Rotary Encoder

11 Key excerpts on "Rotary Encoder"

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  eBook - ePub

  Understanding Position Sensors

  • David Nyce(Author)
  • 2023(Publication Date)
  • CRC Press

    (Publisher)

  13 Encoders

  DOI: 10.1201/9781003368991-13

  13.1 Linear and Rotary

  Linear and rotary position or displacement can be measured and communicated by a device called an encoder without using any form of analog-to-digital (A/D) conversion, because the basic output signal is already in a digital format. Although the term encoder has also been applied to devices based on Moiré patterns from diffraction gratings, as well as laser interferometers, the terms encoder, linear encoder, and Rotary Encoder will be used here in reference to standard industrial sensors based on geometric patterns applied along a linear, angular, or circular scale and detected by any one of several methods. Encoders are available as incremental or absolute reading, and encompass various detection techniques, including brush-type, optical, magnetic, and capacitive types. The most common detection methods for industrial encoders are magnetic and optical. Besides selecting whether the output will be absolute or incremental, encoder designers make trade-offs among important product features, including ruggedness, resolution, physical size, and cost.

  13.2 History of Encoders

  The earliest type of encoder was the brush-type, a linear version of which is depicted in Figure 13.1 . In the figure, flexible mechanical contact fingers rub along a metal pattern printed onto an insulating base. The path of the brush moves over conducting and insulating segments. The conductor pattern is formed onto the base in the same way as printed circuit boards are made for connecting electronic circuits.

  The figure shows a straight binary code. An alternative, the Gray code, is explained later in this chapter. In the figure, the brush holder and four brushes (flexible metallic contact brushes) move along a left-to-right measuring axis. Dark segments represent the electrically conductive areas that would normally be connected to a positive voltage, such as +3.3 or +5 volts DC, to represent a logic one. Light areas are insulated or not electrically connected, and represent a logic zero. So, when a brush is on a light area, the signal from that brush is at logic zero (zero volts), and when a brush is on a dark area, the signal from that brush is at logic one (+3.3 or +5 volts). The brush holder shown is an insulating material, so wires can be connected with each brush to bring their voltages to a measurement or readout circuit. With the brushes in their rightmost position as shown, all four are at logic one, and indicate binary 1111, or decimal 15 (or hexadecimal F). At the leftmost position, binary 0000, or decimal 0 would be indicated. As the brushes move to the right from zero, the indicated position increments by 1 for each new position. These positions can be called digital positions zero through fifteen, or numeral positions one through sixteen. This simple pictorial, having only 4 bits, is shown for simplicity. Normal encoders will usually have at least eight, and up to 18 bits. Eight bits can indicate positions 0 through 255, for a resolution of about 0.4% (that is 100% × 1/256), while 18 bits can indicate 0 through 262,143 (note: 218

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  Mechanical Design and Manufacturing of Electric Motors

  • Wei Tong(Author)
  • 2022(Publication Date)
  • CRC Press

    (Publisher)

  Some typical applications of Rotary Encoders include monitoring motor shaft position and velocity, tracking satellites with telescopes, and detecting robot arm movements. Linear encoders are used in linear motion systems, metrology instruments, precision jig borers, and high precision machine tools, ranging from digital calipers, CMM, CNC milling machines, to X-Y stages, gantry tables, and semiconductor steppers. 8.1.1 Type of Encoder Encoders can be categorized in a number of ways, generally by their working principle, signal generation, method of signal interpretation, and so on. As an example, according to the working principle, encoders can be grouped into several categories such as optical, magnetic, capacitive, and inductive. 8.1.1.1 Optical Encoder Optical encoders rely on optoelectronic components to detect rotary or linear motion. Optical encoders have long been recognized as indispensable displacement/position sensors due to their advantages of high resolution, high accuracy, fast dynamic response, lightweight, and long life (the lifetime is ultimately limited by the LED). Thus, they are particularly suited to high precision industrial automated systems, such as semiconductor fabrication. However, optical encoders often suffer from reduced reliability due to environmental contaminants. Dust, moisture, dirt, and other foreign debris can significantly degrade the accuracy of the measurements and lower the operation reliability. An optical motor encoder is a motion detector that provides feedback signals to a closed-loop control system. Mounted on a motor shaft, the encoder converts the mechanical position of the shaft to digital or analog signals, used to determine the position and velocity of the shaft

