A variable reluctance sensor (commonly called a VR sensor) is a

transducer A transducer is a device that converts energy from one form to another. Usually a transducer converts a signal in one form of energy to a signal in another. Transducers are often employed at the boundaries of automation, measurement, and contr ...

that measures changes in

magnetic reluctance Magnetic reluctance, or magnetic resistance, is a concept used in the analysis of magnetic circuits. It is defined as the ratio of magnetomotive force (mmf) to magnetic flux. It represents the opposition to magnetic flux, and depends on the geom ...

. When combined with basic electronic circuitry, the sensor detects the change in presence or proximity of ferrous objects. With more complex circuitry and the addition of software and specific mechanical hardware, a VR sensor can also provide measurements of linear velocity,

angular velocity In physics, angular velocity or rotational velocity ( or ), also known as angular frequency vector,(UP1) is a pseudovector representation of how fast the angular position or orientation of an object changes with time (i.e. how quickly an objec ...

, position, and torque.

Uses and applications

A VR sensor used as a simple

proximity sensor A proximity sensor is a sensor able to detect the presence of nearby objects without any physical contact. A proximity sensor often emits an electromagnetic field or a beam of electromagnetic radiation (infrared, for instance), and looks for ...

can determine the position of a mechanical link in a piece of industrial equipment. A

crankshaft position sensor A crank sensor (CKP) is an electronic device used in an internal combustion engine, both petrol and diesel, to monitor the position or rotational speed of the crankshaft. This information is used by engine management system An engine contro ...

(in an automobile engine) is used to provide the angular position of the crankshaft to the

engine control unit An engine control unit (ECU), also commonly called an engine control module (ECM), is a type of electronic control unit that controls a series of actuators on an internal combustion engine to ensure optimal engine performance. It does this by re ...

. The engine control unit can then calculate engine speed (

). Speed sensors used in automobile transmissions are used to measure the rotational speed (angular velocity) of shafts within the transmission. The engine control unit or transmission control unit (depending on the particular automobile) uses these sensors to determine when to shift from one gear to the next. A pickup used in an electric guitar (or other musical instrument) detect vibrations of the metallic "strings". See

Pickup (music technology) A pickup is a transducer that captures or senses mechanical vibrations produced by musical instruments, particularly stringed instruments such as the electric guitar, and converts these to an electrical signal that is amplified using an instru ...

for details of this application.

Construction

This sensor consists of a

permanent magnet A magnet is a material or object that produces a magnetic field. This magnetic field is invisible but is responsible for the most notable property of a magnet: a force that pulls on other ferromagnetic materials, such as iron, steel, nickel, ...

, a

ferromagnetic Ferromagnetism is a property of certain materials (such as iron) which results in a large observed magnetic permeability, and in many cases a large magnetic coercivity allowing the material to form a permanent magnet. Ferromagnetic materials ...

pole piece, and coil of wire.

Interface circuits

VR sensors need waveform shaping for their output to be digitally readable. The normal output of a VR sensor is an analog signal, shaped much like a sine wave. The frequency and amplitude of the analog signal is proportional to the target's velocity. This waveform needs to be squared up, and flattened off by a comparator like electronic chip to be digitally readable. While discrete VR sensor interface circuits can be implemented, the semiconductor industry also offers integrated solutions. Examples are th
MAX9924 to MAX9927
VR sensor interface IC from Maxim Integrated products
LM1815
VR sensor amplifier from National Semiconductor an
NCV1124
from ON semiconductor. An integrated VR sensor interface circuit like the MAX9924 features a differential input stage to provide enhanced noise immunity, Precision Amplifier and Comparator with user enabled Internal Adaptive Peak Threshold or user programmed external threshold to provide a wide dynamic range and zero-crossing detection circuit to provide accurate phase Information.

Reluctor rings

To measure angular position or rotational speed of a shaft, a toothed ring made of ferrous material can be attached to the shaft. As the teeth of the rotating wheel (or other target features) pass by the face of the magnet, the amount of magnetic flux passing through the magnet and consequently the coil varies. When the gear tooth is close to the sensor, the flux is at a maximum. When the tooth is further away, the flux drops off. The moving target results in a time-varying flux that induces a proportional voltage in the coil. Subsequent electronics are then used to process this signal to get a waveform that can be more readily counted and timed. This system has been employed in automotive electronic ignition and

ABS braking An anti-lock braking system (ABS) is a safety anti-skid braking system used on aircraft and on land vehicles, such as cars, motorcycles, trucks, and buses. ABS operates by preventing the wheels from locking up during braking, thereby maintaini ...

. By attaching two reluctor rings to a shaft, the torque can be measured. The tooth spacing on reluctor rings may be uniform, or uneven.{{cite web , title=What Is a Reluctor Ring? , url=https://www.reference.com/vehicles/reluctor-ring-87cd9b724e8b3475# , website=Reference , publisher=IAC Publishing, LLC , access-date=7 September 2018 , language=en

Advantages and disadvantages

Although VR sensors are based on very

mature technology A mature technology is a technology that has been in use for long enough that most of its initial faults and inherent problems have been removed or reduced by further development. In some contexts, it may also refer to technology that has not se ...

, they still offer several significant advantages. The first is low cost - coils of wire and magnets are relatively inexpensive. Unfortunately, the low cost of the transducer is partially offset by the cost of the additional signal-processing circuitry needed to recover a useful signal. And because the magnitude of the signal developed by the VR sensor is proportional to target speed, it is difficult to design circuitry to accommodate very-low-speed signals. A given VR-sensing system has a definite limit as to how slow the target can move and still develop a usable signal. An alternative but more expensive technology is

Hall effect sensor A Hall effect sensor (or simply Hall sensor) is a type of sensor which detects the presence and magnitude of a magnetic field using the Hall effect. The output voltage of a Hall sensor is directly proportional to the strength of the field. ...

. Hall effect sensors are true zero-rpm sensors and actively supply information even when there's no transmission motion at all. One area in which VR sensors excel, however, is in high-temperature applications. Because

operating temperature An operating temperature is the allowable temperature range of the local ambient environment at which an electrical or mechanical device operates. The device will operate effectively within a specified temperature range which varies based on the de ...

is limited by the characteristics of the materials used in the device, with appropriate construction VR sensors can be made to operate at temperatures in excess of 300 °C. An example of such an extreme application is sensing the turbine speed of a jet engine or engine cam shaft and crankshaft position control in an automobile.

References

External links

Automotive technologies