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					Autonomous Weather Station
      Electronic Instrumentation and Systems

             Laboratory & Project Module




                                               José Manuel Pinto
                                                María José Quero
                                                    Sergio Ortega
                                                   Julio Sagardoy
                                                          Group 5




                                                               1
1. Introduction
    The main objective of this project is to design, implement and test an Autonomous
Weather Station (AWES from now). Part of this project is to document all the work done. This
document explains and summarizes the proposal of the Group 2 for constructing the AWES.
    This project is intended to be part of the “projects” module of the so-called Electronic
Instrumentation and Systems subject, along with the Guided Laboratory Sessions. These last
are coursed during the first half of the quarter, while the project itself is developed during the
second half, which roughly represents seven working weeks.
    It is important to remark that AWES is not intended to be a final product. At the end of the
quarter, the AWES will be a working prototype. Therefore, those vital processes which are
considered when developing the final product, such as EMI and ergonomics, are not taken into
account at all.

1.1. The AWES
    As told, the AWES is an autonomous weather station. It is not a strict weather station, as it
only contemplates temperature and humidity meteorology factors. Usually complete home-
level weather stations are able to also provide pressure readings, while more advanced
equipment yet provide with wind direction and speed, pluviometer, pyranometer, ceilometer,
between others.
    Figure 1-1 shows the main idea of what the AWES does. The AWES sensors periodically
takes measurements of both temperature and humidity which are then processed and sent to
a computer.




                                    Figure 1-1. AWES general diagram.


1.2. Features
    The AWES provides with various interesting features, which are described below.

1.2.1. Completely autonomous
    The inclusion of the “autonomous” word in the AWES name and definition is, in fact,
because both AWES sensing modes are completely automatic, with no human involvement in
the process at all.
    The only user intervention is just to select the preferred measurement interval.

1.2.2. Two acquisition timing modes
    The AWES module has two modes of obtaining the current weather parameters:
           Continuous mode: Makes the AWES to continuously sense the environment, with
            no intervention by the user.


                                                                                                2
          Periodic mode: Makes the AWES to take a measurement each 10 minutes, with no
           intervention by the user.
    These modes are selectable from the computer program interface, by means of a selector
switch.

1.2.3. Large autonomy
    The AWES is intended to work from two AA batteries. However, it is designed to have
autonomy for more than a year in the continuous-sensing mode and thus even more in the
periodic-sensing mode.

1.2.4. Computer assisted
     The AWES is controlled and its results are displayed in a computer. To avoid the necessary
digital processing hardware stage and software design, a DAS module is provided. Hence, the
output of the acquisition stage is directly connected to the DAS. A LabView program is
provided to display the results and to provide with control tools.
     More information on the DAS module and LabView is provided in ____________________.

1.3. Schedule
    The AWES has been designed and developed during about 7 weeks. Due to the limited
time, a schedule was given, and is shown in the figure below (Figure 1-2).
    Each week has defined tasks to do, so-called “Stage X”. Stages are described in the table.




                                  Figure 1-2. Schedule for the project.


Stage       Week                                            Tasks
                            Understand the project and its requirements.
                            Understand the operating principle and datasheets of the
  1          1
                             sensors, and experimentally check their output.
                            Design the signal conditioning circuits.
                            Build & characterize the signal conditioning circuits.
  2         2, 3            Use the DAS & LabVIEW to acquire and display the output
                             signals.
                            Select the batteries.
                            Design & characterize the power conditioning circuit.
  3         4, 5            Use the DAS & LabVIEW to control and read the power
                             conditioning circuit.
                            Calculate and measure the power consumption.
                            Build the overall hardware & software of the autonomous
  4          6
                             weather station.

                                                                                             3
                               Calibrate the weather station.
                               DEMO of the weather station.
  5            7
                               Individual oral exam.

