What is servo motor?
What is a servo motor?
A servo motor supplies motion in a system that uses feedback to control position, speed or torque. Think of a CNC axis stopping at a programmed coordinate, a robot placing a component, or a packaging machine matching its cutter to a moving product. The useful distinction is the system’s ability to check what actually happened and respond.
What makes the system a servo?
The motor produces mechanical motion. The servo drive controls electrical power, while an encoder or resolver reports shaft position. Speed can be calculated from how that position changes over time. A PLC, motion controller or program inside the drive supplies the command.
In closed-loop operation, the drive compares commanded and measured motion. Suppose a shaft is commanded to 90° but feedback reports 88°: the position error is 2°. The position loop adjusts motion to reduce that error, within the available torque, speed and control limits.
An encoder does not directly measure torque. A typical drive regulates motor current and uses motor data to implement torque control. Applications requiring an independent measurement of actual shaft torque may need a separate torque transducer.
Three control modes, three different jobs
A servo is not limited to positioning. The position, speed and torque modes determine what the axis regulates. Choosing between speed and position control starts with the machine’s task.
| Mode | Purpose | Typical application |
|---|---|---|
| Position control | Reach a commanded angle or location | CNC axis or indexing table |
| Speed control | Track a commanded speed as the load changes | Synchronised feed roller |
| Torque control | Regulate the motor’s turning effort | Winding equipment or controlled tightening |
For a winding axis, torque affects material tension. However, constant torque does not maintain constant tension as roll diameter changes; the required torque must account for the changing radius.
Motor construction and feedback choices
AC and DC labels do not define a high-power versus low-power split. Many industrial servos use brushless permanent-magnet synchronous motors. Brushed DC, linear and direct-drive designs also have their place. Motor construction, drive compatibility and the required operating range matter more than a simple AC/DC label.
An absolute encoder and an incremental encoder provide different kinds of position information. Whether an axis needs a homing routine after power-up depends on the feedback type, its single-turn or multi-turn capability and the machine’s reference scheme. Absolute feedback does not automatically eliminate every homing requirement.
When is a stepper a reasonable alternative?
The servo-versus-stepper decision depends on speed, load changes, duty and budget. A servo can suit rapid reversals and demanding motion tracking. A conventional stepper can be economical for straightforward positioning, and closed-loop stepper systems are available too. Choose for the axis requirements rather than assuming one technology always wins.
Start sizing with the motion, not the wattage
When matching a motor and drive, check continuous torque, peak torque, speed, load inertia and duty cycle. The load and transmission calculation accounts for the mechanism; the wider selection criteria include supply, feedback compatibility and operating conditions. Understanding torque versus power helps avoid a misleading comparison of nameplate wattages.
An energised servo may need current to hold a load at standstill. A motor with a holding brake has an additional mechanical device for holding the stationary shaft. That brake should not be treated as a routine dynamic stopping brake.
Editor’s note: resolution is not machine accuracy
A fine encoder count is not a promise of equally fine load positioning. Gear backlash, belt stretch, coupling stiffness, mounting and drive settings affect the result. Feedback taken at the motor shaft cannot observe every error downstream in the mechanism. Energy use also depends on the actual load and duty cycle; a servo label alone does not establish an efficiency advantage.
Where to go next
Automatic servo tuning · Understanding mechanical vibration · Planetary gearbox basics
Technical references
Oriental Motor — Servo Motor Features · Kollmorgen — What Is a Servo Motor? · Kollmorgen — Current Loop · Kollmorgen — Motor Selection · Oriental Motor — Servo and Stepper Comparison · Kollmorgen — Holding Brakes