High-Accuracy Positioning Technology for Stepper Motors - Development of Load Position Correction Function Using an External Encoder -

This information is current as of February 28, 2025

Stepper motors and servo motors are commonly used for processes that require high-accuracy positioning. Since stepper motors can be used in an open-loop control system without gain adjustment and can perform high-accuracy positioning without vibration during stop, stepper motors are widely used in various industrial applications such as semiconductor manufacturing equipment and medical analysis equipment. However, as semiconductor manufacturing processes have become increasingly miniaturized in recent years, the accuracy requirements for stepper motors have also increased.
Oriental Motor has developed the load position correction function, which uses the advantage of the high-accuracy position detection by an external encoder and the features of positioning under open-loop control. This report describes the high-accuracy positioning technology of stepper motors offered by the CVD Series fully closed-loop control type driver.

1. Introduction

Stepper motors can be used under open-loop positioning control and do not require gain adjustment even when the load fl uctuation is large. As a result, they are easy to use even with a low-rigidity mechanism and can simplify system builds.
However, when stepper motors are combined with mechanisms such as gears or ball screws, it is difficult to meet the accuracy requirements due to lost motion caused by backlash, pitch error, temperature change, and expansion of the mechanism.
As semiconductor manufacturing processes have become more miniaturized in recent years, the accuracy requirements for stepper motors have also increased. In applications where position reliability is important, a method is used in which commands for position adjustment are issued sequentially by the host controller while the position of the stepper motor is monitored by an encoder. However, in this case, the whole system will become more complex and the simplicity of an open-loop system is lost.
Alternatively, a fully closed-loop control method is adopted with a servo motor that directly feeds back the load position through an external encoder. In this case, the cost is higher than that of the stepper motor due to the use of the servo motor. In addition, this system has the disadvantage that the stability of the system is difficult to maintain under a load with low rigidity, and vibration occurs during stops due to the difficulty of gain adjustment.
This means that the system becomes complicated when a stepper motor is used for high-accuracy positioning with an existing method that uses an external encoder. On the other hand, when a servo motor is used, it is necessary to adjust the gain depending on a load condition in order to maintain the stability of the system, and it has been difficult to find a solution that eliminates both the problem of gain adjustment and high cost.
To solve this challenge, Oriental Motor has developed the load position correction function, which uses the advantage of the high-accuracy position detection by an external encoder and positioning under open-loop control.
This report describes the high-accuracy positioning technology of stepper motors based on the CVD Series fully closed-loop control type (see Figure 1, hereinafter referred to as CVD-F) that adopts this function. The CVD-F provides a reliable, high-accuracy positioning system that retains the ease of use and simplicity of stepper motors.

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