1 W08 Sensors and Measurement (2/2) Yrd. Doç. Dr. Aytaç Gören
2 ELK Contents W01 Basic Concepts in Electronics W02 AC to DC Conversion W03 Analysis of DC Circuits (self and condenser) W04 Transistors and Applications (H-Bridge) W05 Op Amps and Applications W06 Midterm W07 Sensors and Measurement (1/2) W08 Sensors and Measurement (2/2) W09 Basic Concepts in Digital Electronics (Boolean Algebra, Decimal to binary, gates) W10 Digital Logic Circuits (Gates and Flip Flops) W11 PLC s W12 Microprocessors W13 Data Acquisition, D/A and A/D Converters. 2 Yrd. Doç. Dr. Aytaç Gören
3 ELK 2018 W08 Contents 1. Photoelectric Sensors 2. Thermal, Heat, Temperature Sensors 3. GPS 4. Flow, Fluid Velocity Sensors 5. Weather, Moisture, Humidity Sensors 6. Electric Current, Electric Potential 7. Magnetic Sensors 3 Yrd. Doç. Dr. Aytaç Gören
4 Connecting Sensors to Microcontrollers *references 9 sensor sensor µc signal timing memory keypad display Analog many microcontrollers have a built-in A/D 8-bit to 12-bit common many have multi-channel A/D inputs Digital serial I/O use serial I/O port, store in memory to analyze synchronous (with clock) must match byte format, stop/start bits, parity check, etc. asynchronous (no clock): more common for comm. than data must match baud rate and bit width, transmission protocol, etc. frequency encoded use timing port, measure pulse width or pulse frequency
5 Digital Sensors As its name implies, Digital Sensors produce a discrete output signal or voltage that is a digital representation of the quantity being measured. Digital sensors produce a Binary output signal in the form of a logic "1" or a logic "0", ("ON" or "OFF"). This means then that a digital signal only produces discrete (noncontinuous) values which may be outputted as a single "bit", (serial transmission) or by combining the bits to produce a single "byte" output (parallel transmission). Compared to analogue signals, digital signals or quantities have very high accuracies and can be both measured and "sampled" at a very high clock speed. The accuracy of the digital signal is proportional to the number of bits used to represent the measured quantity. For example, using a processor of 8 bits, will produce an accuracy of 0.195% (1 part in 512). While using a processor of 16 bits gives an accuracy of %, (1 part in 65,536) or 130 times more accurate. This accuracy can be maintained as digital quantities are manipulated and processed very rapidly, millions of times faster than analogue signals.
6 *references 9 Connecting Smart Sensors to PC/Network Smart sensor = sensor with built-in signal processing & communication Data Acquisition Cards (DAQ) PC card with analog and digital I/O interface through LabVIEW or user-generated code Communication Links Common for Sensors asynchronous serial comm. universal asynchronous receive and transmit (UART) 1 receive line + 1 transmit line. nodes must match baud rate & protocol RS232 Serial Port on PCs uses UART format (but at +/- 12V) can buy a chip to convert from UART to RS232 synchronous serial comm. serial peripheral interface (SPI) 1 clock + 1 bidirectional data + 1 chip select/enable I 2 C = Inter Integrated Circuit bus designed by Philips for comm. inside TVs, used in several commercial sensor systems IEEE P1451: Sensor Comm. Standard several different sensor comm. protocols for different applications
7 Frequency (MHz) offset Yrd. Doç. Dr. Aytaç Gören Sensor Calibration *references 9 Sensors can exhibit non-ideal effects offset: nominal output nominal parameter value nonlinearity: output not linear with parameter changes cross parameter sensitivity: secondary output variation with, e.g., temperature Calibration = adjusting output to match parameter analog signal conditioning look-up table digital calibration T = a + bv +cv 2, T= temperature; V=sensor voltage; a,b,c = calibration coefficients Compensation remove secondary sensitivities must have sensitivities characterized can remove with polynomial evaluation Temperature (C) P = a + bv + ct + dvt + e V 2, where P=pressure, T=temperature T1 T2 T
8 Yrd. Doç. Dr. Aytaç Gören Photoelectric Sensors Photoelectric devices can be grouped into two main categories, those which generate electricity when illuminated, such as Photo-voltaics or Photo-emissives etc, and those which change their electrical properties in some way such as Photoresistors or Photo-conductors. This leads to the following classification of devices. Photo-emissive Cells - These are photodevices which release free electrons from a light sensitive material such as caesium when struck by a photon of sufficient energy. The amount of energy the photons have depends on the frequency of the light and the higher the frequency, the more energy the photons have converting light energy into electrical energy. Photo-conductive Cells - These photodevices vary their electrical resistance when subjected to light. Photoconductivity results from light hitting a semiconductor material which controls the current flow through it. Thus, more light increase the current for a given applied voltage. The most common photoconductive material is Cadmium Sulphide used in LDR photocells.
9 Yrd. Doç. Dr. Aytaç Gören Photoelectric Sensors Photo-voltaic Cells - These photodevices generate an emf in proportion to the radiant light energy received and is similar in effect to photoconductivity. Light energy falls on to two semiconductor materials sandwiched together creating a voltage of approximately 0.5V. The most common photovoltaic material is Selenium used in solar cells. Photo-junction Devices - These photodevices are mainly true semiconductor devices such as the photodiode or phototransistor which use light to control the flow of electrons and holes across their PN-junction. Photojunction devices are specifically designed for detector application and light penetration with their spectral response tuned to the wavelength of incident light.