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  eBook - ePub

  Handbook of Asynchronous Machines with Variable Speed

  • Hubert Razik(Author)
  • 2013(Publication Date)
  • Wiley-ISTE

    (Publisher)

  max ). This figure illustrates a remarkable phenomenon created by the transition zone. This zone defines the operating mode giving the best precision. We therefore find that at low speed the method of period counting is advised. At high speeds, it is the method of frequency counting that is recommended. There is therefore a critical frequency that, if the electronics permit, will change the measurement mode of the rotation speed.

  1.3. The resolver

  A relevant question here is: what is a resolver ?

  A resolver is an analog sensor of angular position. Its make-up is similar to that of a rotary transformer. Traditionally, the measurement of speed and position is done by the use of an optical encoder. Nevertheless, this technique is not without its limits. The precision of the measurement is inversely proportional to the number of lines per rotation. The most frequently used have between 500 and 4,096 lines per revolution. One of the disadvantages is related to the cost, which is exponential depending on the number of lines and therefore on the angular resolution. Its use requires mechanical precautions. It is necessary to avoid mechanical disturbances (vibrations) that lead to premature wearing of the encoder, and implicates the use of specific coupling (and is therefore expensive) that allows mechanical defaults. It is also necessary to ensure that the maximum rotation speed is not exceeded due to the risk of irreversible alterations to the lines, leading to irreparable distortion of the measurements. We must not forget that that an encoder, whichever it is, requires an electronic conversion interface.

  Before going into more detail on the operation and the interface necessary for the reconstruction of the angular position, we can set out some properties of this device.

  A resolver is a rotary transformer whose rotor and stator are both coiled (see Figure 1.8 ).

  Figure 1.8.

  Principle diagram of a resolver

  There is an advantage in the external aspect of the resolver . In effect, this is joined to the electric motor. If this is explosion-proof, then the resolver will also be so. The axis of the said rotor will be coupled with the machine whose speed we want to measure. This will be excited by sinusoidal voltage of constant amplitude and frequency (for example,

  Vexc = V cos

  (ωt ) with V = 7v effective and f

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  eBook - ePub

  Measurement and Instrumentation

  Theory and Application

  • Alan S. Morris, Reza Langari(Authors)
  • 2011(Publication Date)
  • Butterworth-Heinemann

    (Publisher)

  Unlike the incremental shaft encoder that gives a digital output in the form of pulses that have to be counted, the digital shaft encoder has an output in the form of a binary number of several digits that provides an absolute measurement of shaft position. Digital encoders provide high accuracy and reliability. They are particularly useful for computer control applications, but have a significantly higher cost than incremental encoders. Three different forms exist, using optical, electrical, and magnetic energy systems, respectively.

  Optical digital shaft encoder

  The optical digital shaft encoder is the least expensive form of encoder available and is the one used most commonly. It is found in a variety of applications; one where it is particularly popular is in measuring the position of rotational joints in robot manipulators. The instrument is similar in physical appearance to the incremental shaft encoder. It has a pair of discs (one movable and one fixed) with a light source on one side and light detectors on the other side, as shown in Figure 20.5 . The fixed disc has a single window, and the principal way in which the device differs from the incremental shaft encoder is in the design of the windows on the movable disc, as shown in Figure 20.6 . These are cut in four or more tracks instead of two and are arranged in sectors as well as tracks. An energy detector is aligned with each track, and these give an output of “1” when energy is detected and an output of “0” otherwise. The measurement resolution obtainable depends on the number of tracks used. For a four-track version, the resolution is 1 in 16, with progressively higher measurement resolution being attained as the number of tracks is increased. These binary outputs from the detectors are combined together to give a binary number of several digits. The number of digits corresponds to the number of tracks on the disc, which, in the example shown in Figure 20.6 , is four. The pattern of windows in each sector is cut such that as that particular sector passes across the window in the fixed disc, the four energy detector outputs combine to give a unique binary number. In the binary-coded example shown in Figure 20.6

