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A magnetic speed sensor is similar to what is used to detect the speed of vehicles. Generally, a magnetic speed sensor uses a magnetic field to sense the approach of a ferrous metal target, which can be a gear tooth, fan blade, rotor blade, wheel spoke, etc.
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There are a number of different technologies used for a magnetic speed sensor. The most basic is the variable reluctance speed sensor, which will provide a sine wave output. However, there are also a number of options using more sophisticated technologies, such as a Hall Effect Sensor or a magnetoresistive sensor, which provide the user with a digital square wave output.
The signal amplitude will vary with how fast the target is moving, which is how the magnetic speed sensor is able to relay the speed.
Assembly
You can build your own magnetic speed sensor, however it's more convenient and reliable to purchase one. If you make one, you will need a variety of supplies, including a magnet, magnet wire, and a soldering gun. Assembling your own sensor is not recommended as there can be serious complications that can occur and will cause more frustration than anything. In addition, due to the size of most magnetic speed sensors, the magnet wire used is extremely fine and requires special tooling in order to work with it. Most high volume producers of magnetic speed sensors have automated this process, however for custom requirements it is often necessary and more efficient to build the sensor by hand. Training and skill is required to manipulate the wire and other components and achieve the desired outcome.
Uses
Magnetic speed sensors are typically incorporated into tachometers or used as gear tooth speed sensors. Industries that use a wide range of speed sensors include automotive, turbine flowmetering, aerospace, and rail and transit. Each industry has its own unique characteristics and requirements. Many times, speed sensors used in the automotive industry will be epoxy encapsulated, while those in the aerospace may be hermetically sealed for resistance to mositure and environmental conditions. In applications such as the rail and transit industry, the magnetic speed sensor is oftentimes mounted on the underside of the rail car, so it is designed with special cables and tubing to protect the sensor from damage.
Performance
There are specifications to consider when selecting a sensor in order to ensure that the desired signal strength is achieved. This will depend in part upon application considerations, not just the magnetic speed sensor, for example airgap (the distance between the sensing front and the target), target/gear geometry, number of teeth/targets, target material, and the speed at which it is moving. In addition, other criteria such as external noise sources which may interfere with the speed sensor output should be considered in order to achieve optimal performance.
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