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4.3 Sensor Subsystem
The oceanic and atmospheric environmental conditions, which can be categorized as
physical quantities are converted into signals that an observer or an instrument can easily
read and understand, using environmental sensors. These analog data vary in a long range
from the ocean’s substances, solutions and compositions to the atmospheric conditions such
as wind speed, objects’ distance, wind direction and much more. These data are converted
to digital outputs which would be handled in the Command & Data handling subsystem.
In this section six different sensors that contributes to the implementation of the buoy
would be discusses. This section includes different sensors’ alternatives, the feasibility and
testing information.
4.3.1
Alternatives and Trade off
1. Air Temperature
The title of this section explicitly describe the purpose of this sensor, however in general
term the temperature sensor senses the temperature of the atmosphere in terms of the
units, Celsius or Fahrenheit. The inputs of these temperatures would be some analog
voltage, but essentially would be converted to a digital output. The temperature is
interpreted from the digital output based on a voltage to temperature reference and a
mathematical calculation or I2C interface which would be discussed later. In other to
discuss the alternatives and trade offs of the air temperature sensor, it is important to
understand the different types of temperature sensors available.
A. Thermocouple
Thermocouple is pair of junctions from two dissimilar metals. One junction includes the
measured voltage and the other junction includes a voltage from a known reference
temperature. The voltage measurement would be determined by the SeeBack effect. The
SeeBack effect describes the voltage or electromotive force (EMF) induced by the
temperature difference along the copper wire shown in the diagram below. The voltage
output would change with respect to the change in measured temperature from the tip of
the thermocouple. It is normally advisable to make the reference temperature a constant
zero degree Celsius by making a reference junction in ice water, thereby there would be no
change in the reference temperature.
Figure 4.3.1
B. NTC (Negative Temperature Coefficient) Thermistor
Thermistor basically stands for thermal and resistor and they are typically temperature
dependent resistors. The resistance decreases as the temperature increases, in a nonlinear
predefined way, due to the properties of the semiconductor materials used to make the
thermistor. Most NTC thermistors are made from various compositions of metal oxides of
manganese, nickel, cobalt, iron, copper and titanium.
C. RTD (Resistance Temperature Detectors)
The difference between NTC thermistors and RTDs is the material used to make them. The
materials used for RTDs experience an increase in resistance when their temperature
increases. Therefore, RTDs could be referred to as PTC (Positive Temperature Coefficient)
thermistors. The higher the coefficient, the greater an increase in electrical resistance for a
given temperature increase. Both the PTC and NTC thermistors have a high temperature
coefficient.
The most commonly used material for RTDs is platinum, because it has the best
temperature range. They also have low system cost, they are very linear (analog), and can
measure temperature from -55 C to 155 C.
D. Integrated Circuit (analog and digital output)
Analog Integrated Circuits(IC), output voltages or currents that are proportional to the
temperature and digital ICs converts analog signals to digital output, which are basically 1s
and 0s. Some advantages of the ICs temperature sensors over thermocouples, NTC
thermistors and RTDs is that it requires no external circuit to connect to the
microprocessor (microprocessors usually come with analog to digital converters to convert
the analog signal to digital.)
There are a lot of different ICs temperature sensors, all ranging from a 1 to 3 wire interface
of both analog and digital. For example, the DS60 is a 1-wire analog temperature sensor. It
has three pins. One pin is used to power up the device, another is connected to ground, and
there is a 1-wire output pin connected to the microcontroller. Most 1-wire temperature
sensors are analog, except for a few like the DS18S20, which as a built-in ADC (Analog to
digital Converter.)
However, the temperature sensor that would be used in this project is the 2-wire I2C
Interface, DS1621 temperature sensor because it requires no external circuitry for
connections, the pins are easy to solder, the availability at the time for testing, and because
multiple I2C devices/sensors can be connected to an I2C bus. I2C stands for Interintergrated circuits, and it used to communicate with two or more digital devices using
addresses. I2C supports master mode, multi-master mode (to of the devices in mater
mode.), and slave mode, where as the microprocessor could act as the master, and the
sensor could act as the slave. From the18f4520 datasheet it is mentioned that the master can
initiate the data transfer at any time. The master determines when the slave is to broadcast
data by the software protocol. The 2-wire interfaces for I2C are SDA, for serial data input,
and SCK (serial clock.), which schedules and initiates the sending of data.
2. Water Temperature
Sensing the temperature of the water can be done numerous ways, but two major ideas
were considered.
