Electronic Instrumentation

Transcription

1 Chapter 4 Electronic Sensors for Industrial Measurements 1

2 Chapter 4. Electronic Sensors For Industrial Measurements Introduction Position, Displacement and Level Strain and force Velocity and Acceleration Temperature Sensors 2

3 Introduction In this chapter we will focus on the measurement of magnitudes of interest in the industrial/aeronautical environment. Classical electronic sensors will be addressed although some optoelectronic and advanced sensors will be described. IMPORTANT: It is impossibleto cover ALL the sensors and instrumentation systems currently being used. This is an open, high-speed evolving field and the enumeration that follows isn t by any means complete. 3

4 Position, Displacement and Level By positionwe mean the determination of the object s coordinates (linear or angular) with respect to a selected reference. Displacement involves movement from one position to another (the original position of the object acts as the reference). Level is used when a liquid is involved. Position/displacement/level sensors are often part of more complex instrumentation systems as other physical magnitudes can be derived from this measurement (pressure, velocity,.) Types of sensors (some of them): Potentiometric Sensors Capacitive Sensors Inductive and Magnetic Sensors Ultrasonic Sensors Optical/Optoelectronic Sensors 4

5 Position, Displacement and Level Potentiometric Sensors: Based on the linear dependence between total impedance and conductor length. Several technologies: wire bound, conductive plastic, mixed, Both for linear and angular displacement measurements. R 0 l L 0 5

6 Position, Displacement and Level Potentiometric Sensors. Examples : Angular and rotary Sensor: Serial Linear Translation Sensor: Serial

7 Position, Displacement and Level Capacity Sensors: Based on the dependence on Area, distance and dielectric constant of a parallel-plate capacitor. Different sensors architectures can be envisaged based on changes in the distance between plates, active area and dielectric constant between plates. A d ε 7

8 Position, Displacement and Level Capacity Sensors. Examples: Proximity Sensor: E2K-C Linear Position Sensor: D-510 8

9 Position, Displacement and Level Inductive and Magnetic Sensors: Based on the use of magnetic fields and the related currents and voltages induced, with the many advantages associated to the fact that magnetic field can penetrate non-magnetic materials with almost no losses. Several types of sensors: LVDT (Linear Variable Differential Transformer) Linear position Inductive Sensors Transverse Inductive Sensors Hall Effect sensors Other 9

10 Position, Displacement and Level Linear Variable Differential Transformer (LVDT) Based on the use of one static transformers and two secondary windings connected in series opposition so the two induced voltages are opposite in sign. 10

11 Position, Displacement and Level Linear position inductive sensors Based on the change of the inductance of a coil with a mobile core attached to the target. 11

12 Position, Displacement and Level Transverse inductive sensors Based on the change of the inductance of a coil in the presence of a ferromagnetic material (target), that crosses the magnetic field lines. 12

13 Position, Displacement and Level Hall Effect sensors Based on the creation of a transverse hall potential difference in a conductor where a dc current is applied in the presence of a magnetic field. r I B r M.A. Perez et al. Instrumentación Electrónica. Thomson 13

14 Position, Displacement and Level Inductive and Magnetic Sensors. Examples: LVDT: Solartron Technologies Linear position Inductive Sensor: HBM Transverse Inductive Sensor: Pepper+Fulch Hall Effect sensor: Honeywell 14

15 Position, Displacement and Level Ultrasonic Sensors: Based on the use of ultrasonic energy sent towards a target and reflected back. Range is obtained measuring the time difference between the sent pulse and the received signal. Such ultrasonic waves are mechanical acoustic waves covering the frequency range beyond the capabilities of human audition (usually 40kHz- 80kHz). 15

16 Position, Displacement and Level Ultrasonic Sensors. Examples Emitters and Receivers: murata 16

17 Position, Displacement and Level Optical/Optoelectronic Sensors: Optical Sensors are gaining presence due to their advantages as their simplicity, the absence of the loading effect and long operating distances. Usually require at least three essential components: a light source, a photodetector and light guidance devices. Several types of sensors: Optical Range finders Grating Sensors PSD s Other 17

18 Position, Displacement and Level Optical Range Finders: As the ultrasonic systems, range is obtained measuring the time difference between the sent pulse and the received signal. Pulsed and amplitude modulation systems are used. Transmitter Laser diode r APD Reception lens Target d 18

19 Position, Displacement and Level Grating Sensors: An optical displacement transducer can be fabricated with two overlapping gratings which serve as light-intensity modulator. They can be used as proximity sensors or position encoders. Photo-detector Light emitter 19