10 Photoelectric Sensors Yrd. Doç. Dr. Aytaç Gören
11 Yrd. Doç. Dr. Aytaç Gören Photoelectric Sensors Infrared - Active Sensor type = Reflective IR IR detector = Panasonic PNA4602M IR LED type = Narrow focus 10º I/O required = 3 digital lines: 2 outputs, 1 input Range = Approximately 4 to 26" Input voltage = 5vdc 8mA PC board size = 2.3" x.75"
12 Yrd. Doç. Dr. Aytaç Gören Photoelectric Sensors Photovoltaic Photovoltaic light falling on a pn-junction can be used to generate electricity from light energy (as in a solar cell) small devices used as sensors are called photodiodes fast acting, but the voltage produced is not linearly related to light intensity
13 Yrd. Doç. Dr. Aytaç Gören Photoelectric Sensors Photoconductive such devices do not produce electricity, but simply change their resistance photodiode (as described earlier) can be used in this way to produce a linear device phototransistors act like photodiodes but with greater sensitivity light-dependent resistors (LDRs) are slow, but respond like the human eye The Light Dependent Resistor (LDR) is made from a piece of exposed semiconductor material such as cadmium sulphide that changes its electrical resistance from several thousand Ohms in the dark to only a few hundred Ohms when light falls upon it by creating hole-electron pairs in the material. The net effect is an improvement in its conductivity with a decrease in resistance for an increase in illumination. Also, photoresistive cells have a long response time requiring many seconds to respond to a change in the light intensity. LDR
14 Yrd. Doç. Dr. Aytaç Gören Photoelectric Sensors LDR The most commonly used photoresistive light sensor is the ORP12 Cadmium Sulphide photoconductive cell. This light dependent resistor has a spectral response of about 610nm in the yellow to orange region of light.
15 Yrd. Doç. Dr. Aytaç Gören Photoelectric Sensors LDR The resistance of the cell when unilluminated (dark resistance) is very high at about 10MΩ's which falls to about 100Ω's when fully illuminated (lit resistance). To increase the dark resistance and therefore reduce the dark current, the resistive path forms a zigzag pattern across the ceramic substrate. The CdS photocell is a very low cost device often used in auto dimming, darkness or twilight detection for turning the street lights "ON" and "OFF", and for photographic exposure meter type applications.
16 Yrd. Doç. Dr. Aytaç Gören Photoelectric Sensors One simple use of a Light Dependent Resistor, is as a light sensitive switch. LDR This basic light sensor circuit is of a relay output light activated switch. A potential divider circuit is formed between the photoresistor, LDR and the resistor R1. When no light is present ie in darkness, the resistance of the LDR is very high in the Megaohms range so zero base bias is applied to the transistor TR1 and the relay is de-energised or "OFF". As the light level increases the resistance of the LDR starts to decrease causing the base bias voltage at V1 to rise. At some point determined by the potential divider network formed with resistor R1, the base bias voltage is high enough to turn the transistor TR1 "ON" and thus activate the relay which inturn is used to control some external circuitry. As the light level falls back to darkness again the resistance of the LDR increases causing the base voltage of the transistor to decrease, turning the transistor and relay "OFF" at a fixed light level determined again by the potential divider network.
17 Photoelectric Sensors LDR Yrd. Doç. Dr. Aytaç Gören By replacing the fixed resistor R1 with a potentiometer VR1, the point at which the relay turns "ON" or "OFF" can be pre-set to a particular light level. This type of simple circuit shown above has a fairly low sensitivity and its switching point may not be consistent due to variations in either temperature or the supply voltage. A more sensitive precision light activated circuit can be easily made by incorporating the LDR into a "Wheatstone Bridge" arrangement and replacing the transistor with an Operational Amplifier.
18 Photoelectric Sensors LDR Yrd. Doç. Dr. Aytaç Gören In this basic dark sensing circuit, the light dependent resistor LDR1 and the potentiometer VR1 form one adjustable arm of a simple resistance bridge network, also known commomly as a Wheatstone bridge, while the two fixed resistors R1 and R2 form the other arm. Both sides of the bridge form potential divider networks across the supply voltage whose outputs V1 and V2 are connected to the non-inverting and inverting voltage inputs respectively of the operational amplifier. The operational amplifier is configured as a Differential Amplifier also known as a voltage comparator with feedback whose output voltage condition is determined by the difference between the two input signals or voltages, V1 and V2. The resistor combination R1 and R2 form a fixed voltage reference at input V2, set by the ratio of the two resistors. The LDR - VR1 combination provides a variable voltage input V1 proportional to the light level being detected by the photoresistor.
