This week was all about sensors and it was extremely interesting to see how many different sensors are available and used in everyday applications. This week I focused on movement sensors and found that the most used type of movement sensor is called a Passive Infrared Sensor, PIR. This sensor uses heat to detect movement within a given area. This sensor is used in automatic lighting schemes, burglar alarm systems, automatic welcome greetings and even motion detecting remote cameras (see video below). These sensors usually have a range of approximately 20 feet but will practically never wear out, are extremely reliable, incredibly cheap and simple to use.
I thought it was incredibly important to note, especially with the talk of data overload in recent classes the number of readings that single sensors are able to take. Both Junwah Ng , and Xiang Li talked about different senosors, humidity and pressure respectively, but both mentioned the number of measurements each of these sensors can take. Even though the PIR sensor is simple I am sure that there are a number of movement sensors that provide an abundance of measurements that can be recorded to help in disciplines such as traffic studies, population tracking, mass transit security and monitoring and many more.
References:
http://www.gadgetshack.com/motionsensor.html
http://www.ladyada.net/learn/sensors/pir.html
http://www.instructables.com/id/PIR-Motion-Sensor-Tutorial/
<iframe width="420" height="315" src="http://www.youtube.com/embed/dGOgCnlizgU" frameborder="0" allowfullscreen></iframe>
Tuesday, February 19, 2013
Movement Sensors
Prior to this week’s assigned
research, I thought I understood the basic concept of movement sensors and how
and why they were commonly used, yet as I continued doing my research I became
more aware of the importance and dependability that other systems have over
this system. Movement sensors detect
changes in positions of a person or object relative to its environment or the
other way around. There are six types of
motion detectors that are commonly used including: infrared, optics, radio
frequency energy, sound, vibration, and magnetism.
The types of movement sensor methods
are mechanical and electronic methods.
Most people are unaware of the mechanical movement sensors around us,
yet unknowingly we use these daily and vastly.
From the moment we wake up we use movement sensors such as the moment we
turn our alarm clock off, when dialing
the heating time in our microwaves, typing in our computer keyboards or even
the simple motion of flicking the light switch.
Electronic detection sensors are ones that detect optical or acoustical
motion. Examples of when we encounter these
sensors include the ones that activate lighting, activate a camera to turn on, or
even trigger an alarm.
Because of the wide range of
functions of movement sensors, these are the most used type of sensors. Other types of sensors are dependent and rely
on these sensors in order to activate or deactivate. For pressure sensors, like my classmate C.
Meraz explained, a piezoelectric pressure transducer uses crystal to convert motion into an electrical output since “these
sensors are also highly susceptible to shock and vibration”. With
flow sensors, like Brian V says in his blog post, record measurements within a
flow meter of flow of fluids or gases.
These measurements are found by observing the movement of such fluids or gases.
Matthew Tedesco also explains that “aside from mechanical flow meters,
fluid velocity and flow can be measured using optic sensors”. As explained before, optic motion sensors
fall under the movement sensors category.
In conclusion, we can encounter movement sensors in our everyday life and
they also play important roles in other practices.
Sources:
Humidity Sensors
A
humidity sensor measures three different types of humidity; absolute humidity,
relative humidity, and specify humidity. Absolute humidity is the ratio of
water to air, the total mass of water vapor that is in the air at a given time.
Relative humidity (RH) is where many believe is temperature is actually the
saturation level of the air. It’s shown as a percentage of the ratio of the
moisture in the air and the total amount of moisture the air can hold. If the
RH is at 100% then the air cannot hold any more moisture which can mean a high
possibility for rain. The warmer the air is, the more moisture it can hold, the
colder the air the less moisture. Meaning that the RH is affected by the change
in temperature, which can justify why some believe humidity sensors measures
temperature. Specific humidity is the ratio of the mass of water vapor in the
air to the total mass of the mixture of air and water vapor. These humidity
sensors can be used both indoors and outdoors, as well as being available in
both digital and analog forms. The analog sensor uses a system called
capacitive measurement. The sensor would be made of either glass or ceramics.
The insulator material made from polymers, absorbs and releases that water
which changes the level of charge in the capacitor. The measure of the change
in the charge is the humidity of the given area. The digital sensor uses an
electrode based system consisting of polymers, there are two micro sensors that
are calibrated to the humidity of the given area. Which are then converted into
digital format using an analog to digital conversion. There are actually
another type of system called the hygrometer, where there is a pair of
thermometers are used. One is kept wet while the other one is kept dry. The
difference of the two moisture levels are measured. I really liked how in
Nastasha’s blog, she when more in-depth with the components and circuitry of
the sensors. There are many applications that a humidity sensor has. To
regulate humidity in buildings like museums, laboratories, and wine cellars.
Create a sterilized environment for a hospital and even the defogging or
defrosting systems in cars need humidity sensors. Humidity sensors are key into
making a green building as in Jeanine’s blog, she mentioned that “The humidity
sensors will collect data that will be used to determine the comparative advantages
and worth the new technology.”
