Factory networks get ready for real time

Upgrading equipment is one way to improve factory productivity. Sometimes, old plant equipment needs replacing with new equipment designed from the ground up to employ the latest in optimization, control system, and networking technology.

On the network front, recent developments in Ethernet have placed it at the cutting edge of industrial networking. A big reason is the prevalence of Ethernet as an accepted networking standard. This means it's supported by a lot of industrial and commercial computer platforms. Also, because of the wide acceptance of Ethernet, the cost per node for Ethernet devices is rapidly decreasing, making hardware, cabling, and installation relatively inexpensive.

Ethernet on the factory floor can link up control systems to a factory-wide Ethernet network, allowing access to machine-center production and diagnostic information. So the computer in each machine center can be directly accessed from a PC anywhere on the network. When connected to a firewall, Ethernet allows accessing control systems over the Internet, providing remote support.

Ethernet/IP, Ethernet Industrial Protocol, is an open protocol that makes the best use of commercial, off-the-shelf Ethernet hardware in an industrial environment. Ethernet/IP extends the traditional Ethernet standard with elements of time-deterministic messaging, which reduces the chance that critical data may be lost or arrive too late. Ethernet/IP defines both an implicit messaging format (for real-time I/O communications) and explicit messaging (for more conventional message exchange between devices). The implicit-messaging capability was developed because industrial applications have more stringent real-time activity-sequencing requirements than office/commercial applications for which Ethernet was originally designed. Rockwell Automation is behind the new standard and has opened it to gain wider acceptance in the industrial community.

Typically, an Ethernet/IP network uses an active star topology in which groups of devices are connected point-to-point to a switch. The benefit is that it can support both 10 and 100-Mbits/sec products. Network designers can mix 10 and 100-Mbits/sec devices and the Ethernet switch negotiates the speed. Also, Ethernet/IP handles large amounts of messaging data, up to 1,500 bytes/packet.

Ethernet/IP uses standard Ethernet, TCP/IP hardware, and an open-application layer called the control and information protocol (CIP), which is also the application layer used in DeviceNet and ControlNet networks. The protocol makes possible interoperability and interchangeability of industrial automation and control devices on Ethernet/IP.

Ethernet/IP supports both time-critical (implicit) and nontime-critical (explicit) message transfer services of CIP. Exchange of time-critical messages is based on the producer/consumer model where a transmitting device produces data on the network and many receiving devices simultaneously consume the data. Ethernet/IP supports time-critical message exchange (for I/O control), HMI, device configuration and programming, device and network prognostics and diagnostics, and compatibility with SNMP and Web pages embedded in devices.

Keep them logs a-movin'

The new standard was put to the test by the Collins Companies of Portland, Oreg., a producer of forest products. One of the company's mills in Chester, Calif., built in 1941, needed upgrading to process smaller logs that have become a larger portion of the raw materials available. The existing mill layout and equipment couldn't accommodate high-production rates while minimizing wasted wood. The goal is to maximize revenue from logs by favoring high-dollar cuts that produce large, knot-free boards. At the same time, the mill must deal with raw logs of different shapes and sizes, some of them irregularly shaped and with flawed segments.

To support automated optimization of timber processing, many of the machine stations have an optimizing computer that scans incoming material and makes decisions for the machine. The optimizers determine a cutting pattern to attain the highest recovery of wood, or highest lumber value, or both, working within the constraints of each machine center and the mill layout.

For example, a board edger cuts random-width boards from a single slice of a log, called a flitch, to maximize the value of the finished lumber. The machine operator assigns different grades to cross-section segments of each flitch. The optimizing computer combines these inputs with dimensional data from its own scan and tells the machine how to position the saws.

Perceptron Corp., a division of USNR, Woodland, Wash., provided optimization technology and served as lead contractor for the machine control systems. These were designed by Concept Systems Inc. of Albany, Oreg.

Concept Systems developed a common controls platform for each machine station using off-the-shelf hardware. This simplified and decreased the cost of programming, operating, and maintaining the systems. In addition, the system keeps a spare-parts inventory to a minimum. Common elements to the machine designs include hydraulics, optimizers, motion controllers, PLCs, and I/O modules. All PLC programs, motion-controller configurations, and most human-machine interfaces (HMIs) follow the same design architecture.

Selecting components

An Allen-Bradley ControlLogix 5555 PLC platform works with the mill's new networked control-system architecture. The ControlLogix PLCs sport high processing speed, communications flexibility, and a tag-based database structure that supports writing modular, maintainable PLC programs. Plus, the ControlLogix platform supports a range of communication protocols including DH+ (Data Highway Plus, an A-B serial protocol), Remote IO, Modbus, DeviceNet, Ethernet, Ethernet/IP, and a variety of other serial protocols. Communication modules can be used in almost any combination, and the ControlLogix platform can act as a gateway from one protocol to the other.

