Instrumentation Sensors Book
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11.3 Force, Torque, and Load Cells |
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RS Strain Gauge |
Op-amp |
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Zero |
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Balance |
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Output V |
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RD |
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Dummy |
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Gauge |
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Figure 11.13 (a) Configuration for strain gauge elements, and (b) resistive bridge for signal conditioning of a strain gauge.
stages than what is shown may be required, or an ADC will be required to make the signal suitable for transmission. Additional linearization also may be needed, and this information can be obtained from the manufacturers’ device specifications [9].
Example 11.7
A force of 6.5 kN is applied to the end of an aluminum cantilever beam 0.35m from its fixed end. If the beam has a cross-sectional area of 8 cm2, what will be the deflection of the end of the beam?
From (11.12):
∆x = |
Fd |
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6,500 × 035. |
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689. × 1010 × 8 × 10−4 |
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∆x = 0.4 × 10−4 m
Example 11.8
Using the information from Example 11.7, a strain gauge with a gauge factor of 4.3 is attached to the beam. If the resistance of the gauge is 1,200Ω, and the gauge is placed in a bridge circuit with 1,200Ω resistors in the other arms, what would be the change in resistance of the gauge, and the output voltage from the bridge, if the supply to the bridge were 16V?
∆R = 1,200 × 4.3 × 0.4 × 10−4/0.35Ω = 0.6Ω
∆ = − 16 × 1,2006. = − =
V 16 2 8 8002.V 2mV 1200, + 1,2006.
The load cells previously discussed use strain gauges or piezoelectric devices, but they also can use capacitive or electromagnetic sensing devices [10].
A dynamometer is a device that uses the twisting or bending in a shaft due to torque to measure force. One such device is the torque wrench used to tighten bolts
186 |
Position, Force, and Light |
to a set torque level, which can be required in some valve housings to prevent seal leakage or valve distortion. The allowable torque for correct assembly will be given in the manufacturer’s specification. The twist in a shaft from a motor can be used to measure the torque output from a drive motor [11].
11.3.5Force and Torque Application Considerations
In most applications, compensation must be made for temperature effects, which can be larger than the measurement signal. Electrical transducers can be compensated by using them in a bridge circuit with a compensating device in the adjacent arm of the bridge. Changes in material characteristics due to temperature changes also can be compensated using temperature sensors and applying a correction factor to the measurement. This method is difficult with the low-level signals obtained from strain gauges. Vibration also can be a problem when measuring force, but this usually can be corrected by damping the movement of the measuring system. The choice of measuring device will depend upon the application. A far more sensitive strain gauge is the semiconductor strain gauge using the piezoresistive effect. These devices have gauge factors up to 90 times greater than those of the deposited resistive gauges, are very small (0.5 × 0.3 mm), have wide resistance ranges, and unlike deposited resistive devices, do not suffer from fatigue.
11.4Light
11.4.1Light Introduction
Light and its measurement is important as it also relates to the sense of sight, as well as to many industrial applications, such as location, proximity, and linear distance measurement. Light and its measurement is used in many industrial applications for high accuracy linear measurements, location of overheating, temperature measurement (e.g., infrared), object location and position measurements, photoprocessing, scanning, readers (e.g., bar codes), and so forth [12].
11.4.2EM Radiation
Light is an ultrahigh frequency EM wave that travels at 2.998 × 108 m/s. Light amplitude is measured in foot-candles or lux. The wavelength of visible light is from 4 × 10−7m to 7 × 10−7m. Longer wavelengths of EM waves are termed infrared, and
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0.3 m |
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0.3 mm |
0.3 um |
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0.3 nm |
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Wavelength |
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Microwaves |
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Infrared |
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Ultraviolet |
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X - rays |
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Figure 11.14 Electromagnetic radiation spectrum.
11.4 Light |
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shorter wavelengths are called ultraviolet. Light wavelengths are sometimes expressed in terms of angstroms (Å), where 1Å = 1 × 10−10m. The electromagnetic radiation spectrum is shown in Figure 11.14. The relation between the frequency (f) and the wavelength (
) of electromagnetic waves in free space is given by:
c = f |
(11.25) |
where c is the speed of light.
When traveling through any medium, the propagation velocity of EM waves is reduced by the refractive index of the material. The refractive index (n) is defined as:
n = c/v |
(11.26) |
where v is the velocity of EM radiation through the material.
