Chapter 9 230 A number of commercial inductive proximity sensors are also available based on this general technique, some miniaturized for robotic applications. An important but different sort of phenomena can be present if the metal object used to modify the inductance is a ferromagnetic material. Ferromagnetism is an effect in which the magnetic moments of the bound electrons and the nuclei also interact with external fields. In a ferromagnet, the orientation of the magnetic moments in the material can become aligned with external fields, causing an effective field amplifica- tion, which can be very large. In addition, this internal alignment can persist after the external field is removed, or even if its direction is changed. Because the field amplification may be as much as 1000 times, the presence of fer- romagnetic materials greatly increases the sensitivity of inductance coil bridges to their presence. Therefore, such sensors are very much more sensitive to ferromagnetic objects, and one often sees sensors which are tuned to detect a small piece of ferro- magnet attached to a moving metal part. A very common example of the use of a ferromagnetic element is as shown in Figure 9.3.5. In this system, the amount of magnetic field from one coil that is directed towards part of a second coil is dependent on the position of a ferromagnetic element. In the sys- tem shown in the drawing, the two halves of the pick-up coil (wired to V out ) are wound in opposite directions. If the ferromagnet were not present, the flux in each half of the pick-up coil would be equal and opposite, and V out would be zero. When the ferromagnet is positioned in the middle, there is also a complete cancellation of the flux. However, whenever the ferromagnet is displaced, the flux balance is changed, and the net effect is that there will be a voltage across the pickup coil whose amplitude is proportional to the displacement of the ferromagnet from its center position. Figure 9.3.5: Inductive sensor circuit diagram. V ref V out Electromagnetism in Sensing 231 This sort of inductive position sensor is very commonly used in a class of devices called linear variable displacement transducers (LVDT). These transducers can have very good accuracy (much better than 1% of the total range of motion, which is commonly called “stroke”), and are often used for precision position measurement applications, such as flap and rudder position measurements on aircraft. One method of analysis for these systems is based on a magnetic circuit analogy, in which the inductance is seen as the sum of a series of “reluctances,” which are indi- vidually defined as L R R A i i i i = = ∑ 1 where length µ where the length is of a segment of the magnetic circuit, the µ is the permittivity of that segment, and A i is the cross-sectional area of that segment. Figure 9.3.6: A magnetic circuit example #1. b a An example of a device that relies on a magnetic circuit is shown in Figure 9.3.6. A coil is wrapped about one part of a ferromagnetic structure. The magnetic field creat- ed when there is a current in the loop is almost entirely confined within the magnetic material. A moving ferromagnet is placed within the extended legs of the structure. Since the magnetic field will mostly pass through the moving element, the length of the total magnetic circuit is dependent on the position of the moving element. Since the reluctance of the circuit is the sum of the reluctance of the elements, the total inductance that is measured at the coil is also dependent on the position of the moving element. Chapter 9 232 Figure 9.4.1: Hysteresis loop. 0 0 B H Figure 9.3.7: A magnetic circuit example #2. i Ø Ø x Z C Alternatively, these systems may be used in situations where the reluctance is a continuous function of the position of a moving element, as in Figure 9.3.7. The reluctance is inversely proportional to the permittivity, and the inductance is inversely proportional to the sum of reluctances, so the total inductance is maximized when the magnetic circuit has a minimum of air gap. In these systems, the inductance of the total element is increased by a large factor when the moving element is positioned so as to minimize the air gap. 9.4 Magnetic Field Sensors There is another class of instruments used for detection of static magnetic fields. Magnetometers can be made in several ways, and here we will review a couple of spe- cific devices that are of widest commercial use. The flux gate magnetometer relies on measurement of the behavior of a ferromagnet- filled inductor. In the absence of external magnetic fields, current passed through the coil causes the formation of a magnetic field, which acts to polarize the ferromagnetic material. In general, the memory of the ferromagnetic material causes a hysteresis in the relation between the applied field and the polarization