{"id":2444,"date":"2026-10-21T07:45:32","date_gmt":"2026-10-21T07:45:32","guid":{"rendered":"https:\/\/robotcncparts.sytech.site\/servo-actuator\/"},"modified":"2026-10-21T07:52:36","modified_gmt":"2026-10-21T07:52:36","slug":"servo-actuator","status":"publish","type":"post","link":"https:\/\/robotcncparts.sytech.site\/fr\/servo-actuator\/","title":{"rendered":"Servo Actuators and Robot Actuators: How They Work and How They Are Built"},"content":{"rendered":"

A servo actuator is a motor combined with feedback and control, so that it moves a load to a commanded position, speed or torque and holds it there. In a robot, it is usually packaged as a compact joint module: motor, reducer, output bearing, encoder and brake inside one housing. RobotCNCParts does not sell complete actuators; we machine actuator housings, transmission components, shafts and motor mounts for teams who build or customize them. What follows is the mechanical side of an actuator: how it works, the main types, and how its machined parts are built.<\/p>\n

What is a servo actuator?<\/h2>\n

A servo actuator runs a closed loop:<\/p>\n

    \n
  1. Command:<\/strong> the motion controller sends a target position, speed or torque to the servo drive.<\/li>\n
  2. Drive:<\/strong> the drive supplies current to the motor.<\/li>\n
  3. Motor and reducer:<\/strong> the motor turns, usually through a reducer that trades speed for torque, and moves the output.<\/li>\n
  4. Feedback:<\/strong> an encoder (or a resolver) measures the actual position; the drive compares it with the command and corrects the current.<\/li>\n<\/ol>\n

    The loop runs continuously, so the output follows the command even when friction, payload or external forces change. That is the difference with an open-loop drive, which cannot see a lost step. Repeatable positioning and controlled torque are why closed-loop servo control is the default in robot joints.<\/p>\n

    What is the purpose of an actuator?<\/h2>\n

    An actuator turns energy (electric, hydraulic or pneumatic) into controlled motion or force. Sensors and controllers decide where a robot should move; the actuator is the part that moves it. A servo adds feedback, so that the motion is precise and repeatable.<\/p>\n

    Types of robotic actuators<\/h2>\n\n\n\n\n\n\n\n\n\n\n\n
    Type<\/th>\nHow it moves<\/th>\nWhere it is used in robots<\/th>\nPros and cons<\/th>\n<\/tr>\n<\/thead>\n
    Electric rotary servo<\/td>\nMotor and reducer in a joint module<\/td>\nArm, leg and torso joints of humanoids, quadrupeds and cobots<\/td>\nPrecise, programmable; mass and heat grow with torque<\/td>\n<\/tr>\n
    Electric linear servo<\/td>\nMotor driving a ball or roller screw<\/td>\nLinear axes, some leg and torso mechanisms<\/td>\nHigh force and stiffness along one axis; longer package<\/td>\n<\/tr>\n
    Stepper-based<\/td>\nStepper motor, open or closed loop<\/td>\nLight axes, auxiliary motions<\/td>\nSimple and low cost; torque falls with speed<\/td>\n<\/tr>\n
    Quasi-direct-drive<\/td>\nHigh-torque motor with a low-ratio reducer<\/td>\nLegged robots that must absorb impacts<\/td>\nBackdrivable and robust; larger motor<\/td>\n<\/tr>\n
    Series elastic<\/td>\nSpring between reducer and output<\/td>\nJoints that need force control and compliance<\/td>\nSafe contact and torque sensing; lower stiffness<\/td>\n<\/tr>\n
    Hydraulic<\/td>\nPressurized fluid in cylinders or rotary vanes<\/td>\nSome heavy legged robots<\/td>\nVery high force density; pumps, hoses and leaks<\/td>\n<\/tr>\n
    Pneumatic<\/td>\nCompressed air<\/td>\nGrippers and simple on\/off motions<\/td>\nFast and simple; hard to position precisely<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

    Electric rotary modules dominate humanoid and quadruped robots. For legs, many teams choose low-ratio, backdrivable joints; the quasi-direct-drive joint<\/a> explains why. Choosing between robotic actuators usually comes down to torque density, backdrivability and heat.<\/p>\n

