In all manufacturing industries there is a need to collect information for measurement and control of processes and material management. Transducers of many kinds may be installed on machinery, storage tanks, materials and even finished products.
The one common factor in all these applications is the need to return data to a central control or monitoring system. Generally this is accomplished by sending data signals along a network of cables. These may be dedicated to the process data or the signals may be multiplexed over an Ethernet system via suitable network adapters.
There are a number of common problems associated with a cabled solution in an industrial environment, especially in factories and larger industrial facilities where multiple buildings may involved. Cables are expensive and often difficult to install. In addition to the cost of the cable itself, installation over large industrial spaces, often including high ceilings, spans over doors or roads increase the labour costs, create their own health and safety hazards and can affect the continuous smooth operation of plant equipment during installation. In addition to the high costs of installing cabled control systems, maintaining their reliable operation can be very difficult as they are prone to damage from vehicles, building and maintenance works and occasionally vandalism.
Wireless connectivity would seem an obvious alternative to all the downsides of cabled control systems. However the major suppliers of wireless process control systems tend to offer costly, proprietary solutions aimed at large scale production facilities. The high costs and buy-in to complex single source solutions has, in the main, excluded their take-up by small to medium manufacturing, industrial and even agricultural users.
The recent advent of compact, low cost, high reliability, license free short range wireless devices enables a new, low cost approach, to wireless connectivity for industrial control systems. Instrumentation companies, such as Deeter Electronics, are now able to integrate wireless communication into sensors and industrial receivers which can transform industrial control and measuring systems. These emerging industrial wireless sensor communication systems greatly reduce the cost and time of installation, increasing convenience and reliability and enable simple relocation of sensors should factory layout change. Some types of wireless probes are battery powered removing an additional layer of installation complexity by not having to provide mains power in remote locations which would be required for conventional measuring instruments.
A typical wireless sensor system will be based on a wireless enabled probe communicating directly with a “base station” receiver. In addition to wireless enabled probes other sensor inputs such as analog probes, 4-20mA sensors, float sensors or other switch types can be interfaced to a wireless sensor system via a suitable “sender” adapter which transmits the sensor signals over the wireless link to the base station receiver.
A wireless sensor system may also include a number of wireless routers to form a wireless ‘mesh’ network. This improves the reliability of the wireless signal by providing multiple signal paths and allows signals to be automatically re-routed around any broken links in the mesh creating a built-in self-healing capability to further ensure communications reliability.
A modern wireless sensor system will be based on the latest very reliable high power RF transceiver modules combined with a rugged external antenna to achieve the best possible wireless communication range. Range can be further extended by adding one or more wireless “routers” which receive and re-transmit the signal with increased gain. A “base station” acts as the coordinator for the wireless network and will provide a range of outputs for integration into control or monitoring equipment. Typical outputs would include a 4-to-20mA current loop driver, a serial communications output via RS232, RS485 or USB and open-collector transistors for relay operation etc. The complete system will operate in the 2.4GHz ISM (Instrumentation Scientific and Medical) frequency band. This allows the system to be used world-wide without requiring a site radio operating license. Local regulations may restrict the maximum RF transmit power and manufacturers such as Deeter provide different variants of the Wireless System devices for the USA, Canadian and European markets to satisfy these constraints. The IEEE 802.15.4 protocol, as used in the Deeter Wireless Sensor System, allows several networks to share the same frequency without interfering with each other. The protocol uses sophisticated techniques to ensure good communications using very low power signaling. Once the system is installed it will operate reliably for extended periods without any further intervention.
At maximum transmission power, depending on country of use, the distance between sensor node and base station may be up to 4km in an ideal, open field installation. For operation in Europe, regulations restrict the maximum transmission power and a range of up to 1000m may be possible. However, in most practical environments the radio signal will be attenuated by obstructions and by multi-path fading caused by reflections. To improve range, a clearer signal path can often be achieved by increasing the height of the antenna.
Cost and applications
With the cost of a simple single wireless sensor and base station system being less than £500 it’s easy to see what an attractive proposition a wireless solution can be. Simple plug-and-play installation, ease of relocation, removal of costly and hazardous cabling make a wireless sensor system a welcome solution for control engineers in industrial and manufacturing industries such as food and beverages, cooling systems and agriculture.
The Deeter Group has been providing electronic services to industry since 1982. With the formation of Deeter Electronics in 1991 the company manufactures its wireless sensor system in the UK and is a major international supplier of control and instrumentation systems, including electronic sensors, industrial weighing equipment, reed and proximity switches, security switches, termination assemblies and other products and services to manufacturing industry.
The Deeter Group
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