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Optoelectronics/Displays??

Composites beat steel as surveillance system case

Posted: 03 Nov 2008 ?? ?Print Version ?Bookmark and Share

Keywords:digital video surveillance? IP camera? composite material?

It is becoming increasingly popular to use a digital IP-camera connected to a cellular modem for remote monitoring via the Internet. However, the quality of live video that cellular networks can carry is limited, the number of cameras that can be used on a site is often limited and the amount of video that can be moved over time over the network is also limited by cellular carriers' fair usage bandwidth policies.

Traditionally, outdoor industrial video surveillance systems have been installed permanently, usually on lamp poles or telephone utility poles, with a wired video connection to a nearby central monitoring station. Some very expensive systems have used dedicated, very high speed, private, wireless networks. That poses the challenge of how to make it easy and practical for outdoor industrial digital video surveillance systems to use readily available, public wireless communications networks, be moved every few days, and to be as benign in appearance as possible so they don't attract attention.

Boundless has developed the Nail-and-Go, the first ultralow-bandwidth cellular wireless pole camera system for temporary outdoor industrial digital video surveillance.

Rugged enclosures protect cameras and processing hardware in all climates.

Cabinet considerations
The vast majority of enclosures used for outdoor applications are made of painted carbon steel. In the United States, over 90 percent of enclosures are made from steel, stainless steel or other non-ferrous material. The remaining 10 percent are made from non-metallic materials such as thermoset composites and thermoplastic materials.

When we approached enclosure manufacturers with our project, we had four important factors that had to be accommodated: corrosion resistance, extreme temperature fluctuations, thermal management and tensile strength. So we set out to verify if steel or non-metallic materials would meet our needs. Stahlin Non-Metallic Enclosures ended up providing us with evidence we needed?2;composite was the way to go.

The company wanted a sealed, outdoor, weatherproof system that could stand up to prolonged dampness, corrosive atmospheres and salt-water air. We chose stainless steel and aluminum hardware for exposure to the air, and Stahlin's fiberglass housings. Boundless houses its system in white Stahlin enclosures for use in warmer climates and gray enclosures for use in colder climates to help control the amount of solar radiation absorbed. Pigment is embedded throughout the walls of the fiberglass enclosures for stable, scratch-resistant operation. We use thick aluminum angles to protect the mounting flanges.

Composite fiberglass materials are well suited for use in coastal areas such as marinas and other municipal infrastructure. They are not susceptible to attack by halogens such as chlorine, bromine or fluorine. In addition, due to their inert chemical makeup, composite fiberglass materials will also withstand a wide range of aggressive acidic compounds or caustic alkali solutions. Moreover, Stahlin products are made from a patented base material known to provide improved resistance to UV wavelengths.

Defining strength
When designing our system, we knew the units were going to be moved from location to location every couple of days. Hence, we were concerned about the structural integrity of composites. We quickly learned that while tensile strength may be higher in steel, the steel's flexural strength, or its ability to resist denting, is actually quite lower. Steels are ductile in nature and can be stretched and drawn and will retain their shape after a load has been applied.

Most composite or thermoplastic materials will have a lower tensile strength but yield a higher flexural strength or toughness. As a general rule of thumb, the higher the strength the lower the toughness or its ability to resist denting. A good composite tensile strength characteristic is in the range of 10-18kpsi. Flexural strength for non-metallic materials ranges from 1,500-2,000psi and modulus of elasticity is 1,750kpsi.

Denting in a cabinet may seem insignificant, and in most applications where the cabinet is fully secured in a separate room would not cause much concern. However, in environments where the cabinet is exposed to the elements and when it can be moved every couple of days, this is of high importance.

Denting can cause two major concerns in carbon steel products. One is a breach of the protective coating. When this happens, the substrate can corrode causing unsightly stains or in some cases a breach of the enclosure proper exposing the components inside to the elements and potential for system failure.

The other factor is that the dent or deformation can cause the cabinet door and its gasket to lose contact with the cabinet side. This can expose the components to the elements and potential for failure something that we did not want our customers to experience. The Stahlin enclosures we are using have passed field testing for failure against denting.

Additionally, the strength to weight ratio, calculated as the tensile strength (kpsi) to weight (specific gravity) of three common materials used in industrial cabinets, is much higher in composites than carbon steel and even stainless steel. Typically, you will see a 50 percent to 100 percent improvement in the strength to weight ratio of composites over common steels.

Cameras watch from the bottom of the enclosure.

Controlling temperature
Our system could be hung from a pole during winter when temperatures could be -23C or placed in a city during the summer heat with temperatures at 32C to 38C. This temperature range forced us to consider if composite materials could survive. We are now aware that composites have been capable of withstanding the most extreme temperatures without the loss of structural integrity due to its low thermal co-efficient of linear expansion.

In addition, because they are a thermoset-type non-metallic material, they do not change shape under excessive heat loads often seen in exposed outdoor environments. Such exposure to a combination of UV rays and temperature can cause warping and degradation in thermoplastic materials?2;they do not change shape or chemistry under extreme temperatures. The composite enclosures we use are rated -40C to 121C.

Electronic equipment operates best and lasts the longest if its operating temperature is limited. Outdoor applications must stand the rigors of heat and cold, as well as dry and wet. Camera domes that are exposed to the elements must be heated to melt ice and snow, and avoid condensation and fogging.

We wanted a material for outdoor enclosures that helped maintain heat in the cold, but helped dissipate heat when hot. We designed a sealed, thermal management system and heat exchanger that uses low power to operate, and that takes advantage of Stahlin's thermal properties to maintain as narrow an operating temperature range inside the enclosure as possible.

Because composites are insulators, both thermally and electrically, they provide excellent thermal insulating properties in extreme cold and help resist additional heat loads in the summer from UV exposure. While heat transfer of steel is higher than with composites, this is the opposite of what is needed when trying to keep equipment within the enclosure warm in the winter. Additionally, in the summer months of high solar loading, the transfer of UV energy converted to heat energy to the interior can happen more quickly in steels compared with composites, similar to what happens to a car parked in the summer sun.

The company developed the comprehensive Nail-and-Go system to solve these many outdoor digital video surveillance problems. It uniquely solves the communications aspects of using cellular and other public wireless networks, and solves the video monitoring and evidence problems encountered by law enforcement. It can be used in both permanent and temporary applications. It enables companies to carry out more, and more comprehensive, surveillance operations at less risk to personnel, and at less cost. Furthermore, it uses the proper enclosure to protect the system when it is moved often from place to place.

- Steve Morton
CEO and CTO, Boundless Security Systems Inc.





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