Glossary

An optical linear or angle encoder basically consists of a scanning head and a measuring standard. The measurement of a linear or angular movement is executed via an optical process whereby the scanning head scans the measuring standard without contact. In the simplest case, increments (graduated lines) on the measuring standard are counted. The defined spacing between the increments enables the measured length to be derived—similar to a ruler.

With the incremental measuring method, the graduation consists of a periodic grating structure. The position information is obtained by counting the individual increments (measuring steps) from a chosen datum. Since an absolute reference is required to ascertain positions, the linear scales or scale tapes feature an additional track that bears a reference mark. This absolute position on the scale, established by means of the reference mark, is gated with exactly one measuring step. The reference mark must therefore be traversed before an absolute point of reference can be established or before the most recently selected reference point is refound.

With the absolute measuring method, the position value is available immediately upon switch-on of the encoder and can be requested at any time by the subsequent electronics. There is no need to move the axes to find the reference position. The absolute position information is read from the scale graduation, which is formed from either a serial absolute structure or from one incremental track and two absolute tracks. A separate incremental track or the track with the finest graduation structure is interpolated for the position value and at the same time is used to generate an incremental signal.

The measuring standard is a high-accuracy graduation with periodic arrangement of lines and gaps. A line and a gap together are called a grating period.

When scanning the scale grating, sinusoidal signals are generated whose period corresponds to one grating interval.

Number of grating intervals per revolution (angle of rotation measurement).

The sinusoidal measuring signals undergo n-fold subdivision depending on the desired subdivision factor and are converted into square-wave signals by an electronic circuit.

In metrology, the resolution describes the smallest distinguishable measuring step of a measuring system.

A measuring step is the smallest counting step that can be shown in the display unit depending on the grating interval and interpolation factor.

Reference marks serve to identify the count value at a certain position on the measuring length. A signal peak (reference pulse) is generated at this position.

A reference pulse is reproducibly output at exactly one counting step when the reference mark is traversed from either direction.

Accuracy is a crucial feature of an encoder, indicated by accuracy grades (e.g., ±1 µm/m). With exposed linear encoders, the definition of the accuracy grade applies only to the measuring standard.

The accuracy of the measurement is mainly determined by:

• The quality of the graduation
• The scanning quality
• The quality of the signal processing electronics
• The eccentricity of the graduation relative to the bearing
• The guideway error of the scale relative to the scanning unit

These factors of influence are comprised of encoder-specific error and application-dependent issues. All individual factors of influence must be considered in order to assess the attainable total accuracy.

Repeatability is the ability of a motion system to reliably attain a nominal position over many trials under identical conditions and during a short time interval.

The error that is specific to the encoder is stated in the specifications as the system accuracy.

The system accuracy includes the following:

• The homogeneity and period definition of the graduation
• The alignment of the graduation
• The error of the bearing
• The position error within one signal period

Hysteresis error is a deviation between the actual and commanded position caused by elasticity that has accumulated in the motion system. It affects the accuracy and bidirectional repeatability.

During this process, amplitude error, offset error, amplitude differences and phase deviations are cyclically detected and stabilized.

To attain the required resolution, the periodic analog signals A and B are subdivided further with interpolation methods. The interpolation processes operate without error as long as the two sinusoidal output signals are ideal and are electrically phase-shifted compared to each other by exactly 90°. Deviations generate errors that repeat themselves with each period of the scanning signals (signal period).

There are many factors critical for the size of the deviation, such as:

• The fineness of the signal period
• The homogeneity and period resolution of the graduation
• The quality of the scanning filter structures
• The characteristics of the sensors
• The stability and dynamics of the further processing of the analog signals

An Abbe error (tilt error) describes the deviation between a measured value and an actual value in linear metrology. This is a systematic error that can be compensated for by appropriate corrective action.

Degrees of freedom when mounting the scanning head.

The scanning gap describes the defined clearance between the scanning head and the measuring standard (in the Z axis). The gaps themselves as well as their tolerance ranges can vary depending on the encoder or design, and must be adhered to in order to ensure reliable functioning of the encoder in operation.

With exposed encoders, the two main components (i.e., the scanning head and the measuring standard) are supplied separately. They are not protected by an enclosing housing against external influences; however, this makes them much easier to integrate into applications.

These systems are therefore mainly used in clean environments, for example in electronics industry manufacturing areas, semiconductor technology or medical technology applications.

Kit encoders are modular systems in which different individual components can be assembled in a variety of designs. Customizations are also possible.

Such systems enable users to individually realize their own concepts and requirements for a measuring system according to their application. In the latest NUMERIK JENA product generation, the name of the respective product range has the addition "select" in the case of kit systems (e.g., "LIKselect").