Many techniques for measuring the speed of a rotating object are available on the market. In many applications - especially those operating in extreme environmental conditions - the result of selection usually falls on Hall effect speed sensors or variable reluctance (VR) speed sensors.

Speed sensor technology can be used in extreme heat and extreme cold conditions, as well as in dust, grease and other polluted environments. It is important to know which sensor is used in your application. It is important to understand how the Hall effect speed sensor and VR speed sensor work.

Both of these techniques work by rotating the object (target) in the magnetic field, interfering with the magnetic field, and the results are similar.

The basic working principle of the VR sensor VR sensor consists of a coil that passes through the electromagnet, and a gear tooth (or a convex portion of other target) passes through the surface of the magnet, causing a change in the amount of magnetic flux. When the convex portion of the target (for example, the teeth of the gear) moves closer to the sensor, the magnetic flux is maximum at this time. When the projection of the target separates, the magnetic flux drops. The result of the motion of the target causes the magnetic flux to change with time, which induces a proportional voltage in the coil, and the subsequent circuit (signal processing circuit) turns the input signal into a digital waveform (pulse), which makes it easier to count and time control.

Although the VR sensor is based on a very mature technology, it still has some major drawbacks. The first is the price, the coil and the magnet are relatively cheap, but unfortunately the need for additional signal processing circuits, the price is high.

In addition, the magnitude of the signal depending on the VR sensor is proportional to the speed of the target, which makes it difficult to design the circuit to adapt to very low speed signals. A given VR sensor has a definite limitation, that is, to what extent the motion of the target can still produce a useful signal.

In high temperature applications, VR sensors are superior to Hall effect sensors because the operating temperature is limited by the characteristics of the materials used in the device, and the VR sensor with the proper structure enables it to operate above 300 ¡ãC.

An example of an application is to detect the speed of the turbine in a jet engine. Hall effect sensor In the Hall effect speed sensor, when the speed measurement target is turned to the position of the Hall effect sensor, that is, the Hall sensor is located between the target and the magnet, the Hall effect sensor detects the magnetic flux change induced by the target.

Unlike variable reluctance sensors (VR), Hall effect sensors sense the magnitude of magnetic flux, while VR senses the rate of change of magnetic flux. Hall effect technology is characterized by the ability to detect the speed of moving objects at any slow speed. Another important feature of Hall-effect speed sensors is that signal processing circuits are often integrated into the same package, which provides some real-world benefits.

The most important thing is that no additional signal processing circuit is required. Most of the Hall speed sensors directly provide digital signal output, which is compatible with digital logic, ¦ÌC and PLC. An additional advantage is that the signal from the sensor does not have to go far away to the signal processing circuit - often only a few mils. This short distance reduces the interference from the interference source, making the Hall effect speed sensor excellently immune to EMI-induced faults and failures.

The Hall effect speed sensor cannot fully withstand the environmental extremes that VR can withstand. Many universal motion devices operate from -40 ¡ã C to 150 ¡ã C, which limits the application of Hall effect speed sensors. However, because the Hall speed sensor integrates signal processing circuits to measure near zero speed and digital signal output, Hall speed sensors are often the best choice for designers in speed detection applications.