Wireless sensors and networks can make sense even over short distances.
Banner Engineering Minneapolis, Minn.
The operator of a major water park had to monitor the water level in its tank for a performing whale, a critical factor in keeping the beast healthy. Unfortunately, the cable from the level sensor ran for about 10 ft beneath a heavily traveled public walk that was regularly drenched by the splashing whale. When the cable failed, which was frequently, it had to be dug up and replaced, inconveniencing the public and adding to operational expenses. Eventually, a manager got the bright idea to use wireless modules to send data about 20 ft, and walkway excavations stopped.
Anecdotes such as these make people aware that wireless technology, which is frequently used in factories and other industrial settings where I/Os must be sent from a remote area to an operator located several miles away, can also make sense if data only has to travel a few feet.
Wireless traditionally has been thought of as a way to send signals 1 to 3 miles. But it also provides advantages in short-distance applications occasions in which I/O must be transmitted some 200 ft from one side of the plant to another, or even from room to room, because running cable is challenging. For example, plant floors commonly have hazards and obstructions, so companies must buy and install more cable than they would need to go around them. An alternative, running cable beneath the plant floor, calls for new construction and significant downtime. In clean-room, wash-down, explosive, and other environments requiring certification, companies may also need to recertify the environment after each new cable installation. And if these cables are damaged, the costly cycle begins again.
Cables can also pose a logistical nightmare in applications involving rotating machinery, heavy equipment, and hazardous conditions. While cables are often ideal for factory automation, they can also get wrapped up in the moving components they’re meant to monitor or control. In these situations, wireless sensors have simpler installation and maintenance, and provide a convenient, reliable way to keep operations running smoothly without the risk of signal loss from damaged cables.
Wireless networks use a pair of transceivers: a node to gather system data and a gateway controller to concentrate collected information. These transceivers are relatively small and easy to mount throughout a plant, making them particularly convenient for shortdistance applications. Plus, they demonstrate exceptional signal reliability by using bidirectional communications to fully acknowledge data transfer.
For more reliability, the Sure- Cross wireless network from Banner Engineering features deterministic responses, letting it work even if RF communication is unexpectedly obstructed. When a cable is damaged, users can only hope the connected system will revert to a default “off ” condition, rather than crashing the entire application. When a SureCross network is used, wireless transceivers can be configured to respond to signal losses in one of several ways: it can sound an alarm, freeze at the last value recorded, or default to a “safe” mode, among other options.
One reason wireless has not typically been considered in shortdistance applications is its susceptibility to signal interference. Welding, forklift operation, moving machinery, and other obstructions common in plants, as well as weather, can degrade wireless communications. Plus, when two radios are close to each other (generally less than 6 ft apart), there’s a high risk they will interfere with each another. In addition, if there are other signals on the same frequency, such as at 2.4 GHz, which can be used for transceivers and microwaves, the signal will likely be obstructed. But networks now offer several features to help ensure reliable communications and signal strength in these challenging cases.
Frequency Hopping Spread Spectrum (FHSS), for example, helps transceivers successfully deliver I/O despite competing signals from other devices. FHSS lets a wireless signal switch between different frequency channels in a pattern recognized solely by the transceivers assigned to communicate with one another. This technology is particularly well suited for sending small, redundant packets of data in a highinterference environment. Devices can also be equipped with remote antennas, letting radios be mounted within a few feet of one another but sending and receiving signals via antennas above the obstructions or interference.
A SureCross network uses 900 MHz or 2.4 GHz FHSS and binding to deliver secure communications. With binding, each transceiver is assigned a serial number, then configured so that each module knows to only accept signals from a designated device. In addition, a Network ID function lets transceivers differentiate themselves from others nearby. With this function, a gateway can observe other devices operating on the same frequency and, in response, shift its pattern to decrease the risk of interference.
To further avoid interference with other radios, several nodes can be configured to communicate with a single gateway located several hundred feet away. This lets one “brain” monitor and control several machines at once within the same area, making it easier to manage the plant.
To combat signal interference caused by environmental obstructions, including plant operations and weather, companies carry out site surveys to verify and evaluate wireless signal status. It lets users determine the signal strength of their networks and identify locations that suffer from the most interference. This information can be used to improve I/O transmission by adjusting locations. In many cases, simply placing a module as little as 3 ft above the obstruction can make the difference between frequent signal failure and good communications. Banner wireless devices have embedded-site survey capabilities, letting users conduct surveys with no additional software, personnel, or hardware.
Using wires to control conveyors, transfer presses or overhead cranes is often inefficient and ineffective. Although the I/O only needs to travel a couple hundred feet, nearly any cabling used must be able to flex rapidly, sometimes up to 50 times/sec. High-flex cable is expensive and cannot be guaranteed to last in particularly challenging applications. Plus, cabling is extremely difficult to install, especially for overhead installations where extension cords are hard to run.
Wireless networks provide a convenient alternative solution. By using a battery-powered node on the rotating equipment and a gateway at a control panel anywhere from 10 to 200 ft away, a wireless network eliminates the need for high-flex, difficult-to-run cables or costly slip rings. The easy-tomount, compact modules ensure a wireless network will fit into existing production lines.
Wireless is also being used to track inventory. Nodes monitor products moving from one area to the next, covering anywhere from 100 to 100,000 sq ft, without cables cluttering up storage areas. Such tracking systems ensure subassemblies are accounted for as they move from storage to shipping. And tracking data can be sent to specific destinations to provide real-time inventory updates.
The same theory works on the assembly lines in kitting operations. Wireless lets operators call for parts by using a node to communicate with a gateway in a management or storage area tens or hundreds of feet away. Additionally, with discrete I/Os, nodes can be set up so that one button contacts maintenance, another notifies management, and so forth, creating a virtual call center spanning the entire plant, all within a single device. And because one gateway can be used for many nodes, all assembly lines can share a similar setup, so any operator needing parts can use a node to send that message to the same gateway a few hundred feet away. This streamlines production and reduces downtime. As requirements change, the call center becomes mobile, and nodes are easily transferred from one workstation to the next.
Wireless can also identify potential problems before they occur. Nodes devices can be easily mounted to send real-time data from the machine to the operator. The message might be that a bearing or motor, must be replaced. This convenience of a wireless-monitoring system lets it be moved from one critical area to the next, with a single pair of transceivers monitoring several systems at different times, saving time and money.
Simple installation, reliable operation, and sophisticated communications have made wireless a sensible choice for traffic control, entry monitoring, and numerous additional short-distance applications, bridging the gap between a process and its operator. Today, thinking of wireless in terms of miles merely scratches the surface of its capabilities.
A Gateway can receive information from nodes located across the factory or just across the hall.
A node collects data and sends it to the Gateway. Wireless technology delivers reliable communications and monitoring over short distances, providing convenience and cost savings for many applications.
Wireless networks in inventory management systems track items a few feet from the wireless sensor as they are selected for use.
Wireless networks deliver complete facility management by transmitting I/O all across the plant floor, covering anywhere from 100 to 100,000 sq ft at a time.