The notion of modular manufacturing started to take shape in the early ’90s in the automobile industry when U.S. and European manufacturers began feeling economic pressures from the more flexible Japanese manufacturing plants built in the U.S. Several of these plants produce as many as five different automobile models on one line. Leading this build-to-order capability was the installation of machine modules designed for instant changeover based on customer requirements.
This continued push to flexibility has expanded to other industries, forcing machine builders to adopt new, more agile machine designs. This new focus on flexibility most often requires machines with multiple stations, where each station does a specific type of operation, but isn’t necessarily the same for every part or assembly being produced.
The User: the Ultimate Judge
Flexible, modular manufacturing can be found at John Deere Harvester Works, East Moline, Ill. With modular manufacturing, its row crop planter production can produce build versions on one assembly line. Parts manufactured to order by cells adjacent to the assembly line are moved to respective assembly areas when the planter requiring those parts moves into position. “The improvement in factory production volume, finished product inventory cost, and delivery time justified the cost of changing our production methods,” states Bill Fulkerson, technology and information analyst at Deere.
Modular Maximizes Configuration Choices
To stay in tune with this trend toward modular manufacturing, machine builders must develop control schemes with the same modularity and flexibility, while reducing installation and commissioning costs. Improvements from open standards and robustness of fieldbus networks have done much to meet the new modularity criteria.
As this continues, there’s a growing need for higher concentrations of signal capturing via machine-mounted I/O. Pre-terminated field connections mean remote I/O can be installed and debugged quicker because of fewer wiring errors. The resulting decrease in cost is substantial when compared with the cost of wiring to junction box terminal strips.
“The I/O used on machines really is dependent on the machine designer, but for the most part, small, block-style I/O is becoming increasingly popular when the number of points at a node isn’t extensive,” says Dave Cole, president Cole Controls, Grabill, Ind. As a system integrator for PLC and network-based machine control solutions, Cole works with a number of fieldbus systems, many of which are connected directly to remote I/O racks distributed around the machine envelope. Cole also sees a number of installations in which a fieldbus is used as the primary data link between distributed intelligent I/O slaves, but the slaves incorporate a secondary sub-network such as AS-i or HART to connect to various locally distributed I/O blocks.
“There’s a big push to link smart devices via fieldbus or Ethernet versus running discrete wires because of installation costs,” adds Cole, pointing to an application in which Profibus links a remote, 400-hp, Atlas Copco ZR315 screw compressor to a central data-collection terminal. This application, installed at the Pernod-Ricard Seagram-brand distillery in Lawrenceburg, Ind., uses a legacy Siemens 505 PLC as the central data-collection intelligence source to interrogate the compressor over Profibus. The compressor has its own intelligence and distributed I/O. The I/O network within the compressor unit is a proprietary sub-network that collects data on compressor oil temperature and pressure, water temperature and pressure, air temperature and pressure, as well as speed, and then records how long it operates at certain levels.
FIGURE 1: BURNER CONTROL AT THE SOURCE
Wired Ethernet connects distributed control boxes, while an RS-485 sub-network connects VFDs and burner controls to the main network of a walnut dehyrdrating machine. Wireless Ethernet distributes data to the remote PC to monitor trending and history for all operations.
Although simple in concept, this use of distributed I/O optimizes compressor usage, reducing power costs while maximizing usage. “We’ve used Profibus extensively over the years for controlling or monitoring scales, safety barriers and other equipment, but we often have to convert Profibus to another network that’s standard on machines or equipment we purchase,” says John Netzley, senior staff engineer at the Lawrenceburg plant. Atlas Copco compressors use DeviceNet as the standard fieldbus interface. To interface with the Profibus network, Atlas supplied a bridge to convert DeviceNet to Profibus at the compressor.
Cole’s observations are supported by Steven Redcay of Phoenix Contact. “Selecting an I/O system that’s open to multiple networks enables machine builders to standardize on an I/O structure with the flexibility to choose the best network for each individual application,” says Redcay in a recent white paper. “This can shorten design time for the machine builder, and reduce training and maintenance costs for the user. Block-style I/O offers lower installed costs and is optimized for highly distributed digital signals; whereas, modular I/O structures allow higher concentration of signals at one location, but have a higher installed cost.”