of the ferromagnet. We can see this by thinking about the starting situation, where the ferromagnet is unpo- larized and there is no current. If a current is applied, it polarizes the ferromagnet. As the current is increased, the polarization increases until satu- ration. Now, the current may be reduced to zero, and the resulting situation will include some residual polarization of the ferromagnet. If a current in the opposite direction is applied, the polarization is reversed, eventually saturated, and retains some residual reverse polarization when the current is again turned off. A graph of magnetization (B) versus applied magnetic field (H) is shown in Figure 9.4.1. The applied magnetic field is proportional to the current through the electromagnetic field and we see that the mag- netization of the core responds to the applied field with hysteresis. Electromagnetism in Sensing 233 In the absence of an external magnetic field, the hysteresis loop is perfectly symmet- ric, and the graph of voltage versus current would only include the odd harmonics of the drive frequency. If there is an external magnetic field, the hysteresis loop is shifted away from the origin. This is because there is a residual applied magnetic field when the current is off, due to the external magnetic field. One result of this is that the symmetry of the hysteresis loop is spoiled. If the I-V graph is analyzed, there will be a component at the second harmonic of the drive frequency, and the amplitude of this harmonic will be proportional to the component of the external magnetic field vector along the coil axis. Therefore, this device may be used to sense external magnetic fields. In fact, when properly constructed and wired, this sort of sensor can be very sensitive to small changes in external magnetic field. This class of magnetometers is generally used for space science missions, and for all precision terrestrial applications. A miniature flux-gate magnetometer is available from Applied Physics Systems, fea- turing resolution of less than 10 −10 T/sqrt(Hz), very good linearity, and very small size. It is a fairly expensive instrument. This style of magnetometer is also used for a number of prospecting applications. In several important applications, buried objects produce magnetic fields, and instruments that can measure local magnetic field gradients are very useful. Instruments available from Schonstedt feature a pair of flux gate magnetometers operated in a differential mode. If magnetic objects are positioned near the pair, the earth’s magnetic field is distorted and the difference between the two magnetometers does not cancel perfectly. In this mode, the gradiometer can sense the presence of a magnetic object. This kind of instrument is com- monly used by road-repair crews to locate buried cables prior to digging. Another class of magnetometer is called the Hall effect sensor. In the Hall effect sensor, the transport of electrons through an electrical device is affected by the presence of an external magnetic field. As shown in Figure 9.4.2, current flowing from the top to the bottom of a device is deflected to the right, causing a charge build-up, and a measurable volt- age. This sort of sensing approach offers ease of fabrication as a substantial advantage, but does not offer the performance of the flux-gate devices discussed above. In general, a Hall effect sensor can measure down to about 5% of the earth’s magnetic field. Figure 9.4.2: Hall effect sensor. I Magnetic field Chapter 9 234 MBC930 MLC129 2 1 GND V O V CC V O R T R T 34 Figure 9.4.3: Magnetoresistors: Resistive film patterned into Wheatstone bridge on chip (offset trimmed to zero by R T ). (Courtesy Philips Semiconductors.) Instead of measuring the build-up of a Hall voltage, it is also possible to measure the increased resistance of the device due to the deflected electrons. In this case, the Hall- based sensor is called a magnetoresistor. Recent years have seen much research on materials for magnetoresistors at Honeywell and elsewhere. A very important advan- tage of a Hall magnetoresistor is that the resistive film may be easily patterned into geometries that are easily connected into resistance bridges, as shown in Figure 9.4.3. A newer magnetoresistor material, which offers a larger magnetoresistance effect (called the Giant Magnetoresistance Effect or GMR for short), is being used for a number of applications. Several companies are marketing devices based on these new materials. One very important application for miniature magnetic sensors is as the data-read head for disk drives. Clearly, improved sensitivity is important because it can enable increased storage density in disk drives. As a result, GMR read heads have become common on the newer generations of hard disk drives. Electromagnetism in Sensing 235 9.5 Summary We have reviewed