Absolute vs incremental encoders

In short:Use an absolute encoder when the machine must know its position at power-up without moving, which is most servo axes, robot joints and anything that cannot safely home. Use an incremental encoder when a homing move at start-up is acceptable and you want the simplest, cheapest, fastest feedback, typically A/B/Z quadrature on RS-422.

Absolute vs incremental: the core difference

An incremental encoder tells the controller how far it has moved. It outputs two square waves 90° apart (A and B, so the controller can see direction) and usually an index pulse (Z) once per revolution or at a reference mark. The controller counts edges from a known point. Switch off, push the axis by hand, and that count is meaningless until the axis homes again.

An absolute encoder tells the controller where it is. Each position has a unique code read from the scale, so the true position is available the instant power returns, with no movement. Position is sent as a digital word over a serial interface.

The practical question is not which is “better”, but whether your machine can afford to move to find home every time it powers up, and what happens if it cannot.

Single-turn vs multi-turn absolute encoders

A single-turn absolute encoder measures one revolution and then repeats: 361° reads as 1° (POSITAL). That is enough for anything that turns less than one revolution over its full travel: a robot joint output, a rotary table measured at the table, a valve or a pan-tilt head.

A multi-turn encoder also counts complete revolutions. You need one whenever the encoder shaft turns many times over the axis travel: a motor driving a ball screw, a belt or rack, or a motor-side encoder before a gearbox. Counting revolutions with the power off is the hard part, and there are three ways to do it:

POSITAL has used Wiegand-based counting since 2005, and battery-free multi-turn options are now common on servo motor encoders as well. If you choose a battery-backed design, write the battery check into the maintenance schedule and decide what the machine does if it powers up with the count lost.

A useful alternative in robot joints is to avoid multi-turn on the motor altogether. An absolute encoder on the gear output turns less than once over the joint’s range, so it gives absolute joint position with no battery, while a simpler motor-side encoder handles commutation and the speed loop (Allient). See our frameless motor sizing guide for that arrangement.

Encoder interfaces: A/B/Z, BiSS C, SSI, EnDat and others

Incremental (A/B/Z): usually RS-422 differential line driver signals, sometimes TTL, HTL (10–30 V) for PLC inputs, or analogue sin/cos 1 Vpp for interpolation in the drive. Resolution is in pulses per revolution (PPR); counting every edge of both channels gives four times that in counts (CPR) (RLS). Check the output frequency: a 5,000 PPR encoder at 3,000 rpm produces 5,000 × 3,000 / 60 = 250 kHz per channel, which the drive input must accept.

Absolute serial interfaces (Motion Control Tips):

The rule is simple: start from what the drive accepts. A brilliant encoder on an interface the drive cannot read needs a converter, adds latency and complicates safety certification.

Resolution vs accuracy vs repeatability

These three are routinely confused, and the confusion costs money:

• Resolution is the smallest step the encoder reports. For absolute rotary encoders, steps per revolution = 2ⁿ for n bits, so 13 bits is 8,192 steps (RLS).
• Accuracy is the maximum difference between reported and true position, including scale errors, readhead errors and mounting errors. RLS is explicit that higher resolution does not automatically mean higher accuracy.
• Repeatability is how closely the encoder returns the same reading at the same position. RLS’s rule of thumb is that it is typically 5 to 10 times better than accuracy.

For a control loop, resolution matters because it sets velocity-signal noise at low speed. For positioning, accuracy and repeatability matter. A 20-bit encoder reports about a million steps per turn, but that says nothing about whether those steps are in the right place.

Eccentricity: the installation error that dominates ring encoders

For a ring or disc encoder mounted off-centre by e on scale radius r, the peak angular error is roughly e / r radians. A 10 µm mounting error on a 25 mm radius ring gives 10 / 25,000 = 4 × 10⁻⁴ rad, about 83 arcsec, regardless of the encoder’s resolution. Ways to deal with it:

• Specify the shaft and hub run-out the ring needs, and check it on assembly.
• Use a larger diameter ring, since the error falls with radius.
• Use two readheads 180° apart. Renishaw states this cancels odd error harmonics including eccentricity, and quotes better than ±1 arcsec installed for its dual-readhead system (Fierce Electronics).
• Use an encoder with self-calibration or a calibration routine after installation.

