Image Processing Reference
In-Depth Information
must therefore be switched on and off at appropriate times, while maintaining a user friendly interface
(e.g., that responds in a timely manner according to your expectations).
When the HMI is used to asynchronously wake up a device and maybe issue a request to perform
a measurement, the normal duty cycle is changed, which will result in a different power consumption
of the device. he update rate of an HMI is typically a lot faster than the normal duty cycle of the WSN
devices (since it is often used to get a “quick snapshot” of the process). If the HMI is used extensively,
not only will the HMI consume power, but the device will also run according to an entirely different,
from the planned, duty cycle. This could result in early depletion of batteries, or even peak power
consumption that exceeds the bounds of the available power.
Wireless Vibration Monitoring Case Example
The vibration monitoring device has an HMI consisting of two LEDs and one button. The LEDs
areusedtoindicateactivity,suchaspoweringon/of,status(sleepingoractive),andtransmitting
data.hebuttonisusedtoactivateordeactivatethedevice,andforceittostartameasurement,
and consequently transmit the data to its destination.
The device uses low-power LEDs, which have a current consumption of - mA when lit (LEDs
are normally specified for a - mA consumption!); the exact consumption depends on the sup-
ply voltage level (.-. V). Compared to the current consumption of the CPU in active mode,
µA, this is a very high consumption.
To further lower the power consumption of the HMI, it is only activated when a human is inter-
acting with the device. Although it makes no sense to blink an LED if no one is watching, there
are situations when this could be necessary, e.g., if the sensor is in a hard to reach place, but can
be inspected visually. The button is used to interact with the device, and the response is always
indicated by blinking the LEDs in different patterns. his pattern, e.g., short blinks, long blinks,
etc., indicates the internal state of the device (sleeping, measuring, sending, etc.). In order to even
further decrease the power consumption of the LEDS, the digital I/O pins used to control them
are set to an inactive state when they are inactive.
It is important to note that an HMI does not necessarily have to be a physical component within the
device itself. For example, a hand-held unit could communicate (wired or wirelessly) with the device
and request data outside of the normal duty cycle. The same goes for a remote terminal connected
to the network, which would cause additional traffic routed through the network. his means that in
order to make any guarantees in terms of battery lifetime, the topology and usage scenarios need to
be clear for the entire network.
27.6.4 Energy Efficient Protocols
In addition to utilizing low-power electronics and a clever software architecture (i.e., sleep-wake-up
scheme), the communication protocol has a major impact on the final power consumption of the
system. he communication protocol prescribes when the system needs to wake up, how it synchro-
nizes, how long it needs to stay on, etc. Relevant protocol design choices from a power consumption
point of view include:
•
Media access scheme: In a contention-free media access scheme, e.g., TDMA, collisions
are minimized, thus requiring less active time to send the data. The reverse is true for
contention-basedmediaaccesssuchasCSMA,wherethenodeirsthastolistenbeforeit
can send its data (which degrades rapidly when there is lots of communication).
•
Topology: he topology not only affects the timeliness of message delivery, but also the
power consumption. In a multi-hop system, a node may have to wake up to forward data