A. Using a Thermocouple
Remember that a thermocouple needs a reference temperature sensor in other to determine
a gradient temperature output, so what if the reference temperature is acquired from the
sea. The main problem with this, is that the temperature of the sea would change so often
that a précised temperature for both the water and the air temperature would not be
determined. Also the output of the air temperature would depend on the input of the water
temperature, so if there is any fault with retrieving the water temperature that would affect
the air temperature.
B. Temperature Sensor in Resin
Most Water temperature sensors are very expensive; costing hundreds of dollars and most
of the sensors are mounted temperature sensors, whereby the temperature sensors are
calibrated to only function for domestic use. Therefore a suggestion was made to put an air
temperature sensor inside a resin. The resin would then be put under the water, and wires
would connect to the pins of the temperature sensor to read the data – the temperature
would be determined by the temperature of the resin. By doing these, we could avoid water
from destroying the temperature sensor and avoid the expensive cost of water temperature
sensors.
.
What is a Resin?
A resin is a compound that hardens when mixed with a solution. It is not soluble in water,
but soluble in alcohol. There are a number of different classes of resin, depending on what
it is been used for or the chemical composition.
Figure 4.3.2
A DS1621 temperature sensor is inserted inside a non-conductive epoxy (contains resin)
3. Pressure Sensor
Pressure sensor measures the force of fluid or air over an area. It is usually measured in
Pascal (newton/meter2), bars or in pounds per square inch with respect to the type of
pressure measurement. The three main pressure measurements are Differential, Gauge,
and Absolute pressure, which were alternatives for the pressure sensor.
A. Differential Pressure Sensor
Differential Pressure Sensor measures the pressure difference between two measuring
points. Therefore, there are two measuring port on the device that would take the
measurement; the output would then be determined by the difference in the measurement.
The unit for differential pressure is pounds per square inch differential (psid). It measures
in reference to a perfect vacuum, which would have zero psi output.
B. Gauge Pressure Sensor
There two types of gauge pressure measurement. The first type is the vented gauge
pressure measurement which compares the pressure from an opened vent gathering the
ambient pressure, to the atmospheric pressure. The vented input pressure is positive if it
exceeds the atmospheric pressure and it is negative if falls below. Essentially, by adding the
vented gauge pressure with the atmosphere pressure input, an absolute pressure is
retrieved. Now, if the atmospheric measuring point is sealed up, the type is called sealed
gauge pressure measurement. Vented gauge pressure is measured in psivg (pounds per
square inch vented gauge), and sealed gauge pressure is measured in psisg (pounds per
square inch gauge.)
C. Absolute Pressure Sensor
This type of sensor would measure the atmospheric pressure with respect to pure vacuum
with has 0 psig. Since our concentration in this project is gathering atmospheric data, it
seemed like the perfect type of sensor to use because it is uniform. Measuring with the
gauge and differential sensor could also give the desired output, but other considering
needs taken into effect, whether it is doing some addition and calculations with the gradient
of the differential pressure or ignoring the ambient pressure from the gauge sensor.
4. Wind Speed Detector
Due to the $150 budget in the building of the buoy, and the expensive cost of wind speed
sensors, we came up with two ideas to record the speed of the wind.
A. Cups and Mouse
The general idea of the cups and mouse is integrating a computer ball mouse with a 3 cup
anemometer assembly, which would connect through a PVC pipe.
Under a computer ball mouse is a ball, which moves as an individual moves the mouse. The
ball is connected to two rollers that detect the position of the ball; when the ball moves the
roller moves as well. These rollers are then connected to a shaft that spins a disk with hole
in it (essentially, an optical encoder). In front of the optical encoder are infrared LEDs and
the back is an infrared sensor, which detects the motion. The beams of light emitted by the
LEDs (the purpose the two LEDs is catch missed signals by the other) are broken by the
rotation of the encoder, which create digital pulses and is sensed by the infrared sensor. The
signal detected by the infrared sensor is then passed to chip, which transfers the pulses
through the mouse cord. The pulses are related to the distance the mouse as traveled and
the speed. By mounting the cup assembly through the PVC, on the ball, taking cautious
mechanical notation, as in, in a manner which rotation of the cups would spin the ball, one
could read the binary output serially through the cord.
