Stepper Motors Unveiled: From Basic Mechanics to Advanced Control Systems

Highly Reliable Stepper Motors: Unleashing Power and Precision in Motion Control

Stepper motors are often undervalued compared to servo motors, but they offer comparable reliability. These motors sync precisely with pulse signals from controllers to drivers, enabling accurate positioning and speed control. With high low-speed torque and minimal vibration, they excel in short-distance, quick-positioning applications.

Stepper Motors: A Complete Guide to Principles, Types, and Applications

"Stepper motors? Servo motors must perform better." This is a common misconception when discussing stepper motors. In reality, stepper motors excel in diverse applications—from advanced industrial equipment to everyday automated instruments. This article unpacks why they remain a top choice for precision-driven systems.

While some may overlook stepper motors, their role in high-accuracy control is pivotal across industries. They power factory automation (FA), semiconductor/FPD/solar panel manufacturing equipment, medical devices, analytical instruments, precision stages, financial systems, food packaging machines, and even camera aperture adjustments—proving their versatility in demanding, precision-critical environments.

Why do you use a stepper motor?

Easy to use: 34%
Inexpensive: 17%
Simple operations:16%
No need for tuning: 12%
Other: 21%
*# of questionees: 258 (multiple answers allowed)/ researched by Oriental Motor

Key Advantages: User-Friendly, Simple Operation, and Low Cost

According to a survey of stepper motor users, many favor them for their user-friendliness, simple operation, and low cost—benefits directly tied to their structural and system design. The straightforward mechanics and configuration of stepper motors naturally explain these advantages. However, some readers may question their accuracy and torque performance. Such doubts are best addressed through direct comparisons with other control motors like servos. By understanding stepper motor characteristics and matching them to operational needs, users can effectively reduce equipment costs. Below, we break down their key features and technical insights:

High Stopping Accuracy with Fast Low/Mid-Speed Performance

Stepper motors offer exceptional stopping accuracy and enable precise open-loop control. For instance, the RK II Series achieves ±0.05° stopping accuracy (no load) when positioning a rotating table. With no cumulative step errors, they ensure consistent high-precision positioning. Their encoder-free design simplifies the drive system, reducing costs while maintaining reliability.

Point 1
Fantastic Stopping Accuracy

For example, when converting stopping accuracy ±0.05° of a stepper motor to the ball screw mechanism:

Operating Conditions:
• Motor: RK II Series

• Lead of ball screw: 10mm

Stopping Accuracy: ±1.4µ-m

Generally, accuracy of a ground ball screw type is ±10µm. When using a rolled ball screw type, its accuracy declines to ±20µ m, indicating that the stopping accuracy of a stepper motor is much higher than that of ball screw types.

Stepper motors excel in low/mid-speed torque—a key differentiator from servo motors, which deliver consistent torque across mid-to-high speeds and suit long-stroke, high-rotation tasks. Unlike servos, stepper motors feature a non-flat torque curve: peak torque at low/mid-speeds drops off significantly at high speeds. This makes them ideal for short-stroke applications (e.g., limited rotations), where they provide:

• High torque at required speeds for quick positioning in tasks like multi-rotation tables or inching.
• Stable low-speed performance (a challenge for servos), with minimal need for high-speed operation—since short-stroke tasks decelerate and stop before reaching peak RPM.

In short, steppers optimize for torque where it matters most: the low/mid-speed range critical to short-distance, precision-driven applications.

High Responsiveness and Excellent Synchronization

The third outstanding feature of stepper motors is their responsiveness. With open-loop control, which sends one-way commands to the motor, stepper motors can highly synchronize with command signals. In contrast, servo motors rely on encoder feedback, often causing command delays. Stepper motors, however, operate in real-time with incoming pulses, minimizing latency and ensuring rapid response.

This makes stepper motors ideal for applications requiring multi-motor synchronization. For instance, in board transfer systems with two conveyors, each driven by a separate motor, stepper motors can precisely coordinate movements, ensuring seamless board transfer between conveyors.

Point 2
Excellent Low / Mid-Speed Range!

