An incremental encoder is a feedback device that reports motion as a stream of pulses rather than an absolute position. As the shaft turns, it emits pulses on two channels, and by counting those pulses and watching their order the controller tracks how far and which way the shaft has moved. It does not directly say where the shaft is in absolute terms; it says how much it has changed since counting began. This page explains the A and B quadrature channels, the Z index pulse, pulses-per-revolution and quadrature edge multiplication, and why an incremental encoder loses its reference on power loss and must be homed.
Incremental Encoder in one line: An incremental encoder reports relative motion by emitting pulses on two quadrature channels, A and B, as the shaft turns, plus one Z index pulse per revolution. Counting the edges gives distance and the A-to-B phase relationship gives direction. Because it counts change rather than storing absolute position, it loses its reference when power is removed and must be homed at startup.
An incremental encoder produces two square-wave signal channels, called A and B, as the shaft rotates. These channels are offset from each other by a quarter cycle, ninety degrees of phase, which is what quadrature means. That deliberate offset carries the direction information: when the shaft turns one way, channel A leads channel B, and when it turns the other way, B leads A. By watching which channel changes first, the controller knows not just that the shaft moved but which direction it moved.
Counting the pulses on these channels gives the distance moved. Each pulse corresponds to a fixed increment of shaft rotation, so accumulating a running count of pulses, adding when the shaft turns forward and subtracting when it reverses, yields the shaft's position relative to wherever counting started. This is the essence of incremental sensing: it measures change, and the controller integrates that change into a position count.
Many incremental encoders add a third channel, Z, also called the index or marker. It produces exactly one pulse per full revolution at a fixed mechanical angle. The index does not measure motion; it provides a once-per-turn reference mark. During homing, the controller can use the index pulse to establish a precise, repeatable zero, so the running count is anchored to a known physical position rather than to an arbitrary power-on point.
The resolution of an incremental encoder is stated as pulses per revolution, or PPR, sometimes called lines per revolution: the number of complete cycles each channel produces in one full turn of the shaft. A higher PPR means finer resolution, because each pulse represents a smaller slice of rotation, and the controller can therefore distinguish smaller movements. PPR is a fixed property of the encoder's construction.
The effective resolution is better than the raw PPR because of quadrature edge multiplication. Since channels A and B are offset by ninety degrees, one cycle of the pair contains four distinct edges, A rising, B rising, A falling, B falling, spaced evenly. A controller that counts every edge of both channels registers four counts for each cycle, so it resolves four times the PPR. This is often called four-times, or 4x, quadrature decoding, and it is why an encoder rated at a given PPR delivers four counts per line in the position count.
This matters when choosing an encoder for an application. The counts per revolution available to the control system is the PPR multiplied by the quadrature factor, and that number, combined with the mechanics between the encoder and the load, sets how finely the system can resolve the actual position of the moving part. Understanding the multiplication avoids both under-specifying resolution and being surprised by count values four times larger than the nameplate PPR.
The defining limitation of an incremental encoder follows directly from how it works. Because it only counts change from wherever it started, it has no memory of absolute position. When power is removed, the running count is lost, and when power returns, the controller has no idea where the shaft actually is; it only knows it will start counting from zero again, wherever the machine happens to be sitting. The measured position after a power cycle is meaningless until a reference is reestablished.
This is why machines with incremental encoders perform a homing routine at startup. The axis moves, often slowly, until it finds a known reference, a home switch, a hard stop, or the encoder's own Z index pulse, and the controller sets the position count to a defined value at that point. From then on the incremental count is anchored to real physical position and the axis can operate accurately. Homing is a required, sometimes time-consuming, step every time the machine powers up or loses position, which is a real operational consideration.
For monitoring, the practical concern is knowing whether an axis is homed and reliably referenced, because an unhomed incremental axis reports positions that mean nothing. A cloud SCADA platform such as Merobix can surface homing status and startup state alongside the rest of the equipment data, so operators, especially those supervising remote or distributed machines, can see at a glance whether a machine has completed its homing and is ready to run, rather than trusting a position reading that has not yet been anchored.
A and B are two square-wave channels offset by ninety degrees; counting their pulses gives distance and their phase order gives direction, telling the controller which way the shaft turned. Z is the index, producing one pulse per revolution at a fixed angle. Z does not measure motion; it provides a once-per-turn reference used to establish a precise zero during homing.
An incremental encoder only counts change from wherever counting began, so it has no memory of absolute position. When power is lost the count is lost, and at power-up the controller does not know the true shaft position. Homing moves the axis to a known reference and sets the count to a defined value, anchoring the incremental count to real physical position.
Because channels A and B are offset by ninety degrees, each cycle of the pair contains four evenly spaced edges. A controller that counts all four edges resolves four counts per encoder line, giving four times the raw pulses-per-revolution. This 4x quadrature decoding is why the counts per revolution seen by the control system are four times the nameplate PPR.
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