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AGRIS sensors are devices that convert magnetic signals into electrical signals. By utilizing the transformation relationship between magnetic quantities and other physical quantities, with the magnetic field as a medium, they can also convert other non-electrical physical quantity signals into electrical signals. There are two driving methods: one is driven by a permanent magnet, and the other is driven by an electromagnetic coil. The former is mostly used for detection; for example, if a moving part is made of a magnet, it will attract and emit a signal once it approaches the magnetic induction sensor. The latter is mostly used for control; if the electromagnetic coil is energized, the contacts will attract. Using magnetic induction sensors to replace limit switches that rely on collision contact can improve system reliability and service life; therefore, they are often used in programmable logic controllers (PLCs) as signaling devices for position arrival.

AGRIS sensor structure

It consists of two parts: the scale grating and the grating readhead. The figure on the right shows the structure of the linear encoder 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 into direct-receiving readheads (also known as silicon photovoltaic cell readheads, mirror-image readheads, beam-splitter readheads, and metal grating reflective readheads) based on their structural characteristics and application scenarios.

AGRIS sensor principle

The essence of a grating displacement sensor is to measure position using the grating pitch as a standard ruler. High-resolution linear encoder scales are generally expensive and difficult to manufacture. To improve system resolution, Moiré fringes need to be subdivided; grating scale sensor systems mostly adopt 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 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 judgment functions. For example, for a linear encoder scale with 50 line pairs/mm, its grating pitch is 0.02mm. If four-fold subdivision is adopted, 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 usually employed. 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 amplifiers have a phase difference of π/2. To obtain direction judgment 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 linear encoder scale can be determined. By counting the square wave pulses, the displacement and speed of the linear encoder scale can be obtained.

AGRIS sensor usage precautions

When plugging and unplugging the connector between the linear encoder scale sensor and the digital display unit, the power should be turned off. Add a protective cover as much as possible, and promptly clean chips and oil splashed onto the scale to strictly prevent any foreign matter from entering the housing of the linear encoder scale sensor. Regularly check whether the installation and connection screws are loose. To extend the life of the dust-proof sealing strip, apply a thin layer of silicone oil evenly on the sealing strip, taking care not to splash it onto the glass grating etched surface. To ensure the reliability of the linear encoder scale sensor, clean and wipe the scale surface and index grating surface with an ethanol mixture (each 50%) at regular intervals to keep the glass linear encoder scale surface clean. Linear encoder scale sensors are strictly prohibited from severe vibration and dropping to avoid damaging the scale; if the scale breaks, the sensor will fail. Do not disassemble the linear encoder scale sensor yourself, and do not arbitrarily change the relative gap between the main scale and the secondary scale. Otherwise, it may damage the precision of the sensor on one hand, and cause relative friction between the main scale and the secondary scale on the other, damaging the chromium layer and thus the grating lines, resulting in the scrapping of the linear encoder scale. Care should be taken to prevent oil stains and water from contaminating the scale surface to avoid disrupting the distribution of grating lines and causing measurement errors. Linear encoder scale sensors should avoid working in environments with severe corrosion to prevent corrosion of the grating chromium layer and the scale surface, which would degrade the quality of the linear encoder scale.

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