A servo motor is an electric motor combined with a feedback sensor and a closed-loop controller, so that it drives its output to a commanded position, speed or torque and actively holds that value against disturbances. In robotics, servo motors are the most widely used actuators for joints, wheels and axes, because they turn a numeric setpoint from the motion controller into a precise, repeatable movement.
The word describes a system rather than a motor design. The motor inside may be a brushless permanent-magnet machine, a brushed DC motor or, in small hobby units, a tiny geared DC motor; what makes it a servo is that the measured output is continuously compared with the command and the difference is corrected.
How a servo motor closes the control loop
A servo motor works by measuring where the shaft is, comparing it with where it should be and adjusting the motor current until the following error approaches zero. The complete servo system has four parts: the motor, the feedback device (encoder, resolver or potentiometer), the servo drive or amplifier that switches power to the windings, and the control algorithm that runs inside the drive or the motion controller.
Industrial and robotic servo drives use cascaded control loops. The innermost current loop controls winding current, which is proportional to torque; around it sits the velocity loop, and the outermost position loop tracks the trajectory. Each outer loop must be several times slower than the loop inside it, otherwise the loops interact and the axis oscillates. The table below shows the usual structure.
| Loop | Controlled quantity | Typical bandwidth | Usual controller |
|---|---|---|---|
| Current (torque) loop | Winding current, hence torque | About 1-3 kHz | PI with field-oriented control |
| Velocity loop | Shaft speed | About 50-500 Hz | PI, often with filters |
| Position loop | Shaft angle | About 10-100 Hz | P or PID with feedforward |
Feedforward is what separates a well-tuned servo from a merely stable one. The motion controller already knows the planned velocity and acceleration, so it adds the expected torque directly to the command instead of waiting for an error to appear. This can cut tracking error by an order of magnitude on fast, repetitive moves.
Types of servo motors used in robots
Servo motors range from gram-scale hobby units to multi-kilowatt industrial axes, and the type determines the control interface, accuracy and achievable dynamics. Choosing the wrong class is a common source of disappointing robot performance.
| Type | Motor and feedback | Command interface | Typical robotic use |
|---|---|---|---|
| Hobby (RC) servo | Brushed DC, gear train, potentiometer | PWM pulse of 1.0-2.0 ms every 20 ms | Educational robots, small grippers |
| Smart serial-bus servo | DC or brushless, magnetic or optical encoder | Half-duplex serial bus with IDs | Humanoids, research arms, legged kits |
| Industrial AC servo | Brushless PMSM, high-resolution encoder | Fieldbus such as EtherCAT, or pulse-direction | Industrial arms, gantries, AGV drives |
| Frameless servo kit | Rotor and stator built into the joint | Separate drive in or near the joint | Collaborative robots, exoskeletons |
A standard hobby servo maps a 1.5 ms pulse to its center position, with 1.0 ms and 2.0 ms commonly reaching the two ends of a range of about 90-180 degrees, depending on the model. It offers only a position command and no feedback to the host, so it suits light, low-precision tasks. Industrial AC servos, by contrast, report position, velocity, torque and fault status over the network.
Key parameters for sizing a servo motor
Sizing a servo motor means matching its continuous torque, peak torque, speed and inertia to the motion profile of the load. A motor that can reach the peak torque but exceeds its continuous rating over the duty cycle will overheat, even if each individual move looks feasible.
| Parameter | Typical values | What it affects |
|---|---|---|
| Rated (continuous) torque | From under 0.1 N·m to tens of N·m | Heat; must cover the RMS torque of the cycle |
| Peak torque | Often about 3 times rated, for a few seconds | Acceleration capability |
| Rated speed | Commonly 2,000-6,000 rpm for AC servos | Maximum speed before voltage limits |
| Encoder resolution | 17-23 bit absolute: 131,072-8,388,608 counts per revolution | Positioning resolution and speed smoothness |
| Rotor inertia | Low, medium or high inertia variants | Inertia matching and responsiveness |
| Torque constant (Kt) | Stated in N·m/A | Current needed per unit of torque |
The load-to-rotor inertia ratio is the parameter most often overlooked. A gearbox reflects load inertia to the motor divided by the square of the gear ratio, so a 0.5 kg·m² arm link behind a 100:1 reducer appears to the motor as only 0.00005 kg·m². Practitioners commonly keep the reflected ratio below about 10:1, and below about 5:1 for highly dynamic axes, to keep tuning robust.
Servo motor compared with a stepper motor
The servo motor and the stepper motor are the two common choices for electrically positioned axes, and the core difference is that a servo measures and corrects its position, while a typical stepper counts commanded steps and assumes the rotor followed. Related actuators, such as a linear actuator, frequently contain one of these two motors inside.
| Criterion | Servo motor | Stepper motor |
|---|---|---|
| Control principle | Closed loop on measured position | Usually open loop on step count |
| Torque at speed | Near rated torque up to rated speed | Falls steeply as speed rises |
| Response to overload | Error detected and reported | Missed steps go unnoticed |
| Power use at standstill | Current only as needed by the load | Full holding current in most drives |
| Behavior at rest | Small corrective motion possible | Rigid detent-like holding |
| Cost and setup effort | Higher; requires tuning | Lower; minimal tuning |
For lightly loaded, slow axes such as a 3D-printer-style gantry, a stepper is often sufficient. For robot joints with variable loads, high speeds or safety requirements, the servo is the standard choice because the controller always knows the true joint state.
Common servo problems and their causes
Most servo faults in robotic systems come from poor tuning, undersizing or mechanical compliance, not from defective motors. Recognizing the symptom usually points directly to the cause.
| Symptom | Likely cause | Typical remedy |
|---|---|---|
| Audible whine or vibration | Gains too high or mechanical resonance | Lower gains, add a notch filter |
| Following error fault | Acceleration beyond peak torque | Reduce acceleration or resize the motor |
| Overtemperature trips | RMS torque above continuous rating | Recalculate the duty cycle, add gearing |
| Position lost after power-off | Incremental encoder without homing | Use an absolute encoder or a homing routine |
| Jitter in a hobby servo | Noisy pulse signal or sagging supply | Stable supply and clean PWM timing |
A frequent misconception is that higher gains always mean better accuracy. In practice, gearbox backlash, belt stretch and link flexibility limit the usable bandwidth, and pushing gains beyond that limit produces oscillation rather than precision. Joints that need both stiffness and accuracy often use a second encoder on the output side to measure the true joint angle.
Stripped to the essentials
A servo motor is a motor that constantly checks its own position and corrects itself. You tell it where to go, a sensor reports where it actually is, and a small controller pushes harder or eases off until the two match.
Think of a person steering a car to stay in the lane: they look, notice a drift and correct it many times a minute. A servo does the same thing, but thousands of times per second. That is why robot arms can move quickly and still stop at the same spot every time.
Where it is applied
A collaborative robot arm tightening screws on an electronics assembly line uses a frameless brushless servo in each of its six joints, paired with a 100:1 strain wave gear. A 19-bit absolute encoder on each motor gives 524,288 counts per motor revolution, which becomes 52,428,800 counts per joint revolution after the gear, or roughly 0.025 arcseconds per count.
Because each joint also measures motor current, the controller estimates external torque at every cycle. When an operator’s hand touches the arm, the unexpected torque is detected within milliseconds and the arm stops. The result is a robot that holds a repeatability of a few hundredths of a millimeter at the screw head while working safely beside people without a fence.