Table Of ContentSmart Electricity Metering
System
for greenhouseconscious homes
Ryan Alexander
Table of Contents
Introduction........................................................................................................................3
Aim..........................................................................................................................................4
Functional summary.........................................................................................................5
Transmitter module..................................................................................................................5
Receiver module.........................................................................................................................5
Functional description.....................................................................................................6
Power consumption measurement......................................................................................6
Waveform algorithm.................................................................................................................8
Microprocessor........................................................................................................................12
Wireless link.............................................................................................................................12
Computer link...........................................................................................................................12
Network stack implementation.................................................................................14
Packet encapsulation.............................................................................................................14
Implementation of network protocols.............................................................................15
Software.............................................................................................................................18
Overview.....................................................................................................................................18
Structure and operation........................................................................................................18
TX initialisation and association........................................................................................18
TX application layer................................................................................................................20
RX initialisation and association........................................................................................21
RX application layer...............................................................................................................21
Physical specifications..................................................................................................22
Maximum ratings.....................................................................................................................22
Connections...............................................................................................................................22
Testing methodology / Test points...........................................................................24
Hardware....................................................................................................................................24
Software......................................................................................................................................24
User interface...................................................................................................................26
Project timeline...............................................................................................................27
Appendix A: Microcontroller schematics...............................................................29
Appendix B: Software code..........................................................................................32
RX Config_stuff.h....................................................................................................................32
RX non_rf_comms.c...............................................................................................................33
RX RX.c......................................................................................................................................36
TX – mac_attributes.h.............................................................................................................43
TX – non_rf_comms.c...............................................................................................................44
TX – TX.c......................................................................................................................................48
Introduction
Over the past twelve months, the world has seen an exponential increase in the
awareness of climate change and how it will affect society and the environment.
In particular, attention has only recently become focused on what structural
changes are required in the domestic, commercial and industrial environments
in order to mitigate the effects of climate change. The main cause of climate
change has been identified as the increasing quantity of carbon dioxide in the
Earth's atmosphere. According to the Australian Bureau of Statistics, electricity
use accounts for 27% of carbon dioxide emissions in Australia, a figure which
has grown by 42% since 1990. It is therefore essential that carbon emissions are
reduced from electricity use, either through carbon‐neutral electricity
generation technologies or savings through energy efficiency. The aim of this
project is to create a device which will help homeowners and businesses identify
wasteful energy usage in their homes and offices, thereby reducing electricity
use by raising awareness of power consumption and helping Australia move
towards a more sustainable future.
These needs have arisen as a response to two separate markets which have
demonstrated a demand for such a device: the domestic homeowners market
and the energy auditing and reporting industry. The latter is a growing market
driven by the need for businesses to become increasingly conscious of their
carbon footprint. One of the main drivers for this new need is the Federal
Government’s National Greenhouse and Energy Reporting System, which is due
to be implemented in stages from mid‐2008. This will require corporations to
audit the equivalent carbon dioxide emissions of their business activities in
order to provide a streamlined, national framework for emissions reporting. A
key component of this system will be a method of monitoring and recording
electricity usage in commercial operations.
Simultaneously, the Federal Government, following the lead of the Victorian
Government's Department of Primary Industries (DPI), has committed to a roll‐
out of electricity smart meters across the country beginning in 2009 and
continuing until 2012 (the ‘Advanced Metering Infrastructure’ program1). These
meters, to be installed in all domestic, commercial and industrial properties, will
allow remote reading of electricity consumption in discrete intervals, rather than
the current accumulation meters which are monitored every three to six months.
Apart from the cost savings in relation to remote reading of meters, it is hoped
that their implementation will raise awareness of energy consumption and
encourage energy efficiency and demand‐side participation in the national
energy market.
