Optical 3D measuring systems Imaging systems with point-by-point probing
Preliminary note
Optical 3D measuring systems are used as universal measuring and test equipment. In each case, the user must be sure that the optical 3D measuring system in use complies with the required performance specification. In particular, the maximum permissible measurement error shall not be exceeded. In the long run, this can only be ensured by means of harmonised acceptance criteria, and by checking of the equipment at regular intervals. The responsibility for this is shared by the manufacturer of the measuring equipment on the one hand and the user on the other hand.
Quick and easy methods using reasonably priced artefacts are needed for the acceptance and reverification of optical 3D measuring systems of various designs, degrees of automation, and sizes. This can be achieved by means of length standards and artefacts which can be measured in the same way as typical workpieces.
This Part 1 of VDI/VDE 2634 describes practical acceptance and reverification methods for the evaluation of the accuracy of optical 3D measuring imaging systems with point-by-point probing. The definition of the quality parameter length measurement error is similar to that in ISO 10360-2. Separate testing of the probing error is not required as this effect is considered in the determination of the length measurement error.
Part 2 of VDI/VDE 2634 describes systems for surface probing.
This guideline was drafted by the technical committee ”Optical 3D measurement“ of the VDI/VDE Society for Measurement and Automatic Control (GMA) and by the working group ”Close-range photogrammetry“ of the German Association for Photogrammetry and Remote Sensing“.In the joint committee, representatives of well-known users cooperate with members from universities, who specialise in the field of optical 3D measuring systems.
1 Scope
This guideline applies to mobile, flexible optical 3D measuring systems with one or several imaging probes (such as cameras), whose function relies on triangulation (such as photogrammetry). The equipment can be configured by the user to suit a particular measurement task. The workpiece is measured by tactile or optical probing. In both cases, the quantity to be measured is recorded optically by analysing the images of discrete features of the workpiece, or of auxiliary equipment (measuring adapters). Such features are represented, e.g., in the form of circular reflecting measurement marks or light-emitting diodes(LEDs).
This Part 1 of VDI/VDE 2634 describes methods and artefacts for the testing of optical 3D measuring systems with point-by-point probing.The methods are equally suited for
- the acceptance of optical 3D measuring systems
- the reverification of optical 3D measuring systems (for the purpose of checking of test equipment as specified in DIN ISO 9000 through DIN ISO 9004).
The requirements to be met by the artefacts used for acceptance and reverification are specified and examples of artefacts are described. For the purpose of this guideline, an artefact is a linear, two-dimensional, or three-dimensional arrangement of features to be probed. Examples are optically extractable edge intersections, circles, or spheres and surfaces that can be probed mechanically. Artefacts must be calibrated with respect to their dimensions and shape.
A quality parameter is defined in order to validate assess the accuracy of the measuring systems. The permissible limits for this quality parameter is specified by the manufacturer for the acceptance test of the measuring system, and by the user for the reverification, respectively.
The quality parameter serves to specify optical 3D measuring systems and to compare different measuring systems. For its value being dependent on the operating mode and operating conditions, it is recommended to select and specify particular operating modes and operating conditions for acceptance and reverification in order to ensure comparability of the test results. The same quality parameter cannot per se be used for any arbitraty measurement task.
Where no limitations to the operating modes and operating conditions are specified, the specified limit of the quality parameter must be complied with under all possible operating modes and operating conditions. Exceeding the limit value of the quality parameter is possible, and permissible, where specified limitations are not meetmet.
