It is frequently necessary to obtain measured data at different sites simultaneously or topically and to subject them to central processing and/or evaluation. Sensors, which are suitable for obtaining the measured data of interest, are usually placed for this purpose at the corresponding measurement sites. The sensors are connected to a central processing or evaluating unit predominantly in one of two ways, namely, in a cable-based or radio-based manner. Typical applications of such a system of obtaining measured data can be found in the area of safety engineering, especially the atmospheric monitoring of industrial plants, buildings, and the like.
Established, wired systems for connecting sensors to a central evaluating unit are known. For example, a 4-20 mA interface, Ethernet-based LAN applications, various home installation bus systems (LON, EIB) or bus systems from the area of automation engineering (ProfiBus DP) may be used. Cable-based systems usually mean, however, a great effort for installation or they cannot be used for certain installations at all, where installation would meet insurmountable obstacles. A subsequent integration of wired systems in an already existing infrastructure is often also especially costly.
The alternative is often seen in radio-based systems, which are likewise available in established standard applications. WLAN, Bluetooth or ZigBee can be mentioned in this connection, which are frequently used for applications in which wired systems would represent a great installation effort.
A radio-based system for obtaining data in a locally resolved manner requires a functioning radio connection to the central evaluating unit for each measuring site. This radio connection may be built up either directly from the measuring site to the evaluating unit or maintained via various repeaters to increase the range.
It is common to all radio systems that the maximum range, which corresponds to a set transmitting power, may be markedly reduced by structural conditions, for example, by walls or ceilings as well as obstacles in the terrains. The maximum range may thus shrink in WLAN applications from about 100 m to less than 10 m. It is known that the transmitting power can be drastically increased or additional repeaters can be arranged in the transmission path to compensate this reduction in range.
High transmitting powers mean numerous drawbacks, on the one hand. One of these drawbacks is the increase in power consumption, which means a considerable disadvantage especially in radio-based systems, which are often at least partially battery-operated, because a reduction of the use time is inevitably associated with an increase in the transmitting power.
Another drawback is the potential for interference, which originates from transmitters operated at high power. High transmitting power implies the risk that analog signals of sensors are interfered with, which may lead even to the temporary failure of measured data acquisition.
The installation of additional repeaters at closely spaced locations means, on the other hand, an increase in the cost and an increase in maintenance effort. Another drawback of the systems mentioned is the susceptibility or dependence of the data transmission on the functioning of the transmission path itself. A break in the radio connection or interruption of the data line may lead to total loss of the measured data received and evaluation may become impossible. This represents a safety-relevant shortcoming especially in monitoring systems.