German KROHNERT sensors KROHNERT models KROHNERT prices

KROHNERT 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 the 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.

KROHNERT 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. Grating readheads come in many structural forms, classified based on 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).

KROHNERT 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 must be subdivided; grating ruler sensor systems mostly employ 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 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. With four-fold subdivision, counting pulses with a resolution of 5μm can be obtained, achieving 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, a differential amplifier composed of low-drift operational amplifiers is typically used. The 4 photoelectric signal paths obtained from 4 photosensitive devices are fed into the input terminals of 2 differential amplifiers. The two output signals from the differential amplifier 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 velocity of the grating ruler can be obtained.

KROHNERT sensor usage precautions

When plugging or unplugging the connector between the grating ruler sensor and the digital display unit, ensure the power is turned off. Install an external protective cover whenever possible, and promptly clean chips and oil splashed onto the scale to strictly prevent any foreign matter from entering the grating sensor housing. Regularly check whether installation screws are loose. To extend the life of the dust-proof sealing strip, apply a thin, uniform layer of silicone oil to the strip, taking care not to splash it onto the glass grating ruling surface. To ensure reliable operation of the grating ruler sensor, clean and wipe the grating ruler surface and index grating surface periodically with an ethanol mixture (50%) to keep the glass grating surface clean. Grating ruler sensors must not be subjected to severe vibration or impact, as this may damage the grating ruler; if the grating ruler breaks, the sensor will fail. Do not disassemble the grating ruler sensor yourself, and never arbitrarily change the relative gap between the main grating and the secondary grating. Doing so may compromise the sensor's accuracy and cause relative friction between the main and secondary gratings, damaging the chrome layer and thus the grating lines, rendering the grating ruler unusable. Take care to prevent oil and water contamination of the grating surface, as this can disrupt the grating line distribution and cause measurement errors. Avoid operating grating ruler sensors in highly corrosive environments to prevent corrosion of the grating chrome layer and surface, which would degrade quality.

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