Applications of Zigbee Wireless Technology for Industrial Instrumentation

Abstract

Measuring current is fundamental to electrical engineering. Wireless current sensing systems, which do not involve the insertion of a sensor into a circuit, are attractive. This paper describes the implementation and performance of such a system using the recently introduced ZigBee wireless protocol. The system uses a Rogowski coil as the sensor, and a transmitter unit incorporating an integrator, microcontroller and ZigBee transmitter module. The receiver has a further ZigBee module, and a further microcontroller driving a liquid crystal display giving a current readout. Testing shows that the system works, with a linearity error of less than 0.04 A and a potential battery lifetime of eight months

Full text

APPLICATIONS
OF
ZIGBEE
WIRELESS
TECHNOLOGY
FOR
INDUSTRIAL
INSTRUMENTATION
T.
D.
Garnett('),
R.
A.
McMahon(2),
and
E.
Abdi-Jalebi(3)
University
of
Cambridge,
UK
ABSTRACT
Measuring
current
is
fundamental
to
electrical
engineering.
Wireless
current
sensing
systems,
which
do
not
involve
the
insertion
of
a
sensor
into
a
circuit,
are
attractive.
This
paper
describes
the
implementation
and
performance of
such
a
system
using
the
recently
introduced
ZigBee
wireless
protocol.
The
system
uses
a
Rogowski
coil
as
the
sensor,
and
a
transmitter
unit
incorporating
an
integrator,
microcontroller
and
ZigBee
transmitter
module.
The
receiver
has
a
further
ZigBee
module,
and
a
further
microcontroller
driving
a
liquid
crystal
display
giving
a
current
readout.
Testing
shows
that
the
system
works,
with
a
linearity
error
of
less
than
0.04A
and
a
potential
battery
lifetime
of
eight
months.
1
INTRODUCTION
The
sensing
system
described
in
this
paper
is
aimed
at
The
measuring
of
currents
is
fundamental
to
electrical
general-purpose
measurement
of
currents
up
to
10
A
at
power
engineering.
In
most
situations
it
is
not
attractive
power
frequencies.
The
ZigBee
wireless
format
has
to
break
an
existing
circuit
to
insert
a
measuring
device,
been
chosen
for
this
work
as
it
is
optimised
for
low
and
the
device
may
affect
the
electrical
characteristics
power
consumption,
hence
long
battery
life.
In
addition,
of
the
circuit,
for
example
by
imposing
a
voltage
burden.
the
transmission
range
is
relatively
large
and
ZigBee
Detecting
the
current
from
close
proximity
without
can
be
used
in
a
networked
mode.
The
paper
will
breaking
the
circuit
is
a
preferable
alternative.
There
are
describe
the
ZigBee
protocol,
explain
the
system
design,
several
established
methods
of
achieving
this,
including
and
give
test
results.
the
current
transformer
and
the
Hall
probe.
For
this
paper
the
Rogowski
coil
will
be
used,
on
account
of
its
small
size,
light
weight
and
accuracy.
2
THE
ZIGBEE
PROTOCOL
There
are
often
applications
where
remote
sensing
of
a
ZigBee
is
an
intelligent
digital
protocol,
operating
at
current
is
desirable,
for
example
when
the
current
is
in
a
three
frequencies,
with
the
commonest
one
being
at
2.4
moving
part
or
in
a
high
voltage
circuit.
Using
a
radio
GHz.
At
this
operating
frequency,
data
rates
up
to
250
link
between
the
sensor
and
receiver
to
make
the
system
kbit/s
are
claimed.
This
is
a
relatively
low
bandwidth,
wireless
provides
a
convenient
solution.
Wireless
compared
to
other
protocols
such
as
Bluetooth.
systems
also
overcome
the
disadvantages
of wired
However,
the
low
transmission
rate
reduces
power
connections,
with
the
attendant
risk
of
damage
and
consumption,
with
a
typical
module
consuming
45
mA
failure
[1].
Recent
advances
in
wireless
technologies
when
transmitting.
When
not
transmitting,
the
current
have
led
to
the
availability
of
new
systems
specifically
consumption
can
be
reduced
to
less
than
50ptA
in
the
designed
for
the
transmission
of
digital
data
including
so-called
sleep
mode
[3].