20 Position, Displacement and Level Position Sensitive Detectors (PSD): A PSD is a differential current device that gives an output related to the position of a collimated beam on the device surface. It provides one and twodimensional position. Laser/LED Diode D PSD a Target x = 0 x f Receptor Lens 20

21 Force and Strain Force is one of the fundamental quantities to be measured in mechanical, aeronautical and civil engineering. Also, whenever pressure is measured, it requires the measurement of force. There are several methods to measure force. Amongst the most important are: Force-to-displacement conversion (using a spring, for example) Force-to-strain conversion (measuring the deformation of an elastic element). In this sense, strain (unit deformation) measurements are a very common and useful in instrumentation not only for the importance of strain measurements themselves (e.g. structure deformation) but also because a lot of secondary magnitudes can be converted to deformations: specially force and torque. 21

22 Force and Strain Force Measurement Using Position Detectors: Force and pressure can be measured using a system like the one shown in the figure where a spring converts the applied force to a displacement (x). 22

23 Strain Gauges (I) Force and Strain The impedance of a metal wire changes when the material is mechanically deformed. This is called the piezoresistiveeffect and is the origin for a widely used sensors known as strain gauges. In these sensors, the unitary change in resistance is proportional to the elongation (strain) through a parameter known as gauge factor (K) 23

24 Strain Gauges (II) Force and Strain K typically ranges between 2 and 6 for Constatan(a copper/nickel alloy) gauges. For semiconductor strain gauges, K is bigger but their behavior with temperature is much worst. The Strain Gauges are designed to be affected only by elongations in the desired directions. In this sense there are many types of gauges. Example. 24

25 Force and Strain Strain Gauges (III): Signal Conditioning Force R1 R4 R2 R3 Rigid beam They are almost always mounted on a Wheatstone Bridge, taking advantage of the null signal conditioning circuit to cancel temperature variations. 25

26 Force and Strain Load Cells Load Cells consist of a mechanical arrangement that suffers a deformation when a force is applied. The deformation is sensed using strain gauges usually in half or full bridge configuration. They are the most common transducer for weight measurements as well as vibrations and dynamic tests on structures There are several types of load cells (double ended shear beam, single ended shear beam, single column, multi -column) and for a wide range of applications. Example. 26

27 Piezoelectric Sensors Force and Strain The Piezoelectric effect is the generation of electric charge by a crystalline material upon subjecting it to stress. The effect exist in natural crystals as quartz, but currently man-made ceramics (PZT) and polymers are used. Under any stress a charge proportional to the Force applied is generated that results in a voltage across the crystal due to its capacitance: These sensors are widely used for Tactile sensors of great applications in Robotics and other industrial and Aeronautical applications. 27

28 Velocity and Acceleration Although one may think that velocity and acceleration can be extracted from a position measurement, taking derivatives, specially in noisy environments, may result in high errors. For this reason, they are not derived by position measurements but special sensors. Acceleration (Accelerometers) are often related to vibration measurements and the use of a seismic mass. In this sense they are very important in shock analysis, structures characterization and other applications. 28

29 Velocity and Acceleration Accelerometer Types: Capacitive Accelerometers: The movement of the seismic mass is measured with a capacitive displacement transducer to detect the mass displacement with respect to the accelerometer housing (LIS2L02AL). PiezoresistiveAccelerometers: In this case strain gauges are responsible for measuring the strain associated to the mass displacement. They can be used in a broad frequency range. Piezoelectric Accelerometers: In this case the piezo-electric effect is responsible for the direct conversion of mechanical energy (strain) to voltage. They present good off-axis noise rejection, high linearity and wide operating temperature range. Thermal Accelerometers: The seismic mass is heated and the position calculated through the temperature distribution in the accelerometer housing. This principle can be integrated in an IC using gas as the seismic mass (MXD6125Q) 29

30 Gyroscopes Velocity and Acceleration They rely on the principle of conservation of angular momentum. Rotor Gyroscope: The classic system using a massive disk free to rotate about a spin axis. If the gyro platforms rotate around the input axis, the gyro develops a torque around a perpendicular axis that can be measured. Monolithic Silicon Gyroscopes: They are based on MEMS (Microelectromechanical systems) technology and the concept of vibrating gyro based on the Coriolisacceleration. Optical gyroscope: They are based on the sagnaceffect and are implemented both using fiber optics and free-space optics. 30

