Feature Inspection & measurement Choosing 3D measuring technology


Any company designing or manufacturing products with complex geometries will at some stage need to measure prototypes or finished products to ensure they meet design parameters. Ralph Weir, CEO of the 3D optical inspection specialist Phase Vision, discusses the three-dimensional measurement technologies available


T


he range of equipment available comprises three main system types – point-based, stripe-based and area-based, all generating results compatible with the latest CAD software. The purchasing decision depends on factors including measure- ment speed, accuracy needed, size of objects being measured, and measure- ment conditions. For example, many items expand as ambient temperature increases, meaning measurements taken over a long period in inconsis- tent temperature may not be reliable.


Point-based systems Traditionally, co-ordinate measuring machines (CMMs) have a granite base where the object is placed, but this restricts their scope, as no item over- hanging the base can be accurately measured. Meanwhile, their lack of portability means objects must always be brought to them, irrespective of size, weight and fragility. They must also operate in a temperature-con- trolled environment to be accurate. Touch probe measurements can be taken manually using a measurement arm, or with a CMM operated manu- ally or programmed to move automati- cally. Laser trackers typically consist of a spherical mounted retroreflector, containing a complex arrangement of mirrors, in front of a tracker. Point-based systems are accurate, but measure few points, making them less valuable when a detailed under- standing of surface characteristics is required. However, they come into their own where knowledge of the exact position of a few points in rela-


tion to each other is required. Point- based system accuracy is also high- lighted when measuring polished or reflective surfaces, like high-specifica- tion lenses. They also give an excellent idea on the nature of holes, where rela- tively few measurements can provide detail on diameter, and only a few more start to determine ‘roundness’. Where touch probe systems are not ideal is in measuring items whose sur- faces may be affected by contact with the probe. Extended high humidity levels and air currents can impact on laser tracker performance.


Stripe-based systems


Stripe-based systems, such as laser scanners, can be handheld or mounted on an arm or CMM. They usually mea- sure ‘stripes’ up to 100mm, taking thousands of measurements to create point clouds which must then be ‘patched’ together to create a complete representation. ‘Patching’ can be time- consuming unless a high-specification system is used which ‘patches’ auto- matically – but the patching process still introduces errors. This, and their small stripe length, means they are not best suited to measuring large areas. Where they offer advantages is mea- suring where access is restricted, such as car footwells, and longer items such as large turbine blades, where an arm cannot be used as it is not long enough to scan the item without being moved. Laser scanner accuracy, however, relies on operator expertise. They are normally designed to operate at a spe- cific distance from the object, and any deviations can impact on results.


Stripe-based systems measure ‘stripes’ up to 100mm, taking thousands of measurements to create point clouds


Ralph Weir


Materials which absorb or can be penetrated by light, such as casting wax and Rohacell, do not lend well to laser scanning. Effectively the laser goes through the surface, measuring points underneath. Meanwhile, incon- sistencies in laser focus, laser detune and laser speckle mitigate against fast scanning of larger areas.


Area-based scanning


A co-ordinate measuring machine


Phase Vision T: 01509 223632 phasevision.com Enter 304


Touch screen vision sensor has Ethernet connectivity B


anner has introduced the next generation of its iVu touch screen vision sensor family. The iVu Plus range, including the integrated touch screen TG and BCR models, as well as remote versions of both, have been designed to be an


affordable, flexible and user friendly solution for any number of industries and applications. New features including Ethernet connectivity which enables iVu Plus sensors to share inspection data directly with PLCs, PCs or other factory devices, make communications and control easier on the plant floor. The iVu Plus is also capable of storing 30 inspections, accommodating rapid product changeover, while a new Sort sensor function facilitates sorting up to 10 patterns in a single inspection. The remote models provide a solution for applications where the sensor location is in inaccessible, wet or dirty environments. Turck Banner


T: 01268 578888 Factory Equipment SEPTEMBER 2011 www.turckbanner.co.uk Enter 305


Area-based measurement systems such as white light scanners offer advantages over laser scanners in terms of the area they measure – sev- eral metres across if required. A point cloud is generated but with less reliance on operator skill - while the scanned area size reduces the need for ‘patching’. These systems pose issues where the object’s shape cannot be relied upon to dictate the ‘patching’ process such as large, smooth surfaces with few features. Here, a rotating table (for small objects) or photogram- metry targets must be employed. Again, white light scanners can experience problems when measuring reflective or light-absorbent surfaces, although they do offer some control here. However, their ability to measure a complete surface within a few sec- onds means they lend themselves ide- ally to high throughput environments and full integration into production lines where any inaccuracies or irregu- larities can be immediately detected and production stopped if required to allow process rectification.


Meanwhile, their robustness and portability makes them suitable for highly demanding environments – even dirty production areas, aircraft hangars and docksides.


11


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45  |  Page 46  |  Page 47  |  Page 48