The term operating modes denotes adjustment and configuration options of the instrument such as
- type and intensity of illumination
- measuring volume
- type, number and arrangement of the optical sensors used
- type and duration of image acquisition and evaluationprocessing
- type, number and form of features to be analysed
The term conditions of operation denotes external influences on the optical 3D measuring system.These include, for example:
- the temperature and its gradient
- humidity
- vibrations (mechanical)
- electromagnetic interference
- environmental lighting conditions
- dust
2 Symbols
Throughout the text of this guideline, the following symbols are used:
\({E}\) characteristic quality parameter for the length measurement error
\({A}\) constant offset part of the characteristic quality parameter for the length measurement error
\({B}\) maximum value of the quality parameter characteristic for the length measurement error
\({K}\) constant
\({L}\) length to be measured
\({l_m}\) measured value of length
\({l_k}\) calibrated value of length
\({\Delta l}\) length measurement error
3 Principle of acceptance test and reverification
Acceptance test and reverification of optical 3D measuring systems rely on measurements of calibrated artefacts. These shall be designed so that their properties have no significant effect on the quality value parameter to be determined. A test is made to verify that measurement errors lie within the limits specified by the manufacturer or the user.
For both acceptance test and reverification, the maximum permissible length measurement error is the quality parameter to be determined. It is described in terms of a length-dependent limit. In order to determine the length measurement error, the spatial distances between the features to be probed on the artefact are measured and then compared to true dimensions. For this purpose the artefact must be calibrated.
For acceptance test and reverification of the optical 3D measuring systems, the artefacts shall be probed in the same manner as for an actual measurement, i.e. using either tactile or optical probing of the features.
4 Acceptance test
Acceptance test, serving to verify the specified accuracy, shall be subject to contractual agreement between equipment manufacturer and user. It is performed at the manufacturer’s or, alternatively, at the user’s following installation.
Prior to acceptance test, the optical 3D measuring system shall be installed and operated in accordance with the particular operating conditions for acceptance purposes. Required warm-up times have to be considered. The environmental conditions shall correspond to the operating conditions of the optical 3D measuring system. Also, ensure that mounting and fixtures of the artefacts are sufficiently stable. Artefacts shall be allowed to acclimatise to the mean temperature of the measuring volume. Where the mean temperature of the artefacts, or of the components of the optical 3D measuring system deviates significantly from the reference temperature as per DIN 102, appropriate temperature corrections shall be applied, if such corrections are also applied during actual use of the equipment.
A report shall be drafted upon completion of the acceptance test, summarising the test results. It is also recommended to record all measured values in the acceptance report.
4.1 Definition of the quality parameter length measurement error
The three-dimensional length measurement error \({{\Delta}l}\) is obtained from the difference between the measured and calibrated distances between two points.
The limit value \({E}\) of the permissible three-dimensional length measurement error is the quality parameter length measurement error. It is specified as a length-dependent quantity
where \({A}\), \({K}\), and \({B}\) are constants, and \({L}\) is the length to be measured (see Figure 1).
The manufacturer’s specification of the maximum permissible length measurement error, \({E}\) , applies to the artefacts described in Section 4.2. It shall be complied with under all permissible operating modes within the measuring volume specified by the manufacturer. A specification of the length measurement error of an optical 3D measuring system is, therefore, only complete when accompanied by a description of the particular operating modes and operating conditions during acceptance. The quality parameter shall always be complied with when operating modes or operating conditions are changed within permissible limits.
4.2 Artefacts
The quality parameter is determined using one-dimensional artefacts which must have features that are suitable for tactile or optical probing. Examples of suitable artefacts are gauge blocks with circular measurement marks and step gauges. Observe that features to be probed optically must have a minimum size depending on the scale of the image.
The actual dimensions of the artefact shall be known to an uncertainty of less than one-fifth of the maxi-
mum permissible length measurement error specified by the manufacturer for the optical 3D measuring system to be tested. A calibration certificate is required for the artefacts to be used. This calibration certificate shall be used to verify the traceability of the artefact used to national standards.
4.3 Procedure
The quality parameter length measurement error, \({E}\), specified by the manufacturer shall be complied with regardless of the arrangement of the artefacts within the measuring volume. This is verified by sample-testing seven different measuring lines. The artefacts shall be placed along these measuring lines.