1.4. Requirements
      The AWES meteorological requirements were given, and are summarized below:
                                                                                      Sensor
                                 Range             Resolution          Accuracy
      Temperature                                                                      type
      measurement            Min      Max
                                                        0.1ºC           ±0.2ºC          RTD
                            -40ºC     55ºC
       Humidity              Min      Max
                                                         1%               3%         Capacitive
      measurement            5%       100%


      For the conditioning circuit design, some other constraints were also given.
           Power source: 1 or 2 AAA batteries.
           Power conditioning circuit:
                o Bipolar supply voltage.
                o Shutdown mode available, to giving low power consumption when active.
    Apart from the DAS, some ICs were provided to help with the design and to avoid having to
buy them. These ICs are:
             Sensors
                  o Temperature sensor: Honeywell HEL-705-U-0-12-00. This is a RTD PT-1000
                      sensor.
                  o Humidity sensor: Humirel HS1101LF. Gives a capacitive output.
             Signal conditioning:
                  o Operational Amplifier: TI TLC2264AIN.
                  o Timer: Intersil ICM7555IPA. 555-type timer.
             Power conditioning:
                  o Unipolar regulator: Maxim MAX619. A regulated 5V charge pump DC-DC
                      converter.
                  o Bipolar regulator: Maxim MAX660. A voltage doubler or inverter.




                                                                                               4
2. Sensor study
    This chapter describes the working principles of the primary sensors used within the AWES
project. These primary sensors are resistive (for the temperature measurement) and capacitive
type (for the humidity measurement).
    Other kinds of physical measurement principles for primary sensors can be:
         Mechanical: profits dimension changes suffered by some kind of materials in
          presence of humidity, i.e.: organic or synthetic fibers, human hair, etc.
        Based on hygroscopic salts: environment humidity is deduced by means of a
          crystalline molecule which has high affinity with water absorption.
        Conductivity: presence of water on an environment permits current flow through
          a golden grille due to good propagation characteristics of water.
        Infrared: some of the radiation obtained in the water steam is absorbed by two
          infrared power supply.
    However, none of these are used.

2.1. RTD
    Metals have the capability of increasing its electrical resistance by the time its internal
energy raises. This property has been widely used from long time ago in the design and the
build of RTD, whose main element is a thin metal film.
    Its electronic symbol and its external appearance are shown in:




                  +t


                       Figure 2-1. Electronic symbol and external appearances of an RTD.

    The next table shows common RTD metals resistivity and thermal coefficient of the
resistance variation as a result of ambient temperature changes.
                                       Table 2.1. Common RTD metals.

            Metal                      Resistivity (ρ), [Ω, m]              Thermal coefficient (α),
                                                                                  [Ω/Ω/K]
                                                        -8                                -3
        Platinum, Pt                           10.6·10                             3.9·10
                                                       -8                               -3
         Nickel, Ni                            6.84·10                              7·10
                                                      -8                                  -3
        Wolfram, W                              5.6·10                             4.5·10
                                                       -8                                 -3
        Copper, Cu                             1.68·10                             4.3·10


    Platinum has the lowest thermal coefficient while nickel has the highest. In practical terms,
lower thermal coefficient will have a lower sensitivity and vice versa. So nickel will increase its
resistance faster than platinum would, in front of a temperature rise. However, having
platinum more resistivity, thin wires with a notorious resistance can be obtained without being
lengthy. This is important because metals get physically larger with temperature, and
resistance is given by:



                                                                                                       5
    Because of this, in RTDs where nickel is used, changes in its dimensions are more
notorious, and this leads to non-linear changes in resistance-temperature characteristic. That
is why not all metals are used in RTD.

         Static transfer function
    Given by the manufacturer, shows the relationship between input parameter
(temperature) and its output (resistance).
    Figure 2-2 shows a portion of the static transfer function of a platinum PT-1000 RTD,
specifically the Honeywell HEL-700 sensor series. The resistance value at 0ºC is known as R0,
being 1000 Ω in this case.




                           Figure 2-2. Static transfer function of a platinum RTD.

