Transactions on Engineering Sciences vol 16, 1997 WIT Press, ISSN

Transcription

1 Laser computer calibration and quality assurance of certain kinematics and geometrical parameters A. Ryniewicz and J. Gawlik Cracow University of Technology, ALJana Pawla II nr 37, TOWTOfP; fol47vd , ^ox +4< ghryniew@kinga. cyf-kr. edu.pl Abstract The paper presents a computer aided system for certain shape and position guide surface error analysis for quality assurance. Development of new manufacturing techniques of machine parts aims at improving their performance through minimising shape and position errors of kinematics pairs to the technologically justified level. The system described allows the assessment of large-size units, machine tool and robot guides based on accepted criteria. The general version enables the evaluation of errors of straightness and flatness of guiding surfaces, their distance variation, movement velocity, fluctuations and orientation of the travelling unit during its linear motion. The above can be measured with the use of a specially designed measuring system using a laser multi-axis interferometer. 1. Introduction Accuracy of machine tools mainly depends on proper geometry and the possibilities of compensation of sliding assemblies errors. The principal bases in design, manufacture and metrology of machines are flat surfaces. Among many features qualifying those surfaces as guiding elements in sliding assemblies there are geometric tolerances, i.e. dimension, position, shape which have essential significance [1]. It has been obviously clear that the information on a dimension itself is frequently insufficient and has to be implemented with data on features and size of shape errors. Position and shape deviations of guiding units are more and

2 66 Laser Metrology and Machine Performance more commonly used in quality control systems which seek to fulfil operational requirements. The problem is particularly essential for large-size devices and elements. Determination of action mode of particular errors sources demands analysis of systematic and random components of positioning error vector. As a result of many errors of systematic and random characteristics, active in a device, the positioning error vector will be also composed of systematic and random parts. The main share of systematic component is determined by errors caused by improper geometry of travelling units movement resulting mainly from shape and position errors of base and guiding elements. Thus, a system of complex assessment of large-size guiding elements shape and position tolerances has been developed. The system is based on flatness or straightness and parallelnes measurements. The system of computer diagnosis and assessment is particularly applicable in systems of quality control of production lines owing to estimates that are used in the form of synthetic indexes or in graphic form and due to possibility of co-operation with measuring devices through typical computer connections. 2. The main functions of the program The main functions of the program are: automation and communication with the measurement system, control and communication with units or elements, and drives of inspected machine programming data for analysis based on accepted criteria, machine tools or units data correction verification, calibration and quality assurance (guides position correction, calibration of travelling units), + estimation of kinematics parameters 4 estimation of geometrical parameters representation of obtained results. Automation block co-operates with the PC-computer of measurement system or with the measurement system directly through the IEEE interface for automation process control, and setting the task parameters. The block diagram of measurement, calibration and quality assurance system is shown in figure 1. The main parameters of the measurement process are as follows: measurement description, number of measurement sections - measurement lines, adjustment parameters, forming criteria for the obtained results, atmospheric conditions.

3 Laser Metrology and Machine Performance 67 Quality Assurance, 1 Parameters of a Measuring Process: - screen / printer, - measurement description, - number of measurements in a given section, - number of measurements sections, measurements lines. e r I a Parameters of a Calibration Process: - geometrical, - kinematics, Parameters of Quality Assurance: - gemetrical parameters, - kinematics parameters, - positioning parameters Main display read-out: -single, -multiple, - setting time interval between measurement control impulses, - saving to a disc file, - graphical presentation. Figure 1: The block diagram of measurement, calibration and quality assurance system. The computer system described controls any measurement unit. The present version co-operates with the laser interferometer system which performs translational and rotational displacement measurements. The block diagram of the measurement, calibration and quality assurance system is shown in fig. 1. The program controls the main functions and parameters of optical units of the measurement system. When the program co-operates with the laser interferometer or special optical system for large-size machines, measurement block programs controls the main functions of optical measurement units, i.e.: fixing of optical measurement elements on the inspected machine, units of measurement system adjustment,