The mill uses RMC100 motion controllers from Delta Computer Systems Inc., Vancouver, Wash. They're flexible enough so a single motion platform can handle all the motion-control operations in the mill. System integrators would typically use different motion platforms for linear and rotary motion, resulting in greater inventory costs and more complex and lengthy development cycles. The Collins mill incorporates 10 RMCs with a mixture of configurations, controlling a total of 64 linear and rotary axes.

RMC motion controllers also support complex, high-level motion functions such as third-order spline function interpolations. The RMC's spline function makes it easy to program complex motion simply by connecting the dots between data points provided by the optimizing computers. Using splines, the motion controllers guide the movable curve saw to follow the contours of curved logs, providing higher timber output and less waste. A single RMC controller can control up to eight motion axes, with a mixture of inputs coming directly from quadrature feedback devices, synchronous serial interface (SSI) devices, and magnetostrictive linear-displacement transducers (MDTs). With this flexibility, the RMC can control precision hydraulics and high-inertia systems at the same time.

Hydraulic power is used throughout the mill because it is faster and more accurately positions heavy loads. However, hydraulic actuators quite often require more attention to tuning than electric applications. This leads to the additional requirement of a motion controller that can optimize control of precision hydraulic actuators. As with the PLCs, the motion controllers also need to support networking for the entire mill.

Some motion controllers, such as the Delta RMC, are designed with these characteristics in mind. And because they handle closed-loop control, they can deliver precise results. The key to unlocking the full benefits of hydraulic power is using the right motion controller and designing with hydraulics requires sizing and correctly placing hydraulic-system elements. For example, using a hydraulic-pressure reservoir or accumulator that is not the correct size can cause the system to react slowly or work inefficiently. Using incorrectly sized cylinders can also result in suboptimal performance. Using proportional servo valves can allow much better performance and precision than two-position (bang-bang) valves, but the advantage can be lost by mounting the valves too far away from the cylinders.

Connecting the elements

The sawmill networks all the machine centers. Upstream machine centers can interrogate downstream ones to check status and possibly redirect materials or modify cutting solutions to keep processes running, reducing the amount of operator intervention. Control-system programmers and maintenance personnel can remotely access programs and operating parameters, even by the Internet, which streamlines programming, tuning, and troubleshooting.

The mill uses Ethernet as the network throughout. Standard TCP/IP messaging is used for communications between the controllers, the HMI, and development PCs. Ethernet/IP is used for all communications between the PLC, motion controllers, and remote I/O racks. The two protocols reside on the same single-tiered Ethernet network. This ability to schedule high-priority messages as often as every few milliseconds contributes to high performance in machine-control applications.

Plus, the same network can be used for programming and diagnostics. Ethernet/IP makes data requests at determined time intervals and all peer modules on the bus broadcast at determined intervals. Local switches reduce bandwidth loading on the central router because of the potentially large amount of communications between local control system elements within the machine environment.

Each HMI node on the sawmill network is a Dell computer running Delta Computer Systems' motion-controller programming package RMCWin. Concept Systems' engineers use WebX, a Web-based application that lets remote computers access other computers over the Internet. They then run the RMCWin development software, HMI development software, or PLC programming software, and can tune and troubleshoot machine operations remotely. RMCWin can run on an external computer and interrogate the controllers' internal registers on the fly, then plot the actual motion profile versus the target motion.

Coordinating complex motion as processing speed increases is a challenge in any application. Logs must be processed as quickly as possible, yet each has a different size and geometry. The material transport systems position logs a bit differently at the input stage of each machine. PLCs and motion controllers can handle a wide variety of inputs from the operators and optimizing computers without skipping a beat. So production speeds and feedrates are constantly adjusted piece by piece to maintain optimal cutting speed.

Motion controllers provide special capabilities to support the coordination of multiaxis systems. Programmers can link multiple RMC axes so they move together, or so that some axes' motions are slaved to the motion of other axes, in either position or velocity. The RMCs support options, such as differing gains on the same system. They also handle MDT, SSI, and quadrature feedback with no hardware conversion. The controllers provide support for nulling valves with operating dead zones, and slave axes can be set up to move smoothly, even when geared to a master axis that doesn't behave smoothly. The Collins mill application used both the MDT and quadrature feedback types. All the linear positioners used MTS Temposonic transducers (MDT) and all rotary axes use quadrature encoders. Both types provide precise position feedback to within 0.001 in.

Downtime at a mill can be expensive, so a critical goal was to complete the installation and startup of the new mill in the shortest time possible. The RMC motion controller's development software provided tuning and diagnostic tools to quickly support the development of optimal motion programs. The network also allowed for clean and consistent interfaces between system components, which helped speed development.

Owing to the new plant design and state-of-the art control systems and emphasis on optimization, design capacity is 250,000 to 275,000 board feet of production/shift (but Collins employees soon expect to exceed 320,000 bf/shift), setting new records compared to its previous mill (200,000 ft/shift). In addition, Collins has been able to improve the quality of the lumber that is produced and has maximized the yield from available timber.