Image Processing Reference
In-Depth Information
27.9.1.3 Lifetime
Typical quoted lifetime values of industrial WSN devices are often closely related to the lifetime of
the power supply. This means not only that the components of the power supply should not fail
within the specified time frame, but also that the available energy is enough to operate the device for
at least the specified period.
If the power supply is based on a permanent storage such as a battery, the lifetime is highly depen-
dent on how the device is used in the field, i.e., it is dependent on the size of the network, the duty
cycle, the amount of data to transmit, and any asynchronous (nonscheduled) data access. Poor radio
conditions will result in increased retransmits, which further decreases the actual possible operat-
ing time. So, unless you specify these conditions, quoting, e.g., years lifetime of a battery operated
device really does not say anything about how long you can run your device in the field.
To determine the size (Wh) of the permanent energy storage required to reach the desired lifetime,
you also need to account for material aging, leakage, and self-discharge effects.
27.9.1.4 Form Factor
The form factor of the device is a critical parameter when designing WSN devices and the power
supply often has the most dramatic impact on the final form and shape of the device. There must
be some volume or area available in the WSN device, which is appropriate to mount the energy storage
or energy converter. This volume/area has a direct relation to the required energy density of the
storage, or power density if an energy converter is used. The often desired small size and weight of
WSN devices is thus a clear limiting factor when determining the suitability of available power supply
technologies.
27.9.2 Power Management
Depending on the availability, time behavior, and output characteristics of the source some kind of
power management will be necessary as illustrated in Figure ..
Apart from the primary source or energy converter, a power supply solution could involve an inter-
mediate storage (energy buffer), a backup supply (e.g., a battery, which would also allow to keep the
buffer size within reasonable limits), and a control unit in order to cope with I/O power fluctuations
and downtime periods, which are typical for many alternative sources, but also for the power demand
of an event-triggered sensor.
Rechargable storage +
auxiliary energy source +
backup system (battery)
Rechargable storage +
auxiliary energy source
Nonrechargable power
source (battery)
Power
Power
Power
Supply
control
Supply
control
Supply
control
Power fail
output
Power fail
output
Power fail
output
Alternative
energy
source
Electrical
storage
Battery
Energy
Energy
Energy
FIGURE .
Power management.