ZigBee
has
a
transmitting
Wi-Fi,
Bluetooth
and
ZigBee
in
addition
to
established
range
of
up
to
30
m,
but
this
can
be
increased
by
using
radio
(lOOs
MHz)
and
infra-red
methods.
one
of
the
connection
topologies
available
with
the
protocol
[4].
By
using
mesh
networking,
distances
over
The
desire
to
measure
rotor
bar
currents
in
a
rotating
1
km
could
feasibly
be
achieved,
as
ZigBee
allows
up
to
electrical
machine,
known
as
the
Brushless
Doubly
Fed
65,000
nodes
within
an
individual
network.
Other
Machine
(BDFM),
provided
the
initial
motivation
for
network
topologies
include
the
simple
direct
connection
the
study
of
methods
of
wireless
current
measurement.
and
the
star
arrangement,
the
latter
being
ideally
suited
A
system
was
successfully
constructed
using
Rogowski
to
industrial
sensing,
with
multiple
transmitters
coils
to
sense
the
current,
an
analogue
signal
streaming
data
back
to
a
central
receiver
unit.
ZigBee
conditioning
circuit
and
transmission
using
the
also
performs
well
in
electrically
noisy
environments
Bluetooth
protocol
[2].
The
relatively
high
bandwidth
[5].
available
allowed
the
transmission
of
high-resolution
current
waveforms
from
several
sensors.
However,
the
current
drain
of
the
transmitter
was
high,
requiring
3
SYSTEM
DESIGN
inconveniently
frequent
recharging
of
the
battery.
The
overall
system
layout
for
the
prototype
is
shown
in
Figure
1.
748
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Traiisn...t
of
the
integrator
is
shown
in
Figure
2.
The
output
of
the
integrat
or
is
fed
straight
into
a
12
bit
A
/D
converter.
To
Rogowski
nr
AD
Mcotlekeep
the
system
simple,
only
8
bit
resolution
is
used,
so
coil
InertrConverter
Mircnole
that
unnecessary
byte
reconstruction
is
avoided.
------------------------------------------------------------------------------------
A
PIC
microcontroller
is
used
on
the
transmitter
side
to
|
LCD
s
Microcontroller
Module
Module
control
the
A/D
converter
and
the
ZigBee
module,
and
(Receiver)
(Transmitter)
also
to
manipulate
the
current
data.
As
the
root
mean
Recei
e
------------------------------------------------------------------quare
required,
algorithm
__________
created
for
this
calculation.
To
reduce
computational
complexity,
it
was
decided
to
find
the
rms
value
from
the
peak
value
of
the
current
waveform.
An
issue
is
whether
to
send
the
complete
current
waveform
across
The
Rogowski
coil
is
an
air-cored
coil
that
is
used
to
t
thwrels
lik
orpjute
maximum
value.
Itawas
detect
currents
in
wires
that
are
encircled
by
the
coil.
decidedeto
send
the
maximum
valueo
ru
the
When
the
coil
is
placed
around
the
wire
a
voltage
is
.
t
power
consumption
of
the
transmitter
module.
induced
that
is
proportional
to
the
rate
of
change
of
Thereformum
value
ofa
current
was
current.
The
design
and
construction
of
Rogowski
coils
have
been
described
in
detail
[6].
The
equation
determined
using
the
transmitter
PIG.
governing
the
voltage
detected
by
the
coil
is
as
shown
in
Two
XBee
ZigBee
modules
were
used
(transmitter
and
Equation
1.
receiver)
and
were
programmed
with
64
bit
addressing
in
a
unicast
mode.
This
mode
of
operation
means
that
v
=-uonA
-
(1)
only
these
two
modules
can
communicate
with
each
coil
dt
other.
It
also
has
the
effect
that
after
the
sending
of
data,
an
acknowledgement
is
returned
to
the
sender,
reducing
where
pt
is
the
permeability
of
free
space,
n
is
the
the
chance
of
errors
through
lost
packets
of
data.
number
of
turns
per
unit
length
of
the
coil,
A
is
the
Initially
the
sleep
mode
was
not
implemented.
cross
sectional
area
of
the
coil
and
i
is
the
current
in
the
wire.
