Incremental to Absolute Value Encoder: How Position Feedback Works

• 1
  How an Incremental Encoder Builds Position
• 2
  How an Absolute Value Encoder Reads Position Directly
• 3
  Where Multi-Turn Absolute Encoders Earn Their Place
• 4
  Choosing Between Incremental and Absolute Feedback
• 5
  Mounting, Alignment, and Signal Integrity
• 6
  What Encoder Choice Means on the Machine
• 7
  Incremental vs Single-Turn vs Multi-Turn Absolute

An incremental encoder puts a slotted or etched disc between a light source and a photodetector. As the shaft turns, the disc chops the beam into a square wave. The drive electronics count the rising edges. Position exists only as a count kept in memory, not as a value stored on the disc.

Two channels, A and B, sit a quarter of a period apart. That phase offset tells the counter which way the shaft turned. A third channel, Z, fires once per revolution and gives a home reference. Without Z, the count has no fixed origin, so the control has to hunt for a reference on every power-up.

Resolution comes from line count. A 2,500 line encoder with quadrature gives 10,000 counts per revolution, which on a 10 mm pitch ball screw is about 1 µm per count. That number is the smallest step the drive can see, not the accuracy of the machine.

Counting is the weakness. If a pulse is lost to electrical noise, a loose shield, or a power cut, the count drifts. The drive keeps moving to the wrong position and no alarm fires until a hard limit trips. This is why incremental feedback usually wants a homing routine and a battery-backed or mechanical reference.

• 1
  A/B channels90° phase offset sets direction
• 2
  Z channelone pulse per revolution, the home mark
• 3
  Line countsets step size, not machine accuracy

An absolute encoder writes a unique code at every angular position. The disc carries several concentric tracks, each read by its own optical or magnetic sensor. At any instant the sensors read a pattern, and that pattern is the position. No counting, no homing, no memory to lose.

The code is usually Gray code, where only one bit changes between adjacent positions. A plain binary pattern can flip several bits at once at a boundary, and a slight misalignment would then read a wildly wrong number. Gray code removes that failure mode.

Because position is a measured value rather than an accumulated one, a power cut does not erase it. Switch the machine back on and the control already knows where the axis is. For a tool changer, a rotary table, or a lift axis, that saves a reference move and removes a whole class of crash.

An incremental to absolute value encoder change also shifts what the drive can do at low speed. Absolute reading gives a clean position at zero speed, so the loop does not need motion to resolve position. Very slow indexing and holding torque applications get steadier.

A single-turn absolute encoder resolves one revolution. Turn the shaft past 360° and the code repeats, so the control still needs a reference to know how many turns have passed. A multi-turn absolute value encoder adds a turn counter, so it knows both the angle within the revolution and the number of revolutions.

The extra turns are tracked by a second code disc, a geared pair of discs, or an electronic counter with a backup supply. A common arrangement counts 4,096 turns. That covers a ball screw, a gearbox input, or a robot joint through its full travel without any external reference.

The engineering meaning is simple. On a machine with a fixed travel range, the encoder can hold the whole range in absolute terms. After a power loss the axis knows its position to the count, and the control can resume without a homing cycle that might run into a fixture.

Multi-turn units cost more and are usually larger. If your axis homes once per shift and the reference move is safe, a single-turn or incremental device may be the better buy. The decision is about how much a lost reference costs you.

• 1
  Turn count4,096 turns is a typical range
• 2
  No homingresume after power loss without a reference move
• 3
  Trade-offhigher cost, larger body, more wiring

Start with what the axis does when it loses position. If a homing move is cheap and safe, incremental feedback is enough. If a wrong position means a crash, a scrapped part, or an operator standing next to a moving gantry, absolute feedback pays for itself.

Look at the motion profile. An axis that only indexes in whole steps rarely needs absolute reading. An axis that holds a position under load, follows a contour, or is part of a coordinated multi-axis move benefits from position that is known at zero speed and never drifts.

Check the environment. Absolute encoders are less sensitive to the noise that corrupts incremental counts, but they are not immune. Cable routing, shield grounding, and connector sealing still decide whether the signal is clean. A shielded twisted pair run away from servo power cables is the baseline.

Finally, check the drive. Not every servo amplifier accepts every encoder protocol. BiSS, EnDat, SSI, and various serial formats are common, and a mismatch means a new drive or a converter. Confirm the interface before the encoder is ordered, not after.

Mechanical mounting decides most encoder failures. Shaft runout, axial play, and coupling misalignment all load the bearing. Use a flexible coupling rated for the speed, keep runout within the encoder datasheet limit, and never use the encoder shaft as a structural member.

For high-speed spindles, a hollow-shaft or bearingless design removes the coupling entirely. The encoder rides on the motor shaft or a precision stub, so there is no added inertia and no coupling backlash. Speed limits are then set by the scanning electronics.

Cable length and frequency interact. Longer runs attenuate the signal and pick up more noise. Keep encoder cable separate from motor power cable, ground the shield at one end, and respect the maximum cable length in the drive manual. If the run must be long, a serial absolute interface tolerates it better than a raw incremental square wave.

Temperature and contamination matter in machining cells. Coolant mist, chips, and vibration reach everything near the table. Choose an IP rating that matches the location, and route the cable so it does not act as a chip tray or a coolant drain.

On a CNC mill, the encoder sits between the servo motor and the ball screw, so its resolution and the screw pitch together set the smallest commanded step. A fine encoder on a coarse screw still gives a fine step, but backlash and thermal growth of the screw dominate the real accuracy.

On a rotary table, an absolute encoder lets the control know the table angle at power-up. That removes the risk of a homing move driving a fixture into the spindle. On a tool changer, absolute position means the arm knows where it stopped and can recover without a full reset.

Inspection does not replace feedback. A machine can hold ±0.005 mm on the part only if the encoder, the screw, and the structure all agree. We verify parts on the CMM and with in-process checks, and we ask for the encoder and drive details when a customer reports a position problem.

If you are designing a machine and the feedback choice is still open, send us the axis layout. We machine the brackets, housings, and couplings that hold the encoder, and we can hold the bore and face tolerances that keep runout in check.