The Role of Electro Optical Metrology in the Aircraft Industry

Producing and fitting jigs on a new aircraft is a challenging step in aircraft production which becomes increasingly difficult as the plane size increases. The jigs must be set in line and parallel to one another.

The Solution

The alignment of jigs can be achieved using a Micro Alignment Telescope. A reference line of sight is set between the telescope and targets, which are fixed to the jig stations, allowing the jigs to be fixed parallel to one-another. If required, a right angle line of sight can also be provided using optical squares alongside the telescope. This set-up can also be used to sweep out planes at right angles to the reference line of sight.

The line of sight does not necessarily have to correspond to any section of the jig structure. If it is easier the line of sight can be ran parallel to the base’s edge, set horizontal by means of an electronic level or with a stride level mounted to the telescope’s tube. In addition, machine vision systems such as CCD cameras or CCTV can be used to automate the alignment process, deliver considerable time and effort savings. For some applications a laser alignment system could also be used.

Aerospace Engine Applications

Aircraft manufacturers receive aerospace engines from suppliers as modular systems, allowing them to be fitted rapidly and also for aircraft engines to be changed rapidly for servicing. During inspections of the engine it is essential to check the fitting of the engine, specifically the profile of the engine cowlings relative to the datum mounting points.

Any bad fittings in this area could result in interference between the surface of the lower wing and the engine cowling. Conventionally, this surface is difficult to access making these measurements hard to gather however, the Micro Alignment Telescope solves this problem.  

The Micro Alignment Telescope is also a useful tool for engine maintenance. Following an allowed number of flying hours overhaul maintenance must be undertaken. Any distortions, which have occurred during use, must be examined and, where needed and if possible, distorted parts are removed for repairs and re-welding. The Micro Alignment Telescope helps engineers examine distortions as it can be used to check changes in attitude relative to the component data, as well as providing a precision clinometer to help with measurements.

Optical Alignment for Helicopters

Whilst helicopters are not as large as airplanes the technology used to build both is shared. The jigs used to assemble helicopters can be very complicated, requiring accurate manufacturing and calibration.

A common difficult measurement that must be taken is the distance between a flange face to a spacial plane defined by the center point of three bores. When using conventional methods to take this measurement, problems are encountered in terms of both equipment requirements and in handling. The Micro Alignment Telescope, used with a 4 inch offset optical square, provides a simpler and more effective method.

It is possible to test the angle of the helicopter blade with a digital inclinometer. A target can be fixed to a region of the aircraft which can then be used to monitor deflections over time, this can be achieved using a CCD camera attached to the alignment telescope capturing images at set time intervals.

Optical Alignment when Building Satellites

The Micro Alignment Telescope has many application in the space industry from relatively basic tasks, e.g. setting a component as level with a Talyvel leveling system, to more complex challenges, such as setting straight and squaring the assembly jigs and platforms used to launch rockets and satellites.

Alignment of Optical Systems

Optical systems are used in the control systems found in aircraft and satellites. For optical systems to work the components within them must be carefully aligned at high precision to the main optical path, which is a time consuming task.

Using a Micro Alignment Telescope, which possesses useful features such as auto-reflection and zero to infinity focusing, optical components can be set quickly and accurately.

Benefits of Alignment Telescopes

  • Can focus in a straight line from zero to infinity
  • The unique ability to auto-reflect
  • Can set optical and axes parallel in three seconds

For some applications it is possible to use machine vision (CCD or CCTV) to oversee and automate leveling processes. It is also possible to change the telescope’s coating to facilitate detection in the IR region.

Aligning Fighter Aircraft Weapons (Harmonization)

The Problem

For the attack and navigation systems of an aircraft to function effectively they must be correctly aligned to the Longitudinal Fuselage Datums (LFD) of the aircraft. In this instance alignment is called harmonization, and the process is easily carried out using a Micro Allignment Telescope from Taylor Hobson. Harmonization checks must occur any time the aircraft undergoes a change – including disturbance to the display unit, removal of the nose cone, changing the windscreen or modifying the internal or radar systems.

The aircrafts harmonization can be checked using a telescope combined with collimators (for a line of sight) fixed in a sighting frame with an alignment-checking jig. This process is made far easier if machine vision is used to provide digital output and remove human error.

During the building of aircraft the gun pod mounts are aligned along the LFD, and this can be used as a datum. An alignment jig, which includes a telescope, is then fixed to the pods and a sighting board, which includes collimators, is fixed to the front of the aircraft. The inclination between the two is recorded using a clinometer.

The first step in harmonization involves aligning the LFD with the Aircraft Target Board (ATB) to provide a datum, the ATB can then be used to align the Internal Navigation Unit (INU) and Pilots Display Unit (PDU).

The first stage of the harmonization sequence is to align the Longitudinal Fuselage Datum jig (LFD) with the Aircraft Target Board (ATB), to provide an accurate datum. Once this is achieved, the target board is then used to align the Internal Navigational Unit (INU) and Pilots Display Unit (PDU).

Aligning the Aircraft Target Board (ATB)

A Micro Alignment Telescope (focused to infinity) is then fixed in the LFD sighting frame and directed towards the collimator fixed to the ATB. The frame assembly can then be tweaked in azimuth and elevation until the crosslines of the telescope centers to the graticules of the collimator. Any movements that occur after this is recorded and adjusted.

Aligning the Internal Navigational Unit (INU)

The INU has a telescope in its sighting jib, which is then aligned to a collimator on the ATB. Any required realignment is then carried out via re-shimming. A similar method is used to align the FLIR and PDU.

The Electro-Optical Metrology Range

Micro-Alignment Telescope

Used for the checking and setting of:

  • Alignment - e.g., for series of bearings or
  • Parallelism - series of rollers
  • Level/flatness - machine bed foundation
  • Straightness - rails or guideways
  • Squareness- base to a column

When the telescopes optical and mechanical axes are aligned within 3 seconds, an accuracy of 50–70 um at 30 m is possible.

Autocollimators

Used for measurements, e.g.:

  • Straightness - machine tool slides in two axes
  • Angle - indexing the head accuracy
  • Parallelism - slideways
  • Squareness - spindles to slideways

Autocollimators range from visual systems, which are inexpensive, to digital, dual-axis systems which can take measurements in 0.01 seconds, which is equal to 50 nm per meter.

Electronic Levels and Clinometers

Used for the measurements of angles and level measurements:  

  • Squareness –machine columns
  • Level/flatness - granite tables
  • Angle - remote monitoring of movement of structures
  • Straightness & twist - machine slides

Electronic levels and clinometers range from full 360 degree measurement to level measurements in 0.1 second.

This information has been sourced, reviewed and adapted from materials provided by Taylor Hobson.

For more information on this source, please visit Taylor Hobson.

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