Industrial wireless networks and switches
08 February 2013
One of the most important changes impacting industrial and manufacturing plants and facilities is the exploding interest in wireless networking replacing cabling for data transfer between manufacturing switches and controllers
One of the most important changes impacting industrial and manufacturing plants and facilities is the exploding interest in wireless networking replacing cabling for data transfer between manufacturing switches and controllers.
One very common application is in the conversion from wired to wireless for the limit switches or pushbuttons that are used in everything from automotive assembly plants, warehouse doors and gates, elevators, storage tanks, crane booms, forklifts, etc. — anywhere there is a need to sense the presence, absence or position of a moving object or perform an operation such as opening a door.
For example, in a crane application, the limit switch is located on the end of the boom and indicates to the operator when the cable jib is close to the end of the boom and it is not safe to spool the cable further. Because of the constant motion of data cable within a crane arm, wired connections often erode and break. In addition, harsh weather conditions also contribute to the short life of data cables. Wireless systems are immune to those threats.
At airports and outdoor storage facilities, wireless is being used to replace cabling under tarmac and can be used to monitor the access status of hanger doors, opening and closing of gates, and even the operation of fuel dispensers. Wireless networking eliminates the need to tear up runways and parking lots every time there is a problem with a data cable.
In an automotive assembly plant, wireless limit switches are used to monitor and control the movement of robotic assembly devices as well as movement and location of the various sub-assemblies. Because of the constant motion of machines and exposure to hazardous chemicals and solvents, hard wired cabling often breaks.
Wireless switches or pushbuttons are a good fit for retrofit onto safety showers or eyebath stations where alarming for emergency response can be critical or even regulated. Because it is not tethered by wire, or tied to water flow, the installation can be done in minutes.
Using wireless eliminates the need to regularly inspect and replace this cabling. In addition, wireless’s innate flexibility makes it especially valuable in manufacturing plants where the assembly lines are constantly reconfigured and moved around to enable production of new product models. After the line is adjusted, just plug the wireless switches back in and the network is up and running. No need to run new cables.
Conventional wired switches present installation and maintenance challenges, especially in installations that are subject to harsh environments or involve frequent flexing in the wiring. In some cases, traditional wires can represent tripping hazards or can be compromised during normal equipment operation, thus causing expensive machine down-time.
Wireless limit switches can lower equipment costs in a variety of ways. For one, the cost of manufacturing and installation is reduced. Not only is the expense of wiring eliminated, there are no conduits, clips or connectors required to place a limit switch where it is needed. There are no wire routing problems to solve, no need for pulling wire during installation and fewer restrictions on location and placement of the limit switch.
Wireless switch solutions can also reduce maintenance costs. Equipment wiring is less complex with the elimination of wired switches from the mix, simplifying troubleshooting, reducing commissioning time, and increasing system reliability. Switches also become simpler to replace, with no need to disconnect and re-attach wiring and no risk of incorrect wire attachment.
How Wireless Limit Switches Work
Since the development of the 802.15.4 Low-Rate Wireless Personal Area Network (LR-WPAN), developers have specified it for numerous applications within various industries. It has opened up new doors within industrial sectors as little or no underlying infrastructure is needed to adopt wireless technology. This translates to low installation costs as long cable runs and hard wiring are eliminated. The lack of wiring and moving parts increase system reliability, decrease downtime and reduce maintenance. Wear and tear isn’t an issue when wires are not present to flex, bend or abrade due to environmental hazards.
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).
802.15.4 also provides flexibility with data rates of 250, 40 and 20 kbps; addressing modes of 16-bit short and 64-bit IEEE; CSMA-CA channel access; automatic network establishment by the coordinator; fully hand-shaked protocol for transfer reliability; multiple channel configurations within the 2.4GHz ISM, 915MHz ISM and 868Mhz bands; and more.
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.
By integrating a mechanical limit switch with an IEEE 802.15.4 transceiver, 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.
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 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.
Honeywell’s Limitless system alerts the plant manager to low battery levels before there is a serious condition, providing enough time for the maintenance team to check and change the batteries. The good news is that 802.15.4 is a low power technology, and industrial batteries can last for many years without recharging or replacement.
These wireless switches can be encased in packaging to make them extremely resistant to most of the hazards of modern day manufacturing plans and industrial environments. This means they can operate in extremely dirty, noisy and vibration filled areas without any degradation issues.
What Are Our Customers Asking Us?
Over the last year or so, we have seen a marked change in the questions we are being asked. No longer are customers concerned about how wireless works and if it is secure and reliable. They now want to know how they can cost-effectively convert their conventional wired networks to wireless. What is the return-on-investment (ROI) and how long does it take to achieve?
One of the most convincing arguments is in the case of automotive assembly plants. It is often less expensive to completely install a wireless network between a switch and a control system than to track down and repair a bad cable or connection. When the hours of assembly line downtime required for a specialised maintenance technician to search to find the one bad spot is factored in, the advantages of wireless are obvious.
Image 1: Automotive assembly plants have been quick to adopt wireless because of their need for flexible architectures that can be easily and quickly reconfigured for changing automobile models.
Image 2: 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.
Image 3: Honeywell's WDRR Din-Rail or Panel-Mountable Receiver supports up to 14 different remote battery-powered wireless limit switches.
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