Single-Turn vs. Multi-Turn Absolute Encoders for Robot Joint Modules: How to Choose?

When developing humanoid robots, collaborative robotic arms, and exoskeletons, engineers know that accurate joint-position feedback is essential. Absolute encoders are widely used in robot joint modules because they can provide position information immediately after power-up without requiring a conventional homing sequence, provided the encoder and control system are configured correctly.

However, should you choose a single-turn absolute encoder or a multi-turn absolute encoder?

Hello, I'm Theodore Li, Technical Director at HONPINE. In this article, I'll use the HONPINE HPJM Harmonic Joint Actuator PRO Series to explain the key differences between these two encoder configurations, how they behave in real-world applications, and how to select the right option for your robot joint.

Choosing the wrong encoder configuration can lead to position ambiguity after power loss, incorrect robot posture, unstable standing, repeated homing procedures, and unexpected joint-position errors after manual movement.

The key is to understand one fundamental difference: a single-turn absolute encoder measures angular position within one revolution, while a multi-turn absolute encoder also tracks the number of revolutions. However, whether multi-turn information is retained after power loss depends on the encoder's specific memory technology and backup-power arrangement.

What Is the Difference Between Single-Turn and Multi-Turn Absolute Encoders?

A single-turn absolute encoder identifies the angular position within one complete revolution, typically from 0° to 360°. It does not independently identify how many complete revolutions the shaft has made.

A multi-turn absolute encoder provides both the angular position within one revolution and the accumulated turn count, allowing the system to determine the absolute position across multiple revolutions within its supported measurement range.

Single-Turn Absolute Encoder: Position Within 360°

A single-turn absolute encoder reports the shaft's angular position within one revolution. For example, the same angular reading can occur after the shaft has rotated through different numbers of complete turns.

If a robot joint is mechanically restricted to a limited angular range and its control system can reliably determine the joint's position within that range, a single-turn encoder may be sufficient.

For many humanoid robot joints, such as knees, hips, shoulders, and torso-pitch joints, mechanical travel limits make single-turn feedback a practical option.

Multi-Turn Absolute Encoder: Angular Position Plus Turn Count

A multi-turn absolute encoder tracks the angular position within one revolution as well as the number of revolutions across its supported range.

This additional information is particularly valuable for joints that can rotate through multiple revolutions or may be manually moved while the drive is powered off.

However, not all multi-turn encoders retain turn-count information in the same way. Some use nonvolatile or energy-harvesting technologies, while others rely on a backup battery to maintain electronic multi-turn counting.

For battery-backed encoders, turn-count retention depends on the backup supply remaining within its specified operating conditions. If the backup power is lost, the stored turn count may become unavailable, depending on the encoder design.

HONPINE HPJM PRO Series: Two Absolute Encoder Configurations

The HONPINE HPJM Harmonic Joint Actuator PRO Series offers different encoder configurations to meet the position-feedback requirements of robotic applications. Two configurations worth comparing are the dual-encoder single-turn version and the battery-backed multi-turn version.

The important distinction is not simply the number of encoders. It is whether the configuration can retain the joint's accumulated turn count when the main power supply is disconnected.

HPJM Single-Turn Absolute Encoder Joint Module — Dual-Encoder Version

Hardware configuration: Dual-encoder feedback with single-turn absolute-position measurement and no encoder backup-battery interface, according to the specified configuration.

How It Works？

The single-turn encoder provides the angular position within one revolution. It does not retain an accumulated multi-turn count after power loss.

When the robot restarts, the encoder can report its current single-turn angle. If the joint remains within a known mechanical travel range and the controller can uniquely determine the joint's position from that reading, the system can recover its operating position without a conventional homing procedure.

However, if the joint is manually rotated through a complete revolution or moved into a position that produces an ambiguous single-turn reading, the controller may no longer be able to determine the true mechanical position from the single-turn measurement alone.

Practical Engineering Advantages

1. Suitable for joints with limited angular travel

For joints operating within a defined mechanical range, such as ±180° or ±120°, single-turn feedback may provide sufficient position information, provided the mechanical limits and control logic prevent position ambiguity.

2. Simplified backup-power requirements

A configuration without an encoder battery interface does not require an encoder backup battery for multi-turn counting. This removes battery aging and battery-maintenance concerns associated with battery-backed turn-count retention.

3. Practical for production-oriented robot joints

When the joint's mechanical range is constrained and its position can be reliably reconstructed from single-turn feedback, a single-turn configuration can simplify the system architecture and reduce maintenance requirements.

Limitations of Single-Turn Absolute Encoders

The main limitation is the absence of an independently retained multi-turn count.

If the joint is manually moved while powered off and the movement causes the same single-turn reading to correspond to multiple possible mechanical positions, the controller may not know the actual joint position.

For example, a reading of 30° could correspond to 30°, 390°, or another equivalent angle, depending on the joint's permitted range and reference convention.

This does not mean every power cycle causes position errors. The risk arises when the mechanical position can change in a way that cannot be resolved from the available single-turn feedback and other reference information.