the basic principles of induction, and examined several examples of devices that use this principle to measure the position or presence of objects. We have also looked at flux-gate and magnetoresistance magnetometers, and looked at some products based on both. In general, a wide variety of magnetic sensor-based instruments are available. The emergence of thin-film sensors is important for the disk-drive industry, and other applications of thin-film magnetometers can be expected. This page intentionally left blank 237 C H A P T E R 10 Flow and Level Sensors William Hennessy, BMT Scientific Marine Services, Inc. Flow sensors are used in many monitoring and control applications, to measure both air and liquid flows. There are many ways of defining flow (mass flow, volume flow, laminar flow, turbulent flow). Usually the amount of a substance flowing (mass flow) is the most important, and if the fluid’s density is constant, a volume flow measure- ment is a useful substitute that is generally easier to perform. There are numerous reliable technologies and sensor types used for this purpose. Some technologies have been applied to both air and liquid flow measurements, as their principles of operation hold true in either application. Other technologies lend themselves to being airflow or liquid flow specific. In this chapter, we will discuss several of the most commonly used techniques for measuring both airflow and liquid flow. Complementary to flow measurement is level measurement. Used together, flow and level sensors answer the basic question of “how much” in laboratories and industries worldwide. Both mea- surement processes also share the distinction of being fairly complicated. 10.1 Methods for Measuring Flow Flow rate is typically obtained by first measuring the velocity of a fluid in a pipe, duct, or other structure and then multiplying by the known cross-sectional area at the point of measurement. Methods for measuring airflow include thermal anemometers, differential pressure measurement systems, and vortex shedding sensors. Methods used for measuring liquid flow include differential pressure measurement systems, vortex shedding sensors, positive displacement flow sensors, turbine based flow sen- sors, magnetic flow sensors, and ultrasonic flow sensors. Thermal Anemometers Thermal (or “hot wire”) anemometers use the principle that the amount of heat re- moved from a heated temperature sensor by a flowing fluid can be related to that fluid’s velocity. These sensors typically use a second, unheated temperature sensor to compensate for variations in the air temperature. Hot wire sensors are available as single point instruments for test purposes, or in multi-point arrays for fixed installa- Chapter 10 238 tion. These sensors are better at low airflow measurements than differential pressure types, and are commonly applied to air velocities from 50 to 12,000 feet per minute. Differential Pressure Measurement Differential pressure measurement sensor technologies can be used for both airflow and liquid flow measurements. A variety of application-specific sensors used for both airflow and pressure measurements are on the market, as well as differential pressure sensors used for liquid measurements. Differential pressure flowmeters are the most common type of unit in use, particularly for liquid flow measurement. The operation of differential pressure flowmeters is based on the concept that the pressure drop across the meter is proportional to the square of the flow rate; the flow rate is found by measuring the pressure differential and taking the square root. Differential pressure flow devices, like most flowmeters, have a primary and second- ary element. The primary element causes a change in the kinetic energy, to create the differential pressure in the pipe. The unit must be correctly matched to the pipe size, flow conditions, and the properties of the liquid being measured. In addition, the measurement accuracy of the element must be good over a reasonable range. The secondary element measures the differential pressure and outputs the signal that is converted to the actual flow value. For airflow measurements, common differential pressure flow devices include Pitot tubes (see Figure 10.1.1) and numerous other types of velocity pressure-sensing tubes, grids, and arrays. All of these sensing elements are combined with a low differential pressure transmitter to produce a signal proportional to the square root of the fluid velocity. A Pitot tube consists of two tubes that measure pressure at different locations within a pipe. One tube measures static pressure, usually at the pipe wall, and the other measures impact pressure (static pressure plus velocity head). The faster the flow rate, the larger the impact pressure. Pitot tubes use Static Pressure Total Pressure Ve locity Pressure Static Pressure 8 Holes - 0.04" DIA Equally spaced free from Burrs Section A Inner Tubing 1/6" 00x21 B8S GA Copper A A Outer Tubing 5/16" 0Dx18 B8S GA Copper Figure 10.1.1: The Pitot tube. [...]