Design Article
RF in Action: Wireless switches offer unlimited benefits
Todd Hanson, Honeywell Sensing and Control
6/20/2011 1:32 PM EDT
Making Wireless Worry-Free
The IEEE 802.15.4 wireless PAN standard defines a low-duty wireless signaling scheme targeting basic instrumentation and control applications. The standard has been in place for a number of years with several generations of RF devices already deployed. As a result, the IEEE 802.15.4 specifications are both robust and field-proven to eliminate crosstalk between network links as well as resistant to interference from out-of-network users in the same frequency band such as Bluetooth™ and portable phones.
Because the 802.15.4 is a PAN standard, operation is distinct from and independent of familiar wireless LAN systems. There is no exchange of information or other interaction between a PAN and the LAN, so there is no need for corporate IT department involvement. Further, the PAN systems are independent of one another. So, for example, if two construction cranes with 802.15.4 switch networks are operating at the same construction site, there is no risk of interference or interaction.
802.15.4 radios operate in the 2.4 GHz band using low power, short-duration signaling. Because the radio spectrum and power levels are approved for unlicensed operation worldwide, system developers do not need to be concerned with such things as compliance testing.
How to Design-In Reliability
802.15.4 supports a simple star configuration network configuration with each switch in the network communicating with a single monitor/receiver. Communications links are point-to-point between switch and monitor; there is no signaling between switches. This network architecture keeps the switches isolated from one another so there is no opportunity for the behavior of one switch to affect the behavior of any other.
To enforce the isolation of their communications links, 802.15.4 radios can be set up to require registration with their network monitor. The registration or “pairing” process is simple and needs be performed only once to establish the linkage. This process takes less than 5 seconds. Once established, the “pairing” persists even if power is lost to the monitor or a battery is replaced in the switch. This not only keeps system maintenance simple, it ensures that the network is self-healing. The monitor is able to automatically re-establish any dropped link to a registered switch. Replacement or reconfiguration of the switches can be done through a clearing and re-pairing process.
Three Ways to Ensure 802.15.4 Security
Security is always a concern. The above pairing process follows the security techniques outlined in IEEE 802.15.4. When registration is activated, the monitor provides the switch with several pieces of information. The first is the monitor’s individual 16-bit network identification number, which the monitor chooses at power-up to avoid conflict with other networks in the vicinity. The second is a unique 16-bit address for the switch, which the monitor creates. The third piece of information is a 128-bit encryption key that the pair will use to encode future communications. Once the information is exchanged, the switch and monitor use it to address and encode the signals they exchange.
The combination of addressing and encryption ensures the uniqueness and security of each switch’s communications channel. No two monitors have the same network address, so monitors and switches will not respond to signals originating in other networks. A monitor will not confuse signals within its network, nor will switches respond to signals intended for other switches. No switch has another switch’s address, so switches will not react to one another’s signals.
Even if an addressing error occurs somewhere, the network will not react to the erroneous signal because the encryption keys for decoding the message will not match. The use of encryption further ensures that no unregistered node can successfully insert erroneous signals into a network or decode signals from a network, making the network secure from both eavesdropping and hacking.
Once a switch and monitor have been paired, the two continually exchange information as long as both have power. The switch sends a switch position message to the monitor each time the position changes, accompanied by a “health” status message that indicates battery level and RF signal strength. In addition, the switches send out “health status” messages at regular intervals. This interval is typically 30 seconds but is configurable to meet specific installation needs; shorter intervals can reduce battery life.
Using a 802.15.4 radio network to connect limit switches to a central controller frees industrial equipment developers and users from the costs and restrictions of wired switch solutions and expands the range of possible uses and placement of switches. It also reduces the wire complexity within a control cabinet. The commissioning and lifetime cost of switches drops while reliability and integrity rise.
About the Author
Todd Hanson is a Director of Strategic Marketing and Wireless Solutions at Honeywell Sensing and Control (S&C). In this role, Todd is responsible for S&C’s wireless solutions as well as the overall S&C industrial business. He actively collaborates with other Automation and Control Solutions businesses to formulate a One Honeywell strategy where S&C supports the other businesses to help drive total Honeywell solutions.