    Linear actuator vs servo actuator<\/h2>\n

    “Linear” describes the motion: a straight line. “Servo” describes the control: closed-loop feedback. The two words are not opposites. A linear actuator that only extends and retracts when powered is not a servo; the same actuator with an encoder and a servo drive becomes a linear servo actuator.<\/p>\n\n\n\n\n\n\n\n\n
    <\/th>\nLinear actuator<\/th>\nServo actuator<\/th>\n<\/tr>\n<\/thead>\n
    Describes<\/td>\nThe type of motion<\/td>\nThe control method<\/td>\n<\/tr>\n
    Motion<\/td>\nStraight line<\/td>\nRotary or linear<\/td>\n<\/tr>\n
    Feedback<\/td>\nOptional<\/td>\nAlways (encoder or resolver)<\/td>\n<\/tr>\n
    Example<\/td>\nScrew-driven push rod<\/td>\nRobot joint module, linear servo axis<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

    Inside a robot actuator: the joint module<\/h2>\n

    A rotary robot actuator for a humanoid or quadruped joint stacks its parts on one axis. Each one sets part of the actuator’s behavior.<\/p>\n

    Frameless torque motor<\/h3>\n

    The stator and rotor are supplied without their own housing or bearings. The stator is bonded or pressed into the actuator housing; the rotor sits on the hollow shaft. The motor’s air gap is only as uniform as the stator bore is concentric to the bearings.<\/p>\n

    Reducer<\/h3>\n

    The reducer multiplies torque and reduces speed. A strain wave (harmonic) gear (wave generator, flexspline, circular spline) gives a high ratio and near-zero backlash in a compact ring. A planetary stage (sun, planets, ring gear, carrier) is efficient and backdrivable at low ratios. A cycloidal or RV reducer is very stiff and shock tolerant, but heavier. Ratio, backlash (in arcmin) and torsional stiffness are the figures to compare; our article on RV and harmonic reducers<\/a> compares the two main joint reducers, and the robot gearbox components<\/a> we machine are listed by reducer type.<\/p>\n

    Output bearing<\/h3>\n

    A crossed roller bearing often carries the output, because one bearing takes radial, axial and tilting loads. Its seat in the housing sets the joint’s stiffness.<\/p>\n

    Encoders<\/h3>\n

    A motor-side encoder measures the rotor for commutation and speed control. An output-side encoder measures the actual joint angle after the reducer, so it sees backlash and wind-up. Absolute encoders keep position at power-up; incremental encoders need a homing move.<\/p>\n

    Holding brake<\/h3>\n

    A spring-applied brake holds the joint when power is off, so a humanoid arm or leg does not fall.<\/p>\n

    Driver electronics<\/h3>\n

    Many modules integrate the servo drive in the housing, which adds a heat source and a cable interface.<\/p>\n

    Housing<\/h3>\n

    The housing holds all of the above on one axis, carries the loads into the next link and conducts heat away. It is usually a machined part, specific to each robot. See our machined actuator housings for robot joints<\/a>.<\/p>\n

    \"Diagram<\/p>\n

    How actuator housings and gear stages are built<\/h2>\n

    This is where a robot actuator’s performance is decided on the machine, not in the catalog.<\/p>\n

    Housing.<\/strong> The front and rear bearing seats are machined in one setup where the geometry allows (5-axis or mill-turn), so they share one axis. The stator bore is held concentric to those seats to keep the air gap even. Wall thickness is a trade-off between mass and heat path: a thinner wall saves grams but raises motor temperature. Sealing faces and O-ring grooves are machined to the seal maker’s dimensions for the IP class required (IEC 60529). Bearing seats follow the bearing maker’s fit recommendation (ISO 286, often H6 or H7 for the outer ring). After hard anodizing, bearing bores are masked or finished after coating.<\/p>\n

    Gear-stage transmission parts.<\/strong> Planetary gear housings<\/a> carry a ring-gear seat concentric to the output bearing. Planet carriers need accurate pin-hole positions, because every planet must share the load. Bearing seats, gear shafts and output shafts<\/a> are ground on their journals. Typical steels are 42CrMo (through-hardened) and 20CrMnTi or 8620 (carburized); 17-4PH is used where corrosion resistance matters. In RV designs, the crankshafts, pin housing and carrier of the RV reducer components<\/a> need the same position and fit control. Gears themselves are machined to the customer drawing on request, to the grade on that drawing.<\/p>\n

    Motor mounts and output flanges.<\/strong> Pilot diameters locate the motor and the output; bolt-circle position sets how well they line up with the bearing axis. Our custom motor mounts<\/a> are machined to the same datums as the housing.<\/p>\n

    Finishes and identification.<\/strong> Hard anodizing on aluminum wear faces, masked bearing bores, black oxide or electroless nickel on steel parts, and laser marking for joint IDs and revisions, so that parts from different prototype builds are never mixed.<\/p>\n

    \"Metal<\/p>\n

    How to specify a robot actuator<\/h2>\n

    Before choosing a module or drawing your own, fix these values:<\/p>\n