   The next equation shows this relationship between temperature and resistance:

                                                   Eq. 2.1

   A, B, and C coefficients are given in the datasheet. However, due to the insignificance of B
and C when compared to the lower-order A coefficient, the transfer function has been
approximated to a first-grade equation, where A = 0.00375 K-1 and so:

                                                   Eq. 2.2


         The HEL-705 sensor
     The RTD used in this project is the Honeywell HEL-705 sensor (Figure
2-3), which is a 2-wire, TFE Teflon PT-1000. The TFE Teflon construction
allows a measurement range of -200ºC to 260ºC, whilst maintaining a ±0.5ºC
(0.8% of temperature) accuracy.
     Its linearity is specified to be ±0.1ºC of full scale for temperatures
ranging -40ºC to +125ºC. This is not acceptable, and thus a linearization
circuit is included in the acquisition (See 3.1).                                      Figure 2-3.
                                                                                     Honeywell HEL-
     A self-heating of <15mW/ºC is specified, while a 1mA feeding current is
                                                                                      700 PT-1000
recommended for maximum performance whilst maintaining an acceptable                     sensor
self-heating. See 3.1 for more design information.



                                                                                                      6
    The OMM requirements involve a 0.1ºC resolution and an accuracy of 0.2ºC, but this
sensor resolution is inherently 0.2ºC and an accuracy of 0.5ºC1. So using the HEL-700 will make
impossible to meet the OMM requirements.

2.2. Humidity
     One of the most typical parameter that should be installed and processed in an
Autonomous Weather Station is the humidity. For this project, the value calculated will be the
relative humidity (RH) which contains the air mass in relationship with the absolute maximum
humidity that would be admitted without condensation and remaining same temperature and
atmosphere pressure conditions.
     Therefore, a sensor that is able to measure this parameter will be used to obtain a value
for the relative humidity. Let’s know more about humidity sensors.
     The sensor that has been supplied for the project follows the capacitive physical principle,
therefore, depending on relative humidity of the environment the sensor that will act as a
capacitor, will give different capacitance values.
     The supplied sensor is from Humirel, model HS1101LF
     Its humidity operating range goes from 0 to 100%. If SMC (Servei Meteorològic de
Catalunya) recommendations are checked, the range that has to be measured by the sensor
ranges from 5 to 100%. Thus, it completely covers all requirements specified by SMC.
     Taking into account that the capacitance of the sensor when it is at 55% of Relative
Humidity is 180 pF, below are the characteristic formulas that follows the sensor in terms to
obtain theoretically new values of capacitance depending the RH and vice versa.

             Polynomial Response


             Reversed Polynomial Response

    Where X =




    1
        Specified at 0ºC.

                                                                                               7
3. Conditioning circuits
    This chapter shows the design and calculations of the conditioning circuits for the AWES.

3.1. Temperature sensor

3.1.1. Circuit design and characterization
   As seen, the primary sensor used for temperature measurements is a PT-1000 type, which
means that it has a resistance of 1000 Ω at 0ºC. As the temperature range is from -40ºC to
55ºC, applying the RTD static transfer function (see 2.1):

    Would give:



    And thus the sensitivity is:



    This resistance range is processed to obtain a bipolar voltage meeting ADC requirements.
    The global circuit is shown below (Figure 3-1):




                            Figure 3-1. Acquisition circuit for temperature sensor.


          Obtaining a voltage variation from a resistance variation
     The first stage of the acquisition circuit is taken straight out from the HEL-700 datasheet.
The recommended circuit uses a 1 mA current driver circuit to feed the RTD. As the real circuit
is to be fitted with commercial-value, 5% tolerance resistors, and sourced with ±5 V, it has to
re-designed.
     In short, what the circuit does is to provide a fixed 1.04 mA current to the RTD. Then, the
voltage difference that appears in the RTD is converted to bipolar values and finally –second
stage- it is amplified and filtered to meet the ADC requirements.
     It is necessary to calculate the maximum self-heating (Joule effect) when the HEL-700 is
fed with 1.04 mA:



                                                                                                8
    So feeding with 1.04 mA will not affect the accuracy requirements.
    The first stage of the circuit is shown below. It shows the theoretical voltage levels:




                Figure 3-2. First stage of the adaptation circuit for the temperature measurement.

