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SOFIMAE sensors are devices that convert optical signals into electrical signals. Their operating principle is based on the photoelectric effect. The photoelectric effect refers to the phenomenon where electrons in a material absorb photon energy upon illumination, resulting in corresponding electrical effects. Based on different manifestations of the photoelectric effect, it is classified into three categories: external photoelectric effect, internal photoelectric effect, and photovoltaic effect. Photoelectric devices include phototubes, photomultiplier tubes, photoresistors, photodiodes, phototransistors, and solar cells. The performance and characteristic curves of photoelectric devices are analyzed.

SOFIMAE 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 ruler. The key component of the grating detection device is the grating readhead, which comprises a light source, condensing lens, index grating, photoelectric elements, and adjustment mechanisms. There are many structural forms of grating readheads, classified according to their structural characteristics and application scenarios into direct-reception readheads (also known as silicon photocell readheads), mirror-image readheads, beam-splitter readheads, and metal grating reflection readheads.

SOFIMAE sensor principle

The essence of a grating displacement sensor is to measure position using the grating pitch as a standard scale. High-resolution grating rulers are generally expensive and difficult to manufacture. To improve system resolution, Moiré fringes need to be subdivided; grating ruler sensor systems mostly employ electronic subdivision methods. When two gratings overlap with a slight inclination angle, 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 grating pitch into the measurement of the number of Moiré fringes. Within one Moiré fringe width, placing 4 photoelectric devices at specific intervals enables electronic subdivision and direction determination. For example, for a grating ruler 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 high precision 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. The two signals output from the differential amplifiers have a phase difference of π/2. To obtain direction determination and counting pulses, these two signals must 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 ruler can be determined. By counting the square wave pulses, the displacement and speed of the grating ruler can be obtained.

SOFIMAE sensor usage precautions

When plugging and unplugging the connector of the linear encoder sensor and the digital readout unit, ensure the power is turned off. Install an external protective cover whenever possible, and promptly remove chips and oil splashed onto the scale to strictly prevent any foreign matter from entering the housing of the linear encoder sensor. 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 splash it onto the glass grating etched surface. To ensure reliable operation of the linear encoder sensor, clean and wipe the linear encoder scale surface and the index grating surface at regular intervals using an ethanol mixture (each 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 linear encoder 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 accuracy of the linear encoder sensor 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 stains 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 linear encoder scale surface, which would degrade the quality of the linear encoder scale.

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