An 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. Together, the A and B signals indicate both the occurrence of and direction of movement. Many incremental encoders have an additional output signal, typically designated index or Z, which indicates the encoder is located at a particular reference position. Also, some encoders provide a status output (typically designated alarm) that indicates internal fault conditions such as a bearing failure or sensor malfunction. Unlike an absolute encoder, an incremental encoder does not indicate absolute position; it only reports changes in position and the corresponding direction of movement for each change. 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. Incremental encoders report position increments nearly instantaneously, which allows them to monitor the movements of high speed mechanisms in near real-time. Because of this, incremental encoders are commonly used in applications that require precise measurement and control of position and velocity.
Quadrature outputs
An incremental encoder generates its A and B output signals using a quadrature encoding technique. When the encoder moves at a constant velocity, the A and B signals are square waves with a 90° phase difference between them, allowing detection of both movement and direction. At 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. 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.
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. Static, unchanging signals are output on A and B when the encoder is motionless. 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.
Conceptual drawings of quadrature encoder sensing mechanisms Quadrature encoder outputs can be produced by a quadrature-offset pattern read by aligned sensors (left diagram), or by a simple pattern read by offset sensors (right diagram).
Resolution The resolution of an incremental encoder is a measure of the precision of the position information it produces. 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. 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). This differs from the measurement resolution, which refers to the smallest change in position that the encoder can detect. Each signal edge on output A or B corresponds to a discrete position change. Because one full square-wave cycle on A (or B) includes four edges—rising A, rising B, falling A, and falling B—the measurement resolution is one-fourth of the distance represented by a full cycle. For example, a linear encoder with a resolution of 1000 pulses per millimeter has a per-cycle resolution of 1 μm (1 mm / 1000 cycles), yielding a measurement resolution of 250 nm (1 μm / 4).
Symmetry and phase
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 and the duty cycle would be 50%) with a phase difference of exactly 90° between A and B signals. In real encoders, however, due to sensor imperfections and speed variations, the pulse widths are never exactly 180° and the phase difference is never exactly 90°. Furthermore, the A and B pulse widths vary from one cycle to another (and from each other) and the phase difference varies at every A and B signal edge. Consequently, both the pulse width and phase difference will vary over a range of values. For any particular encoder, the pulse width and phase difference ranges are defined by "symmetry" and "phase" (or "phasing") specifications, respectively. For example, in the case of an encoder with symmetry specified as 180° ±25°, the width of every output pulse is guaranteed to be at least 155° and no more than 205°. Similarly, with phase specified as 90° ±20°, the phase difference at every A or B edge will be at least 70° and no more than 110°.
Signal types Incremental encoders employ various types of electronic circuits to drive (transmit) their output signals, and manufacturers often have the ability to build a particular encoder model with any of several driver types. Commonly available driver types include open collector, mechanical, push-pull and differential RS-422.
Open collector
Open collector drivers (using an NPN transistor or open drain drivers using an n-type MOSFET) allow operation over a wide range of signal voltages and often can sink significant output current, making them useful for directly driving current loops, opto-isolators and fiber optic transmitters. Because it cannot source current, the output of an open-collector driver must be connected to a positive DC voltage through a pull-up resistor. Some encoders provide an internal resistor for this purpose; others do not and thus require an external pull-up resistor. In the latter case, the resistor typically is located near the encoder interface to improve noise immunity. The encoder's high-level logic signal voltage is determined by the voltage applied to the pull-up resistor (VOH in the schematic), whereas the low-level output current is determined by both the signal voltage and load resistance (including pull-up resistor). When the driver switches from the low to the high logic level, the load resistance and circuit capacitance act together to form a low-pass filter, which stretches (increases) the signal's rise time and thus limits its maximum switching frequency.
Mechanical
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