How to select a harmonic actuator

In short:Buy an integrated harmonic actuator (strain-wave gear, motor, encoder and often brake and drive in one housing) when you need a compact high-ratio rotary axis working quickly and the catalogue sizes fit; build your own joint from a gear set and frameless motor when mass, shape or volume pricing demand it. Size on rated and repeatable peak torque at the output, check speed at the chosen ratio, then match the encoder, bore, brake and drive interface to the machine.

What an integrated harmonic actuator contains

An integrated harmonic actuator packages a complete rotary servo axis in one housing:

• A strain-wave gear giving roughly 30:1 to 160:1 in one stage with no backlash (see our strain-wave vs planetary vs cycloidal comparison for how the gear works).
• A servo motor, usually a frameless motor built around a hollow shaft.
• A crossed-roller output bearing that carries the load’s radial, axial and tilting forces.
• Feedback: a motor-side encoder, and on many units an output-side encoder too.
• Often a holding brake, and on some units the servo drive and a fieldbus interface.

The result is a compact, hollow, backlash-free joint for robot arms, positioners, rotary tables, antenna and camera axes, and laboratory automation.

Step 1: integrated actuator or build your own joint?

As a rule of thumb, start with an integrated actuator unless one of these is true: the catalogue sizes do not fit your envelope, mass or bore; the joint must be structurally part of the arm; or the production volume justifies a custom joint. If you are building your own, our frameless motor sizing guide for robot joints and frameless torque motor selection guide cover the motor side.

Step 2: size the output torque

Work at the actuator output. Build the torque cycle from gravity, inertia and friction:

• Gravity torque = Σ mᵢ × g × rᵢ for each mass at its lever arm, at the worst-case angle
• Inertia torque = J_load × α
• Peak torque = gravity + inertia + friction at the worst point in the move

Then compare against the actuator’s torque limits. The gear inside follows the same hierarchy as a component set. Harmonic Drive LLC, for example, defines rated torque at 2,000 rpm input, a repeated peak limit for acceleration and deceleration, an average torque limit, and a momentary peak reserved for collisions and emergency stops that should never occur in the normal cycle. Exceeding it can cause ratcheting and lead to flexspline fatigue failure (Harmonic Drive LLC).

So check:

• Average or RMS torque (including holding against gravity during dwells) against the rated or continuous torque
• Peak cyclic torque against the repeated peak
• Collision torque against the momentary peak, limited by the drive’s torque limit

Actuator continuous ratings are thermal as well as mechanical, and they depend on mounting. Harmonic Drive LLC’s FHA-Mini continuous and 50% duty ranges, for example, apply only with the actuator mounted on a 150 × 150 × 6 mm or 200 × 200 × 6 mm plate, at 0 to 40 °C ambient (Harmonic Drive LLC). If your bracket is smaller, plastic or enclosed, derate.

Step 3: choose the ratio and check speed

Ratio choice is a trade between output speed and torque from the same frame. A higher ratio gives more output torque and finer resolution but lower top speed. Harmonic actuators are typically offered in a few ratios per frame; across the actuator ranges we supply they run from 51:1 to 161:1.

Check the output speed your move needs against the maximum output speed at that ratio, not the series headline. Then check reflected inertia: load inertia divided by the ratio squared, compared with the motor inertia (Motion Control Tips). At 100:1 even a large load reflects very little, which is one reason these actuators tune easily.

Step 4: accuracy, repeatability and stiffness

Read the definitions. Harmonic Drive LLC quotes one-way positioning accuracy as the maximum difference between commanded and actual stop position approached from one direction, to JIS B-6201, and notes that because input error is divided by the ratio, most output error comes from the gear’s transmission error (Harmonic Drive LLC). Its FHA-Mini sizes list 90 to 150 arcsec.