Example: Torque of a motor frame size 85 mm is equivalent to a rated torque of a 400 W servo motor when 1000 r/min.

Torque in an even lower speed range can be up to 5 times higher. For a shortdistance positioning, having high torque in the low / mid-speed range is essential.

Point 3
High Responsiveness!

Suitable Applications

Other than an inching application with frequent starting and stopping, stepper motors are suitable for positioning of image check processors that dislike vibrations, cam drives that would be difficult to adjust with servo motors, and low rigidity mechanisms such as a belt drive. Furthermore, cost is reduced significantly by replacing a ball screw drive to a belt drive.

Advantage of Great Features

Besides cost reduction, stepper motors have many advantages in terms of performance. The following chart shows the converting torque of the RKII Series example to that of typical servo wattage ranges. Further down, detailed information on stepper motors, such as basic structure, system, and example applications, is introduced for more information on stepper motors.

Basics of Stepper Motors

Operation & Structure

A stepper motor rotates with a fixed step angle, just like the second hand of a clock. Highly accurate positioning can be performed with open-loop control thanks to the mechanical structure within the motor.

Accurate Positioning (Number of Steps)

While having full control of rotation and speed, the simple structure of stepper motors is achieved without using electrical components, such as an encoder within the motor. For this reason, stepper motors are very robust and have high reliability with very few failures. As for stopping accuracy, ±0.05° (without cumulative pitch errors) is very accurate. Because positioning of stepper motors is performed by open-loop control and operated by the magnetized stator and magnetic rotor with small teeth, stepper motors have a higher follow-up mechanism toward commands than that of servo motors. Also, no hunting occurs when stopping stepper motors. They are also excellent in belt drives, which have low rigidity.

Useful for Speed Control and Position Control

When pulses are input to a driver through a pulse generator, stepper motors position according to the number of input pulses. The basic step angle of 5-phase stepper motors is 0.72° and 1.8° for 2-phase stepper motors. The rotating speed of the stepper motor is determined by the speed of the pulse frequency (Hz) given to the driver, and it is possible to freely change the motor rotation by simply changing the number of input pulses or frequencies to the driver. Stepper motors not only serve as position control motors, but also as speed control motors with high synchronization.

Stepper Motors Uses:

• High frequency, repetitive positioning of fixed step angles
• Positioning that requires long stopping time due to width adjustment, etc.
• Fluctuating loads and changing rigidity
• Positioning that divides 1 cycle
• Motor shafts that requires synchronous operation

Operating System

Simple Control With No Sensor or Feedback

Because it is possible to perform accurate positioning and position control while synchronizing with the number of command pulses and speed, there is no need for devices, such as a sensor, for positioning. Therefore, the entire system is simple to build. If advanced control, such as an interpolation operation, is not required, the built-in controller function type driver is recommended. Cost is reduced by eliminating controllers, such as a pulse generator and PLC positioning modules.

Built-in Sensor Closed-Loop Type

Although high accuracy positioning is possible with open-loop control, what would happen if a problem occured? In order to avoid such pitfalls, an encoder type or built-in sensor closed-loop control type motor (AR Series) can be used.

Can the Cost be Further Reduced?

The common issue among design engineers is cost reduction. Is there really no way to further reduce cost? To find out a cost reduction test, with specification enhancements, was conducted based on the ball screw mechanism. The following explains the details of the test:

Mission

Linear Motion Mechanism

1. Further Increase Speed
2. Further Reduce Cost
[Conditions of the Originally- Planned Equipment] Mechanism: Ball Screw + Servo Motor Conditions such as a load, speed, and lead, shown on the right, are determined based on the servo motor attached with ball screws and steel plate.

Plan

Change the Mechanism to Belt Pulley
• Ball screw if trying to increase speed => Belt mechanism may be more suitable => 1000mm/sec to 1500mm/sec is possible with the belt mechanism. Change to belt if there is no issue with positioning accuracy. • Reduce cost significantly if changing to belt is possible => Belt is inexpensive but its low rigidity may affect stability of servo motor operation, even with automatic tuning.

