The semiconductor industry and automation technology increasingly require more precise and faster machines in order to satisfy growing demands on miniaturization, quality, and manufacturing cost reduction. Linear motors gradually are becoming more important in such highly dynamic applications with one or more feed axes. The benefits of this direct drive technology are low wear, low maintenance, and higher productivity.

Motor current of a direct drive with position encoder (a) with low interpolation error, and (b) with high interpolation error.
However, this increase in productivity is possible only if the control, the motor, the machine frame, and the position encoder are optimally adjusted to one another. Direct drives place rigorous demands on the quality of the measuring signals.

Heat generation of a linear motor controlled with an encoder (a) with low interpolation error, and (b) with high interpolation error.
Optimum measuring signals reduce vibration in the machine frame, stop excessive noise exposure from velocity-dependent motor sounds, and prevent additional heat generation, allowing the motor to realize its maximum mechanical power rating. The efficiency of a linear motor is therefore greatly influenced by the selection of the position encoder. Encoders with optical scanning methods provide benefits in the accuracy, speed stability, and thermal behavior of a direct drive.

Design of Direct Drives

The decisive advantage of direct drive technology is the very stiff coupling of the drive to the feed component without any other mechanical transfer elements. This allows significantly higher gain in the control loop than with a conventional drive.

On direct drives, there is no additional encoder for measuring the speed. Both the position and speed are measured by the position encoder: linear encoders for linear motors, and angle encoders for rotating axes. Since there is no mechanical transmission between the speed encoder and the feed unit, the position encoder must have a correspondingly high resolution in order to enable exact velocity control at slow traversing speeds. The velocity is calculated from the distance traversed per unit of time. This method, which is also applied to conventional axes, represents a numerical differentiation that amplifies periodic disturbances or noise in the signal. The combination of significantly higher control loop gain, as is used particularly with direct drives, and insufficient encoder signal quality, can result in a dramatic decline in drive performance.

Signal Quality of Position Encoders

Modern encoders feature either an incremental, which means counting, or an absolute method of position measurement. The path information is transformed in the encoder into two sinusoidal signals with 90° phase shift. Both methods require that the sinusoidal scanning signals be interpolated in order to attain a sufficiently high resolution. Inadequate scanning, contamination of the measuring standard, and insufficient signal processing can lead to a deviation from the ideal sinusoidal shape. As a consequence, during interpolation, periodic position error occurs within one signal period of the encoder’s output signals. These position errors within one signal period are referred to as “interpolation error.” On high-quality encoders, it is typically 1% to 2% of the signal period.

If the frequency of the interpolation error increases, the feed drive can no longer follow the error curve. However, the current components generated by the interpolation error cause increased motor noises and additional heating of the motor.

A comparison of the effects of linear encoders with low and high interpolation error on a linear motor illustrates the significance of high-quality position signals. The linear encoder used here generates only barely noticeable disturbances in the motor current: the motor operates normally and develops little heat. If at the same controller setting, the interpolation errors of the same encoder are increased through poor adjustment, significant noise arises in the motor current. This causes an increased amount of noise and heat generated in the motor.