Renishaw Plc · LSE:RSW
Renishaw — Technical Presentation (September 2018)
33 slides from Renishaw's technical presentation, Sep 11, 2018
Slide 1: Laser step diagonal testing
Laser step diagonal testing H-5650-2057-01-B 11/09/2018 1
Slide 2: Introduction
Introduction • This presentation reviews the step or vector diagonal measurement method for machine calibration. • It explains the technique and then highlights some fundamental flaws in this method of measurement. • It is recommended that the introductory Renishaw presentation entitled “Laser diago
Slide 3: Laser diagonal tests - B5.54 and ISO230-6
Laser diagonal tests - B5.54 and ISO230-6 • Laser body diagonal tests for machining centres are described in B5.54 and ISO 230-6 standards. • These tests use a laser interferometer to measure the linear positioning accuracy of the machine as it moves along each of its four body diagonals in turn. •
Slide 4: Laser diagonal tests - B5.54 and ISO230-6
Laser diagonal tests - B5.54 and ISO230-6 • A number of equi-spaced target positions are defined along each body diagonal. • The machine is then moved along the diagonal from one target position to the next. • The laser measures the linear positioning error at each target position. • The measurement
Slide 5: Laser diagonal tests - presentation of results
Laser diagonal tests - presentation of results • The results are analysed to compute the mean forward and mean reverse errors at each target position along each diagonal. B = 3.7µm • These are then inspected to E = 7.4 µm identify the systematic error of positioning (E) and the maximum reversal erro
Slide 6: Laser diagonal tests - step diagonal method
Laser diagonal tests - step diagonal method • It has been proposed in a number of technical papers1,2 that the original laser diagonal test can be enhanced by using a special step sequence to move between target positions on the body diagonals. • This method is referred to as the step diagonal or ve
Slide 7: Laser diagonal tests - step diagonal method
Laser diagonal tests - step diagonal method • In the original diagonal test (as described in • In the step diagonal test the machine’s X, Y and Z ISO 230-6 and B5.54) the machine’s X, Y and Z axes are moved individually, in turn, and a laser axes are moved simultaneously in order to move reading is
Slide 8: Laser diagonal tests - step diagonal method
Laser diagonal tests - step diagonal method • The step diagonal method therefore takes 3 times as many laser readings as a normal laser diagonal calibration. • It has been claimed1,2 that, in addition to the normal diagonal displacement error results, this additional data can be processed to give re
Slide 9: Step diagonal tests - laser alignment
Step diagonal tests - laser alignment Spindle • In order to maintain alignment of the laser beam (as the machine moves along the stepped diagonal path), the laser Mirror is directed onto an angled plane (flat) mirror fixed to the positions machine spindle. • The figure shows successive positions of
Slide 10: Step diagonal tests - theory
Step diagonal tests - theory • If the laser beam and mirror are aligned perfectly and there are no pitch, yaw and roll errors in the machine, then the theory of operation is stated as follows: • When the X axis moves the laser will measure the combined effect of errors in the linear and straightness
Slide 11: Step diagonal tests - problems
Step diagonal tests - problems • However, there are two fundamental problems… • Firstly, the vast majority of machines do have roll, pitch and yaw errors. These errors will contaminate the results by introducing additional error terms which the proposed calculation methods don’t resolve. • Secondly,
Slide 12: Step diagonal tests - effect of mirror misalignment
Step diagonal tests - effect of mirror misalignment • The figure shows the side view of a laser aligned at 45° to the machine’s X and Z axes. • The movement of a misaligned mirror (shaded) is compared with that of a perfectly aligned mirror (outline). • The mirror has been misaligned by a small angl
Slide 13: Step diagonal tests - effect of mirror misalignment
Step diagonal tests - effect of mirror misalignment • It might be thought that this error is very small, unfortunately this is not the case, because the error accumulates. • For example, • If the plane mirror is misaligned by an angle of just 40 arcseconds or 0.2 mm/m (a typical alignment tolerance)
Slide 14: Step diagonal tests - effect of mirror misalignment
Step diagonal tests - effect of mirror misalignment • It has been proposed that, because this error is constant, it can be removed by software, e.g. by linear regression/slope removal, etc. However, there are two flaws in this approach. • Firstly, the error will only be constant if the angle of the
Slide 15: Step diagonal tests - effect of mirror misalignment
Step diagonal tests - effect of mirror misalignment • It might be thought that by reversing the axis movement sequence it could be possible to separate the errors caused by mirror misalignment from any linear travel errors in the axis. • Unfortunately this is not the case. In both instances the lase
Slide 16: Step diagonal tests - regular diagonal results
Step diagonal tests - regular diagonal results • Although the step diagonal technique cannot reliably measure the linear positioning errors in an axis, it can provide systematic error in diagonal positioning and reversal error results, in accordance with the B5.54 and ISO 230-6 standards. B = 3.7µm