Humidity Sensors
Week 8
2/19/13
Humidity Sensors
Humidity
sensors, or hygrometers, come in multiple forms depending on the desired
measurement (absolute vs. relative), cost factor and level of accuracy. These sensors are used to measure the
moisture content in the air. Measuring
the relative humidity is most practical because the reading depends on an
associated temperature; where a ratio of current moisture content to the
maximum moisture content at that temperature is developed. This makes the most sense as we are dealing
with both moisture and ambient temperature when designing a room to be as
comfortable as possible. In this way,
data from a hygrometer will be fed into an HVAC systems computer which will
either humidify or dehumidify the air based on the requested conditions.
Due
to their ability to be used in multiple scenarios, low cost and relatively high
and stable accuracy, the capacitive humidity resistor is the most commonly used
type. In this style, a film capacitor is
sandwiched between a ceramic and a dielectric polymer. The polymer will absorb or release moisture
based on the relative conditions. The
addition or subtraction of water will change the capacitance of the capacitor
which can be converted to a digital reading.
These sensors have an average accuracy of about +/- 2%.
The
least accurate humidity sensor is called the metal-paper coil and gives a good
visual of the principle that makes the more accurate types work. In this style, a piece of paper that has been
saturated with salt is attached to a metal coil. When humidity increases, the paper absorbs
water and changes the shape of the coil.
These changes are calibrated so a dial can be used to provide relative
readings. Although the accuracy is
limited to +/- 10%, these gauges are cheap and require no digital feedback to
function.
Brian did a good job in his post to point out
the range of options from the most accurate to least accurate. He refers to the idea that “primitive”
methods of something physically changing such as the length of a hair have been
superseded by more technological methods that use electrical properties and
digital conversion. In the end, however,
these primitive methods still do the job that they were designed to do.
Originally,
I only thought of hygrometers being used in conjunction with HVAC systems for
air condition quality purposes. Jeanine’s post made me realize that their uses go far beyond just providing comfort. Having a controlled humidity level in a hospital
scenario is very important for controlling the spread of airborne germs and
bacteria. Industries like greenhouses
can benefit by controlling the amount of moisture introduced to the plant
life. The uses obviously stretch much
further than home comfort in thermostats.
Pressure Sensors
Pressure is defined as force per unit area that a fluid exerts on its surroundings. A pressure measurement can further be described by the type of measurement being performed. There are three types of pressure measurements: absolute, gauge, and differential. Absolute pressure measurement is measured relative to a vacuum. Gauge pressure is measured relative to ambient atmospheric pressure. Differential pressure is similar to gauge pressure, but instead of measuring relative to ambient atmospheric pressure, differential measurements are taken with respect to a specific reference pressure.
A pressure sensor, sometimes called a pressure transmitter, is a transducer that converts pressure into an analog electrical signal. Because of the great variety of conditions, ranges, and materials for which pressure must be measured, there are many different types of pressure sensor designs. Often pressure can be converted to some intermediate form, such as displacement. The sensor then converts this displacement into an electrical output such as voltage or current. As C. Meraz introduced in her post, the three most universal types of pressure transducers of this form are the strain gage, variable capacitance, and piezoelectric. Chunyi Wang adds other two types of sensor named resonant wire pressure sensor and Pirani gauge sensor. Besides, according to G.Carpenter’s blog, there are three types of electrical outputs available for pressure sensors: millivolt, amplified voltage, and 4-20 mA.
Figure above provides an overall orientation to the scientist or engineer who might be faced with the task of selecting a pressure detector from among the many designs available. This table shows the ranges of pressures and vacuums that various sensor types are capable of detecting and the types of internal references (vacuum or atmospheric pressure) used, if any.
Common causes of pressure sensor failure include dynamic impact that results in sensor overload, spikes that cause a hole or tear in the diaphragm, moisture ingress whereby liquid seeps in through an interface and between a cable and the sensor, extreme temperature, stress when being calibrated, and wear and tear that produces drift and failure. Advances in pressure sensors should be focus on miniaturization with integration of electronics and control capabilities into the same chip as the sensor resulting from the new, smaller form factors. By reducing the size of the sensor, pressure sensors can be used in more area.
source:
http://www.omega.com/literature/transactions/volume3/pressure.html
Humidity Sensors
Developments in
semiconductor technology have made possible humidity sensors that are
incredibly accurate, durable, and inexpensive.
The three most commons types of humidity sensors are capacitive, resistive,
and thermal conductivity.
Resistive humidity
sensors measure the change in impedance of a medium which has an inverse
exponential relationship to relative humidity.
Mediums used include conductive polymers, salt, or treated substrate. These sensors typically use ceramic as a
coating for protection from condensation.
The voltage output provided from the sensor become directly proportional
to the relative humidity when signal conditioning is applied.
