Laser Alignment Theory Essay
Paper type: Essay
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Modern optical metrology uses precise lines and planes in space from which measurements are made. Because this method creates these features using light it has become known as optical tooling. The 35 year old historical development and current technology of laser instrumentation as used in optical tooling is discussed in detail.
This includes the how the measurements are made, applications, and the technology of alignment lasers and position sensing targets developed for sensing position within a laser beam or plane.
The various geometries used to make alignment measurements are discussed in detail. Applications are discussed and the challenges each poses are discussed. The challenge of long range alignment and the effect of the turbulent atmosphere on the measurement process is discussed along with methods of handling the associated errors them. WHAT IS OPTICAL TOOLING? Optical tooling is a means establishing and utilizing a line of sight (LOS) to obtain precise reference lines and reference planes from which accurate measurements are made with position sensitive targets.
1] Formerly the measurements were done by a person interpreting a scale or optical micrometer by looking through an alignment telescope; today the lines and planes are created by a laser; the measurements are digital and require no interpretation. Optical tooling uses the principle that light travels in straight lines to produce precise measurements that cannot be reached by manual or mechanical methods. Level lines can be established over great distances so accurately that every point is exactly perpendicular to the force of gravity.
Plumb lines can be set to a given level. Right angles can be produced quickly and precisely with auxiliary components. In the assembly, maintenance and calibration of industrial equipment or in the alignment of precision systems, one or as many as four basic questions always must be answered: is it straight, is it flat, is it plumb or is it square? A number of techniques have been developed to make these measurements; however, many of them result in inaccuracies so great that proper operation of the equipment involved will be compromised or seriously endangered.
The science of optical metrology and alignment makes it possible to achieve the highest degree of accuracy in answering these four important questions. It is no longer necessary to interpret readings or to make constant adjustments and calculations. In laser alignment applications, direct, precision measurements are made rapidly and consistently. Straightness In aligning several points, a tight wire is often used as a reference line. This technique has numerous drawbacks and introduces inaccuracy. First of all, wire has weight, which causes it to sag; over long distances this sag can become considerable.
In addition, wire vibrates, can bend or kink, and when stretched in the area to be measured, equipment cannot be moved around for fear of disturbing the wire reference line. Even a gentle breeze can cause the wire to move sideways a considerable amount; the aerodynamic drag on a thin wire is huge. In laser alignment, the LOS of is established by a laser beam instead of a tight wire. The invisible LOS reference has no weight, cannot sag, kink, or be disturbed, nor is it a safety hazard. It constitutes a precise, unvarying reference, determining straightness to within thousandths of an inch.
Flatness In order to determine flatness a shop level and a straightedge was employed in the past. However, over large horizontal areas, the shop level must be moved from part to part. Consequently, one can only tell the degree of flatness of each individual surface upon which the level is place. Whether all objects in a large area are flat is still in doubt. Flatness over a considerable area must be assured in the erection of large machinery, surface tables and large machine tools. Conventional bubble levels simply do not offer the degree of precision required. Laser levels”, a termed that has fallen into generic use, offer a way to produce a level datum over a wide area. Laser technology has overcome the many disadvantages of bubble levels and assures levelness to within a few thousandths of an inch over hundreds of feet. This high degree of levelness is accomplished by horizontally sweeping the laser beam manually or via a motor driven rotary stage. This revolving line of laser light becomes a horizontal “plane of sight”, giving a precise horizontal reference datum, sometimes called a waterline. Squareness
Perfect squareness implies that one plane forms a 90° angle with another intersecting plane. When a steel square is used to test for this condition, the results can be very misleading. Such measurements rely upon the trueness of the steel square, which can vary from square to square with time. In addition, steel squares have a definite limit in their physical dimensions and consequently the testing of very large surface becomes inaccurate, slow and cumbersome. Laser alignment overcomes all these disadvantages and offers a quick and precise method for determining squareness.