4 68 Laser Metrology and Machine Performance measuring force control, contact with the measured surface. The main functions of measurement block program are shown in figure 2. Measuring force Fixing of optical measurement elements I Ring units for measurement system adjustement J 1 t L System for measuring unit loghudinal and trasverse location control Contect with a measured surface Figure 2: The main functions of measurement unit. 3. The programme for analysis and quality assurance Below is presented the programme of analysis of test results obtained from measurements. In the first stage, after data transfer from measuring device they are verified in the data storage programme DANWE. Its diagram is presented in figure 3. Then the final data file is made and introduced into main part of program ANALIZA which block diagram is presented in figure 4. ] j From Laser Interferonicter \ ^^ j k Print out of Subsequent Pages with (:heck Points Paranmeters Points coordinates» Inclinationi Asperitiesheight Data Input 4 From Disc i j Data Review 4 r^o+o r*orrection i r Final Data File "*"" Registration i f FromKeyboard ^Automatic: Division of Data for Print out - Pages and Selection of a Page! parameters Figure 3: Block diagram of DANWE programme.

5 Laser Metrology and Machine Performance 69 Measurement data input Travelling units parameters block Surface parameters block Kinematics errors functions Obtained criterions of analysis Print out block: - quality assurance - correction parameters. Figure 4: Diagram of ANALIZA program. The program is composed of three principal options [4]: option I - forms data into relevant files and distributes for two groups: coordinates of measuring points and data determining mutual position of guides, option II - related to analysis of the guide surface - its shape deviations - in the basic version of the programme it concerns straightness deviation in some sections and position in the adopted reference system. At the same time the graphic surface image is presented: 3D or contour line. In this part is also the block of positioning accuracy that is used for analysis of selected positioning parameters of travelling units ( in the case of positioning errors control )and the block for estimation the kinematics parameters. option III - calibration and quality assurance designed for guides or travelling units systems assessment. After the guide describing parameters were introduced the characteristics of its assessment are obtained from the point of view of appeared shape and position deviations in the form of characteristics: Global Out-of Parallelness Factor

6 70 Laser Metrology and Machine Performance Guide Errors Function Harmonic Characteristics 3.1 Guides assessment Assessment of guides is performed on the base of obtained characteristics: Global Out-of Parallelness Factor Guide Errors Function Harmonic Characteristics Global Out-of Parallelness Factor -GOF Shape and position deviations are determined e.g. by flatness deviations, parallelnes or perpendicular deviations, or otherwise, by complex deviations determined as a difference of maximum and minimum distance between real surface points and the reference plane in boundaries of a partial area. When establishing new criteria it is possible to take an advantage of evaluated in this block shape and position errors and chosen movement parameters. In that case the criteria must be given in the form of function. The subprogram, in its basic version, calculates correction parameters, which can be used (e.g. in case of shape and position errors) for achieving guiding elements position so that existing parallelness and orthogonality errors of the guide surfaces are minimised, the flatness and straightness errors of the guides being measured. Upon made analysis one can conclude that in order to determine complex position and shape error of guiding elements, it is necessary to take into consideration both flatness or straightens errors and the variation of the actual distance between inspected guide surfaces. The developed method is suitable for the assessment and correction of the robots and processing machines, especially the great size one, by means of measuring variation of the distance between inspected surfaces and their straightens error. These parameters are determined simultaneously in chosen guide segments and the errors of parameters are calculated with reference to assumed base lines or planes. The global out-of-parallelness factor (GOF) was defined in order to create an overall qualitative and quantitative description of the complex error of the guides. GOF is the function describing the actual measured distances between guides and their straightness errors, in comparison with parallelness errors, have much stronger influence on static and dynamic positioning of travelling units (there is a possibility of defining GOF replacing straightness with parallelness). GOF is defined as a vector : where : GOF = (1)

