If a linear encoder is used for measurement of the slide position (see figure 2), the position control loop includes the complete feed mechanics. This is referred to as closed-loop operation. Play and inaccuracies in the transfer elements of the machine have no influence on position measurement. This means that the accuracy of the measurement depends almost solely on the precision and location of the linear encoder.
Various conditions of application together with growing feed rates and forces lead to constant changes in the ball screw’s thermal condition. Local temperature zones develop on a recirculating ball screw, which change with every change in position and decisively reduce accuracy in the semi-closed loop. High reproducibility and accuracy over the entire traverse range of linear axes can therefore be achieved only through operation in a closed loop. The results are precise workpieces and a drastically reduced rejection rate.
Figure 3: Torque motor – Rotary axes with torque motors are operated in a closed loop.
As an alternative to using a recirculating ball screw, linear feed axes can also be driven by linear motors. In this case, the position of the machine axis is also measured directly by a linear encoder mounted on the axis slide. For highly dynamic and at the same time quiet operation, linear motors already depend on high-resolution, accurate linear encoders. For this type of drive, the advantages of closed-loop operation apply unrestrictedly.
Position acquisition on rotary axes
The basic principle for linear axes also applies to rotary axes. Here, too, the position can be measured with a rotary encoder on the motor, or with a high-accuracy angle encoder on the machine axis. If the axis position is measured using a rotary encoder on the feed motor, it too is referred to as a semi-closed loop, because the transmission error of the gear mechanisms cannot be compensated through a closed position loop.
Error of the mechanical transmission in rotary axes is caused by eccentricity of the gear wheels, play, or friction, and elastic deformations in the tooth contacts and the bearings of the transmission shafts. Beyond this, most pre-stressed transmissions are subject to significant friction that heats the rotary axes and can therefore -- depending on the mechanical design -- result in positioning error.
In a semi-closed loop, the error in the transmission of rotary axes leads to substantial positioning error and to significantly reduced repeatability. The errors of the rotary axes are transferred to the geometry of the workpiece, which can greatly increase the number of rejected parts.
The positioning accuracy and repeatability of rotary axes can be decidedly improved with the use of precise angle encoders. Since the axis positions are no longer measured on the motor, but rather directly on the rotary axes of the machine, this is referred to as closed-loop operation. Errors of rotary axis transmissions have no influence here on positioning accuracy. The accuracy with which a rotary axis can move to a certain axis position over a long period is also decidedly increased. Economic manufacturing with minimal scrap is the result.
The rotary axes that are driven directly by a torque motor play a special role. The torque motors’ special design permits very high torque without additional mechanical transmission. Rotary axes with torque motors require a high-resolution angle encoder immediately on the machine axis. They are always operated in a closed loop.
Editor’s Note: This is the first in a two-part series. Part two of this article will appear in our January Fluid Power supplement and online at designnews.com.
Dr. Jens Kummetz is head of application development at DR. JOHANNES HEIDENHAIN GmbH, in Traunreut, Germany.