31 Temperature Sensors Temperature is one of the main magnitudes to be measured in industrial environments. Taking a temperature essentially requires the transmission of a small portion of the object s thermal energy to the sensor. In this sense, the influence of the measurement in the object s temperature is sometimes of relevance. Many physical and chemical phenomena are found to be functions of temperature, and thus many physical principles can be used to this measurement. Types of sensors (some of them): Thermoresistive sensors (RTD, Thermistors) Thermocouples Semiconductor PN Junction Sensors and IC s Optical Sensors (Pyrometers) Other 31

32 Temperature Sensors Resistance Temperature Detectors (RTD): Based on the temperature dependence of resistivity of all metals and alloys. Although virtually all metals can be employed, platinum is used almost exclusively: predictable response, long-term stability and durability. All RTD s have positive temperature coefficients. High accuracy and expensive sensors, they are standard used for most of the temperature range of industrial interest. 32

33 Temperature Sensors Resistance Temperature Detectors (RTD): According to the International Temperature Scale (ITS-90), precision temperature instruments should be calibrated at reproducible equilibrium states of some materials. From the value of the resistance at those points, the coefficients for the interpolation polynomial are calculated: Attention should be paid to the self-heating of the sensor!! Example: HEL

34 Thermistors: Temperature Sensors They are also thermoresistivesensors but fabricated with metal-oxide materials that behave like semiconductors. They usually present a negative temperature coefficient (NTC), although PTC (Positive temperature coefficient) thermistors are also available. Low-accuracy and low stability sensors, but low cost, they present an exponential-like dependence on temperature. 34

35 Thermistors: Temperature Sensors Same way as RTD, the parameter for the curve-fitting (in this case an exponential) is done using reproducible equilibrium states of some materials. Attention should also be paid to the self-heating of the sensor!! Example: T67-NTC 35

36 Temperature Sensors Thermocouples (I) They consist of a junction, often spot welded, between two dissimilar metal wires. The different thermoelectric properties of such metals produce a EMF (electromagnetic force) when two junctions are kept at different temperatures (Seebeckeffect). Thermoelectric voltage Thermocouple measurement Thermocouple I=(V1-V2)/R,, R circuit resistance Thermocouple measurement V = Vm Vr if T3 = T4 V=S (Tm-Tr) 36

37 Temperature Sensors Thermocouples (II): Types Materials Range (ºC) Sensitivity (µv/ºc) Type (ANSI) Pt(6%) / Rh Pt (30%) / Rh B W (5%) / Re W (26%) / Re C Chromel Constantan E Iron - Constantan E Chromel Alumel K Pt(13%) / Rh Pt R Pt(10%) / Rh Pt S Cu Constantan T Materials: Platinum (Pt), Rhodium (Rh), Rhenium (Re), Tungsten (W), Chromel (Ni-Cr), Alumel, Constantan 37

38 Temperature Sensors Thermocouples (III): Cold Junction Compensation V(out) = Vm Vr + V(comp) = S1 (Tm Tr ) + S2 Tr 38

39 Temperature Sensors Semiconductor PN junction Sensors (I) Based on the dependence of the semiconductor band-gap voltage with temperature. Easy integration sensors, they can be embedded in any electronic circuits. Origin for ICs temperature Sensors. 39

40 Temperature Sensors Semiconductor PN junction Sensors (II) In this case I S1 is different to I S2 due to the different size of the TRTs 40

41 Temperature Sensors IC Temperature Sensors Schemes like the one described are usually integrated in monolithic Integrated Circuits (IC) for low-cost temperature sensors. They are also used for cold-junction compensation in thermocouple-based temperature sensors Example: LM335 41

42 Temperature Sensors Pyrometers These instruments provide a no-touch means of estimating surface temperatures Based on blackbody radiation Laws: As the temperature of the Blackbody increases, the peak of maximum spectral emittance shifts systematically to shorter wavelengths. As the temperature increases, the area under the spectral emittance curves increases Wien s displacement Law Stefan-Boltzmann Equation These results have to be corrected as practical surfaces have non-unity emissivity (they are function of λ), but after correction the peak for the emission curves follows quite nicely Wien s law 42

43 Summary In this chapter we have described some of the most important sensors for magnitudes of relevance in the industrial/aeronautical fields. This brief revision has allow us to realize the magnitude of this open, high-speed evolving field. The selection of the proper sensor depends heavily on the application and other design constrains (budget, environmental conditions). Time should be spent on choosing the best transducer for our applications. That will probably save us a lot of time and money. 43