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  eBook - ePub

  Sensor Systems

  Fundamentals and Applications

  • Clarence W. de Silva(Author)
  • 2016(Publication Date)
  • CRC Press

    (Publisher)

  Any transducer that generates a coded (digital) reading of a measurement can be termed as an encoder. Shaft encoders are digital transducers that measure angular displacements and angular velocities. Applications of these devices include motion measurement in performance monitoring and control of robotic manipulators, machine tools, industrial processes (e.g., food processing and packaging, pulp and paper), digital data storage devices, positioning tables, satellite mirror positioning systems, vehicles, construction machinery, planetary exploration devices, battlefield equipment, and rotating machinery such as motors, pumps, compressors, turbines, and generators. High resolution (which depends on the word size of the encoder output and the number of pulses generated per revolution of the encoder), high accuracy (particularly due to noise immunity and reliability of digital signals and superior construction), and relative ease of adoption in digital systems (because transducer output can be read as a digital word), with the associated reduction in system cost and improvement of system reliability, are some of the relative advantages of digital transducers in general and shaft encoders in particular, in comparison with their analog counterparts.

  11.2.1  Encoder Types

  Shaft encoders can be classified into two categories depending on the nature and the method of interpretation of the transducer output: (1) incremental encoders and (2) absolute encoders.

  Incremental encoder: The output of an incremental encoder is a pulse signal, which is generated when the transducer disk rotates due to the motion that is measured. By counting the pulses or by timing the pulse width using a clock signal, both angular displacement and angular velocity can be determined. With an incremental encoder, displacement is measured with respect to some reference point. The reference point can be the home position of the moving component (say, determined by a limit switch) or a reference point on the encoder disk, as indicated by a reference pulse (index pulse) generated at that location on the disk. Furthermore, the index pulse count determines the number of full revolutions.

  Absolute encoder

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  Measurement and Instrumentation Principles

  • Alan S. Morris(Author)
  • 2001(Publication Date)
  • Butterworth-Heinemann

    (Publisher)

  The maximum number of tracks possible is ten, which limits the resolution to 1 part in 1000. Thus, contacting digital shaft encoders are only used where the environmental conditions are too severe for optical instruments. Magnetic digital shaft encoder Magnetic digital shaft encoders consist of a single rotatable disc, as in the contacting form of encoder discussed in the previous section. The pattern of sectors and tracks consists of magnetically conducting and non-conducting segments, and the sensors aligned with each track consist of small toroidal magnets. Each of these sensors has a coil wound on it that has a high or low voltage induced in it according to the magnetic field close to it. This field is dependent on the magnetic conductivity of that segment of the disc that is closest to the toroid. These instruments have no moving parts in contact and therefore have a similar reli-ability to optical devices. Their major advantage over optical equivalents is an ability to operate in very harsh environmental conditions. Unfortunately, the process of manufac-turing and accurately aligning the toroidal magnet sensors required makes such instru-ments very expensive. Their use is therefore limited to a few applications where both high measurement resolution and also operation in harsh environments are required. 398 Rotational motion transducers 20.1.5 The resolver The resolver, also known as a synchro-resolver , is an electromechanical device that gives an analogue output by transformer action. Physically, resolvers resemble a small a.c. motor and have a diameter ranging from 10 mm to 100 mm. They are friction-less and reliable in operation because they have no contacting moving surfaces, and consequently they have a long life. The best devices give measurement resolutions of 0.1%. Resolvers have two stator windings, which are mounted at right angles to one another, and a rotor, which can have either one or two windings.