Three things decide what you will get at the tool:

1. Transmission error of the gear, which a motor-side encoder cannot see.
2. Wind-up under load: strain-wave gears are softest at low torque, so a gravity-loaded arm deflects more than the zero-backlash label suggests.
3. Bearing tilt under moment load, multiplied by the arm length.

An output-side encoder measures the joint angle after the gear, so it captures transmission error and wind-up. If it is absolute, the joint also knows its position at power-up without a battery or homing run (Harmonic Drive LLC). For cobots, humanoid joints and anything that must not home on start-up, dual encoders are the default.

Step 5: bore, bearing and mechanical interface

• Hollow bore: list every cable, tube and fibre that passes through, with connectors if they must pass during assembly. Larger-bore versions exist when a standard bore is too small.
• Moment load: check the tilting moment from the load’s offset against the allowable moment of the crossed-roller bearing. FHA-Mini sizes, for example, are rated from 15 to 75 Nm (Harmonic Drive LLC).
• Mounting: flatness and stiffness of the bracket affect both accuracy and heat dissipation.
• Cable exit: rear or side exits change the envelope and the cable bend radius.

Step 6: feedback, brake and drive compatibility

Encoder. Single-turn absolute, multi-turn absolute or incremental, and which protocol: BiSS-C, Nikon A, SSI or A/B/Z. Multi-turn matters if the joint can turn more than one revolution while powered down. See absolute vs incremental encoders.

Brake. Specify a holding brake on any gravity-loaded or externally loaded axis. Check its holding torque at the output, its release voltage (often 24 VDC) and whether it is rated for emergency stops.

Drive. For actuators with a separate drive, the drive must support:

• Motor voltage and current (low-voltage DC units from 24–48 VDC, mains units from 110–220 VAC or higher)
• The encoder protocol, and a second encoder input for dual-encoder versions
• Brake control and the motor thermal sensor
• Your fieldbus and safety functions (STO, and SS1 or SLS if needed)

Compatibility is not automatic. Harmonic Drive LLC, for example, lists different compatible drives for each FHA-Mini encoder option (Harmonic Drive LLC).

Integrated-drive actuators carry the drive inside, with EtherCAT or CANopen and often STO. They remove drive matching, cut cabling to one power and one network connection per joint, and suit multi-joint arms and mobile robots on a DC bus. The trade-off is that you are tied to their fieldbus, their tuning tools and their safety implementation.

Worked example: horizontal axis on a lab sample handler

Requirement: A horizontal-axis arm on a laboratory sample handler. Gripper and sample 2.0 kg at 0.40 m. Arm 1.5 kg, centre of mass at 0.20 m. Move 90° in 0.6 s (triangular profile), then dwell. Gripper must repeat to ±0.1 mm. 48 VDC bus, EtherCAT controller already in the machine. Pipework and gripper cables must pass through the axis. Must hold position on power loss and know its position on start-up.

Gravity torque (worst case, arm horizontal): 2.0 × 9.81 × 0.40 + 1.5 × 9.81 × 0.20 = 7.85 + 2.94 = 10.8 Nm

Inertia: gripper as a point mass 2.0 × 0.40² = 0.32 kg·m²; arm as a rod about its end, 1.5 × 0.40² / 3 = 0.08 kg·m²; total 0.40 kg·m².

Kinematics (θ = 1.571 rad, t = 0.6 s):

• α = 4θ / t² = 17.5 rad/s², so inertia torque = 0.40 × 17.5 = 7.0 Nm
• Peak speed ω = 2θ / t = 5.24 rad/s = 50 rpm

Peak torque: 10.8 + 7.0 = 17.8 Nm accelerating upwards at horizontal.

Continuous torque: if the arm holds horizontal for most of the cycle, the axis carries about 10.8 Nm almost continuously, so the rated (continuous) torque must exceed that with margin at the actual mounting.

Accuracy: ±0.1 mm at 0.40 m is ±2.5 × 10⁻⁴ rad, about ±52 arcsec. Wind-up under 10.8 Nm and gear transmission error both count, so an output-side encoder is worth having.

