Image Processing Reference
In-Depth Information
27.3.4.2 Low-Power Design
One important aspect of WSN is to keep the power consumption of a node at a minimum, while at
thesametimeprovidingthebestpossibleperformancetothesystemusers.helow-powernature
of WSN poses serious restrictions on the power consumption of the HW as well as on the SW
architecture.
Integrating functionality can be one way to lower the overall power consumption, but can put
severe limitations on the modularity of the system. his not only affects the HW/SW maintenance of
the device, but may also limit your options concerning what to power up/down and when, which in
turn can affect the overall power consumption as well as performance of the device negatively.
Although WSN devices are autonomous entities, a local human-machine interface (HMI) is
often useful or necessary during commissioning and troubleshooting. he specific low-power design
directly limits your options concerning the use of a local HMI, e.g., liquid crystal display (LCD), light
emitting diode (LED), buttons, etc.
Designing for low power consumption involves choosing low-power components. his may seem
trivial,butisotenacomplexissueasitusuallyinvolvescompromisesontheperformance.Typically,a
low-power CPU runs on reduced clock cycle with fewer on-chip features than its more power hungry
counterparts. he trick is to choose elements with just enough performance to do the job. In addition,
“over specifying” components to create margins in your design can lead to cost and maintenance
issues as the number of suppliers tend to shrink rapidly.
Finally, the low-power design more or less dictates the required average and peak power that needs
to be available in the system and this may have some serious consequences on the design of the power
supply solution, including choice of power source.
Lessreallyintrinsicallysafe(IS)moreintheWSNworld,andthemoreyoutakeaway,themore
challenging the design task becomes. In Section ., we invest some more “energy” in describing the
intricacies of low-power design.
27.3.4.3 Packaging
The packaging of the WSN device represents one of the most costly parts of the total product. The
environmental conditions and the device rating (IP class, EX, temperature range, etc.) all influence
the required ruggedness of the device and prescribe what material to use in the housing and how well
it must be sealed off to protect the internal components from dust and moisture.
heuseofalocalHMI,whetheritisanLCDorasingleLED,canhaveadramaticimpactonthe
overall power consumption of the device and thus needs to be accounted for in the low-power design.
If the device is to be able to operate in a hazardous environment, great care needs to be taken
throughout the design of the device. An intrinsically safe (IS) design is commonly required. The
basic principles of an IS design are simple and easy to grasp, but challenging to achieve in practice.
It requires long experience in the field as well certification by a notified body. Making IS adapta-
tions can have a dramatic impact on the power consumption and the form factor of the device (due
to additional protection and power limiting components). Often the maximum power that can be
provided by the power supply needs to be limited. he major requirement for batteries operating in
ATEX applications is for intrinsic safety. Specifically, this calls for a battery design that prevent both
excessive heat buildup and the danger of an electric spark. he modularity of the device is governed
by the need to define barriers with clear interfaces in the design to contain and transfer energy in a
controlled and safe manner. he maximum surface temperature of any component on a circuit board
under fault conditions must be considered.
The form factor of the device components is often given indirectly, either through product design
guidelines to foster harmonization and common look and feel or by the requirement to utilize an
already existing housing. This means that you often have a given space where you need to squeeze
in your HW components, which can have a dramatic impact on the required level of component
integration (single chip solutions, double-sided printed circuit boards (PCBs), etc.). It is common
 
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