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  Sensors and Signal Conditioning

  • Ramón Pallás-Areny, John G. Webster(Authors)
  • 2012(Publication Date)
  • Wiley-Interscience

    (Publisher)

  8.1 POSITION ENCODERS Linear and angular position sensors are the only type of digital output sensors that are available in several different commercial models. Incremental encoders 433 434 8 DIGITAL AND INTELLIGENT SENSORS Linear movement 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 Cliffs, 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 differentiates them, and these regions are arranged in a repetitive pattern (Figure 8.1). Sensing that property by a fixed head or reading device yields a definite output change when there is an increment in position equal to twice the pitch p. A disk with diame-ter d gives m nd Yp (8.1) 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 differentiation 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

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  Sensors for Mechatronics

  • Paul P.L. Regtien, Edwin Dertien(Authors)
  • 2018(Publication Date)
  • Elsevier

    (Publisher)

  The optical sensors described in this section are digital in nature. They convert, through an optical intermediate, the measurement quantity into a binary signal, representing a binary coded measurement value. Sensor types that belong to this category are optical encoders, designed for measuring linear and angular displacement, optical tachometers (measuring angular speed or the number of revolutions per unit of time) and optical bar code systems for identification purposes.

  Optical encoders are composed of a light source, a light sensor and a coding device (much the same as the general setup in Fig. 7.1 ). The coding device consists of a flat strip for linear displacement or disk for angular displacement, containing a pattern of alternating opaque and transparent segments (the transmission mode) or alternating reflective and absorbing segments (the reflection mode). Both cases are illustrated in Fig. 7.12 .

  Figure 7.12 Optical encoders in (A) transmission mode and (B) reflection mode.

  The coding device can move relatively to the assembly of transmitter and receiver causing the radiant transfer between them switch between a high and a low value. In the transmission mode the encoder consists either of a translucent material (e.g., glass, plastic, and mylar), covered with a pattern of an opaque material (for instance a metallization), or just the reverse, for instance a metal plate with slots or holes.

  Two basic encoder types are distinguished: absolute and incremental encoders. An absolute encoder gives instantaneous information about the absolute displacement or the angular position. Fig. 7.13 gives examples of absolute encoder devices in transmission mode.

  Figure 7.13 Absolute encoders: (A) linear encoders with 4-bit dual code and with 4-bit gray code, (B) readout system with collimator, and (C) angular encoder disc.

  Each (discrete) position corresponds to a unique code, which is obtained by an optical readout system that is basically a multiple version of Fig. 7.12A . The acquisition of absolute position with a resolution of n bits requires at least n optical tracks on the encoder and n

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  eBook - ePub

  Introduction to Sensors for Electrical and Mechanical Engineers

  • Martin Novák(Author)
  • 2020(Publication Date)
  • CRC Press

    (Publisher)

  An IRC is a relative sensor. After power-up, the absolute position is not available. Another device is required to set the reference position, such as an end switch. Devices equipped with relative sensors require homing after power up. On the other hand, relative sensors have typically higher resolution than absolute Rotary Encoders.

  Figure 7.47 shows an IRC used to measure the angular position of a tram wheel. Another industrial example is an IRC in a torque sensor shown in figure 7.48 . An IRC code disc is shown in figure 7.49 . An industrial IRC is shown in figure 7.50 .

  FIGURE 7.47: An IRC used to measure the angular position of a tram wheel—photo author

  FIGURE 7.48: An IRC in a torque sensor—photo author

  FIGURE 7.49: IRC code disc, photo author

  FIGURE 7.50: IRC—photo author

    7.7Absolute Rotary Encoders

  An absolute Rotary Encoder operates in a similar way as an IRC. However, it has more tracks on the code wheel. Each track encodes 1 bit. An example of the code wheel is shown in figure 7.51 for an 2-bit encoder (4 positions per revolution).

  FIGURE 7.51: Code disc for a 4=bit absolute Rotary Encoder—left with binary code, right with Gray code

  The absolute position is encoded in Gray code. Gray code is a special form of binary encoding; it changes only 1 bit at a time for each transition. This allows simple error detection. An example of Gray code for 2 bits is shown in table 7.2 .

  TABLE 7.2: Binary vs. Gray code for 2 bits

  Binary	Gray
  00	00
  01	01
  10	11
  11	10

  The available space in the encoder housing limits the achievable resolution. Therefore absolute Rotary Encoders have typically smaller resolution and are more expensive than relative IRCs.