    The following calculations were made taking into account the theoretical values of the
resistances, although figures show the actual final values used in the circuit.2
    The voltage at the Vref node is:



    The U1A used for linearization and current feeder is forming a non-inverting amplifier,
with variable gain set by the fixed resistance R3 and the RTD. This variable gain is given by:



     Thus, knowing that the gain runs from 1.85 to 2.206, and that a reference voltage of 1.04V
is set at both inverting and non-inverting inputs, the output Va will range from 1.93 V to 2.30 V
(Figure 3-3) depending on the RTD resistance.




    2
     In order to simplify the explanations, these calculations have been made by neglecting relatively
low currents, which actually do not affect that much to the results. 4.2 section shows more accurate
current calculations.

                                                                                                     9
                          2.4
                         2.35
                          2.3
                         2.25
                          2.2

                Va (V)
                         2.15
                          2.1
                         2.05
                            2
                         1.95
                          1.9
                                850     910      961      1009     1058     1110     1161   1206
                                                       RTD resistance

                                Figure 3-3. Voltage at Va testpoint vs. RTD resistance

    Because the ADC expects bipolar signals, it is necessary to “center” this function. In order
to maintain symmetry, the signal is centered at 7.5ºC (the mid-point between -40ºC and 55ªC).
This is equivalent to:

    Following the circuit designed before, the resulting voltage at Va for this resistance is:




     So whatever the signal is, it needs to be dropped 2.11V. This is accomplished by using a
differential amplifier circuit. This circuit takes the Vref voltage, doubles and subtracts it from
the non-inverting entrance, which is connected to the non-inverting input.
     The output for this differential amplifier is given by:



    Serial R6 and R12 resistors (Figure 3-2) are termed Ra, and R7 and R11 are termed Rb.
    By forcing Ra = Rb and R5 = R8:



    And so, by using 56 kΩ for R5 and R8, and series 27 kΩ + 1 kΩ, the required circuit is
obtained. The resistances were taken big enough in order not to have loading effects due to
the input low impedance inherent to the differential amplifiers set. This also makes the input
current flowing through the resistances negligible (0.027 mA in front of more than 1 mA).

         Amplifying and filtering
    As told, the ADC has four preamplifier settings. Each one permits having an input dynamic
range of ±1.25V, ±2.5V, ±5V and ±10V. In order to obtain maximal resolution, the signal needs
to be spanned to the maximum permitted dynamic range. As the supply voltages are of ±5V,
these were the maximum allowable signal values.
    In order to comply with that, the signal needs to be amplified. This is accomplished by
using another non-inverting amplifier, shown in Figure 3-4:




                                                                                                   10
                            Figure 3-4. Non-inverting amplifier (resistances have the final value) with noise filtering.

    Theoretically, the gain for this amplifier must be:



    By taking R9 = 39kΩ and R10 = 1k5Ω, a gain of 27 is obtained.
    The result after using the final values is shown in the graph below:
                      6
                                                                                  4.983525         4.867
                      4                                                  3.7195875         3.612
                                  y = 0.028088x - 28.890000
                      2                                           2.287125        2.175
    Vanalog_in (V)




                                                         0.826575        0.649                                             Vanalog
                      0                                                                                                    theoretical
                                             -0.5497125
                          800            900          1000 -0.739 1100              1200                     1300          Vanalog
                     -2               -1.8979125     -2.116                                                                measured
                                -3.330375                 y = 0.028074x - 29.017216
                                              -3.542
                      -4
                     -5.015625       -5.011
                      -6
                                                           Resistance (ohm)

    The sensitivity slopes are:




   The filter was designed to prioritize noise filtering instead of fast system response time,
because a meteorological station such as the AWES will not expect fast temperature changes
and can stand a relatively large response time.
   By designing a LPF with 50Hz corner frequency, the response time is:




3.1.2. Calibration
    Sensor calibration has been done in order to eliminate those errors caused by resistance
tolerances, remaining non-linearity, and other factors. The calibration curve was done at two
given points (5ºC and 40ºC). However, it is important to remark that for a precise calibration of
the circuit more calibration points should have been taken.

