STM switch German STM model STM price

STM switches are components that convert changes in non-electrical quantities (such as speed and pressure) into changes in electrical signals. Depending on the non-electrical quantity being converted, they can be classified as pressure switches, speed switches, temperature switches, etc., and serve as parts and accessories for measurement, control instruments, and equipment.

STM switch 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 component of the grating detection device is the grating readhead, which comprises a light source, a condenser lens, an index grating, photoelectric elements, and an adjustment mechanism. There are many structural forms of grating readheads. Based on the structural characteristics of the readhead and the application scenarios, they are classified into direct-receiving readheads (also known as silicon photovoltaic cell readheads, mirror-image readheads, beam-splitter readheads, and metal grating reflective readheads).

STM switch principle

The essence of a grating displacement measurement switch is to use the grating pitch as a standard ruler for position measurement. High-resolution linear encoder scales are generally expensive and difficult to manufacture. To improve system resolution, Moiré fringes need to be subdivided; grating scale switch systems mostly adopt electronic subdivision methods. When two gratings overlap with a slight inclination angle, Moiré fringes are generated in the 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 functions. For example, for a linear encoder scale with 50 line pairs per 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 typically used. The 4 photoelectric signal paths 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 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 linear encoder scale can be determined. By counting the square wave pulses, the displacement and speed of the linear encoder scale can be obtained.

STM switch usage precautions

When plugging or unplugging the connector of the linear encoder switch and the digital display 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 switch. Regularly check whether the mounting and connection screws are loose. To extend the service 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 engraved surface of the glass grating. To ensure the reliability of the linear encoder switch, clean and wipe the surface of the linear encoder scale and the index grating at regular intervals using an ethanol mixture (50%) to keep the glass grating scale surface clean. The linear encoder switch must not be subjected to severe vibration or impact, as this may damage the linear encoder scale; if the scale fractures, the linear encoder switch will fail. Do not disassemble the linear encoder switch yourself, and never arbitrarily change the relative spacing between the main grating scale and the secondary grating scale. Otherwise, this may compromise the accuracy of the linear encoder switch and cause relative friction between the main and secondary grating scales, damaging the chrome layer and thus the grating lines, resulting in scrapping of the linear encoder scale. Take precautions 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 switch 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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