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Rockwell sensors are measurement feedback devices that operate based on the optical principle of gratings. Grating scales are frequently used in the closed-loop servo systems of CNC machine tools for detecting linear or angular displacement. The measurement output signal is a digital pulse, characterized by a large detection range, high detection accuracy, and fast response speed. For example, in CNC machine tools, they are commonly used to detect the coordinates of tools and workpieces to observe and track tool path errors, thereby compensating for tool motion errors. Depending on the manufacturing method and optical principle, grating scales are divided into transmissive gratings and reflective gratings.

Rockwell sensor structure

It consists of two parts: the scale grating and the grating readhead. The figure on the right shows the structure of the grating scale. The key part of the grating detection device is the grating readhead, which consists of a light source, condensing lens, index grating, photoelectric elements, and adjustment mechanisms. There are many structural forms of grating readheads, which are classified according to their structural characteristics and application scenarios into direct-reception readheads (also known as silicon photovoltaic cell readheads), mirror-image readheads, beam-splitter readheads, and metal grating reflective readheads.

Rockwell sensor principle

The essence of a grating displacement sensor is to measure position using the grating pitch as a standard ruler. High-resolution grating scales are generally expensive and difficult to manufacture. To improve system resolution, Moiré fringes need to be subdivided; grating scale sensor systems mostly use electronic subdivision methods. When two gratings overlap with a slight inclination, Moiré fringes are generated in a direction roughly perpendicular to the grating lines. As the grating moves, the Moiré fringes move up and down accordingly. This converts the measurement of the grating pitch into the measurement of the number of Moiré fringes. Within the width of one Moiré fringe, placing 4 photoelectric devices at certain intervals enables electronic subdivision and direction determination. For example, for a grating scale with 50 line pairs/mm, the grating pitch is 0.02mm. If four-fold subdivision is used, counting pulses with a resolution of 5μm can be obtained, which achieves very high accuracy in general industrial measurement and control. Since displacement is a vector, both its magnitude and direction must be detected; therefore, at least two photoelectric signals with different phases are required. To eliminate common-mode interference, DC components, and even harmonics, differential amplifiers composed of low-drift operational amplifiers are typically used. The 4 photoelectric signals obtained from 4 photosensitive devices are sent to the input terminals of 2 differential amplifiers respectively. The two signals output from the differential amplifier have a phase difference of π/2. To obtain direction determination and counting pulses, these two signals need to be shaped, first into square waves with a duty cycle of 1:1. Then, by comparing the phase of the square waves, the movement direction of the grating scale can be determined. By counting the square wave pulses, the displacement and speed of the grating scale can be obtained.

Rockwell sensor usage precautions

When plugging and unplugging the linear encoder sensor and the digital readout connector, ensure the power is turned off. Install an external protective cover whenever possible, and promptly remove chips and oil that splash onto the scale to strictly prevent any foreign matter from entering the linear encoder sensor housing. Regularly check whether the mounting and connection screws are loose. To extend the service life of the dust-proof sealing strip, apply a thin, even layer of silicone oil to the strip, taking care not to let it splash onto the glass grating etched surface. To ensure the reliability of the linear encoder sensor, clean and wipe the linear encoder scale surface and the index grating surface at regular intervals with an ethanol mixture (50%), keeping the glass linear encoder scale surface clean. The linear encoder sensor must not be subjected to severe vibration or impact, as this may damage the linear encoder scale; if the scale fractures, the sensor will fail. Do not disassemble the linear encoder sensor yourself, and never arbitrarily change the relative spacing between the main scale and the secondary scale. Doing so may compromise the sensor's accuracy and cause relative friction between the main and secondary scales, damaging the chrome layer and thus the grating lines, which renders the linear encoder scale unusable. Take care to prevent oil and water from contaminating the linear encoder scale surface, as this can disrupt the distribution of the grating lines and cause measurement errors. Avoid operating the linear encoder sensor in environments with severe corrosive effects, to prevent corrosion of the grating chrome layer and the scale surface, which would degrade the quality of the linear encoder scale.

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