Transcript SENSORS
Physical Principles of Sensing SOPHOMORE CLINIC I FALL 2004 1 Definition A sensor is a device that receives a stimulus and responds with an electrical signal. Fig 1.1 Level control system. A sight tube and the operator’s eye form a sensor. FALL 2004 SOPHOMORE CLINIC 2 What are some quantities that can be sensed? Motion, position, displacement Velocity and acceleration Force, strain Pressure Flow Sound Moisture Light Radiation Temperature Chemical presence These quantities are the stimulus. FALL 2004 SOPHOMORE CLINIC 3 The Response is an Electrical Signal When we say electrical we mean a signal which can be channeled, amplified and modified by electronic devices: • Voltage • Current • Charge FALL 2004 SOPHOMORE CLINIC The voltage, current or charge may be describe by: • • • • Amplitude Frequency Phase Digital code 4 Any sensor is an energy converter This conversion can be direct or it may require transducers. Fig 1.2 Example: • A chemical sensor may have a part which converts the energy of a chemical reaction into heat (transducer) and another part, a thermopile, which converts heat into an electrical signal. FALL 2004 SOPHOMORE CLINIC 5 Physical Principles of Sensing Charges, fields & potentials Capacitance Magnetism Induction Resistance Piezoelectric effect FALL 2004 SOPHOMORE CLINIC Seebeck and Peltier effects Thermal properties of materials Heat transfer Light 6 Types of Sensor Direct • A sensor that can convert a nonelectrical stimulus into an electrical signal with intermediate stages. Thermocouple (temperature to voltage) Indirect • A sensor that multiple conversion steps to transform the measured signal into an electrical signal. A fiber-optic displacement sensor: Current photons current FALL 2004 SOPHOMORE CLINIC 7 Today’s Topic: Physical Principles that are Used to Effect a Direct Conversion of Stimuli into Electrical Signals FALL 2004 SOPHOMORE CLINIC 8 Electric Charges, Fields, & Potentials Any charged object is subject to a force when in the region of an electric field. • A field can be used to detect the presence of charge or the opposite can be true and the force on a charge determined to detect a field. f E q0 E FALL 2004 SOPHOMORE CLINIC q 4 0 r 2 9 Other Geometries E 2 0 r =charge/unit length FALL 2004 E 2 0 The field is strongest at areas of highest curvature =charge/unit area SOPHOMORE CLINIC 10 Electric Dipole Dipoles are found in crystalline materials and form a foundation for piezoelectric and pyroelectric detectors. • The dipole is a combination of 2 opposite charges placed 2a apart. The electric field is the vector sum to the two fields. E qa p 4 0 r 3 8 0 r 3 p represents the dipole moment In the presence of an E field the dipole will develop a torque pE FALL 2004 SOPHOMORE CLINIC 11 Capacitance Two isolated conductive objects of arbitrary shape which can hold an electric charge is called a capacitor. • An E field is developed between the two conductors. q 0 A C V d FALL 2004 SOPHOMORE CLINIC 12 Capacitor as Displacement Sensor If the inner conductor can be moved in and out, the measured capacitance will be a function of l. 2 0l C ln b / a FALL 2004 SOPHOMORE CLINIC 13 Dielectric Constant The material between the plates of the capacitor can also be used to sense changes in the environment. • When vacuum (or air) is replaced by another material, the capacitance increases by a factor of , known as the dielectric constant of the material • The increase in C is due to the polarization of the molecules of the material used as an insulator. q 0 A C V d FALL 2004 SOPHOMORE CLINIC 14 Example – A Water Level Sensor The total capacitance of the coaxial sensor shown below is the capacitance of the water-free portion plus the capacitance of the water-filled portion. As the level of the water changes, the total capacitance changes. Ch C free C filled Ch FALL 2004 SOPHOMORE CLINIC 2 0 H h1 lnb / a 15 Magnetism There are two methods of generating a magnetic field: • Permanent magnets (magnetic materials). • The magnetic field generated by a current. Force is generated on a test magnet in the field of magnetic materials. A compass needle will respond to the magnetic field generated by a current. Magnetic field, B “flux” is the field density, B FALL 2004 SOPHOMORE CLINIC 16 Sources of Magnetic Field Electric current sets a circular magnetic field around a conductor. Moving electron sets a field, superposition of field vectors results in a combined magnetic field of a permanent magnet. Magnets are useful for fabricating magnetic sensors for the detection of motion, displacement, and position. FALL 2004 SOPHOMORE CLINIC 17 Induction A phenomenon related to magnetism is induction, the generation of voltage from a changing magnetic field. • If the coil has no magnetic core, the flux is proportional to current and the voltage proportional to di/dt. v FALL 2004 SOPHOMORE CLINIC d n B dLi di L dt dt dt 18 Resistance If we apply a battery across two points of a piece of material, an E field will be set up where E=V/l V i E V 1 Va j l i / a li Va l l R il a a R The tendency of the material to resist the flow of electrons is called its resistivity, , and we say that the material has a particular electrical resistance, R. FALL 2004 SOPHOMORE CLINIC 19 Sensitivity of Resistance To Temperature: Specific resistivity of tungsten as a function of temperature. 