A joint actuator combines a brushless motor, a reduction and a driver in one module. Picking one comes down to five questions.
1. How much torque, and for how long?
Every joint module lists two torque figures. Peak torque is what it can deliver for short bursts, such as a jump or a fast arm move. Rated torque is what it can sustain without overheating. Size the peak figure to your most demanding motion and keep the continuous load, such as holding a leg or an arm against gravity, below the rated figure. A common starting point is to calculate the torque at each joint in your worst-case pose and add a 30–50% margin.
2. Low ratio or high ratio?
The gear ratio changes how the joint behaves, not just how strong it is.
- Low ratios (about 6:1 to 10:1), quasi-direct drive. The joint can be pushed back by the load, which lets a robot absorb impacts and sense contact through motor current. This is why most quadrupeds and humanoid legs use them.
- High ratios (30:1 to 100:1 and up), harmonic, cycloid or multi-stage planetary. Stiffer and more precise with higher holding torque, but hard to backdrive. Suited to arms, wrists and positioners.
Compare humanoid and legged robot joints with robot arm and cobot joints.
3. Voltage and power
Most modules run on 24 V or 48 V. Higher voltage reaches higher speed at the same current, which usually means less heat in the wiring. Pick one bus voltage for the whole robot so every joint shares one power system.
4. Communication and control
- CAN or CAN FD is the most common bus for joint modules, and many support an impedance-style mode (position, velocity, stiffness and damping in one command) used by legged-robot controllers.
- EtherCAT offers faster, deterministic cycles for many joints and is common in industrial arms.
- RS485 appears on simpler or lower-cost modules.
Check that the vendor publishes the protocol and example code, and that the update rate you need is supported.
5. Encoders and feedback
A single motor-side encoder loses the output position on power-up unless the joint is homed. A dual encoder adds an output-side encoder so the joint knows its absolute position immediately. Resolution of 14 bits or more is typical.
Compare before you buy
Use the torque and gear-ratio filters in Robot Joint Actuators, then add two to four modules to Compare to see weight, torque density and communication side by side.