Selection logic:

• 48 VDC bus and an existing EtherCAT master point to an integrated-drive harmonic actuator with dual absolute encoders and a brake option.
• Pick the smallest frame whose rated torque exceeds about 11 Nm and whose repeated peak exceeds 18 Nm with at least 30% margin, then confirm 50 rpm is within the maximum output speed at the chosen ratio.
• Confirm the bore takes the pipework and cables, and check the moment load from the arm’s offset against the output bearing rating.
• Specify the holding brake. Set the drive torque limit so a collision with the sample rack stays below the momentary peak.

Candidate: the MGM series harmonic actuator with integrated EtherCAT drive (24–48 VDC, dual BiSS-C absolute encoders, STO, brake option), in a frame chosen against the figures above. If the axis were later re-engineered for high volume, the build-your-own route would use a hollow-shaft strain-wave set such as Leaderdrive LHS with a frameless motor and a hollow capacitive encoder.

Products in our range to consider

• MAH harmonic servo actuator: hollow-shaft strain-wave actuator with motor, encoder and brake, six frames, up to 557 Nm rated, published accuracy under 10 arcsec.
• MAT large-bore harmonic actuator: frameless 16-pole motor for a larger through-hole, bores to 45 mm, repeatability 7–8 arcsec.
• MGM harmonic actuator with integrated drive: built-in EtherCAT servo drive with STO and optional FSoE, 24–48 VDC, up to 267 Nm rated.
• MGU robot-joint harmonic actuator: compact joint with EtherCAT or CANopen drive, dual 20-bit encoders and optional torque sensor, from 0.35 kg.
• Leaderdrive LHS / LHSG: hollow-shaft strain-wave gear units for building your own joint, ratios 30:1 to 500:1.
• Leaderdrive LCS / LCSG: cup-type strain-wave component sets; LCSG lifts torque by around 30%.
• Leaderdrive LHD: ultra-flat hollow-shaft strain-wave gear for short joints.
• Akribis ADR-F: 48 VDC frameless motor kits, 45–90 mm OD, to pair with a gear set in a custom joint.

What else the build needs

• Servo drive for actuators without an integrated drive, matched to voltage, encoder protocol and brake. See servo drives.
• Motion controller or EtherCAT master for multi-axis coordination. See motion controllers.
• Power supply sized for peak current on a DC bus, with braking resistor or energy absorption for regenerative loads.
• Slip ring if the axis rotates continuously and cables cannot pass through the bore. See slip rings and how to select a slip ring.
• Encoders for a custom joint. See encoders and how to select an encoder.
• Cabling with the right flex rating and bend radius for moving joints.

Things that catch engineers out

• Holding torque counted as zero. A gravity-loaded arm holding still is running at near-continuous torque.
• Series maximum speed used for every ratio. Top output speed falls as ratio rises.
• Momentary peak used as a design load. It is for collisions and emergency stops only.
• Ignoring wind-up. Zero backlash is not zero deflection; budget it, or fit an output encoder.
• Drive and encoder mismatch. The protocol and the dual-encoder input must be supported.
• Mounting on a small or plastic bracket. Continuous ratings assume a defined heatsink.
• No brake on a vertical or gravity axis. Do not rely on the gear’s resistance to back-driving.

Checklist

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

• Integrated actuator or component build, and the reason
• Masses, lever arms and orientation (worst-case gravity torque)
• Load inertia and the motion profile (angle, time, dwell)
• Peak, continuous and collision torque at the output
• Output speed required
• Accuracy and repeatability at the tool, and the arm length
• Moment load on the output bearing
• Bore size and what passes through it; cable exit direction
• Encoder: single or dual, absolute or incremental, protocol
• Brake required, and its voltage
• Bus voltage, drive (separate or integrated), fieldbus and safety functions
• Environment: IP rating, temperature, cleanroom or washdown
• Quantities and production timing

For the wider family, see our gearboxes overview.