The IEEE 802.15.4 wireless PAN standard defines a low-duty wireless signaling scheme targeting basic instrumentation and control applications. The standard has been in place for a number of years with several generations of RF devices already deployed. As a result, the IEEE 802.15.4 specifications are both robust and field-proven to eliminate crosstalk between network links as well as resistant to interference from out-of-network users in the same frequency band such as Bluetooth™ and portable phones.
Because the 802.15.4 is a PAN standard, operation is distinct from and independent of familiar wireless LAN systems. There is no exchange of information or other interaction between a PAN and the LAN, so there is no need for corporate IT department involvement. Further, the PAN systems are independent of one another. So, for example, if two construction cranes with 802.15.4 switch networks are operating at the same construction site, there is no risk of interference or interaction.
802.15.4 radios operate in the 2.4 GHz band using low power, short-duration signaling. Because the radio spectrum and power levels are approved for unlicensed operation worldwide, system developers do not need to be concerned with such things as compliance testing.
How to Design-In Reliability
802.15.4 supports a simple star configuration network configuration with each switch in the network communicating with a single monitor/receiver. Communications links are point-to-point between switch and monitor; there is no signaling between switches. This network architecture keeps the switches isolated from one another so there is no opportunity for the behavior of one switch to affect the behavior of any other.
To enforce the isolation of their communications links, 802.15.4 radios can be set up to require registration with their network monitor. The registration or “pairing” process is simple and needs be performed only once to establish the linkage. This process takes less than 5 seconds. Once established, the “pairing” persists even if power is lost to the monitor or a battery is replaced in the switch. This not only keeps system maintenance simple, it ensures that the network is self-healing. The monitor is able to automatically re-establish any dropped link to a registered switch. Replacement or reconfiguration of the switches can be done through a clearing and re-pairing process.
Three Ways to Ensure 802.15.4 Security
Security is always a concern. The above pairing process follows the security techniques outlined in IEEE 802.15.4. When registration is activated, the monitor provides the switch with several pieces of information. The first is the monitor’s individual 16-bit network identification number, which the monitor chooses at power-up to avoid conflict with other networks in the vicinity. The second is a unique 16-bit address for the switch, which the monitor creates. The third piece of information is a 128-bit encryption key that the pair will use to encode future communications. Once the information is exchanged, the switch and monitor use it to address and encode the signals they exchange.
The combination of addressing and encryption ensures the uniqueness and security of each switch’s communications channel. No two monitors have the same network address, so monitors and switches will not respond to signals originating in other networks. A monitor will not confuse signals within its network, nor will switches respond to signals intended for other switches. No switch has another switch’s address, so switches will not react to one another’s signals.
Even if an addressing error occurs somewhere, the network will not react to the erroneous signal because the encryption keys for decoding the message will not match. The use of encryption further ensures that no unregistered node can successfully insert erroneous signals into a network or decode signals from a network, making the network secure from both eavesdropping and hacking.
Once a switch and monitor have been paired, the two continually exchange information as long as both have power. The switch sends a switch position message to the monitor each time the position changes, accompanied by a “health” status message that indicates battery level and RF signal strength. In addition, the switches send out “health status” messages at regular intervals. This interval is typically 30 seconds but is configurable to meet specific installation needs; shorter intervals can reduce battery life.
Using a 802.15.4 radio network to connect limit switches to a central controller frees industrial equipment developers and users from the costs and restrictions of wired switch solutions and expands the range of possible uses and placement of switches. It also reduces the wire complexity within a control cabinet. The commissioning and lifetime cost of switches drops while reliability and integrity rise.
About the Author
Todd Hanson is a Director of Strategic Marketing and Wireless Solutions at Honeywell Sensing and Control (S&C). In this role, Todd is responsible for S&C’s wireless solutions as well as the overall S&C industrial business. He actively collaborates with other Automation and Control Solutions businesses to formulate a One Honeywell strategy where S&C supports the other businesses to help drive total Honeywell solutions.
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janine.love
6/20/2011 2:01 PM EDT
You asked for it! I've been asking around for RF in Action articles by popular request. Hope this one fits the bill. If you have questions or comments, please sound off in the comments section below.
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thingstodo
6/22/2011 10:32 AM EDT
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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Hgrover
6/27/2011 5:07 PM EDT
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.
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WKetel
6/30/2011 6:14 PM EDT
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.
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Limitless Greg
7/18/2011 11:35 AM EDT
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.
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