Image Processing Reference
In-Depth Information
storage. This enables the power supply to provide an even power supply level in spite of a vary-
ing “incoming” power, and to handle intermittent peak consumptions. he size of this intermediate
storage and the time it takes to charge it directly impact the “maximum” duty cycle.
Although battery technology is progressing, the battery still constitutes a major part of the foot-
print of the device if we are to reach the - years lifetime that are typically quoted. he same goes for
the energy-scavenging solutions, which usually contain some form of intermediate storage as well.
Typical parameters to consider when choosing battery technology are operating temperature
range, energy density, nominal voltage, and Watt hours (for a given cell size). However, there is more
than meets the eye here as well. For example, in some types of lithium technologies, passivation may
lead to voltage delay, which is the time lag that occurs between the application of a load on the cell
and the voltage response. This behavior becomes more pronounced when you have very low duty
cycles (since the passivation layer grows thicker).
Ifbatterychangeintheieldistobesupported,specialcareneedstobetakentoallowseparationof
thebatteryfromthedevice.hisisespeciallytrickyifthedeviceisgoingtobedeployedinahazardous
environment, where any spark must be prevented.
Low-power design is all about powering up/down various parts of the system. In Section ., we
find out that there is a lot more to consider than on/off.
27.4 Reference Case
As with other resource-constraint embedded systems, “the devil is in the details.” In this section, we
present one specific use-case of WSN in industrial automation. This application will be used as a
reference case for the remainder of the chapter to exemplify some of the details that really matter
when making the design choices.
The application is “wireless condition monitoring of AC motors using vibration analysis.” In the
following sections, all references to this specific use-case will be highlighted with a border:
Wireless Vibration Monitoring Case Example
27.4.1 Wireless Vibration Monitoring Application
The goal of the vibration monitoring application is to detect early signs of bearing degradation on
small AC motors (less than  kW), located on offshore oil/gas rigs, in order to be able to schedule
maintenance. The motors drive equipment such as pumps, air compressors and fans, and the load
conditions as well as the operating environmental conditions will vary (Figure .).
The application consists of several parts as illustrated in Figure .: vibration-monitoring sensor
nodes, repeater nodes, a gateway, and an asset management system (vibration analysis software).
For the remainder of the chapter, we will only refer to the WSN part of the application, which
consists of the sensor and router nodes. he gateway is actually also part of this but is considered to
be part of the infrastructure and often regarded as a COTS component. It is (usually) mains pow-
ered, and does not face the same design challenges as the resource-constraint sensor nodes. he need
for repeater nodes depends on the communication technology and the operating environment of
the WSN.
he WSN part of the application has the following design goals and requirements:
Low cost : the sensor/repeater nodes are very restricted in what they are allowed to cost,
which has a large impact on component selection and packaging. his also has the effect
that the number of nodes that will be deployed in the field should be minimized; i.e., the
range of the wireless communication should be as long as possible.
 
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