Overview
Speed sensors convert the rotational speed of an object into an electrical output. Common types include optical, eddy-current, magnetoresistive, Hall, capacitive, and proximity sensors. Magnetoresistive, eddy-current, and Hall speed sensors all use electromagnetic induction as their basic measurement principle and are classified as magnetic-based speed sensors. These sensors are widely used in industrial production, including power generation, automotive, aerospace, textile, and petrochemical applications, to measure and monitor the rotational speed of mechanical equipment for automation and control. Establishing reliable calibration methods for magnetic-based speed sensors is an important metrology task. This article explains the measurement principles of magnetic-based speed sensors.
Basic Measurement Modes
Both active and passive magnetic-based speed sensors operate non-contact. Magnetoresistive and eddy-current sensors detect rotating targets made of magnetic or magnetically permeable materials that have protrusions or notches. Their working principle is shown in Figure 1(a). The magnetoresistive type is based on the magnetoresistive effect, while the eddy-current type is based on eddy-current effects. As the speed wheel rotates, the gap between the gear and the sensor varies periodically, causing the magnetic flux to change periodically. The sensor then detects periodic pulse signals.
Hall sensors require a magnetic element to be mounted on the rotating object to alter the magnetic field near the sensor so the sensor can capture the motion. The working principle is shown in Figure 1(b). As the sensor experiences changes in magnetic flux density, a Hall voltage is generated in the sensing element when magnetic lines pass through it. Internal circuitry conditions and amplifies this signal and outputs pulse signals.
As the measured object rotates, the speed sensor outputs pulses corresponding to rotational speed (approximately sinusoidal or rectangular). Counting instruments convert those pulses into a displayed speed value.
1. Hall Sensors
1.1 Characteristics
Hall sensors are magnetic-based sensors that use a Hall element to convert a measured quantity into an electromotive force via the Hall effect. Hall elements are sensitive to magnetic fields in a static state and have simple structure, small size, low noise, wide frequency range (from DC to microwave), large dynamic range (output voltage variation up to 1000:1), and long lifetime, which makes them widely used.
1.2 Working Principle
A metal or semiconductor plate placed in a magnetic field generates an electromotive force perpendicular to both the current and the magnetic field when current flows through it. This physical phenomenon is the Hall effect.
The Hall voltage can be expressed by the standard Hall equation shown in the figure.
Hall sensors are used for direct and indirect detection. Direct applications measure the magnetic field or magnetic properties of the target itself. Indirect applications use an externally applied magnetic field on the target as an information carrier to convert many non-electrical physical quantities such as force, torque, pressure, stress, position, displacement, velocity, acceleration, angle, angular velocity, count, speed, and timing variations into electrical signals for detection and control.
1.3 Applications
With constant bias current and thickness, the Hall voltage is a function of magnetic flux density. Applications include gaussmeters for magnetic field strength measurement, Hall tachometers for speed measurement, counters for magnetic products, Hall encoders, and small-displacement-based Hall accelerometers and micropressure gauges.
With constant bias current and magnetic flux, the Hall voltage is a function of thickness, used in angular displacement instruments.
With constant thickness, the Hall voltage is proportional to the product of bias current and magnetic flux, which enables analog multipliers and Hall-based power meters.
1.4 Selection Considerations
1. Magnetic field measurement. For high magnetic field accuracy better than ±0.5%, gallium arsenide (GaAs) Hall elements are typically selected because of higher sensitivity (about 5–10 mV/100 mT), low temperature error, and compact size. For lower accuracy and less strict size requirements, silicon or germanium Hall elements are suitable.
2. Current measurement. Most Hall elements can be used for current measurement. For high accuracy, choose gallium arsenide Hall elements. For lower accuracy, GaAs, silicon, or germanium Hall elements can be used.
3. Speed and pulse measurement. For speed and pulse detection, integrated Hall switches and indium antimonide Hall elements are commonly used. Replacing motor brushes with indium antimonide Hall elements in video recorders and cameras has extended service life.
4. Signal processing and measurement. The Hall voltage is proportional to the control current and magnetic flux and varies with the sine of the angle between the magnetic flux and the Hall element surface. These characteristics are used to build function generators and, using the proportionality to the product of control current and magnetic flux, to make power meters and energy meters.
5. Tension and pressure measurement. Sensors built with Hall elements often offer better sensitivity and linearity than sensors made from other materials.
2. Magnetoresistive Effect Sensors
Magnetoresistive elements are similar to Hall elements, but operate based on the magnetoresistive effect (also called the Gauss effect) in semiconductor materials. The difference between magnetoresistive and Hall effects lies in the direction of the induced electrical response relative to current: the Hall voltage is a transverse voltage perpendicular to the current, while the magnetoresistive effect manifests as a change in resistance along the current direction.
A magnetoresistive element is placed in a magnetic field; when it moves relative to the field, element resistances R1 and R2 change. If connected in a bridge circuit, the output voltage is proportional to the resistance change.
The magnetoresistive effect depends on material properties and geometry. Materials with higher mobility exhibit stronger magnetoresistive effects. A smaller length-to-width ratio amplifies the effect.
Magnetoresistive elements can measure displacement, force, acceleration, magnetic field, and other parameters.