A measuring volume of 2000 mm × 2000 mm × 1500 mm (length × width × height) is recommended for acceptance testing. Other measuring volumes are permissible. At least five test length shall be tested on each measuring line (see Figure 2). The longest test length on each measuring line shall be at least as long as the shortest side of the measuring volume. The longest length to be tested should be at least two-thirds of the body diagonal through the measuring volume. Where no artefacts of suitable length are available, this length may be represented by two overlapping artefacts.
The longest test sections shall be so arranged on the measuring lines that one end point of any one of these test sections comes to lie in each corner of the measuring volume. Also, there should be at least one parallel measuring line for each side of the measuring volume. Figure 3 shows one possible arrangement of the measuring lines. When determining the measurement results, the test sections shall not be introduced as known quantities.
Where auxiliary equipment (such as measuring adapters) is used for the measurements, these shall be included in the test. Otherwise, auxiliary measuring equipment shall be expressly excluded.
The measuring lines may be recorded one after the other in separate sets of images. The arrangement of the sensors and the manufacturer’s scale definitions shall be kept unchanged during all measurements. Operating modes and operating conditions should be identical. It is recommended to represent the entire measuring volume by an identical arrangement of object points in each case.
4.4 Calculation of results
For a particular measurement, the length measurement error,\({{\Delta} l}\), is obtained from the difference between the measured value, \({l_m}\) (indication of the optical 3D measuring system or computer printout) and the calibrated value, \({l_k}\) , of length as per
Length measurement errors may be represented graphically (see Figure 1).
4.5 Inerpretation of results
The quality parameter will be complied with, if no length measurement error \({{\Delta} l}\) exceeds, the value of the maximum permissible length measurement error \({E}\) in magnitude. In case of excess errors occurring for not more than one measuring line, the measurements of all test lengths of this particular measuring line may be repeated once. No further excess errors shall occur then, or the measuring system has failed acceptance.
5 Reverification
Reverification of optical 3D measuring systems serves to ensure long-term compliance with limits for the length measurement error as specified by the user. By comparing the results of successive reverification measurements, it is possible to analyse trends with respect to changes in instrument characteristics. This allows conclusions to be drawn, regarding both preventive maintenance of the optical 3D measuring system and the reverification interval. Operating modes and operating conditions must be approximately identical for trend analyses.
Prior to reverification, the optical 3D measuring system shall be put in operation as described in the instruction manual. Observe any warm-up time required. The environmental conditions shall correspond to the operating conditions of the optical 3D measuring system. Also, ensure that mounting and fixtures of the artefacts are sufficiently stable. Artefacts shall be allowed to acclimatise to the mean temperature of the measuring volume. Where the mean temperature of the artefacts, or of the optical 3D measuring system deviates significantly from the reference temperature as per DIN 102, appropriate temperature corrections shall be applied, if such corrections are also applied during actual use of the equipment.
5.1 Measurement procedure
The recommended procedure for reverification 3D measuring systems is analogous to that described for their acceptance (see Section 4). However, the quality parameter length measurement error may be specified by the user to satisfy his requirements. The user may reduce the number of measuring lines as well as the number of test sections.
5.2 Evaluation
Evaluation with the aim of determining the quality parameter is performed in the same manner as for acceptance test. The distances between the features to be probed are calculated and compared to the calibrated distances. The magnitudes of the differences between measured and calibrated distances shall not exceed the quality parameter specified by the user. In case of excess errors occurring for not more than one measuring line, the measurements of all test lengths of this particular measuring line may be repeated once.
If the quality parameter is exceeded, the measuring system shall be marked as usable to a limited extent only, and appropriate corrective action shall be taken.
5.3 Reverification interval and documentation
The reverification interval shall be specified individually by the user of an optical 3D measuring system. It is determined by the system components, the required measurement uncertainty, and the environmental conditions at the varying sites where the instrument is used.
Reverification shall be performed according to a specified testing schedule, depending on the stability of the instrument components, and includes visual inspections (checking for damage). A report shall be drafted upon completion of the test, summarising the test results. It is also convenient to record all measured values in the checking report.
Bibliography
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