B. Cups and motor
The idea of the motor is basically the same as with the mouse; the difference is that the
motor would represent the ball and the rollers would touch the motors’ axles. The fault in
this method is that the motor would produce fiction against the wind. In order to avoid this,
one could loosely up the axle of the motor internally. But an advantage of the cups and
mouse over the cups and motor is that it is already structured; that is, it would take more
time developing a basic structure in mounting the cups assembly on the motor, than it
would with the cups and mouse.
5. Salinity Detector
What is Salinity? Salinity is the measurement of the concentration of dissolved salt in
water. The concentration is the amount of salt in the water (weight) and could be expressed
as parts per million (ppm), or in grams/milliliter (grams/mililiter = g/ml =
milligrams/micro-liter = mg/ul. 1ug/ml = 1mg/l = 1 ppm.) Our interest with the salinity
sensor is measuring the amount of salt in the water, but it important to note that the salt
would also include the impurities in the gulf from factors like the dissolved substances like
mud in the water. Salinity can be measured using a hydrometer or a refractometer but
these devices are used for lab experiment only. Another method used to measure salinity is
deeping two electrodes water, then connecting the electrodes to a voltage supply, and a
multimeter in other to measure resistane across the electrode or the current that flows
through the electrodes.
Figure 4.3.3
In other to measure the resistance or current the fundamental idea that needs to be used is
the idea of conductivity. Conductivity is usually measured in Siemens or in mho; it is also
the reciprocal of resistivity. It is more ideal to measure the resistance in terms of the
resistivity because of the length and the cross sectional area of the electrodes, R = (P*L)/A,
which as a unit of ohm*cm ( P stands for the resistivity, L stands for the length, and A is the
cross sectional area.) The cross sectional area (A)) = radius2*pi or (diameter2 * pi)/4.
When the electrodes are inserted in the water they create positively charged hydrogen ions
and negatively charged oxygen ions. The positively charged ions migrate to the cathode
electrode and collect electrons causing an addition of more electrons at that end. The
negatively charged ions at the same time migrate to the anode electrode to drop off
electrons and oxides (loss of electrodes, gain of holes.) The separation of these ions is called
electrolysis. Salt that dissolves in water break into positively charged sodium ions which
are attracted by the cathode electrode, and negatively changed chloride ions which are
attracted by the anode electrode. If the negative charged ions discover that there are excess
holes by the anode, the chloride ions drop off their electrons into the holes. The movement
of these electrons in and out of the electrodes creates the flow of current in and out.
Obviously, a very important instrument in this sensor is the electrode. Therefore, finding
the right electrode as to be taken into consideration. Since electrodes are typically
conductors, any material that conducts electricity can be used in this measurement. Two
important factors that needs to be known, when looking at conductors are: 1. The small
amout of resistance (the smaller the resistance, the greater the condutivity.) 2. The
temperature of the melting point because, the more the temperature, the slower the metal
disintegrate over time because of the thermal conductivity of the material. Here are a list
of three good conductors.
A . Copper
Copper is part of the elements in the chemical element table. Its symbol is Cu and its
atomic number is 29. The electrical resisitivy at 20 C is 16.78 nΩ·m. Its thermal conductivity
is (300 K) 401 W·m−1·K−1 and its melting point is at 1,084.6 °C.
B.
Aluminum
The atomic number of aluminum is 12. Its symbol is Al. The electrical resistivity at 20 C is
26.50 nΩ·m. Its thermal conductivity is (300 K) 237 W·m−1·K−1, and its melting point is at
660.32 °C.
C. Pure Tungsten
Pure tungsten is a steel-gray to tin-white metal. It atomic number is 74. It has the highest
melting point of all the chemical elements. The electrical resistivity at 20 C is 52.8 n Ω·m,
which is still quite good. Its thermal conductivity is (300 K) 173 W·m−1·K−1, and its melting
point is at 3,407 °C.
6. Humidity Sensor
Humidity measures the amount of water vapor in the air. Below explains two most common
terms used for humidity, which corresponds to the alternatives.
A. Absolute Humidity
Absolute Humidity is the mass of water vapor over the volume of the air or gas
(Mass/Volume.) It is usually measure in terms of grams per cubic meter or grains per cubic
foot. Absolute humidity could be calculated if the relative humidity (RH) is known.
B. Relative Humidity
This sensor measures how much water vapor is present in the air, over the amount of water
vapor that would be in the air if the air were saturated. This is measure in percentage.
Humidity is usually referred to in terms of percentage; therefore for the design we are
using the HIH-3610 which measures in percentage.
4.3.2
Feasibility
Two main factors that are very important in the construction of this low cost buoy are the
cost of the devices and the timeline given to complete the implementation.