19 Photoelectric Sensors LDR As with the previous circuit the output from the operational amplifier is used to control a relay, which is protected by a free wheel diode, D1. When the light level sensed by the LDR and its output voltage falls below the reference voltage set at V2 the output from the opamp changes state activating the relay and switching the connected load. Likewise as the light level increases the output will switch back turning "OFF" the relay. The hysteresis of the two switching points is set by the feedback resistor Rf can be chosen to give any suitable voltage gain of the amplifier. The operation of this type of light sensor circuit can also be reversed to switch the relay "ON" when the light level exceeds the reference voltage level and vice versa by reversing the positions of the light sensor LDR and the potentiometer VR1. The potentiometer can be used to "pre-set" the switching point of the differential amplifier to any particular light level making it ideal as a simple light sensor project circuit. Yrd. Doç. Dr. Aytaç Gören
20 Yrd. Doç. Dr. Aytaç Gören Opto-switches Photoelectric Sensors opto-switch consist of a light source and a light sensor within a single unit. Two common forms are the reflective and slotted types A reflective opto-switch A slotted opto-switch Opto-switch sensor
21 Photoelectric Sensors Yrd. Doç. Dr. Aytaç Gören In example above, the speed of the rotating shaft is measured by using a digital LED/Optodetector sensor. The disc which is fixed to a rotating shaft (for example, from a motor or wheels), has a number of transparent slots within its design. As the disc rotates with the speed of the shaft, each slot passes by the sensor inturn producing an output pulse representing a logic level "1". These pulses are sent to a register of counter and finally to an output display to show the speed or revolutions of the shaft. By increasing the number of slots or "windows" within the disc more output pulses can be produced giving a greater resolution and accuracy as fractions of a revolution can be detected. Then this type of sensor arrangement could be used for positional control.
22 Yrd. Doç. Dr. Aytaç Gören Photoelectric Sensors Photodiode Circuits *references 10
23 Yrd. Doç. Dr. Aytaç Gören Photoelectric Sensors Photodiode Circuits Photodiodes are very versatile light sensors that can turn its current flow both "ON" and "OFF" in nanoseconds and are commonly used in cameras, light meters, CD and DVD-ROM drives, TV remote controls, scanners, fax machines and copiers etc, and when integrated into operational amplifier circuits as infrared spectrum detectors for fibre optic communications, burglar alarm motion detection circuits and numerous imaging, laser scanning and positioning systems etc. *references 6
24 Yrd. Doç. Dr. Aytaç Gören Photoelectric Sensors Photodiodes
25 Yrd. Doç. Dr. Aytaç Gören Light Sensor photoconductor light R Photoelectric Sensors Photodiode light I
26 Yrd. Doç. Dr. Aytaç Gören Photoelectric Sensors Semiconductor light sensors include: photodiodes, phototransistors, photodarlingtons. Like diodes, all transistors are light-sensitive. Phototransistors are designed specifically to take advantage of this fact. The most-common variant is an NPN bipolar transistor with an exposed base region. Here, light striking the base replaces what would ordinarily be voltage applied to the base so, a phototransistor amplifies variations in the light striking it. Note that phototransistors may or may not have a base lead (if they do, the base lead allows you to bias the phototransistor's light response. All of these have similar noise performance, but phototransistors and darlingtons have better sensitivity (more current for given light input). Phototransistor: lux Photodarlingtons up to 100x this sensitivity. Phototransistors
27 Photoelectric Sensors Phototransistors Yrd. Doç. Dr. Aytaç Gören20 Basically, a phototransistor can be any bipolar transistor with a transparent case. There are some variations provide advantages. For example, a focusing lens can be built into the case for directional sensitivity. Coatings can be applied to block some higher or lower wavelengths. The transistor itself may provide higher gain, or higher frequency, or lower capacitance, etc. The diagram above illustrates the frequency response of silicon phototransistor junctions, along with the spectral output of an infrared LED.
28 Yrd. Doç. Dr. Aytaç Gören20 Photoelectric Sensors a photo-darlington Phototransistors Phototransistor is basically a photodiode with amplification. The Phototransistor light sensor has its collector-base PN-junction reverse biased exposing it to the radiant light source. Phototransistors operate the same as the photodiode except that they can provide current gain and are much more sensitive than the photodiode with currents are 50 to 100 times greater than that of the standard photodiode and any normal transistor can be easily converted into a phototransistor light sensor by connecting a photodiode between the collector and base.
29 Photoelectric Sensors Phototransistors Yrd. Doç. Dr. Aytaç Gören20 Phototransistors consist mainly of a bipolar NPN Transistor with its large base region electrically unconnected, although some phototransistors allow a base connection to control the sensitivity, and which uses photons of light to generate a base current which inturn causes a collector to emitter current to flow. Most phototransistors are NPN types whose outer casing is either transparent or has a clear lens to focus the light onto the base junction for increased sensitivity.
30 Yrd. Doç. Dr. Aytaç Gören Photoelectric Sensors Photovoltaic Cells Photovoltaic cells are made from single crystal silicon PN junctions, the same as photodiodes with a very large light sensitive region but are used without the reverse bias. They have the same characteristics as a very large photodiode when in the dark. When illuminated the light energy causes electrons to flow through the PN junction and an individual solar cell can generate an open circuit voltage of about 0.58v (580mV). Solar cells have a "Positive" and a "Negative" side just like a battery.
