What contributes to increased reliability and durability of Dynamic Message Signs?
Reliability and durability of Dynamic Message Sign can be assured by:
a) choice of verified components from verified suppliers (LED’s and LED module components)
b) operating most critical components (LED, power supplies) under low stress
c) Assuring safe environmental conditions for the components of LED module and LEDs
d) Assuring long-term durability of components exposed to environmental influences
e) Using manufacturing procedure that assures proven and certified level of quality
Since variable message signs differ in the size of visible area, the best way to evaluate operation under low stress is to evaluate power consumption per square meter.
In order to avoid misunderstanding if consumption in Watts or Volt-Ampers is relevant and at which exact point in the system, the best and easiest for method for assuring low stress is calculating power through current consumption of LED’s.
Simply calculate the number of pixels in 1 sqare meter of the offered sign and multiply it by the current through each LED on white color. Currents through each LED are visible in manufacturer’s CE test report.
The lower the total value of current, the more efficient the sign is, meaning that less energy is used to produce desired optical output defined with EN12966 optical classes.
Improving reliability and longevity of Variable Message Sign using sealed enclosures.
In the transportation industry, electronic systems are becoming ubiquitous. Besides street light and traffic light, there are myriad of other systems: VMS, detectors, cameras, network AP, sensors just to name some.
Operating conditions are harsh as these devices are not only exposed to environment, but also to effects of proximity to heavy traffic. Vibrations from heavy vehicles, corrosive exhaust gases, snow plow impact, salt during winters etc.
Enclosures that need to be large like the one used for various control cabinets and VMS are usually rated below IP66. It is difficult to ensure weathertightness of such enclosures as they are expected to be periodically maintained and components inside need active cooling by convection.
On the other hand, smaller devices as cameras or traffic detectors, even some traffic lights and street lights are in watertight enclosures. Common denominator of these devices is that they are expected to be maintenance free, expected lifetime is very long, they are low powered and do not require active convection cooling.
Sealed enclosures are superior in terms of reliability and longevity in harsh environment. Industry is inclined to use such enclosures once device fall into criteria discussed above. Looking at the criteria of price, simplicity or power consumption, modular VMS unit is more similar to camera than to VMS, so it makes sense to use watertight design.
Technical aspects of positive impacts of watertight enclosure design are further discussed below.
About 20% of electronic assembly’s failures are moisture related, right after heat and vibration effects.
Moisture related failures are totally eliminated with sealed (IP67 and IP66) enclosures, therefore, devices in sealed enclosures are expected to be at least 20% more reliable and durable than devices in non-sealed enclosure.
Resistance to environmental effects, primarily moisture and corrosive volatiles is handled on several levels in typical electronic device.
1. Electronic component level
Majority of electronic components are manufactured in sealed packaging. Potting or sealing material is often epoxy resin or silicone. Even this package offers protection against splashes and temporary submersion, under constant high levels of relative humidity corrosion might occur.
Typical example is shown in figure 5 on the link below.
Beside corrosion, other moisture related problem occurs at component level. Moisture absorption, swelling and delamination of plastic is common as well as metallic dendrites and whisker formation.
2. PCB assembly level
Electronic assembly without additional conformal coting protection is usually declared up to 85% relative humidity. For higher levels of protection, conformal coating is employed.
Conformal coating is effective on perfectly clean surfaces and only temporary exposure to water. Small amounts of impurities left under coating can make thig worse.
Enclosure can be sealed or open/vented.
Vented enclosures require complex set of climate control systems to keep humidity levels in safe levels.
Figure 14. shows such cabinet.
Filters, fans, heaters are devices that requires periodic maintenance and in general they have lower life expectancy than entire device.
In vented enclosure, moisture is not the only problem, especially in polluted areas. Road vehicles are source of hydrocarbon volatiles, nitrogen and Sulphur oxide which can form acidic environment in contact with moisture. This leads to deterioration of sealed and conformal coating protected components.
Sealed enclosure guaranties 100% effectiveness against pollutants: as explained on page 8 of document:
One somewhat surprising effect of sealed enclosures is the reduction in reactive gas concentrations within the enclosure. It might be logical to conclude that within an enclosure the gas concentration would come to equilibrium to the outside gas concentration. However, this is not the case. The gas concentrations under discussion are in the ppb range. Within a well-sealed enclosure, the reactive gas molecules get absorbed and react with materials in the enclosure thereby depressing the gas concentration.
Sealed enclosures also have some limited exchange of moisture with environment. However, time for moisture to reach equilibrium between internal air an environment is very long. Therefore night/day, humid/dry season do not create occasional high humidity spikes in atmosphere in the enclosure.
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