A.P. Kuznetsova, H.-J. Koriathb, A.O. Dorozhkoa,b
aMoscow State University of Technology “STANKIN”, 127055, Moscow, Russia
bFraunhofer Institute for Machine Tools and Forming Technology IWU, Reichenhainer Str. 88,
09126 Chemnitz, Germany
Abstract. Existing control methods for the thermal stability of machine tools are
based on two fundamentally different principles: - changes in the position of objects of the impact; - change of the thermal state of the objects of the impact. It is substantiated and experimentally shown that the thermal spindle displacement for any machine design solution occur no more than three types of laws out of sixteen possible types of laws. A new concept and algorithm of an sensorless thermal controller for the spindle axis displacement is proposed.
Peer-review under responsibility of the International Scientific Committee in the person of the Conference Chair Prof. Steffen Ihlenfeldt.
Keywords: Machine tools; Accuracy; Temperature; Deformation; Control
1 Introduction
Nomenclature
r
G deformation
T temperature
Thermal deformations of a metal-cutting machine cause deviations in the rela- tive movement of the tool and the workpiece from the ideal (nominal or speci- fied). Tool and workpiece are the final links in the kinematic chain as for geo- metrical shaping as for the coordinate thermo-physical structure of the ma- chine. Error caused by the thermal position / orientation shift of units and parts of machine are proportional to their functional thermal state [1,5-8,10]. Summarizing the state of research on thermal processes in machines and re- flecting various methods for modifying the thermal robustness and thermal be- havior of machine tools, their parts, components and units, figure 1a shows a general scheme for principles and methods to control the thermal precision of machine tools, as its main functional characteristics. Exceptional attention was
given to general principles and methods, but less to special solutions resulting in positive effects for one machine tool, their parts, components and units.
a) b)
Fig. 1 a) General scheme for principles and methods to control the thermal precision of machine tools; b) Classification for modification methods of the thermal state of machine tools.
Analysing numerous research studies over the past 30 years, we have identi- fied, synthesized and formulated the structure of interrelations and different levels for control methods of thermal phenomena in machines. Fig.1 b shows a classification for modification methods of the thermal state of machine tools, derived from generalizations and split in six classes of control structures, cover- ing most of the areas of the performed studies.
Review, synthesis and analysis of the implementation of control methods aim- ing at effects on the thermal state of metal-cutting machine tools dedicated to generalizing work [1,2,3,9,10]. Following Fig. 1 b a classification of research re- sults with relevant publications aiming at the following targets is proposed:
a) Reduction in the number of heat sources [1,5-8]; b) Thermal flow management [1,2,3,6,7,10];
c) Design of thermo-robust machine structure [1,2,5,6,7,9]; d) Compensation of thermal errors [1,2,3,9,10,17,18,19];
e) Error compensation control (depending only on temperature, or else on the position/orientation and the temperature) [1,2,3,9,10,16,17,18,19]. It is also noted that the existing compensation methods are based on:
f) Direct thermal position/orientation error measurement methods [2,9,17,19];
Metal-cutting machine tool
Temperature Deformation (linear, angular) Determination method And / or Model Position State And / or Modification method
Thermal precision of machine tool
Modification methods of the thermal state of metal-cutting machines
Limitation Reduction
Modification of thermal state
Compensation Regulation Configu- rational Struc- tural Techno- logical Thermo- physical Thermo- mechanical Opera- tional Direct Indirect
Temperature Thermo-elastic deformation (displacement)
Metal-cutting machines and their various components and mechanisms Modification of thermal behavior Output precision characteristics of metal-cutting machines
Actions on parameters Modified factors Modification type Modified parameters Modification target
g) Indirect temperature measurement methods at local points of the ma- chine and the calculation based on the thermal model [1,2, 7,8,9,17,19]; h) Indirect parameter measurement methods according (e.g. rotation fre-
quency, motion velocity along the axes, etc.) and calculation based on the created or selected mathematical model [1,2,4,5,7,8,10];
i) Combined methods from the above listed [1,4,5,7,21,22];
j) Other methods, including combinations of previous ones [1,4,5,7,22-25]. Methods that link interacting parameters as mathematical methods to create control or influence models on the thermal behavior of machine tools and im- prove the manufacturing accuracy are (see fig. 1a,b):
k) Linear and non-linear regression [1,2,3,17,18,19]; l) Neural networks [20];
m) Fuzzy logic [1,2]; n) Transfer function [1,22]; o) Grey system theories [1,2]; p) B-spline [1,2];
q) Uniform transformation matrices [1,2,3,17,18,19].
Here, action types on parameter modification include the classes: limitation, reduction, compensation, and regulation of parameter modifications. Modifica- tion type factors are: configuration, structural, technological, thermo-physical, thermo-mechanical, and operational. Limitation methods are linked to a limited effect of thermal sources on thermal active machine components. Thermal sources (e.g. electrical drives and hydraulic unit) are separated from the ma- chine tool aiming at limited heat input, or isolated aiming at limited effect from external heat sources and the cutting process. Reduction methods oriented to- wards a reduced heating temperature and/or thermo-elastic deformation. Ap- plication solutions are internal heat source cooling, increased heat convection surface (grooves), increased thermal convection parameter (air exchange), use of low dimension expansion materials, and creation of thermo-elastic defor- mations in required and defined directions etc.
Compensation methods change the relative position or orientation between machine units and parts (usually the tool and work piece) based on information (indirect or direct) about the operational temperature and/or thermo-elastic machine deformations. Today compensation methods are wide used as easy and effective means for an increased precision of metal cutting machine tools. Practical systems based on: - the machine (part, unit) temperature measure- ment for resulting axis control, - the machine temperature or a different factor
(e.g. power loss, rpm) measurement for indirect deformation estimation, - the machine temperature measurement and comparison to a standard and/or mathematical model for thermal machine deformations, - a factor measure- ment for indirect deformation estimation, and last – the measurement of ma- chine unit linear deformations.