Jul 19, 2023

What is the Difference of Single-field Scanning and Four-Field Scanning of Optical Scales?


Optical Linear Encoders are used for machine tools, automation techniques, measuring, and inspection equipment. Closed linear encoders prevent dust, debris, and splash liquid,  it is ideal for machines and systems operating in harsh operating environments associated with pollution. Due to their compact design, they are also very effective in direct drive and assembly automation. Optical Open Linear Encoders are used for fast, accurate machines and systems, such as production and measurement equipment in the semiconductor industry, super-precision machines, linear metering, direct-drive, and precision equipment.

The most important requirement of these applications is:

- High positioning precision 

- High mobile speed

- High machine availability 

- Fine-tuning speed control

Single-field scanning is characterized by significantly reducing the sensitivity to pollution and higher quality output signals.

Optoelectronic scanning

Measurement standards are regularly constructed by linear encoders according to the principle of photoelectric scanning. In the imaging scanning principle, structured rulers move relative to indexed gratings with the same or similar structure. Injected light is modulated: if the gap is aligned, the light passes through. If the line of one grating gap with the other, no light passes through. Photovoltaic cells convert, these changes convert light intensity into electrical signals.

Photoelectric scanning was performed according to the imaging principle

The optical system is essentially sensitive to all types of pollution. With the new single-field scanning principle, it can be decisively improved


Four-Field Scanning generates the signal


Photoelectric scanning was performed according to the imaging principle, with Four-Field Scanning


The scanning line has a scanning field whose grating offset each other by one-quarter of the grating period. This corresponding photovoltaic cell produces a sinusoidal signal, phase-shifting 90° (electric) to each other. These scanning signals are not at first with respect to zero-line symmetry. For this reason, the photovoltaic cell is connected in the push-pull circuit where the two output signals I 1 and I 2 are symmetrically relative to the zero line and an electric phase shift 90°.

A single-field scan generates a signal

The scan line has a large area grating with a slightly different period from the ruler. This produces an optical beat along the length of the scanning field: at some locations, lines remerge to let light through. In locations where the other lines and gaps overlap, resulting in shadows. In between, the gap is only partially covered. This leads to an optical filter that allows a shape very close to the uniform signal of the sinusoidal wave. A large-area, specially structured photoelectric sensor replaces a single photovoltaic cell to generate four 90° electric phase shift scanning signals.

Photoelectric scanning was performed according to the imaging principles, with single-field scanning


Imaging of light / dark field scanning markings and scale structure light sensors


Advantages of single-field scanning

Not sensitive to pollution

The large scanning area over the entire width of the grating ruler and the continuous arrangement of multiple scanning fields make the encoders of single-field scans extremely insensitive to contamination. Results pollution tests of the corresponding control demonstrate that the encoder continues to provide high-quality signals even if large area pollution is simulated. The position error is still well below the value specified for the encoder accuracy level.

In many cases, depending on the contamination, this even prevents encoder failure beyond four scans.

The example below shows the contamination affecting the output signal. The XY representation on the oscilloscope in which the signal forms a Lissajous graph. The ideal output signal is represented as a concentric inner circle. The deviation in the form and position of the circle causes a position error through a signal cycle (see measurement accuracy), so go directly into the measurement results. The size of the corresponding circle with the amplitude of the output signal can vary over a certain range without affecting the measurement accuracy. In encoders with single-field scans, however, only a small amplitude of change was seen.In the XY display, only slight diameter changes -- which is a determined sign of extremely low positional error. This type of contamination has a very significant effect on four scans: because the second scan involves fields, the XY display shows an extremely eccentric ellipse. This causes the encoder to completely fail at that position.

Effect of pollution on the output signals


Better electrical signal

In practice, the encoder output signal is distorted due to defects in manufacturing, assembly, and optical scanning operations, as well as the negative effects of changes in environmental conditions. The distortion of the signal leads to the SDE, which repeats the changes in the encoder raster at each cycle. Changes in the signal background level (UA_o ", UB_o") are usually caused by defects or contaminants in the encoder measurement scale. Different signal offsets may also be related to the improper adjustment of the electronic components. This distortion leads to an eccentric Lissajous curve, and this distortion causes an eccentric Lissajous curve, as shown in Figure (a) below. Due to uneven or inconsistent illumination of the photodetector, different peak amplitude of A and B signals (UA, UB), this error leads to an elliptic Lissajous curve as shown in Figure (b). The phase shift at the 90 ° electrical angle makes the curve elliptical, as shown in Figure (c), and the main cause of this error is the tilt between the grating of the scanning line and the main measuring ruler. All high harmonics caused by optical effects and electronic devices will make the signal, not perfect sinusoidal curves, and this type of error forms a non-circular Lissajous curve, as shown in Figure (d).


The Lissajous curve of optical encoder signal with relative segmentation error (SDE): (a) setting error of electronic components; (b) amplitude error; (c) phase shift error; (d) signal shape


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