Chapter 7 190 RELATIVE OPTICAL ABSORPTION 600 800 1000 Hb Hb O 2 Hb CO Figure 7.2.3: Optical absorption spectra. WAVELENGTH, nm CHEMFETs Another method for chemical detection in solutions or in gas relies on the charge transfer that can occur during a chemical reaction on a surface. In these devices, the surface of interest is a metal electrode which is actually the exposed gate of a field ef- fect transistor (FET). Since the conduction from source to drain in a FET is modified by charge on the gate, this device can be a remarkably sensitive detector of certain absorbed species. These sensors are called CHEMFETs. Of course, to build a selective detector, it is necessary to select a metal electrode that allows only one chemical reaction to occur. Simple metals are not very selective, so simple CHEMFETs suffer from a lack of selectivity—meaning that they respond to many different chemical species. One way to improve the selectivity of such a sensor is to follow a biological example, and to coat the electrode with molecules that are in- deed very selective. Antibodies are molecules that tend to react only with a particular (virus) molecule, and are more chemically selective than any simple metal electrode. Finally, there are a number of medical applications that rely on detection of oxygen in the bloodstream. Unfortunately, the bloodstream is a difficult place to work because white blood cells interpret the presence of almost any foreign matter as an invading organism, and tend to form scabs on all surfaces of such objects. Blood does exhibit a detectable change in color upon the absorption of oxygen, and blood oxygen may be crudely measured by looking at blood color. For example, a sensor that measures blood reflectivity at 700 nm and at 800 nm ought to be able to measure the blood oxygen content very accurately. The measurement at 800 nm is used to cancel out effects of scab overcoating. One possible implementation is a fiber-op- tic system that transmits light of two colors (700 and 800 nm), and senses the reflected light intensity as a measure of blood oxy- gen. Such a system is often used during surgical procedures but is not typically used for long-term implants. Chemical Sensors 191 CVP INJECTION PORT THERMISTOR BALLOON TRANSMITTING FIBER-OPTIC RECEIVING FIBER-OPTIC SAMPLING AND PRESSURE MONITORING LUMEN BALLOON INFLATION LUMEN DISTAL (PA) LUMEN PROXIMAL (CVP) LUMEN OPTICAL MODULE CARDIAC OUTPUT COMPUTER CONNECTOR TO OXIMETRY INSTRUMENT Figure 7.2.4: Opticath® catheter. (Courtesy of Hospira, Inc.) One device uses an LED emitter and a pair of detectors, each mounted looking out the side of a 1-mm thick catheter. The emitter and detector are separated by a few mil- limeters, so this instrument samples to a depth of a few millimeters, and is not badly affected by an overcoating of “scab.” This same technique can be applied to the measurement of skin color. Summary There are many different applications for chemical sensors, and many techniques which can be applied for any given application. In general, chemical sensing devices do not compare favorably to biologically developed detectors. Devices generally suf- fer from a lack of sensitivity, selectivity, and speed. For some applications, the signals of interest are large and easy to detect. For others, it is very tough going. Research can be expected to grow in detection of toxins in groundwater, vehicle emissions, biotoxins in public settings, and a large variety of chemicals in manufacturing process control. This page intentionally left blank 193 C H A P T E R 8 Capacitive and Inductive Displacement Sensors Mark Kretschmar and Scott Welsby, Lion Precision 8.1 Introduction Noncontact sensors and measurement devices—those that monitor a target without physical contact—provide several advantages over contacting devices, including the ability to provide higher dynamic response to moving targets, higher measurement resolution, and the ability to measure small fragile parts. Noncontact sensors are also virtually free of hysteresis, the error that occurs with contacting devices at the point where the target changes direction. With these noncontacting sensors there is no risk of damaging a fragile part because of contact with the measurement probe, and parts can be measured in highly dynamic processes and environments as they are manufactured. Noncontact sensors are based on various technologies including electric field, elec- tromagnetic field, and light/laser. Two complementary sensor technologies will be discussed in detail in this chapter: capacitive—electric field based, and inductive (eddy current)—electromagnetic field based. A capacitive or inductive sensor consists of a probe, which is the actual physical device that generates the sensing field, and a driver, the electronics that drive the probe and generate the resulting output voltage propor- tional to the measurement. In some sensors, the driver is physically integrated in the probe itself. Capacitive and inductive noncontact sensors have many similar characteristics as well as some characteristics unique to each technology. In the following pages we will discuss those things which are common to each of the technologies, compare those things which are different, and look at applications for each and at the unique solutions that are possible