Absolute vs. incremental encoder — differences, operating principle and applications

Absolute and incremental encoders differ primarily in how they determine position. An incremental encoder generates pulses corresponding to a change in position and requires these pulses to be counted, whereas an absolute encoder assigns a unique value to each position, allowing it to determine the position even after the system is restarted.

In practice, the choice between these solutions depends on whether the machine needs to know its position immediately after power-up, what resolution the application requires, how the controller receives the signal, and whether more than one shaft revolution needs to be tracked. Both types of encoders are widely used in automation, but they address slightly different requirements.

What is an encoder and what is it used for?

An encoder is a position transducer that converts rotary or linear motion into an electrical signal that can be read by a controller. Based on this signal, position, direction of movement, speed, and in some applications acceleration can be determined.

In automation, encoders are used, among other things, for:

• motor shaft position control,
• rotational speed measurement,
• feedback in servo drives,
• linear axis position monitoring,
• synchronization of multiple drives,
• length or displacement measurement,
• position control of rotary tables and robots.

Selecting the right solution starts with determining what motion information the controller actually needs. Encoders for industrial automation are available in both incremental and absolute designs, so the technology can be matched to the way a particular axis operates.

How does an incremental encoder work?

An incremental encoder, also known as a relative encoder, generates a defined number of pulses as the shaft moves. The controller counts the successive pulses and uses them to determine the change in position.

A typical incremental encoder provides:

• channel A,
• channel B,
• optional reference channel Z.

Channels A and B are phase-shifted relative to each other. This makes it possible to determine not only the number of pulses generated, but also the direction of rotation.

If an encoder generates, for example, 1000 pulses per revolution, one pulse corresponds to:

360° / 1000 = 0,36°

In practice, the way the edges of signals A and B are counted can further increase the effective number of positions detected by the controller.

What happens after a power failure?

An incremental encoder provides information about a change in position, not the absolute position of the shaft. If the position was determined by counting pulses, this information may be lost when the controller is switched off.

Therefore, after restarting the machine, it is often necessary to:

1. execution of the homing procedure,
2. movement to the reference sensor,
3. set the initial position,
4. resume counting pulses.

This is not a problem in many machines that perform a reference procedure at startup anyway.

In such applications, incremental encoders provide a simple and effective way to measure speed and relative changes in position.

How does an absolute encoder work?

An absolute encoder assigns a unique digital value to each shaft position. The controller does not have to calculate the current position by counting pulses from a starting point—it receives direct position information.

This is the most important practical difference compared with an incremental encoder.

If the machine is switched off and the shaft changes position while stopped, a properly selected absolute encoder can provide the current position after restart without requiring a full homing procedure.

This can be particularly important in:

• robots,
• vertical axes,
• multi-axis machines,
• rotary tables,
• automated storage systems,
• handling systems,
• applications where homing would be time-consuming or unsafe.

Single-turn absolute encoder – how does it work?

A single-turn absolute encoder determines the shaft position within one full revolution.

For example, a 12-bit encoder can distinguish:

2¹² = 4096 pozycji na obrót

This corresponds to a theoretical angular resolution of approximately:

360° / 4096 ≈ 0,088°

After another full revolution, however, the position values start again from the beginning.

Such a solution is sufficient when the angular position within the 0–360° range is what matters, for example in:

• turntables,
• valves,
• rotary axes,
• adjustment mechanisms,
• single joints.

Multi-turn absolute encoder – how does it differ?

A multi-turn encoder records not only the position within one revolution, but also the number of complete revolutions made.

This allows the controller to determine the shaft position over a much larger range.

If the encoder provides:

• 12 bits of single-turn position,
• 12 bits for the number of revolutions,

it can distinguish 4096 positions within one revolution while simultaneously tracking 4096 complete revolutions.

This solution is particularly useful for linear axes where one encoder revolution corresponds to a small displacement of a screw, drum, or gear.

The difference between single-turn and multi-turn versions is one of the key criteria when selecting absolute encoders for machines that need to know their position without re-homing.

Absolute vs. incremental encoder – key differences

Criterium	Incremental encoder	Absolute encoder
Position information	Change in position	Specific position
Signal	A/B pulses, optionally Z	Digital position value
Position after power loss	Usually requires re-establishing	May be available without homing
Reference procedure	Often required	Often unnecessary
Speed measurement	Very simple	Possible based on position changes
Direction of movement	From A/B phase	From changes in position value
Counting multiple revolutions	Controller must maintain a counter	Possible directly with a multi-turn version
Integration complexity	Usually lower	Usually higher
Cost	Usually lower	Usually higher
Typical application	Speed measurement, simple axes, drives homed at startup	Robotics, servo systems, axes requiring position knowledge after restart

The most important practical difference therefore concerns behavior after the system is switched off.

If the machine can easily perform homing after every startup, an incremental encoder may be entirely sufficient. If the position must be known immediately after startup, an absolute encoder is the appropriate direction.

What does an encoder operating diagram look like?

Regardless of the technology, a simplified diagram can be assumed:

shaft movement → sensing element → position reading → electronics → output signal → controller

In an incremental encoder, the measurement result takes the form of successive pulses:

movement → A/B pulses → controller counter → relative position

In an absolute encoder:

movement → position code → value transmission → absolute position

The difference therefore lies not only in the sensor design, but also in how the control system interprets the information.

Optical encoder – can it be absolute and incremental?

Yes. The term “optical” describes the sensing technology, whereas “absolute” and “incremental” describe how position information is encoded.

An optical encoder may use:

• light source,
• code disc,
• fotodetektor,
• signal-processing electronics.

In an incremental encoder, the disc contains regularly spaced areas that generate pulses.

