Probing is the best choice over scanning if you have to inspect a part with a limited number of geometrical parameters.Click image to enlargeby Daniel Brown & Bob cramblitt

New portable 3D scanning and optical CMM technologies allow even novice operators to capture data anywhere

Portability, reliability and simplicity: the three attributes that have historically brought new engineering and manufacturing technologies into the mainstream are working their magic on the quality control and inspection market.

What used to be the province of technicians in lab coats working in hermetically sealed rooms well away from the shop floor is now undergoing an historic democratization. Portable 3D laser scanners and optical CMMs now enable even novice operators to capture QC and inspection data for practically anything, anywhere.

The new generation of portable 3D scanning and optical CMM systems offer five major breakthroughs that are making them the go-to solutions for manufacturers worldwide:

  • Dynamic referencing that makes the scanner or CMM system impervious to involuntary movement, relocation of the part being scanned, and environmental disturbances, such as vibrations on the shop floor.
  • Automatic alignment that eliminates inherent errors in any manual alignment process.
  • Intelligent measurement processes that optimize data and ensure greater accuracy.
  • Easy, onsite calibration that can be done by a novice operator in less than five minutes.
  • Wireless, low-cost and rugged optical targets that reduce set-up complexity while maintaining accuracy.

These five breakthroughs deliver bottom-line business benefits, including faster measurements that speed time-to-market and the ability to ensure quality in every facet of product lifecycle management (PLM).

Making an impact
The need for fast, reliable and portable measurement is growing rapidly in industrial development and production, impacting multiple quality control and inspection applications throughout the product lifecycle.

The biggest impact is being seen in the most common type of quality control: production part inspection. Hand-held optical technologies are enabling companies to integrate quality control and inspection directly into the production process for a part or assembly, saving time and money.

Beyond production part inspection, new portable 3D scanning and optical CMM technologies impact an increasingly wide variety of quality control applications across almost every type of industry, such as first-article inspection, inline inspection, tool and jig inspection and adjustment, mould and die inspection, dynamic measurement, damage assessment, and maintenance, repair and overhaul (MRO).

Assessing options
The technologies currently available for 3D quality control and inspection have strengths and weaknesses.

Despite some vendors’ claims of “all-in-one” solutions, potential buyers of QC and metrology systems should aim to meet 80 per cent of their measurement needs while reducing the pressure on quality control personnel, limiting the need for inflexible equipment such as fixed CMMs, decreasing the time needed for training and implementation, and increasing the number of inspections that can be performed on the shop floor or in the field.

Here’s a look at the options available, how they work, and their advantages and limitations.

Measuring arm
Fixed-probe or touch-trigger probe heads mounted on an arm are positioned by mechanical encoders to capture discrete points on the object.

  • Advantages
  • Versatility: Many different tools, including 3D scanning heads, can be mounted on arms, enabling integration of scanning and probing
  • Good accuracy within its working volume
  • Can capture data in places not
    visible to a laser tracker beam or optical tracker

 

  • Limitations
  • Typically requires a qualified technician to operate
  • Needs to be fixed on a surface and use a physical link (arm) for positioning
  • Susceptible to vibrations, temperature and other environmental factors
  • Lacks flexibility in where it can be used
  • Not efficient for capturing complex surface shapes
  • Extending measurement volume requires leapfrogging, a lengthy, cumbersome operation that
    reduces accuracy

Laser tracker
A stabilized laser beam aims at a prism, which reflects light back in the direction from which it came. A laser head records the distance from the prism and the two angles corresponding to the target direction.

  • Advantages
  • Great precision and good reliability
  • Freedom of movement by removing the physical link between the scanner and the object being scanned
  • Good for large-volume measurement

 

  • Limitations
  • Cost: $80,000 to $250,000
  • Typically requires a qualified technician to operate
  • Sensitivity to obstacles cutting off the light beam
  • Difficult or impossible to reach areas hidden from line of sight
  • Extending the scanning parameters is difficult and can add uncertainty to measurements

Photogrammetry
This option uses triangulation to extract measurements from a series of photos of an object or an environment.