7 Laser Metrology and Machine Performance 71 B = ^M (2) FA - surface distances function (measured values) FB - parallelness or straightness error function (in assumed area) AQ, BQ -base distance and flatness values. F - function describes parallelness errors of the guide surface calculated with reference to assumed regression planes (mean or adjacent). A and B functions give the values of position correction parameters for the chosen set of points Guide Errors Function and Harmonic Characteristics According to the adopted procedure of GOF the deviations of parallelness are determined commonly with straightness or flatness for the area tested. After a prescribed processing of measurements a discrete results set is obtained in the form of co-ordinates set (x,y,z) of particular points describing the surface under control. In the case of empirical data the described parallelnes function of a surface is given by a series of its values for selected co-ordinates: zv=f(*,,yj) (3) where : i= 1...M j = l...n Thus the guide errors function is to be formulated. The function domain is the range (from 0 to L) of the total length of a guide in a measuring plane. Practical use of numerical values listed in a table is difficult thus it is graphically represented as guides straightness or parallelness characteristics in the form of the guide errors spectrum function for different size of base element in the form of the Guide Errors Function. Supported by above function the guide accuracy error may be determined ace. to [2]: BD^ = \maxd, + min^_j (4) where : d - distance between guides or representative planes Extreme values for particular operational ranges of guides also may be presented e.g. in the form of segment factors: where : n - number of measurements in a segment. This function, when a level of discretization is adopted represents a signal delivering information on guides errors.

8 72 Laser Metrology and Machine Performance The function signal has been processed with application of harmonic analysis with use of Fourier series [3]. Base parameters for ten harmonic components have been determined. 3.2 Guides position correction The part of the programme designed for guides position correction enables the determination correction parameters that define position change of controlled guides as a common single guiding assembly (unit) for each guide separately in relation to adopted reference element. The programme in its basic version has been prepared for position correction of flat guides in relation to reference plane representing the guides as a whole or planes that represent its defined segments according to accepted criterion's. In the first version the criterion of minimisation of guides surfaces parallelnes deviation (with existing shape errors) has been accepted. Correction parameters define co-ordinates of check points for the whole guides [5] or in determined control segments. 4. Analysis of errors connected with a measured unit Mechanical design of CNC machine tools [6] (especially machining centres) and measuring machines are fairly similar. In both cases there are slideways enabling displacements of machine units along axis (usually perpendicular to each other). It is therefore obvious that description of errors caused by not ideal displacements of moving units will be also similar. So will be applied techniques and measurement methods aiming to identify these errors. In both cases accuracy of displacements of moving units can be determined taking advantage of position errors 6p in measuring points. The position error 5p has both "spatial" and "vector" aspect although displacement is carried out theoretically only along one axis of a co-ordinate system. Position error is presented in figure 5. Position error is mainly caused by manufacturing errors of slideways and units which move on them. Consequently, a tool or a measuring probe actually move along a spatial profile instead of a straight line (assuming one axis motion). Deviation from a straight line is characterised by the position error 5p. A resultant vector 5p consists in fact of many elementary component errors, both translational and rotational. It can be defined as follows: (8J; = fj(lj) %); = gj(pj) (6) where: (6\);: i-th translational error, (6\); : i-th rotational error, fj : functions of translational errors, g; : functions of rotational errors,

9 Laser Metrology and Machine Performance 73 lj : generalised co-ordinates (x u y u z), (Jj : rotation angle around a given axis, i 3 N, i 3 <1..9> p. i- f,-- > Pzx n R! Pyx PZZ ; P. *6 '-'tzx X ^r» &tyx Figure 5: Position error. The conclusion is that for 5p to be fully described it is necessary to determine 18 component functions. In practice both f; and g; functions have to be found by measurements and, if necessary, measurement data has to be described using a relevant approximating function (e.g. trigonometric functions). In special cases these 18 error components have to be complemented by 3 angles showing deviations from orthogonality of a co-ordinate system. Measurements of the above mentioned errors have to feature high accuracy if the results are to be used as a basis for a machine tool settings correction or a CMM software correction of errors. The reform laser interferometer based measurement systems are the most suitable for this purpose. 5. Test of vertical guides elements Below are presented some results [2] of measurements of two flat guides of vertical lathe. In figure 6 is presented the Guides Error Function for central (middle) sections of guides. The characteristics make possible determination of straightness deviations in relation to adjacent reference straight line and their mutual position in the base system.

