Variable Reluctance Speed Sensors - VR Sensors

Principles of Operation – Variable Reluctance Speed Sensors

Variable Reluctance (VR) sensors offer a unique solution to many system measurement problems by providing an uncomplicated, accurate, reliable, versatile counting device.

A variable reluctance sensor is composed of a winding wound around a cylindrical magnetic material, typically made of some type of ferrous material that is referred to as a pole piece. A magnet is attached behind the pole piece, creating a magnetic field through the pole piece and winding. This magnetic field projects out from the pole piece front, also known as the sensor tip. When ferrous material passes through and disrupts this magnetic field, electricity (a sine wave) is generated. They are passive devices and do not require any external source of power to generate a signal.

Therefore, when the tip of the sensor is placed near a moving or rotating device made of ferrous metal such as a gear or rotor, a simple technique for measuring rotational speed is created. The frequency of the signal is directly proportional to the speed of rotation. The amplitude of the signal is affected by the speed of rotation, the material being sensed and the distance, known as the “air gap”, between the sensor tip and the rotating object.

Most manufacturers of magnetic sensors base their design on having very small air gaps of .005″ to .015″. Increasing the size of this gap produces a fall-off of signal voltage. In practical application, such small gaps could be hazardous, especially if the object being sensed is not concentric or tends to have a loss of concentricity with increased speed or motion. MSI’s Premium line of VR sensors employs a special “focusing tip” which enables the sensor to operate at larger gaps in excess of .060″. The closer the sensor is to the object being sensed, the greater the amplitude. MSI sensors, however, allow the sensor to be farther from the target without rapid signal deterioration as in other sensors.

Motion Sensors also manufactures a variation of variable reluctance sensors known as inductive magnetic sensors. These differ from standard VR sensors in that the magnet is not in the unit, but in the object being sensed. They are used when the rotating device sensed cannot be made of ferrous materials. For instance, if the target to be sensed is made of nonmagnetic stainless steel, brass, aluminum, or plastic, a centerpole magnet may be placed in the center of the rotating device. Alternatively, magnets can be imbedded (equally spaced) into the target’s outer rim or diameter. Inductive sensors will sense the magnetic fields and produce a signal. They are also designed to provide this capability at air gap distances substantially greater than standard VR sensors, as would be required for centerpole magnet locations.

In summary, variable reluctance sensors convert mechanical motion to electric energy without direct contact when positioned near a turning rotor, gear, shaft, or other regularly moving device. The output signal can be fed into a digital counter, totalizer, analog converter, tachometer, overspeed or underspeed control, oscilloscope, or other monitoring and control device. The sensor provides a simple, reliable, inexpensive transducer for highly sophisticated control systems.