Robot Joint and Actuator Motor Selection Guide

Design your motor in the configurator →

Selecting a motor for a robot joint or a precision actuator is a different problem from selecting one for continuous rotation. A conveyor motor mostly needs to sustain a torque at a speed; a joint motor needs to hold position accurately, move smoothly through a small range, and often do both while carrying the load statically for long periods. This guide walks through the constraints specific to that class of application.

BLDC or Stepper: Start Here

The first fork is whether the application actually needs continuous, electronically commutated rotation with closed-loop position feedback, or whether open-loop step-based positioning is enough. The BLDC actuators vs. stepper motors comparison covers this in detail — as a rule of thumb, a joint that needs high speed, high efficiency, or closed-loop torque control points toward BLDC; a joint with a lighter duty cycle where open-loop positioning is acceptable, and cost is the binding constraint, can still be well served by a stepper. Most articulated robot joints and precision actuators with any meaningful continuous load land on the BLDC side of that line.

Direct Drive or Geared

Once BLDC is the choice, the next decision is whether the joint is direct drive or driven through a gearbox. A gearbox multiplies torque, letting a smaller motor serve a given load, but it introduces backlash, adds mechanical compliance between the motor and the load, and adds its own maintenance items. For a joint where backlash-free positioning and high control bandwidth matter more than motor size or cost, direct drive is usually worth the larger motor it requires — see the direct-drive motor sizing guide for how to size one. For a joint where load is high relative to the available envelope and some backlash is tolerable, a geared BLDC motor keeps the motor itself smaller.

Positioning Accuracy: Air Gap and Cogging Both Matter

A joint's positioning accuracy is bounded by more than its encoder resolution. The motor's own air gap affects torque ripple and, if it varies with manufacturing tolerance, can introduce position-dependent torque variation that a control loop has to fight rather than being able to command around. Cogging torque, the reluctance-based torque a motor produces even unpowered, shows up most at the low speeds and standstill holding that joint applications spend most of their time at — a motor with meaningful cogging will feel jerky exactly where smoothness matters most. A coreless stator, with no iron teeth for the magnets to interact with, removes cogging outright rather than reducing it, which is why it is a common choice for gimbals, camera mounts and other joints where smooth low-speed motion is the whole point.

Feedback and Control Bandwidth

The encoder or resolver feeding the control loop needs resolution matched to the joint's positioning requirement, and — for a direct-drive joint specifically — needs no gear reduction to amplify its counts, unlike a geared design where a coarser encoder on the motor side can still deliver fine positioning at the load after the gearbox. Control bandwidth, how aggressively the loop can be tuned before instability, is limited by mechanical compliance in the drivetrain: a direct-drive joint's stiff, ungeared coupling generally allows higher bandwidth than a geared equivalent, where gearbox compliance sits between the commanded and actual position.

Thermal at Standstill

A robot joint that holds a static load, an arm extended against gravity, for instance, spends long periods at or near zero speed with essentially no rotor-driven airflow. This is the same low-speed thermal problem covered in the direct-drive motor sizing guide, and it applies whether or not the joint is geared: the continuous holding torque a motor can actually sustain depends on its real cooling path, not on a datasheet figure that may assume forced air the application doesn't have.

Selection Checklist

  • Continuous holding torque at the actual mounting and cooling condition, not a peak or forced-air figure.
  • Positioning accuracy required, translated into an air gap tolerance and an acceptable level of cogging and ripple.
  • Backlash tolerance — if none is acceptable, direct drive is very likely the right call regardless of the larger motor it needs.
  • Encoder resolution matched to the positioning requirement, with no gear reduction to lean on if the joint is direct drive.
  • Control bandwidth the application's move profile actually needs, which bounds how much mechanical compliance (gearbox or otherwise) the drivetrain can tolerate.
  • Duty cycle at standstill, since this is usually where the thermal ceiling, not the winding's rated current, decides what the motor can actually hold.

Configure a motor for your joint or actuator →

Table of Contents