Recommended Applications

The HPJM single-turn configuration is worth considering for:

• Humanoid robot knee joints

• Hip joints with defined mechanical travel limits

• Shoulder joints with restricted rotation

• Torso-pitch joints

• Collaborative robot joints with limited angular travel

Exoskeleton joints operating within a predefined motion range

For these applications, single-turn feedback can be a practical choice when the mechanical design, control strategy, and operating procedure ensure reliable position recovery.

HPJM Multi-Turn Absolute Encoder Joint Module — Battery-Backed Version

Hardware configuration: Multi-turn absolute-position feedback with a dedicated encoder backup-battery interface, according to the specified configuration.

How It Works？

The battery-backed multi-turn configuration uses backup power to maintain electronic multi-turn counting when the main power supply is disconnected.

When correctly configured and operated within the encoder's specified backup-power conditions, the encoder can retain the accumulated turn count and report both the current single-turn angle and the multi-turn position after restart.

This can reduce the need for homing or manual position reconstruction after power cycles and make development workflows more convenient.

The ability to move a joint while powered off without losing its absolute position depends on the encoder's actual multi-turn retention capability, the backup battery's condition, and the permitted measurement range.

Practical Engineering Advantages

1. Easier position recovery after power loss

When multi-turn information is retained correctly, the controller can recover the joint's absolute position after restart without relying solely on the single-turn angle.

2. More convenient prototype development

During robot development, engineers frequently adjust joint positions, manually guide mechanisms, and repeat motion tests. Multi-turn position retention can simplify these workflows when joints may be moved through multiple revolutions while the main power is off.

3. Better suited to multi-revolution joints

For joints that require more than one complete revolution, multi-turn feedback provides position information that a single-turn encoder cannot supply on its own.

This is particularly relevant to wrist joints or other rotary mechanisms designed for continuous or multi-revolution movement.

Limitations of Battery-Backed Multi-Turn Encoders

The principal consideration is backup-battery maintenance.

If the battery ages, becomes depleted, or cannot provide sufficient power under the specified environmental conditions, the encoder may lose its accumulated turn count. The exact behavior depends on the encoder design and its fault-handling logic.

A lost multi-turn count should not automatically be assumed to trigger a safe or seamless fallback to single-turn operation. The controller must detect and handle any encoder status or position-validity fault according to the manufacturer's documentation.

For this reason, engineers should verify battery life, replacement intervals, low-battery diagnostics, operating-temperature limits, and the recovery procedure if multi-turn data becomes invalid.

Recommended Applications

The HPJM battery-backed multi-turn configuration is worth considering for:

Robot joints that rotate through multiple revolutions

Wrist joints designed for continuous or unrestricted rotation within their mechanical limits

Robot prototypes that are frequently moved manually during development

Teaching and demonstration systems that require repeated manual positioning

Applications where retaining joint position after power-off movement is an important operational requirement

For any joint that can rotate continuously, confirm that the encoder's supported turn-count range and the complete mechanical design meet the application's requirements.

How to Choose the Right Absolute Encoder for Your Robot Joint？

The choice between single-turn and multi-turn absolute encoders should be based on the joint's mechanical travel range, expected movement while powered off, position-recovery requirements, and maintenance strategy.

Choose a Single-Turn Absolute Encoder If...

The joint has a clearly defined mechanical travel range.

The control system can determine the actual joint position from the available single-turn feedback.

The joint is not expected to move into an ambiguous position while the system is powered off.

You want to avoid battery-dependent multi-turn counting.

The application prioritizes a simpler maintenance arrangement.

Choose a Battery-Backed Multi-Turn Absolute Encoder If...

The joint can rotate through multiple complete revolutions.

The joint may be manually moved while the main power supply is disconnected.

The application requires multi-turn position information after restart.

Prototype testing involves frequent manual movement and repeated power cycles.

You can implement appropriate battery monitoring and maintenance procedures.

Questions to Ask Before Selecting an Encoder

Before finalizing the encoder configuration for an integrated robot joint actuator, engineers should confirm:

What is the joint's maximum mechanical rotation range?

Can the joint be moved manually while the main power is off?

Does the encoder retain multi-turn information without external backup power?

What happens if the encoder battery becomes depleted?

Does the controller validate absolute position before enabling motion?

Is a homing procedure required after an encoder fault or loss of position information?

These questions help prevent position ambiguity and improve the reliability of startup and recovery procedures.

Conclusion: Match the Encoder to the Joint's Mechanical Design

Single-turn and multi-turn absolute encoders serve different engineering requirements.

A single-turn absolute encoder can be a practical choice for robot joints with limited angular travel, provided the mechanical design and control system can reliably determine the joint's position. A battery-backed multi-turn encoder is more suitable when accumulated rotation must be retained across power interruptions, particularly for multi-revolution joints and applications involving frequent manual movement.

For the HONPINE HPJM Harmonic Joint Actuator PRO Series, the right configuration depends on the joint's rotation range, position-retention requirements, and maintenance strategy.

At HONPINE, we provide harmonic joint actuators for humanoid robots, collaborative robotic arms, exoskeletons, and other precision motion-control applications.

Contact HONPINE to learn more about the HPJM PRO Series and select an encoder configuration that matches your robot joint design.