Servo Guide

How to read actuator specs

Actuator listings mix units and quote numbers under different conditions, so two servos with the same headline figure can behave very differently. A few rules make the numbers comparable.

Torque. Hobby makers quote kg·cm, robotics makers quote N·m. 1 N·m is about 10.2 kg·cm, and 1 kg·cm is about 0.098 N·m. A 20 kg·cm servo can hold 20 kg at 1 cm from the shaft, or 2 kg at 10 cm.
Stall versus rated torque. The headline number is almost always stall torque, the most the motor can push while stopped, drawing its highest current and heating fast. A servo can work continuously at only a fraction of that, often 20–30% for hobby servos. Brushless actuators usually list peak and continuous (rated) torque separately.
Speed. Hobby servos list the time to turn 60° with no load, such as 0.10 s/60°. That converts to about 10 ÷ (seconds per 60°) rpm, so 0.10 s/60° is about 100 rpm. Under load the motor slows down, and at stall it stops.
Voltage. Torque and speed rise with voltage, so a figure only means something next to the voltage it was measured at. Many "high-voltage" (HV) servos quote their best numbers at 7.4 or 8.4 V.
Gears. Plastic gears are light and cheap but strip under shock loads. Metal gears survive falls and stalls but add weight. A high gear ratio gives more torque and precision but makes the joint hard to push by hand (not backdrivable, see quasi-direct drive) and slow to react to impacts.
Feedback. An ordinary hobby servo uses a potentiometer inside to hold the angle it is told, but reports nothing back. Smart servos send back position, speed, load and temperature over a data bus. Brushless actuators use high-resolution magnetic encoders and can control torque directly.

PWM, serial bus and CAN

How an actuator is wired and commanded matters as much as its torque.

PWM (pulse) servos take one signal wire each. A pulse every 20 ms, usually 1–2 ms long, sets the target angle. Any microcontroller or a PCA9685 board can drive them, but the controller never learns whether the servo actually got there.
Serial bus servos (DYNAMIXEL, Feetech, Hiwonder, Kondo) are chained one to the next on a shared data line, each with its own ID. One cable carries power and data, and the controller can read back every joint's position and load, which is what makes teleoperation and imitation learning possible on low-cost arms such as the SO-101.
CAN bus and EtherCAT are used by brushless joint actuators in dynamic legged robots and humanoids. They carry fast torque commands and encoder readings at hundreds to thousands of updates per second, which reinforcement-learning controllers need.
Step and direction signals drive stepper motors through a driver chip, one pulse per microstep. Without an encoder the controller assumes every step happened.

Choosing by robot type

Robot	Typical actuators	Why
Desk toys and small walkers (under 1 kg)	Micro PWM servos (SG90, MG90S), micro bus servos	Light and cheap, and the loads are tiny
Hobby quadrupeds and hexapods (1–3 kg)	20–35 kg·cm digital servos such as the DS3218, Hiwonder or Feetech bus servos	Enough torque for a slow, statically stable gait at a modest price
Low-cost arms for AI and teleoperation	Feetech STS3215, DYNAMIXEL XL330 and XL430	Position feedback on every joint, daisy-chained wiring
Kid-size humanoids	DYNAMIXEL XM430 and MX, Kondo KRS, Hiwonder HX	High torque in a compact case, with feedback for balance
Dynamic legged robots and humanoids	Brushless quasi-direct-drive actuators (Unitree, CubeMars, RobStride)	Low gear ratios absorb impacts and allow fast torque control
Wheeled rovers	DC gear motors with encoders, hub motors	Continuous rotation, simple drivers, long runtime
Desktop and 3D-printed arms	NEMA 17 and NEMA 23 steppers	Precise open-loop positioning and cheap drivers

Categories

How to read actuator specs

PWM, serial bus and CAN

Choosing by robot type