Servo motors — DC and AC servo systems

Reference notes

A servo motor is a motor designed for precision position or velocity control in a closed-loop system. Use Next → to walk through DC servo architecture, AC servo (PMSM-based) industrial standard, the cascaded control-loop structure, sizing considerations, and modern trends (linear servos, direct-drive, fieldbus, AI tuning).

What makes it a SERVO

Three things, together — servo is a SYSTEM, not just a motor. (Note: a series-wound DC motor is NOT a servo unless paired with feedback and a matched drive.)

1. Motor design — low-inertia rotor + high torque for fast acceleration, low electrical and mechanical time constants.
2. Position / velocity feedback — optical encoder (2–8 M counts/rev), resolver, or tachometer.
3. Matched servo drive — closes inner current, velocity, and position loops at high rates.

DC servo motors

• Stator: permanent magnets (small servos) or separately excited wound field (larger). PM construction gives linear T = K_t · I and back-EMF E = K_e · ω, and eliminates the field-circuit time constant. Separately excited wound-field DC servos allow controllable field current for field-weakening above base speed.
• Rotor: low-inertia armature with shallow slots, or moving-coil ironless design for very high response.
• Feedback: tachometer for velocity loop, encoder for position loop.
• Drive: linear amplifier (small servos < 100 W) or PWM-based chopper / H-bridge.
• Bandwidth: 50–500 Hz. Position accuracy < 1 arc minute.
• Applications: instrumentation, robot wrists, antenna positioners, hobby RC servos, small machine-tool feed axes.

AC servo motors — modern industry standard

• Construction: small PMSM (SPM or IPM) with surface or interior permanent magnets, distributed stator winding for sinusoidal back-EMF.
• Built-in high-resolution encoder: 2–8 million counts per revolution, often absolute (known position at power-up).
• Drive: matched servo amplifier with field-oriented control.
• Bandwidth: 100–1000 Hz. Position accuracy < 1 arc second.
• Brands: Yaskawa Sigma, Mitsubishi MR-J, Siemens SIMOTICS, ABB MotiFlex, FANUC alpha, Allen-Bradley Kinetix.
• Applications: CNC machine-tool axes (every axis is a servo), industrial robots, semiconductor wafer steppers, packaging machinery, printing presses.

Cascaded control structure

Each outer loop must have bandwidth roughly 10× slower than its inner loop for stability margins. Feedforward of velocity and acceleration commands directly into inner loops reduces position error during dynamic moves without raising gain (which would amplify noise).

Sizing servo motors

• Load inertia ratio J_load_reflected / J_motor (where J_reflected = J_actual / N² for gear ratio N). < 5 : 1 easy; 5–30 : 1 careful tuning; > 30 : 1 limits bandwidth, needs gear reduction or larger motor.
• Peak vs continuous torque — peak typically 3-5× continuous, available for short moves and acceleration. Continuous thermally limited.
• RMS torque over duty cycle must stay below continuous rating.
• Regenerative energy — decelerating loads return energy; drives include DC-bus caps and external brake resistors.
• Gearing — gear ratio N reduces reflected inertia by N² and required motor torque by N. Optimum N ≈ √(J_load / J_motor).

Modern trends

• Linear servos — linear synchronous / induction motors with linear encoders. Zero backlash. Used in CNC slides, semiconductor steppers, pick-and-place.
• Direct-drive torque motors — large-diameter PMSM for high T at low ω. Robot joints, CNC rotary tables, telescope mounts. Zero backlash.
• Real-time fieldbus — EtherCAT (dominant), PROFINET, Powerlink, Sercos III. Synchronize many axes within sub-ms / sub-µs jitter.
• Integrated intelligence — vibration suppression, adaptive notch filters, on-board gain auto-tuning, predictive maintenance.
• AI auto-tuning — ML algorithms learn load characteristics during commissioning, optimize loop gains in seconds vs hours of manual tuning.