Thermal conductivity
humidity sensors calculate the difference between thermal conductivities of dry
air and air containing water vapor to measure absolute humidity. Because of their ability to measure absolute
humidity, these sensors are also referred to as absolute humidity sensors. This sensor consists of two negative
temperature coefficient thermistor elements within a DC bridge circuit. One element gets sealed in dry nitrogen and
the other remains exposed to the environment.
Absolute humidity is directly proportional to the difference between the
resistances of each element.
Capacitive sensors are
the only type of full-range sensor that can accurately measure to 0% relative
humidity. This type of relative humidity
sensor is most commonly used in industrial, commercial, and weather
applications and is used over wide ranges of temperature due to their low
temperature effect. Capacitive sensors
measure the change in dielectric constant which is proportional to the
environment’s relative humidity. A
0.2-0.5 pF change in capacitance is related to a 1% change in relative
humidity. This type of sensor would most likely be used in HVAC applications.
Regardless of the type of relative
humidity sensor, all outputs are affected by both temperature and perfect of
relative humidity. When higher accuracy
or wide operating temperature ranges are considered, temperature compensation is
included in the application. Relative humidity integrated circuits, or RHIC,
have linear voltage outputs that are a function of the supply voltage, percent
of relative humidity, and temperature.
This allows for the sensor to translate supply and output voltages to
determine the true relative humidity of an area.
Temperature Sensors - Thermocouple
As the other groups mentioned, resistive temperature
detectors (RTD), simple mercury thermometers, and more technologically advanced
infrared sensors are all used to sense temperature. I would like to discuss a
different type of temperature measurement – thermocouples. As Elda mentioned,
thermocouples tend to be less accurate than RTDs, but they are still widely
used in industry. They are simple and easy to understand. They have some
advantages too which include a wide temperature range, robustness, rapid responsiveness,
and lack of self heating.
Like most scientific inventions, the thermocouple was
invented by accident. An Estonian physician accidentally discovered the ability
to sense temperature by the effect of joining two different metals together.
When two different metal wires are joined together and a temperature difference
exists along them, they generate voltage, which is indicative of the
temperature difference. Figure 1 presents a simple thermocouple diagram: the
junction where the metals meet is called the measurement junction, or the hot
junction. That point should be exposed to the temperature we would like to
measure. The wires should then be placed in what’s referred to as the reference
junction, or the cold junction. At that point the wires are generally inserted
in a bath of ice water to maintain a constant 0 degrees Celsius. Thermocouples
measure the relative temperature between the two junctions, and therefore the
reference junction must be known, and is usually kept at 0 degree Celsius.
Figure 1: Thermocouple Diagram
The metals used are indicative of the sensitivity,
temperature range, and voltage range measured by the thermocouple. Table 1 has
this information about the common types of thermocouple. The types also
indicate the error in measurements. As mentioned before, the error in
measurement can be significant when using thermocouples. Figure 2 shows the
possible error for four different thermocouples for the temperature range of 0
to 400 degree Celsius.
Table 1: Types of Thermocouples
Figure 2: Error in Thermocouple Measurements
What is actually measured when using a thermocouple is the
voltage created by the difference in temperature. In order to interpret this
date, one needs to know how to convert the voltage data to meaningful
temperature data, which is, again, dependent on the type of thermocouple. The
seedback coefficient is the voltage change per degree Celsius in μV per degree Celsius, and it is
represented in Table 2 for the different thermocouple types at 25 degree Celsius. The seedback
coefficient is not constant, though, which makes the fitted graphs used to
interpret the temperature nonlinear. Software needs to collect the voltage data
and have a function imbedded within it used to convert these measurements to
useful temperature data.
Table 2: Seedback Coefficient at 25 Degree Celsius
References:
http://cds.linear.com/docs/Application%20Note/an28f.pdf
http://www.analog.com/library/analogDialogue/archives/44-10/thermocouple.pdf
How flow sensors work?
Flow sensors are used for wide range of fluids in multiple
industries for various measurements. There are many types of flow sensors and
they all measure “volume or area per unit time”. And depending of the sensor
type there are multiple ways this volume per unit time can be measured, but
they all use basic concept of fluid flow principles with concept such as the
Bernoulli’s principle. Some of the basic flow sensors used today are orifice
meter, venture meter, flow nozzle, and pitot tubes which us the principle of difference
in pressure from the Bernoulli equation. Then there are sensors which use
direct force to measure the flow which include rotameter, turbine meter, propeller
flow meter, coriolis mass flow meter. Using pressure differences and direct
force are the most common methods used to measure flow rate but there are other
complicated methods such as ultrasonic flow meters, magnetic flow meter,
calorimetric flow meter, gear flow meter, thermal flow meter, and couple more.
All the flow sensors which use pressure difference use
Bernoulli equation which is
Where the condition on both sides of the sensors are inversely
related to each other, therefore the equation can be manipulated into the
pressure drop across the flow sensor is equal to velocity of the flow squared.
