Calibration and Certification
– Nicolas Marmet, Senior Systems Engineer, Shapegrabber
Most devices used to make measurements require calibration and can be certified to a certain standard. 3D scanners are no exception. Calibration ensures that the scanner meets specifications and is operating optimally, while certification provides proof that the scans meet specifications. In many cases, scanner certification may be required to meet ISO requirements, or may be required by certain customers. The following information is important to consider when purchasing or maintaining an industrial 3D scanner.
Calibration is the process by which a measuring device is compared to an independent reference of known value, usually one which is more accurate and/or authoritative, and is then adjusted to best match the reference.
A 3D scanner is composed of multiple elements, and regular calibration of 3D scanners is necessary to ensure their accuracy. The calibration and certification of a 3D system typically involves three elements:
- Optical Subsystem calibration
- System calibration
- System certification
Optical subsystem calibration is necessary to ensure that the raw data generates accurate coordinates. For example, in a Shapegrabber scanner, the scanhead (composed of a camera-laser assembly) will generate accurate x-z coordinates. Optical subsystem calibration is typically performed in an optics calibration lab using specialized equipment, procedures and qualified technical staff, who will perform such tasks as:
- Verify the subsystem geometry and alignment, and adjust if necessary.
- Take a large number of measurements and generate compensations for ever present but subtle optical and geometric imperfections.
- Verify the newly calibrated subsystem with certified and NIST-traceable objects, such as gauge blocks and balls bars, and create a certificate of calibration with accuracy results.
Most ISO 9000 certified organizations require a periodic recertification of measuring devices. Always find out what calibration and certification services or capabilities the 3D scanner vendor offers.
Many 3D scanners include an optical subsystem, as described above, attached to some type of motion subsystem, which will move or rotate the optical system while performing scans. System calibration is necessary to properly place the reference frame of the optical subsystem into the reference frame of the motion subsystem. The motion platform may also include, for example, an index platter, and its location and orientation can also be calibrated.
System calibration is usually performed by the user with a calibration object provided by the scanner manufacturer for this purpose. With a Shapegrabber scanner, for example, a calibration plate and a calibration software wizard are used to periodically perform the system periodically – the process takes about 15 minutes. Be sure to find out what process and tools your 3D scanner vendor offers.
Certification is the process by which a calibrated measuring device is compared to an official reference and a statement regarding the performance of the device is issued. Traditional measuring tools such as coordinate measurement machines (CMM) have a certification process and a well-established industry of providers and standards are available to do it.
For 3D scanners, however, industry standards are still being developed and it is left to each scanner manufacturer, or sometimes the actual user, to determine the appropriate certification process. For example, Shapegrabber has partnered with an independent, third-party, ISO 17025-certified lab to provide NIST-traceable certifications to their customers.
In order to certify a 3D scanner for specific accuracies, it is not enough to know only the optical subsystem accuracy, since the motion subsystem also affects accuracy. To certify the system, a series of measurements using certified objects must be taken with the complete scanner system, and the resulting accuracy is calculated.
In a typical field calibration of a 3D scanner, a technician will travel to the customer site and will perform tasks such as:
- Perform preventative maintenance of the structure, electronics components, motors and linear motion subsystems.
- Make a series of measurements and perform adjustments to optimize the accuracy. This will optimize linearity, orthogonality and scale.
- A second series of measurements is then taken to produce a certificate.
Annual system certification is an excellent means of maximizing your 3D scanner investment and its performance. Before making a purchase decision, find out what the vendor’s certification capabilities, process, and service offerings include.