Example 11.9
What is the wavelength of light in Å, if the wavelength is 493 nm?
1Å = 10−10m
493 nm = 493 × 10−9 ÷ 10−10Å = 4,930Å
Intensity is the brightness of light. The unit of measurement of light intensity in the English system is the foot-candle (fc), which is one lumen per square foot (lm/ft2). In the SI system, the unit is the lux (lx), which is one lumen per square meter (lm/m2). The phot (ph) is also used, which is defined as one lumen per square centimeter (lm/cm2). The lumen replaces the candela (cd) in the SI system. A 1 cd or 1 lm source is defined as a source that emits radiation at such an intensity that there is 1/683W passing through 1 sr of solid angle by monochromatic light with a frequency of 340 × 1012 Hz. Because a point source emits radiation in all directions, the intensity (I) on the surface of a sphere radius (R) is given by:
I = |
P |
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where P is the power of the source in watts.
This also shows that the intensity of a point source decreases as the inverse square of the distance from the source.
The decibel (dB) is a logarithmic measure used to measure and compare amplitudes and power levels in electrical units, sound, and light. When used for the comparison of light intensity, the equation is as follows:
Light level ratio in dB = 10log10 |
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where Φ1 and Φ2 are the light intensities at two different points.
The change in intensity levels at varying distances from a source is given by:
188 Position, Force, and Light
Change in levels = 10log |
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where d1 and d2 are the distances from the source to the points being considered.
Example 11.10
Two points are 43 and 107 ft from a light bulb. What is the difference in light intensity at the two points?
Difference = 10log10 |
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X-rays should be mentioned at this point, since they are used in the process industry, and are electromagnetic waves. X-rays are used primarily as inspection tools. The X-rays can be sensed by some light sensing cells, and can be very hazardous if proper precautions are not taken.
11.4.3Light Measuring Devices
Photocells are used for the detection and conversion of light intensity into electrical signals. Photocells can be classified as photovoltaic, photoconductive, photoemissive, and semiconductor.
Photovoltaic cells develop an EMF in the presence of light. Copper oxide and selenium are examples of photovoltaic materials. A microammeter calibrated in lux is connected across the cells and measures the current output.
Photoconductive devices change their resistance with light intensity. Examples of these materials are selenium, zirconium oxide, aluminum oxide, and cadmium sulfide.
Photoemissive materials, such as mixtures of rare Earth elements (e.g., cesium oxide), liberate electrons in the presence of light.
Semiconductors are photosensitive, and are commercially available as photodiodes and phototransistors. Light generates hole-electron pairs, which cause leakage in reversed biased diodes, and base current in phototransistors. Commercial high resolution optical sensors are available with the electronics integrated onto a single die to give temperature compensation, and a linear voltage output with incident light intensity. Visible light intensity to voltage converters (TSL 250), Infrared (IR) light to voltage converters (TSL 260), light to frequency converters (TSL 230), and light intensity to digital converters are commercially available. Note that Texas Instruments manufactures the TSL family.
11.4.4Light Sources
Incandescent light is produced by electrically heating a resistive filament, or by the burning of certain combustible materials. A large portion of the energy emitted is in the infrared spectrum, as well as the visible spectrum.
Atomic type sources cover gas discharge devices, such as neon and fluorescent lights.
Laser emissions are obtained by excitation of the atoms of certain elements.
11.4 Light |
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Semiconductor diodes, such as LEDs, are the most common commercially available light sources used in industry. When forward biased, the diodes emit light in the visible or IR region. Certain semiconductor diodes emit a narrow wavelength of visible rays, where the color is determined by material and doping. A list of LEDs and their color is given in Table 11.2.
11.4.5Light Application Considerations
Selection of sensors for the measurement of light intensity will depend upon the application. In instrumentation, requirements include: a uniform sensitivity over a wide frequency range, low inherent noise levels, consistent sensitivity with life, and a means of screening out unwanted noise and light.
In some applications, such as the sensing of an optical disk, it is only necessary to detect absence or presence of a signal, which enables the use of simple and inexpensive sensors. For light detection, the phototransistor is widely used, because of the ability with integrated circuits to put temperature correction and amplification in the same package, for high sensitivity. The device is cost-effective and has good longevity.
