project21 final paper
TRANSCRIPT
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ECE445 Senior Design
Final Report
Prepaid Electricity Meter
Ankit Chandak
Yen-Chia Huang
Pattaramon Vuttipittayamongkol
Department of Electrical and Computer Engineering
University of Illinois at Urbana-Champaign
04 May 2011
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ABSTRACT
Our team designed and built a prepaid energy meter with a GSM module. The GSM
module provides a mode of communication between the user/meter and the utility. This
will enable the user to recharge his electricity account from home. This will also enable
the user to carry his electricity account with him, eliminating the need to set up a new
account every time the user changes homes. The GSM module will also get real time
electricity rates and enable the utility to keep a check on electricity theft. Our model of
the energy meter samples the current and voltage waveform using the ADC available on
the PIC microcontroller and computes real power, taking into account, the power factor
of the load.
The device was implemented with GSM to transmit the exchanged data between the end-
users, which are a utility company and a customer. The keypad is used to get banking
information from an electricity customer, and the LCD will display the user’s account
balance and the present electricity rate. The LED light will blink when the balance goes
below the threshold to remind the user to refill the account. If the balance goes down to
zero, the power will be cut off.
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TABLE OF CONTENTS
1. INTRODUCTION ....................................................................................................................3
1.1 Objective .............................................................................................................................3
1.2 Specifications ......................................................................................................................3
1.3 Subprojects .........................................................................................................................4
1.3.1 LCD Display ..............................................................................................................4
1.3.2 Keypad .......................................................................................................................4
1.3.3 Current Sensing Module ............................................................................................4
1.3.4 Voltage Sensing Module ...........................................................................................4
1.3.5 Power Measurement Module .....................................................................................4
1.3.6 GSM Module .............................................................................................................5
1.3.7 User Interface Program .............................................................................................5
2. DESIGN PROCEDURE ...........................................................................................................6
2.1 Current Sensing Module Design .........................................................................................6
2.2 Voltage Sensing Module Design ........................................................................................6
2.3 Power Measurement Module Design ..................................................................................7
3. DESIGN DETAILS ..................................................................................................................8
3.1 LCD Display .......................................................................................................................8
3.2 Keypad ................................................................................................................................8
3.3 Current Sensing Module .....................................................................................................8
3.4 Voltage Sensing Module .....................................................................................................9
3.5 Power Measurement Module ..............................................................................................9
3.6 GSM Module ....................................................................................................................10
3.7 User Interface Program .....................................................................................................10
4. DESIGN VERIFICATION .....................................................................................................12
4.1 LCD Display .....................................................................................................................12
4.2 Keypad ..............................................................................................................................12
4.3 Current Sensing Module ...................................................................................................12
4.4 Voltage Sensing Module ...................................................................................................14
4.5 Power Measurement Module ............................................................................................14
4.6 GSM Module ....................................................................................................................15
4.7 User Interface Program .....................................................................................................15
5. COST ......................................................................................................................................16
5.1 Parts ..................................................................................................................................16
5.2 Labor .................................................................................................................................16
6. CONCLUSIONS ....................................................................................................................17
REFERENCES .......................................................................................................................18
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1. INTRODUCTION
We designed and built a prepaid electricity meter. This meter will be the medium
between an individual household and the utility company for conducting money
transactions and exchanging the current electricity rate and usage. Although prepaid
meters have been out in the market nowadays, we are aiming for a better performance in
term of coverage range and wireless technology seems to handle this issue well.
1.1 Objective
Upon completion, we would like our meter to be able to do the following things:
1. Measure electricity consumption accurately.
2. Display real time account balance.
3. Communicate with the utility company to:
Let the user recharge his electricity account from the meter
Using a previously used card
Using a new card
Update rates for electricity as and when required
Perform a daily/hourly verification of electricity consumption
4. Warn the user of low account balance by flashing an LED
5. Cut power off when there is zero credit on the account.
In order to complete the afore mentioned tasks, we plan to make accurate power
measurements, interface an LCD display as well as a keypad with the central
microcontroller unit and implement a transceiver module in our system.
