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.
The answer to your question- the Limitless™ product will have difficulty functioning in an area where the 2.4 GHz ISM band is completely consumed on a consistent basis. Some examples include high power industrial microwaves and high bandwidth video transmission on 2.4 GHZ. So you are correct, wireless products are not completely immune to interference, but the chances are small that an issue will arise assuming you survey the site before implementation. Also, unless the interference source is constantly on, our system can recover to a large extent due to the retries and acknowledged transmissions that we built into our system. Please contact our technical representatives if there are further questions at 800-537-6945.
Have you ever experienced a receiver becoming "de-sensitized" because of a strong signal nearby? It is a problem in some areas, even with receivers costing hundreds of times more than the wireless switch control units. I am talking about another signal of amplitude high enough to move the first stage or stages out of the intended operating area. It is not an everyday-everywhere problem, but it certainly does happen. It winds up that certain wireless-sensor security systems can be paralyzed this way. So the fact is that they are not interference immune, only interference resistant.
There are diagnostic functions when using a Limitless™ monitor/receiver that can help isolate a problem with a particular switch. Please review the attached Troubleshooting Section in Section 10 which references other sections for review at the attached link: http://sensing.honeywell.com/index.cfm/ci_id/157464/la_id/1/document
In answer to your question regarding interference issues, there are commercially available devices (i.e. packet sniffers) that allow you to look at all of the RF signals in a localized area and this can be accomplished via a site survey; please reference Section 6.5.3 at the same link above. The WDRR Series product also has the capability to provide an RF signal strength indication for each individual Limitless™ input (i.e switch). The IEE802-15.4 protocol that the Limitless™ product uses was specifically designed for wireless product use in the Industrial market. It has many features within it to minimize interference along with our implementation features, i.e., 128 bit encryption key used by each limit switch; automatic channel/PAN ID and energy scanning at power-up. Testing in the actual application against the customer’s particular requirements is also recommended in proving product suitability.
The Limitless™ products are not meant for use in any human safety or fail safe application. Please contact our technical representatives if there are further questions at 800-537-6945.
I have seen literature on wireless switches and transducers for a couple of years now. My concern is maintenance. How do I know that the switch is still working, and how do I troubleshoot? An electrician uses a meter on the contacts of a wired switch to determine if it is active or not. In the installation drawings, I'd have specified that the switch be wired so that ON = OK and OFF = alarm or not OK or cable fault. How is this done with wireless?
The added problem - how do I determine interference issues? Is there a device available that allows me to receive or 'eavesdop' the 'signal' that the switch is transmitting, on a handheld device, and verify good signal? I am not concerned with false positives - I need to know that the X-ray machine in the lab is not intermittently wiping out my switch signal.
You describe using this on a crane application. A crane installation using the wireless switch for man safety would convince me hat these switches are a 'real solution', reliable enough for me to test in a real application.