Harmonic Drive Actuator Accuracy: Why It Should Be Evaluated at Four Levels

The accuracy of a harmonic drive actuator is often misunderstood because several different performance metrics are used to describe its motion. Encoder resolution, repeatability, unidirectional positioning accuracy, and bidirectional positioning accuracy are related, but they are not interchangeable.

For example, a high-resolution encoder may provide extremely fine measurement increments, but this does not mean the actuator's output shaft can achieve the same level of absolute positioning accuracy. Mechanical transmission errors, structural compliance, bearing characteristics, encoder configuration, and control parameters all influence the final result.

For engineers selecting a harmonic rotary actuator, understanding these four levels of accuracy helps prevent overestimating performance based on a single specification.

1. Resolution: How Small Is the Theoretical Measurement Increment?

Resolution describes the smallest detectable or commandable increment of a measurement or control system. In a rotary actuator, it is often associated with encoder resolution.

Consider a rotary actuator equipped with a 23-bit single-turn encoder. The encoder divides one revolution into approximately 8.39 million theoretical counts. The corresponding angular increment is approximately 0.154 arcseconds at the encoder shaft.

If this encoder is installed on the motor side of a harmonic drive actuator with a 101:1 reduction ratio, the theoretical output-side increment would be approximately 0.0015 arcseconds, assuming an ideal transmission ratio and no additional quantization or processing limitations.

These figures may look impressive, but they should not be interpreted as the actuator's actual positioning accuracy.

The encoder measures angular movement at its installation point. The complete actuator also contains mechanical components that can introduce transmission errors, elastic deformation, friction, and other deviations. The output-side encoder configuration, feedback architecture, controller, and measurement method also affect the practical result.

Key takeaway: Resolution describes the fineness of measurement or command increments. It does not independently establish the accuracy of the actuator's output position.

2. Repeatability: Can the Actuator Return to the Same Position?

Repeatability describes how closely an actuator returns to the same position when the same positioning operation is repeated under specified conditions.

For industrial automation, repeatability is often one of the most useful specifications because equipment may repeatedly perform the same motion cycle over thousands or millions of operations.

However, repeatability must be evaluated under defined test conditions. Test direction, load, speed, temperature, mounting rigidity, control settings, and the measurement method can all affect the result. It is therefore important to check how a manufacturer's stated value was measured before comparing products.

Why Repeatability Matters in Practical Applications？

Repeatability is particularly relevant to applications such as rotary indexing, repetitive inspection, dispensing, screwdriving, and component handling. If an actuator repeatedly reaches a consistent position, the machine can often compensate for a known systematic offset through calibration.

Nevertheless, good repeatability does not necessarily mean high absolute positioning accuracy. An actuator may return to nearly the same position every time while consistently stopping slightly away from the intended target.

When evaluating a specification such as ≤10 arcseconds, engineers should confirm the applicable model, operating conditions, test method, and whether the figure represents unidirectional or bidirectional repeatability.

3. Unidirectional Positioning Accuracy: How Close Is the Actual Position to the Target?

Unidirectional positioning accuracy describes the deviation between a commanded target position and the measured position when the target is approached from a specified direction.

Unlike repeatability, which evaluates consistency across repeated movements, positioning accuracy also reflects systematic deviations between the commanded and actual positions.

In a harmonic drive actuator, positioning errors can be affected by transmission characteristics, mechanical deformation, manufacturing tolerances, assembly conditions, and the control system. Periodic error components may also occur during rotation. Their magnitude and pattern depend on the specific design and operating conditions.

Can Error Compensation Improve Positioning Accuracy?

In some systems, a measured error map or compensation table can reduce repeatable systematic errors. The controller can use the measured relationship between commanded positions and actual positions to apply corrections.

However, compensation cannot automatically eliminate all positioning errors. Its effectiveness depends on the stability of the error pattern, measurement quality, controller capability, operating load, temperature, and other conditions. Errors that change with direction, load, or time may be more difficult to compensate fully.

For this reason, engineers should distinguish between a manufacturer's unidirectional positioning accuracy specification, a compensated system result, and the overall positioning performance of the finished machine.

4. Bidirectional Positioning Accuracy: Does Direction Change the Result?

Bidirectional positioning accuracy evaluates positioning performance when the target is approached from both directions. It is important in applications where the actuator frequently reverses direction or must position accurately from either side of a target.

Mechanical transmission behavior, friction, elastic deformation, and direction-dependent errors can cause the measured position to differ depending on the direction of approach. The actual impact depends on the actuator's design, control strategy, load, and test procedure.

Bidirectional positioning accuracy should not be assumed to be identical to unidirectional positioning accuracy. Engineers should check whether the published specification includes both directions and how the manufacturer defines and measures the result.

For rotary indexing, scanning, inspection, and other applications involving frequent reversals, this metric may be particularly important. Testing the actuator under representative operating conditions can help establish whether its performance meets the application's requirements.

HONPINE Harmonic Rotary Actuator Specifications

HONPINE offers external-driver harmonic rotary actuators in solid-shaft and hollow-shaft configurations. These configurations address different installation requirements, including structural support, through-axis cable routing, and integration with automation equipment.

The following table summarizes selected model ranges and their stated specifications. Exact performance depends on the model and configuration, so engineers should verify the relevant datasheet before final selection.

When comparing the specifications, do not treat repeatability and unidirectional positioning accuracy as equivalent values. They describe different performance characteristics and should be assessed against the machine's actual requirements.

Encoder resolution should also be evaluated as part of the complete feedback and control system, rather than as a substitute for measured output accuracy.

