Digital control improves tool machine

New developments in the field of CNC-controlled machine tools aim primarily at increasing the productivity of the machines, i.e. the machining time must be shortened and the contour accuracy must be increased. The new completely digitized drive technology fulfils these demands in an ideal manner because it offers faster acceleration rates, greater path accuracy and a higher drive availability rate.The achievable production accuracy and operating speed depend primarily on the position-controlled motion axes of the machine tool. To fulfil the growing demands with regard to dynamics and accuracy, the motion axes require minimal inertia, hysteresis and elasticity. These properties depend primarily on the driving mechanism, i.e. on the transmission gears or belts.

High positioning accuracy, a high maximum speed and a large torque through correspondingly adapted overall mechanical/ electrical solutions must be the aim of all new developments. A modular three-phase main drive is presented here as a typical example of such an extensive assignment.

Motor and spindle bearing united

In the past a servomotor, that is swivelled into the C-axis, positioned the main spindle during milling (Fig. 1a). According to the new development presented here, the motor and spindle bearing is integrated into a direct drive (Fig. 1b). However, the associated high demands expected of the controlling quality were no longer compatible with conventional analog technology. This was therefore the major consideration for Grundig-Gildemeister Automation GmbH to jointly develop with IAM GmbH (now sci-worx GmbH - Industrial Systems) a fully digitized control for direct drives in the C-axis, and thus a CNC-control for machine tools. This would give the motor the demanded characteristics of a servo drive.







Fig. 1: Different drive mechanisms                    

A field-oriented control process was implemented for the asynchronous motor used for the main spindle drive (Fig. 2). This, together with high-resolution position acquisition, established the preconditions to achieve excellent results regarding

   Positioning accuracy
   Speed control bandwidth
   Dynamics and rigidity
   Motor efficiency

Fig. 2: Field-orientated control process

Experiences gained with the integrated drive control in the tool machine

The first test of an integrated drive control (IAR) on the motor test stand for several drives furnished promising results. This was then followed by Gildemeister testing. The integrated drive control was used to establish the C-axis capability of the main drive and the machining quality of workpieces in connection with two different mechanical drive configurations: A separate main drive (7.5 kW) which drives the spindle via a belt transmission, and a direct drive (10 kW) that is integrated as a hollow-shaft built-in motor in the spindle box.

The IAR integrated drive technology significantly improved the dynamics, quality and rigidity, even with the mechanically less favourable separate main drive. Moreover, additional mechanical drive damping could be eliminated. The IAR integrated drive technology, in conjunction with the direct drive, by far exceeded all the properties regarding dynamics, quality and rigidity associated in the past with this particular type of machine tool. As a result of its extreme rigidity, this drive proved to be ideal for workpiece positioning while milling. Nowadays, this control is incorporated in the CTX-product range of Gildemeister AG.

The main advantages of the digital solution include parameter-true control, convenient setting into operation and parameterization of the drive. Further tests showed that the next innovation step in machine development must be on the mechanical side to ensure that the resulting precision will significantly improve workpiece quality. It is now well worthwhile to especially concentrate on backlash and elasticities in the chuck, turret, tool, motor and machine bed. It is essential to eliminate inaccuracies that are not associated with the control circuit of the control and therefore cannot be compensated by way of the software. The accuracy and dynamics in connection with the workpiece are increased still further with the IAR integrated drive control. Rapid traverse speeds of up to 60 m/min at a resolution of 0.003 mm can now be theoretically immediately implemented with this fully digitized control. Torque-dependent compensation of the spring rigidity of the x- or z-axes is now possible with IAR integrated drive control. Other advantages are the result of the close coupling with CNC control software that is now possible. Path profiles can be transmitted with all the necessary pre-controls to the drive control. Inversely, interpolation software of all state variables of the drive is now available. Digital interfaces thus open the way to optimal close meshing between the drive and the higher-ranking software.

A detailed description of this development and the measurement results are given in the following joint publications with Grundig Gildemeister Automation GmbH:

Eckhardt, V.; Ehrenberg, J.; Hentschke, B.; Korinth, L.:
Die Grenzen liegen jetzt bei der Mechanik.
Elektronik 1993, No. 18, pages 66..81.