One method is to use a transparent penta prism in conjunction with a simple alignment laser. This optical element will split the beam from the laser into two parts; one beam passes through the prism undeviated, the other beam is reflected at a perfect 90 degree angle. This will be described in more detail later. Other systems use three independently mounted lasers that are orthogonal to each other. Plumb Classically, a plumb bob is used to establish a single vertical reference line. Of course, as vertical distances increase, the plumb bob becomes cumbersome and inaccurate. It takes a long time for the plumb bob to settle.
Also, it can easily be swayed by vibration, air currents, and other disturbances which are bound to be encountered. In the laser alignment method there are several ways to produce a plumb reference; it can be a plane or a line. To form a plumb line, an alignment laser with autocollimating capability is used with a pool of almost any liquid. Autocollimation senses the angle of an external mirror by reflecting its beam back into the laser head. A position sensor, beamsplitter and lens measure the angle of the reflected beam. When the laser is adjusted such that the internal sensor reads 0 in both axes, then the laser is producing a plumb line.
For example, if a heavy machine tool is being surveyed, the two reference points which determine the LOS should be located off of the machine. If for any reason the machine were to move or deflect all measurements would be in error. The two reference points should be located close enough to be convenient to use and/or out of the way of other people working in the area. Transits and alignment telescopes first made these types of measurements. But the problem with transits and telescopes is that they require a person to interpret a scale placed on the object of interest; and usually a second person is holding the scale against the object.
It is a two person job that takes time and much training to accomplish successfully. It is also subject to errors. This type of alignment measurement, commonly called straightness, is the most basic of all alignment applications. The figure below shows an alignment laser source on the left whose collimated beam is striking a position sensor target on the right. The target can freely slide and make measurements of straightness of the structure to which it is attached. [pic] Another common requirement is to establish another LOS perpendicular or parallel to the original LOS.
To establish a perpendicular a special prism is used: a penta prism. A penta prism has the property that rotation around its axis does not deviate the reflected beam at all; it does not have to be critically mounted. Penta prisms are often called optical squares, an appropriate term. To establish a parallel LOS to an existing LOS typically involves tooling bars if the distance is relatively short, say a meter or less. These bars are made of steel and hold electronic targets at a precise distance from a center. Using two of them with the original LOS establishes a parallel LOS.
If the distance between the two LOS is large, then it can be done using the penta prism twice; the first time to turn the beam 90 degrees, followed by a certain distance, and concluded by turning the beam back 90 degrees. Care must be taken that two LOS are truly parallel; usually using a level reference datum makes the task much easier. The next alignment application involves measuring the alignment error between two different LOS datums; the typical application is to determine the lateral offset and angular error between two shafts.
The shafts essentially define the two LOS’s. The measurement consists of setting up the source on one shaft and parallel to it. The targets are placed on the second shaft and surveyed. Then the shafts are rotated 180 degrees and surveyed again; the difference is twice the shaft offset. If the target is placed at two axial locations and measured for offset, the difference in the offsets divided by twice the axial separation is the angular error in radians. The figure below shows a typical method to measure shaft alignment errors using a laser and target. [pic]
A more sophisticated alignment application is to sweep a laser beam quickly to generate a plane of light. The advantage of this is that many targets can be aligned using one laser source. In simple straightness applications the target location is restricted to the active area of the position sensor. In swept plane alignment, the targets are using sensitive in only one dimension. A typical application to establish a level plane is to put three or more targets at the same (desired) waterline location and adjust the structure the targets are on until all targets read the same.
The targets for swept plane alignment can be static, meaning they require the laser beam to be directed in to them constantly. Usually the laser beam is swept by hand by rotating a knob on the laser source. If the laser plane is moving at high speed, say once a second or faster, then the targets must capture and hold the position of the laser beam as the beam sweeps by. The problem becomes harder to accomplish at longer distance because the beam is on the detector for such a short periods of time. The figure below shows a horizontally swept level laser beam scanning by several targets placed on a machine bed.