![]() In real encoders, however, due to sensor imperfections, the pulse widths are never exactly 180° and the phase difference is never exactly 90°. When moving at constant velocity, an ideal incremental encoder would output perfect square waves on A and B (i.e., the pulses would be exactly 180° wide) with a phase difference of exactly 90° between A and B. Incremental encoders exhibit symmetry and phase errors due to sensor imperfections For example, a 1000 pulse-per-mm linear encoder has a per-cycle measurement resolution of 1 mm / 1000 cycles = 1 μm, so this encoder's resolution is 1 μm / 4 = 250 nm. Since each square-wave cycle on A (or B) encompasses four signal edges (rising A, rising B, falling A and falling B), the encoder's measurement resolution equals one-fourth of the displacement represented by a full A or B output cycle. Every signal edge on A or B indicates a detected position change. This is in contrast to the measurement resolution of the encoder, which is the smallest position change that the encoder can detect. In the case of rotary encoders, resolution is specified as the number of pulses per revolution (PPR) or cycles per revolution (CPR), whereas linear encoder resolution is typically specified as the number of pulses issued for a particular linear traversal distance (e.g., 1000 pulses per mm). Encoder resolution is typically specified in terms of the number of A (or B) pulses per unit displacement or, equivalently, the number of A (or B) square wave cycles per unit displacement. The resolution of an incremental encoder is a measure of the precision of the position information it produces. Linear encoder the R signal indicates the encoder is located at its reference position In the case of a rotary encoder, the frequency indicates the speed of the encoder's shaft rotation, and in linear encoders the frequency indicates the speed of linear traversal.Ĭonceptual drawings of quadrature encoder sensing mechanisms Static, unchanging signals are output on A and B when the encoder is motionless. The frequency of the pulses on the A or B output is directly proportional to the encoder's velocity (rate of position change) higher frequencies indicate rapid movement, whereas lower frequencies indicate slower speeds. In the case of a rotary encoder, the phase difference is +90° for clockwise rotation and −90° for counter-clockwise rotation, or vice versa, depending on the device design. Īt any particular time, the phase difference between the A and B signals will be positive or negative depending on the encoder's direction of movement. The pulses emitted from the A and B outputs are quadrature-encoded, meaning that when the incremental encoder is moving at a constant velocity, the A and B waveforms are square waves and there is a 90 degree phase difference between A and B. The direction of motion is indicated by the sign of the A- B phase difference which, in this case, is negative because A trails B.Īn incremental encoder employs a quadrature encoder to generate its A and B output signals. Because of this, incremental encoders are commonly used in applications that require precise measurement and control of position and velocity. Incremental encoders report position changes nearly instantaneously, which allows them to monitor the movements of high speed mechanisms in near real-time. Consequently, to determine absolute position at any particular moment, it is necessary to send the encoder signals to an incremental encoder interface, which in turn will "track" and report the encoder's absolute position. Unlike an absolute encoder, an incremental encoder does not indicate absolute position it only reports changes in position and, for each reported position change, the direction of movement. Also, some encoders provide a status output (typically designated alarm) that indicates internal fault conditions such as a bearing failure or sensor malfunction. Many incremental encoders have an additional output signal, typically designated index or Z, which indicates the encoder is located at a particular reference position. ![]() Together, the A and B signals indicate both the occurrence of and direction of movement. Introduction to incremental encoders, from VideoWiki script Incremental EncoderĪn incremental encoder is a linear or rotary electromechanical device that has two output signals, A and B, which issue pulses when the device is moved.
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