1. The Cost
Four of the sensors that would be used in retrieving the data are packaged ICs. These ICs
are known to be very cheap and basically priceless, for instances, the cost of getting 114
HIH-4000 humidity sensors (which has the same features as the HIH3610 humidity sensor)
from digi-key is only for $23 in U.S currency. However, the price of these devices might
very with the size, for example the 37.60 millimeter to 29.85 millimeter MXP4115AP
pressure sensor, which is the biggest IC sensor in our design cost only $15 U.S dollars from
freescale semiconductors.
The salinity and the wind speed sensor on the other hand include a combination of a few
different materials to contribute to their price. However, there are only a few devices that
would have a significant cost, just like in the case of the salinity sensor, where the main
components are the electrodes, which has considerably high price. A cheap pure tungsten
electrode could range from $3 U.S dollars to $20 dollars; this is essentially, the main
component of the device. The wind speed sensor on the other hand consist of three main
components – the ball -mouse, the cups and the PVC to connect the devices; combining the
three, adds to a cost under $20, a prices division ratio of 1 to 12 to 4 dollars, respectively.
Therefore, as far as cost is concerned, having all this sensors would be feasible.
2. TimeLine
The time for the completion of this project is one year. The timeline mainly depend on the
testing, accuracy and the mechanical aspect/configuration of each sensor. Because this
phase as already been initiated, the timeline should be no obstacle.
4.3.3
Design Documents
1. Testing
The testing phase includes codes and experiments that would contribute to the
implementation of the buoy system.
A. Coding
The Appendix shows a complete code to program the humidity and pressure sensor, using a
basic C programming language (the MPLAB software to compile the code.)
The program shows a basic structure in implementing analog devices while using the C18
library as a reference graph. Basically, it begins with initializing the input and
communication ports, opening respective sensor functions, retrieving the data, and
outputting it. Due to the fact that the humidity and pressure sensors do not have integrated
analog to digital converters, the mathematical equations below would be used t convert the
analog data into readable terms that a person could understand. The datasheets of the two
sensors contain the graphs that show a relationship between the analog input and the
actual values.
Pressure Conversion
voltage = ( ( ((float)value)/1023 ) * 5 );
pressure = ( (1/.009)( (voltage/5) + .095 ) );
Humidity Conversion
voltage = ( ( ((float)value)/1023 ) * 5 );
humidity = ( (1/.0062)( (voltage/5) - 0.16 ) );
Figure 4.3.4
Figure 4.3.5
B. Experiment
For the Salinity Sensor, certain procedure had to be taken in order to find a relationship
between the resistivity of the electrode and the amount of salt in water. This procedure was
tried using aluminum and copper but because of their thermal conductivity, they started to
disintegrate, therefore there was no constant reading of the output voltage. The same
procedure was tried using a tungsten electrode and a more steady output was retrieved.
Procedure
1. Connect the circuit (see circuit below)
.
2. Pour a 50,000uS/cm calibrated solution in a beaker up to 200ml (make sure the
temperature is about 25C to avoid worry about specific conductivity.)
3. Record the DC output Voltage.
4. Pour the Solution out of the beaker until it reaches about 100ml in the beaker.
5. Add some distilled water in the beaker to reach 200ml. (This basically reduces the
conductivity by half.)
6. Measure the DC output voltage.
7. Repeat procedure (4 – 6) until 20 measurements are made.
8. Plot the Data: the concentration of Salt in the water, VS electrode impedances.
NOTE: PPM = (Electrical Conductivity (EC)) * 500.
Circuit for testing:
Figure 4.3.6
The purpose of the operational amplifier is to give a cleaner and stable output, it was not
necessary to include this in the testing.
Result:
Figure 4.3.7
Salinity Graph
V1-V2
5.9
Series1
5.85
5.8
3.0518
24.414
97.656
781.25
6250
50,000
5.75
COnductivity
The graph above shows 17 measurements that were taking following the procedure. Notice
that right between the voltage 5.85 and 5.9 there is a huge difference between the graphed
data. The reason for this is because a one minute elapsed time was given before each
measurement, except for this data. For this data, a 5 minute elapsed time was given to
make a comparison. Other than that, the graph gives a mathematic formula of y = ln(x).
However, further testing need to be taking, for accuracy.
2. Schematics
The schematic for the MXP4115AP pressure sensor, HIH3610 humidity sensor, and DS1621
temperature sensor are showed in the Appendix.