- In actual practice, the work surface whose tilt is to be obtained forms the reflective surface, & the displacement x is measured by a precisions microscope that is directly calibrated to the tilt values of inclinationΘ.
- The optical system of the autocollimator is shown in fig.; the target wires are illuminated with an electric bulb and serve as a source of light because it is not convenient to visualize the reflected image of a point and then measure the displacement x properly.
- After being reflected from the surfaces being measured, the image of the illuminated wire is formed in the same plane as the wire.
- The eyepiece system containing the micrometer microscope apparatus consists of a pair of setting lines, which can be used to measure the displacement of the image by setting it to the original cross lines and then moving over them to the image.
- Typically, calibration is supplied with the instruments. thus, the angle of inclinations of the reflecting surface towards the micrometer scale can be read directly.
- Autocollimators are fairly accurate and can read up to 0.1 seconds, and can be used for distances of up to 30 meters.
What Is the Autocollimator Method?
- The autocollimator is an optical instrument used to measure small angular differences.
- This device is very sensitive to changes in angles and provides a very accurate measurement of angular difference. It is basically a combination of an infinity telescope and a collider.
- Autocollators are mainly used to align optical components and measure the optical and mechanical deflection.
Principle of Autocollimator:
- The two main principles used in autocollimators are (a) projection and refraction of a beam parallel to light by a lens (b) the change in the direction of the reflected angle on the plane is accompanied by a change in the angle of reflection of the surface.
- To understand this, let us imagine a convergent lens with a point source of light o on our theory, as shown in fig.; a. When a beam of light strikes a flat reflective surface, one part of the beams is absorbed, and the other part is reflected back.
- If the angle of incidence is zero, that is, incident rays fall perpendicular to the reflective surface, then the reflected rays reproduce the original path.
- When the reflective plane is tilted at fixed angles, the total angle through which the light is deflected is twice the angle through which the mirror is tilted.
- Thus, alternatively, if the incident rays are not at right angles to the reflective surface, they can be brought to the focal plane of the light sources, reflecting the reflecting plane at half the angle of reflection as shown in fig. b.
- Now, from the diagram, oo’ = 2θ × f = x, where f is the focal length of the lens.
- Thus, by measurings, the linear distance x, the inclination of the reflecting surface θ can be determined. The position of the final image does not depend upon the distance of the reflector from the lens.
- If, however, the reflector is moved too long, the reflected ray will then completely miss the lens, and no image will be formed.
Parts of Autocollimator:
Six parts of the autocollimator:-
- Light Source.
- Reflecting Surface.
- Diverging Lens.
- Beam Splitter.
- Target Graticule.
- Micrometer microscope
#1. Light Source-
the light source is used to generates the light rays so that it reaches the reflectors.
#2. Reflecting Surface-
it is the surfaces that act as a workpiece for the autocollimators. Using the autocollimator, the angles of tilt of this reflecting surface is to be measured. It reflects the parallels light rays coming through an objective lens.
#3. Diverging Lens-
the diverging lens, which is also knowns as the objective lens, is used to parallelize the light rays coming through the beams splitter so that parallels light rays reach the reflectors.
#4. Beam Splitter-
the beams splitter is used to split the light rays and direct them towards the objective lens.
#5. Target Graticule-
The light rays, after getting reflected, reached this target graticule, and the distance between the incident & reflected ray is traces in this target’s graticules.
#6. Micrometer Microscope-
it is used to see points of incidents and reflected rays in the target graticules clearly and measure the distance between them.
Working of Autocollimator:
- In autocollimators, the reflective surface is the surface whose inclination is to be measured using this instrument. the distance between the source of light & the reflected ray in the focal plane is measured using a micrometer microscope.
- First, the light source is illuminated, & the rays of the light line are extracted from the intersection points of the cross line target gratitude, which is placed in the focal plane of the objective lens.
- After that, the beam of the beam reaches the beam splitter, and the v ray beam is the gate that will direct the light rays towards the objective lens.
- The objective lens will parallel the light rays, & the light rays will move towards the reflectors.
Now there can be two cases:
Case 1: The reflector is perpendicular to the ray of light.
- When the reflected light rays reach the reflector, and the reflector is perpendicular to the light rays, the light rays will reflect back to their original path.
- These lights rays are then brought to focus in the plane of the target’s graticule, coincident with the intersection of the cross lines of the grillage.
- Some part of the reflected light passes directly through the beam splitter, and therefore the image of the return of the target crossline is therefore visible through the eyepiece, thus acting as a telescope centered at infinity.
Case 2: The reflector is tilted at some angle.
- If the reflector is tilted at an angle, the reflected light rays from the reflector double the tilt angle. After reflection, the light rays are focused in the plane of the target granule but linearly displaced from the intersection of the crossline by a distance of 2 * (angle of inclination) (focal length of the objective lens).
- The linear displacements of the graticule image are measured by an eyepiece graticule and a micrometer microscope or an electronic detector system that uses a visual or digital autocollimator.
- Most autocollimators are calibrated so that the distance measured in the angle of inclination does not need to be changed. It converts to the autocollimator itself, and the tilt angle can be read directly into the autocollimator.
- The factors responsible for the autocollimator’s basic sensitivity and angular measurement range are the focal length and effective aperture.
Types of Autocollimator:
Two types of autocollimators:-
- Visual Autocollimator.
- Digital Autocollimator.
#1. Visual Autocollimator-
- In the visual autocollimator, the angle of inclination of the reflecting surface is measured by observing graduation through an eyepiece.
- As the focal length of the visual autocollimators increases, the angular resolution increases, & the field of view decreases.
#2. Digital Autocollimator-
- In the digital autocollimator, micrometer adjustment is provided for the setting, but the coincidence of setting the static and target image is detected photo-electrically.
- This autocollimator is used in the lab. It has very high precision, provides real-time measurement, and is very user friendly.
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Applications of Autocollimator:
- Direct testing of machine tool slides.
- Measuring very small angles.
- Checking for similarities.
- Checking the base of the column.
- Checking the flatness of bed plates and surface tables.
- Measuring very small displacement.
- Checking for small linear displacement.
Advantages of Autocollimator:
- It has very high accuracy.
- It can measure a wide range of angles.
- It is very easy to install and operate.
- Calibration surpassing international standards.
- It can be used to visually or electronically view the result, i.e., on the computer screen.
- A wide range of available accessories and levels.
Disadvantages of Autocollimator:
- Maintenance is required regularly.
- It is time-consuming.
- It requires sample cutting and processing for tracing by the detector.