For technicians who use optical scales for the first time, often ask about the concept of the grating period, and it is easy to confuse the relationship between the grating scale period and the actual measurement resolution. The introductions of different optical scale brands also have a different emphasis. For example, HEIDENHAIN's selection mainly emphasizes the grating period and measurement step distance; PRECIZIKA and FAGOR samples mainly emphasize the resolution.
Therefore, to understand the optical scales, you should first distinguish the concept and internal relationship of the Grating period, the Signal Period, the Measuring step, and the Resolution.
The Grating Period (pitch) of the optical scales, we also call it the physical grating pitch. Grating refers to the measurement reference of a grating ruler or encoder, that is, a periodic grating. Optical encoders generally have two types of substrates, namely metal tape and glass tape. The grating is a series of stripes and slits made on a metal or glass ruler. The width of one stripe and one slit is called the grating pitch, and the common grating pitch is 20 μm and 40 μm.
For example, HEIDENHAIN has registered several trademarks for precision gratings, including:
- AURODUR： Etch lines on gold-coated steel strip, typical grating period 40 µm
- METALLUR： Contamination-resistant gold-plated wire, typical pitch 20 µm；
- DIADUR： Ultra-hard chrome wires on a glass substrate (typical pitch 20 µm) or three-dimensional chrome wire grid on a glass substrate (typical pitch 8 µm);
- SUPRADUR: Phase grating: optical three-dimensional flat grating lines; excellent anti-fouling ability; typical grating pitch does not exceed 8 µm;
- OPTODUR Phase Grating： Optical three-dimensional flat grating lines, ultra-high reflective performance, typical grating pitch does not exceed 2 µm.
Precision engraving machine equipment is required for the manufacture of grating masters. Whether the edges of the engraved grating lines are clear and uniform is the key to whether the reading head can output high-quality signals after photoelectric scanning. The grating quality of the grating scale is also an important factor determining the accuracy of the optical scales.
Signal Period: Every time the reading head scans a grating pitch, a sine wave or square wave signal period is generated, and this signal is subdivided by an electronic circuit (built-in or external subdivision box of the reading head), such as 5, 10, 50, 100 times the subdivision, so you can achieve very high resolution.
(signal period of sine cosine and square wave)
Measuring step (Messschritt) refers to the minimum distance that the system motion axis can move. For the 1VPP signal, the measuring step depends on the external subdivision signal. The number of measurement steps means that the position information is obtained by calculating the number of increments (measurement steps) from a certain point.
Resolution is the smallest physical motion that can be measured. It is defined as the distance of a single count. For linear encoders, the resolution is expressed in µm/count or nm/count; for rotary encoders, the resolution value is measured in counts/revolution, arc seconds/count, or microradians/count. The meaning of resolution is equivalent to the measurement step. It's just that different manufacturers have different usage habits for the concept. For example, one pass of the HEIDENHAIN system is expressed as the measuring step, and the expression of the PRECIZIKA system is the resolution.
The resolution can be calculated by the grating pitch and the subdivision multiplier. The subdivision multiplier here refers to the sum of the internal subdivision and the external electronic subdivision of the grating ruler. Generally, the resolution (measurement step) referred to by the manufacturer includes After the external 4-fold frequency subdivision in the subsequent electronic circuit, including PRECIZIKA, HEIDENHAIN, RSF, if there is no special instruction, the resolution includes the 4-fold subdivision in the subsequent electronic circuit. For example, the grating period of the grating ruler is 40μm, and the internal interpolation is 50 times, then its resolution is 40/(50*4)=0.2μm.
Therefore, when calculating the resolution of the grating ruler and measuring the step distance, you can use the method: resolution = grating period/sum of interpolation
The concepts mentioned above are the basic concepts of the grating scales. According to the parameter characteristics of the grating ruler, the function and performance of the grating ruler can be analyzed and determined. In the following articles, we will also introduce how these conceptual parameters play a role in the practical application of grating rulers from the perspective of practical application principles.