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3D Measurement: A Multifaceted Endeavour Featured

  • Thursday, 23 July 2015 08:44

Developments in multisensor 3D metrology systems promise much for businesses who are looking to maximise the potential of their equipment. By Khoo Chin Hock, product line manager at Hexagon Metrology Asia Pacific

Selecting any kind of equipment for a metalworking factory environment is a balancing act, and metrology systems are no different. There is no shortage of technology out there — in fact there is an almost bewildering choice. When researching options to measure a particular part, buyers may be faced with several different inspection methods, all with their own merits and drawbacks, and that is before they have even begun to think about which supplier to go with.

Usually, a quality inspection system will be chosen based fundamentally on what needs to be measured, accounting for the workpiece size, the type of application, the tolerances or accuracy levels required and the throughput speed. But there is always a degree of compromise.

For example, the most accurate way to measure is perhaps not likely to be the quickest, so you might give up a little on tolerance to gain something in speed. Of course, the ideal scenario would be to have the best of everything, and while this is unlikely to happen overnight, metrology manufacturers are now offering increasingly flexible multisensor measurement equipment that has the potential to help ease the decision-making process.

New Sensors

The concept of using more than one probe type on a single Coordinate Measuring Machine (CMM) is not new. In fact, systems combining tactile and non-contact probes have been around since the 1980s. Early versions were not particularly successful as the software and controllers struggled to match the potential of the hardware, diminishing some of the advantages gained by the presence of both probes.

However, progression in these technologies mean that many CMMs today can be fitted with a number of different types of sensor — and it is not unusual for companies to utilise this option by using an automatic probe changer.

Progress does not stop there of course. In recent years, there has been an explosion of new types of sensors for CMMs, and this is where true multisensor systems — CMMs with more than one inspection method physically integrated within the system — come into their own.

Specialised software and machine controllers enable seamless switches between the sensors so that all the available methods can be reconciled within a single inspection program leveraging the relative advantages of each type of sensor collaboratively on a complicated part.

Typically, the multisensor CMM has been heavily associated with 2D vision sensors, but in fact you can now equip this sort of machine with a whole metrology arsenal to give a complete 2D and 3D measurement setup:

    • Touch trigger sensor: Perhaps the most commonly used type of probe, a touch-trigger sensor returns a single measurement point on contact with the part
    • Analogue scanning sensor: Offers a tactile scanning option, returning multiple measurement by moving the probe tip over the surface
    • Vision sensor: Image-processing non-contact probe which uses the pixels in a camera image to measure multiple points simultaneously at high speed
    • Chromatic white light sensor: A non-contact sensor that uses focused white light to return a highly-precise measurement point even on reflective or light-absorbing surfaces
    • Laser point sensor: Laser-based sensor that returns single data points without physical contact with the part
    • Laser line sensor: Returns multiple data points using a laser line sweep across the surface of the part

Measurement For The Future

So what advantage is to be gained by combining sensors to measure a single part? At the most basic level, two or more sensors can be used within one program to inspect different areas of the workpiece.

Low density data can be taken by a touch-trigger probe in areas where simple geometry is sufficient, while higher-density data like that offered by laser scanning can be employed where more detailed knowledge of form is necessary. But in contrast, sometimes the solution to a problem can be more complex.

For example, if a manufacturer wants to take high-density scan data of the contours of a surface with very high accuracy, very quickly and maintain 100 percent inspection, simply combining two inspection methods within the program would not achieve the desired outcome.

Analogue scanning would offer the accuracy but not the speed, while laser scanning would offer the speed but not the accuracy. In this case it might be necessary to operate the sensors in sequence. The laser probe could be used to scan the volume of parts required at the speed required against some specified limits. If the data gathered indicated that there may be a problem, the system could be programmed to automatically switch to the analogue probe for more detailed analysis.

As the sensors, controllers and software have moved on, so has the CMM hardware itself. In the past, the flexibility offered by these sensors has been at the expense of some level of part accessibility — the cluster of sensors making part programming more difficult for the operator and increasing the risk of collision. But now, manufacturers are catching up and really working on technologies to better integrate the multisensor capabilities.

For example, Hexagon Metrology has introduced the Dual-Z concept to its Optiv Performance and Optiv Reference line machines, so that the different sensors can be mounted on completely independent Z-axes. The idea is to minimise the impingement of any sensor that is not in use on the measurement volume of the machine, ensuring optimum part accessibility and allowing the full potential of the different sensors to be realised.

Also introduced recently is the through-the-lens laser sensor, which combines a vision sensor with a laser on the same optical path to enable easy shifting between the measurement methods while also saving space on the machine.

Globally, mature multisensor systems have found a niche in the medical equipment sector, where the non-contact measurement options are particularly attractive to the manufacturers of small, fragile and sterile workpieces.

However, the increasing sensor options have seen their appeal widen and demand from the booming consumer electronics industry has also helped to drive adoption. Part of the reason for this demand is the fact that vision sensors are ideally suited to measuring microelectronic components like printed circuit boards.

However, supporting industries will also face new metrology challenges as supply chains extend — for example the metalworking industry might face demand to produce the micromachined LED moulds that enable the tiny electronic diodes to be cast.

So while adoption rates for multisensor systems in the APAC region have yet to catch up with those of Europe or the US, the predicted growth of electronics manufacturing in over the next few years and the subsequent diversification of other supplier industries look set to accelerate the growth of this multifaceted measurement technique in this part of the world.

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