METHOD FOR ESTIMATING ABSOLUTE DISTANCE OF TRACKING LASER INTERFEROMETER AND TRACKING LASER INTERFEROMETER

Abstract

To provide a method for estimating an absolute distance L between a tracking laser interferometer and a retroreflector 70, the tracking laser interferometer including; the retroreflector 70 for reflecting and returning incident measurement light A in an incident direction; and a two-axis rotating mechanism 40 for rotationally moving in an exit direction of the measurement light A so that optical axes of the measurement light A and return light B are collimated, which outputs a measurement value according to an increase or decrease in the distance between the interferometer and the retroreflector 70, wherein the absolute distance L between the interferometer and the retroreflector 70 is estimated by performing arithmetic operation based on an angular position variation θ2 of the two-axis rotating mechanism 40 when a deviation amount d of the return light B from the retroreflector 70 relative to a predetermined position is given as a predetermined value d2. This enables an appropriate increase or decrease of the control amount of a tracking control according to a distance between the interferometer and the retroreflector without forcing an operator to perform a troublesome origin return operation and adding an expensive absolute distance sensor.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

The preferred embodiments will be described with reference to the drawings, wherein like elements have been denoted throughout the figures with like reference numerals, and wherein;

FIG. 1 is a view showing an overall structure of a conventional tracking laser interferometer;

FIG. 2 is an optical path view showing a conventional optical system;

FIG. 3 is an optical path view showing a conventional arrangement in which a retroreflector moves in a direction perpendicular to an optical axis;

FIG. 4 is an optical path view showing a conventional arrangement in which a retroreflector moves in a direction of an optical axis of a tracking laser interferometer;

FIG. 5 is a view illustrating an overall structure of a tracking laser interferometer according to the present invention;

FIG. 6 is an optical path view showing a principle of estimating an absolute distance according to the present invention; and

FIG. 7 is an optical path view illustrating an embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to the drawings, an embodiment of the present invention is described in detail below.

The present invention utilizes an advantage that a retroreflector has the following characteristics.

• (1) Incident light and reflecting light are parallel.
• (2) Positions of the incident light and the reflecting light become point symmetric with respect to the center point of the retroreflector as a datum point.
• (3) An optical axis of the light incident on the center point of the retroreflector coincides with an optical axis of its reflecting light.

FIG. 5 shows an overall structure of a tracking laser interferometer according to an embodiment of the present invention. A change made from the conventional example of FIG. 1 is that a sensor (not shown) for outputting an angle signal is mounted on a two-axis rotating mechanism 40 for controlling an orientation of an optical system 10, and thus a controller 50 can receive: a deviation output signal d indicating a deviation amount of return light back to a laser interferometer measuring device 20; a distance signal measured by the laser interferometer measuring device 20; and an angle signal from the two-axis rotating mechanism 40.

Referring to FIG. 6, the following describes a principle of estimating an absolute distance according to the present invention.

When an optical system 10 of a tracking laser interferometer tracks a retroreflector 70, a point around which the two-axis rotating mechanism 20 rotates is given as a rotation center O. The measurement light A from the laser interferometer measuring device 20 is reflected at the retroreflector 70. When a distance between the measurement light A and return light B is d (also referred to as optical axis deviation amount), which is equivalent to a deviation amount of the return light, the return light B propagating to a light spot position detecting element 34 enters Q₁.

In order to set the distance d between the measurement light A and the return light B to 0, the two-axis rotating mechanism 40 is controlled so that the return light at a point Q₁ is returned to Q₀ on a light receiving surface of the light spot position detecting element 34. In this case, when a rotational angle ∠P₀OP₁ of the optical system 10 is θ, the following equation is established.

d=2L tan θ  (1)

With this equation, an absolute distance L between the initial point O (=the rotation center O) and a measurement object P₀ can be found.