  Absolute Rotary Encoders have typically a resolution of 210 to 212 bits = 1024 to 4096 positions / revolution. An example of a code wheel for an absolute position encoder with resolution of 10 bits (1024 positions/revolution), Gray code, is shown in figure 7.52 .

  FIGURE 7.52: Code wheel for an absolute Rotary Encoder, 10 bit (1024 positions/revolution), Gray code, generated with [41 ]

    7.8Microwave position sensor (radar)

  This sensor type is used to measure both large and short distances. Based on the required range the suitable principle is selected. For short distances, Frequency Modulated Continuous Wave (FMCW); for large distances, Time of Flight (TOF).

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  Mechatronics

  A Foundation Course

  • Clarence W. de Silva(Author)
  • 2010(Publication Date)
  • CRC Press

    (Publisher)

  Sensors and Transducers 435 of the stator have a carrier component at the supply frequency and a modulating compo-nent corresponding to the rotation of the disk. The latter (modulating component) can be extracted through demodulation and converted into a proper pulse signal using pulse-shaping circuitry, as for an incremental encoder. When the rotating speed is constant, the two modulating components are periodic and nearly sinusoidal with a phase shift of 90° (i.e., in quadrature). When the speed is not constant, the pulse width will vary with time. Angular displacement and angular velocity are determined as in the case of an incre-mental encoder by counting the pulses. Very fine resolutions (e.g., 0.0005°) may be obtained from a digital resolver, and it is usually not necessary to use step-up gearing or other tech-niques to improve the resolution. These transducers are usually more expensive than opti-cal encoders. The use of a slip ring and brush to supply the carrier signal may be viewed as a disadvantage. 6.14.2 Digital Tachometers A pulse-generating transducer whose pulse train is synchronized with a mechanical motion may be treated as a digital transducer for motion measurement. Pulse counting may be used for displacement measurement and the pulse rate (or pulse timing) may be used for velocity measurement. According to this terminology, a shaft encoder (particularly, an incremental optical encoder) may be considered as a digital tachometer. According to the popular terminology, however, a digital tachometer is a device that employs a toothed wheel to measure angular velocities. A schematic diagram of a digital tachometer is shown in Figure 6.54. This is a mag-netic induction, pulse tachometer of the variable-reluctance type. The teeth on the wheel are made of a ferromagnetic material. The two magnetic-induction (and variable-reluctance) proximity probes are placed radially facing the teeth, at quarter-pitch apart (pitch = tooth-to-tooth spacing).

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  Mechatronics

  An Integrated Approach

  • Clarence W. de Silva(Author)
  • 2004(Publication Date)
  • CRC Press

    (Publisher)

  Another way to measure rectilinear motions using a rotary sensor is to use a modified sensor that has the capability to convert a rectilinear motion into a rotary motion within the sensor itself. An example would be the cable extension method of sensing rectilinear motions. This method is particularly suitable for measuring motions that have large excursions. The cable extension method uses an angular motion sensor with a spool rigidly coupled to the rotating part of the sensor (e.g., the encoder disk) and a cable that wraps around the spool. The other end of the cable is attached to the object whose rectilinear motion is to be sensed. The housing of the rotary sensor is firmly mounted on a stationary platform, so that the cable can extend in the direction of motion. When the object moves, the cable extends, causing the spool to rotate. This angular motion is measured by the rotary sensor. With proper calibration, this device can give rectilinear measurements directly. As the object moves toward the sensor, the cable has to retract without slack. This is guaranteed by using a device such as a spring motor to wind the cable back. The disadvantages of the cable extension method include mechanical loading of the moving object, time delay in measurements, and errors caused by the cable, includ-ing irregularities, slack, and tensile deformation. 7.6.7 Binar y Transducers Digital binary transducers are two-state sensors. The information provided by such a device takes only two states (on/off, present/absent, go/no-go, etc.); it can be represented by one bit. For example, limit switches are sensors used in detecting whether an object has reached its limit (or, destination) of mechanical motion, and are useful in sensing presence/absence and in object counting. In this sense, a limit switch is considered a digital transducer. Additional logic is needed if the direction of contact is also needed. Limit switches are available for both rectilinear and angular motions.

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