Chapter 14 310 with a large number of slits cut through it. The detector emitter pair is mounted so that the slits cause an oscillation in the signal—and the rotary position can be de- termined by counting the peaks in the signal. This is called an optical encoder, or an incremental encoder, and it is widely used in electric motors, as shown in Figure 14.1.3. Figure 14.1.3: Incremental encoders. (Courtesy BEI Technologies, Inc.) Incremental Disc Electronics Board Code Disc Bearing Housing Assy Light Source Mask Photodetector Assy Most phototransistors and photodiodes have their peak sensitivity in the near infrared (see Figure 14.1.4). The peak sensitivity occurs near the cutoff wavelength (near 1 µm) and extends to shorter wavelengths. The location of this peak sensitivity is due to the energy of the “bandgap” in silicon, and is not easily adjusted. Wavelength (nm) Relative Intensity (% of Peak) 800 900 1000 1100 140 120 100 80 60 40 20 Figure 14.1.4: Typical photodiode spectral response. Optical and Radiation Sensors 311 Table 14.1.1: Bandgaps of some semiconductors. Material Bandgap (eV) ZnS 3.6 CdS 2.41 CdSe 1.8 CdTe 1.5 Si 1.12 Ge 0.67 PbS 0.37 InAs 0.35 Te 0.33 PbTe 0.3 PbSe 0.27 InSb 0.18 Photosensors can be made from other electronic materials with different bandgaps, as shown in Table 14.1.1. None of these materials are as widely available as silicon, and costs for detectors made from InSb can be substantially higher. There is another important consideration to keep in mind when selecting photosen- sors. In addition to the photocarriers in the device, thermally generated carriers can be produced. The distribution of energies generated by thermal processes is dependent on the thermodynamics of the device, and on the temperature. Because of this rela- tionship, increasing the temperature causes an increase in the number of thermally generated carriers. Conversely, reducing the bandgap of a room-temperature device will also cause an increase in the number of thermally generated carriers. Silicon de- tectors work well at room temperature, but heating to more than 100°C starts to cause substantial increases in “dark current.” Detectors made from materials other than sili- con may offer increased cutoff wavelength, but may also require cooling below room temperature. In general, there is a nearly linear relationship between the maximum operating tem- perature and the cutoff energy for the detector. By selecting a material with a cutoff energy one-fifth that of silicon (such as InSb), it is necessary to cool the device to about one-fifth of the maximum operating temperature of silicon (cooling to 77K is optimal for InSb). This tradeoff between cutoff and operating temperature imposes severe cost issues for operation of devices at fairly long wavelengths. If cooling is affordable, a large selection of materials and devices with “engineered band-gaps” is available. The tremendous interest in devices with cutoff wavelengths near 10–20 µm is a direct result of the DOD interest in infrared detectors for night Chapter 14 312 vision. It turns out that the peak of the infrared spectrum for objects at room tempera- ture is in this region, and so the maximum contrast in thermal detection is available by producing devices with sensitivity in this region. There is a simple relationship between the temperature of an infrared source and the peak wavelength of the blackbody spectrum. λ m T = 2898 where the wavelength is in microns, and the temperature is in Kelvin. So, for room temperature, the maximum wavelength is near 10 microns. Figure 14.1.5: Typical spectral response of IR detectors. (Courtesy of Electro Optical Industries, Inc.) THERMISTOR BOROMETER (300K) PYROELECTRIC DETECTOR (300K) THERMOCOUPLE THERMOPILE (300K) PbSe (300K) WA VELENGTH (µm) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 D* (cm • Hz ½ /W) 10 12 10 11 10 10 10 9 10 8 GeAu (77K) PbSe (77K) Ge (300K) Ge:Hg (4.2K) Si:Ga (4.2K) HgCdTe (77K) HgCdTe (77K) PbSe (196K) PbS (77K) PbS (300K) InSb (77K) Ge (77K) Ge (196K) InAs (196K) PbS 2π STERADIANS FIELD OF VIEW 300K BACKGROUND TEMPERATURE ( ) IDEAL LIMIT OF PHOTOVOLTAIC DETECTORS IDEAL LIMIT OF PHOTOCONDUCTIVE DETECTORS (196K) GOLAY CELL (300K) HgCdTe (77K) InAs (77K) Of the materials most studied, the clear winner is Mercury Cadmium Telluride (MCT). It may be formulated to have cutoff between 10 and 20 microns, and offers excellent properties for infrared detection. In particular, it offers low dark current, high absorptivity, and low carrier scattering. Unfortunately, it is difficult and ex- pensive to manufacture. As should be expected for anything containing mercury, its fabrication process is an environmental nightmare, and the basic material is not com- patible with electronics. As a result, it is “bump-bonded” onto silicon substrates for Optical and Radiation Sensors 313 readout and signal processing. In addition, it must be operated at or below 77K, which imposes operational complications. A commercial imaging system based on MCT detector arrays generally costs near $100,000 at the present time. Most military applications (ballistic missiles, aircraft imaging systems, satellite systems) can afford this set of costs and complications, but commercial and civilian applications are generally cost-constrained. Therefore, recent research activities have focused on other materials which might be less expensive to make and operate. InSb does not offer sensitivity in the 10–20 µm region, but is more easily made than MCT, is electronics compatible, and can be operated near 100K. Research to extend the operation to higher temperatures is underway throughout academia and industry. Overall, the relationship between cutoff and operating temperature is fairly strict. MCT, which has been the focus of billions of dollars of materials research effort, has only been slightly extended to higher temperatures. There is not tremendous hope that InSb or other materials will benefit from a large change in operating requirements. The other type of optical detector, the thermal detector, does offer some hope for this problem. Thermal Detectors Thermal detectors operate by absorbing the infrared radiation and measuring the temperature rise of the detector with a thermometer. Generally, the performance of thermal detectors is limited by the availability of sensitive and small heat capacity thermometers. An important advantage of thermal infrared detectors is due to the absence of any relationship between the wavelength of the absorbed radiation and the response of the detector. Any energy absorbed causes a response in the detector. Therefore, it is pos- sible to use a thermal infrared detector at room temperature to detect radiation from room temperature blackbodies. However, it is important to note that if the conditions allow use of a quantum detec- tor, such a detector will outperform a thermal detector by several orders of magnitude. Thermal detectors come into their own in situations that don’t allow quantum detec- tors. Since the thermometer is mounted within the infrared detector structure, it is connect- ed to a temperature reference by a finite thermal conductance. This finite conductance imposes dynamic constraints on the system behavior, and we may analyze the situa- tion as follows: Chapter 14 314 Assume we have a thermometer that is a thermistor with a temperature coefficient given by: α = ∆ ∆ 1 R R T Figure 14.1.6: Voltage divider. R L R L V s V in This thermistor is mounted in an electrical circuit with a load resistor with a resis- tance of R L , and is biased by a dc voltage of V in . The electrical circuit is shown in Figure 14.1.6. As in all voltage dividers, we have: V V R R R V V R R R R out in t L t out in t L L t = + ≈ > >for The sensitivity of this system is given by: Sensitivity = ∆ ∆ = V T V R R out in t L α Figure 14.1.7: Thermal circuit. Power In G T c Optical and Radiation Sensors 315 However, we must consider the thermal characteristics of this system as well. In this case, we model the thermometer as a finite heat capacity attached to an object by a finite thermal conductance. Infrared power is deposited into the thermometer, causing the temperature of the thermometer to oscillate. This thermal situation can be mod- eled as a thermal circuit as shown in Figure 14.1.7. By energy balance, the energy gained is equal to the change in energy of the ther- mometer: P G T T C T t in c − − ( ) = ∆ ∆ O Now, we assume that the power and the thermometer temperature oscillate: P P P e T T T e in in in i t c c c i t = + = + 1 2 1 2 ω ω We insert these expressions into the energy balance equation, and we have: P P e G T T e T i CT e in in i t c c i t c i t 1 2 1 2 2 + − + − ( ) = ω ω ω ω O We can take the constant and oscillating parts to be independent, and we have: P G T T P G T i CT in c in c c 1 1 2 2 2 0− − ( ) = − ( ) = O ω These reduce to: T T P G T P i C G c in c in 1 1 2 2 = + = + O ω So the sensitivity of this device to changes in infrared absorbed power is: Sensitivity = ∆ ∆ = ∆ ∆ ∆ ∆ = + V P V T T P V R R i C G in t L α ω 1 To improve the sensitivity, it is important to choose a thermometer with a large tem- perature coefficient and a small heat capacity. We can see from this expression that the response of the detector will have a simple 1-pole response, which is to say that it is frequency-independent below the cutoff frequency and decreases as 1/f above the cutoff. Its response is exactly the same as that of an electrical low-pass filter. Chapter 14 316 There are several different infrared detectors based on this detection concept. In fact, almost every well-established thermometer has also been optimized as an infrared detector. Phototransistor Example The phototransistor is a device that operates by converting