In the first part of this column, I mentioned a wind power startup project I’d been working on, in which redundant control and monitoring stations need to be tied into a redundant central controller. Interconnect is a challenge because the stations are separated by 100 ft or more, and lightning strikes not only are possible but inevitable and frequent.
I decided on a wireless interconnection strategy since it would offer complete electrical isolation of the stations and even more. The previous column explained how to go wireless, but the trick is ensuring security and creating adequate redundancy.
I’m using the Jennic JN5139 module product with an integrated SMA connector for antenna connection. The JN5139 is a low-power, low-cost wireless microcontroller integrating a 32-bit RISC processor, a fully compliant 2.4 GHz IEEE 802.15.4 transceiver, 192 kB of ROM, RAM sizes from 8 kB to 96 kB and extensive analog and digital peripherals. The device also integrates hardware MAC and AES encryption accelerators and mechanisms for security key and program code encryption.
The Jennic encryption coprocessor implements the NIST-approved Advanced Encryption Standard (AES) using a 128-bit nonce (number used once) and a 128-bit key for encryption. The purpose of the nonce is to allow implementation of a counter, date or otherwise disposable portion of the key so if an intruder simply repeats an earlier message, the decryption will fail since an old nonce would be embedded in the packet.
By using the AES coprocessor and a standard security model for the nonce/key pair, completely secure communications can be realized between the stations without dramatically increasing CPU load.
What about jamming? 802.15.4 is based on a spread-spectrum technique that makes the radios frequency-agile in the 2.4 to 2.5 GHz band. They jump from one frequency to another to avoid interference with other radios operating in the same region of space and frequency. This reliability feature also dramatically improves their jam-tolerance. A jamming system would have to transmit on frequencies spanning 100 MHz—from 2.4 to 2.5 GHz—to jam operation, and this is a difficult and costly proposition. Further, the module units support antenna diversity—the use of dual antennas located in different locations or orientations. The antennas can be selected alternately to avoid destructive interference nodes or blocked pathways in the environment.
My redundant control system is inherently easier to design with the wireless communications, since “cutover” between one weather station and another, or between one motion control system and another, or even one main controller and another, only involves changing the addressing field in a transmitted packet. There are no muxes, relays, routers or switches to interpose and control between the terminal stations. This eliminates software headaches, as well as more single-point-of-failure hardware devices that so often complicate our best-designed redundant systems.
To experiment with Zigbee, I’d certainly recommend starting with the $500 Jennic Home Monitoring evaluation kit that contains five AA-powered evaluation boards, seven Zigbee radio modules and a fantastic set of C-based development tools with enough sample code and documentation to get you started on becoming a Zigbee expert. While the offerings from many other vendors are excellent, I’m always in the mode of delivering full-featured applications very quickly. Having great documentation, copious sample code and a fully working set of demo hardware that can form the core of my own design is about the only way I can feel comfortable about trying something radically new.
Give it a whirl!
Ned Lecky is a mechanical and electrical engineer with 25 years of hands-on experience in control systems and machine vision. He is owner of Lecky Integration.