When calculating the flow using the different sensors the area of the opening on
both sides of the sensor, density of the liquid, and pressure readings from the
sensors are known an can be plugged into the Bernoulli equation to find the
velocity because V1 = V2 = V. Below are few images portraying
how some of this sensors operate and how the pressures differences (dp) can be
measured.
Orifice Plate Flow Sensor
Venturi Tube Flow Sensor
Flow Nozzles Flow Sensor
Sensors using direct force to measure velocity use methods
of balancing forces with in systems where the force applied by the fluid flowing
through the sensor is measured and manipulated with a proportion factor to get
the flow of fluid in the system. Below are some images which portray how the forces
applied by the fluid are with few different types of direct force flow sensors.
Rotameter: resistance of gravity force of the bolt is being
measured here.
Turbine meter: work is being measured here where work equals
force times distance, and the distance id known so the force can be calculated
from the work measured by the sensor.
Sources:
Infrared Thermometers
Mike does a
great job summing up a wide variety of temperature sensors. Back in high school I learned that the
bimetallic strip was used to control thermostats. The expansion of the metal at a specific
temperature would close a circuit, thus turning on/off the HVAC system. Now we have NEST, which is more accurate and
efficient.
I would like
to expand on Infrared (IR) Thermometers.
These thermometers are essentially a laser gun that can read the
temperature of an object without any contact.
This type of thermometer is has an increasing popularity in the food
industry. Chefs are using them to read
the temperature of food so that they don’t have to puncture the food and
because it gives a fast and accurate reading.
Every object
emits an invisible infrared energy. IR
is located on the electromagnetic spectrum between visible light and
microwaves. There are three ways to
transfer this invisible heat: reflected, transmitted, and emitted. The emitted energy is the only type of energy
that can be used to get the actual surface temperature. This is a disadvantage to this type of thermometer. As I mentioned before this thermometer is
becoming popular in the food industry.
If the food is under a heating lamp the temperature measured will also
include that of the lamp. Therefore,
when taking the temperature of the food it should be in low light, or the light
should be covered with a cloth.
Depending on
how advanced the IR thermometer the emissivity value can be altered based on
the material. These values can be looked
up in charts. As a comparison,
emissivity of aluminum and water are 0.77 and 0.95, respectively.
Again these
thermometers are growing in the food service industry. They are also great for
monitoring equipment. For example, they
can be used to find hot or cold spots detecting leaks in HVAC equipment.
Sources:
http://www.allqa.com/IR.htm
http://www.thermoworks.com/emissivity_table.html
http://www.buzzle.com/articles/laser-thermometer-how-does-it-work.html
Flow Sensors (Fluidic-flow measurement sensors)
Flow sensors are devices that sense the rate of fluid or gas flow. The sensors installed in different field varied depending on the properties of the fluid or gas being measured. Sensors have different meters and principles therefore, the most appropriate should be chosen for the desire application. The most common sensors in our daily life are the ones used to measure water flow and electrical consumption at our home.
During my research through AccessScience I found two types of Fluidic-Flow measurement sensors known as fluidic-oscillator meter and fluidic flow-sensor. The fluidic-oscillator meter works on the principle of Coanda effect. The Coanda effect of a jet fluid attaching to a nearby surface, and it remains attached even when the surface curves away from the initial fluid direction. In the case of the fluidic-oscillator meter the fluid comes into the device and the fluid attaches to one of the side walls (see attach figure). Part of the flow splits off and goes through the feedback passage forcing the incoming flow to attach to the other side of the sidewall. The frequency of the oscillation back and forth is proportional to the volume flow through the meter. The sensor records the oscillations and transmits the signal to record the flow. These types of meters can be used for fluids and flow meters. The fluidic-flow sensor measures the flow of gas. It consists of air or another gas directed from an outer nozzle onto two small openings. The flow of the gas being measure will bend the air or gas and therefore changes the relative pressure on the two ports. This activates the signal and allows recording the gas velocity.
Rita Pauliushchyk on her blog decided to focus on the common types of flow sensors such as flow of water and energy consumption of a building or household. These are sensors we use/activate every day for our usage. This sensors are the one in charge of saying how much we have consume at home shown in our monthly bills. Sensors although they are measuring fluid, gas, air they are also helping to control the usage. Sensors are today recording and serving data to make building more efficient and sustainable. They are a powerfull device that is making buildings sustainable. It is important to know which sensors to install accordingly to the application and characteristics.
Flow Sensors
Flow sensors find various uses in the Building Automation systems. One of the increasingly more common uses of flow sensors is to measure the chilled water, heating water, and electrical energy consumption of a building or a tenant. Sometimes it is used to verify energy consumption and utility costs. Measuring water energy consumption requires temperature and flow measurements. In the following application, a flow meter is used to measure the liquid flow through the pipes. Flow sensors are also often utilized to measure air velocity indoors. This provides for a controlled ventilation of living areas as well as an optimization of energy costs. Flow sensors are also employed in water and wastewater management. Measurement of water, wastewater, and gray water used by a building provides an understanding of the building’s carbon footprint.