31 Photoelectric Sensors Cameras Yrd. Doç. Dr. Aytaç Gören20 Two solid-state camera types: CCD and CMOS. A charge-coupled device (CCD) is a device for the movement of electrical charge, usually from within the device to an area where the charge can be manipulated, for example conversion into a digital value. A CCD image sensor is an analog device. When light strikes the chip it is held as a small electrical charge in each photo sensor. The charges are converted to voltage one pixel at a time as they are read from the chip. Additional circuitry in the camera converts the voltage into digital information. CCD is the more mature technology, and has the widest performance range. 8 Mpixel size for cameras Low noise/ high efficiency for astronomy etc. Good sensitivity (low as lux, starlight) CCDs require several chips,but are still cheap ($50 +) Most CCDs work in near infrared and can be used for night vision if an IR light source is used. (*ref.12, 13)
32 Yrd. Doç. Dr. Aytaç Gören20 Photoelectric Sensors CMOS (Complementary Metal-Oxide Semiconductor) is a technology used in fabricating integrated circuit chips. A CMOS imaging chip is a type of active pixel sensor made using the CMOS semiconductor process. Extra circuitry next to each photo sensor converts the light energy to a voltage. Additional circuitry on the chip may be included to convert the voltage to digital data. CMOS cameras are very compact and inexpensive, but haven t matched CCDs in most performance dimensions. Start from $20(!) Custom CMOS cameras integrate image processing right on the camera. Allow special functions like motion detection, recognition. Cameras
33 Temperature Sensors Yrd. Doç. Dr. Aytaç Gören The most commonly used type of all the sensors are those which detect Temperature or heat. These types of temperature sensor vary from simple ON/OFF thermostatic devices which control a domestic hot water system to highly sensitive semiconductor types that can control complex process control plants. We remember from our school science classes that the movement of molecules and atoms produces heat (kinetic energy) and the more movement, the more heat is generated. Temperature Sensors measure the amount of heat energy or even coldness that is generated by an object or system, and can "sense" or detect any physical change to that temperature producing either an analogue or digital output.
34 Temperature Sensors Yrd. Doç. Dr. Aytaç Gören Temperature sensors consist of two basic physical types: Contact Temperature Sensor Types - These types of temperature sensor are required to be in physical contact with the object being sensed and use conduction to monitor changes in temperature. They can be used to detect solids, liquids or gases over a wide range of temperatures. Non-contact Temperature Sensor Types - These types of temperature sensor use convection and radiation to monitor changes in temperature. They can be used to detect liquids and gases that emit radiant energy as heat rises and cold settles to the bottom in convection currents or detect the radiant energy being transmitted from an object in the form of infra-red radiation (the sun). The two basic types of contact or even non-contact temperature sensors can also be sub-divided into the following three groups of sensors, Electromechanical, Resistive and Electronic.
35 Yrd. Doç. Dr. Aytaç Gören Sensors Sensor operation small prism-shaped sample of single-crystal undoped GaAs attached to ends of two optical fibers light energy absorbed by the GaAs crystal depends on temperature percentage of received vs. transmitted energy is a function of temperature Can be made small enough for biological implantation
36 Yrd. Doç. Dr. Aytaç Gören Thermal Sensors The Thermostat The Thermostat is a contact type electro-mechanical temperature sensor or switch, that basically consists of two different metals such as nickel, copper, tungsten or aluminium etc, that are bonded together to form a Bi-metallic strip. The different linear expansion rates of the two dissimilar metals produces a mechanical bending movement when the strip is subjected to heat. The bi-metallic strip is used as a switch in the thermostat and are used extensively to control hot water heating elements in boilers, furnaces, hot water storage tanks as well as in vehicle radiator cooling systems. The Bi-metallic Thermostat The thermostat consists of two thermally different metals stuck together back to back. When it is cold the contacts are closed and current passes through the thermostat. When it gets hot, one metal expands more than the other and the bonded bimetallic strip bends up (or down) opening the contacts preventing the current from flowing.
37 Yrd. Doç. Dr. Aytaç Gören Thermal Sensors There are two main types of bi-metallic strips based mainly upon their movement when subjected to temperature changes, "snap-action" types that produce an instantaneous "ON/OFF" or "OFF/ON" type action on the electrical contacts and the slower "creep-action" types that gradually change their position as the temperature changes. Snap-action thermostats are commonly used in homes for controlling the temperature of ovens, irons, immersion hot water tanks and on walls to control the domestic heating system. Creeper types generally consist of a bi-metallic coil or spiral that slowly unwinds or coils-up as the temperature changes. Generally, creeper type bi-metallic strips are more sensitive to temperature changes than the standard snap ON/OFF types as the strip is longer and thinner making them ideal for use in temperature gauges and dials etc. One main disadvantage of the standard snap-action type thermostats when used as a temperature sensor, is that they have a large hysteresis range from when the electrical contacts open until when they close for example, set to 20 o C but may not open until 22 o C or close again until 18 o C. So the range of temperature swing can be quite high. Commercially available bi-metallic thermostats for home use do have temperature adjustment screws that allow for a desired set-point and even its hysteresis level to be pre-set and are available over a wide operating range. The Thermostat
38 Thermal Sensors The Thermistor The Thermistor is another type of temperature sensor, whose name is a combination of the words THERM-ally sensitive res-istor. A thermistor is a type of resistor which changes its physical resistance with changes in temperature. Thermistors are generally made from ceramic type semiconductor materials such as oxides of nickel, manganese or cobalt coated in glass which makes them easily damaged. Most types of thermistor's have a Negative Temperature Coefficient of resistance or (NTC), that is their resistance value goes DOWN with an increase in the temperature but some with a Positive Temperature Coefficient, (PTC), their resistance value goes UP with an increase in temperature are also available. Their main advantage is their speed of response to any changes in temperature, accuracy and repeatability. Thermistors are passive resistive devices which means we need to pass a current through it to produce a measurable voltage output. Then thermistors are generally connected in series with a suitable biasing resistor to form a potential divider network and the choice of resistor gives a voltage output at some pre-determined temperature point or value. Yrd. Doç. Dr. Aytaç Gören
39 Thermal Sensors The Thermistor Yrd. Doç. Dr. Aytaç Gören The following thermistor has a resistance value of 10KΩ at 25 o C and a resistance value of 100Ω at 100 o C. Calculate the voltage drop across the thermistor and hence its output voltage (Vout) for both temperatures when connected in series with a 1kΩ resistor across a 12v power supply. At 25 o C At 100 o C by changing the fixed resistor value of R2 (in our example 1kΩ) to a potentiometer or preset, a voltage output can be obtained at a predetermined temperature set point for example, 5v output at 60 o C and by varying the potentiometer a particular output voltage level can be obtained over a wider temperature range.
