Laser cross sensor for flexible, large scale machining reduces fixture costs

Citation

(2003), "Laser cross sensor for flexible, large scale machining reduces fixture costs", Sensor Review, Vol. 23 No. 3. https://doi.org/10.1108/sr.2003.08723caf.003

Laser cross sensor for flexible, large scale machining reduces fixture costs

Laser cross sensor for flexible, large scale machining reduces fixture costs

Keywords: Machine vision, Lasers, Sensors

Originally developed by Meta Vision Systems for Boeing's factory in Long Beach, California, to enable feature location and machine guidance in aircraft production, a high speed, non-contact position sensor that is broadly applicable to production of large components throughout manufacturing industry has been launched by Meta (Plate 2).

Employing a pair of laser stripes at right angles to each other, the new Digital MXS sensor is able to identify the position of a feature to within a few microns and use the feedback signal to send offsets to the control driving a robot or machine tool. In this way, the exact location of the feature as well as the orientation of the often-complex surface at that particular point is known. So countersinking or tapping a hole, for example, can be performed accurately and normal to the surface. Where this is not the case, a skewed countersink could compromise the strength of a subsequent riveting operation, while if tapping is not co-linear the tool might break and the component may have to be scrapped.

Plate 2 Meta's new Digital MXS sensor uses cross laser stripes to locate features for CNC machining, eliminating the need for expensive fixturing. It also provides in-process quality control capability

Where components are of relatively modest size, such a system is not needed. The part is fixtured in a known position on the table of the machine tool and the program controlling its axes always presents the tool at the exact position and correct orientation.

However, for big and sometimes flexible components such as that found in the aerospace industry, machine tools are very large and expensive and so too are the fixtures for clamping the parts. There is also a considerable cost associated with storing and periodically calibrating the large number of fixtures needed. Moreover, batch runs are often short, so set-ups can consume a large proportion of production time. It is not surprising, therefore, that manual production methods are still widely used, as they do not require the part to be fixtured accurately – merely supported.

One way to combine such versatility with the productivity benefits of automated CNC machining, and to achieve an agile production environment, is to use a robot equipped with a spindle and fit a Digital MXS sensor or similarly equip a conventional machining centre. This approach eliminates the need for precise location of the part in an expensive fixture; only simple, low cost clamps are needed to hold the workpiece.

With a contact probe there is always a risk of damage due to collision with the component, but this is eliminated with a non-contact sensor. For the same reason, the latter can safely approach the component more quickly than is possible with a touch sensor, eliminating the cycle time penalty of slow-approaches. Absolute reliability of operation is assured, as the Digital MXS includes filters to remove light wavelengths other than those of the laser, so changes in the ambient lighting conditions have no effect on sensor operation.

An additional advantage of using this vision technique is that in-process measurement may be incorporated as a matter of course. When the sensor locates a hole centre, for instance, the data fed back to the machine control include, in addition to its position, information such as hole diameter, size of countersink, angle of the component surface and the height of the sensor (and hence of the tool) from it. The latter information can further speed productivity by over-riding the CNC program to enable faster rapid infeed to the next cut.

Two versions of the Digital MXS sensor are available with 10 and 20 mm field of view, having nominal stand-off distances of 30 and 63.5 mm, respectively. The former is the most accurate, enabling height, centre detection and tool angle measurement to within ±4.3 microns, ±3.6 microns and ±0.3° of arc. Accuracy of hole diameter measurement is ±7.6 microns and that of countersink depth is ±22 microns.

For further information, please contact: Mike Wilson, Managing Director, Meta Vision Systems Ltd, Oakfield House, Oakfield Industrial Estate, Eynsham, Oxfordshire OX29 4TH, UK. Tel: 01865 887900; Fax: 01865 887901; E-mail: sales@metamvs.com; Web site: www.meta-mvs.com