Wireless instrumentation solutions are becoming more popular as the equipment becomes more flexible and sophisticated. Typically, the following are the reasons most commonly cited for choosing a wireless solution:
* Where wiring is impossible, difficult – or too expensive.
* In remote locations where no existing power source exists – and thus a battery-driven wireless sensor would be appropriate.
* In hazardous environments containing volatile chemicals and flammable liquids and/or gases and vapours, and where a Class 1 Division 1 certification might be required.
* Where the unit being monitored is mobile or moving around.
Wireless system components
Wireless instrumentation uses radio link protocol to communicate between base radio units and field units. Field units would include all manner of sensors, for measuring the physical parameters being monitored. The base units accumulate and distribute data, serving as a link to various control systems (DCS, PLC, PC and Mimic panels). Repeaters can be used to extend the transmission range, and additional/optional antennas can be used for either directional or omni-directional signal enhancement in cases where obstructions are in the Fresnel zone.
Different requirements call for different arrangements
The topology between field units and the base unit can be simple peer-to-peer for a standalone application, star topologies, mesh topologies or even wired fieldbus sections being repeated wirelessly to a gateway, etc.
Different signal modulation schemes
Over time different modulation schemes were developed to minimise communication errors by making the system less vulnerable to interference from other signals and any other noise that may be present in the band being used. Some of these techniques include:
* Frequency hopping spread-spectrum (FHSS) – alters the wireless carrier frequency to suppress possible interference from other radio signals, while at the same time minimising possible interference to other radio signal devices.
* Frequency shift keying (FSK) – Like FM, but the modulating signal is a digital signal, so instead of varying continuously, the carrier 'jumps' between frequencies representing logic levels.
* Time division multiple access (TDMA) allows several field units (using the same carrier frequency) to communicate to one base radio while maintaining deterministic communication timing for process variable information and the capability of communicating unit configuration information.
* Multiple frequency hopping patterns allow several base radios to co-exist in a single radio cell without interfering with each other.
Measures to ensure reliable communication include the following:
* Message Synchronisation Bit Stream to match the communication timing of the base radios to field units.
* Manufacturer Identification Code to ensure message security between base radio and field unit.
* Send-Receive Acknowledgement, where every communication initiated by the base radio or by the field unit is acknowledged by its intended receiver.
Choosing the appropriate radio scheme
The expected range between a field unit and the base radio or repeater is often the deciding factor in this technology. The transmission power permitted in the 868 to 870 MHz ISM (industrial scientific medical) band, for instance, is very low (10 to 25 mW) – and there are duty cycle requirements to be considered as well. As a result the expected range is only 300 m.
Many vendors offer 900 MHz – which is unfortunately in the cellular band in South Africa, and thus not allowed.
The 2,4 GHz band is another ISM band available to South Africa. The expected range reduces (non-linearly) with increasing bandwidth, so the transmission power has to be increased to obtain the required distance. This means that the battery life of remote, battery-powered sensors will shorten as bandwidth and distance between antennas increase.
Large range of wireless sensors and I/O units available
Wireless sensors are available for pressure, level, flow, temperature/thermocouple, acoustic valve leakage detection. There are also input devices: 4 to 20 mA, 0 to 10 V, including digital, and output devices: 4 to 20 mA, including digital.
SCADAGroup offers advice and design information for those looking for wireless instrumentation that connects to PLC and DCS equipment.
For more information contact Jaco Hoogenboezem, SCADAGroup, +27 (0)83 282 5706, [email protected], www.scadagroup.co.za
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