40 Thermal Sensors Resistive Temperature Detectors (RTD) Yrd. Doç. Dr. Aytaç Gören Another type of electrical resistance temperature sensor is the Resistance Temperature Detector or RTD. RTD's are precision temperature sensors made from high-purity conducting metals such as platinum, copper or nickel wound into a coil and whose electrical resistance changes as a function of temperature, similar to that of the thermistor. Also available are thin-film RTD's. These devices have a thin film of platinum paste is deposited onto a white ceramic substrate. RTD Resistive temperature detectors have positive temperature coefficients (PTC) but unlike the thermistor their output is extremely linear producing very accurate measurements of temperature. However, they have poor sensitivity, that is a change in temperature only produces a very small output change for example, 1Ω/ o C. The more common types of RTD's are made from platinum and are called Platinum Resistance Thermometer or PRT's with the most commonly available of them all the Pt100 sensor, which has a standard resistance value of 100Ω at 0 o C. However, Platinum is expensive and one of the main disadvantages of this type of device is its cost.
41 Thermal Sensors Resistive Temperature Detectors (RTD) Like the thermistor, RTD's are passive resistive devices and by passing a constant current through the temperature sensor it is possible to obtain an output voltage that increases linearly with temperature. A typical RTD has a base resistance of about 100Ω at 0 o C, increasing to about 140Ω at 100 o C with an operating temperature range of between -200 to +600 o C. Wire-wound Elements Thin Film Elements Coiled elements These elements work with temperatures to 660 C. These Elements works with temperatures to 300 C. These Elements works with temperatures to 850 C. Yrd. Doç. Dr. Aytaç Gören
42 Yrd. Doç. Dr. Aytaç Gören Thermal Sensors Thermocouples are thermoelectric sensors that basically consists of two junctions of dissimilar metals, such as copper and constantan that are welded or crimped together. One junction is kept at a constant temperature called the reference (Cold) junction, while the other the measuring (Hot) junction. When the two junctions are at different temperatures, a voltage is developed across the junction which is used to measure the temperature sensor. Thermocouples The Thermocouple is by far the most commonly used type of all the temperature sensing devices due to its simplicity, ease of use and their speed of response to changes in temperature, due mainly to their small size. Thermocouples also have the widest temperature range of all the temperature sensors from below -200 o C to well over 2000 o C. /thmcple_theory.cfm
43 Thermal Sensors Thermocouples Yrd. Doç. Dr. Aytaç Gören The principle of operation is that the junction of the two dissimilar metals such as copper and constantan, produces a "thermo-electric" effect that produces a constant potential difference of only a few millivolts (mv) between them. The voltage difference between the two junctions is called the "Seebeck effect" as a temperature gradient is generated along the conducting wires producing an emf. Then the output voltage from a thermocouple is a function of the temperature changes. If both the junctions are at the same temperature the potential difference across the two junctions is zero in other words, no voltage output as V 1 = V 2. However, when the junctions are connected within a circuit and are both at different temperatures a voltage output will be detected relative to the difference in temperature between the two junctions, V 1 - V 2. This difference in voltage will increase with temperature until the junctions peak voltage level is reached and this is determined by the characteristics of the two dissimilar metals used.
44 Yrd. Doç. Dr. Aytaç Gören Thermal Sensors Thermocouples Thermocouples can be made from a variety of different materials enabling extreme temperatures of between -200 o C to over o C to be measured. With such a large choice of materials and temperature range, internationally recognized standards have been developed complete with thermocouple color codes to allow the user to choose the correct thermocouple sensor for a particular application.
45 Thermal Sensors Thermocouples Yrd. Doç. Dr. Aytaç Gören The three most common thermocouple materials used above for general temperature measurement are Iron-Constantan (Type J), Copper-Constantan (Type T), and Nickel-Chromium (Type K). The output voltage from a thermocouple is very small, only a few millivolts (mv) for a 10 o C change in temperature difference and because of this small voltage output some form of amplification is generally required. The type of amplifier, either discrete or in the form of an Operational Amplifier needs to be carefully selected, because good drift stability is required to prevent recalibration of the thermocouple at frequent intervals. This makes the chopper and instrumentation type of amplifier preferable for most temperature sensing applications.
46 Thermal Cameras Yrd. Doç. Dr. Aytaç Gören
47 Yrd. Doç. Dr. Aytaç Gören Flow, Fluid Velocity Sensors A flowmeter is an instrument used to measure linear, nonlinear, mass or volumetric flow rate of a liquid or a gas.