when using them together. We will start with capacitive sensors. Probe Driver Sensor Figure 8.1.1: Noncontact sensor system. Chapter 8 194 8.2 Capacitive Sensors Capacitive sensors are noncontact devices used for precision measurement of a con- ductive target’s position or a nonconductive material’s thickness or density. When used with conductive targets they are not affected by changes in the target material; all conductors look the same to a capacitive sensor. Capacitive sensors sense the sur- face of the conductive target, so the thickness of the material is not an issue; even thin plating is a good target. Capacitive sensors are widely applied in the semiconductor, disk drive and precision manufacturing industries where accuracies and high frequen- cy response are important factors. When sensing nonconductors they are popular in packaging and other industries to detect labels, monitor coating thickness, and sense paint, paper, and film thicknesses. Capacitive displacement sensors are known for nanometer resolutions, frequency responses of 20 kHz and higher, and temperature stability. They typically have mea- surement ranges of 10 µm to 10 mm although in some applications much smaller or larger ranges can be achieved. Capacitive sensors are sensitive to the material in the gap between the sensor and the target. For this reason, capacitive sensors will not function in a dirty environment of spraying fluids, dust, or metal chips. Generally the gap material is air. Capacitive technology also works well in a vacuum, but the sensors must be properly designed for the peculiarities of a vacuum environment to prevent the probes from compromis- ing the vacuum. Under some circumstances they can be used while immersed in a fluid but this is not common. When used with a conductive target, capacitive sensors are usually factory calibrated. Using capacitive sensors with nonconductive materials requires experimentation to determine the sensor’s sensitivity to the material and the technology’s suitability for the measurement. Capacitive Technology Fundamentals Capacitance is an electrical property that exists between any two conductors that are separated by a nonconduc- tor. The simplest model of this is two metal plates with an air gap between them. When using capacitive sensors, the sensor is one of the metal plates and the target is the other. Capacitive sensors measure changes in the ca- pacitance between the sensor and the target by creating an alternating electric field between the sensor and the target and monitoring changes in the electric field. Figure 8.2.1: A capacitor is formed by the target and the capacitive probe’s sensing surface. Typical Capacitor Conductive Plates Nonconductive Gap Capacitive Probe Target Capacitive and Inductive Displacement Sensors 195 ~130% of sensing surface diameter Figure 8.2.2: “Spot size” on the target is about 30 percent larger than the probe’s sensing surface area. Capacitance is affected by three things: the sizes of the probe and target surfaces, the distance between them, and the material that is in the gap. In the great majority of applications, the sizes of the sensor and target do not change. When used with con- ductive targets, the gap material does not change. The only remaining variable is the distance between the sensor and target, so the capacitance is an indicator of the gap size, or the position of the target. Capacitive sensors are calibrated to produce a cer- tain output change to correspond to a certain change in the distance between sensor and target. This is called the sensitivity. Target Considerations The electric field generated by a capacitive sensor typically covers an area on the target approximately 30 percent larger than the sensor area. Therefore, best results are obtained when the target is at least 30 percent larger than the sensing area of the probe. Sensors can be specially calibrated to smaller targets when the application demands it. When used to measure nonconductive ma- terials, the gap between the sensor and a conductive target is held constant and the material to be measured is passed through the Nonconductive Material Grounded Conductive Reference Output Nonconductive Material Thickness Figure 8.2.3: When measuring nonconductors, the electric field from a capacitive sensor passes through the nonconductive material on its way to a conductive target. Chapter 8 196 gap. This way the gap is unchanging and the only remaining capacitance variable is the gap material. The output of the sensor will change with changes in the material’s thickness, density, or composition. Holding two of these variables constant enables measurement of the third; for example, when a strip of plastic has a constant composi- tion and density, changes in the capacitance can only indicate a change in thickness. 