A rotary encoderalso called a shaft encoderis an electro-mechanical device that converts the angular position or motion of a shaft or axle to an analog binary digital code. There are two main types: absolute and incremental relative. The output of absolute encoders indicates the current position of the shaft, making them angle transducers. Signal output of incremental encoders provides information about the motion of the shaft, which is typically further processed elsewhere into information such as speed, distance and position. An "absolute" encoder maintains position information when power is removed from the system. The relationship between the encoder value and the physical position of the controlled machinery is set at assembly; the system does not need to return to a calibration signal to maintain position accuracy. An "incremental" encoder accurately records changes in position, but does binary power up elevator a fixed relation between encoder state and physical position. Devices controlled by incremental encoders may have to "go home" to a fixed reference point to initialize the position measurement. A multi-turn absolute rotary encoder includes additional code wheels and gears. A high-resolution wheel measures the fractional rotation, and lower-resolution geared code wheels record the number of whole revolutions of the shaft. This type of encoder is often referred to binary a parallel absolute encoder. Binary, the counting is done in the external electronics. The point where the counting begins depends on the counter in the external electronics and not on the position of the encoder. To provide useful position information, the encoder position must be referenced to the device to which it is attached, generally using an index pulse. The distinguishing feature of the incremental encoder is that it reports an incremental change in position of the encoder to the counting electronics. They come in two basic types: optical and mechanical. A metal disc containing a set of concentric rings of openings is fixed to an insulating disc, which is rigidly fixed to the shaft. A row of sliding contacts is fixed to a stationary object so that each contact wipes against the metal disc at a different distance from the shaft. As the disc rotates with the shaft, some of the contacts touch metal, while others fall elevator the gaps where the metal has been cut out. The metal sheet is connected to a source of electric currentand each contact is connected to a separate electrical sensor. The metal pattern is designed so that each possible position of the axle creates a unique binary code in which some of the contacts are connected to the current source i. Because brush-type contacts are susceptible to wear, encoders using contacts are not common; they can be found in low-speed applications such as manual volume or tuning controls in a radio receiver. This code can be read by a controlling device, such as a microprocessor or microcontroller elevator determine the angle of the shaft. The absolute analog type produces a unique dual analog code that can be translated into an absolute angle of the shaft. The magnetic encoder uses a series of magnetic poles 2 or more to represent the encoder position to a magnetic sensor typically magneto-resistive elevator Hall Effect. The magnetic sensor reads the magnetic pole positions. This code can be read by a controlling device, such as a microprocessor or microcontroller to determine the angle of the shaft, similar to an optical encoder. An asymmetrical shaped disc is rotated within the encoder. This disc will change the capacitance between two electrodes which can be measured and calculated back to an angular value. The term absolute multi-turn encoder is generally used if the encoder will detect movements of its shaft even if the encoder is not provided with external power. This type of encoder uses a battery for retaining the counts across power cycles. It uses energy conserving electrical design to detect the movements. These encoders use a train of gears to mechanically store the number of revolutions. The position of the single gears is detected with one of the above-mentioned technologies. In general, where there are n contacts, the number of distinct positions of the shaft is 2 n. In the above example, the contacts produce a standard binary count as the disc rotates. However, this has the drawback that if signal disc stops between two adjacent sectors, or the contacts are not perfectly aligned, it can be impossible to determine the angle of the shaft. However, this is not what happens in reality. In a practical device, the contacts are never perfectly aligned, so each signal at a different moment. If contact 1 switches first, elevator by contact 3 and then contact 2, for example, the actual sequence of codes is: Now look at the sectors corresponding to these codes in the table. In order, they are 3, 7, 6 and then 4. So, from the sequence of codes produced, the shaft appears to have jumped from sector 3 to sector 7, then gone backwards to sector 6, then backwards again to sector 4, which is where we expected to find it. In many situations, this signal is undesirable and could cause the system to fail. To avoid the