Error Analysis of Digital Level Instrument
The digital level is composed of an optical mechanical part and an electronic device, and its error is not only caused by the above two items alone, but also includes the error caused by the combination of the two. Among them, the errors generated by the optomechanical part are well known. It mainly includes a. error of circular level; b. running error of focusing lens; c. error of collimation axis caused by vertical axis tilt; d. compensation error of automatic compensator. The following mainly discusses the errors produced by electronic equipment and the combination of the two.
1 Errors caused by the physical characteristics of the line array detector (CCD)
1.1 The intensity of the light causes the error effect of the image contrast of the barcode ruler
The digital level is measured according to the position and ratio of the barcode image on the detector. The physical characteristics of the CCD determine that it can be used in situations such as too strong or too weak light, uneven illumination on the surface of the barcode scale, strong light flicker at the moment of observation, and external heat flicker. Under any circumstances, the contrast of the scale imaging will be greatly reduced, and it will also cause local distortion, which will cause measurement errors and even make it impossible to read.
1.2 The influence of artificial light
The detector of the digital level uses the infrared light part (the CCD array is most sensitive to infrared light) to receive and detect the barcode image. Therefore, when measuring under artificial light, such as when the infrared light component is weak, it will cause measurement errors and even fail to read. In this regard, Zeiss DINI series digital levels use visible light to receive and detect barcode images, so they are not affected by this.
2 Errors in signal analysis and processing
2.1 The error of the signal analysis method
The digital level uses the correlation method for measurement. According to experience, the correlation (alignment) accuracy between the reference signal (pseudo-random code) stored in the instrument and the measurement signal (pseudo-random code) is about 1% of the symbol width (or code wavelength) . The symbol width of the barcode scale is 2.025mm, so the relative accuracy is about 0.02mm.
2.2 The error of calculating the maximum correlation coefficient
When the digital level is correlated, in the accuracy correlation search area, the measurement signal and the reference signal are correlated with all eight digits, because the amplitudes of the two signals are different, and the maximum correlation coefficient is calculated point by point in the height and distance coordinate system: the maximum correlation position The accuracy depends on the size of the grid, related interpolation calculations, ignoring the impact of missing barcodes.
2.3 Measurement signal processing (image processing) error
The measurement signal processing of the digital level is the key link to obtain high-precision level measurement. The quality of the image of the barcode image on the CCD array and the quality of the processing technology determine the measurement accuracy to a large extent. The main error effects that cause image errors include: (1) errors caused by the physical characteristics of the CCD array; (2) errors caused by barcode information loss caused by the blocked ruler; (3) image resolution errors caused by accurate focusing; (4) Image deformation error caused by scale tilt; (5) Image error caused by external condition change; (6) Error of electronic equipment when measuring signal formation, etc.
Obtaining a stable, clear, moderately contrasted, and complete barcode image is the premise of measurement signal generation and processing, and is the key to measurement. Generally speaking, image processing is done by the built-in software of the level. Its error depends on the advanced nature of software algorithm and technology.
3. The error of TV alignment axis (i angle)
The influence of the error of the TV collimation axis (angle i) on leveling is the same as that of the error of the i angle of the optical level in theory, but the TV collimation axis (i angle) lacks the absolute value of the optical collimation axis (i angle) in the digital level. Calibrate the nature of the horizontal line of sight. However, the influence of TV quasi-axis (i angle) on the accuracy of leveling is certain, and it changes with the change of external conditions. Although it can be weakened by using the same length of front and rear sight distances, and the digital level can also be automatically corrected by the program set in the machine, but the external conditions are changing at any time, measuring the i angle at any time and correcting it will not only affect the work efficiency, but also the correction number cannot be linearly simulated. of.
4 Errors caused by changes in external conditions
The measurement system composed of digital level and barcode ruler works under the constantly changing external conditions. Changes in external conditions will cause errors in various components of the instrument. This effect is often manifested as the comprehensive influence of each component and its combination. The errors caused by the influence of external factors mainly include:
(1) The influence of the change of the collimation axis (i angle);
(2) The influence of atmospheric vertical refraction;
(3) The influence of the vertical displacement of the instrument and scale;
(4) The influence of ground vibration;
(5) The influence of the ground electromagnetic field, etc.
5 Errors of conventional levels overcome by digital levels
(1) There is no reading error, and there is no artificial reading error;
(2) Multi-barcode (can be regarded as multi-division) measurement, weakening the scale division error;
(3) Automatic multiple measurements to weaken the influence of changes in external conditions;
(4) Realize the integration of internal and external industries, and realize automatic recording, checking, processing and storage.