1.2 Specifications
An electricity meter provides an interface between the utility and the customer.
Successfully implemented, our product will benefit the end customer as well as the
electric utility in the following ways:
The customer can recharge his account wirelessly from his home.
The customer can use a different card every time. He will not have to worry about
his default credit card already been maxed out.
The device will show the remaining balance so that the user knows how much he
has consumed and can plan ahead to not overuse the budget and know when he
needs to refill the account.
The display will also show the current electricity rate. Since electricity rates vary
throughout the day, the user can cut down on consumption when the rate is high.
The utility companies will have a better idea of electricity demand. This will help
them to plan ahead.
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The utility companies would be able to collect the expenses from customers in
advance, so they will no longer have to deal with late payments or non-paying
customers.
Since the meter will send daily/hourly consumption data to the utility company, it
will help reduce electricity theft.
Our product will have the following features:
Wireless capabilities to recharge the account from home.
Provision of a keypad to recharge the account from a different card every time.
LCD to display real time account balance and the current electricity rate.
LED to warn the user of low account balance.
Automatic shut-down feature once the account balance is zero
1.3 Subprojects
The design was broken into many modules, which each perform specific tasks.
1.3.1 LCD Display
The LCD display is used for displaying user account information, account balance and
power usage. It also acts as an interface between user and power meter.
1.3.2 Keypad
The keypad allow user to input his/her account number, credit number and PIN number.
1.3.3 Current Sensing Module
The current sensor needed to monitor the current flow by measuring and reporting the
actual current usage and the current phase angle to the microcontroller. The current
sensor needed to operate accurately and linearly in order to obtain the accurate usage and
consequently the accurate power usage. Lastly, the current sensor was expected to be able
to hold the maximum current of 10 Amperes.
1.3.4 Voltage Sensing Module
The voltage sensor needed to measure voltage and the phase angle of the voltage across
the load accurately, and it was expected to behave linearly in some specific voltage range.
1.3.5 Power Measurement Module
The power measurement module takes the scaled-down signals from the voltage divider
and the current transducer and computes the real power consumed by the load. These
power consumptions readings then are used to compute the real time balance for the
customer.
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1.3.6 GSM Module
GSM module transmits user’s account information from power meter to utility company
and also receives data from utility company.
1.3.7 User Interface Program
The user interface program is consist of a finite state machine and was loaded onto PIC
micro-controller. The state machine jumps from one state to another based on different
conditions.
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2. DESIGN PROCEDURE
2.1 Current Sensing Module Design
The current sensor was designed to connect directly to the load on the input side and to
the microcontroller on the output side. The input to the entity was the value of the voltage
drop across a shunt resistor and the output to the microcontroller was the voltage that was
proportional to the input voltage. The ratio of the input and output voltage would depend
on the model of the current transformer used in the circuit. Because the 120 AC volts
would be apply to the load but the maximum input voltage that the microcontroller we
used was 5 Volts, we then were looking for a current transformer that had the turn ration
of at least 1: 25 including any error, in order to scale the 120 AC supply voltage down
within a 5 volt constraint.
After searching for a proper current transformer, the main component of this module, we
decided to go with the LAH 25-NP current transducer, which has all the properties within
our constraints. The LAH 25-NP current transducer works with both DC and AC. It can
be configured to have a turn ratio of 1:1000, 2:1000, or 3:1000. Different turn ratio comes
with different nominal and maximum primary currents; thus after considering all the
factors, we chose the configuration of 1:1000 turn ratio with 25A nominal current and
55A maximum current in order to have the optimize result even though any of these three
configurations would work fine with our circuit.
2.2 Voltage Sensing Module Design
The voltage sensor module was designed to be connected to the power supply on the
input side and to the microcontroller on the output side. As this module would be
connected directly to the power supply, we wanted to insert an isolated voltage amplifier
or optocoupler, which will conduct the current signal without electrical connection
between the input and output in order to prevent high voltages or rapidly changing
voltages on one side of the circuit from damaging components on the other side. After
searching for a proper optocouple, we carefully decided to use the HCPL-7800A
Isolation Amplifier, which had a broad ambient operating temperature of
and worked accurate and linearly in the range of -200 to 200 mV.
