Servo vs Stepper Motor: How to Choose for Motion Control
Both servo and stepper motors move a load to a defined position, but they get there very differently. Steppers are open-loop, simple, and cheap. Servos use closed-loop feedback, maintain torque at high speed, and can detect and correct position errors. Choosing the wrong one wastes money or causes production problems.
Contents
Overview
Stepper and servo motors are both used for precision positioning in industrial machines, 3D printers, CNC equipment, conveyors, and robotics. The fundamental difference is in how position is controlled:
- Stepper motor: An open-loop system. The driver sends a fixed number of step pulses; the motor moves a fixed angle per pulse. There is no feedback confirming the rotor reached the commanded position.
- Servo motor: A closed-loop system. An encoder (or resolver) on the rotor continuously reports actual position back to the drive, which adjusts current to eliminate any position error.
This difference cascades into everything else: cost, torque at speed, heat, tuning complexity, and fault detection capability.
Stepper Motors
How They Work
A stepper motor is a brushless DC motor with a high pole-count rotor (typically 50 rotor teeth for a 1.8° motor). The stator windings are energised in sequence; each electrical step advances the rotor by one tooth pitch divided by the number of phases. For a standard 2-phase, 50-tooth motor: 360° / (50 × 4) = 1.8° per full step, giving 200 steps/revolution.
The driver controls two phases (A and B) with positive and negative polarity — four full-step states. Microstepping varies the current ratio between the two phases, creating intermediate positions between full steps. Common microstep resolutions: 1/2, 1/4, 1/8, 1/16, 1/32, 1/64, 1/128, 1/256 of a full step.
Torque Characteristics
A stepper motor's torque decreases steeply with speed. Maximum torque is at stall (0 RPM) — called holding torque. As the driver increases step frequency, inductance limits the rate of current rise in each phase, reducing effective current and therefore torque. Above the pullout speed, the motor stalls.
Typical NEMA 23 stepper (57 mm frame): holding torque 1.0–3.0 N·m at full rated current; torque at 600 RPM may be only 30–40% of holding torque.
Current and Heat
Stepper drivers apply rated current continuously to maintain holding torque — even when stationary. This causes significant heating. Many drivers feature an automatic idle-current reduction (typically to 50% after ~100 ms of no step pulses) to reduce heat when the motor is holding position.
Servo Motors
How They Work
A servo motor is a brushless AC or DC motor combined with a position feedback device — typically an incremental or absolute encoder (10,000–1,000,000 pulses per revolution in modern systems). The servo drive runs three nested control loops:
- Current loop (innermost, ~20 kHz) — regulates phase currents to control torque
- Velocity loop (~1–4 kHz) — controls motor speed using encoder velocity feedback
- Position loop (outermost, ~1–4 kHz) — drives position error to zero using encoder position feedback
Because the drive knows actual position at all times, it can apply maximum torque to accelerate toward the target, then precisely decelerate and settle — a profile called a trapezoidal or S-curve move profile.
Torque at Speed
Unlike a stepper, a servo motor delivers near-rated torque across its entire speed range up to the rated speed. Above rated speed (into the field weakening or constant-power region), torque decreases, but the motor can still run — just at reduced torque. High-speed capability is a key servo advantage for fast cycle-time machines.
