Two ways to position a shaft
Both servo and stepper motors are used where a shaft must stop at controlled positions, but they get there by different routes. A stepper motor is built to move in fixed increments. A servo motor is an ordinary-looking motor combined with a feedback device and a drive that keeps correcting. The difference in approach explains nearly everything else about them.
How a stepper works
A stepper has multiple windings that are energized in sequence, pulling the rotor from one position to the next. A common hybrid stepper takes 200 full steps per revolution, which is 1.8 degrees per step (360 divided by 200). Drives can divide each step further with microstepping. At 16 microsteps per full step, the same motor takes 3,200 steps per revolution, or about 0.11 degrees each. Microstepping makes motion smoother, but the extra positions are not all equally accurate, because holding torque at the in-between points is lower.
In basic use, the controller sends a pulse for each step and a direction signal. There is no feedback, which is called open-loop. As long as the motor can produce the torque needed, each pulse produces one step and position is known by counting. If the load exceeds available torque, for instance during rapid acceleration or a jam, the motor can miss steps. The controller continues to count, and the real position drifts away without any warning.
Characteristics:
- High torque at low speed and strong holding torque at standstill.
- Torque falls as speed rises. Winding inductance limits how fast current can build, so steppers are usually best below a moderate speed.
- Resonance. Steppers can vibrate at certain speeds and can stall if they pass through them poorly. More on this appears in the discussion of motion tuning.
- Simple and economical for light or moderate loads.
- Always draw current. Even at rest, a stepper usually holds with full or reduced current, so it can run warm.
How a servo works
A servo motor is paired with a feedback device, typically an encoder, and a servo drive. The controller commands a position or speed, the drive compares it with the encoder reading, and adjusts the motor current to remove the error. This is closed-loop control. Servo motors can be AC or DC types, with brushless designs very common in industrial machines.
Characteristics:
- Torque available over a wide speed range. Servos usually keep useful torque to high speed, and they often tolerate short bursts of peak torque well above their continuous rating, for rapid acceleration.
- Responds to the load. If the load is heavier than expected, the drive supplies more current, up to its limit, or raises a fault if it cannot follow. It reports a following error instead of silently drifting.
- Smooth running. Servo motors generally run smoothly and quietly at speed.
- Higher cost and more setup. There are more parts, and the drive must be tuned to the load.
- Uses power in proportion to demand. At rest with no load, it draws little.
Because servo systems depend on feedback, the quality of the encoder and the stiffness of the mechanics matter. A servo connected to a springy mechanism can oscillate if poorly tuned, which again leads into tuning.
Closed-loop steppers
A middle option exists: a stepper motor fitted with an encoder and a drive that watches for lost steps. Some versions only flag an error. Others vary current with load and correct position. These offer more reassurance than a plain open-loop stepper, with less complexity than a full servo, though performance and cost sit between the two.
Side-by-side comparison
| Feature | Stepper (open loop) | Servo |
|---|---|---|
| Feedback | None, or optional | Required |
| Torque at high speed | Falls off quickly | Holds up well |
| Risk of unnoticed missed positions | Yes | Low, faults reported |
| Setup | Simple | Needs tuning |
| Cost | Generally lower | Generally higher |
| Typical sizes | Small and medium | Small to very large |
| Holding at standstill | Strong, with current | Held actively by the loop |
These are general tendencies. A well-chosen stepper in a modest application can be just as reliable as a servo, and a poorly tuned servo can perform worse than a stepper.
How to decide
Questions that help:
- How fast and how heavy is the move? High speed and fast acceleration of larger loads favor servo.
- What happens if a position is missed? If a missed step would scrap product or cause damage, favor feedback.
- Is the load predictable? Constant, light loads suit steppers. Varying loads suit servos.
- How much does smooth running at high speed matter? Servos are usually better.
- What is the budget, including time to set up? Steppers are often quicker to commission.
- How many axes and how much duty? Long, continuous duty at high speed generally points toward servo.
Typical stepper uses include small positioning stages, label and dosing equipment, and light-duty 3D printing or laboratory automation. Typical servo uses include fast packaging machines, machine tools and robots, where speed, responsiveness and reliability under changing loads matter.
Either type pairs with mechanics such as ball screws, belts or gearboxes. Selecting gear ratios and checking how the load’s inertia compares to the rotor’s inertia is part of proper sizing, and the choice should rely on manufacturer data and a qualified engineer’s calculations. This article is general information, not engineering advice.