There are various types of flow sensors for HVAC systems available on the market today. In order to make an appropriate selection of a flow sensor, it is important to have a clear understanding of the requirements of a particular application. Characteristics that should be considered: familiarity of the staff with the type of product and its calibration procedure, maintenance, type of fluid, characteristics of the fluid, minimum and maximum pressure and temperature values and etc. Figure below illustrates different types of flow sensors:
Differential pressure flow meters are possibly the most commonly used type. The calculation of fluid flow is performed by reading the pressure loss across a pipe restriction. As a fluid passes through, it accelerates, and the energy associated with this acceleration is obtained. The pressure differential head is measured. Different types of differential pressure flow meters:
Additionally, Jalpesh has shown various very detailed diagrams of differential flow meters in his blog post.
Maria has decided to focus on fluidic flow measurement sensors. These are much more advanced than the basic differential flow meters I have described. It also sees as though they are more applicable for applications with gaseous fluids.
http://www.omega.com/literature/transactions/volume4/T9904-07-DIFF.html#diff_1
Flow Sensors
For the majority of this term,
this class has focused on intelligent buildings and new technology. However, as
engineers we deal with lots of basic measurements and calculations we encounter
in an everyday work environment. An example of these measurements, is the
measurement of flow done through the use of flow meters and sensors. Flow sensors are detecting elements within a
flow meter that record the flow of fluids or gases. In the
figure below, there’s a variety of flow sensors that measure liquid flow, but vary in the form of which they measure the
flow.
As can be seen on the left hand side, the rotor in turbine flow meters measures
the flow because the rate of the flow causes a proportional movement in the rotary
wheel. The rate at which the wheel is spinning, is also the rate of the flow. Magnetic
flow meters as can be seen on the bottom right side of the image above, operate
on Faraday’s law of electromagnetic induction. This means that the flow meters
are triggered by conductive liquids because the flow is measured as a counter reaction
to the conductivity. This counter reaction is a voltage that is produced by a current applied to coils mounted on or
outside the flow pipe. The voltage produced is a magnetic field that is
proportional to flow rate, an and its measured by electrodes in the system. Thermal
flow meters as pictured above (second one down, left hand side) measure mass
flow directly. The thermal flow meters measure flow by heating the liquid
within, and take the rate at which it takes to dissolve. Other thermal sensors
just input heat into a system, and measure the amount of energy used for the
system to stay at that temperature. This type of thermal system is more often
used for gases, along with multivariable differential pressure transmitters.
These type of meters are based on temperature sensors, which measure the heat
within the moving medium, along with velocity to calculate the rate.
I found it neat that the multivariable differential pressure
transmitters, can act as temperature sensors as well. They can measure pressure
and temperature , to calculate mass flow. This was really interesting because
it shows an overlap within sensors, since the flow meters use resistive
temperature detectors (RTDs), which EldaCifligu describes as temperature sensors. Like Matthew Tedesco stated, “aside from mechanical flow meters, fluid velocity
and flow can be measured using optic sensors.” None of the meters above show this type of sensor because this
“laser-based interferometry is often used for air flow measurement but not for
liquid flow.
Sources:
http://en.wikipedia.org/wiki/Flow_sensor
http://www.pc-control.co.uk/flow_sensors.htm
Monday, February 18, 2013
Thermography
I think that Mike did an excellent job of going over the history of temperature sensors and how they work. When I was assigned this post, I immediately though of a thermistor, however it is important to think about the history of temperature sensor, and that the analog approach has worked very well for a long time. I personally got very interested while reading Elda's post when she talked about the use of infrared thermometers. Elda talks about how infrared thermometers are used to detect temperatures without the need for surface contact. This brought me to the idea to discuss thermography as a type of "temperature sensor."
A paper written by Maldague shows there are two main types of thermography, passive and active. These two types of thermography are discussed as a means of Non-Destructive Evaluation Techniques (NDT). The idea behind active thermography is to send a wave of energy (in the form of heat), and see how the heat moves in the observed medium. Passive thermography on the other hand uses energy already found in the observed medium such as the difference in temperature of a pipe or electric box that generates or takes heat to show how the materials around it act. Thermography can easily be used to determine if an electrical box is overheating and needs attention. It can also be used for firefighters to determine what is going on in a household in the event of a fire, and where people are located inside the burning structure.
The most basic description of thermography, is the use of an infrared camera to detect the radiation from a given material, that then is translated to a surface temperature. It is important (according to NDT sources) to know the emissivity of the material observed to get an accurate reading of what temperature is being observed. If the wrong emissivity is chosen, than the entire process of thermography can become a fruitless exercise.
One can see each different way of measuring temperature has its pros and cons, and there is no one solution fits all approach for measuring temperature.