8.3 Inductive Sensors Inductive sensors, also known as eddy current sensors, are noncontact devices used for precision measurement of a conductive target’s position. Unlike capacitive sen- sors, inductive sensors are not affected by material in the probe/target gap so they are well adapted to hostile environments where oil, coolants, or other liquids may appear in the gap. Inductive sensors are sensitive to the type of target material. Copper, steel, aluminum and others react differently to the sensor, so for optimum performance the sensor must be calibrated to the correct target material. Inductive sensors are known for nano- meter resolutions, frequency responses of 80 kHz and higher, and immunity to contaminants in the measurement area. They typically have measurement ranges of 0.5mm to 15mm although in some ap- plications much smaller and larger ranges can be achieved. Inductive sensors’ tolerance of contaminants make them excellent choices for hostile environments or even for operating while immersed in liquid. An inductive sensor’s magnetic field creates electrical currents within the target material and therefore the targets have a minimum thickness requirement. Details are provided in the next section. Inductive Technology Fundamentals While capacitive sensors use an electric field for sensing the surface of the target, inductive sensors use an electromagnetic field that penetrates into the target. By pass- ing an alternating current through a coil in the end of the probe, inductive sensors generate an alternating electromagnetic field around the end of the probe. When this alternating field contacts the target, small electrical currents are induced in the target ~300% of probe coil diameter Electromagnetic field penetrates target surface Figure 8.3.1: Inductive sensors use electromagnetic fields. Capacitive and Inductive Displacement Sensors 197 material (eddy currents). These electrical currents, then, generate their own electro- magnetic fields. These small fields react with the probe’s field in such a way that the driver electronics can measure them. The closer the probe is to the target, the more the eddy currents react with the probes field and the greater the driver’s output. Inductive sensors are affected by three things: the sizes of the probe coil and target, the distance between them, and the target material. For displacement measurements the sensor is calibrated for the target material and the probe size remains constant, leaving the target/probe gap as the only variable. Because of its sensitivity to material changes, eddy current technology is also used to detect flaws, cracks, weld seams, and holes in conductive materials. Target Considerations Inductive sensors are sensitive to different conductive target materials. Sensors must be calibrated to the specific material with which they will be used. Some materials behave similarly and others differ significantly. There are two basic types of target materials: ferrous (magnetic) and nonferrous (not magnetic). Some inductive sensors will work with both materials, while others will only work with one type or the other. Some ferrous materials include iron, and most steels. Nonferrous materials include aluminum, copper, brass, zinc and others. Inductive sensors are frequently used to monitor rotating targets such as crankshafts and driveshafts. However, measurements of rotating ferrous targets generate small errors because of tiny variations within the target material. This is called electrical runout or magnetic runout. These errors are quite small, on the order of 0.001 mm, which is negligible in the measurement of larger motions such as driveshafts. But in- ductive sensors are not well suited to high resolution measurement of rotating ferrous targets where they are expected to measure changes of 0.0001 mm. Ideally, the target’s measured surface must offer an area three times larger than the probe’s diameter. This is because the electromagnetic field from an inductive sensor’s probe is approximately three times the probe’s diameter. Sensors can be specially calibrated to smaller targets when the application demands it. Another target consideration is the thickness of the target material. Because electro- magnetic fields penetrate the target, there is a minimum thickness requirement for the target. The minimum thickness is dependent on the electrical and magnetic properties of the material and on the frequency at which the probe is driven. As the frequency goes up, the minimum thickness goes down. This table lists some minimum thick- nesses for common materials with a typical 1 MHz drive frequency. Chapter 8 198 Copper 0.2 mm Aluminum 0.25 mm 304 Stainless Steel 0.4 mm Brass 1.6 mm 1040 Steel 0.008 mm 416 Stainless 0.08 mm Iron 0.6 mm 8.4 Capacitive and Inductive Sensor Types Capacitive and inductive sensors are available in three basic types: proximity switch- es, analog output, and linear output. Proximity switches simply provide an on or off output to indicate whether or not the target is present in front of the probe. The distance from the probe to the target re- quired to activate the proximity switch may be adjustable or may be fixed. Proximity switches do not provide any indication of the target’s actual position, only whether or not its position is within the set proximity. Proximity sensors often have the driver electronics integrated in the probe body. They are inexpensive and readily available but they are not suited to precision positioning applications that require continuous readings of the target position. Proximity Switch Output Normally Low Distance from Probe to Target Sensor Output Figure 8.4.1: Proximity type sensors only provide off or on outputs which are triggered by the target position. Analog output sensors provide a continuous analog output voltage that changes pro- portionately to the changes in the probe/target gap. Common output ranges are 0 to 10 VDC, ±10 VDC, 0–20 mA, or 4–20 mA. With analog sensors, the relationship of the output to the changing gap is not linear. While the output is not linear, it is repeat- able, allowing for the accurate detection of a repeated position of the target. Analog output sensors frequently have gain and offset adjustments for adjusting the sensor to each application. Adjustable setpoint outputs are often provided on this type of sensor. These allow the user to set target position points at which digital outputs are activated. [...]