It is a special form of telescope that is used to measure small angles with a high degree of resolution. It is used for various applications such as precision alignment, verification of angle standards, and detection of angular movement, among others. It projects a beam of collimated light onto a reflector, which is deflected by a small angle about the vertical plane. The light reflected is magnified and focused on to an eyepiece or a photo detector. The deflection between the beam and the reflected beam is a measure of the angular tilt of the reflector. Figure 1 illustrates the working principle of an autocollimator.
The reticle is an illuminated target with a cross-hair pattern, which is positioned in the focal plane of an objective lens. A plane mirror perpendicular to the optical axis serves the purpose of reflecting an image of the pattern back on to the observation point. A viewing system is required to observe the relative position of the image of the cross-wires. This is done in most of the autocollimators by means of a simple eyepiece. If rotation of the plane reflector by an angle q results in the displacement of the image by an amount d, then, d = 2fq, where f is the focal length of the objective lens.
It is clear from this relationship that the sensitivity of an autocollimator depends on the focal length of the objective lens. The longer the focal length, the larger the linear displacement for a given tilt of the plane reflector. However, the maximum reflector tilt that can be accommodated is consequently reduced. Therefore, there is a trade-off between sensitivity and measuring range. The instrument is so sensitive that air currents between the optical path and the target mirror can cause fluctuations in the readings obtained. This effect is more severe when the distance between the two increases. Therefore, an autocollimator is housed inside a sheet-metal or a PVC plastic casing to ensure that air currents do not hamper measurement accuracy.
Autocollimators may be classified into three types:
1. Visual or conventional autocollimator
2. Digital autocollimator
3. Laser autocollimator.
In this type of autocollimator, the displacement of the reflected image is determined visually. A pinhole light source is used, whose reflected image is observed by the operator through an eyepiece. Visual collimators are typically focused at infinity, making them useful for both short and long-distance measurements. The plane reflector is one of the vital parts of an autocollimator, because a mirror that is not flat will defocus the return image, resulting in poor definition of the image. High-quality mirrors with a flatness tolerance of 1 μm per 100 mm are used. Most visual collimators have a resolution of 3–5″ over a distance of 1.5 m.
Fig.2 Visual Autocollimator
The following are some of the typical applications of visual autocollimators:
1. Determination of angular measurements up to 3″
2. Determination of straightness of machine guide ways
3. Determination of parallelism of slide movements with respect to guide ways
4. Flatness estimation of machine tables, surface plates, etc.
5. Verification of right angle prisms for angular errors
6. Angle comparisons of reflecting surfaces
A digital autocollimator uses an electronic photo detector to detect the reflected light beam. A major advantage of this type of collimator is that it uses digital signal processing technology to detect and process the reflected beam. This enables the filtering out of stray scattered light, which sharpens the quality of the image? The illuminated target reticle slit is imaged back in its own plane through the objective lens and reflecting mirror. It is then re-imaged onto a vibrating slit by means of a relay lens. A photocell positioned behind the vibrating slit generates an output, which captures both the magnitude and the direction of rotation of the mirror from a central null position. These instruments have a resolution of up to 0.01 arc-second and a linearity of 0.1%. Since the output is digital in nature, it can be transferred to a data acquisition system, thereby facilitating storage and further processing of data. Another major advantage is that it can also measure angles of dynamic systems to a high degree of resolution, thanks to high sampling rates of digital electronic systems.
Fig.3 Digital Autocollimator
The following are some of the applications of a digital autocollimator:
1. Angular measurement of static as well as dynamic systems
2. Alignment and monitoring of robotic axes
3. Verification of angular errors of rotary tables, indexing heads, and platforms of machine parts
4. Remote monitoring of alignment of large mechanical systems
Laser autocollimators represent the future of precision angle measurement in the industry. Superior intensity of the laser beam makes it ideal for the measurement of angles of very small objects (1 mm in diameter) as well as for long measuring ranges that extend to 15 m or more.
Fig.4 Laser Autocollimator
Another marked advantage is that a laser autocollimator can be used for the measurement of non-mirror-quality surfaces. In addition, the high intensity of the laser beam creates ultra-low noise measurements, thereby increasing the accuracy of measurement.
Fig.5 Laser Autocollimator
Figure 4 illustrates the construction details of a laser collimator. All the components of a laser collimator are housed in a precisely machined barrel. The line of sight, in this case, laser beam, is precisely cantered, thanks to the holding and supporting fixture. A laser beam is produced by a continuous wave plasma tube. It is coherent and has a diameter of approximately 8–10 mm. The optical axis is exactly in line with the mechanical axis of the barrel. The rear lens along with the spatial filter ensures a sharp beam. The laser beam is aimed at the target. The instrument barrel, in addition to the laser emitter, contains a beam splitter and an array of photoelectric sensors. These are arranged in such a manner that the sensors do not receive laser emissions but receive only the return beam from the mirror.
Unlike in the visual autocollimator, in a laser collimator, the internal target is a bi-cell array of sensors. Two sensors are provided to measure displacement in each axis. The sensor output is converted to angular displacements by a logic circuit. Most laser collimators have a resolution of ±3 arc-seconds.