Slide 17: Step diagonal tests - regular diagonal results
Step diagonal tests - regular diagonal results • However, the method has some disadvantages compared with using a regular linear laser interferometer and retro-reflector, as shown on right. • The system is harder to align accurately to the machine diagonal. • Any misalignment of the laser beam to th
Slide 18: Step diagonal tests - error compensation
Step diagonal tests - error compensation • The step diagonal method does not give reliable linear positioning accuracy data for the machine’s axes. • However, the technique can give reliable diagonal displacement accuracy results in accordance with B5.54 and ISO 230-6. • It is therefore possible to
Slide 19: Step diagonal tests - error compensation
Step diagonal tests - error compensation Although the diagonal test results have been improved, is it true that the machine’s positioning accuracy must have been improved too? • Not necessarily • Unfortunately, the combined weaknesses of the step diagonal method and ISO 230-6 or B5.54 diagonal tests
Slide 20: Error compensation example
Step diagonal tests - error compensation example • Consider a 1m x 1m x 1m machine which has perfect X and Y axes, but the Z axis over-travels by 100 µm/m. All three axes are perfectly square to one another and contain no other errors. 1000.1 mm • The diagonal results will show all four body diagona
Slide 21: Step diagonal tests - error compensation example
Step diagonal tests - error compensation example • It is possible to apply a single compensation value of -33 µm/m to all three axes to improve the B5.54 or ISO 230-6 diagonal test results. 1000.067 1000.1 mm mm • This will produce a good diagonal test result, but it hasn't fixed the problem with th
Slide 22: Conclusions
Conclusions 11/09/2018 RENISHAW& 22 apply innovation”
Slide 23: Step diagonal tests - conclusions
Step diagonal tests - conclusions • The sensitivity of the step diagonal method to mirror misalignment means that the method cannot be used, in isolation, to reliably determine the linear positioning errors in a machine. Such errors must be measured using conventional linear laser measurements taken
Slide 24: Further developments
Further developments 11/09/2018 24
Slide 25: Further developments - Renishaw paper
Further developments - Renishaw paper • Following on from the original work at Renishaw, described earlier, there have been a number of developments. • In 2003 Renishaw submitted a detailed technical paper covering this topic to the Journal of Precision Engineering, where it was independently review
Slide 26: Further developments - NIST paper
Further developments - NIST paper • In 2005 J.A. Soons at NIST (National Institute of Standards and Technology, USA) performed a detailed theoretical analysis of the step diagonal test. The results of this work were presented at the Lamdamap 2005 Conference in the UK and published4 in the proceeding
Slide 27: Further developments - ASME B5.54 revisions
Further developments - ASME B5.54 revisions • In 2005 a new edition of ASME B5.54 was published. The new edition advises against using laser diagonal measurements, in isolation, to assess machine performance. • In 1992 ASME B5.54 in section 5.9.2 stated: – Volumetric Performance Using Laser Diagonal
Slide 28: Further developments - ASME B5.54 revisions
Further developments - ASME B5.54 revisions • ASME B5.54* now also recommends a minimum test set for quickly estimating the performance of a three axis machine tool. • Recommended minimum test set 1. Contouring performance 2. Linear positioning 3. Diagonal 4. Precision test using circular tests in a
Slide 29: Further developments - RCMT paper
Further developments - RCMT paper • In 2006 a paper, by O. Svoboda of the Research Center for Manufacturing Technologies (RCMT) at the Czech Technical University in Prague, was published5 in the Journal of Precision Engineering. The abstract of this paper states; – This paper describes the results o
Slide 30: Further developments - Kyoto University papers
Further developments - Kyoto University papers • In 2009/10 two papers6,7 were published in the Journal of Precision Engineering by the Micro Engineering Department of Kyoto University. • These papers confirm the conventional step diagonal method is subject to a significant estimation error caused b
Slide 31: Further developments - Korean papers
Further developments - Korean papers • In 2012 the Korean University of Science and Technology and Korean Institute of Machinery and Materials Technology 09/10 published a paper8 in the International Journal of Machine Tools & Manufacture. • The paper shows a revised data capture method along the li
Slide 32: Overall conclusion
Overall conclusion • Step diagonal (vector) methods, used in isolation, cannot reliably determine the linear positioning errors in a machine. • It is necessary to take additional, conventional, linear laser measurements, parallel to at least two of the machine’s X, Y and Z axes, in order to determin
Slide 33: References
References 1. C. Wang, "Laser vector measurement technique for the determination and compensation of volumetric positioning errors, Part 1: Basic theory", Review of Scientific Instruments, Vol 71, No 10, October 2000 2. C. Wang, G.Liotto, "A laser non-contact measurement of static positioning and dy