Development of a Wireless Distributed
have learned that a new set of rules must be used in order that these
systems work as reliably as a conventional wired system.
Technologies has been in the HVAC control systems business since 1985
and has systems installed world wide.
the last five years, Walker Technologies has developed a
viable wireless BMCS (Building Management and Control System)
that is being used successfully in several applications. These include
both stand alone fully wireless systems and as part of new
or existing installed “wired” BMCS systems. However, because it
is wireless, we have learned that a new set of rules must be used in
order that these systems work as reliably as a conventional wired
system. See more information and PDF’s at http://www.walkersys.com/WIRELESS
ADVANTAGES OF WIRELESS BMCS
a reliable wireless communication strategy is implemented in a BMCS,
then there are huge advantages for both retrofit and new building
construction. The major advantage of wireless systems is
substantially reduced costs for installation, servicing and future
expansion. Applied to multi unit facilities such as malls and
MDU’s, wireless gives owners a cost effective route to gaining the
advantages of a BMCS to monitor and reduce energy use.
- No cost for installing and commissioning
of network LAN
components. No issues with ground loop on communication network. No
requirement for shielded cable.
- Installed system is infinitely
expandable. – Just add more wireless controllers and they will
- All BMCS control panels are located in a
location. A wireless mesh network links to all other controllers. This
central control panel allows simple servicing and expansion and removes
the need for EMCS network communications cabling.
- No engineering drawings or as-built
drawings are needed when
servicing or supporting the system at a later date. No lost or
unrecorded control panels.
- The structure of a wireless based
control system results in less
need for programming within the main control panels. This also reduces
initial installation cost as well the cost of ongoing support. In
many cases, a diligent owner can support the system in house by simple
replacement of controllers.
THINGS THAT ARE DIFFERENT IN A WIRELESS BMCS
with a comparison of wired versus wireless telephone connections,
wireless links in a communicating BMCS system will always be less
reliable than a direct wired link. This difference must be taken
into consideration at the outset when developing and deploying such a
system, but, as we have with cell phones, we can adapt to the
- WIRELESS EXPERTISE
Installation, management and support of an installed wireless
control system requires a new set of skills, not those normally held by
HVAC technicians. These skills are currently the domain of IT
personnel. Having an IT expert manage the wireless components of
the installation makes sense.
- WIRELESS SIGNAL
A new paradigm for wireless strategy is required. It is not
possible to close a control loop across a wireless network. With
our wired I/O BUS or SmartLAN, our contractors normally could close
control loops over these communication links because throughput was
determined, -- as long as the real time response of the control
loop could handle it. Just as your cell phone can drop out
annoyingly, you can expect wireless links within a controls system to
be intermittent or to fail periodically. This drop out must be
considered as part of the strategy for system communications.
Within a wireless network, each wireless control device must be
autonomous. It must perform all its control functions internally
with its own wired I/O. This could be in concert with a regular
stream of wireless command data such as setpoint, and other tuning
parameters, however it must also have a fall back strategy for periods
when it detects a loss of wireless command data. The best application
of wireless is for systems of supervised unitary controllers. The
practise of wiring several control loops into a single main controller
does not work well with wireless. However, by moving the control
programs from software in the main panels to the unitary controller a
simpler, more serviceable control system is created. This gives huge
advantages for the end user on the long term. More complex
control strategies can still be implemented successfully by having
control presets in the devices that are commanded by the supervisory
control system. Presets and setpoint commands need to be saved in
EPROM (Erasable Programmable Read-Only Memory) memory in the wireless controller so that operation after a
power failure is assured.
- POWER FAILURE
Considerable thought must be put into
defining operation of control
devices after a power failure. Although a ZigBee network will
usually reboot and re establish communication paths fairly quickly, it
could take several minutes before devices start receiving valid
commands. This mandates that a clear power fail recovery strategy needs
to be in place.
-Occupied or setback state?
If a device recovers in its unoccupied state and the area is
really occupied then the tenant will not be happy. So a wireless
controller must wake up as occupied and using the setpoint knob as the
device setpoint. This is also the case if the setpoint knob is changed
by the occupant at any time. Reversion to non-occupied state can occur
after a valid supervisory command is received and also after a time
period which, depending on system conditions, may be up to 12 hours.
This is further impacted by fact that the head end may need data from
controllers to “calculate” the occupied state.