A paper written by Maldague shows there are two main types of thermography, passive and active. These two types of thermography are discussed as a means of Non-Destructive Evaluation Techniques (NDT). The idea behind active thermography is to send a wave of energy (in the form of heat), and see how the heat moves in the observed medium. Passive thermography on the other hand uses energy already found in the observed medium such as the difference in temperature of a pipe or electric box that generates or takes heat to show how the materials around it act. Thermography can easily be used to determine if an electrical box is overheating and needs attention. It can also be used for firefighters to determine what is going on in a household in the event of a fire, and where people are located inside the burning structure.
The most basic description of thermography, is the use of an infrared camera to detect the radiation from a given material, that then is translated to a surface temperature. It is important (according to NDT sources) to know the emissivity of the material observed to get an accurate reading of what temperature is being observed. If the wrong emissivity is chosen, than the entire process of thermography can become a fruitless exercise.
One can see each different way of measuring temperature has its pros and cons, and there is no one solution fits all approach for measuring temperature.
Week 7: Movement Sensors
When analyzing movement sensors, they can be broken down into two main types of movement sensors: active motion detectors and passive motion detectors. Active movement sensors utilize sensors that emit a type of signal that is then reflected back and detected. Passive movement sensors do not actually emit a signal like active sensors, but instead detect the signals being emitted by the objects in their field of view and have a predetermined baseline reading for the surrounding area.
Active movement sensors are mainly used in places where a rapid response from a change in detection is needed (ultrasonic or microwave). For example, a motion sensor for a car garage uses an active ultrasonic radar-like sensor that emits sound waves. When the sound waves are emitted, the sensor detects the pattern or the time it takes for those waves to bounce off of its surroundings and return back to the sensor. When an object is approaching the garage door or entering the space under the garage door while it is closing, the sound waves are emitted and bounce off of the object crossing the path of the sensor causing the return time to be faster. This is then computed by the sensing system and tells the garage door mechanism to open so as to allow a car to enter or to prevent the door from closing on something or someone.
Passive movement sensors detect a change in their surroundings by reading the energy of their surroundings. These types of sensors come in various forms, some of the most common being infrared or photo sensors. These sensors detect and measure the energy that is being produced by the objects in their line of sight and are commonly used in home security systems or businesses to warn them of someone entering the facilities. Bodies that generate heat, be they humans or animals, also generate infrared energy (for humans usually ranging between 9 and 10 micrometers). With this range in mind, infrared sensors are programmed to detect emissions within the range of 8 and 12 micrometers with the use of a photo detector. This sensor measures the light being transmitted to it, converts it into an electrical current that is sent to the sensor’s processing core. “The alarm is triggered when the photo detector detects large or fast variations in the distribution of the emitted infrared energy.” (1). This form of detection allows for the movement sensor to ignore slight variations in heat that occur over the length of the day, such as the slow change in temperature of objects in the sensor’s field of view as their temperatures cool down over night.
These sensors are very inexpensive and can offer a very secured environment when combined to cover all entry points of the desired area.
I found it interesting in my classmate Matthew Tedesco's post (http://ae-510-ay12-13.blogspot.com/2013/02/flow-measurement-of-fluids-including.html) how he explained the various applications a specific type of sensor may have. Movement sensors aren't limited to only sensing movement, but as Matthew described, optic sensors (which are a type of movement sensor) can also be used to measure mechanical flow, fluid velocity and flow.
Sources:
(1) http://www.ehow.com/how-does_4596955_motion-sensor-work.html
(2) http://home.howstuffworks.com/home-improvement/household-safety/security/burglar-alarm2.htm
(3) http://home.howstuffworks.com/home-improvement/household-safety/security/question238.htm
Active movement sensors are mainly used in places where a rapid response from a change in detection is needed (ultrasonic or microwave). For example, a motion sensor for a car garage uses an active ultrasonic radar-like sensor that emits sound waves. When the sound waves are emitted, the sensor detects the pattern or the time it takes for those waves to bounce off of its surroundings and return back to the sensor. When an object is approaching the garage door or entering the space under the garage door while it is closing, the sound waves are emitted and bounce off of the object crossing the path of the sensor causing the return time to be faster. This is then computed by the sensing system and tells the garage door mechanism to open so as to allow a car to enter or to prevent the door from closing on something or someone.
Passive movement sensors detect a change in their surroundings by reading the energy of their surroundings. These types of sensors come in various forms, some of the most common being infrared or photo sensors. These sensors detect and measure the energy that is being produced by the objects in their line of sight and are commonly used in home security systems or businesses to warn them of someone entering the facilities. Bodies that generate heat, be they humans or animals, also generate infrared energy (for humans usually ranging between 9 and 10 micrometers). With this range in mind, infrared sensors are programmed to detect emissions within the range of 8 and 12 micrometers with the use of a photo detector. This sensor measures the light being transmitted to it, converts it into an electrical current that is sent to the sensor’s processing core. “The alarm is triggered when the photo detector detects large or fast variations in the distribution of the emitted infrared energy.” (1). This form of detection allows for the movement sensor to ignore slight variations in heat that occur over the length of the day, such as the slow change in temperature of objects in the sensor’s field of view as their temperatures cool down over night.
