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Selection box01 January 2004To end up with the optimum enclosure, a specifier must consider numerous variables from size, operational environment, to function and key personnel issues. John Wilkins, Marketing Services Manager, Rittal Ltd, boxes clever Enclosures for electronic systems come in a wide variety of shapes and sizes,
from simple plastic boxes, instrument cases to large floor standing cabinets and outdoor enclosures. Custom enclosures are specifically designed for use in a particular application or environment but the same design constraints and considerations need to be applied to all enclosures containing electronics.
But how do you go about specifying an enclosure? By answering a series of questions the basic style, size and shape for the mechanical housing can be ascertained. To start one must decide if the electronics will be contained on a single board, mother/daughter board sandwich, or as a multiple board, bussed system
with a backplane.
One must also consider the application for the system and whether there is a need for portability. Is the
application a “stand-alone” unit or part of a larger overall system? And what are the aesthetic considerations? By answering such questions, the basic style of enclosure, whether a simple box, a rack-mount shelf or subrack, an instrument case or outdoor enclosure, can be determined.
Other considerations are: the system’s need to withstand shock, vibration, moisture or dust ingress; EMC issues; resistance to external damage; and the ergonomic needs of the user. Answering these questions
will narrow the field from the range of options.
If the unit is to be portable, it will need space for either batteries, or mains conversion. The PSU can need to
be a plug-in for easy exchange, or N+1 redundant to ensure continuity of supply following a unit failure. The
developer must also think about the heating effect of the power supplies and how the heat will be dissipated.
Then one must look at how data will be received by the system and the means of response and control of the process under consideration. Is there an “industry standard” cabling structure involved? Wireless, infrared, or fibre-optic links may be used to keep the unit remote from the action. Copper connections will require different space and handling both inside and outside of the enclosure wall, and wireless/infra-red options may need additional circuitry (with accompanying space) as well as suitable sites for antennae or target windows.
The need for cooling provides us with more potential enclosure problems. If we need to keep dirt, moisture and EMC out, how do we bring cooling air to the enclosure? As processing power and system power increases, so does the need to remove the heat generated. Even the batteries in small, portable, units will contribute to heat generation. There will also be local thermal effects between the enclosure and any near neighbours.
In 482.6mm (19in) racks, it is no use adding a super-cooled system if it receives an air supply from the hot
exhaust of an adjacent unit. For this reason it is common to ventilate rack-mounted equipment from the bottom at the front and exhaust at the rear and top. This keeps the cool air at the front, and the hot air at the back for maximum cooling effectiveness. The rack may even be supplied with conditioned air to maintain the inlet temperature for all mounted systems. Some parts of the system may require heat sinks, and appropriate air-flows. Some new server systems are considering liquid cooling (some now available from Rittal) that improves the system’s heat management and performance.
Even a simple box can become inefficient when its set-up means a disproportionate amount of time is
needed to replace a battery. Having to remove a dozen screws to access the battery compartment may prevent unauthorised access, but think about the effect on the user who is in a hurry. Box and system design needs to allow access where necessary, and to restrict it elsewhere. In the 482.6mm environment, the cabinet may include a lockable door, while inside, the subracks contain “hot-swappable” circuit boards, power supplies and fan systems. These are partly enabled by utilising the subrack rear for input/output cabling, to leave the front clear for service exchange. This is one of the underlying principles of Rittal CompactPCI systems.
The major factors to improve reliability are the cooling and environmental considerations. Properly cooled systems generally increase the Mean Time Between Failures (MTBF) of critical components. Consideration of the environment the electronics will normally operate under will ensure mechanical, EMC, dust and liquid
protection, reduces the risk of external factors damaging the innards. Portable equipment that needs a fork-lift to move it is no use for remote site operation as test equipment, and will almost certainly be damaged “in
transit”. You may also need to plan for expansion. Some customers expect to be able to add on extras as they become available without having to replace the complete model. For example, it may be necessary to build a system with only three PCBs in a 21-PCB housing to allow for expansion. The enclosure or housing for electronic equipment performs several functions, and selection of the right enclosure could enhance the overall performance of the system. Perhaps the best way to ensure the housing is optimised is to discuss the requirements at an early stage in the product design with engineers from a major enclosure manufacturer.
Rittals Microcomputer Packaging Systems “competence” centre at Eckental, near Nuremberg, designed
for the creation and production of integrated systems, contains a computerised control system for quotation to production and assembly, process plant with test and specific design capabilities in backplanes, power
supplies and industrial computer solutions.
Computers are becoming faster and more powerful with a corresponding increase in the generation of heat,
which can dramatically shorten the working life of components. Heat generated from compact server rack
systems can no longer be kept under control using standard air-cooling, which is why the fastest processors available have not been installed in compact servers until now. This was the case at the Max-Planck Institute (MPI) for bio-physical chemistry in Gîttingen, until the worlds first liquid cooled server enclosure, developed by Rittal, was installed. MPI works with a 3D Kryo-electron microscope, which examines the structures of biological macromolecules. The 3D structure is calculated from 10,000 - 40,000 images. In order to be able to produce even more precise results, up to 500,000 images per molecule are necessary, needing enormous computing which in-turn requires intense cooling-power. Dr. Henry Stark, head of the working group at MPI summed the problem up as “Adding even more conventional systems would be very expensive and pushed us to our financial limits as well as taking up valuable space”.
Liquid cooling appeared to be the most suitable way out of this dilemma and had more or less taken the
decision to develop such a server enclosure by themselves. However, a visit to CeBIT Exhibition in the spring of 2002 saved the MPI scientists the anticipated long and costly development, as Rittal was exhibiting
a working, liquid cooled IT rack, which was filled with industrial PC ATX racks. Around six months after the
initial discussions, Rittal installed a server rack tailored to meet the high requirements of MPI. Thirty-four dual Athlon mainboards (MP 2000+) with their combined computing power (which makes it approximately
number 450 in the “Top 500” fastest computers), will increase MPIs image processing capacity in the future. Heatsinks fitted on the CPU are responsible for the immediate heat dissipation at the precise point where it occurs. Incoming and outgoing pipelines have been integrated into the enclosure, and in order to ensure a specific supply of liquid, are interconnected with an external, central re-cooling unit. The Rittal CMC Module (Computer Multi Control) provides additional safety by monitoring the incoming and outgoing
nominal and actual water temperatures as well as the flow-rate. The enclosure is the first step in a new era
of revolutionary server cooling and has proven a great interest to specialist commentators around the world. Contact Details and Archive...Related Articles...Most Viewed Articles... |
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