48 Yrd. Doç. Dr. Aytaç Gören Flow, Fluid Velocity Sensors Types of Flowmeters Turbine Differential Pressure Coriolis Mass Ultrasonic Electromagnetic Thermal
49 Flow, Fluid Velocity Sensors Turbine flowmeters Yrd. Doç. Dr. Aytaç Gören Turbine flowmeters use the mechanical energy of the fluid to rotate a pinwheel (rotor) in the flow stream. Blades on the rotor are angled to transform energy from the flow stream into rotational energy. The rotor shaft spins on bearings. When the fluid moves faster, the rotor spins proportionally faster. Ref: reference 14
50 Flow, Fluid Velocity Sensors Differential Pressure Yrd. Doç. Dr. Aytaç Gören Differential pressure flowmeters use Bernoulli's equation to measure the flow of fluid in a pipe. Differential pressure flowmeters introduce a constriction in the pipe that creates a pressure drop across the flowmeter.
51 Flow, Fluid Velocity Sensors Coriolis mass flowmeters Yrd. Doç. Dr. Aytaç Gören Coriolis mass flowmeters measure the force resulting from the acceleration caused by mass moving toward (or away from) a center of rotation
52 Flow, Fluid Velocity Sensors Ultrasonic flowmeters Yrd. Doç. Dr. Aytaç Gören Ultrasonic flowmeters use sound waves to determine the velocity of a fluid flowing in a pipe with uses Doppler Effect. When the fluid moves faster, the frequency shift increases linearly. The transmitter processes signals from the transmitted wave and its reflections to determine the flow rate.
53 Flow, Fluid Velocity Sensors Magnetic flowmeters Yrd. Doç. Dr. Aytaç Gören Magnetic flowmeters use Faraday s Law of Electromagnetic Induction to determine the flow of liquid in a pipe.
54 Flow, Fluid Velocity Sensors Thermal flowmeters Yrd. Doç. Dr. Aytaç Gören Thermal flowmeters use the thermal properties of the fluid to measure the flow of a fluid flowing in a pipe or duct.
55 Global Positioning System (GPS) Yrd. Doç. Dr. Aytaç Gören The Global Positioning System (GPS) is a spacebased satellite navigation system that provides location and time information in all weather, anywhere on or near the Earth, where there is an unobstructed line of sight to four or more GPS satellites. A GPS receiver calculates its position by precisely timing the signals sent by GPS satellites high above the Earth. Each satellite continually transmits messages that include -the time the message was transmitted -satellite position at time of message transmission The receiver uses the messages it receives to determine the transit time of each message and computes the distance to each satellite. These distances along with the satellites' locations are used with the possible aid of trilateration, depending on which algorithm is used, to compute the position of the receiver. This position is then displayed, perhaps with a moving map display or latitude and longitude; elevation information may be included.
56 Global Positioning System (GPS) Yrd. Doç. Dr. Aytaç Gören All satellites broadcast at the same two frequencies, GHz (L1 signal) and GHz (L2 signal). The satellite network uses a CDMA spread-spectrum technique where the low-bitrate message data is encoded with a high-rate pseudorandom (PRN) sequence that is different for each satellite. The receiver must be aware of the PRN codes for each satellite to reconstruct the actual message data. The C/A code, for civilian use, transmits data at million chips per second, whereas the P code, for U.S. military use, transmits at million chips per second. Subframes Description Satellite clock, GPS time relationship Ephemeris (precise satellite orbit) Almanac component (satellite network synopsis, error correction) GPS message format
57 Global Positioning System (GPS) Yrd. Doç. Dr. Aytaç Gören The receiver uses messages received from satellites to determine the satellite positions and time sent. The x, y, and z components of satellite position and the time sent are designated as [x i, y i, z i, t i ] where the subscript i denotes the satellite and has the value 1, 2,..., n, where Knowing when the message was received, the receiver computes the message's transit time as Note that the receiver indeed knows the reception time indicated by its on-board clock, rather than. Assuming the message traveled at the speed of light (c) the distance traveled is (t r t i )c. Knowing the distance from receiver to satellite and the satellite's position implies that the receiver is on the surface of a sphere centered at the satellite's position. Thus the receiver is at or near the intersection of the surfaces of the spheres. In the ideal case of no errors, the receiver is at the intersection of the surfaces of the spheres. Let b denote the clock error or bias, the amount that the receiver's clock is off. The receiver has four unknowns, the three components of GPS receiver position and the clock bias [x, y, z, b]. The equations of the sphere surfaces are given by:
58 Global Positioning System (GPS) Yrd. Doç. Dr. Aytaç Gören or in terms of pseudoranges,, as These equations can be solved by algebraic or numerical methods.
59 Yrd. Doç. Dr. Aytaç Gören Weather, Moisture, Humidity Sensors A hygrometer is an instrument used for measuring the moisture content in the environmental air, or humidity. Most measurement devices usually rely on measurements of some other quantity such as temperature, pressure, mass or a mechanical or electrical change in a substance as moisture is absorbed. From calculations based on physical principles, or especially by calibration with a reference standard, these measured quantities can lead to a measurement of humidity. Modern electronic devices use temperature of condensation, or changes in electrical capacitance or resistance to measure humidity changes.
