Limit switches have been around for decades, protecting heavy equipment and providing important position information. They are used in everything from crane booms to gates, lifts to storage tanks — anywhere there is a need to sense the presence, absence or position of a moving object. In a crane application, the limit switch is located on the end of the boom. The limit switch could be used to indicate 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.
In the last few years, limit switches have become enabled by wireless using technologies such as IEEE 802.15.4 to transmit information from the remote switch to a receiver, which then converts the signal to ones used by standard controllers. Converting switch solutions to a wireless mode addresses a variety of customer needs for lowered cost and increased limit switch installation options, giving early adopters a competitive advantage in the design of next generation industrial and transportation equipment.
Benefits of Wireless Switching
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 limit switches can also reduce maintenance costs. Equipment wiring is less complex with the elimination of wired switches from the mix, simplifying troubleshooting and reducing commissioning time. Further, going wireless increases system reliability by eliminating the potential for having continuity issues with switch wiring or connectors. Switches also become simpler to replace, with no need to disconnect and re-attach wiring and no risk of incorrect wire attachment.
Global limit switches are an essential element of industrial and transportation controls, monitoring position and presence of doors, booms and valves. Conventional wired switches, however, 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.
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.
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