press rolls,'" said Joe Butz, electrical engineer at Coastal Machinery. "By making the press rolls intelligent, we can make the planer machine more intelligent."
Typically, press rolls for planer machines have been operated by two hydraulic cylinders: An operator manually positions the rolls using one hydraulic cylinder, and the second cylinder applies pressure. It was not possible to maintain both constant force and position of the roll with a single cylinder. These older systems based on two-position or "bang-bang" valving could not adapt easily to respond to varying thicknesses of boards, and boards were often split or damaged by press rolls that moved too quickly without feedback control. In addition, running lumber at high speed through the old press rolls resulted in maintenance problems and shorter machine life due to repeated hydraulic shock and vibration. Mill operators often had to make difficult tradeoffs between maximizing board feet of lumber through the mill and maintaining quality of the output.
In the new design, Coastal replaced two hydraulic cylinders with a single one, and changed manual positioning to automatic positioning with improved control of force on the wood. This improves throughput and reduces damage to the wood while cutting cost of machine maintenance dramatically.
To control the motion, Coastal Machinery's engineers chose the RMC100 motion controller from Delta Computer Systems. The RMC100 is unique in its ability to control both pressure and position, and a single RMC can control up to eight motion axes. In the Coastal Machinery system, the RMC controls either six or eight axes depending on whether there are three or four feed rolls to and from the planer. (Each roll involves two axes of control: one for pressure and one for position.) Each hydraulic cylinder is operated via a Bosch NG6 servo valve, based on inputs from a Balluff magnetostrictive displacement transducer (MDT) and two pressure transducers per cylinder to control the net force in each cylinder (Fig. 1 shows the key elements of the hydraulic circuit.)
The same hardware that gives the new Coastal Machinery press rolls automatic control offers the ability to build new features into the machines. For example, the system can measure the thickness of the wood going through the planer, something that was never possible before without the addition of custom-built electronic systems. By tracking this information, users can also apply statistical process control analysis to output and plot machine performance as a maintenance tool. Another advantage of automatic board height sensing is the ability to detect "thin boards," which can cause handling problems in the lumber transfer mechanisms, and momentarily adjust roll height without operator intervention.
Coastal Machinery engineer Joe Butz has had previous experience using Delta Computer Systems motion controllers, finding them very easy to use and able to produce quick design results. "We prototyped the initial control system on a temporary electrical panel," said Joe. "We demonstrated correct hydraulic motion in our shop in five to six hours time. Then we installed the system in a mill and were showing results two to three hours later the same day. The next day the prototype press roll was used in a full production run with no interruptions to our customer's process."
The new control system is also the first press roll controller to use Ethernet as a fieldbus interface to connect major elements of the system (see Fig. 2). This offers the significant advantage of being able to easily connect the system to laptop computers for programming, debugging, and maintenance. Also, because Ethernet is used so extensively in the PC market, this connection has become rock solid and inexpensive to implement. Coastal plans to ship low-cost Ethernet network hubs with most of the machines the company installs. An Ethernet interface is supported directly by the Delta Computer Systems RMC controllers, and Delta's easy to use RMCWin motion programming and optimizing software runs directly on attached PCs.
Figure 1. Detailed Hydraulics Schematic
Figure 2. Major System Blocks and Connections
