Increasing the NC axes positioning accuracy through positioning error software compensation

Transcription

1 Increasing the NC axes positioning accuracy through positioning error software AVRAM GEORGIA CEZARA 1, NICOLESCU FLORIN ADRIAN 1, DUMITRACHE RUJINSKI ALEXANDRU 2, POPA STELIAN 1 1 Machine and Manufacturing Systems Dept.; 2 Materials and Welding Technology Dept. University POLITEHNICA of Bucharest 313 Splaiul Independentei, Sector 6, Bucharest, ROMANIA cezara.avram@yahoo.com; afnicolescu@yahoo.com; ad_rujinski@yahoo.com Abstract: The paper presents experimental research developed by the authors in order to determine the positioning accuracy and the repeatability in positioning of X axis of a gantry robot used for ultrasonic nondestructive testing. The results will be included in the global performance analysis of the gantry robot the analyzed NC axis belongs to. The experimental setup included the analyzed gantry robot (standard-duty ULTRA PAC Ultrasonic immersion system, produced by NDT Automation) and the RENISHAW LASER ML 10 calibration system, used for the linear measurements required to determine the positioning error and the repeatability in positioning for the gantry robot s X axis mobile element as well as to evaluate the robot s X axis improvement in positioning accuracy through error software. Key-Words: industrial robot, numerically controlled axes, functional optimization, laser measurement, positioning accuracy, error software 1 Introduction The final goal of the research developed is the structural and functional optimization of the IR s NC axes. A preliminary stage of this activity is to identify the functional features of the real system which represents the study s subject. For this purpose the first part of the paper presents the setting of the positioning accuracy and repeatability in position of the mobile element of the X axis of the RI. The second part of the paper illustrates the research approach targeting a first level of IR performance s optimization through positioning error s software and the improvement obtained (experimentally determined) for the X axis mobile element positioning accuracy. travel speed being V max X, Y = 5 /sec = 127 mm/sec; V max Z = 1.25 /sec = mm/sec. 2 The subject of the research A gantry robot (Fig. 1) for ultrasonic nondestructive testing (NDT Automation standard - duty ULTRA PAC TM Ultrasonic immersion system) was considered the subject of analysis. Technical data for this system is as follows [1]: - overall dimensions: 53 L X 37 W X 57 H; - scanning envelope: 42.5 L X 29.5 W X 18 H Each of the 3 NC axis is driven by a stepper motor and it includes a lead screw type mechanism for transforming the nature of motion (Lead X,Y = 12.7 mm, Lead Z = 2.54; mm) the limit values for linear Fig. 1. The subject of the research a gantry robot 3 Measuring equipment To determine the positioning error of IR s X NC axis studied in this phase, linear measurements with RENISHAW ML 10 laser calibration system were performed. The laser measurement system s components are presented in Fig 2 [2]: ISBN:

2 Fig Tripod; 2 - ML10 Laser; 3 - EC10 unit; 4 Optics system; 5- Linear interferometer; 6 - Linear reflectors; 7 Mobile element; 8 Material temperature sensors; 9 Air temperature sensors, 10 - Printer; 11 - PC; 12 PCM 20 interface or PC10 interface card Establishing the duty cycle - stroke and speed of the mobile element along IR s NC X axis); Performing the measurement in association with RENISHAW Laser 10 software application to capture target points position in order to determine the positioning error and repeatability in position for the studied IR S NC X axis (through laser linear measurement procedure); Results interpretation and evaluation of the necessaries corrections; Positioning error s software implementing in IR control system and measuring of the positioning accuracy to evaluate improvements gained by error s. 4.1 Mounting the RENISHAW ML 10 laser calibration system According to the technical specifications for the equipment installation [2]; [3] the arrangement of the optical system is as shown in Fig. 4 and Fig.5. Measurement of linear positioning accuracy and repeatability has been achieved by comparing the position shown by the control system of RI for the studied axis and the actual position of the mobile element, measured by the laser system according to Fig. 3 experimental setup [2], [3]. Fig. 4. The arrangement of the optical system Fig. 3. Principle of linear measurement 4 The experimental procedure In order to determine the positioning accuracy and repeatability in position for the IR s X axis mobile element (to identify the functional features of the real system) and increase the studied IR s operating performance following major steps were performed: Setup the RENISHAW ML 10 laser calibration system for linear measurements of X axis mobile element s linear positioning accuracy and repeatability (Fig. 4; Fig. 5); Fig. 5. The arrangement of the optical system - detail 4.2 Establishing the duty cycle In order to highlight the positioning error of the mobile element along the X axis the following duty cycle was selected: stroke required to be traveled by the mobile element along the X axis - S X = 700 mm; ISBN:

