Overview

Linear actuators are used in machine tools because of their high-speed performance, high acceleration and deceleration, and accurate positioning. There are coreless types of linear actuators, as well. They generally have a smaller thrust force than core linear actuators, but they do not produce cogging, so they only have a small amount of thrust variation. Because of this property, they are used in fields where high-accuracy positioning is necessary, like with head drives of packaging machines or the slight movements of precision stages.
Static thrust variations at the translation position of the actuator have an effect on determining the position accurately. The static thrust is determined by the amount of current, so its current characteristics need to be obtained.
This Application Note explains how to obtain the current characteristics and the translation position characteristics of the static thrust in a voice coil motor, which is a type of coreless linear actuator.

Translation Position Characteristics of Static Thrust

The translation position characteristics of the static thrust at a supply current of 0.05 A are shown in fig. 1. The Lorentz force density distribution and magnetic field distribution of the coil at translation positions of 0 mm and 1 mm are shown in figures 2 and 3.
From fig. 1, it is apparent that the static thrust reaches its peak at a translation position of 1 mm, when the coil is directly beneath each magnetic pole. From fig. 2, it is apparent that the Lorentz force density inside the circle at the translation position of 0 mm is small compared to the translation position of 1 mm. From fig. 3, it is apparent that the magnetic field in the circle at the translation position of 0 mm is weak, and its direction is not perpendicular to the translation direction. Because the direction and size of the current in the coil is constant and the static thrust (Lorentz force) is determined from the strength and direction of the magnetic field, the static thrust is reduced by the magnetic field distribution in the coil.

Current Characteristics of Static Thrust

The current characteristics of the static thrust are shown in fig. 4. The figure shows the linearity of the static thrust against the supply current. According to the slope of this line, the thrust constant is 6 N/A. This means that it is possible to control the static thrust with the supply current value.