Figure 11.15 shows the schematic symbols used for optoelectronic sensors, and Table 11.3 gives a comparison of photosensor characteristics.
Table 11.2 |
LED Characteristics |
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Material |
Dopant |
Wavelength (nm) |
Color |
GaAs |
Zn |
900 |
IR |
GaP |
Zn |
700 |
Red |
GaAsP |
N |
632 |
Orange |
GaAsP |
N |
589 |
Yellow |
GaP |
N |
570 |
Green |
SiC |
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490 |
Blue |
λ |
λ |
Photoresistor |
Photodiode |
λ |
λ |
Phototransistor |
Photovoltaic (solar) cell |
Phototube
Photomultiplier
Figure 11.15 Schematic symbols for optosensors.
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Table 11.3 Summary of Optosensor Characteristics |
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Response |
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Photoconductive |
Photoresistor CdS |
0.6–0.9 |
100 ms |
Small, high sensitivity, |
Slow, hysteresis, |
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low cost |
limited temperature |
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Photoresistor CdSe |
0.6–0.9 |
10 ms |
Small, high sensitivity, |
Slow, hysteresis, |
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low cost |
limited temperature |
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Semiconductor |
Photodiode |
0.4–0.9 |
1 ns |
Very fast, good |
Low-level output |
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linearity, low noise |
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Phototransistor |
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Solar cell |
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11.5Summary
A number of important sensors used in process control, for applications other than for fluid characteristics, were discussed in this chapter. Sensors for measuring linear and rotational position, speed, and acceleration were introduced. These devices have many applications in process control. They can be used as transducers, to convert linear motion into electrical signals, using potentiometers, magnetic coupling, or capacitive changes. Other devices are used for absolute or incremental position control and measurement, velocity, vibration, and acceleration. Optical sensors and magnetic devices are used as proximity detectors for positioning, and the reading of bar codes and magnetic strips.
The measurement of force, torque, and load measurements are important in process control. Stress, strain, and Young’s Modulus play an important part in the measurement of loads and torque. The sensitivity of strain devices and load cells is enhanced by the gauge factor, but is much higher using piezoelectric elements. Extra elements are incorporated in strain gauges and load cells to compensate for temperature effects, when used in a bridge circuit.
Light can be produced from several sources. Many materials are light sensitive, and can be used to measure light intensity. Semiconductor devices are produced to emit specific frequencies or different colors of light, and integrated semiconductor devices with temperature compensation and conditioning also can be used to convert light into voltage, frequency, or digital signals.
Definitions
Absolute position is the distance measured with respect to a fixed reference point, and can be measured whenever power is applied.
Acceleration is the rate of change of speed, for linear motion or for rotational motion.
Angular motion is a measure of the rate of rotation. Angular velocity is a measure of the rate of rotation when rotating at a constant speed about a fixed point, and angular acceleration is measured when the rotational speed is changing.
Definitions |
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Angular position is a measurement of the change in position of a point about a fixed axis, measured in degrees or radians.
Arc-minute is an angular displacement of 1/60 of a degree.
Couple occurs when two parallel forces of equal amplitude, but in opposite directions, are acting on an object to cause rotation.
Force is a term that relates the mass of an object to its acceleration. Incremental position is a measure of the change in position, and is not referenced to a fixed point. If power is interrupted, the incremental position is lost.
Mass is a measure of the quantity of material in a given volume of an object.
Rectilinear motion is measured by the distance traversed in a given time, velocity when moving at a constant speed, or acceleration when moving is changing in a straight line.
Torque occurs when a force that is acting on a body tends to cause the body to rotate.
Velocity or Speed is the rate of change of position. This can be a linear measurement or an angular measurement.
Vibration is a measure of the periodic motion about a fixed reference point.
Weight of an object is the force on a mass due to the pull of gravity.
References
[1]Clifford, M. A., “Accelerometers Jump into the Consumer Goods Market,” Sensors Magazine, Vol. 21, No. 8, August 2004.
[2]Kulwanosli, G., and J. Schnellinger, “The Principles of Piezoelectric Accelerometers,” Sensors Magazine, Vol. 21, No. 2, February 2004.
[3]Hoffman, F.J., “A New Dimension in Encoder Technology,” Sensors Magazine, Vol. 19, No. 5, May 2002.