From
the
equation
it
can
be
seen
that
it
is
The
current
resolution
on
the
LCD
display
was
0.04
A,
necessary
to
integrate
the
induced
voltage
to
obtain
the
determined
by
the
8
bit
resolution
of
the
A/D
converter.
current.
The
integrator
circuit
also
acts
as
a
low
pass
Therefore,
given
that
the
maximum
current
to
be
filter,
reducing
high
frequency
noise.
The
transfer
measured
is
10
A,
a
three-digit
display
was
chosen
with
function
for
the
integrator
is
shown
in
Equation
2.
the
highest
possible
value
being
9.99A.
To
operate
the
LCD,
the
segments
and
backplane
connection
are
v
-R
supplied
with
square
waves.
For
a
segment
to
appear
out
=
2
(2)
'on',
the
drive
signal
for
the
segment
must
be
1800
out
vin
RI
(1
+
foCC
R2)
of
phase
with
respect
the
backplane
connection.
There
are
two
methods
for
driving
the
LCD,
namely
direct
and
The
pole
is
set
at
2.8
Hz,
so
that
the
integrator
acts
as
an
multiplexed.
As
there
are
only
23
segments
to
be
ideal
integrator
with
an
error
of
0.2%
from
the
ideal
controlled
and
many
free
pins
on
the
PIC
were
available,
value
at
50
Hz.
The
performance
of
the
integrator
has
direct
drive
was
chosen.
Direct
drive
from
the
also
been
modelled
in
NIATLAB.
The
circuit
diagram
microcontroller
has
the
distinct
advantage
that
no
external
LCD
drive
chip
was
required,
hence
reducing
power
consumption
and
overall
cost.
It
is
important
to
ensure
that
no
DC
component
appears
across
the
segments,
so
bit
toggle
instructions
together
with
timer
loops
were
used
on
the
PIC
to
produce
suitable
square
|
112M
waves.
Ivcc
Rogowski
Coil
Rl
3
552
A/D
Converter
As
well
as
driving
the
LCD,
the
receiver
PIC
has
to
scale
the
data
received
from
the
transmitter
unit
to
give
the
correct
calibration.
Calibration
was
initially
carried
c41cs
1
lc6lc7
~~~out
using
the
theoretical
sensitivity
of
the
coil
[6]
and
I
~~~~~~~~the
gain
of
the
integrator.
This
gives
a
value
of
10.6
for
4.uIQO
.u
O
the
scale
factor.
An
experimental
calibration
was
also
carried
out
using
a
current
transformer
and
an
around
oscilloscope;
this
calibration
includes
the
effect
of
any
Figure
2
-
Integrator
circuit
error
due
to
the
Rogowski
coil.
A
value
close
to
10.6
of
11
was
found
for
the
scale
factor,
which
was
749
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programmed
into
the
PIC,
along
with
a
look
up
table,
so
A
Current
Data
that
the
correct
segments
would
be
turned
'on'
for
the
10
-
Linear
(ideal
Trend)
current
values.
,
The
PIC
also
inserts
the
decimal
point
cLinear(CurrentData)
/l
after
the
first
digit.
l
__l
Figures
3
and
4
show
the
Rogowski
coil
and
receiver
>
unit
respectively.
____
'.,~~
~ ~ ~ ~ ~ ~ ~
~
~~
~
~~~~
°
2
4
6
10
12
Am
meter
Reading
/A
Figure
5
-
Linearity
test
with
wireless
receiver
Figure
3
-
Rogowski
coil
current
against
ammeter
reading.
current
in
both
cases
is
drawn
by
these
modules.
Without
the
ZigBee
module,
the
current
in
the
transmitter
falls
to
11.9
mA,
indicating
that
the
module
in
fact
uses
more
than
45mA
when
transmitting.
One
of
the
main
benefits
of
ZigBee
is
its
low
power
consumption.
However,
this
can
only
be
realized
when
the
sleep
mode
associated
with
the
modules
is
employed.
was
found
that
the
current
drawn
fell
to
13.8
mA.
As
the
transmitter
draws
11.9
mA
in
the
absence
of
the
module,
this
suggests
that
the
module
draws
1.9mA
when
asleep,
which
is
more
than
the
data
book
value
of
50
ptA
[3].