  • Advantages
  • Enables large-scale inspection (1 - 10 m or larger)
  • Great accuracy (from 0.015 mm/m to 0.025 mm/m, depending on the size of what’s being measured)

 

  • Limitations
  • Captures only low-density data (only target points are measured)
  • Only surface points are measured with default targets; a specific accessory is needed for each type of geometric entity measured (two aligned targets for an axis, target pins for hole location, for example)
  • Requires a level of expertise
  • Significant time delay between acquisition and generating results

Fixed-location structured light scanner
This system projects a pattern of light onto a part and processes how the pattern is distorted when light hits the object being scanned. Either an LCD projector or a scanned or diffracted laser beam projects the light pattern. One or more sensors record the projected pattern.

  • Advantages
  • Generates very high-quality data
  • Excellent resolution, capturing even the smallest of features
  • Speed: Can capture millions of points in seconds

 

  • Limitations
  • Typically requires a qualified technician to operate
  • Requires multiple scans in most cases to cover
    all angles of complex parts, lengthening acquisition time
  • Not portable
  • Expensive to acquire, implement and maintain

Portable optical CMM
Optical CMM cameras track reflectors mounted to a hand-held device and the part being scanned to provide dynamic referencing in 3D space.

  • Advantages
  • Complete wireless portability for freedom of movement
  • No rigid setup required; the part and optical tracker can be moved freely during measurement.
  • Works well in unstable conditions (not impacted by vibrations, extreme temperatures or part movement)
  • Advanced systems can use low-cost passive optical targets
  • Part-size range of 1-3 meters (extendable to 10 meters) with any type of material
  • Can be paired with a 3D scanner

 

  • Limitations
  • Difficult or impossible to reach areas hidden from line of sight
  • Some use expensive wired or powered LED targets
  • Requires a minimum stand-off distance between the part and optical tracker

Portable 3D scanner
Portable laser scanners project laser lines on an object and white-light devices project a light and shade pattern on the object. Both types of scanners analyze the resulting deformed projections to extract 3D scan data.

  • Advantages
  • Easy to transport and use
  • Does not require fixed measuring setup during data acquisition
  • Captures complex or free-form surfaces
  • Can combine multiple positioning methods, providing the accuracy of positioning targets with the flexibility of object features and texture positioning
  • Can acquire data from unstable or moving objects
  • Speed: The most advanced can capture half a million points per second and rebuild the 3D triangle mesh in real-time during scanning

 

  • Limitations
  • Requires powder to be applied for capturing parts with extremely reflective or transparent materials
  • Self-positioning is done on a local area, making it possible for errors to stack up as the scanning volume increases
  • Lower resolution than high-end fixed-location scanners

Scanning is ideal when you need to inspect a part with complex shapes.Click image to enlargeProbe, scan or both?
An oft-asked question when assessing available 3D technologies for quality control and inspection is: “do I want to probe or scan”?

3D optical probing and scanning each have their strengths and are extremely complementary.

In general, probing is best when one needs to:

  • Inspect a part that has a limited number of geometrical parameters
  • Measure a part that has no complex surfaces
  • Measure depressions (such as the interior of a cylinder)
  • Take precise measurements of some points to obtain geometric alignment
  • Inspect a shiny part without needing to prepare it with powder
  • Periodically adjust or inspect an assembly or control jig
  • conduct a simple and repetitious inspection performed by an untrained operator
  • Probing will also sharply reduce the amount of data to be processed.

Scanning is ideal when one needs to:

  • Inspect a part with complex shapes
  • Get detailed information, such as that needed for conformity mapping
  • Perform non-contact measurement
  • Measure a complete part or assembly
  • Perform a best-fit alignment requiring many points

For those who need both types of data acquisition, portable optical systems that scan and probe are available. These systems combine the benefit of optics (speed, portability, reduced sensitivity to the measurement environment, and higher measurement volume) while remaining compatible with familiar and proven probing procedures. SMT

Daniel Brown is senior product manager at Creaform and Bob Cramblitt is principal with Cramblitt & Company.

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