What is the difference between a servo motor and a stepper motor?

A servo motor runs closed loop: an encoder measures the shaft position and the drive corrects any error, so it keeps full torque at speed and does not lose position under load. A stepper runs open loop and simply trusts that each step happened. Steppers are cheaper and hold strongly at standstill; servos are faster, smoother and more efficient.

What does encoder resolution mean for a servo motor?

Encoder resolution is how many distinct positions the encoder reports per revolution. A 17-bit encoder gives 2^17 = 131,072 counts per turn, about 0.003° per count. Higher resolution lets the drive control position and low speed more smoothly, though mechanical backlash and loop tuning still limit real accuracy.

What does position loop gain do in a servo drive?

Position loop gain, in 1/s, sets how strongly the drive responds to position error. At constant speed the following error is roughly speed divided by the gain, so higher gain makes the axis stiffer and more accurate. Too much gain causes overshoot and oscillation. In the simulator, change the gain and load and watch the following error.

Servo Motor Simulator | Curvo Studio

What is a servo motor?

Working principle

A servo motor is not a distinct type of machine so much as a complete closed-loop package: a motor, a position or speed sensor, and a controller that continuously compares the commanded value with the measured one. The controller drives the motor with whatever current is needed to reduce that error to zero. Most modern industrial servos use a permanent magnet synchronous motor with a high-resolution encoder, while hobby servos use a small brushed DC motor, a gearbox and a potentiometer.

The feedback loop

The feedback loop is what separates a servo from an ordinary motor. Position, velocity and current are usually regulated in nested loops, the current loop being the fastest and the position loop the slowest. Because the controller sees the actual shaft position, it corrects for load disturbances, friction and manufacturing tolerance automatically. A stepper simply assumes it arrived; a servo knows whether it did, and keeps pushing until it has.

Torque, speed and dynamic response

A servo drive can briefly deliver two to three times continuous torque, which is what makes fast acceleration and short settling times possible. Rated torque is available continuously from zero up to base speed, and above that the drive enters field weakening and trades torque for speed. Tuning the loop gains sets the trade-off between response speed and stability: too little gain and the axis lags, too much and it overshoots or oscillates.

Where it is used

Servo motors run anything where motion must be accurate, repeatable and fast: industrial robots, CNC machine tool axes, pick-and-place machines, packaging and printing lines, and camera gimbals. Small hobby servos with a PWM pulse-width command move RC surfaces and robot joints. Compared with a stepper the servo costs more and needs tuning, but it does not lose steps, runs cooler at standstill and holds its accuracy at high speed.

Frequently asked questions

What is the difference between a servo motor and a stepper motor?: A servo motor runs closed loop: an encoder measures the shaft position and the drive corrects any error, so it keeps full torque at speed and does not lose position under load. A stepper runs open loop and simply trusts that each step happened. Steppers are cheaper and hold strongly at standstill; servos are faster, smoother and more efficient.
What does encoder resolution mean for a servo motor?: Encoder resolution is how many distinct positions the encoder reports per revolution. A 17-bit encoder gives 2^17 = 131,072 counts per turn, about 0.003° per count. Higher resolution lets the drive control position and low speed more smoothly, though mechanical backlash and loop tuning still limit real accuracy.
What does position loop gain do in a servo drive?: Position loop gain, in 1/s, sets how strongly the drive responds to position error. At constant speed the following error is roughly speed divided by the gain, so higher gain makes the axis stiffer and more accurate. Too much gain causes overshoot and oscillation. In the simulator, change the gain and load and watch the following error.