60 Yrd. Doç. Dr. Aytaç Gören Weather, Moisture, Humidity Sensors Energy consumption: 80uW (at 12bit, 3V, 1 measurement / s) RH operating range: 0 100% RH T operating range: C ( F) RH response time: 8 sec (tau63%) Output: digital (2-wire interface) Maximal accuracy limits for relative humdity and temperature:
61 Yrd. Doç. Dr. Aytaç Gören Pyranometer A pyranometer is a type of actinometer used to measure broadband solar irradiance on a planar surface and is a sensor that is designed to measure the solar radiation flux density (in watts per metre square) from a field of view of 180 degrees. A typical pyranometer does not require any power to operate.
62 Yrd. Doç. Dr. Aytaç Gören Pyranometer The solar radiation spectrum extends approximately from 300 to 2,800 nm. Pyranometers usually cover that spectrum with a spectral sensitivity that is as flat as possible. To make a measurement of irradiance, it is required by definition that the response to beam radiation varies with the cosine of the angle of incidence, so that there will be a full response when the solar radiation hits the sensor perpendicularly (normal to the surface, sun at zenith, 0 degrees angle of incidence), zero response when the sun is at the horizon (90 degrees angle of incidence, 90 degrees zenith angle), and 0.5 at 60 degrees angle of incidence. It follows that a pyranometer should have a so-called directional response or cosine response that is close to the ideal cosine characteristic. (1) sensor, (2, 3) glass domes, (5) cable, standard length 5 m, (9) desiccant.
63 Yrd. Doç. Dr. Aytaç Gören Pressure Sensor A pressure sensor measures pressure, typically of gases or liquids. Pressure is an expression of the force required to stop a fluid from expanding, and is usually stated in terms of force per unit area. A pressure sensor usually acts as a transducer; it generates a signal as a function of the pressure imposed. For the purposes of this article, such a signal is electrical. Diaphragm (Upper electrode) Lower electrode Types Absolute pressure sensor This sensor measures the pressure relative to perfect vacuum. Gauge pressure sensor This sensor measures the pressure relative to atmospheric pressure. A tire pressure gauge is an example of gauge pressure measurement; when it indicates zero, then the pressure it is measuring is the same as the ambient pressure.
64 Yrd. Doç. Dr. Aytaç Gören Pressure Sensor Vacuum pressure sensor This term can cause confusion. It may be used to describe a sensor that measures pressures below atmospheric pressure, showing the difference between that low pressure and atmospheric pressure (i.e. negative gauge pressure), but it may also be used to describe a sensor that measures low pressure relative to perfect vacuum (i.e. absolute pressure). Differential pressure sensor This sensor measures the difference between two pressures, one connected to each side of the sensor. Differential pressure sensors are used to measure many properties, such as pressure drops across oil filters or air filters, fluid levels (by comparing the pressure above and below the liquid) or flow rates (by measuring the change in pressure across a restriction). Technically speaking, most pressure sensors are really differential pressure sensors; for example a gauge pressure sensor is merely a differential pressure sensor in which one side is open to the ambient atmosphere. Sealed pressure sensor This sensor is similar to a gauge pressure sensor except that it measures pressure relative to some fixed pressure rather than the ambient atmospheric pressure (which varies according to the location and the weather).
65 Yrd. Doç. Dr. Aytaç Gören Pressure Sensor Pressure-sensing technology There are two basic categories of analog pressure sensors. Force collector types These types of electronic pressure sensors generally use a force collector (such a diaphragm, piston, bourdon tube, or bellows) to measure strain (or deflection) due to applied force (pressure) over an area. Piezoresistive strain gauge Uses the piezoresistive effect of bonded or formed strain gauges to detect strain due to applied pressure. Common technology types are Silicon (Monocrystalline), Polysilicon Thin Film, Bonded Metal Foil, Thick Film, and Sputtered Thin Film. Generally, the strain gauges are connected to form a Wheatstone bridge circuit to maximize the output of the sensor. This is the most commonly employed sensing technology for general purpose pressure measurement. Generally, these technologies are suited to measure absolute, gauge, vacuum, and differential pressures. Capacitive Uses a diaphragm and pressure cavity to create a variable capacitor to detect strain due to applied pressure. Common technologies use metal, ceramic, and silicon diaphragms. Generally, these technologies are most applied to low pressures (Absolute, Differential and Gauge)
66 Yrd. Doç. Dr. Aytaç Gören Pressure Sensor Electromagnetic Measures the displacement of a diaphragm by means of changes in inductance (reluctance), LVDT, Hall Effect, or by eddy current principle. Piezoelectric Uses the piezoelectric effect in certain materials such as quartz to measure the strain upon the sensing mechanism due to pressure. This technology is commonly employed for the measurement of highly dynamic pressures. Optical Techniques include the use of the physical change of an optical fiber to detect strain due to applied pressure. A common example of this type utilizes Fiber Bragg Gratings. This technology is employed in challenging applications where the measurement may be highly remote, under high temperature, or may benefit from technologies inherently immune to electromagnetic interference. Another analogous technique utilizes an elastic film constructed in layers that can change reflected wavelengths according to the applied pressure (strain). . Potentiometric Uses the motion of a wiper along a resistive mechanism to detect the strain caused by applied pressure.
