Incremental vs Absolute Encoders: How to Choose Feedback for a Servo Motor

When choosing a servo motor, buyers usually start with power, torque and speed.

Those specifications determine whether the motor can move the load, but they do not tell the servo system where the shaft actually is.

That job belongs to the feedback device.

In most servo systems, the motor works with an encoder or resolver that sends position and speed information back to the servo drive. The drive uses that information to control the axis in a closed loop.

For an OEM or machine builder, encoder selection affects more than positioning accuracy. It can also affect startup behavior, homing, wiring, drive compatibility and how the machine recovers after power is removed.

The most common question is therefore not:

"Which encoder has the highest resolution?"

A more useful question is:

"What feedback information does this machine actually need?"

Why Does a Servo Motor Need Feedback?

A servo motor is normally part of a closed-loop motion-control system.

The controller sends a position, speed or torque command. The servo drive supplies current to the motor, while the feedback device reports what the motor is actually doing.

The drive continuously compares commanded motion with measured motion and adjusts the motor output.

This is what allows a servo axis to perform controlled acceleration, positioning and speed regulation.

Without suitable feedback, the drive cannot determine rotor position with the accuracy or reliability required by the application.

That is why the encoder should be selected together with the motor and servo drive, rather than treated as a separate accessory.

What Is an Incremental Encoder?

An incremental encoder produces a series of electrical pulses as the motor shaft rotates.

The drive counts those pulses to determine how far the motor has moved and how fast it is turning.

A typical incremental encoder may provide A and B channels for direction and position counting, together with a reference or index signal.

Its main characteristic is that position is measured relative to a known reference point.

After power is removed, the controller normally does not retain the motor's absolute mechanical position purely from incremental pulse information.

For many machines, that is not a problem.

The axis can perform a homing routine when the machine starts.

When Does an Incremental Encoder Make Sense?

Incremental feedback is widely used because it is straightforward and suitable for many industrial applications.

It can work well when:

• the machine already performs a homing cycle at startup;
• the absolute shaft position after power loss is not required;
• the application uses conventional indexing or speed control;
• the existing servo drive supports the encoder interface;
• cost and system simplicity are important.

Packaging machines, conveyors, general automation axes and some robot mechanisms can all use incremental feedback successfully when the machine architecture includes a reliable reference procedure.

The decision depends on the machine, not on whether incremental technology is considered more basic.

What Is an Absolute Encoder?

An absolute encoder reports an identifiable shaft position rather than only producing relative movement pulses.

This means the controller can determine where the motor shaft is within the encoder's measuring range.

That can be particularly useful when the machine should know its position after startup without performing a complete homing sequence.

Absolute encoders are generally divided into single-turn and multi-turn types.

Single-Turn Absolute Encoders

A single-turn absolute encoder identifies the angular position within one mechanical revolution.

For example, if a robot joint rotates less than one complete revolution during normal operation, a single-turn encoder may provide enough positional information.

Once the motor rotates beyond one full turn, however, the encoder alone does not necessarily tell the controller how many complete revolutions have occurred unless another method is used.

Multi-Turn Absolute Encoders

A multi-turn encoder records both:

angular position within the current revolution; and

the number of shaft revolutions within its specified range.

This is useful for machinery where the axis can rotate many turns and the machine needs to retain the complete mechanical position.

Examples can include ball-screw axes, material-handling equipment and automated machinery where returning to a reference sensor after every power cycle would interrupt production.

The value of a multi-turn encoder is therefore not simply "higher accuracy."

Its main advantage is position awareness across multiple revolutions.

Absolute Feedback Can Reduce Homing Requirements

Consider a machine with several servo axes.

With incremental feedback, each axis may need to move to a reference sensor after startup.

For a simple machine, this takes only a few seconds.

For a large automated system, however, homing may require multiple mechanisms to move in a specific sequence.

That can increase startup time and may be inconvenient if the machine stops unexpectedly in the middle of a production cycle.

An absolute encoder can allow the control system to recover position information more directly.

But this benefit only matters if the machine actually needs it.

There is little reason to pay for an advanced multi-turn feedback system on an axis that can safely and quickly home every time it powers up.