Encoder Types
- Incremental encoder — outputs A/B quadrature pulses; loses position on power loss; requires homing on startup
- Absolute single-turn encoder — knows position within one revolution; homing only needed for multi-turn applications
- Absolute multi-turn encoder — tracks position across multiple revolutions using battery backup or mechanical gear counting; no homing required
Torque–Speed Characteristics
The torque–speed curve is the most important factor in motion system selection:
| Condition | Stepper Motor | Servo Motor |
|---|---|---|
| At standstill (holding) | Maximum rated torque | Rated torque (current-limited) |
| At low speed (<300 RPM) | High torque, good | Full rated torque |
| At mid speed (300–1000 RPM) | Torque dropping, 40–70% of holding | Full rated torque |
| At high speed (>1000 RPM) | Very low torque, stall risk | Full torque to rated speed (e.g. 3000 RPM) |
| Under sudden overload | Steps lost — position error undetected | Drive alarms, position error fault |
Side-by-Side Comparison
| Parameter | Stepper Motor | Servo Motor |
|---|---|---|
| Control type | Open-loop (no feedback) | Closed-loop (encoder feedback) |
| Position feedback | None (assumed) | Encoder / resolver |
| Typical accuracy | ±3–5% of 1 full step (open-loop) | ±1 encoder count (sub-0.01°) |
| Torque at high speed | Poor | Excellent |
| Motor cost | Low (£20–£200) | Medium–high (£100–£2000+) |
| Drive cost | Low (£15–£100) | Medium–high (£150–£3000+) |
| Tuning required | None (open-loop) | Yes — PID gain tuning |
| Heat at standstill | High (full current) | Low (near-zero current) |
| Noise/vibration | Audible at low speeds | Smooth across speed range |
| Power range | Typically <1 kW | 10 W to hundreds of kW |
| Missed step detection | No | Yes (following error alarm) |
| Typical applications | 3D printers, label applicators, low-speed conveyors | CNC, robotics, pick-and-place, packaging |
When to Use Each
Choose a stepper motor when
- Speed is low (under 500–600 RPM in operation)
- Load is consistent and predictable (no sudden overload)
- Cost is the primary driver
- The application can tolerate a homing routine on power-up
- Examples: filament extruders, label applicators, simple XY gantries, valve positioners
Choose a servo motor when
- High speed and full torque are both required
- Position accuracy is critical and must be verified (not assumed)
- The load varies or there may be sudden mechanical disturbances
- Smooth motion with low vibration is needed (e.g., camera slides, medical devices)
- Power is above ~0.5–1 kW (steppers are rarely practical above this)
- Examples: CNC machine axes, robot joints, high-speed pick-and-place, flying saw, winding machines
Drive Selection
Stepper Drives
Stepper drives accept step/direction (STEP/DIR) pulse inputs from a motion controller or PLC high-speed output. Key specifications:
- Phase current rating — must match motor rated current (set via DIP switch or software)
- Supply voltage — higher voltage gives better high-speed torque (faster current rise through inductance)
- Microstep resolution — typically selectable up to 1/256
- Idle current reduction — reduces heat at standstill
Popular stepper driver ICs: Texas Instruments DRV8825, Trinamic TMC2209 (with StealthChop for silent operation), Gecko G201X.
Servo Drives
Servo drives accept position, velocity, or torque commands via:
- Analogue ±10 V (velocity or torque command)
- Step/direction pulse train (position command — simplest PLC interface)
- EtherCAT, PROFINET, or CANopen (cyclic synchronous position/velocity/torque)
Servo drives require PID gain tuning: proportional (Kp), integral (Ki), and derivative (Kd) for position and velocity loops. Most modern drives include auto-tuning routines that measure motor inertia and set initial gains.
Common industrial servo systems: Siemens SINAMICS S210/S120, Allen-Bradley Kinetix 5500/5700, Mitsubishi MELSERVO-J5, Yaskawa Sigma-7, Beckhoff AX5000.
Closed-Loop Steppers
A hybrid technology exists: closed-loop stepper (also called step-servo). A standard stepper motor is fitted with an encoder; the driver monitors actual position and increases current or retries if it detects a position error. This provides:
- Missed-step detection and correction
- Lower heat (current reduced proportional to load)
- Higher top speed than open-loop operation
Closed-loop steppers remain less capable than true servo systems at high speed and high power, but they fill a useful middle ground for applications where open-loop stepper accuracy is insufficient but full servo cost is hard to justify. Examples: Leadshine EtherCAT closed-loop stepper modules, Oriental Motor AZ series.
Frequently Asked Questions
- Can a stepper motor lose steps?
- Yes. A stepper motor can miss steps if the load torque exceeds its holding torque, especially at higher speeds where torque drops sharply. Because there is no feedback, the controller cannot detect or correct missed steps. This is why critical positioning applications use servo motors or stepper motors with encoder verification.
- What is the difference between a servo motor and a servo drive?
- A servo motor is the mechanical device (rotor, stator, encoder). A servo drive (amplifier) is the electronics that power the motor and close the position, velocity, and current control loops. Both together form a servo system. You cannot run a servo motor without a matching drive.
- Why does a stepper motor get hot?
- Stepper drivers apply full phase current continuously to hold rotor position, even when stationary. This generates significant I²R heat. Using a driver with automatic current reduction (idle current reduction) at standstill reduces heating by 30–70%.
- What resolution can a stepper motor achieve?
- A standard 1.8° stepper has 200 full steps per revolution. With microstepping drivers (1/16 or 1/32 microstep), resolution reaches 3200–6400 steps/rev. However, microstep accuracy degrades at low holding torque; real positioning accuracy is typically ±3–5% of one full step regardless of microstep setting.