These sensors are very inexpensive and can offer a very secured environment when combined to cover all entry points of the desired area.
I found it interesting in my classmate Matthew Tedesco's post (http://ae-510-ay12-13.blogspot.com/2013/02/flow-measurement-of-fluids-including.html) how he explained the various applications a specific type of sensor may have. Movement sensors aren't limited to only sensing movement, but as Matthew described, optic sensors (which are a type of movement sensor) can also be used to measure mechanical flow, fluid velocity and flow.
Sources:
(1) http://www.ehow.com/how-does_4596955_motion-sensor-work.html
(2) http://home.howstuffworks.com/home-improvement/household-safety/security/burglar-alarm2.htm
(3) http://home.howstuffworks.com/home-improvement/household-safety/security/question238.htm
Temperature Sensors
A temperature sensors are devices that measure temperature of a medium. As Mike S. mentions in his blog, there are many types of temperature sensors used from simple home purposes to extremely accurate and precise scientific uses. Thermocouples, resistive temperature detectors, infrared thermometers, bimetallic devices, liquid expansion thermometers, and state-of-change devices are the some of the basic types of temperature sensors used for simple or more complex purposes.
As Mitchell Butler and John Scanlon mention in their blogs, the most basic liquid expansion thermometers, especially mercury thermometers, are the most common temperature sensors that is very easy to use and accessible to everyone. The main elements of the liquid expansion thermometers are the mercury-in-glass sensor which expands and contracts when there is a temperature change and the means of converting this change into a temperature reading. Although accurate, mercury-in-glass thermometers are delicate and mercury is a hazardous material.
Resistive temperature detectors (RTDs) are the most common sensors used in laboratory and industrial purposes. RTDs use resistors to record resistance values as the temperature changes. They are very accurate and have a wide temperature range which is why they are used for heavy duty purposes. They are preferred over a thermocouple or a thermistor sensor because of their high accuracy and stability for many years.
Infrared thermometers are non-contact temperature measurement devices that detect the energy emitted by the medium and convert the energy factor into a temperature reading. Infrared thermometers are very useful for temperature measurements of moving objects, or when non-contact measurements are required due to hazardous material; more conventional thermometers do not seem useful in situations like that.
As mentioned above, temperature sensors are used for multiple purposes. One use of distributed-temperature sensors is to measure the temperature profile of oil reservoirs, which has been done for many years now. The information provided by temperature sensors are very important for the process control of the oil tanks; they provide visual data to optimize the well performance as well as detect flow and viscosity behind the reservoir casing. Nowadays, there are many temperature sensors used in HVAC systems of buildings in order to efficiently maintain comfortable indoor conditions.
Sources:
http://www.omega.com/prodinfo/temperaturemeasurement.html
http://www.omega.com/Temperature/pdf/RTD_Gen_Specs_Ref.pdf
http://en.wikipedia.org/wiki/Infrared_thermometer
http://www.accessscience.com.ezproxy2.library.drexel.edu/content.aspx?searchStr=Temperature+sensors&id=YB980600
As Mitchell Butler and John Scanlon mention in their blogs, the most basic liquid expansion thermometers, especially mercury thermometers, are the most common temperature sensors that is very easy to use and accessible to everyone. The main elements of the liquid expansion thermometers are the mercury-in-glass sensor which expands and contracts when there is a temperature change and the means of converting this change into a temperature reading. Although accurate, mercury-in-glass thermometers are delicate and mercury is a hazardous material.
Resistive temperature detectors (RTDs) are the most common sensors used in laboratory and industrial purposes. RTDs use resistors to record resistance values as the temperature changes. They are very accurate and have a wide temperature range which is why they are used for heavy duty purposes. They are preferred over a thermocouple or a thermistor sensor because of their high accuracy and stability for many years.
Infrared thermometers are non-contact temperature measurement devices that detect the energy emitted by the medium and convert the energy factor into a temperature reading. Infrared thermometers are very useful for temperature measurements of moving objects, or when non-contact measurements are required due to hazardous material; more conventional thermometers do not seem useful in situations like that.
As mentioned above, temperature sensors are used for multiple purposes. One use of distributed-temperature sensors is to measure the temperature profile of oil reservoirs, which has been done for many years now. The information provided by temperature sensors are very important for the process control of the oil tanks; they provide visual data to optimize the well performance as well as detect flow and viscosity behind the reservoir casing. Nowadays, there are many temperature sensors used in HVAC systems of buildings in order to efficiently maintain comfortable indoor conditions.
