During the last decade, the telecommunications industry has made phenomenal advances toward replacing wired connectivity with wireless. As the technology evolved, it overcame both technical challenges and political barriers to deployment. A variety of fully defined and robust networking protocols emerged that function well over these wireless technologies. Government agencies worldwide settled frequency allocations and regulatory issues to allow worldwide operation of those wireless networks.
These advances in wireless telecommunications have opened the door for applications beyond telecommunications to “cut the cord” and to go wireless. For instance, one of the protocols that arose, the IEEE 802.15.4 standard, fully defines a robust wireless personal area network (PAN) that specifically targets low-power, low-bandwidth signaling from switches such as those commonly utilized in industrial monitoring and control. Further, wireless telecommunications spurred semiconductor design and manufacturing to develop highly integrated, low-power RF components for wireless communications.
As a result of these achievements, it is now possible to cost effectively convert a basic component in industrial monitoring to wireless: the limit switch. The advent of wireless limit switches also provides equipment developers with an expanded range of applications. By using wireless, designers can now place switches in locations where it is impractical or impossible to run wires back to the control console. They also can increase their design’s flexibility by allowing switch positioning to adapt to installation requirements, even if that position must be modified over time.
Wireless connectivity even simplifies the retrofit of existing systems with additional limit switches as needed for improved functionality or to meet new regulatory requirements.
Based on 802.15.4 point-to-point communications, Honeywell's Limitless™ switches can be configured to potentially allow up to sixteen devices to communicate with one receiver module. The wireless signal is received by either a panel-mount receiver, or an industrial DIN-rail module, that then converted to an output. Outputs can be LEDs, buzzers, or standard electrical signals used by traditional controllers.
How Do Wireless Limit Switches Work?
By integrating a mechanical limit switch with an IEEE 802.15.4 transceiver, these wireless limit switches can communicate their position changes to a monitoring receiver that can handle multiple switches in a star-configuration network. Every network, and each switch within the network, has a unique identification number. These identification numbers allow a switch and its associated monitor to encode their signals so that the communications link between them is both private and virtually immune to crosstalk from other switches or networks.
802.15.4 radios provide excellent results in large single room and outdoor installations with a relatively open line-of-sight between switch and monitor. A 35-dB link margin ensures that minor obstacles or even intense precipitation will not compromise communications. Depending on composition, the signal can penetrate intervening walls in some installations.
An 802.15.4 radio allowable operating range is more than 1000 feet (304 meters). In extreme conditions such as heavy precipitation, rain or snow, the signal could be reduced by approximately 75 feet (23m).
To ensure reliable operation and provide for almost limitless options for installation, 802.15.4 draws so little power that the switches can be operated by industry standard batteries rather than depending on situation-dependent, un-reliable, and expensive energy scavenging. With the proper design, a wireless switch should be able to operate for several years without a battery replacement or re-charging.
CAPTION: Honeywell’s WDRR Din-Rail or Panel-Mountable Receiver supports up to 14 different remote battery-powered wireless limit switches.
The monitor/receiver unit that forms the other half of a wireless switch installation can be designed to support a single switch or multiple switches. For example, the Honeywell Limitless(TM) WDRR receiver can support up to 14 different remote battery-powered wireless limit switches. In addition to switch activation status, the controller can also monitor the signal strength and battery levels for each individual switch on its network.
Another interesting application for limit switch networks is prevention of storage tank overflowing. As the tank fills up, the fluid level forces a change in the position of the limit switch. The wireless limit switch then sends a signal to the pump controller to start pumping out the tank to lower the level. When the fluid level drops to a safe level, the switch then sends a signal to the controller to turn off the pump. The new Honeywell Limitless WDRR wireless receiver can control up to 14 tanks and 14 separate pumps.