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JJF 1951-2021 PDF in English


JJF 1951-2021 (JJF1951-2021) PDF English
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JJF 1951-2021: PDF in English

JJF 1951-2021
JJF
NATIONAL METROLOGY TECHNICAL SPECIFICATION
OF THE PEOPLE’S REPUBLIC OF CHINA
Calibration Specification for Optical 3D Measuring Systems
Based on Structured Light Scanning
ISSUED ON: DECEMBER 28, 2021
IMPLEMENTED ON: JUNE 28, 2022
Issued by: State Administration for Market Regulation
Table of Contents
Introduction ... 5
1 Scope ... 6
2 Terms and definitions ... 6
3 Overview ... 7
4 Metrological characteristics ... 8
5 Calibration conditions ... 9
5.1 Operation modes and environmental conditions ... 9
5.2 Calibration software ... 9
5.3 Etalons for calibration ... 9
6 Calibration items and methods ... 10
6.1 spherical form probe error PF, size probe error PS ... 10
6.2 Flat form probe error F ... 12
6.3 Sphere-spacing error, SD ... 13
7 Processing of calibration results ... 14
8 Recalibration interval ... 14
Annex A Examples for assessment of sphere-spacing error uncertainty ... 15
Annex B Format for inside page of calibration certificate ... 18
Calibration Specification for Optical 3D Measuring Systems
Based on Structured Light Scanning
1 Scope
This calibration specification applies to the calibration of optical 3D measuring systems
based on structured light scanning (hereinafter referred to as structured light measuring
systems).
2 Terms and definitions
The following terms and definitions apply to this Specification.
2.1 point cloud [data]
A set of spatial coordinate points that are obtained by measurement to characterize
contour features and are associated with each other.
2.2 ball bar, dumb bell
An etalon consisting of two spherical targets of the same diameter connected by a rigid
structure.
2.3 ball plate
A standard measuring tool consisting of a series of standard balls with different
diameters and a fixed bottom plate.
2.4 structured light
A beam of light with a defined pattern projected onto the profiled surface to be measured.
2.5 spherical form probe error; PF
The range of statistical variation in the radial distance between the measured point and
the fitted sphere.
2.6 size probe error; PS
The difference between the standard ball diameter obtained by point cloud fitting and
the reference value.
2.7 flat form probe error; F
In the vertical direction of the point cloud fitting plane, the maximum distance between
all points in the point cloud.
2.8 sphere-spacing error; SD
The difference between the measured value and the reference value of the center
distance between two balls.
3 Overview
The structured light measuring system is a non-contact measurement device. By
projecting the structured light onto the surface of the measured object, and by collecting
the point cloud of the structured light pattern on the surface of the measured object, the
surface contour features of the measured object are calculated and obtained. The
structured light measuring system is mainly composed of a camera (including a lens
group), a structured light projection device, a calibration board, and measurement
software. The composition of a typical structured light measuring system is shown in
Figure 1.
Structured light measuring systems are divided into single-view system and multi-view
system. A single-view system refers to a structured light measuring system that does
not change the relative position of the structured light measuring system and the
measured object during measurement. The multi-view system refers to structured light
measurement in which point clouds are collected from different directions of the
measured object by changing the relative position of the structured light measuring
system and the measured object during measurement, and all point clouds are
transformed into a unified coordinate system for data processing system. The multi-
view system can be composed of multiple structured light measurement subsystems
installed in different directions of the measured object. It can also be constructed by
moving the single-view system to different directions of the measured object.
5 Calibration conditions
5.1 Operation modes and environmental conditions
Before calibration, the operation mode needs to be set, including the type and brightness
of lighting, the measurement range, the type, quantity and distribution of sensors used
in the system.
Environmental conditions, including environmental vibration, background light,
ambient temperature and its uniformity, rate of change, shall be considered in the
uncertainty assessment. At the same time, there shall be no other environmental factors
affecting the measurement.
5.2 Calibration software
1) The matching (data acquisition and data processing) software of the equipment
shall be used in the calibration process.
2) Set the point spacing, shutter time, sparse point cloud parameters, rejection rate,
and fitting algorithm for image acquisition and processing.
NOTES:
1. When sparse point cloud is required, it shall be carried out according to the instruction manual.
If these parameters are not specified by the manufacturer, no sparse point cloud is performed.
2. The rejection rate is set at 0.3%.
3. Unless otherwise specified by the manufacturer, the fitting algorithm is recommended to use the
least squares method.
5.3 Etalons for calibration
Etalons for calibration shall be made of ceramic, steel, aluminum or other rigid material.
It shall have a diffuse surface (non-volumetric scattering). The etalons used to calibrate
The sphere-spacing error SD shall be measured at 7 different positions in the entire
measurement range. It is recommended to arrange and measure the etalon as shown in
Figure 5.
For all measurement positions, use the fixed radius fitting method to fit all the center
positions of the sphere. Each view takes a center coordinate in each ball bar.
When calculating the sphere-spacing of the ball bar at each position, the two ball center
coordinates of the ball bar at each position shall be obtained from the measurement
results of different viewing angles.
See 6.3.1 for the calculation method of the sphere-spacing error.
7 Processing of calibration results
Calibration certificate is issued to the calibrated structured light measuring system. The
calibration certificate shall meet the requirements of 5.12 in JJF 1071-2010. Calibration
results and their uncertainties list the calibration items and data. Indicate necessary
information such as point spacing, shutter time, sparse point cloud parameters,
proportion of deleted points, fitting algorithm, manufacturer and version number of the
software used.
8 Recalibration interval
The user decides the recalibration interval according to the actual usage. It is
recommended that the interval between recalibration shall not exceed 1 year.
Where,
u(Lai) - the standard uncertainty component introduced by measurement repeatability;
u(Lr) - the standard uncertainty component introduced by etalon.
A.4 Standard uncertainty components
A.4.1 Standard uncertainty component u(Lr) introduced by etalon
A.4.1.1 Standard uncertainty component introduced by ball bar reference value
The measurement uncertainty of the reference value of the ball bar is 5.0μm, and the
inclusion factor k=2, then
A.4.1.2 Standard uncertainty component introduced by ball bar temperature
variation
The coefficient of linear expansion of the ball bar is (8±1)×10-6℃-1. The length is
99.984mm. The temperature measurement error is better than ±0.5℃. According to
uniform distribution, then
A.4.1.3 Standard uncertainty component introduced by measurement error of
linear expansion coefficient of ball bar
The coefficient of linear expansion of the ball bar is (8±1)×10-6℃-1. Obey uniform
distribution in the half-width interval 1×10-6℃-1. The ambient temperature of the
laboratory is estimated by an average deviation of 5°C, then:
A.5 Composite standard uncertainty
See Table A.1 for a list of standard uncertainties.
......
Source: Above contents are excerpted from the PDF -- translated/reviewed by: www.chinesestandard.net / Wayne Zheng et al.