1 http://www.dpi.vic.gov.au/dpi/dpinenergy.nsf/ at 12/4/2008
Both of these drivers are contributing to further innovation in the energy
monitoring and auditing sector. More products are required to provide services
which allow home‐owners and commercial bodies to further understand their
carbon footprint and take action to mitigate their impact on climate change.
Aim
To design, develop and implement an electricity metering device for home or
commercial use which transmits data to a display for use in energy monitoring
and auditing.
Functional summary
Transmitter module
Test point 5 Test point 1
Test point 2
Op-Amp (2) SPI (3)
V and I signals ADC ZigBee RF230
Transformers(2)
AVRmega8
Load
Receiver module
Test point 3
Test point 2
SPI (3) USART (3)
ZigBee RF230 AVRmega8 MAX232
Test point 4
RS232
Computer
Functional description
Power consumption measurement
The Smart Meter measures both the current and voltage of the test circuit. Using
instrument transformers, the test circuit characteristics can be measured non‐
invasively. The resulting analog signals are fed into an LM348 op amp configured
as a summer circuit to scale the signals down to a 0‐5v logic level. This provides
an interface into the Atmel AVRmega8 microprocessor, where both current and
voltage signals are input into two channels of the on‐board 10‐bit ADC.
The functional specification for the Smart Meter required it to measure a circuit
operating at no more than 12v AC, in order to mitigate the potential for
hazardous electric shocks. In order to measure a voltage of such a circuit, a
transformer was used to provide electrical isolation from the primary circuit.
This ensured that the sensitive microprocessors on the PCB were not directly
exposed to the primary circuit.
Fig 1: Voltage measurement circuit
The configuration of the voltage transformer and the op‐amp is shown above. As
the voltage from the voltage transformer oscillates from a positive voltage to a
negative voltage, it must have an offset added to it to allow it to interface to the
op‐amp, which can only measure voltages within its voltage supply range of 0‐5v.
To acheive this, the op‐amp is configured as a summer circuit which sums
together the voltages present at its + input. One of the input voltages is the signal
from the voltage transformer. The voltage emanates from the voltage
transformer’s centre tap and is halved using a voltage divider circuit, lowering it
by a further factor of two. The other input voltage is a reference voltage provided
by another voltage divider circuit, which remains constant at 2.5v. This offset, or
bias voltage, represents 0v at the voltage transformer output. When the test
circuit is enabled, the output of the op‐amp will swing between 0v and 5v, which
allows it to interface to the Atmel AVRmega8’s ADC.
Fig 2: Current waveform at A (lower
line) and at B (higher line)
There were multiple possible configurations for measuring current in the test
circuit. A load resistor could be used in series with the test load to convert the
current information into voltage information. However, this would result in
unnecessary losses in the load resistor and also present a shock hazard if the
load inadvertently disconnected from the circuit. A Hall effect sensor was
considered, as it requires no contact with the test circuit; it detects the voltage
Fig 3: Current measurement circuit
induced along a strip of metal.
However, the current measurement circuit is similarly configured to the voltage
measurement circuit. It utilises a Talerma AC1005 current transformer which is
rated to measure up to 10A in the primary circuit. The current transformer
produces a corresponding ‐2.5v to 2.5v voltage swing at its outputs which is
summed with a bias voltage to produce a 0‐5v signal range as above.
Waveform algorithm
• Waveform Analysis
In order to calculate the instantaneous power consumption of the test circuit, it
is necessary to determine the voltage and current at any given time. In general,
for sinusoidally‐varying voltage and current waveforms, this can be determined
by calculating the root mean square (RMS) values, using one of two approaches:
1) Calculate the maximum signal deviation from the zero‐crossing level of
the waveform (i.e. I or V )
peak peak
2) Calculate the maximum variation in signal (i.e. I orV )
p‐p p‐p
Option 1) is viable if the zero‐crossing level of the waveform is known. This level
should correspond to the bias voltage applied to the + input of the op‐amp, which
should correspond to the output of the op‐amp if no voltage is present across the
test circuit. However, if there is an unforseen failure or error which changes the
bias voltage, this option may return incorrect data.