Because the microcontroller we decided to use had the maximum input voltage of 5
Volts, we would need to add a voltage divider somewhere in between the 120 voltage
supply and the microcontroller.
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Figure1. Complete Current and Voltage Sensor circuit
2.3 Power Measurement Module Design
The power consumed by the load is calculated by sampling the current and voltage
waveforms for their peak values. The peak values are then used to compute the RMS
values of the signals. Next, the power factor is calculated as explained in the next section.
Once we get the RMS values for the current and voltage signals and the power factor of
the load, we easily compute the real power consumed by the load.
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3. DESIGN DETAILS
3.1 LCD Display
We use 16x2 Lumex LCD display for our meter. This LCD uses 5V power supply and
the contrast is adjustable. We interface LCD with PIC micro-controller using 4-Pin
control mode. For LCD driver, we use the code from CCS forum; the detail will be
included in reference section.
3.2 Keypad
In order to interface hex keypad and PIC micro-controller, we need to use matrix
decoding. One of disadvantages of using this keypad is the debouncing issue, and we are
able to solve it using the software method. We use the keypad driver code also from CCS
forum.
3.3 Current Sensing Module
Figure2. Current Sensor Circuit
The current sensor circuit was designed to measure the current passing through the load.
To scale down the output voltage from the load, the LAH 25-NP was placed in between
the load and the PIC microcontroller as seen in Figure 1. Also, we a shunt resistor was
needed to be placed in between the current transducer and the PIC microcontroller
because the output of the current sensor was a voltage, and thus the current through the
load could be obtained by the division of the output voltage value over the shunt resistor
value.
The turn ratio of 1:1000 of the current transducer results in a maximum of 0.12 Volts on
the output side. Thus, the voltage limit to the PIC16F877A microcontroller is in the
proper range. The voltage will never exceed 0.12 Volts as a positive voltage across the
load indicates that the output side of the load will have a voltage lower than 120 Volts.
The output flowing can be obtained by the equation:
(Eq1)
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3.4 Voltage Sensing Module
Figure3. Voltage Sensor Circuit
Knowing from the HCPL-7800A that it behaved accurately and linearly in the range of -
200 to 200 mV, we scaled down the input voltage from the 120 V supply to be within the
range by placing a voltage divider circuit in between. The circuit divider consisted of two
resistors that their values were measured and recorded for the purpose of later power
calculation. We calculated the ratio of the two resistors from
(Eq2)
Then, we got the ratio of R1:R2 999:1.
Because the input to the optocouple would not exceed 0.2 Volts, the output to the
microcontroller would be in the proper range, which was within 5 Volts.
3.5 Power Measurement Module
We start power measurement by sampling the input voltage and current signals and
sampling them to get their peak values. These peak values are then used to compute the
RMS values of these signals using the following relation:
(Eq3)
The code to sample the waveforms is included in the appendix.
After obtaining the RMS values for the input signals, we find out the phase difference
between the two signals to compute the power factor for real power calculation.
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Figure4. Voltage and current waveforms for power factor calculation
The power factor was calculated using the following algorithm and the code is included
in the appendix.
Both the voltage and the current signals are sent through a comparator with the reference
voltage set at 0V. Now, the half-period of the waveform is calculated by setting the
counter when the voltage comparator reads a ‘1’. Let this value be C1. After this, a
logical construct is used to set the counter only when both the voltage comparator and the
current comparator are set as ‘1’. This value is named as C2. The difference between C1
and C2, calibrated against time (since the frequency is known) gives the phase difference.
The cosine of this phase difference gives the power factor.
After obtaining the power factor and the RMS values of the voltage and current signals,
real power can be computed as follows.
(Eq4)
3.6 GSM Module
Our original idea is to connect the GSM module (GM862) to PIC micro-controller. First
we test the AT commands by connecting GSM to computer using Putty. The result is that
we are able to send and receive messages using AT command without any major issues.
