Image Processing Reference
In-Depth Information
system has a cyclical transmission, e.g., as in WISA, a distribution pattern with steps can be noted
(---- curve).
In practical applications, a large difference between the two curves can be noted by compar-
ing the error rate for keeping a certain time limit, like sketched with the vertical bar. A factor
of results which means in the round table application that with one wireless system (----
WISA) there is an error, e.g., a production stop or even more severe problem, to be expected less
then once a year, in the other case nearly once a week. A dramatic difference was seen from the
user side.
The wireless communication subsystem transmits messages between the sensors/actuators (S/A)
and a base station. he communication requirements derived from the above include
•
Fast response times (generally less than ms).
•
(Practically) guaranteed upper time limit for the response time (e.g., ms on the wireless
interface with a miss probability of less than
−
under normal operating conditions).
•
Guaranteeahighreliabilityofdatatransmissionevenintheunfriendlyenvironmentof
factories, where radio propagation may be affected by many obstacles and where vari-
ous sources of interfering signals must be expected, e.g., with other users of the same
frequency band.
•
Have low power consumption, which is crucial due to the limited capacity of any wireless
power supply.
•
Serve a large number (up to ) of sensors/actuators to one base station.
•
Permit coexistence of multiple cells: Up to devices in a single machine have to be
supported.
•
Have a practically not limited number of sensors/actuators in a large area (
>
deviceper
m
volume to cover , nodes in a factory).
•
Permit coexistence with other wireless systems.
-
Be robust.
-
Eicientuseofthevaluablefrequencybandresource.
Currently, the most suitable radio frequency band available for high-bandwidth, short-range wireless
communication is the industrial, scientific, and medical (ISM) frequency band at . GHz. Radio
transceivers operating in this band are readily available, e.g., radios adhering to the IEEE ..
standard (physical layer as used in Bluetooth).
The ISM Band also has a good balance between possible disturbances (more likely at lower fre-
quencies, see Figure .) and a good range. Effective range is lower at higher frequencies with
a given transmit power. Electromagnetic interference is a concern, however measurements in dif-
ferent environments (see Section ..) have shown that typical industrial electromagnetic noise
is significantly reduced above GHz, with only arc welding providing broad band noise up to
. GHz.
The protocols of existing standardized wireless systems designed for other purposes do not
satisfy the requirements of the sensor and actuator level in machines used in automated factories:
•
Wireless local area networks (WLANs) (IEEE .) [] and Bluetooth (IEEE ..)
[,] and also forthcoming systems [], including ultrawideband (UWB) systems, are
all designed for high throughput between a small number of terminals, with less stringent
latency, reliability, and power requirements.
•
ZigBee (IEEE .. based) [] cannot reliably serve a high number of nodes within
the specified cycle time.
