German Harrow sensor Harrow model Harrow price

Harrow sensors are devices or units that can sense pressure signals and convert them into usable output electrical signals according to a specific pattern. Pressure sensors typically consist of a pressure-sensitive element and a signal processing unit. Depending on the type of test pressure, pressure sensors can be classified into gauge pressure sensors, differential pressure sensors, and absolute pressure sensors. Pressure sensors are the most commonly used sensors in industrial practice, widely applied in various industrial automation control environments, including water conservancy and hydropower, railway transportation, smart buildings, production process control, aerospace, military industry, petrochemicals, oil wells, electric power, ships, machine tools, pipelines, and many other industries.

Harrow 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. The key part of the grating detection device is the grating readhead, which is composed of a light source, condensing lens, index grating, photoelectric elements, and adjustment mechanisms. There are many structural forms of grating readheads, which are divided 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.

Harrow sensor principle

The essence of a grating displacement sensor is to measure position using the grating pitch as a standard ruler. High-resolution linear encoders are generally expensive and difficult to manufacture. To improve system resolution, Moiré fringes need to be subdivided; grating sensor systems mostly use electronic subdivision methods. When two gratings overlap with a slight 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 with 50 line pairs/mm, its 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, a differential amplifier composed of low-drift operational amplifiers is 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 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 can be determined. By counting the square wave pulses, the displacement and speed of the linear encoder can be obtained.

Harrow sensor usage precautions

When plugging and unplugging the connectors between the linear encoder 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. Strictly prevent any foreign matter from entering the housing of the linear encoder sensor. Regularly check whether the mounting screws are loose. To extend the service life of the dust-proof sealing strip, apply a thin, uniform layer of silicone oil to it, taking care not to splash it onto the glass grating etched surface. To ensure the reliability of the linear encoder sensor, clean and wipe the glass scale surface and the index grating surface at regular intervals using an ethanol mixture (50%), keeping the glass scale surface clean. The linear encoder sensor must not be subjected to severe vibration or impact, as this may damage the scale; if the scale breaks, the sensor will fail. Do not disassemble the linear encoder sensor yourself, and never arbitrarily change the relative gap 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, resulting in scrapping of the scale. Prevent oil stains and water from contaminating the scale surface to avoid disrupting the distribution of grating lines, which causes measurement errors. Avoid operating the linear encoder sensor in environments with severe corrosion to prevent corrosion of the chrome layer and the scale surface, which would degrade the quality of the scale.

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