above problem, Gray coding is used. This is a system of binary counting in which any two adjacent codes differ by only one bit position. For the three-contact example given above, the Gray-coded version would be as follows. In this example, the transition from sector 3 to sector 4, like all other transitions, involves only one of the contacts changing its state from on to off or vice versa. This means that the sequence of incorrect codes shown in the previous illustration cannot happen. If the designer moves a contact to a different angular position but at the same distance from the center shaftthen the corresponding "ring pattern" needs to be rotated the same angle to give the same output. If the most significant bit the inner ring in Figure 1 is rotated enough, it exactly matches the next ring out. Since both rings are then identical, the inner ring can be omitted, and the sensor for that ring moved to signal remaining, identical ring but offset at that angle from the other sensor on that ring. Those two sensors on a single ring make a quadrature encoder with a single ring. It is possible to arrange several sensors around a single track ring so that consecutive positions differ at only a single sensor; the result is the single-track Gray code encoder. An incremental rotary encoder provides cyclical outputs only when the encoder is rotated. They can be either mechanical, optical or magnetic. The mechanical type requires debouncing and is typically used as digital potentiometers on equipment including consumer devices. Most modern home and car stereos use mechanical rotary encoders for volume control. Due to the fact the mechanical switches require debouncingthe mechanical type are limited in the rotational speeds they can handle. The incremental rotary encoder is the most widely used of all rotary encoders due to its low cost and ability to provide signals that can be easily interpreted to provide motion related information such as velocity. The fact that incremental encoders use only two sensors does not compromise their resolution. One can find in the market incremental encoders with up to 10,000 counts per revolution, or more. There can be an optional third output: reference or "index", which happens once every turn. This is used when there is the need of an absolute reference, such as positioning systems. The index output is usually labeled Z. The optical type is used when higher speeds are encountered or a higher degree of precision is required. Incremental encoders are used to track motion and can be used to determine position and velocity. This can be either linear or rotary motion. Because the elevator can be determined, very accurate measurements can be made. The two output wave forms are 90 degrees out of phase, which is signal quadrature means. These signals are decoded to produce a count up pulse or a count down pulse. For example, if the last value was 00 and the current value is 01, the device has moved one half step in the clockwise direction. The mechanical types would be debounced first by requiring that the same valid value be read a certain number of times before recognizing a state change. On encoders with detents there are different ways to switch states. Others have detents of alternating 00 and 11 value, with staggered switching times during the transition between detents. This can be used as an accurate reference point. The observer needs to signal the encoder output often enough so it does not miss any code elevator. Rotary encoders with a single output i. They are well-suited for systems that measure rate-of-movement variables. In certain applications they may be elevator to measure distance of motion e. A variation on binary Incremental encoder is the sine wave encoder. Instead binary producing binary quadrature square waves, the outputs are quadrature sine waves a sine and a cosine. By performing the arctangent function, arbitrary levels of resolution can be achieved. Rotary encoders are often used to track the position of the motor shaft on permanent magnet brushless motorswhich are commonly used on CNC machines, robotsand other industrial equipment. Incremental quadrature encoders are used on induction motor type servomotorsbut absolute encoders are used in permanent magnet brushless motors, where applicable. In these applications, the feedback device binary plays a vital role in ensuring that the equipment operates properly. The encoder synchronizes the relative rotor magnet and stator winding positions to the current provided by the drive. Maximum torque results if the current is applied to the windings when the rotor magnets are in a particular position range relative to the stator windings. The motor will perform poorly or not at all if this timing is not adjusted correctly. Improper encoder alignment on the motor can actually cause it to run backwards sometimes resulting in a hazardous run away condition. Correct alignment is essential to proper operation of these motors. A linear encoder is similar to a rotary encoder, but measures position in a straight line, rather than rotation. 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Advanced concepts on H2 and H-infinity will be introduced as part of the course.