By calculating the maximum variation in signal, Option 2) avoids the above
problem in Option 1). As long as the input signal into the AVRmega8 is within its
input range of 0‐5v, changes in the bias voltage will not affect the result of the
calculation. This ensures that failures in the circuit or fluctuations in the voltage
regulator output will not affect the accuracy of the measured voltage or current
signal.
P = V I
RMS RMS * RMS
V = V / √2
RMS p‐p
I = I / √2
RMS p‐p
The on‐board 10‐bit Analog to Digital Converter on the AVRmega8 is used to
convert the voltage and current analog input signals into digital data which feeds
into the waveform algorithm.
The performance requirement of the meter, specified in the Design Document, is:
“The Smart Meter shall be able to measure power consumption at a minimum
resolution of 5w.”
This corresponds to:
ΔIp = P/Vp = 5w/(240v*√2) = 0.030A
ΔIp = Ip*sinθ
0.030 = 10*√2sinθ //rated current is 10A RMS
sinθ = 0.002
θ = 0.12°
360°/0.12° = 3000 samples/cycle = 3kHz
For 50 cycles per second (50Hz),
3000 samples/cycle x 50 cycles/second = 150000 samples/second = 150kHz
The clock speed of the AVRmega8 is 16MHz. The ADC can be run in 'free‐
running' mode as long as the ADC sampling speed is 3kHz or greater. This can be
achieved by pre‐scaling the ADC clock. To achieve maximum sampling resolution
(10 bits), the ADC clock must run between 50kHz and 200kHz (the slower the
clock speed, the more time for the successive approximation circuitry to settle to
a more precise value). However, it takes around 13 ADC clock cycles for one
conversion to occur. This means that:
150kHz*13 = 1.95MHz
is the slowest the ADC clock can run while still sampling enough waveform
values per second to accurately measure the signal amplitude. Choosing an ADC
clock pre‐scale value of 8, the resulting ADC clock speed will be:
16000/8 = 2MHz.
which is about 10 times the minimum ADC frequency necessary to guarantee
accurate 10‐bit sampling. As a result, it is unclear whether the performance
requirement specified in the Design Document will be met, as it depends on the
ability of the successive approximation registers in the ADC to settle to an
accurate value before the next sample is taken.
• Waveform algorithm operation
The performance requirement stated that:
“The Smart Meter shall be able to transmit power consumption data at a minimum
rate of 1Hz.”
In order to achieve this requirement, a software algorithm has been created to
evaluate the waveform algorithm at one second intervals, calculate the power
consumption of the test circuit and pass the data onto the network stack for
transmission.
As specified above, it is necessary to record 3000 samples/waveform cycle. The
Design Document specified a 5% tolerance so that accurate measurements could
be maintained in the event of over‐frequency events in the power system which
would increased the period of a waveform cycle, which would potentially cause
the peak voltage/current measurement to occur outside of the 3000
samples/waveform.
However, it is only necessary to record the data from one sine wave at any one
time. There are 3000 samples / cycle which would create:
3000 samples / cycle x 10 bits/sample = 30 000 bits/cycle = 3750 bytes/cycle
Taking into account a tolerance margin in the event of over‐frequency
fluctuations:
3750 bytes/cycle + 5% tolerance = 3900 bytes
would be required.
A better approach would be to write only the current sample of each sine wave
to memory. In order to determine the RMS value of the input voltage or current
sine wave, it is necessary to capture the peak value of each sine wave cycle. This
could be achieved by comparing the previous sampled value to the current
sampled value and determining the maximum value when the current sampled
value starts to decrease (i.e. after 90° on a sine wave). This maximum value could
then be reset for the next cycle to measure a new voltage or current value. This
approach is shown below:
Description:Smart Electricity Metering System for greenhouseconscious homes Ryan Alexander
