Its a bit disconcerting to know that there is something like this parameter on a cnc and I haven't heard of it till now....
googling brought up some stuff ( links below)
'http://thesis.lib.ncu.edu.tw/ETD-db/ETD-search/view_etd?URN=90323119'
then some more at
'http://journals.pepublishing.com/content/l5t68377h7052281/'
the text is copied here....
Résumé / Abstract
One of the most significant sources of tracking error for an X-Y table is static friction, a nonlinear disturbance at low velocity. In traversing a circular profile, an X-Y table encounters zero velocity crossings at ninety degree intervals around the circle, leaving relatively large tracking errors referred to as «quadrant glitches.» To learn the control input which eliminates errors caused by stiction, repetitive control, a subclass of learning control, is employed. Experiments, conducted on the X-Y bed of a CNC milling machine, demonstrate that near-perfect tracking can be achieved in twelve cycles or less. Of particular interest is the velocity command input generated by repetitive control. The input contains feedforward information for developing perfect trajectories at low velocities
Revue / Journal Title
Journal of dynamic systems, measurement, and control ISSN 0022-0434 CODEN JDSMAA
Source / Source
1993, vol. 115, no2A, pp. 279-284 (24 ref.)
Langue / Language
Anglais
and here
As the development in industry, time and quality are more and more important in manufacturing industry. Therefore, automatic manufacturing machines are tending to high speed and high precision. This thesis is aimed to eliminate tracking error of traveling a circular profile on the CNC machine tools. Friction is one of the most significant source of nonlinear disturbance for the motion control which caused by the relative motion of different contact surface at the low velocity. The nonlinear component of friction such as static friction (stiction) and the Coulomb friction should be overcome so that the tracking error will be eliminated. When the X-Y tables are tracking a circular profile, quadrant glitches appear at ninety degrees intervals, i.e. the motion of one axis has a zero velocity crossing and reverses direction.
The quadrant glitches were due to friction brought about transient phenomenon. Therefore, we will solve the transient phenomenon. We will make the system to reach steady state quickly, we may anticipate compensating a value when the motion of one axis has a zero velocity crossing and reverses direction. We will observe the system dynamic before traversing a circular profile, and to obtain a compensated value on off-line.
In addition, we designed the feedforward and P feedback tracking controllers. The research adopts feedforward controller to improve the tracking performance of the system.
Finally, the above servo loop control of tracking a circular profile on CNC machine tools is verified by the simulation and experimental results.
and more here
N Uchiyama1
1 Department of Mechanical Engineering, Toyohashi University of Technology, 1-1 Hibarigaoka, Tempaku, Toyohashi, Aichi 441-8580, Japan,
[email protected]
Abstract
Contouring control that employs tracking error components normal to desired contour curves as feedback signals is an effective method of improving contouring accuracy in machine tool control. In this paper, a contouring control method for biaxial feed drive systems is presented, which transforms tracking errors in each feed drive axis into errors that are reference to a moving frame around the desired position. The method is extended to include a referenced adjustment function for reducing inherent contour errors in the moving frame approach. Because friction is a major disturbance in motion control of feed drive systems, a friction compensation method for the contouring control systems is proposed. Since properties of friction cannot be predicted accurately, an estimation method of the friction model parameters is developed, and a robust controller for the estimation errors is designed. The effectiveness of the proposed method is demonstrated by experimental results with circular contour curves.
Keywords
machine tool, feed drive system, contouring control, non-linear friction
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Hope some of you can make more sense of it that I .
mjm