3 X axis speed - V X =100 mm/s; X axis acceleration - acc x = 400 mm/s Performing the measurement while using the RENISHAW Laser 10 software application [2] Setting the parameters necessary to perform linear measurement to determine the lost motion and displaying measurement results to determine it To perform this step and the next 3 steps the procedure is similar: - Entering the desired values for the first target (FIRST TARGET = 300 mm), last target (LAST TARGET = 1000 mm) and interval size (25 mm) as shown in fig. 6 and automatic calculation of the total number of target points (29 points); - Select the type of measurement (Linear), the number of double travels (1 for error positioning measurement and 5 for repeatability measurement; Bidirectional measurement (each target is investigate twice per double travel, i.e. each target is measured from both directions - Fig. 7 a, b) and setting the parameters for performing the target points position capture (Fig. 8). Fig. 6. Target Setup a. Capture Initialization b. Linear target sequence - Bidirectional Fig. 7. Settings for Linear target sequence Fig. 8. Auto Data Capture Setup Fig. 9. Capturing target points position During the process of capturing the position of each target selected point, the window is updated as soon as data is generated for each target point the system was moved to (Fig. 9) [1],[2]. Following the capture of each target point s position, the results can be analyzed through the diagrams [2] shown in Fig. 10 and Fig Setting the necessary parameters to perform linear measurements for determining positioning error and display measurement results Measurements for evaluation of the positioning error were performed using two hypotheses: without taking over the backlash at the end of stroke before starting the return stroke (Fig 10), and taking over the backlash by exceeding 1000 mm target point and return to it after taking over the backlash before starting the return stroke (Fig.11). The first assumption allows evaluation of the lost motion based on measurement analysis. For the analyzed NC axis the lost motion mean value identified (based on the measurements) is of microns (Fig 10). The second assumption allows determining the positioning error for the bidirectional run performed for the X axis, in the same points, while taking over the lost motion (Fig. 11). ISBN:

4 Fig ANALYSIS PLOT backlash determination Fig ANALYSIS PLOT - after taking over the backlash before starting the return stroke Fig ANALYSIS TRIPLET PLOT before positioning error Fig ANALYSIS TRIPLET PLOT - after positioning error ISBN:

5 NO. TARGET I II III Axis position FORWARD DIRECTION REVERSE DIRECTION before software Compensation values after software NO. TARGET Axis position before software Compensation values after software Table 1. Experimental results before ( = microns) and after positioning error software ( = 45.3 microns) ISBN:

6 Setting the necessary parameters to perform linear measurements to determine repeatability in position for the X axis mobile element and display the results The procedure is similar with the one performed for establishing the positioning error (Fig. 6, Fig. 7, Fig. 8), the difference being that in this case the interval size is 50 mm and the number of bidirectional travels is 5. After the settings have been made, capture of the target points positioning has been performed, the results being shown in Fig 14. Fig. 14. ANALYSIS PLOT Repeatability determination 5 Results interpretation and conclusions From the experimental data obtained, the following conclusions can be drawn: IR s X axis evaluation allowed the identification of a lost motion average value of microns (fig 10); Evaluation of NC axis positioning accuracy (analyzed through the bidirectional run procedure on total stroke) allowed the identification of the total value of microns positioning error, and the measured error value distribution in three distinct areas: I, II, III (Fig 11, Fig. 12, Table 1). Area I and Area III correspond to the areas where the positioning error determined has as unique source the cumulative pitch error for the X axis lead screw. In contrast, the second area includes, in addition to the cumulative pitch error, the errors due to the X axis lead screw wear. In the absence of the error components caused by the screw s wear, the positioning error chart could highlight that the maximum value of this error is about 100 microns (red mark in Fig 11) obtained through the cumulative pitch error chart linearization subsequent to the translation of Area III to continue the same line with Area I. Considering the actual level of wear on the X axis lead screw, the increasing positioning accuracy of X axis mobile element can be achieved by: replacing the screw with an unused one, selective use of only Area 1 and 3 of the X-axis stroke (both solutions being money spending / non-functional by eliminating frequently used scanning areas), respectively looking for implementing a positioning error software procedure - positioning error being identified by measurements (solution that does not lead to occurrence of additional costs and allows to exploit the full functionality of the analyzed gantry robot s NC X axis. Taking into account all above, we proceeded to identifying the error corresponding to each measurement point; we calculated the required corrections and, respectively we have entered them in the command program of the mobile element along X axis (Table 1). The results of the final measurement performed after positioning error software show a decrease from microns to 45.3 microns (by 87.16%) in the positioning error (Fig. 13, Table 1). In conclusion, we can say that in addition to the identification of IR NC axis technical condition, the research carried out resulted in increasing the positioning accuracy for X axis by 87.16%. In the next stage, new research procedures will be developed to achieve the functional optimization of IR s Y and Z axes and respectively concerning the overall level of functionality (IR s volumetric error ). In parallel, studies for IR s NC axes structural optimization will be conducted. 6 Acknowledgements The work has been funded by the Sector Operational Program Human Resources Development of the Romanian Ministry of Labor, Family and Social Protection through the Financial Agreement POSDRU/107/1.5/S/ References: [1] NDT, ULTRAPACII System User, Installation & Technical Manual, NDT Automation a division of MISTRAS Group Inc, USA, [2] RENISHAW, Laser calibration system - User guide Issue 5, Renishaw, [3] Anania F.D., Ispas C., Mohora C., An Algorithm for CAD correction of the work pieces based on machine tool errors, Proceedings Annals of DAAAM for 2007, 2007, Zadar, Croatia, ISBN ISBN: