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Digital and Intelligent Sensors

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Digital and Intelligent Sensors 8 DIGITAL AND INTELLIGENT SENSORS The overwhelming presence of digital systems for information processing and display in measurement and control systems makes digital sensors very attrac- tive. Because their output is directly in digital form, they require only very simple signal conditioning and are often less susceptible to electromagnetic interference than analog sensors. We distinguish here three classes of digital sensors. The ®rst yields a digital version of the measurand. This group includes position encoders. The second group relies on some physical oscillatory phenomenon that is later sensed by a conventional modulating or generating sensor. Sensors in this group are some- times designated as self-resonant, variable-frequency,orquasi-digital sensors, and they require an electronic circuit (a digital counter) in order to yield the desired digital output signal. The third group of digital sensors use modulating sensors included in variable electronic oscillators. Because we can digitally measure the oscillation frequency, these sensors do not need any ADC either. Digital computation has evolved from large mainframes to personal com- puters and microprocessors that o¨er powerful resources at low cost. Process control has similarly evolved from centralized to distributed control. Silicon technology has achieved circuit densities that permit the fabrication of sensors that integrate computation and communication capabilities, termed intelligent or smart sensors. 8.1 POSITION ENCODERS Linear and angular position sensors are the only type of digital output sensors that are available in several di¨erent commercial models. Incremental encoders 433 434 8 DIGITAL AND INTELLIGENT SENSORS Disk Figure 8.1 Principle of linear and rotary incremental position encoders. (From N. Norton, Sensor and Analyzer Handbook, copyright 1982, p. 105. Reprinted by permis- sion of Prentice-Hall, Englewood Cli¨s, NJ.) are in fact quasi-digital, but we discuss them here because they are related. Each September issue of Measurements & Control lists the manufacturers and types of encoders. 8.1.1 Incremental Position Encoders An incremental position encoder consists of a linear rule or a low-inertia disk driven by the part whose position is to be determined. That element includes two types of regions or sectors having a property that di¨erentiates them, and these regions are arranged in a repetitive pattern (Figure 8.1). Sensing that property by a ®xed head or reading device yields a de®nite output change when there is an increment in position equal to twice the pitch p. A disk with diame- ter d gives pd m 8:1 2p pulses for each turn. This sensing method is simple and economic but has some shortcomings. First, the information about the position is lost whenever power fails, or just after switch-on, and also under strong interference. Second, in order to obtain a digital output compatible with input±output peripherals in a computer, an up± down counter is necessary. Third, they do not detect the movement direction unless additional elements are added to those in Figure 8.1. Physical properties used for sector di¨erentiation can be magnetic, electric, or optic. The basic output is a pulse train with 50 % duty cycle. A toothed wheel or etched metal tape scale of ferromagnetic material yields 8.1 POSITION ENCODERS 435 Figure 8.2 Di¨erent sensors for magnetic incremental position encoders. (a) Coil and magnet (courtesy of Orbit Controls). (b) Toroidal core. a voltage impulse each time it passes by a ®xed coil placed in a constant mag- netic ®eld, as shown in Figure 8.2a. The resulting signal is almost sinusoidal, but it is easy to convert it into a square wave or just to determine its zero crossings. There is a minimal and maximal velocity that determines the appli- cation range for this method, which is used, for example, in antilock braking systems (ABS) in cars. Alternatively, an AMR or GMR sensor (Section 2.5) can replace the coil to obtain a change in resistance whose amplitude does not depend on the turning speed. Figure 8.2b shows another inductive system but this time based on a toroid with two windings. One winding is used for exciting, using currents between 20 kHz and 200 kHz, and the other is used for detection. There are two output states: ``1'' when no voltage is detected and ``0'' when a voltage with the excit- ing frequency is detected. The moving element includes regions with magne- tized material. Each time one of these regions passes in front of the reading head, the core saturates because the ¯ux emanating from the material adds to the ¯ux created by the exciting signal. When the core saturates, the secondary winding does not detect any voltage (e dF=dt, with F constantÐsaturation 436 8 DIGITAL AND INTELLIGENT SENSORS Figure 8.3 Silver-in-glass technology for contacting incremental position encoder (courtesy of Spectrol). ¯ux): state ``1.'' When there is a region with no magnetization in front of the reading head, the secondary winding detects a voltage induced by the primary, state ``0.'' A Hall e¨ect sensor (Section 4.3.2), magnetoresistor (Section 2.5), or Wiegand sensor (Section 4.2.6) can replace the toroidal core. Inductive en- coders are sensitive to stray magnetic ®elds. Electric encoders can be capacitive or contacting encoders. Capacitive en- coders use a patterned strip such as that in the InductosynTM (Section 4.2.4.3) but without shielding between the scale and the ruler. This yields a cyclic change in capacitance with a period equal to the pitch, which can be as low as 0.4 mm. The contacting encoder in Figure 8.3 has a moving element formed by an alumina substrate with a fused glass layer and a conductive palladium±silver pattern screen printed on top of the glass. When kiln-®ring during the fabrica- tion process, the conductive pattern sinks into the glass to yield a 5 mmto8mm step-height di¨erential between the conductor and the insulator surface. The wiper is from precious metal. Former encoders with copper foil etched on printed circuit board had 25 mmto35mm steps, which increased wear and contact bouncing. This silver-in-glass technology o¨ers low cost, ruggedness, high-resistance to corrosion, and life expectancy of up to ®fteen million cycles, far above the 100,000 cycles of former PCB designs. Optical encoders can be based on opaque and transparent regions, on re- ¯ective and nonre¯ective regions, and also on interference fringes. Whatever the case, the ®xed reading head includes a light source (infrared LED), and a photodetector (phototransistor or photodiode). Main problems result from dust-particle buildup, time and temperature drifts for optoelectronic compo- nents, and vibration e¨ects on focusing elements. High-performance sensors have either a lens or an aperture to provide collimated light output and minimal spurious re¯ections. When opaque and transparent regionsÐchromium on glass, slotted metal, and so forth (Figure 8.4a)Ðare used, the emitter and the receiver must be placed on each side of the moving element. In contrast, when relying on re¯ec- tive and nonre¯ective zonesÐfor example, polished steel with an engraved surface (Figure 8.4b)Ðthe emitter and the detector must be on the same side of the coding element. Glass disks are more stable, rigid, hard, and ¯at than metal disks, but are less resistant to vibration and shock. Reference 1 discusses the design of encoders based on integrated optical subsystems. Interference fringe encoders are based on moire patterns. To create them 8.1 POSITION ENCODERS 437 Figure 8.4 Incremental optical encoder. (a) With opaque and transparent sectors. (b) With re¯ective and nonre¯ective sectors. from a linear movement, we can use a ®xed and a movable rule having lines inclined with respect to each other (Figure 8.5). If the inclination a is such that tan a p=d, a relative displacement p (line pitch) produces a vertical displace- ment d of a dark horizontal fringe. If the relative inclination is n times higher, n dark horizontal fringes appear. Interference fringes from a rotary movement can be obtained from two superimposed disks, one ®xed and the other one mov- able, one with N radial lines and the other one with N 1. Interference fringes are also obtained if both disks have N lines but one is o¨ center, or one has N lines with a di¨erent inclination. A light emitter±detector pair yields an almost sinusoidal signal with N cycles/turn for a rotary encoder. Incremental encoders have resolution ranges from 100 counts/turn to 81,000 counts/turn and accuracy up to 3000. When the detector o¨ers two sinusoidal outputs with di¨erent phase shifts, interpolation using various phase-shifting methods can increase the resolution by a factor of up to 256. One interpolation method [2] digitally measures the phase from the quadrature outputs. p p Figure 8.5 Optical incremental encoder based on interference fringes (moire patterns). The horizontal dark fringe moves in the vertical direction when the sliding rule moves horizontally. 438 8 DIGITAL AND INTELLIGENT SENSORS Va Vp cos Ny 8:2 Va Vp sin Ny 8:3 where Vp is the amplitude of the output voltage, N is the number of steps (pitches) in one turn, and y is the current shaft angle, which can be calculated from arctan V =V y b a 8:4 N This is a periodic quantity that gives 2p rad each 360=N angle increment, so that we also need an incremental counter to determine the actual angle. To measure the phase, the 2p rad phase plane is divided in several sectors and the sector corresponding to each Va, Vb pair is stored in a ROM.
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