In an absolute encoder, a track arrangement or another encoding method allows a specific value to be assigned to a given position.

Therefore, both absolute and incremental encoders can use:

• optical,
• magnetic,
• inductive,
• other technologies, depending on the design.

The sensing technology should be selected separately from the method used to encode position.

How to select an encoder – absolute or incremental?

The key is to determine how the machine should behave after startup and what information the controller requires.

1. Does the position need to be known immediately after power-up?

If so, an absolute encoder is the natural choice.

This applies, for example, to an axis that cannot be safely moved to a home sensor after every restart.

If the homing procedure is simple and takes only a few seconds, an incremental encoder may be a more economical solution.

2. Does more than one revolution need to be tracked?

If the machine needs to know the total shaft position after multiple revolutions, a multi-turn absolute encoder should be considered.

Examples include:

• drive screw,
• unwinding drum,
• winch,
• linear axis,
• mechanizm wieloobrotowy.

3. What resolution is required?

Resolution determines the smallest change in position that the system can distinguish.

For incremental encoders, it is often specified as:

• PPR,
• pulses per revolution,
• number of signal cycles.

For absolute encoders, it is usually expressed in bits.

For 10 bits:

2¹⁰ = 1024 pos.

For 12 bits:

2¹² = 4096 pos.

For 16 bits:

2¹⁶ = 65 536 pos.

However, the number of positions alone does not indicate the accuracy with which the mechanism operates. When selecting an encoder, the encoder resolution must be distinguished from the accuracy of the entire system, which is also affected by sensor geometry, mounting, backlash, and drive mechanics.

4. How fast does the shaft rotate?

At high speeds, check:

• the encoder’s maximum mechanical speed,
• output frequency,
• the controller’s maximum input frequency,
• cable length and quality,
• electrical signal type.

At high speed, a high-pulse-count incremental encoder may generate a signal frequency exceeding the capabilities of a PLC input or counter.

5. What interface does the controller support?

For incremental encoders, common interfaces include:

• TTL,
• HTL,
• differential signals,
• A/B/Z.

Absolute encoders can communicate via various serial interfaces and industrial networks.

The selection should take into account not only the encoder itself, but also the inputs available on the PLC, servo drive, or motion controller.

when should you choose an incremental encoder?

An incremental encoder is particularly suitable when:

• speed measurement is the main requirement,
• information about a position change is needed,
• the machine performs homing,
• the system should be relatively simple,
• solution cost is an important factor,
• the controller has a high-speed counter input.

Typical applications include:

• conveyors,
• auxiliary drives,
• shaft speed control,
• simple positioning axes,
• material length measurement,
• mechanism synchronization.

When should you choose an absolute encoder?

An absolute encoder is particularly useful when:

• the position must be known immediately after startup,
• homing is undesirable or impossible,
• shaft movement while the system is off must be taken into account,
• multi-turn measurement is required,
• the application requires highly reliable position information,
• the encoder is used as feedback in a precision automation system.

Such requirements arise in, among other areas:

• robotics,
• servo drives,
• CNC machines,
• automated warehouses,
• rotary tables,
• vertical axes,
• positioning systems.

When choosing absolute measurement, it is then necessary to decide whether a single-turn version is sufficient or whether multiple complete revolutions need to be tracked. This is one of the fundamental considerations when selecting an absolute encoder.

Is an absolute encoder always better?

No. Having more information available does not automatically mean that a more sophisticated encoder will be the more advantageous solution.

If the application:

• always performs a homing procedure,
• primarily needs speed information,
• does not need to know the position while stopped,
• uses a simple controller,

an incremental encoder may meet all requirements with simpler integration.

Conversely, using an absolute version is justified when absolute position information actually adds value to the process.

Absolute or incremental? Start with how the machine operates

The most important criterion is not the resolution or price of the device itself, but how position information is used by the machine.

If the controller must know the position immediately after startup, an absolute encoder is the appropriate direction. When the main requirement is measuring position change or speed and homing is not a problem, an incremental encoder may be a simpler and sufficient solution.

Before selecting an encoder, it is worth determining:

• whether the position should be absolute or relative,
• whether the machine can perform homing,
• the number of revolutions to be tracked,
• the required resolution,
• the expected accuracy,
• the maximum shaft speed,
• wymagany interfejs,
• environmental conditions,
• mounting method.

AMG’s offering includes both incremental and absolute encoders, making it possible to match the measurement method to the control architecture and the actual requirements of the application.

FAQ

How does an absolute encoder differ from an incremental encoder?

An incremental encoder generates pulses corresponding to a change in position and requires them to be counted. An absolute encoder provides a specific position value, so it can determine the position without counting movement from a reference point.

What happens to the position of an incremental encoder after a power failure?

An incremental encoder itself does not store the absolute position of the shaft. If the controller loses the pulse counter information, a homing procedure is usually required after restart.

Is an absolute encoder more expensive than an incremental encoder?

Usually, yes, because it requires a more sophisticated encoding method and electronics. However, cost also depends on resolution, sensing technology, interface, mechanical design, and operating conditions.

What is the difference between a single-turn and a multi-turn absolute encoder?

A single-turn encoder determines the position within one full revolution. A multi-turn encoder additionally tracks the number of revolutions made, allowing it to determine absolute position over a much larger range.

What is an encoder used for in automation?

An encoder is used to measure position, speed, and direction of movement. It can be used as a feedback element in a servo drive, for linear axis control, length measurement, machine synchronization, or shaft speed monitoring.

How should you select an absolute or incremental encoder for an application?

If the position must be known immediately after startup or without performing homing, an absolute encoder is worth considering. If speed or relative position change is sufficient and the machine can perform a reference procedure, an incremental encoder is often sufficient.