Incremental encoder
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 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 ; 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, or by a simple pattern read by offset sensors.
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 pulses per unit displacement or, equivalently, the number of A square wave cycles per unit displacement. In the case of rotary encoders, resolution is specified as the number of pulses per revolution or cycles per revolution, whereas linear encoder resolution is typically specified as the number of pulses issued for a particular linear traversal distance.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 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, yielding a measurement resolution of 250 nm.
Symmetry and phase
When moving at constant velocity, an ideal incremental encoder would output perfect square waves on A and B 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 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" 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 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
drivers 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, whereas the low-level output current is determined by both the signal voltage and load resistance. 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 the signal's rise time and thus limits its maximum switching frequency.
Mechanical
Mechanical incremental encoders use sliding electrical contacts to directly generate the A and B output signals. Typically, the contacts are electrically connected to signal ground when closed so that the outputs will be "driven" low, effectively making them mechanical equivalents of open collector drivers and therefore subject to the same signal conditioning requirements.The maximum output frequency is limited by the same factors that affect open-collector outputs, and further limited by contact bounce and by the operating speed of the mechanical contacts, thus making these devices impractical for high frequency operation. Furthermore, the contacts experience mechanical wear under normal operation, which limits the life of these devices. On the other hand, mechanical encoders may be relatively inexpensive and have no internal, active electronics. These attributes make mechanical encoders a good fit for hand-operated controls and a variety of other low duty, low frequency applications.
Push-pull
s typically are used for direct interface to logic circuitry. These are well-suited to applications in which the encoder and interface are located near each other and powered from a common power supply, thus avoiding exposure to electric fields, ground loops and transmission line effects that might corrupt the signals and thereby disrupt position tracking, or worse, damage the encoder interface.Differential pair
Differential RS-422 signaling is typically preferred when the encoder will output high frequencies or be located far away from the encoder interface, or when the encoder signals may be subjected to electric fields or common-mode voltages, or when the interface must be able to detect connectivity problems between encoder and interface. Examples of this include CMMs and CNC machinery, industrial robotics, factory automation, and motion platforms used in aircraft and spacecraft simulators.When RS-422 outputs are employed, the encoder provides a differential conductor pair for every logic output; for example, "A" and "/A" are commonly used designations for the active-high and active-low differential pair comprising the encoder's A logic output. Consequently, the encoder interface must provide RS-422 line receivers to convert the incoming RS-422 pairs to single-ended logic.
Principal applications
Position tracking
Incremental encoders are commonly used to monitor the physical positions of mechanical devices. The incremental encoder is mechanically attached to the device to be monitored so that its output signals will change as the device moves. Example devices include the balls in mechanical computer mice and trackballs, control knobs in electronic equipment, and rotating shafts in radar antennas.An incremental encoder does not keep track of, nor do its outputs indicate the current encoder position; it only reports incremental changes in position. Consequently, to determine the encoder's position at any particular moment, it is necessary to provide external electronics which will "track" the position. This external circuitry, which is known as an incremental encoder interface, tracks position by counting incremental position changes.
As it receives each report of incremental position change, an encoder interface will take into account the phase relationship between A and B and, depending on the sign of the phase difference, count up or down. The cumulative "counts" value indicates the distance traveled since tracking began. This mechanism ensures accurate position tracking in bidirectional applications and, in unidirectional applications, prevents false counts that would otherwise result from vibration or mechanical dithering near an AB code transition.