What Are Servomotors?

Servomotors are rotary or linear actuators designed specifically for use within closed-loop control systems, where a feedback sensor continuously reports the motor's position, velocity, or torque to a controller that adjusts the drive signal to minimize error. The term "servo" is not a property of the motor hardware alone but of its operating context: a conventional motor becomes a servomotor when paired with a position or velocity sensor and integrated into a feedback loop. Servomotors are characterized by their ability to hold precise positions, execute controlled velocity profiles, and respond rapidly to changing commands, making them distinct from induction or stepper motors used in open-loop or simple on-off applications.

The discipline draws from electrical machine design, power electronics, and control theory. Advances in permanent-magnet materials, digital signal processors, and high-resolution encoders over the past four decades have made high-performance servo drives compact and cost-effective across a broad range of industries.

Motor Types and Construction

Servomotors are divided primarily into AC and DC types, with each category further subdivided by commutation method. DC brushed servomotors use mechanical commutators, where carbon brushes maintain electrical contact with rotating segments; they are relatively simple and deliver high starting torque but require periodic brush replacement. Brushless DC (BLDC) servomotors replace mechanical commutation with electronic switching, eliminating wear and reducing electrical noise while improving efficiency and allowing higher operating speeds. AC synchronous servomotors use permanent-magnet rotors that lock to the rotating magnetic field of the stator, offering high torque density and precise speed regulation. Advanced Motion Controls' analysis of servo motor types identifies brushless DC and AC synchronous motors as the dominant choices for modern precision applications, noting that brushless designs offer "high efficiency, low maintenance, reduced noise, and compact size" compared to brushed alternatives. Linear servomotors omit the rotary-to-linear conversion mechanism by producing translational force directly, benefiting applications in semiconductor manufacturing and precision positioning stages.

Encoder Feedback Systems

The feedback sensor is as important as the motor itself in determining overall servo performance. Rotary encoders are the most common feedback device, producing digital pulse trains that a controller counts to derive angular position and velocity. Incremental encoders generate relative position data referenced to a known home position, while absolute encoders output a unique code for every shaft angle, providing position information without requiring a homing cycle after power-on. Resolvers, which are analog transformers that output sine and cosine signals proportional to shaft angle, are preferred in aerospace and military applications because of their mechanical robustness and resistance to contamination. Resolution is a key specification: a 10,000-count-per-revolution encoder provides angular measurement granularity of 0.036 degrees, and many modern drives interpolate encoder signals to achieve effective resolutions an order of magnitude higher. Research on servomotor control principles published in Applied Sciences discusses how encoder resolution directly limits achievable positioning accuracy and the bandwidth of the velocity feedback loop.

Control and Drive Electronics

The servo drive, also called the servo amplifier, sits between the control computer and the motor, converting low-power command signals into the high-current waveforms that drive the motor windings. Modern digital servo drives implement the control algorithms entirely in firmware, running current, velocity, and position loops at update rates in the tens of kilohertz. Field-oriented control (also called vector control) is the standard technique for AC and BLDC servomotors, decomposing the stator current into torque-producing and flux-producing components for independent regulation. The IEEE Robotics and Automation Society's work on motion control reflects how advances in servo drive technology have enabled increasingly capable robotic systems, where tight servo bandwidth is essential for safe, precise interaction with unstructured environments.

Applications

Servomotors have applications in a wide range of precision motion fields, including:

• CNC machine tools and additive manufacturing equipment, where multi-axis servo coordination follows programmed toolpaths
• Industrial robotics, providing torque and position control at every joint of an articulated arm
• Semiconductor fabrication, driving the high-resolution positioning stages used in lithography and inspection
• Medical devices, including surgical robots, infusion pumps, and prosthetic limb actuators
• Aerospace and defense, operating flight control surfaces, radar antennas, and electromechanical actuators