Button
cell
batteries
and
AAA
batteries
were
considered
Figure
4
-
Receiver
unit
as
power
sources.
Two
button
cells
will
give
the
minimum
required
supply
voltage
of
3V.
However,
the
maximum
peak
discharge
current
is
specified
as
only
2mA,
which
is
inadequate
unless
transmission
is
4
SYSTEM
PERFORMANCE
infrequent
and
a
large
capacitance
is
placed
across
the
supply
to
supply
the
peak
current
during
transmission.
The
system
was
tested
extensively
to
validate
its
Three
AAA
batteries
were
used
instead,
giving
a
peak
performance.
discharge
current
of
1.5A
and
a
total
voltage
is
4.5V,
which
is
more
than
sufficient.
The
average
capacity
is
4.1
Linearity
Test
1250
mAh,
which
means
the
batteries
would
last
62.5
The
unit
was
set
up
to
measure
the
current
in
a
single
hours
if
there
is
continuous
transmission.
If
the
modules
conductor.
The
current
was
varied
so
that
the
were
in
sleep
mode
for
9900
of
the
time
then
the
lifetime
relationship
between
the
values
shown
by
the
display
would
approach
eight
months.
unit
and
a
conventional
series-connected
ammeter
could
be
found.
The
results
are
shown
in
Figure
5
and
it
can
4.3
Supply
Current
Waveforms
be
seen
that
the
receiver
unit
current
values
are
never
The
supply
current
waveforms
for
both
the
receiver
and
more
than
0.04
A
away
from
the
trend
line
for
the
data.
transmitter
were
analysed
by
putting
a
2
Q
resistor
in
series
with
the
return
circuit
to
the
power
supply.
The
4.2
Supply
Current
Consumption
voltage
waveform
across
this
resistor
is
a
replica
of
the
By
attaching
an
ammeter
in
series
with
one
of
the
input
current
drawn
and
the
waveform
was
observed
with
an
terminals
to
the
transmitter
and
receiver
in
turn,
the
oscilloscope.
current
drains
were
determined.
It
was
found
that
the
current
consumed
by
the
transmitter
was
63.0
mA
and
A
typical
supply
waveform
with
the
transmitter
on
is
by
the
receiver
was
58.5
mA.
The
XBee
manual
[3]
shown
in
Figure
6.
gives
the
current
consumption
for
the
XBee
modules
as
45mA
while
transmitting,
showing
that
most
of
the
750
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T=k
I
stop
M
P,
0
CROR
it
is
susceptible
to
transients,
which
will
give
.
4
,
,
X
anomalously
high
readings.
*<*>t,6a X
lr~~~ype
The
Rogowski
coil
output
is
very
noisy
as
is
shown
in
Figure
7.
However,
when
the
integrator
output
is
observed
(Figure
8),
it
can
be
seen
that
the
noise
has
i
ii
ti
S
8been
removed
as
the
integrator
acts
as
a
low
pass
filter.
>J2
.
.
.E.When
the
Rogowski
coil
is
detecting
no
current,
the
C
or
1
noise
level
is
+/-2
mV.
.K
Stop
M
Post
6.300rs
CURSOR
CHI
6m
M
0OOM
CHI
I-368mn
source
Figure
6
-
Receiver,
with
transmitter
on,
supply
current
waveform.
elXa
From
this
image
it
should
be
noted
that
current
spikes
are
occurring
at
6.3
Hz,
which
is
exactly
the
rate
of
transmission
across
the
wireless
link.
When
the
transmitter
is
off,
no
spikes
occur,
and
the
spikes
in
CXJsor2
transmitter
current
are
wider
when
the
receiver
unit
is
_ ______
off.
This
is
a
consequence
of
the
unicast
mode
of
CL
operation,
where
the
data
is
sent
up
to
three
times
until
an
acknowledgment
is
received.
The
average
current
for
Figure
8
Integrator
output.
the
transmitter
unit
is
66
mA,
which
is
close
to
the
value
measured
with
an
ammeter.
A
further
disadvantage
of
the
method
of
deriving
the
rms
value
from
a
peak
value
arises
with
non-sinusoidal
4.4
Transmission
Range
current
waveforms.