In order to interface GSM module with PIC micro-controller, we have to remove the
solder jumper on the GSM evaluation board to separate the connection between USB
ports and the module. We use hardware UART channel of PIC which consist of two
signal lines, TX and RX. However, when we send AT commands to GSM, we never get
any signal back from it. We also try using logic analyzer to test if GSM ever responds to
the input signal from PIC. The result from logic analyzer shows that GSM did output
signal. However, we were not able to read it inside the PIC micro-controller.
3.7 User Interface Program
The user interface program consists of six states from state zero to state five. When the
meter first starts, it always starts from state zero which does all the initialization. Once
initialization process is done, it automatically goes to next state which is state one. In
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state one, the meter asks user to input account number and account PIN number. This
information will be sent to utility company through GSM module. Once the confirmation
signal is received by power meter, it also extracts account balance from utility company.
If the account balance is normal, the meter goes into normal operation mode. If the
account balance is low, it will remind user to recharge. Once the balance goes to zero, the
user can choose to recharge or shut down the meter.
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4. DESIGN VERIFICATION
4.1 LCD Display
For testing LCD display, first we power up the LCD and test the limitation of adjustable
contrast. Finally we decide to use 0V for contrast input. Secondly, we send data from PIC
micro-controller and test if LCD displays the correct value. After we are able to interface
keypad with PIC, we also try inputting data from keypad and test if LCD displays the
correct value.
4.2 Keypad
The most important thing for testing keypad is to make sure we have solved the
debouncing issue. The way we test it is by constantly pressing one key and make sure
LCD still displays the correct value.
4.3 Current Sensing Module
Figure5. Current Transducer
To test if the current sensor module worked properly, we powered the LAH 25-NP
current transducer with +12V and -12V supplies, inserted three different values of loads
and three different values of input voltages from the function generator across each load
for each test, then measured the output current using a multi-meter in the ammeter mode.
By apply equation (3.1.1), we could calculate the expected output current of each test.
Then, we compared the expected and the observed currents to see how much the error
was. The table 1 below shows the results from the testing procedures. The theoretical and
observed results are to be compared in the two right most columns.
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Table1 Results from the Current Sensor Circuit Testing
R1
(Ω)
Vin
(V)
Expected Iin
(mA)
Experiment Iin
(mA)
Expected Io
(mA)
Experiment Io
(mA)
Error*
(%)
1488 12 8.064516129 8.05 0.0080645 0.00804 0.303801
1488 20 13.44086022 13.54 0.0134409 0.01353 0.662902
1488 25 16.80107527 16.98 0.0168011 0.0172 2.374249
744 12 16.12903226 16.12 0.016129 0.0164 1.680203
744 20 26.88172043 26.86 0.0268817 0.027 0.440076
744 25 33.60215054 33.54 0.0336022 0.03353 0.214867
38.9 12 308.4832905 308.33 0.3084833 0.309 0.167497
38.9 20 514.1388175 514.01 0.5141388 0.521 1.334503
38.9 25 642.6735219 642.47 0.6426735 0.656 2.073603
The plot of expected and the observed currents from the table 1 is shown below to make
it easier to compare those values. We can see that most parts of the lines overlap, which
indicates that the errors are very small.
* Error (%) = –
(Eq5)
Figure6 Plots Comparing the Expected and Observed Current from table1
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4.4 Voltage Sensing Module
To make sure that the voltage sensor module worked properly, we tested the HCPL-
7800A give us accurate result and linear input-output relationship. We input the isolated
amplifier with different voltages and measure the output voltages. Table 888 shows the
results and the gain
.
Table2. HCPL-7800A Isolated Voltage Amplifier Testing Results
R1 (Ω) R2 (Ω) Input (V) Output (V) Vo/Vin
300 300 0.05 0.394 7.88
300 300 0.1 0.794 7.94
300 300 0.15 1.194 7.96
300 300 0.2 1.594 7.97
300 300 0.3 2.395 7.98
300 300 0.31 and above 2.56 7.31
These results verified that the chip behaved linearly when the output voltage was no
higher than 0.3 Volts and the average gain was around 7.95.
