Linear actuator

Electrical generator or motor structure – Dynamoelectric – Linear

Reexamination Certificate

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Details

C310S017000

Reexamination Certificate

active

06198179

ABSTRACT:

This application corresponds to Japanese Patent Application No. 10-51340, filed on Feb. 16, 1998, which is hereby incorporated by reference in its entirety. This application also corresponds to Japanese Patent Application No. 11-15073, filed on Jan. 25, 1999, which is also hereby incorporated by reference in its entirety
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a small-scale linear actuator for exemplary use in a process for component mounting and measuring or the like, such as a process for mounting and measuring a chip component using a probe.
2. Description of the Related Art
Generally, a linear actuator comprises a fixed element and a moving element. For instance, the fixed element can comprise a yoke and a magnet, and the moving element can comprise a bobbin and an electromagnetic coil. When current is injected into the coil, the action of the magnetic field and the current produces thrust on the moving element. As a result, the moving element moves in the direction of the thrust while maintaining a very small gap between the moving element and the fixed element. When the direction of current flow is reversed, the thrust is reversed. By using a lightweight moving element and appropriately controlling the electrical current, it is possible to achieve excellent responsiveness and precise position control.
It is preferable that the acting force (thrust) of the actuator to an outside is constant within a movable stroke area in case of a mounting process or an inspection process. However, in the configuration described above, the magnetic flux acting on the coil varies depending on the position of the moving element, and consequently, the resultant force acting on the actuator also varies within the stroke range.
In order to make the thrust of the actuator constant, the length of the magnet in the stroke direction may be lengthened so that the coil can move within a constant magnetic flux area of the magnet. However, in this structure, when power is switched off due to an emergency shutdown or the like, thrust is lost and the moving element stops in a position other than the desired position. In particular, when the linear actuator is used as a Z-axis actuator in the process of mounting and measuring or the like, there is a possibility that a dead load will cause the moving element to drop below the lower limit of the stroke, thereby causing damage to the device or product, or causing injury to the operator. Thus, the fact that the actuator stops in a position not intended by the designer gives rise to problems of safety and product quality.
Furthermore, when the actuator remains stationary in one place for a long time due to malfunction or while waiting for items to be processed, heat is generated in the actuator, leading to problems of poor performance and shortened life span.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide a linear actuator wherein a fixed load can be obtained within the stroke range and at least the problems mentioned above can be solved or reduced.
In order to achieve the above objects, a first aspect of the present invention comprises a linear actuator comprising a fixed element located at a fixed position, a moving element provided movable in the straight line direction, a coil provided with one of the fixed element and the moving element, and a magnet provided with the other of the fixed element and the moving element. The magnet generates the magnetic flux in vertical direction to the coil. The linear actuator also comprises means for applying opposing force to a thrusting force applied to the moving element. The means has a predetermined reaction characteristic in the stroke direction of the moving element. The present invention provides a linear actuator wherein a constant load stroke area is provided in at least a part of stroke area of the moving element so that the reaction characteristic of the means and the thrusting force applied to the moving element are balanced.
The present invention is characterized in that the force applying means (for instance, a spring) is provided which has the reaction force opposing to the thrust of the moving element. However, when the actuator is activated and the position of the moving element changes, the opposite force from the force applying means also changes, so that the resultant load which the actuator applies to the outside is different. Therefore, according to the present invention, balance is ensured between the characteristics of the opposite force of the force applying means and the characteristics of the thrust acting on the moving element, thereby achieving a constant load to the outside within the stroke range.
As in the second aspect of the present invention, it is preferable that a cylindrical magnet magnetized in a radial direction defined by a yoke is provided with one of the fixed element and the moving element, a cylindrical coil is provided with the other of the fixed element and the moving element in a concentric circle at inner or outer peripheral side of the cylindrical magnet, and the moving element is movable in the axial direction of the cylindrical magnet. In this case, since the magnetic flux of the magnet functions over the whole periphery, enabling to generate a large thrust force by a small current.
As shown in
FIG. 1
, when the density distribution of the magnetic flux is constant as in the third aspect of the present invention, it is preferable that a constant load stroke area is in an area where one end of the coil in the stroke direction is located at an outside position from one end c of the magnet in the stroke direction, and the other end b of the coil in the stroke direction is located at an inside position from both ends, c, d of the magnet in the stroke direction. In such a case, when the reaction force of the force applying means is made to vary with a constant inclination in accordance with the variation of the coil, the thrust applied to the coil is increased in proportion. Thus, the thrust and the reaction force are cancelled, thereby making the load to outside constant.
In general, the magnetic flux distribution is not always constant. As shown in
FIG. 2
, there is a case having an area, whose magnetic flux distribution is not constant, at both ends thereof. According to a fourth aspect of the present invention, a constant load stroke area is determined in an area where one end of the coil in the stroke direction is located in the outside end b of the coil in the stroke direction is located within a substantially constant area c′ to d′ of the magnetic flux of the magnet. Namely, the thrust applied to the coil can be vary in proportion by preventing both ends of the coil from moving in the area whose magnetic flux is not constant.
When a constant load stroke area includes a pair of a coil and a magnet, the constant load stroke area is limited by a dimension of the coil in the stroke direction. According to a fifth aspect of the present invention, a first and a second coil, a first and a second magnets which generate a magnetic flux in the vertical direction to the coils, are provided. The first coil and the second coil, and the first magnet and the second magnet are mutually connected with a predetermined interval therebetween, so that when one end of the first coil in the stroke direction is at an outside position from one end of the first magnet in the stroke direction and the other end of the first coil is at an inside position from both ends of the first magnet in the stroke direction, both ends of the second coil in the stroke direction are at an outside position from both ends of the second magnet in the stroke direction, and when both ends of the first coil in the stroke direction are at an inside position from both ends of the first coil in the stroke direction, one end of the second coil in the stroke direction is at an outside position from one end of the second magnet in the stroke direction and the other end of the second coil is at an inside position fro

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