L=d/(2 tan θ)   (2)

Specifically, from a state shown in FIG. 2 in which the measurement light A from the laser interferometer measuring device 20 is reflected at the center point P₀ of the retroreflector 70 and optical axes of the measurement light A and the return light B coincide, the optical system 10 is rotated as shown in FIG. 7 such that the return light B passes to an outside line Q₂ of the light spot position detecting element 34, i.e., a sensor for measuring the distance between the measurement light A and the return light B as an optical axis deviation amount, and deviates from the measurement range of the light spot position detecting element 34. Further, the amount of rotation of the optical system 10 when the return light B is incident on the outside line Q₂ is θ₂.

In this case, since the measurement range of the light spot position detecting element 34 for measuring the optical axis deviation amount is known, an absolute distance L to the measurement object can be determined by substituting the amount of rotation θ₂ of the optical system 10 and the optical axis deviation amount d₂ (=Q₀Q₂) for equation (2).

As such, in the laser interferometer measuring device having the two-axis rotating mechanism, the absolute distance can be estimated by an angle position sensor attached to the interferometer and the optical axis deviation amount of the return light. Further, based on the absolute distance, the laser interferometer measuring device can be preset.

This enables interruption at an arbitrary position and restart at the position of measurement of an absolute distance. Thus, even when the laser light is blocked and the measurement is interrupted, remeasurement of the absolute distance can be performed instantly.

Further, based on the distance between the measurement light and the return light and the absolute distance, it is possible to calculate an angle required to move the laser beam to the center point of the retroreflector. In the tracking control of the laser interferometer measuring device having the two-axis rotating mechanism, it is necessary to increase or decrease the control amount according to a distance between the interferometer and the retroreflector. The present invention enables the interferometer to know the absolute distance autonomously and constantly. Further, with the information, the interferometer can maintain the closed loop control system to be constantly in an optimal condition. As a result, the interferometer can move at a high speed and achieves measurement with high accuracy.

In the present embodiment, since the estimation of an absolute distance is performed using the outside line Q₂ of the light spot position detecting element 34, the estimation can be performed easily with high accuracy. Note that, the estimation of the absolute distance can be performed using another point of the light spot position detecting element 34.

It should be apparent to those skilled in the art that the above-described embodiments are merely illustrative which represent the application of the principles of the present invention. Numerous and varied other arrangements can be readily devised by those skilled in the art without departing from the spirit and the scope of the present invention.

Claims

1. A method for estimating an absolute distance of a tracking laser interferometer, the tracking laser interferometer including; a retroreflector for reflecting and returning incident measurement light in an incident direction; and a two-axis rotating mechanism for rotationally moving in an exit direction of the measurement light so that optical axes of the measurement light and return light are collimated, which outputs a measurement value according to an increase or decrease in distance between the interferometer and the retroreflector, wherein a deviation amount of the return light from the retroreflector relative to a predetermined position is given as a predetermined value, andthe absolute distance between the interferometer and the retroreflector is estimated by performing arithmetic operation based on an angular position variation of the two-axis rotating mechanism.

2. A tracking laser interferometer including; a retroreflector for reflecting and returning incident measurement light in an incident direction; and a two-axis rotating mechanism for rotationally moving in an exit direction of the measurement light so that optical axes of the measurement light and return light are collimated, which outputs a measurement value according to an increase or decrease in distance between the interferometer and the retroreflector, the tracking laser interferometer further including;position detecting means for outputting a deviation signal according to a deviation amount of the return light from the retroreflector relative to a predetermined position;angular position detecting means for outputting an angular position variation of the two-axis rotating mechanism; andmeans for estimating an absolute distance between the interferometer and the retroreflector by performing arithmetic operation based on the angular position variation when a deviation amount of the return light from the retroreflector relative to a predetermined position is given as a predetermined value, whereinthe absolute distance thus estimated is substituted for a measurement value.

3. The tracking laser interferometer according to claim 2, wherein the absolute distance is estimated by using an outside line of the position detecting means.

4. The tracking laser interferometer according to claim 2, wherein the estimation of the absolute distance and the substitution of the measurement value are automatically performed.

5. The tracking laser interferometer according to claim 2, wherein the absolute distance is automatically set by detecting discontinuity of the absolute distance due to intermittent blocking of the measurement light.