incoming photons to electrons in the base of a bipolar transistor. As for any such transistor, the base current causes a larger collector-emitter current to flow, which is detected by a circuit. The easiest way to detect a current is to use a resistor to convert it to a voltage, as shown in Figure 14.1.8. LM − + +5 LM − + +5 LM − + +5 LM − + +5 5V 50 k 31 k 33 nF 1 M Figure 14.1.8: Phototransistor circuit. In this case, an oscillator circuit is powering a light-emitting diode, causing a light oscillation at 1 kHz. The phototransistor is pointed at the LED, and detects the os- cillation in the incident light. The circuit converts the current to a voltage with a pull-down resistor, buffers the signal, high-pass filters the signal, and then converts it to a square wave with a comparator. This circuit is one of many possible such circuits, and may be considered typical. We can see that the signal at the beginning of the circuit reflects the oscillation as well as the background illumination (dc and 60-Hz components). Some of the filter design is intended to reduce the sensitivity to these “noise” components while preserving the sensitivity to the signal at 1 kHz. Optical and Radiation Sensors 317 The variable resistor at the front of the circuit is an important degree of freedom. The current flowing through the transistor cannot exceed the saturation current: I V V R s t bias pd ω = − 0 6. where V bias is the total collector-emitter voltage, and R pd is the value of the pull-down resistor. If the background is very bright, the current flowing in the device may al- ready be very close to the saturation value, and any additional signal illumination will not produce much additional signal. Depending on the amount of background illumination, it is possible to saturate the detector, thereby reducing the sensitivity to signals. We use a variable resistor here to allow adjustment so that the detector is biased at a point of good performance. It is possible to obtain such detectors in side-by-side emitter-detector pairs, which cause a signal only if a reflective object is nearby. Depending on the biasing and the background illumination, it is possible to detect objects at a range of more than 1 cm. 14.2 Thermal Infrared Detectors In recent years, the Department of Defense (DOD) has invested a great deal of research and development funds into detection techniques that allow long-wave detec- tion from uncooled platforms. An additional focus of this work has been techniques that are compatible with the formation of dense arrays. One interesting device that has emerged due to this investment has been the Uncooled Detector arrays made by Honeywell. These detectors are based on the simplest thermal design—a resistance thermometer. What is novel about this device is that it combines the best microfabrication technol- ogy with good thermometer technology and electronics integration. A drawing of the microbolometer is shown in Figure 14.2.1. The basic idea is to use silicon microfabrication techniques (such as those in many accelerometers) to make an isolated thermal structure with very little heat capacity. As we saw in the ther- mometer discussion, the thermal infrared detector is improved by minimizing the heat capacity. Chapter 14 318 Figure 14.2.1: Microbolometer. (Courtesy Infrared Solutions, www.infraredsolutions.com) 50 µm 0.5 µm 2.5 µm Y-metal X-metal B E IR Radiation Monolithic Bipolar Tr ansistor Silicon Nitride and Vanadium Oxide In the final device, a flake of silicon nitride with dimensions of 50 µm × 50 µm × 0.5 µm is floated above a silicon substrate. This flake is supported by a pair of legs, and is coated with a resistive material with a good thermal coefficient of resistance. Under- neath the flake is a transistor that is used to connect the current-measuring circuit to the device using a conventional row-column addressing technique. The device cur- rent is passed out to a processing circuit on the perimeter of the device by the x and y metal leads. In this device, much research went into developing a technique for depositing the nitride on top of a transistor, for releasing the devices with very high yield, and for obtaining a sensitive thermometer in the form of a deposited metal film. This resistor is made from vanadium oxide, which offers a TCR of about 1% near room tempera- ture. The resistance change is a result of a structural phase transition in vanadium oxide above room temperature, so this device must be held near room temperature to allow operation with good sensitivity. Having developed this technology, Honeywell has gone on to make dense arrays (200 × 200), and to continue optimizing the performance of the devices. In the last couple of years, a complete camera system has been demonstrated. This base technology has been offered for licensing, and is presently being commercialized by several manufac- turers of infrared imaging systems. This device does not out-perform the MCT imager, but it does enable operation at room temperature, and might be available at low cost with further development. [...]