Single field scanning linear grating ruler

Jingshan linear grating ruler is widely used as a position measurement system and measurement and detection equipment in machine tools, conveying equipment, and automation systems. The enclosed linear grating ruler is insensitive to dust, chips, and cutting fluid pollution, making it an ideal choice for machine tools and equipment working in heavily polluted environments. Due to its compact structure, it has become an effective measuring device for direct drive and automated assembly production lines. Open type linear grating ruler is suitable for high-speed and precision machine tools and systems, such as production and measurement equipment in the semiconductor industry, ultra precision machine tools and measuring machines, as well as precision measurement equipment for linear measurement and direct drive systems.

The most important requirements for these applications are:

·High positioning accuracy

·Fast movement speed

·High reliability of machine tools

·Precision adjustable speed control

The refined single field scanning linear grating ruler can meet these requirements. The prominent feature of single field scanning is its strong anti pollution ability and high output signal quality.

Photoelectric scanning

Like most Coffin encoders, the linear grating ruler also uses the working principle of a photoelectric scanning grating structure as a measurement reference. In the principle of imaging scanning, such as in the G-series enclosed linear grating ruler, the grating ruler and scanning grating move relative to each other, and the scanning grating has the same or similar grating structure as the grating ruler. Modulation of light beam: If the slit is aligned, the light passes through. If the engraved lines of one grating align with the slits of another grating, light cannot pass through. Photovoltaic cells convert these changes in light intensity into electrical signals.

Signal generation based on imaging scanning principle

The scanning method determines the quality of the output signal, and therefore also determines the positioning accuracy and supported motion speed. The characteristics of optical systems determine that they are insensitive to various types of pollution. If the latest single field scanning principle is adopted, it can significantly improve the positioning accuracy and motion speed. The diagram shows a comparison between single field scanning and four field scanning methods.

Generation of single field scanning output signal

The scanning mask has only one large-area grating, and its grating spacing is slightly different from that of the grating ruler. It generates light shots along the length direction of the scanning field: the grating lines overlap at some positions, allowing the light beam to pass through. The grating lines and slits at other positions overlap, and the light beam is obstructed. The slit between the above two positions only partially overlaps. This optical filtering effect forms a uniform and consistent signal with a shape very close to a sine waveform. It does not use multiple independent photovoltaic cells, but instead uses a large-area photovoltaic cell, especially a grid like structure of the photoreceptor, to generate four phase differences of 90. Scanning signal of electronic angle.

The image of the bright and dark fields of the scanning mask and grating ruler on the grating structure photoreceptor

Advantages of single field scanning

Not sensitive to pollution

The large-area scanning of the entire grating width and continuous scanning of multiple scanning fields make the single field scanning principle insensitive to pollution. The corresponding pollution testing also proves this point: even in simulations of larger pollution areas, the grating ruler can still provide high-quality output signals. The position error is much lower than the error value required for the accuracy level of the grating ruler. In most cases, based on the contamination situation, single field scanning can even avoid the failure of the grating ruler, which cannot be achieved by a four field scanning grating ruler.

This example demonstrates the impact of pollution on the output signal.

The XY coordinate display signal of the oscilloscope forms a Lissajous plot. The ideal output signal is a concentric circle. Deviation from the shape and position of the ideal circle is caused by positional errors within one signal cycle (see "measurement accuracy"), which directly affects the measurement results. The size of the circle corresponds to the amplitude of the output signal, and it can vary within a certain limit without affecting the measurement accuracy. People can find that the amplitude variation of the grating ruler using the single field scanning principle is very small. The x-coordinate shows a slight change in the diameter of the circle, indicating a small positional error. This pollution has a significant impact on the four scanning fields: because it involves two scanning fields, the XY coordinate display shows an ellipse with severe eccentricity. This will cause the grating ruler to completely fail at that position

Higher signal quality

The large-area scanning of the entire grating width and continuous scanning of multiple scanning fields make the single field scanning principle insensitive to pollution. The corresponding pollution testing also proves this point: even in simulations of larger pollution areas, the grating ruler can still provide high-quality output signals. The position error is much lower than the error value required for the accuracy level of the grating ruler. In most cases, based on the contamination situation, single field scanning can even avoid the failure of the grating ruler, which cannot be achieved by a four field scanning grating ruler.

The single field scanning principle can greatly avoid these errors: the large-area scanning field and the optical filtering principle of the special grid structure make the scanning signal generated very uniform and the sine waveform is very regular throughout the entire range of motion. Therefore, it can greatly reduce the position error of a single signal cycle. The high-quality output signal of single field scanning allows for higher electrical and mechanical motion speeds. Due to the slight influence of speed on signal amplitude, a stable output signal can ensure subdivision even at higher motion speeds.

This is clearly displayed in the XY coordinates of the oscilloscope+: the output signal of the single field scanning linear grating ruler shows a regular circular shape and low signal noise.

It is a prerequisite for achieving the following requirements

Small position error within a single signal cycle

● High quality direct drive control, i.e. controllability of speed control

● Higher and more precise

● High repeatability