Control system mounting options
In any machine control design, the designer is faced with choices regarding the mounting of individual components. Mounting options can be broken down into several basic categories: wall-mount, rail-mount, panel-mount, rack-mount, or cage-mount. Each type of mounting has unique attributes that designers must consider.
Wall-mounting is the direct attachment of devices to the walls of the control cabinet. Typically, wall-mounting is performed by drilling and tapping holes that match a hole pattern in the package of the device. Screws are then used to secure the device to the wall. Wall-mounting is very flexible, since holes can be drilled and tapped just about anywhere, but it is also labor-intensive, particularly when numerous devices must be mounted.
Rail-mounting is an improvement over wall-mounting. Instead of drilling and tapping holes for each device, holes are drilled and tapped only for the rail. The devices are then clipped onto the rail and can be positioned by sliding along it. The most common rail standard is the 35 mm DIN rail (EN50022)1. Two less widely used sizes are 15 mm (EN 50045) and 7.5 mm (EN50023).
|On-machine controls can make use of wall, rail, panet, rack, and cage mounting systems. Source: Siemens Energy and Automation|
Panel-mounting is used when devices must be accessed from outside of the cabinet while the wiring remains inside. Panel-mount devices are typically mounted in rectangular or circular cutouts in the cabinet. Most rectangular panel-mount devices adhere to standard cutout dimensions such as 1/32, 1/16, 1/8, and 1/4 DIN cutouts. Other standards exist for circular cutouts. The principal benefit of panel-mount components is the interface that they provide to the human. The principal drawback is the need to make cutouts in the cabinet.
Rack-mounting is typically used to mount major system components. The most common rack-mount standard is EIA 310-D, which defines the ubiquitous 19-in rack 2. Rack-mounting is frequently used to mount industrial computers and machine controllers. It is also widely used by IT departments to mount network and computing infrastructure components. In scientific applications, rack-mounting is commonly used to mount test equipment. Unsurprisingly, numerous industrial controls can be purchased in rack-mount form factors. Rack-mounting has enjoyed widespread use because it provides a number of benefits including ease of installation/removal and an organized and orderly appearance of the installed components. Its principal drawback is the additional cost of the rack and, in many cases, higher costs for rack-mountable components.
Cage-mounting is the mounting of cards (individual circuit boards or board stacks) within a card cage. The cards slide on rail guides and typically plug into a backplane. When used, a backplane carries both power and signals from one card to the next. Use of a backplane can eliminate a great deal of point-to-point wiring. The card cage itself may be rack, wall, panel, or even rail-mounted. Industry-standard and vendor-specific card cages exist. One of the more common card cage standards is the Eurocard standard (IEEE 1101.10)3.
The principal drawback of cage-mounting is the additional cost of the cage, and backplane. This additional cost is offset by reduced cost of individual modules, which need not include provision for power supplies, cooling airflow, inter-module connections, and protective enclosures. These supporting facilities are provided by the cage. For installations where several modules are needed, the overall installation cost can be lower for cage-mount systems than other solutions.
It is not uncommon for a machine design to use all of the above mounting techniques. How and where to use them depends on design constraints and designer preferences, with the driving factor being component cost, assembly cost, maintainability, availability of components, corporate or industry standards, or even aesthetics.
|Michael N. Thompson is principal development analyst for The Timken Company. Contact him at firstname.lastname@example.org .|