10 74 Laser Metrology and Machine Performance Characteristics G O F g?sgggggggggggg,_:.^..;(s,>co^-ooi>i*-o'o<\!coxr ooooooooooooooooooo Figure 6. GOF characteristics of vertical lathe columns guides. In figure 7 is presented characteristics of Global Out-of-Parallelnes Factor. Bar chart of harmonic analysis Guides I section V section IV section section II section I Mean value Harmonic components Figure 7. Bar chart of harmonic analysis. The curve denoted A represents function of A parameter that describes changes of distance between guides when the curve denoted B - the B parameter of particular measurement segments straightness. It may be stated when A curve

11 Laser Metrology and Machine Performance 75 is analysed that the surfaces of tested guides have increasing positive parallelness deviation in the first stage (starting from the base) and then bring together and at the same time incline in the direction of coordinates system origin. Diagram of B that feature straightness allow to state that guides in lower part are convex and in middle part and partially in the upper part are concave. Parallelness and straightness deviations for the whole guides or an optional tested segment could be read from diagrams. The programme enables harmonic analysis of selected sections of one or a number guides. Analysis results, figure 8, give basic parameters of dominating and residual harmonics. Figure 8 represents the characteristics of the guide position correction determined according to the mean regression. It has been stated from the characteristics that upper part has to be shifted in relation to base for 550 um to obtain minimum value at the whole guide parallelness at existing shape errors. Position Correction Guide I Guide lenght section V section IV section III section II section I Mean value 6. Final remarks Figure 8. Correction of the guides position. The programme makes possible quality assessment of processing devices guides or kinematics parameters of travelling units. The assessment is based on shape deviations defined by characteristics: Global Out-of-Parallelnes Factor Guide Error Function Harmonic Characteristics Kinematics parameters errors

12 76 Laser Metrology and Machine Performance At the same time parameters of guides position correction are presented for adopted criterion's. GOF is a functional set determining base errors of shape and position due to relevant analysis of measured deviation values given in analytical or graphical mode or their approximating functions. GOF allows for determination of selected shape or position parameters for the whole controlled surface or its segment. At the same time harmonics are determined and accordingly distributed into dominating - basic and remnantresidual. Such procedure makes possible qualitative and quantitative assessment of a single guide or guiding assembly (unit). GOF has an open modular structure i.e. may be composed of any number of arguments or values sets of functions (or describing functions) with any determined interfunctional relation, according to adopted regression elements. The system of assessment may be applied for various type of processing devices (machines) with free configuration of guiding assembly (elements or units) and its task is to improve operational properties of the equipment at the stage of final technological inspection in quality ensuring systems. 7. References 1. Ryniewicz A.: Application and adaptation laser interferometer systems for measurements of certain travelling units parameters, CPBP 0220 rapport nr II, III, Politechnika Krakowska ITMiAP Krakow Ryniewicz A.: Introduction to the project of computer aided accuracy measurements of the processing machines parts, rapport nr II, III, Politechnika Krakowska ITMiAP Krakow Inspection and control rapport's of the processing machines, F. O. RAFAMET, KuYnia Raciborska Ryniewicz A.: Computer laser system for analysis and geometrical quality assurance of large-size units in production engineering - International Conference & Exhibition: The World of New Technologies, Jagiellonian University and Centre of Japanese Art and Technology MANGHA, Krakow V.I Ryniewicz A.: Automation of measurement laser system for analysis and geometrical quality assurance of large-size units in manufacturing - 7-th International DAAAM Symposium: Product & Manufacturing, Flexibility, Integration, Intelligence, Technical University of Vienna X.I Ryniewicz A., Kowalski M., Rewilak J., S^adek J.: Laser measurement of quides and travelling units of CMMMs and CNC machine tools - 7-th International DAAAM Symposium: Product & Manufacturing, Flexibility, Integration, Intelligence, Technical University of Vienna X.I 996