As more semiconductor machines incorporate linear motors, it is crucial to select the right position encoder. Encoders with optical scanning methods enhance the accuracy, speed stability, and thermal behavior of a direct drive.
By Dr. Jens Kummetz,
Marketing Application Development,
Dr. Johannes Heidenhain GmbH
The semiconductor industry continues to demand tighter precision and faster operating speeds from machines in order to satisfy growing demands on quality, production, and size reduction. Linear motors are becoming more important in such highly dynamic applications that use one or more feed axes. The benefits of direct drive technology are low wear, low maintenance, and more throughput.
However, this increase in throughput is possible only if the control, the motor, the machine frame, and the position encoder fit one another. Direct drives place rigorous demands on the quality of the measuring signals. High quality 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.
Design of direct drives
The decisive advantage of direct drives is the very stiff coupling of the drive to the feed component without any other mechanical transfer elements. This configuration allows significantly higher gain in the control loop than with a conventional drive.
Direct drives do not need an additional encoder to measure speed. Both position and speed are measured by the position encoder: linear encoders for linear motors, angle encoders for rotating axes. Because there is no mechanical transmission between the speed encoder and the feed unit, the position encoder must have a correspondingly high resolution 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 the significantly higher control loop gain used with direct drives and insufficient encoder signal quality can reduce drive performance.
Signal quality of position encoders
Modern encoders have either an incremental, which means counting, or an absolute method of position measurement. In the encoder, path information is transformed 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 are referred to as “interpolation error.” On high-quality encoders the error is typically 1% to 2% of the signal period.
During interpolation, position errors can occur within one signal period of an encoder’s output signals.
The interpolation error can produce several effects:
—Generation of heat and noise. If the frequency of the interpolation error increases, the feed drive can no longer follow the error curve. The current components generated by the interpolation error
increase 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 LIDA linear encoder, for example, generates 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, which can cause more noise and heat generated in the motor.
—Dynamic behavior. Digital filters will smooth the position signals for direct drives. However, the additional phase delay caused by filtering in the speed-control loop must be kept to a minimum, otherwise the dynamic accuracy decreases.
Position encoders with optimum signal quality help to reduce the use of filters, which maintains the control bandwidth.
Position Encoders for direct drives
Linear encoders that generate a high quality position signal with low interpolation errors are essential for optimal direct drive operation in the electronics industry. Encoders that use photoelectric scanning are ideally suited for this task, since they can scan very fine graduations.
The right encoder will generate minimal disturbance in the motor current.
Encoders with optical scanning measure periodic structures known as graduations. The substrate material is glass, steel, or—for large measuring lengths—steel strips. These fine graduations—graduation periods from 40 μm to under 1 μm are typical—are manufactured in a photolithographic process. They have high edge definition and excellent homogeneity—a fundamental prerequisite for low interpolation error, and therefore for smooth operating performance and high control loop gain.
By the nature of their design, the measuring standards of exposed linear encoders are less protected from their environment. The manufacturer should therefore always uses tough gratings made in special processes.
Here’s a look at the heat generation of linear motor contolled with an encoder
TOP: With low interpolation error
BOTTOM: With high interpolation error
In the DIADUR process, hard chrome structures are applied to a glass or steel carrier. The AURODUR process applies gold to a steel strip to produce a scale tape with a hard gold graduation.
In the SUPRADUR process that we use, a transparent layer is applied first over the reflective primary layer. Then an extremely thin, hard chrome layer is applied to produce a grating. Scales with SUPRADUR graduations have proven to be particularly insensitive to contamination because the low height of the structure leaves practically no surface for dust, dirt or water particles to accumulate.
These production technologies ensure an enduringly high signal quality suitable for the use of direct drives in demanding applications.
Optimal scanning
The scanning method and the high quality of the grating share responsibility for low interpolation error. In single-field scanning, the output signals are generated from one scanning field. This large field and the special optical filtering through the structure of the scanning reticle and photosensor
generate scanning signals with constant signal quality over the entire range of traverse. Constant signal quality is necessary for:
—Low signal noise
—Low interpolation error
—High traversing speed
—Good control loop performance for direct drives
—Low motor heat generation
To put it simply, the imaging scanning principle functions by means of projected-light signal generation: two scale gratings with equal grating periods are moved relative to each other—the scale and the scanning reticle. The carrier material of the scanning reticle is transparent, whereas the graduation on the measuring standard may be applied to a transparent or reflective surface.
When parallel light passes through a grating, light and dark surfaces are projected at a certain distance. An index grating with the same grating period is located here. When the two gratings move in relation to each other, the incident light is modulated: if the gaps are aligned, light passes through. If the lines of one grating coincide with the gaps of the other, no light passes through. Photovoltaic cells convert these variations in light intensity into electrical signals. The specially structured grating of the scanning reticle filters the light current to generate nearly sinusoidal output signals.
Linear encoder scales made with SUPRADUR graduations tend to be less sensitive to contamination because they have few surface structures.
In the XY representation on an oscilloscope the signals form a Lissajous figure. Ideal output signals appear as a concentric inner circle. Deviations in the circular form and position are caused by position error within one signal period and therefore go directly into the result of measurement. The size of the circle, which corresponds to the amplitude of the output signal, can vary within certain limits without influencing the measuring accuracy.
In single-field scanning, the output signals are generated from one scanning field. This large scanning field, and the optical filtering through the structure of the scanning reticle and photosensor generate scanning signals with constant signal quality over the entire travel range.
On direct drives, deviations from the circular form cause acoustic noise, reduce control quality and increase heat generation.
This chart shows a selection of position encoders for direct drives and the maximum values of interpolation error with respect to the signal period.
Lower sensitivity to contamination
Production facilities and handling devices for the semiconductor industry demand high acceleration and compact designs. Such requirements usually mean exposed measuring systems that operate without friction and, because they operate without their own housing, can be designed to be very small and low in mass. Special scanning methods and production techniques provide tough protection against contamination even without sealing the encoder.
The specially structured grating of the scanning reticle filters the light current to generate nearly sinusoidal output signals. An XY representation of the signals on an oscilloscope takes the form of a Lissajous figure. Ideal output signals appear as a concentric inner circle. Deviations in the circular form and position are caused by position error within one signal period.
Many exposed linear encoders operate with single-field scanning where only one scanning field generates the scanning signals. Local contamination on the measuring standard (such as fingerprints from mounting or oil accumulation from guideways) influences the light intensity of the signal components, and therefore of the scanning signals, in equal measure. The output signals do change in their amplitude, but not in offset and phase positions. They stay highly interpolable, and the interpolation error remains small. The large scanning field additionally reduces sensitivity to contamination. In many cases this can prevent encoder failure.
Thus, optical encoders with low sensitivity to contamination need an optimal scanning method, a large scanning field, and contamination-tolerant graduation.
Very small signal periods usually come with very narrow distance tolerances between the scanning head and scale tape. However, several varieties of encoders provide ample mounting tolerances in spite of the small signal periods. Within the mounting tolerances, therefore, changes in the signal amplitude remain negligible.
HEIDENHAIN Corp.
www.heidenhain.com
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