Why 10 Arcseconds Can Matter in Precision Automation？

Ten arcseconds is a small angular displacement, but its practical significance depends on the distance from the axis of rotation to the point where positioning error matters.

Ten arcseconds is approximately 4.85 × 10⁻⁵ radians. For a point located 0.5 m from the rotational axis, this corresponds to a tangential displacement of approximately 0.024 mm. At a radius of 1 m, the corresponding displacement is approximately 0.048 mm.

These calculations illustrate how angular error can translate into linear displacement. They do not guarantee the accuracy of the complete machine, which may also be affected by structural deflection, mounting errors, calibration, thermal effects, and other sources of error.

Semiconductor Equipment and Metrology Applications

Semiconductor equipment can require precise rotary positioning for wafer handling, inspection, alignment, and other automated processes. In these systems, actuator performance must be considered alongside the accuracy and repeatability requirements of the complete mechanism.

For metrology and inspection rotary tables, the positioning requirement may be particularly demanding. Engineers should evaluate not only the actuator's repeatability and positioning accuracy, but also output stiffness, runout, mounting geometry, vibration, thermal stability, and the measurement system.

EFEM and Vacuum Robot Applications

Equipment front-end modules (EFEMs) and semiconductor handling systems may require compact rotary actuators that provide space for cables, vacuum lines, pneumatic lines, or other utilities.

Hollow-shaft harmonic rotary actuators can simplify routing through the axis of rotation. Depending on the model, larger through-holes may help reduce cable congestion and make equipment integration easier.

Environmental requirements should also be checked carefully. IP65 protection or a customized IP67 configuration may be relevant in some installations, but the required protection level must be confirmed for the actual actuator and its connectors. For vacuum environments, lubricant outgassing, material compatibility, and operating conditions should be reviewed with the manufacturer rather than inferred from the actuator's general specifications.

Applying Harmonic Rotary Actuators in 3C Automation

In 3C manufacturing—covering computers, communications equipment, and consumer electronics—rotary actuators may be used in indexing tables, bonding systems, dispensing equipment, screwdriving stations, and precision assembly lines.

The appropriate accuracy level depends on the process tolerance, the distance between the rotational axis and the working point, the fixture design, and the total error budget of the machine.

For example, a linear tolerance of ±0.05 mm at a radius of 0.5 m corresponds to an angular tolerance of approximately 20.6 arcseconds, if angular positioning error is the only error source. A 10-arcsecond angular error corresponds to approximately 0.024 mm of tangential displacement at the same radius.

This comparison can help engineers assess whether an actuator specification is suitable as part of the system design. It should not be interpreted as a guaranteed machine-level tolerance, because other sources of error may consume part of the available error budget.

External-driver architecture may also be useful when the drive electronics need to be installed in a control cabinet or when the machine's electrical and thermal layout favors separating the driver from the actuator. However, external-driver and integrated-driver solutions should be compared using their actual motor performance, transmission characteristics, control bandwidth, cooling conditions, installation constraints, and total system requirements. Neither architecture is inherently superior in every application.

How to Select the Right HONPINE Actuator for Your Application？

The selection process should begin with the required output performance and mechanical interface, rather than a single encoder or accuracy number.

Semiconductor Metrology and Inspection Rotary Tables

For precision rotary tables, evaluate positioning accuracy, repeatability, torsional stiffness, runout, thermal behavior, and the rigidity of the mounting structure. A solid-shaft configuration may suit designs that prioritize a particular mechanical interface or support arrangement. Confirm the selected model's specifications against the system's error budget.

EFEM and Vacuum Handling Systems

For EFEM and wafer-handling applications, a hollow-shaft actuator may help route cables and utilities through the rotational axis. Select the bore size according to the actual routing requirements, while also checking load, stiffness, positioning performance, environmental compatibility, and installation space.

Cleanroom Systems Requiring Multiple Utility Lines

When several cables, pneumatic lines, or other services must pass through the rotary axis, a large hollow-shaft configuration may simplify integration. Compare the usable through-hole diameter, actuator outer dimensions, mounting interfaces, and the available internal routing space for each specific model.

3C Rotary Indexing, Bonding, and Screwdriving

For 3C automation, choose an actuator based on the process tolerance, cycle time, payload, rotational inertia, and required repeatability. Where the process has a relatively broad tolerance, a more economical model may be sufficient. Where the working point is far from the rotational axis or the process tolerance is tight, evaluate the angular error budget carefully.

Applications Requiring Positioning Accuracy Near 5 Arcseconds

If an application requires positioning accuracy at approximately the 5-arcsecond level, do not select a transmission type based solely on a general rule. Request model-specific test data and confirm the test conditions, output measurement method, load, control configuration, and compensation strategy.

A high-precision planetary actuator or another transmission solution may be worth evaluating when its measured performance, stiffness, and dynamic characteristics better fit the application. The final decision should be based on verified specifications and system-level testing, not on transmission category alone.

Conclusion: Evaluate Accuracy as a Complete System Performance

Harmonic drive actuator accuracy should not be judged by encoder resolution alone. Resolution describes the fineness of measurement increments; repeatability describes consistency across repeated movements; unidirectional positioning accuracy describes deviation when approaching from one direction; and bidirectional positioning accuracy considers positioning from both directions.

For semiconductor equipment, EFEM systems, and 3C automation, the most appropriate actuator is the one whose verified performance matches the machine's accuracy requirements, mechanical constraints, and operating environment.

When comparing HONPINE harmonic rotary actuators, check the specifications for the exact model and configuration, and evaluate them together with the feedback system, controller, mounting structure, and actual operating conditions. This approach provides a more reliable basis for actuator selection than relying on a single headline accuracy value.