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Building Automation

Wireless network spoils grocer

Mesh technology helps keep track of temperature fluctuations in refrigeration systems.

Perishable food begins the process of deterioration the moment it is picked, bottled or packaged. Milk can turn sour, meat spoils. Ice cream can melt and vegetables wilt. In the constant war against spoilage, no factor is more critical than temperature control.

Alfred Holzheu, owner of El Rancho Marketplace, replaced a "rat's nest of wires" with a wireless sensor network. Sensor nodes mounted inside refrigeration and freezer units collect temperature and humidity information and send it wirelessly over the IP network.

Alfred Holzheu knows this as well as anyone. The second-generation owner of El Rancho Marketplace, an upscale supermarket in Solvang, Calif., had been experiencing odd problems with his meat and ice cream cases. "We would see random temperature anomalies," Holzheu says. "The temperatures would just go up and we couldn't tell when it would happen or why." In a business already known for notoriously low margins, a single such incident could mean a loss of thousands of dollars.

Holzheu's 28,000-square-foot store, which brings in about $14 million in annual revenues, has a compressor rack that was installed in 1994. The system normally worked well, but its controller provided only a rudimentary data-collection mechanism accessible over dial-up lines. Staff members were supposed to periodically check all thermometers, but ensuring this was done on a regular basis was difficult. So, when one of the anomalies occurred-typically during the night-they were hard-pressed to find an explanation.

"The compressors are on the roof and, at night, the cold would exacerbate the problem and shorten the cooling cycle," Holzheu recalls. "If our meat cutter got in at 4 a.m. and saw that the meat he had cut the day before had lost its fresh color, he knew the temperature had gone up."

Staff, however, did not know why it had gone up. Was the evaporator coil malfunctioning? Did the defrost cycle last too long?

Furthermore, the system was "a rat's nest of wires," as Holzheu describes it, running from the thermistors (resistors used to measure temperature changes) in the refrigerator and freezer cases to the CPC controller unit in the machine room. As storage cases were moved or replaced, or sections of the store remodeled, Holzheu says, "It became impossible over time to know what was connected to what, and sometimes the wrong wires would be cut."

WIRELESS SENSOR NETWORKING

So Holzheu, who had spent time in Silicon Valley as a technology products distributor, was intrigued when Dave Retz, CEO of regional Internet consultant and service provider Comware International, told him about an emerging technology known as wireless sensor networking. Retz proposed installing a system developed by Arch Rock Corp. of San Francisco.

Arch Rock's Primer Pack/IP wireless sensor network is designed as a complete, easy-to-deploy
solution that lets users monitor physical conditions, such as temperature and humidity, and have the collected data immediately sent to a desktop browser or displayed on a mobile device, just as with any other Web application. Battery-powered sensor nodes-miniature computers that monitor the desired conditions-communicate with one another over a self-organizing wireless mesh network based on the IEEE 802.15.4 radio standard.

Primer Pack/IP implements the IETF 6LoWPAN standard for IPv6 communication over wireless personal-area networks, and uses TinyOS 2.0, the latest version of the de facto standard embedded operating system created especially for sensor networks, to conserve maximum power while letting the remote wirelessly connected nodes remain responsive. Arch Rock's mesh wireless networking protocols provide triple redundancy in the data-collection path for reliable transmission.

One of the first dilemmas in setting up the wireless sensor network for El Rancho was whether to put the entire sensor node inside the refrigeration unit or to mount only the sensor itself inside and run a wire to the electronics outside. "I didn't know if the radio signal would be able to get through the refrigerator wall," says Retz, who was also concerned that the node electronics would prove sensitive to the moisture and condensation inside the case and that node batteries would be difficult to replace.

Alternatively, drilling holes in the refrigeration units would be an installation nightmare and lessen the effectiveness of the insulation that helps keep ice cream at 20-degree temperatures and meat at 36 degrees. Ultimately, Arch Rock engineers suggested that a drop-in unit, placed entirely inside each refrigeration and freezer unit, was the answer.

PLACEMENT DIFFICULTIES SOLVED

Each sensor node was placed into a hermetically sealed, watertight plastic case containing desiccant to absorb any moisture. A magnet attached to each unit allows it to be mounted onto the inner wall of the refrigerator. Instead of replacing batteries inside the food cases every few months, El Rancho staffers swap out the sensor nodes from spares in inventory, recycling the newly designated spares back into the rotation next time around.

Sensor nodes now keep track of temperature in various areas of the market: meat cases, ice cream freezers and produce storage units. Each node both collects data and relays it to a neighboring node. That node then relays it to another neighbor, and so on until it reaches a node designated as the Arch Rock gateway.

Because Arch Rock's mesh network does not require that any particular node be in range of the gateway, node placement is completely flexible. The gateway communicates via a wireless Internet connection from El Rancho to Comware, where all the data is put into a server database. On the server is a software program designed to grab the collected data every five minutes, manipulate and analyze it, while recognizing alarm conditions, such as whether the temperature in a refrigerator is too high at a given time.

The Arch Rock software also can be configured to recognize patterns, as well as critical departures from those patterns, and what they might mean. When one of those conditions occurs, the wireless sensor network system proactively sends alerts to designated El Rancho staff-with a numerical code sent to a pager, a beeping text message sent to a manager's cell phone or an e-mail message sent to a desktop or mobile device.

In the future, Holzheu is considering expanding the wireless sensor network to monitor not only temperature but also humidity and lighting, to maximize energy savings. He also is working on ways to potentially generate electricity inside the refrigerators, powering internal sensor nodes without relying on batteries.

For more information from Arch Rock (click here)