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)