Sources:
http://www.omega.com/prodinfo/temperaturemeasurement.html
http://www.omega.com/Temperature/pdf/RTD_Gen_Specs_Ref.pdf
http://en.wikipedia.org/wiki/Infrared_thermometer
http://www.accessscience.com.ezproxy2.library.drexel.edu/content.aspx?searchStr=Temperature+sensors&id=YB980600
Temperature Sensors
While initially, sensing temperature may seem like a simple concept (and it's been getting done for a very long time now) the actual details of what we are sensing can become quite confusing. In its most basic sense, temperature is a measure of the movement of molecules within a body of matter. That matter can be solid liquid or gas, and more frequent movements translate to higher temperatures (with absolute zero at the bottom of the scale, meaning no particle movement at all) The reasons we care about temperature are varied, but mainly we want to be comfortable or we want to ensure that a certain process is happening by manipulating the temperature. Of course our perception of comfort is based on our own sensing of temperature by biological processes far more complex than the processes we use to quantify temperature.
Obviously the thermometer is the simplest way to sense and understand temperature. We figured out pretty early on that certain fluids expand when they are heated, so that their volume is a function of temperature. By controlling and marking this expansion of volume we were able to quantify temperature. As John points out, we were later able to recognize the same principle applies to solids, and we began to use this to control the movment of thin metal strips, which would in turn control some basic circuitry, mainly for switching on and off some electrical process. Of course, this is the principle by which most early thermostats operated.
As we progressed in knowledge of electrical properties of materials, it was found that the resistance of certain conductive materials decreases as we increase the temperature, and so we developed thermistors made of ceramics or polymers to effectively convert a change in temperature to a change in resistance that could be easily measured. We also knew that the current changes between to metals at different temperatures, so a slightly more complicated Thermocouple circuit was developed, where this change in current can be measured across dissimilar metals. Furthermore we found that different metals behaved more predictably for certain temperature ranges, so a number of sets of metals were used for varying applications. Finally, the resistance in a coiled wire was found to be more accurately measurable, so RTD's (made of a homogeneous wire wrapped around a ceramic core) became prominent. These could be configured differently to accomodate different temperature ranges and provided high accuracy and repeatability.
Mike does a great job of explaining the usefulness of all of the sensors mentioned above. However, there are still some applications in which even RTD's and Thermocouples cannot be used. For example measuring the surface temperature of an object that is moving or one that cannot be disturbed becomes quite difficult with the sensors mentioned above. Rather than measuring a change in a specific material's response to a temperature, Pyrometers are able to measure a temperature directly as a function of an object's electromagnetic radiation. The methods by which this is acheived can become rather complex, but in their simplest form all pyrometers consist of a lense to concentrate radiation into a point or array of points and an absorber, which chemically translates radiation into terms of flux. It is at this point that the line between sensing and transducing becomes blurred for me. David gives a good explanation of these principles as applied to an array of infrared sensors (thermography). By some means of transduction the radiation is translated to thermal energy and the thermal energy is translated to an electrical current. The figure below helps explain the process.
References:
http://drexel.summon.serialssolutions.com/search?s.q=thermal+radiation+sensor
Pressure Sensors
A pressure sensor can detect pressure and then convert it to
electricity signal for display. So it acts as a transducer that generates an
electrical signal as a function of the pressure imposed [1]. According to
G.Carpenter’s blog, the electrical outputs of pressure sensor can be classified
into three types: (1) sensors with millivolt output. These sensors are common
and economic but need regulated power supplies and not suitable for noisy
environment because the outputs are nominally around 30mV, easily being interrupted.
(2) sensors with amplified voltage output.
By using the integral signal conditioning
the outputs are amplified, ranged
from 0-5Vdc to 0-10Vdc so they are more steady than type (1); (3) Sensors with
4-20mA output. The signal is the most steady so they are suitable for the long transition
distance(1000+ft)[2].
Pressure sensors can use different technologies to detect
pressure. The most common method is to measure strain due to applied force over
an area also named force collector. For example, as C.Meraz introduced, variable
capacitance and piezoelectric sensors are different force collector types. I
want to discuss other types of sensors using other properties to infer pressure.
Resonant wire pressure sensor uses the difference of resonant
frequency to measure pressure. The input pressure is detected by the high
pressure and low pressure diaphragms on the right and left of the unit[3]. Usually,
the resonant wire oscillates at its natural frequency. When the pressure
changes, the wire tension will change accordingly and the resonant frequency
also changes. A digital counter circuit is used to detect the shift and
transform the signal to pressure value. The advantage of this technology is it
can provide very stable readings over time[1].
Another type of sensor uses the changes in thermal
conductivity of a gas[1]. The typical application is Pirani gauge, which was
invented in 1906 by Marcello Pirani[4]. The method measures heat loss of a
filament to indirectly determine the pressure of gas. For example, within high
pressure, there should be more molecules present in the same volume and the
chance to collide with heated metal wire potentially high, resulting in more
efficiency removing heat than low pressure. Since the thermal conductivity and
heat capacity of the gas may affect the readout, the sensor needs to be
calibrated before using. The advantage of this method is its accuracy—between
0.5Torr to 10-4 Torr[4].
Subscribe to:
Posts (Atom)