Moreover, with the input limit and the gain, the amplified output to the microcontroller
would not exceed 5 volts. Therefore, the voltage sensor module had been proved to be
proper to add to the rest of the circuit of the project.
4.5 Power Measurement Module
We tested the power measurement module by sending two 60 Hz sine wave signals from
a function generator. We chose a function generator for testing purposes as it gave us the
freedom of choosing the frequency and amplitude of input signals, thus helping in
debugging sampling and phase measurement algorithms. Following is the data obtained
while measuring the peak value of the input signal:
Table3. Data from scaling input current and voltage signals with scaling factor
Input Value Reading Scaling Factor Peak Value
0.5 11 1/23 0.48
1 24 1/23 1.04
2 46 1/23 2.00
3 70 1/23 3.04
4 91 1/23 3.95
5 114 1/23 4.95
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The phase measurement algorithm was tested by building the circuit and testing it for
various different capacitive and inductive loads and measuring the power factor with the
help of the watt-meter in the power lab and verifying the result against the power factor
calculated by our algorithm. Following is a snapshot of this procedure:
Figure7. Phase angle measurement testing
The two different values of the power factor are tabulated below:
Power Factorwatt-meter Power Factorour meter
-0.93 -0.93
-0.76 -0.74
0.13 0.15 Table4. Power Factor comparison
4.6 GSM Module
When connecting the GSM module to computer using Putty, we use AT command to
make sure we can perform bi-directional communication. We also test if there is any data
loss during the process. The result was perfect.
4.7 User Interface Program
Our user interface program is a finite state machine. For testing it, we make sure it goes
from one state to another based on the desired condition and never get stuck at any states.
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5. COST
5.1 Parts
Table5. Parts cost
Parts For Quantity Price($)/Unit Total($)
HCPL 7800A
Isolated Voltage
Amplifier
Power
Measurement 1 10 10
LAH25NP Current
Transducer
Power
Measurement 1 23 23
Power IC ADE
7753
Power
Measurement 1 5 5
PIC 16F877A
Micro-controller MPU 1 8 8
GM 862 GSM
Module Transceiver 1 20 20
4x4 HEX Keypad User
Interface 1 10 10
2x16 LCD User
Interface 1 15 15
Capacitor
(100nF*2, 0.1uF*3,
1uF*1, 10uF*2,
24uF*2)
10 0.32 3.2
Resistor (8ohm*1,
10ohm*1,
1kohm*1,
10kohm*1,
2.4kohm*1
5 0.32 1.6
5.2 Labor
Table6. Labor cost
Name Salary ($/hr.) # of Hours Cost of Labor
(Salary*hrs.*2.5) ($)
Ankit Chandak
40 200 20000
Pattaramon
Vuttipittayamongkol 40 200 20000
Yen-Chia Huang
40 200 20000
Total cost = Parts + Labor = $60,120
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6. CONCLUSIONS
Our team was successfully able to build a power measurement system, complete with
voltage dividers, current transformers and a phase angle measurement algorithm. We
were able to interface an LCD display as well as a matrix keypad with our system and
also successfully implement a user interface program. This user interface notifies the user
of his current account balance and assists him in recharging/managing his account. We
also tested the GSM module successfully using a terminal emulator application (PuTTY).
However, we weren’t able to interface the GSM module with the microcontroller.
Further work on this project can lead to the development of a utility-side program that
can manage the utility database and assist them to address account recharge requests.
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References
PCM programmer, “Flexible LCD driver”
http://www.ccsinfo.com/forum/viewtopic.php?t=24661
Ahmed, “Flexible Keypad Driver”
http://www.ccsinfo.com/forum/viewtopic.php?t=26333
Lukas Hoffmann, “PIC 16F877A Tutorials for Pitt Robotics Club”
http://www.pitt.edu/~sorc/robotics/Lukas%20PIC%20Tutorial.doc