To
illustrate
the
effect,
the
output
The
range
of
operation
of
a
wireless
system
is
very
current
from
a
triac-based
light
dimmer
was
sensed.
important
to
the
user.
To
find
the
range
of
the
system,
Although
the
integrator
again
removed
the
effects
of
the
transmitter
was
operated
from
the
power
supply,
transients,
the
displayed
rms
value
of
the
current
was
in
with
the
receiver
unit
being
powered
by
batteries.
The
error
by
10%.
receiver
was
then
moved
within
the
laboratory,
so
that
at
all
times
there
was
a
clear
line
of
sight
to
the
transmitter,
to
avoid
any
possible
unwanted
attenuation.
6
DISCUSSION
It
was
found
that
the
maximum
distance
of
operation
was
about
22
m.
6.1
System
performance
The
practicality
of
using
a
ZigBee-based
system
for
the
4.5
Noise
Interference
and
1rms
Calculation
remote
sensing
of
current
has
been
demonstrated.
The
method
that
was
implemented
for
finding
the
rms
Experience
with
the
demonstration
system
has
value
of
the
current
is
a
simple
technique
using
highlighted
issues
for
further
consideration.
maximum
values.
However,
it
has
the
disadvantage
that
6.2
Supply
Current
Consumption
Tek
J
____
The
current
consumption
was
63
mA
and
58.5
mA
for
+1_
v
llo
VWllolL
WllJlllLlUlType
TN
Ythe
transmitter
and
receiver
units
respectively,
and
the
E__._=
transmitter
drew
13.8
mA
in
the
sleep
mode.
Therefore,
to
take
full
advantage
from
the
ZigBee's
relatively
low
________
t
Xpower
consumption,
the
sleep
mode
must
be
used.
In
addition,
attention
must
be
paid
to
reducing
the
current
x
'l
<t1
V
consumption
of
the
PIC
and
analogue
circuits.
4
1
#
1
#
6
l
twism
.2ftY
.
-
For
the
sleep
mode
to
operate
correctly,
one
of
the
*~~~~~~~~~~~~~mdlsms
tyo
th
whol
time
whl
hte
.
Xw"v
~~~~~~~~~~wakes
periodically
tO
check
for
new
dlata.
For
this
-
--;tW';|3|pEsysemith
Dlll
1
units
are
theW
wrong
way
round,U
as
h
transmitter
should
be
asleep,
but
this
is
also
the
unit
that
7
~~~~~~~~~sends
the
data.
Therefore
another
sleep
mode
must
be
Figure
-
Rgowski
coi
output.
ncorporated
where
the
pins
on
the
modules
are
used
to
wake
the
units
up
[3].
With
the
sleep
mode
operating
for
751
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99%0
of
the
time,
a
battery
life
of
eight
months
can
be
corresponds
to
a
delay
of
5.6pts
between
the
operations,
achieved.
which
therefore
introduces
an
overall
DC
term,
degrading
the
display.
Further
study
of
the
issue
of
power
supply
would
be
beneficial.
It
was
found
in
this
paper
that
powering
the
One
way
to
remove
this
problem
would
be
to
control
the
system
off
button
cell
batteries
was
not
feasible
unless
segments
of
the
LCD
through
an
LCD
driver
chip.
This
special
measures
are
taken.
One
interesting
option
is
to
would
remove
the
delay,
so
therefore
eliminating
the
make
the
transmitter
unit
run
as
a
parasite,
using
power
fading
problem,
but
would
increase
the
cost
and
power
electro-magnetically
coupled
from
the
current
carrying
consumption
of
the
system.
Another
method
that
could
wire.
be
used,
that
still
uses
the
PIC
directly,
would
be
to
toggle
the
segments
as
bytes
rather
than
with
individual
6.3
Noise
Interference
and
1rms
Calculation
bits.
This
could
be
done
using
the
complement
The
integrator
was
found
to
be
effective
in
reducing
the
instruction,
which
would
reduce
the
timing
errors
noise
level.
The
residual
noise,
+/-2
mV,
was
less
than causing
the
DC
voltage
considerably.
the
resolution
of
the
A/D
converter
of
12
mV,
so
therefore
the
noise
is
not
a
problem.