... sensors measure this quantity in various ways Some magnetometers measure total magnitude but not direction of the field (scalar sensors) Others measure the magnitude of the component of magnetization along their sensitive axis (uni-directional sensors) This measurement may also include direction (bi-directional sensors) Vector magnetic sensors have two or three bi-directional sensors Some magnetic sensors... magnets) The sensor output will be a nominal 6.0 volts As magnet #1 continues toward the sensor, the field will become more and more positive until the sensor output reaches 9. 0 volts This approach offers high accuracy and good resolution as the full span of the sensor is utilized The output from this sensor is linear over a range centered on the null point Figure 15.1.17: Bipolar head-on position sensor. .. important in your specific application and environment The types of position sensors covered here include: ■ ■ Contact devices • Limit switches • Resistive position transducers Non-contact devices • Magnetic sensors, including Hall effect and magneto-resistive sensors • Ultrasonic sensors • Proximity sensors • Photoelectric sensors Limit Switches Limit switches are electromechanical contact devices... limit switches and potentiometers, involve physical contact with the object being sensed These are called contact position sensors Contact position sensors often prove to be the simplest, lowest cost solution in applications where contact with the target is acceptable Sensor manufacturers have employed a much wider variety of approaches and technologies to develop non-contact position sensors, which have... for longer sensor- to-magnet distances A unique aspect of using magnetic sensors is that measuring magnetic fields is usually not the primary intent Another parameter, such as wheel speed or the position of a part, is sought Magnetic sensors don’t measure these parameters directly but extract them from changes in magnetic fields The enacting input has to create or modify a magnetic field Once the sensor detects... field, which has the function of keeping the sensors in saturation mode, minimizing effects of stray magnetic fields and providing a linear operating range for selected sensor pairs Unlike other incremental sensors, this technology is absolute reading; no reference point is required Position is accurately known at any time as well as at power-on When each sensor in the array is connected to the supply... sensors: bipolar and unipolar Bipolar sensors require positive gauss (south pole) to operate and negative gauss (north pole) to release Unipolar sensors require a single magnetic pole (south) to operate Release is obtained by 333 Chapter 15 moving the south pole away from the sensor Analog sensors operate by proximity to either magnetic pole Ratiometric linear Hall effect sensors are small and versatile... inexpensive IR sensors, and a family of devices adequate for imaging systems are emerging 320 CHAPTER 15 Position and Motion Sensors 15.1 Contact and Non-contact Position Sensors Adolfo Cano Muñoz, Product Manager, Honeywell Sensing and Control Introduction Position sensors play an increasing role in our daily lives They are abundant in our homes, in our cars, and in our work places As sensing technology. .. Figure 15.1.18: Biased headon position sensor Slide-by actuation is shown in Figure 15.1. 19 A tightly controlled gap is maintained between the magnet and the sensor As the magnet moves back and forth at that fixed gap, the field seen by the sensor becomes negative as it approaches the north pole, and positive as it approaches the south pole This type of position sensor features mechanical simplicity,... output 338 Position and Motion Sensors Figure 15.1.22 illustrates supply for an NPN (current sinking) sensor In this circuit configuration, the load is generally connected between the supply voltage and the output terminal (collector) of the sensor When the sensor is actuated, turned ON by a magnetic field, current flows through the load into the output transistor to ground The sensor s output voltage is . axis about which the lever rotates. Position and Motion Sensors 327 Resistive Position Sensors Resistive position sensors, also called potentiometers or simply position transducers, were originally. (77K) Ge (77K) Ge ( 196 K) InAs ( 196 K) PbS 2π STERADIANS FIELD OF VIEW 300K BACKGROUND TEMPERATURE ( ) IDEAL LIMIT OF PHOTOVOLTAIC DETECTORS IDEAL LIMIT OF PHOTOCONDUCTIVE DETECTORS ( 196 K) GOLAY CELL. might be available at low cost with further development. Optical and Radiation Sensors 3 19 Another very important technology for low-cost uncooled infrared detectors has emerged in recent years