0 1 t t0 is the temperature coefficient of resistivity. FALL 2004 SOPHOMORE CLINIC 20 Application for Temperature Indication using a Laminate of Materials with Two Different ’s. FALL 2004 SOPHOMORE CLINIC 21 To Strain: Strain changes the geometry of a conductor and its resistance. F dl E Ee a l Stress = Young’s Modulus x strain (1) R (2) dR Se l dl v (3) dR S e (4) dR R R1 See 2 R R1 v l2 v ldl S e l l dl dl dl S e R S e a l a l l Since length is changing the factor of 2 in the second equation becomes a variable which depends on the material. FALL 2004 SOPHOMORE CLINIC 22 To Moisture: For the hygristor, the resistance of the polymer changes with the absorption of water molecules. FALL 2004 SOPHOMORE CLINIC 23 The Piezoelectric Effect The piezoelectric effect is the generation of electric charge by a crystalline material upon subjecting it to stress. FALL 2004 SOPHOMORE CLINIC 24 Piezoelectric Sensor Because a crystal with deposited electrodes forms a capacitor the voltage developed can be expressed as: Qx d x V Fx C C Piezoelectric crystals are direct converters of mechanical energy into electrical energy. Where dx is the piezoelectric coefficient in the x direction and Fx is the applied force in the x direction. Laminated 2-layer piezoelectric sensor FALL 2004 SOPHOMORE CLINIC 25 Pyroelectric Effect dPs PQ dT PQ is the pyroelectric charge coefficient and Ps is related to the charge developed on the electrodes when the sensor is subjected to heat flow. If the sensor has the “capacitor” form: Pyroelectric materials are crystals capable of generating an electrical charge in response to heat flow. Q PQ AT Q r 0 A V h then C PQ AT Q PQ AT h V PQ T C C r 0 A / h r 0 FALL 2004 SOPHOMORE CLINIC 26 The material loses its usefulness at the Curie Temp – the point at which polarization disappears. The electric charge reaches its peak nearly instantaneously and then decays with a thermal time constant, T FALL 2004 SOPHOMORE CLINIC 27 Seebeck and Peltier Effects The Seebeck effect is a direct conversion of thermal energy into electric energy. The varying temperature along the bar is a source of electromotive force (voltage) and current will flow. This is the principle behind the thermocouple. FALL 2004 SOPHOMORE CLINIC 28 Thermoelectric Loops VAB A B T If a loop of conductor has points at 2 different temperatures, current flows. But if there is a single conductor no measurable net current flows. If a loop of conductor has points at 2 different temperatures, again current flows. If the loop is composed of 2 different conductors, measurable net current flows due to a difference in the Seebeck coefficients. A and B are the Seebeck coefficients FALL 2004 SOPHOMORE CLINIC 29 Output Voltage from Standard Thermocouples FALL 2004 SOPHOMORE CLINIC 30 TYPES – MATERIALS – TEMP RANGES Thermocouple Type Names of Materials Useful Application Range B C E J K N R S T Platinum30% Rhodium (+) Platinum 6% Rhodium (-) 2500 -3100F 1370-1700C W5Re Tungsten 5% Rhenium (+) W26Re Tungsten 26% Rhenium (-) 3000-4200F 1650-2315C Chromel (+) Constantan (-) 200-1650F 95-900C Iron (+) Constantan (-) 200-1400F 95-760C Chromel (+) Alumel (-) 200-2300F 95-1260C Nicrosil (+) Nisil (-) 1200-2300F 650-1260C Platinum 13% Rhodium (+) Platinum (-) 1600-2640F 870-1450C Platinum 10% Rhodium (+) Platinum (-) 1800-2640F 980-1450C Copper (+) Constantan (-) -330-660F -200-350C FALL 2004 SOPHOMORE CLINIC 31 The Peltier Effect The Peltier effect concerns the reversible absorption of heat which usually takes place when an electric current crosses a junction between 2 dissimilar metals. It can produce heat or cold depending on the direction of electric current through the junction. FALL 2004 SOPHOMORE CLINIC 32 Light (Electromagnetic Radiation) E h h c E is the energy of the radiation c = c x 108 m/s h = 6.63 x 10-23 J-s is the wavelength of the radiation UV and visible photons have relatively high energy levels and are easily detected. In the far IR the energies become very small and thermal detectors are used. FALL 2004 SOPHOMORE CLINIC 33 Conclusions A sensor is a device that receives a stimulus and responds with an electrical signal. The final stage of any sensor is dependent upon the electrical properties of the sensor materials. The materials introduced today are used in the design and fabrication of many different types of sensors. FALL 2004 SOPHOMORE CLINIC 34