[4]Salt, H., “A New Linear Optical Encoder,” Sensors Magazine, Vol. 16, No. 11, November 1999.
[5]Massa, D. P., “Choosing an Ultrasonic Sensor for Proximity or Distance Measurement,” Sensors Magazine, Vol. 16, No. 2, February 1999.
[6]Young, W. C., Roark’s Formulas for Stress and Strain, 6th ed., McGraw-Hill.
[7]Shigley, J., Mechanical Engineering Design, McGraw-Hill, 1963, pp. 284–289.
[8]Nagy, M. L., C. Spanius, and J. W. Siekkinen, “A User Friendly High Sensitivity Strain Gauge,” Sensors Magazine, Vol. 18, No. 6, June 2001.
[9]Emery, J.C., “Simplifying the Electrolytic Balance Load Cell,” Sensors Magazine, Vol. 19, No. 6, June 2002.
[10]Bruns, R. W., “The Helix Load Cell,” Sensors Magazine, Vol. 15, No. 5, May 1998.
[11]Andreescu, R., M. Gupta, and B. Speelman, “A Magnetostrictive Torque Sensor,” Sensors Magazine, Vol. 21, No. 11, November 2004.
[12]Johnson, C. D., Process Control Instrumentation Technology, 7th ed., Prentice Hall, 2003, pp. 273–315.
C H A P T E R 1 2
Humidity and Other Sensors
There are many sensors, other than level, pressure, flow, and temperature sensors, which may not be encountered on a daily basis, but still play an equally important part in process control in high technology industries and in operator protection. This chapter discusses several sensors that are very important in modern processing. They are:
1.Humidity;
2.Density, specific weight, and specific gravity;
3.Viscosity;
4.Sound;
5.pH;
6.Chemical.
12.1Humidity
It is necessary to control the amount of water vapor present in many industrial processes. Textile, wood, and chemical processing is very sensitive to humidity.
12.1.1Humidity Introduction
Humidity is a measure of the relative amount of water vapor present in the air or a gas.
Relative humidity (Φ) is given by:
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Amount of water vapor present |
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Relative humidity = |
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Maximum amount of water vapor soluble in |
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the same volume of air or gas (P and T constant) |
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where P is the pressure and T is the temperature. |
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An alternative definition using vapor pressures is: |
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Relative humidity = |
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water vapor pressure in air or gas × 100 |
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water vapor pressure in saturated air or gas(T constant) |
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Humidity and Other Sensors |
The term saturated means the maximum amount of water vapor that can be dissolved or absorbed by a gas or air at a given pressure and temperature.
Specific humidity, humidity ratio, or absolute humidity can be defined as the mass of water vapor in a mixture in grains (where 7,000 gr = 1 lb), divided by the mass of dry air or gas in the mixture. The measurement units could also be defined as a ratio (pounds of water vapor per pound of dry air), or be defined in SI units (grams of vapor per kilogram of dry air).
Humidity ratio = |
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mass(water vapor) |
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0622. |
P(water vapor) |
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P(water vapor) |
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(12.5)
where P (water vapor) is pressure, and P (air or gas) is a partial pressure. The number 0.622 is a conversion factor between mass and pressure.
Example 12.1
Examples of water vapor in the atmosphere are as follows:
•Dark storm clouds (cumulonimbus) can contain 10 g/m3 of water vapor.
•Medium density clouds (cumulus congestus) can contain 0.8 g/m3 of water vapor.
•Light rain clouds (cumulus) can contain 0.2 g/m3 of water vapor.
•Wispy clouds (cirrus) can contain 0.1 g/m3 of water vapor.
In the case of the dark storm clouds, this equates to 100,000 tons of water vapor per square mile for a 10,000-ft-tall cloud.
12.1.2Humidity Measuring Devices
Humidity can be measured using materials that absorb water vapor, giving a change in their characteristics that can be measured; by measuring the latent heat of evaporation; by dew point measurement; or by microwave absorption.
12.1.2.1Psychrometers
A psychrometer uses the latent heat of vaporization to determine relative humidity. If the temperature of air is measured with a dry bulb thermometer and a wet bulb thermometer, then the two temperatures can be used with a psychrometric chart to obtain the relative humidity, water vapor pressure, heat content, and weight of water vapor in the air. Water evaporates from the wet bulb, trying to saturate the surrounding air. The energy needed for the water to evaporate cools the