MEMS and Sensors

VR vs. Hall Effect Speed Sensors

14 September 2018

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Magnetic speed sensors are a durable, efficient, reliable method of measuring the speed of a rotating object. They are used in a wide range of applications, are environmentally friendly and cost-effective. For many applications, deciding between a variable reluctance (VR) or Hall-effect type of magnetic speed sensor is the first and most critical decision to make. Each type has its advantages and disadvantages and is appropriate for particular applications. It is important to understand the respective technologies as well as the relative strengths and weaknesses of both in order to make an informed decision on which type of magnetic speed sensor is needed.

VR Speed Sensors

VR speed sensors consist of a coil of wire wrapped around a magnet. A varying magnetic flux is created when a ferrous target moves within proximity of the magnet. This varying magnetic flux induces a current in the coil, producing an analog signal. For speed applications, the ferrous target is usually a spinning wheel with geared teeth. As each gear tooth approaches and passes the face of the magnet, the magnetic flux increases. As a gear tooth moves away from the magnet, the magnetic flux decreases. The varying magnetic flux induces a proportional voltage change in the coil which in turn creates a sinusoidal analog signal. The analog signal is then sent to a signal processor to produce a digital output.

Hall-effect Speed Sensors

Hall-effect speed sensors work by exploiting the fact that, when in the presence of a magnetic field, the electrons in a current are deflected in a direction perpendicular to both the current and the magnetic field. This deflection, the Hall-effect, creates a voltage across the width of the conductor that can be measured to indicate the magnetic flux’s strength. Hall-effect speed sensors thus have a Hall-effect sensor positioned between a magnet and a ferrous target gear wheel. As the gear wheel turns the magnetic flux increases, creating an increasing Hall voltage. When the Hall voltage hits a particular threshold a signal is produced. It is important to note that unlike the VR speed sensor, the magnetic flux does not need to be varying in order to produce a signal. In addition, the generated signal is digital, so external signal conditioners are not required.

Speed Sensor Applications

Both types of sensors described above have advantages and disadvantages and are best suited for particular applications. The VR speed sensor, due to its simple design and external signal conditioning circuitry, tends to be less expensive and more durable. This type of sensor is better suited for high temperatures and harsh conditions because its simpler design allows it to accommodate the material restrictions such conditions require. In some cases, VR speed sensors can operate in temperatures as high as 300° C. VR sensors are better suited for high-speed applications since the signal it produces is proportional to the change in the magnetic flux. At slow speeds, this signal becomes noisy and difficult to condition. Particularly because the signal processing is being done at a distance from the sensor.

With Hall-effect speed sensors, the signal processing electronics are often integrated into the same package as the transducer, making the signals less susceptible to EMI interference. Furthermore, since the Hall-effect speed sensors are detecting the magnitude of the magnetic flux rather than its variation, it can detect targets moving at very slow speeds, or even the presence of nonmoving targets. With the signal processing electronics built in, the Hall-effect speed sensor outputs a digital signal that doesn’t require additional conditioning.

However, this does mean that an input voltage is required for operation, which is not the case for a VR speed sensor. With the integrated signal processing, Hall-effect speed sensors cannot operate at quite the extreme conditions that VR speed sensors can. However, Hall-effect speed sensors are fairly resilient in their own right, able to withstand temperatures of around 150° C.

An application that involves high EMI interference and low speeds, perhaps in the healthcare industry, would be well suited for a Hall-effect speed sensor. By contrast, the high speeds and extreme temperatures of a jet turbine are better suited for a VR speed sensor. Some applications are less clear, and it may be a good idea to talk over the intended application and operating conditions with a supplier so that they can assist.

Figure 1. Magnetic speed sensors detect changes in magnetic flux due to the proximity of ferrous teeth on a rotating gear to a magnet in the sensor. Source: Sensoronix Inc.Figure 1. Magnetic speed sensors detect changes in magnetic flux due to the proximity of ferrous teeth on a rotating gear to a magnet in the sensor. Source: Sensoronix Inc.

Conclusion

Magnetic speed sensors are an effective way of measuring rotational speed, particularly in harsh environments. Whether to choose a VR speed sensor or a Hall-effect sensor depends on the operating conditions and applications. Both types of magnetic speed sensors have their strengths and weaknesses that should be considered carefully when deciding on the technology for a particular application. In the cases where the distinction is less clear, consulting a qualified manufacturer is advised to guarantee that the most reliable yet cost-effective sensor is used.

Sensoronix, Inc., (ISO9001-AS9100) is a leading manufacturer of non-contact magnetic sensors with its headquarters in Irvine, Calif., offering a wide range of standard and customized magnetic sensor solutions providing for precise measurement of speed, direction, position and proximity. Sensoronix offers engineering and manufacturing services from prototype to production for low and high volume requirements. Sensoronix has provided high-quality sensors for a variety of applications in various industries ranging from aerospace and military to automotive, healthcare and industrial.

Sensoronix Key Differentiators:

- Over 30 years of experience in the design of high precision magnetic sensors

- Wide range of standard and customized solutions for military and aerospace

- Pioneer in development of Hall-effect technology for highly sensitive applications replacing VR technology

- Large global footprint in U.S, Europe, Asia, Australia and Latin America

- Emphasis on R&D for new and innovative designs

- Highly efficient engineering workflow management to ensure quality and efficiency

- Has maintained its reputation in the industry for high-quality sensor products and services with hundreds of customers globally

-Cost-effective solutions while maintaining highest level of quality

For more information, visit sensoronix.com.



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