Variable Reluctance Speed Sensors

Can VR speed sensors be used with nonferrous target materials?

Standard variable reluctance speed sensors require a ferrous metal target to function. The magnetic field generated by the sensor must be disrupted by a material that responds to it, like steel or iron. If your target is made from aluminum, stainless steel, brass, or plastic, a standard VR sensor will not produce a usable signal. In those cases, an inductive magnetic sensor, which relies on a magnet embedded in or near the target rather than in the sensor itself, is the appropriate alternative. We can help you determine the right sensing technology for your specific target material.

What is the typical air gap specification for a variable reluctance speed sensor?

Most VR speed sensor applications require an air gap of 0.005" to 0.015" between the sensor tip and the gear tooth or target surface. Wider gaps are possible depending on target velocity, mass, and the sensor's coil design, but the signal amplitude drops significantly as the gap increases. For OEM applications with nonconcentric shafts or high-speed targets, proper air gap specification is critical, and our engineering team accounts for this during the design phase to prevent signal loss in service.

What output does a VR sensor produce, and how is it processed?

Variable reluctance speed sensors produce an analog sinusoidal (AC) output signal. The frequency of that signal is directly proportional to the target's rotational speed, while its amplitude varies with speed and the air gap distance. In most control system integrations, this analog signal passes through a signal conditioner or analog-to-digital converter before the data is usable by a digital controller or monitoring system. Some applications use the raw sine wave directly with frequency counters or tachometers. We can recommend compatible signal conditioning approaches based on your system architecture.