67 Yrd. Doç. Dr. Aytaç Gören Pressure Sensor Where they are used Pressure sensing This is where the measurement of interest is pressure, expressed as a force per unit area Altitude sensing This is useful in aircraft, rockets, satellites, weather balloons, and many other applications. All these applications make use of the relationship between changes in pressure relative to the altitude. p = ( h) ; p[pa], h[m] Flow sensing This is the use of pressure sensors in conjunction with the venturi effect to measure flow. Differential pressure is measured between two segments of a venturi tube that have a different aperture. The pressure difference between the two segments is directly proportional to the flow rate through the venturi tube. A low pressure sensor is almost always required as the pressure difference is relatively small. P = pressure, ρ = density of the fluid, g = standard gravity, h = height of fluid column above pressure sensor Level / depth sensing A pressure sensor may also be used to calculate the level of a fluid. This technique is commonly employed to measure the depth of a submerged body (such as a diver or submarine), or level of contents in a tank (such as in a water tower). For most practical purposes, fluid level is directly proportional to pressure
68 Yrd. Doç. Dr. Aytaç Gören Pressure Sensor Where they are used Leak testing A pressure sensor may be used to sense the decay of pressure due to a system leak. Ratiometric Correction of Transducer Output Piezoresistive transducers configured as Wheatstone bridges often exhibit ratiometric behavior with respect not only to the measured pressure, but also the transducer supply voltage. where: is the output voltage of the transducer. is the actual measured pressure. is the nominal transducer scale factor (given an ideal transducer supply voltage) in units of voltage per pressure. is the actual transducer supply voltage. is the ideal transducer supply voltage. Correcting measurements from transducers exhibiting this behavior requires measuring the actual transducer supply voltage as well as the output voltage and applying the inverse transform of this behavior to the output signal:
69 Yrd. Doç. Dr. Aytaç Gören Electric Current, Electric Potential Hall effect sensor A Hall effect sensor is a transducer that varies its output voltage in response to a magnetic field. Hall effect sensors are used for proximity switching, positioning, speed detection, and current sensing applications. In its simplest form, the sensor operates as an analogue transducer, directly returning a voltage. With a known magnetic field, its distance from the Hall plate can be determined. Using groups of sensors, the relative position of the magnet can be deduced. Electricity carried through a conductor will produce a magnetic field that varies with current, and a Hall sensor can be used to measure the current without interrupting the circuit. Typically, the sensor is integrated with a wound core or permanent magnet that surrounds the conductor to be measured. Frequently, a Hall sensor is combined with circuitry that allows the device to act in a digital (on/off) mode, and may be called a switch in this configuration. Commonly seen in industrial applications such as the pictured pneumatic cylinder, they are also used in consumer equipment; for example some computer printers use them to detect missing paper and open covers. When high reliability is required, they are used in keyboards.
70 Yrd. Doç. Dr. Aytaç Gören Electric Current, Electric Potential The Hall effect comes about due to the nature of the current in a conductor. For a simple metal where there is only one type of charge carrier (electrons) the Hall voltage V H is given by Hall effect sensor where I is the current across the plate length, B is the magnetic field, d is the depth (thickness) of the plate, e is the electron charge, and n is the charge carrier density of the carrier electrons. The Hall coefficient is defined as where j is the current density of the carrier electrons, and the induced electric field. In SI units, this becomes is
71 Yrd. Doç. Dr. Aytaç Gören Magnetic Sensors Active (emitting) Metal detectors Follows metallic strips on or under the floor Magnetometer Magnetic Resonance Imaging (MRI) Magnetic Sensor Passive (sensors only) Compass Magnetic field sensor ( oscillating current) Wheatstone bridge configuration that converts magnetic fields into a millivolt output. These wheatstone bridges are passive components that don t emit any fields or broadband noise. Resolution: The magnetic sensors feature very low noise floors for their size. Typical resolution ranges from 27 to 120 microgauss (for HMC). *reference 15
72 Yrd. Doç. Dr. Aytaç Gören Magnetic Sensors MRI Magnetic resonance imaging (MRI), nuclear magnetic resonance imaging (NMRI), or magnetic resonance tomography (MRT) is a medical imaging technique used in radiology to visualize detailed internal structures. MRI makes use of the property of nuclear magnetic resonance (NMR) to image nuclei of atoms inside the body. An MRI machine uses a powerful magnetic field to align the magnetization of some atomic nuclei in the body, and radio frequency fields to systematically alter the alignment of this magnetization. This causes the nuclei to produce a rotating magnetic field detectable by the scanner and this information is recorded to construct an image of the scanned area of the body. Magnetic field gradients cause nuclei at different locations to rotate at different speeds. By using gradients in different directions 2D images or 3D volumes can be obtained in any arbitrary orientation.
73 Yrd. Doç. Dr. Aytaç Gören Magnetic and Radio Sensors MRI
74 Thanks, Assist. Prof. Dr. Aytaç Gören. References for this week 1. Practical Applications of Statistical Methods in the Clinical Laboratory, Roger L. Bertholf, Ph.D., DABCC, Mechatronics Principles and Applications, Godfrey C. Onwubolu 4. THE MECHATRONICS H A N D B O O K, E d i t o r - i n - C h i e f, Robert H. Bishop Sensors, Prof. A. Mason 10. Electrical and Electronic Systems Pearson Educaton Ltd Design Realization lecture 18, John Canny. 13. Wikipedia Yrd. Doç. Dr. Aytaç Gören
SENSORS a.k.a. Interfacing to the Real World: Review of Electrical Sensors and Actuators Andrew Mason Associtate Professor, ECE Teach: Microelectronics (analog & digital integrated Circ., VLSI) Biomedical
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