Homing incremental axes

An incremental axis needs a homing sequence at start-up. Common methods:

• Home switch plus index pulse: move to a switch, then on to the next encoder index for a repeatable zero. The most common approach.
• Hard stop with torque or current limit: cheap, but repeatability depends on mechanics, and it loads the hardware every start.
• Reference marks on a linear scale: a single mark may need long travel to find. Distance-coded marks are spaced so the controller knows its absolute position after crossing two of them; Heidenhain quotes a maximum of 20 mm travel on its LS and LIP 500 scales and 80 mm on LB (HEIDENHAIN).

Homing is a safety question as much as a time question. If the axis carries a tool inside a fixture, a vertical load, or a robot arm among people, an unplanned homing move can be dangerous. That is often what pushes the specification to absolute.

Decision table

Worked example: a vertical pick-and-place axis

Axis: vertical Z axis on a ball screw, 10 mm lead, 400 mm stroke, servo motor with brake. The gripper works inside a fixture, so moving blind at power-up could crash it. Required positioning accuracy at the gripper ±20 µm. Restarts happen several times a shift.

Turns over the stroke: 400 / 10 = 40 revolutions. A single-turn absolute encoder would not know which of the 40 turns it was on, so the choice is multi-turn absolute or incremental with homing.

Is homing acceptable? No. Homing upward from inside a fixture risks the gripper catching the part, and homing time at every restart costs throughput. Multi-turn absolute it is. A 12-bit turns counter (4,096 turns) covers the 40-turn stroke with room to spare.

Resolution check: even a 17-bit single-turn resolution gives 10 mm / 131,072 ≈ 0.08 µm per count at the load. A 2,500-line incremental encoder (10,000 CPR) would give 1 µm. Resolution is not the limiting factor; the ball screw’s lead accuracy and thermal growth are. Spend the money on the screw grade, not extra bits.

Battery or not: a battery-backed counter would work, but adds a maintenance item and a re-homing risk if a machine sits unpowered for a long period. A battery-free multi-turn (Wiegand or geared) is the cleaner choice for an OEM machine shipped to many sites.

Interface: match the drive. If the drive accepts BiSS C, a BiSS C multi-turn encoder or a servo motor with a battery-free multi-turn absolute encoder fitted as standard.

Result: servo motor with a battery-free multi-turn absolute encoder on BiSS C or the drive’s native protocol, and a power-off brake. Servo motors with this option include the Leadshine ELM2 range; for integrated stepper axes, the Novanta Liberty MDrive offers absolute options.

Example encoder families

• Absolute optical rings and scales: Renishaw RESOLUTE, with BiSS C and drive-specific protocols.
• Incremental optical: Renishaw TONiC and VIONiC for high-resolution linear and rotary axes.
• Absolute magnetic, through-bore: Renishaw RLS AksIM-4.
• Absolute inductive, sealed: Novanta Mini Ultra IncOder and Midi Ultra IncOder.
• Absolute capacitive, ultra-thin: Netzer VLX.
• Magnetic heads and tapes: Bogen AKS16 (absolute, multi-turn variant) and IKS11 (incremental).

Checklist

Send these to our engineers, or enter them in the spec builder:

• Rotary or linear; shaft, hollow shaft, ring or kit; available diameter and bore
• Total travel in revolutions or millimetres
• Whether the axis may move at power-up, and whether a homing sequence is acceptable
• Required accuracy and repeatability at the load, not just resolution
• Maximum speed (rpm or m/s) and the drive’s input frequency or clock limits
• Interface the drive or controller supports (A/B/Z RS-422, HTL, sin/cos, BiSS C, SSI, EnDat, HIPERFACE DSL, drive-specific)
• Battery acceptable or not for multi-turn
• Functional safety requirement (SIL or PL level)
• Environment: temperature, contamination (oil, dust, water), shock and vibration, IP rating
• Mounting tolerances achievable: run-out, ride height, alignment
• Cable length to the drive

See also our encoders overview.