It
should
also
be
noted
that
due
to
the
A/D
converter
only
operating
at
8bit
resolution,
the
display
had
a
Calculating
the
rms
value
of
the
current
from
the
peak
limited
current
resolution,
with
no
number
between
value
is
simple
but
is
only
accurate
for
sinusoidal
0.08A
and
0.
12A
for
example.
waveforms,
as
demonstrated
by
the
test
with
the
output
of
a
light
dimmer.
The
time
response
for
the
system
is
also
very
slow
at
present,
typically
1
s,
as
the
system
7
REFERENCES
uses
100,000
data
values
for
each
maximum
term
to
ensure
that
the
true
maximum
is
always
found.
An
1.
T.
Ciardiello,
"Wireless
Communications
for
alternative
solution
would
be
to
use
a
peak
detector
Industrial
Control
and
Monitoring",
Wireless
circuit
after
the
integrator
to
eliminate
the
time
delay.
Conference,
IEE
Computing
and
Control
Engineering,
April/May
2005,
pp.
12-13.
Calculating
the
true
rms
value
of
the
current
would
2.
E.
Abdi-Jalebi,
P.
C.
Roberts,
and
R.
A.
McMahon,
overcome
both
the
restriction
on
wave
shape
and
the
"Real-time
rotor
bar
current
measurement
using
a
problem
of
slow
response.
However,
due
to
the
limited
Rogowski
coil
transmitted
using
wireless
function
library
on
the
PIC
microcontroller
used,
a
more
technology,"
vol.
5.
18th
International
Power
powerful
PIC
would
have
to
be
employed.
System
Conference
(PSC2003),
October
2003,
pp.
1-9.
6.4
Wireless
Data
3.
XBee
/
XBee-Pro
OEM
RF
Modules
Manual,
Whether
to
carry
out
the
signal
processing
to
find
the
Maxim.
maximum
value,
or
true
rms
value,
of
the
current
on
the
4.
"What
Exactly
Is
ZigBee?",
IEE
Communications
transmitter
or
receiver
side
of
the
wireless
link
is
a
Engineer,
August/September
2004,
pp.
44-45.
critical
issue.
In
the
system
described
in
this
paper,
the
5.
N.
Baker,
"ZigBee
and
Bluetooth
Strengths
and
maximum
was
found
on
the
transmitter
unit,
so
that
the
Weaknesses
for
Industrial
Applications",
IEE
ZigBee
module
data
rate
could
be
kept
low
(6.3
Hz),
Computing
and
Control
Engineering,
April/May
thereby
reducing
the
current
consumption
to
a
minimum.
2005,
pp
20-25.
However,
this
meant
that
the
PIC
on
the
transmitter
was
6.
E.
Abdi-Jalebi
and
R.
A.
McMahon,
"Simple
and
constantly
active,
resulting
in
a
high
current
practical
construction
of
high
performance,
low
consumption.
The
alternative
would
be
to
have
a
cost
Rogowski
transducers
and
accompanying
relatively
inactive
transmitter
PIC,
but
constantly
send
circuitry
for
research
applications,"
vol.
1.
22nd
data
ay
a
higher
rate.
It
should
be
noted
that
the
current
IEEE
Conference
on
Instrumentation
and
consumption
for
the
PIC
is
much
lower
than
the
45
mA
Measurement
Technology
(IMTC2005),
May
2005,
drawn
by
the
ZigBee
module,
so
former
method,
as
pp.
354-358.
used
in
this
work,
is
preferable.
This
is
also
reinforced
by
the
fact
the
ZigBee
modules
are
designed
to
combine
a
low
data
rate
with
a
low
power
consumption.
AUTHOR'S
ADDRESS
6.5
LCD
For
contact
purposes
the
second
author
should
be
It
is
noticed
that
after
a
certain
amount
of
time
the
LCD
contacted
at:
began
to
fade
quite
significantly,
becoming
almost
Department
of
Engineering
invisible
when
viewed
straight
on.
The
LCD
is
operated
Cambridge
using
the
toggle
instruction
on
the
PIC.
From
the
CB2
JRL
backplane
toggle
instruction
to
the
last
segment
toggle
United
Kingdom
instruction
there
are
28
operations
in
between,
which
Email.
ram1
@eng.
cam.
ac.
uk
752
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