Encoder Resolution Is Not the Same as Machine Accuracy

This is one of the most common misunderstandings in servo selection.

A higher-resolution encoder gives the control system finer position information.

It does not automatically guarantee that the final machine achieves the same positioning accuracy.

Machine accuracy is also affected by:

• bearing play;
• gearbox backlash;
• ball-screw lead error;
• coupling stiffness;
• structural deflection;
• thermal expansion;
• servo tuning;
• mechanical vibration.

A 23-bit encoder on a mechanically flexible system does not turn the entire machine into a 23-bit positioning system.

Encoder resolution should therefore be selected as part of the complete mechanical and control design.

What Does "17-Bit" or "23-Bit" Mean?

Absolute encoder resolution is often expressed in bits.

A higher bit count provides a greater number of measurable positions within one revolution.

For a potential buyer, the exact mathematical resolution is less important than the application question:

Does the feedback resolution provide enough usable position information for the machine's required accuracy and control performance?

For many industrial applications, using the highest available resolution can add cost without creating a meaningful improvement in final machine performance.

The servo drive must also support the selected encoder protocol.

Compatibility comes first.

Optical vs Magnetic Encoders

Feedback devices can also use different sensing technologies.

Optical Encoders

Optical encoders use an optical sensing system to detect rotor position.

They are widely used in industrial servo applications and can provide fine position resolution.

The complete encoder assembly, sealing and operating environment should still be considered when selecting one for the machine.

Magnetic Encoders

Magnetic encoders detect shaft position using magnetic sensing.

They are commonly used where compact construction and robust feedback are priorities.

Modern magnetic encoders are available in incremental and absolute configurations, including high-resolution single-turn and multi-turn designs.

The correct choice should be based on the drive interface, environmental conditions and required feedback performance rather than assuming one sensing technology is universally better.

Jiangnan's current servo catalogue includes both optical and magnetic encoder options across its product families.^[1]

Where Does a Resolver Fit?

A resolver is another rotor-position feedback device used in servo and industrial motor systems.

Unlike a digital encoder that directly produces digital position data, a resolver uses an electromagnetic sensing principle and requires the drive or interface electronics to interpret its signals.

Resolvers are often considered where environmental robustness is important.

However, the servo drive must have the correct resolver interface.

This makes compatibility particularly important.

A resolver cannot simply replace an encoder because the mechanical connection looks similar.

Start with the Servo Drive

When replacing a servo motor, one of the first questions should be:

What feedback interface does the existing drive support?

This matters because motor and drive compatibility is not defined by power and voltage alone.

The servo drive needs to understand the encoder's:

• signal type;
• power supply;
• communication method;
• resolution;
• wiring;
• position format.

If the drive only supports a particular incremental interface, installing an absolute encoder does not automatically give the machine absolute positioning.

Likewise, changing encoder type may require changes to the drive parameters, cable and controller configuration.

For replacement projects, the servo-drive model is therefore one of the most useful pieces of information a customer can provide.

How Jiangnan Electric Handles Encoder Options

Jiangnan Electric's current servo motor portfolio is designed around multiple application requirements rather than one universal feedback configuration.

The company's servo catalogue includes encoder codes for several feedback types, including:

• reduced-wiring encoder configurations;
• 2,500-line incremental encoders;
• 5,000-line incremental encoders;
• 17-bit encoders;
• 20-bit encoders;
• 23-bit encoders;
• magnetic incremental encoders;
• 17-bit single-turn magnetic absolute encoders;
• 23-bit single-turn magnetic absolute encoders;
• 17-bit multi-turn magnetic absolute encoders;
• 23-bit multi-turn magnetic absolute encoders;
• POSITAL encoder options;
• SICK encoder options;
• resolver feedback.^[1]

This does not mean every encoder is available on every motor model.

The feedback option still needs to be confirmed against the motor series, servo drive and application.