... complete voltage mode sensor system schematic The sensor range and DTC are fixed by the components in the voltage mode sensor s internal amplifier (It is assumed that the DTC of the signal conditioner is greater Figure 11.2.4: Voltage mode sensor system than that of the force sensor. ) Piezoelectric Force Sensor Construction The basic mechanical construction of general purpose quartz forces sensors consist... Chatsworth, CA 91311 Endevco Corporation – 3 070 0 Rancho Viejo Rd., San Juan Capistrano, CA 92 675 11.3 Strain Gage Sensors Technology Fundamentals1 Sensors based on foil strain gage technology are ideally designed for the precise measurement of a static weight or a quasi-dynamic load or force The design of strain gage-based sensors consists of specially designed structures that perform in a predictable and repeatable... value, R, in the sensor s built-in circuitry Charge mode 256 Figure 11.2.2: Decay due to discharge time constant (DTC) Force, Load and Weight Sensors sensors, which do not contain built-in circuitry require an external amplifier (“charge amplifier”) to set the DTC Some charge amplifiers provide an adjustable DTC It should also be mentioned how the range of the force sensor is determined For sensors that... by examining the performance specifications for the sensor For sensors which do not contain built-in circuitry, the range is set by the external amplifier There are a number of trade-offs between each type See Reference 2 for additional information Sensor Types Charge Mode, High-Impedance, Piezoelectric Force Sensor A charge mode piezoelectric force sensor, when stressed, generates an electrostatic charge... Level Sensors Technological advances must also be taken into consideration One common mistake is to select a design that was popular for a given application years ago, assuming that it is still the best choice Many changes and innovations may have occurred in recent years in flowmeter technology for that particular application, making the choice much broader 10.3 Installation and Maintenance Airflow sensors... energy absorbed by the coating will cause the meter to become inoperative 248 Flow and Level Sensors 10.4 Recent Advances in Flow Sensors A recent study conducted by Flow Research and Ducker Worldwide showed that a major shift is taking place in the field of flow measurement, to “new technology flowmeters “New technology flowmeters are defined in the study as magnetic, ultrasonic, Coriolis, vortex, and... controller (PLC), etc These sensors are useful in sensing the levels of a variety of aqueous and organic liquids and slurries, and liquid chemicals such as quicklime There are also dual probe capacitance level sensors that can be used to sense the interface between two liquids that have very different dielectric constants These sensors are rugged, easy to use, contain no moving parts, and are simple to... Unlike through-air radar systems, GWR is an invasive technology It appears similar to RF Admittance sensor technology, but it does not have the same capabilities of coping with extremes of pressure, temperature or product coating Pulses of electromagnetic energy are emitted from the base of the transmitter down the waveguide (a 252 Flow and Level Sensors cable or rod) When the signal reaches a point... Sensing Technology When choosing a method for any particular application, many factors beyond initial costs must be taken into consideration The most important factors that sensor manufacturers need to know about a level measurement application are: ■ ■ ■ ■ name and characteristics of the material to be measured, whether solid, liquid, slurry, powder, or granular The dielectric constant K is of particular... force sensor generate an electrostatic charge only when force is applied to or removed from them In other words, if you apply a static force to a piezoelectric force sensor, the electrostatic charge output initially generated will eventually leak away and the sensor output ultimately will return to zero 255 Chapter 11 The rate at which the charge leaks back to zero is exponential and based on the sensor s . mea- sure reverse flow. Ultrasonic Flow Sensors Ultrasonic flow sensors can be divided into Doppler sensors and transit, or time-of-travel, sensors. Doppler sensors measure the frequency shifts. and reflected signals. Figure 10.1 .7: LDA configuration. Particle Light detector Lens Lens Laser Flow with particles Scattered light Beam splitte r Flow and Level Sensors 245 A Bragg cell is often. and vortex shedding sensors. Methods used for measuring liquid flow include differential pressure measurement systems, vortex shedding sensors, positive displacement flow sensors, turbine based