Thermographic images of a rotary table with an optical (left) and a magnetic (right) position encoder show that the magnetic encoder generates more heat.

Direct drives place rigorous demands on position-signal quality. Optimum measuring signals increase the quality of the machined workpiece surface, reduce vibration in the machine frame, stop excessive noise exposure from velocity-dependent motor sounds, and prevent additional heat generation in the motor. Direct-drive efficiency is 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.

Construction of direct drives

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

On direct drives, there is no additional encoder for measuring the speed. Both the position and the speed are measured by the position encoder: linear encoders for linear motors, 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 for exact velocity control at slow traversing speeds.

Higher k_V factors also increase the influence of the position encoders’ signal quality on positioning behavior. Direct drives therefore require position encoders with small signal periods and high signal quality.

Signal quality of position encoders

Modern encoders feature an incremental (which means counting) or 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. Incomplete scanning, contamination of the measuring standard, and insufficient signal conditioning can lead to the signals deviating from the ideal sinusoidal shape.

A control loop for a typical rotary direct drive torque motor shows the angle encoder providing feedback to the controller. Both position and speed are meadured by the angle encoder.
Select figure to enlarge.

Interpolation generates shortwave errors whose periodic cycle is within one signal period of the encoder’s output signals. These errors are termed “position errors within one signal period,” or also “interpolation errors.” On highquality encoders they are typically 1 to 2% of the signal period.

Interpolation error not only affects the position accuracy, but also influences the speed stability of the drive. The speed controller calculates the nominal currents used to brake or accelerate the drive depending on the error curve. At low feed rates, the feed drive lags the interpolation error. Since direct drives have a greater control bandwidth due to the kV factor, they can lag the interpolation error over a larger speed range. This can lead to wave-like errors on the workpiece surface during cutting processes. The wavelength and amplitude depend on the speeds of the machine axes involved in the feed rate.

Generation of heat and noise

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 increase motor noise and heat.

A comparison of the effects of an optical angle encoder and a magnetic modular rotary encoder on a rotary table with direct drive emphasize the meaning of high-quality position signals. The angle encoder with 16,384 lines generates only barely noticeable disturbances in the motor current, and the motor operates normally and develops little heat. Because of the magnetic scanning principle, the rotary encoder has notably fewer signal periods. Significant disturbances occur in the motor current at the same controller settings, causing an increase in motor noise and heat.

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