Feedback Options Across Jiangnan Servo Families

Jiangnan's current website separates its servo products into Low-Inertia, Medium-Inertia and High-Inertia Servo Motors, and multiple product pages list encoder compatibility as part of the configuration.^[2]

For example, the low-inertia product family supports incremental optical, incremental magnetic, absolute and resolver feedback on relevant configurations. The catalogue specifically identifies broad feedback-device compatibility as one of the low-inertia range's design features.^[1]

The medium-inertia C Series also supports incremental optical, magnetic incremental, absolute encoder and resolver options.^[1]

For high-inertia products, Jiangnan's D/E/S Series includes incremental and absolute magnetic encoder options for relevant models.^[1]

This makes encoder selection part of the motor configuration rather than an afterthought.

A Practical Example: 40B Series

Jiangnan's 40B Series illustrates how feedback options fit into a compact low-voltage servo platform.

Published catalogue data for the 40B platform includes:

• 24 V and 48 V drive configurations;
• 0.1 kW rated power;
• 0.32 N·m rated torque;
• 3,000 rpm rated speed;
• 5,000 rpm maximum speed;
• IP65 protection;
• 2,500-line or absolute feedback configurations.^[1]

For an AGV or compact robotic system, the final encoder choice would still depend on what the control system needs to know after startup and which feedback protocol the servo drive supports.

The motor specifications alone do not decide that.

How Should an OEM Choose?

A practical encoder-selection process can begin with five questions.

1. Does the machine need to remember position after power is removed?

If no, an incremental encoder may be sufficient.

If yes, an absolute encoder may be more appropriate.

2. Does the shaft rotate more than one revolution while position must be retained?

If yes, consider whether multi-turn absolute feedback is required.

3. Does the machine perform a homing cycle?

If homing is fast, safe and already part of the machine sequence, incremental feedback may remain entirely practical.

If homing is difficult or undesirable, absolute feedback can have more value.

4. What feedback does the servo drive support?

This can immediately eliminate incompatible choices.

Always confirm the actual drive model and interface before finalizing the motor.

5. What level of positioning performance does the machine genuinely require?

Do not choose an encoder only because it has the highest bit count.

Select enough feedback performance for the mechanical system and control requirement.

What Information Should You Send with a Servo Motor RFQ?

For an OEM project, providing feedback requirements early can prevent unnecessary redesign later.

Useful information includes:

• application or machine type;
• servo-drive brand and model;
• required motor power;
• rated and peak torque;
• rated and maximum speed;
• incremental or absolute feedback requirement;
• single-turn or multi-turn requirement;
• encoder resolution, if specified;
• brake requirement;
• flange and shaft dimensions;
• operating environment;
• cable-length requirements;
• expected quantity.

For a replacement project, photographs of the encoder connector and the existing motor nameplate can also be useful.

A motor that fits mechanically but uses the wrong feedback interface is not a drop-in replacement.

Choose Feedback Around the Machine

Incremental, absolute and resolver feedback are not simply three levels of encoder quality.

They solve different control problems.

Incremental feedback can be practical and cost-effective when the machine can home normally.

Single-turn absolute feedback is useful when the angular shaft position should be known immediately.

Multi-turn feedback becomes valuable when the control system needs to retain position across several shaft revolutions.

Resolvers offer another option where the drive and operating environment favor that type of feedback.

The right encoder is therefore the one that matches:

machine behavior → startup requirement → position information → servo drive → motor configuration.

For OEM and replacement projects, Jiangnan Electric can review the customer's servo drive, motion requirements, mechanical drawing and existing feedback interface before confirming a suitable motor and encoder configuration.

Product Information Sources

[1] Jiangnan Electric / Zhongke Servo, English Servo Motor Catalog supplied for this article. PDF pages 8 (encoder codes), 10 (low-inertia feedback options), 24 (40B specifications), 36 (C Series), and 43 (high-inertia feedback options).

[2] Jiangnan Electric website: www.jnelectricmotor.com

Disclaimer

The information in this article is provided for general reference only. Product specifications, performance, configuration options and suitability must be evaluated in detail for the intended application and actual operating conditions. Before selecting, purchasing or using a product, please confirm the relevant details with Jiangnan Electric and refer to the applicable technical documentation and agreed specifications. This article does not replace product-specific instructions or a professional engineering assessment.