-Other tuning parameters and supervised
Hopefully this information has been saved in
non volatile memory and
verified by the head end prior to the power outage so “last values”
will automatically be used until new ones are received.
RESULTS OF THE DEVELOPMENT PROCESS.
Walker Technologies developed our wireless system we used the ski
chalets on Mt Washington in Courtenay, B.C. The mountain usually gets a
large amount of snow (up to 500cm) over the winter so we expected large
variations in wireless connectivity. We built the mesh network between
five chalets using high powered radios to jump the distance between
chalets. To determine occupancy we polled motion detectors on
each baseboard thermostats-- usually four to 10 in each chalet.
then automatically changed the chalet’s occupied state based on the
number of zones that were occupied. This setup proved to be an
extremely good mechanism to underline the differences between wireless
and wired communications and helped us lay out the rules in the
connections are not like wired
While we know that if a wired connection fails there is a problem,
usually mechanical, that can be fixed. When a wireless connection
fails, it will likely fix itself, but unfortunately we can not be sure
when that will happen.
- Wireless Full
Duplex is a conflict of terms.
You can not expect a full duplex wireless connection to be full
duplex. Wireless connections across a mesh network are one
way. A full duplex connection in a mesh network consists of two
single wireless connections, one from the source to the destination
across the wireless network and a second one from the destination back
to the source. It is more or less guaranteed that each signal
path will be different; with different routing, different obstructions
and different reflections between source and destination. It can
not be assumed that both connections will be active at all times and
this must be figured into the strategy.
If we are
using zone occupancy to determine occupied state of a group
of zones, loss of feedback information may cause the head end system to
make the wrong decision. In making control decisions a wireless
head end must also consider the age of its data.
connections are time sensitive.
We all know what it is like to use our cell phone while driving (hands
free). Our conversation will always be impacted by time as we drive in
and out of cell zones. This also impacts fixed wireless
links. Wireless communications vary throughout the day and
are affected by temperature, humidity, barometric pressure and
humanity, not to mention the delivery truck that just parked out front.
We never know
when there will be an interruption of the wireless data
stream between two points, so as a general strategy we need to
continuously re-transmit information. The time delay for this
retransmission depends on the requirements of the system, but getting
data updates every few minutes is generally appropriate. This
also gives us a critical piece of information that allows us to
dynamically monitor the integrity of the wireless network and the
specific data from individual controllers.
- The age of wireless
data must be considered
One piece of information that is crucial when looking at any data
received over the wireless network is the age of the data. This
is important not only for inbound data from devices sending status
information back to the head end, but also for devices receiving
setpoint and scheduling information from the head end. At both
ends, the devices need to be aware of transmission status and have a
course of action to take when communication fails either momentarily or
way to monitor communication status is to keep track of
the time since last transmission. In our systems this is known as
the Last Transmission Time (LTT). We use minutes as the metric – one to
two minutes is normal, several minutes is warning of some issue. An
hour or so may indicate a problem and the individual zone should be
At the head end, an alarm
triggered by a high LTT from any device will
give and automatic annunciation of communication issues. The same
information reported in the feedback from devices gives continual
assurance that these devices are receiving commands. In training the
end user, it is straight forward to educate them on the importance of
paying attention to LTT and what to do if this gets too high.
- Point to point and
global commands both have their
We have found
that within a ZigBee mesh network, point to point
commands from devices back to the head end seem to work well, allowing
report data to their assigned controller within a one minute time
frame. Data is sent in small packets every 30 seconds from each
controller. These packets are received randomly so some collisions will
occur. This strategy is the best fit because we can not successfully
create a timely master slave relationship using a mesh network.
commands such as setpoint and schedule information sent out
to individual controllers, we do have control of the command frequency
so we use a token passed between head end controllers to ensure that
outbound commands are sequenced over time. Each controller also
sends data out with delays between packets. By including address
information in the command packets these out going commands can be sent
In a mesh
network, global commands propagate to all controllers through
the network and so, tend to “ring” in the wireless network for a short
time. However global information gets through to the destination
controller much more reliably and often when a controller has a high
LTT for data coming back it is still receiving global command data that
is sent out to all controllers.
a new set of rules is followed for the structure and interconnection
of devices used, a wireless BMCS can be successfully deployed.
Because a wireless system is less expensive and generally less complex,
the cost of ownership is lower. Wireless allows BMCS systems to
be applied cost effectively in areas where use of these systems was
previously too expensive.
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