Image Processing Reference
In-Depth Information
also increase interference for other systems like wireless-LAN, and increase delay as the network size
grows and collisions force retransmissions.
Secondly, there is no path diversity meaning that in case a link is broken; a new path from source
to destination must be set up. his increases both delay and overhead, and eventually will consume
all bandwidth available.
Battery operation for routers with many peers is not realistic, since the CSMA/CA forces it to keep
itsreceiveronforalargepartoftheframe.Keepinmindthatindustrialusersrequireseveralyears
of battery life, since frequent battery change on thousands of sensors is not an option.
Security in ZigBee can to a great extent be set up to meet the requirements from industrial users,
although care must be taken to use equipment from vendors which support the necessary security
mechanisms. A good guideline for ZigBee security is found in Ref. [].
In summary, the harsh operating environment, battery operation, and lifetime requirement render
ZigBee a nonviable option for the industrial automation application. WirelessHART on the other
hand is very suitable as it is designed to be a very robust communications technology in the presence
of interference and fading. In addition, WirelessHART has features that allow it to efficiently coexist
in a “crowded” frequency band, i.e., it is a “nice neighbor.”
27.6 Low-Power Design
In this chapter, we dive more deeply into the issues related to low-power design of hardware, software,
and the communication protocol.
27.6.1 Basic Principle
The aim is to use the available resources within the acceptable limits of the specification and never
have anything powered up, if it does not have to be. The task reduces to switching units, such as
the sensor, CPU, and transceiver on and off with the right timing. Assume a node needs to wake
up at regular intervals and transmit its sensor value if it differs from the last value by more than a
predetermined amount. After the value has been sent over the radio channel, the unit waits for an
acknowledge message indicating that the packet has been correctly received.
The required behavior of the node is best explained using what is known as a state diagram: a
schematic representation of the state the node is in, the events that may cause it to move from one
state to another, and the actions associated with each state transition.
Figure . illustrates a typical high-level state diagram for a sensor node, in which the “Measure”
and “Send” states together form an operational mode. he transition from active to sleep is managed
by a sleep-wake-up scheme, which has been derived from the measurement duty cycle requirement.
A natural prerequisite to implement an efficient sleep-wake-up scheme is that the active/sleep state
of various system components can be controlled externally. If and how this is accomplished can vary
greatlybetweencomponentsandcanhavequiteanimpactonthedutycycleofthedevice.
Wireless Vibration Monitoring Case Example
The vibration sensor is build around the following controllable components:
•
CPU
: the microcontroller has several controllable low-power modes.
•
External real-time clock
(
RTC
):theclockcanbepoweredonandof,aswellas
controlled using its “normal” software interface.
•
Radio chip
: the radio is a black-box that only provides on/off control, as well as
a reset possibility.
•
Analog filter electronics
: these filters can be powered on and off.