... continuous position information is required Linear Sensor Output Figure 8.4.3: Linear output sensor provides accurate, linear representation of the target position 10 Sensor Output 8 6 4 2 0 0 2 4 6 8 10 Target Displacement 199 Chapter 8 8.5 Selecting and Specifying Capacitive and Inductive Sensors Selecting the proper sensor starts by determining which of the three sensor types discussed previously is appropriate... measurement errors for parts that have a different dimension than the masters Figure 8.7.1: Calibrating a nonlinear sensor with two part masters creates errors at nonmastered points Output Measured Part Potential Error Calculated Dimension Actual Dimension Sensor Output Calculated Sensitivity Master Part Measurements For more precise measurements with nonlinear sensors, an array of master parts is measured... factors outside of the sensor itself, it is rare for sensor specifications to include accuracy 8 .6 Comparing Capacitive and Inductive Sensors Capacitive and inductive sensors each have unique characteristics Below is a comparison of typical parameters for standard sensors This is intended to provide a general idea of how the technologies differ This data is by no means exhaustive; sensors are available... Analog Sensor Output vs Ideal Straight Line 10 Figure 8.4.2: Analog sensor outputs are proportional to the target position but not in a linear fashion Sensor Output 8 6 4 2 0 0 2 4 6 8 10 Target Displacement Linear output sensors produce a proportional output voltage whose relationship to the changing gap is linear Common output ranges are 0 to 10 VDC, ±10 VDC, 0–20 mA, and 4–20mA Linear output sensors... Changes in the output of the sensor indicate changes in position of the target When using linear sensors, the output change of the sensor is multiplied by the sensitivity of the sensor to produce a dimensional value Some sensing systems are available with integral displays that convert the sensor output and display the dimensional value 2 06 Capacitive and Inductive Displacement Sensors Position Window Figure... of the label edge is accomplished using a differential sensor The sensor actually has two sensing areas that are driven by the same circuit The sensor only activates its output when there is a difference between the two sensors (This particular configuration and application is covered under a U.S Patent.) Sensor 2 Difference Label Edges Figure 8.7. 16: Differential capacitive measurement The advantage... analog type sensor is sufficient Interpreting the Output Converting the output of the sensor into dimensional units is accomplished with this simple formula: Dimension = Output × Sensitivity When using linear sensors the calculation is straightforward A linear sensor s sensitivity is listed in calibration certificates or is otherwise listed on the sensor 204 Capacitive and Inductive Displacement Sensors... the center of the bottle Optical sensors were used to perform this function Then in the 1990s, clear labels on clear backing became common and optical sensors were no longer functional Because capacitive sensing only senses changes in density or thickness, and is not affected by color, it is an ideal solution Sensor 2 Sensor 1 Label Motion Sensor 1 Output In label sensors, the detection of the label... sensor is measuring the current state of the part Measurement then proceeds with changes in the sensor output indicating changes from this initial condition Linear or Analog Whether an analog sensor or linear sensor is necessary will depend on the required accuracies and specifications of the application Generally, where the application is intended to produce a specific dimensional measurement of a particular... theory Available from: Lion Precision 563 Shoreview Park Road St Paul, MN 551 26 USA 65 1-484 -65 44 www.lionprecision.com 222 CHAPTER 9 Electromagnetism in Sensing Dr Thomas Kenny, Department of Mechanical Engineering, Stanford University 9.1 Introduction This chapter discusses the basic principles behind the use of electromagnetism in sensing Since many established sensor types rely on electromagnetism, . together. We will start with capacitive sensors. Probe Driver Sensor Figure 8.1.1: Noncontact sensor system. Chapter 8 194 8.2 Capacitive Sensors Capacitive sensors are noncontact devices used for. Stainless Steel 0.4 mm Brass 1 .6 mm 1040 Steel 0.008 mm 4 16 Stainless 0.08 mm Iron 0 .6 mm 8.4 Capacitive and Inductive Sensor Types Capacitive and inductive sensors are available in three. information. Figure 8.4.2: Analog sensor outputs are proportional to the target position but not in a linear fashion. Linear Sensor Output 0 2 4 6 8 10 0 2 4 6 8 10 Target Displacement Sensor Output Figure