                                                                                                                                11
   The results to the calibration are shown below (Figure 3-5):


                 4
                                      y = 0.0286171x - 29.4897143
                 3                           R² = 1.0000000

                 2

                 1

                 0
                  1000            1050              1100              1150        1200
                -1


                             Figure 3-5. Temperature circuit calibration curve.

   This calibration curve is used in the LabView program to correct the temperature reading.

3.2. Humidity sensor
    Since the sensor’s capacitive value obtained has to be processed and inserted on the DAS
(Data Acquisition System), a conditioning circuit is necessary to be implemented. The
information acquired by the sensor has to be added in a signal which will be the responsible to
transfer it to de DAS. This is the reason why a timer 555 (INTERSIL CM7555) has been used to
achieve this data transfer process.




                                                                                            12
4. Power supply and consumption

4.1. Supply

4.1.1. Voltage Inverter
    Due to the need of bipolar voltages for the amplifier, an IC granting inverted, stable
voltages is provided. This IC is from MAXIM, model MAX660.
    MAX660 is a monolithic, charge-pump voltage inverter, which is capable of converting a
+1.5 to +5.5V input to a corresponding -1.5V to -5V output. It only needs to external capacitors
in order to adjust its internal oscillator frequency.
    Figure 4-1 shows the designed circuit, extracted from the MAX660 datasheet.




                                   Figure 4-1. Voltage inverter circuit

    The only design consideration is for the external capacitor selection. Following the
datasheet instructions, capacitor selection is done by considering three factors:
        1. MAX660 output resistance.
        2. Capacitor equivalent series resistance (ESR)
        3. Maximum voltage drop compared to Vin.
    Some tables are given for optimum capacitor selection, but it is mandatory to know the
capacitor Equivalent Series Resistance (ESR), which is not available for the capacitors used.
However, some measures were taken with 10uF capacitors in order to ensure correct
operation (Figure 4-3).
    The output resistance and the maximum voltage drop depend on the required output
current, which was calculated to be around 15mA. Figure 4-2 shows this relationship, including
the expected efficiency.




                             Figure 4-2. Output voltage drop vs. load current

    So a 94% and about -4.9 V are expected at the output.

                                                                                             13
    Next figure (Figure 4-3) shows the Iin/Iout relationship and the calculated performance.
This calculation was done using 10uF capacitors and different resistances at the output,
simulating the circuit load. Values next to the curve represent this performance.
    The performance is calculated by:




                              60
                              50   99.3
                                           99.78
                              40                   99.87
                   Iin (mA)




                                                            99.57
                              30
                                                                     99.42
                              20
                                                                             99.29
                              10                                                     99.18
                                                                                         98.48
                               0
                                51.62 45.03    39.1    33.23 24.02 16.87 12.13          7.16
                                                         Iout (mA)

                                     Figure 4-3. Iout/Iin curve and performance.

   So the performance of the MAX660 is better than expected.
   The ripple of the MAX660 is about 160mV, as seen in Figure 4-4:




                                           Figure 4-4. Ripple in MAX660.


4.2. Consumption
    In order to calculate battery duration, total circuit consumption must be evaluated. The
calculations are divided within the circuit functional blocks: temperature measurement,
humidity measurement and regulator/inverter performances.

4.2.1. Temperature measurement circuit consumption
   As seen in Figure 4-5, TLC2264 consumes itself around 850 uA when supplied with 5 V.




                                                                                                 14
                  Figure 4-5. TLC2264 current consumption.

This leads to:




                 Figure 4-6. Temperature circuit current flows.




                                                                  15
   TLC2264 bias currents are of the picoampere order, and thus are neglected. So is the
amplifier output current flowing to the DAS, as its impedance is extremely high.
   Summing the input currents:



   FALA ACLARAR EL TEMA DE LAS CORREITNES SUPPLY, QUE PASA CON ELLAS SI LA
CORRIENTE OUTPUT ENTRA O SALE, Y COMO SE SUMA LUEGO ESO.




                                                                                    16

				
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