Problems

1. Difference in Stopping Accuracy between Screw and Belt... How Much Stopping Accuracy Is Required?
2. Impact of Low Rigidity... Impact on Settling Time, Avoiding Tuning Problem
• Better stopping accuracy with the screw. No problem to change to the belt? => The application's required stopping accuracy is ±0.05 ~ 0.1mm, which is not as accurate as the one for the screw. Therefore, it should be okay to replace with the belt.
• If changing to the belt, rigidity on the mechanism gets low, thus the servo motor movements becomes unstable. => Among positioning motors, stepper motors do not have a built-in encoder. For this reason, they require no adjustment and are strong against low rigidity. Their movements are stable regardless of fluctuating loads. If the output is the same, consider stepper motors.

Estimation

Mechanism: Belt Pulley + Motor: Try with Stepper Motor

Problems

1. Difference in Stopping Accuracy between Screw and Belt... How Much Stopping Accuracy Is Required?
2. Impact of Low Rigidity... Impact on Settling Time, Avoiding Tuning Problem
• Better stopping accuracy with the screw. No problem to change to the belt? => The application's required stopping accuracy is ±0.05 ~ 0.1mm, which is not as accurate as the one for the screw. Therefore, it should be okay to replace with the belt.
• If changing to the belt, rigidity on the mechanism gets low, thus the servo motor movements becomes unstable. => Among positioning motors, stepper motors do not have a built-in encoder. For this reason, they require no adjustment and are strong against low rigidity. Their movements are stable regardless of fluctuating loads. If the output is the same, consider stepper motors.

Estimation

Mechanism: Belt Pulley + Motor: Try with Stepper Motor

• Transportable Mass -> Max. permissible load 7kg • Traveling Speed -> Improved to 800 mm/ sec Motor => By changing from stepper motor to servo motor, reduced cost by 50%! Mechanism => By changing from ball screw to belt mechanism, reduced cost by 7%!

Results

There Was Much Room for Cost Reduction!
By conducting a zero-based review of the mechanism as well as the motor selection based on characteristics, we managed to increase specifications and reduce cost, even with the motor size became slightly larger. In the past, motor selection was done based on its ease-of-use or familiarity. After this exercise, differences of operations between servo motors and stepper motors became clear. It was surprising that stepper motors are more affordable than expected. There must be room for cost reduction of other devices using this method. This exercise reacknowledged that the well-balanced selection between motor specifications and cost, while maximizing motor characteristics is the key.

Which Has Higher Stopping Accuracy - Stepper Motor or Servo Motor?

Customer Inquiry: Looking for a motor with good stopping accuracy. How much of a difference is there between stepper motors and servo motors?

Assumption: The AC servo motor NX Series is equipped with a 20-bit encoder, thus it should have a fine resolution, and good stopping accuracy.

First, it is necessary to clarify the difference between resolution and stopping accuracy: Resolution is the number of steps per revolution and it is also called a step angle for stepper motors. It is needed when considering how precise the required positioning needs to be. Stopping accuracy is the difference between the actual stop position and theoretical stop position.

Does this mean that the AC servo motor equipped with a high accuracy encoder has better stopping accuracy than stepper motors?
Not quite. In the past there was no issue with the concept of "stopping accuracy of servo motors being equal to encoder resolution within ±1 pulse." However, recent servo motors are equipped with the 20 bit encoder (1,048,576 steps) which has a very fine resolution. Because of this, errors due to the encoder installation accuracy have a huge effect on stopping accuracy. Therefore, the concept of stopping accuracy has slightly started to change.
According to the comparison charts, stopping accuracy between stepper motors and AC servo motors is almost the same (±0.02º ~ 0.03º). Accuracy depends on the mechanical precision of the motor for stepper motors, thus if stop position can be done per 7.2º, positioning is done by the same small teeth on the rotor at all times, according to the motor structure. This makes it possible to further improve stopping accuracy.
However, stepper motors may generate displacement angle depending on the load torque value. Also, depending on the mechanism condition, AC servo motors may have wider hunting width as a response to gain adjustments. For these reasons, some caution is required.

Comparison of Stopping Accuracy Between Stepper Motors and AC Servo Motors