Inductive and Hall Effect RPM Sensors Explained
RPM sensors in today’s vehicles, mainly are using for measuring the rpm and determining the position of crankshaft or camshaft at engine management systems, as well as measuring the speed (rpm) of the wheels at ABS systems, ESP systems, etc. The RPM sensors typically can be Hall or inductive type. The operation of these sensors is fundamentally similar in all instances, although the construction can vary depending on the type of sensor, its intended use or manufacturer application.
Inductive Sensor – Operating Principles and Specification
The inductive sensor, also known as magnetic pickup sensor, during the operational work, as result of inductive effect, in the sensor’s coil is producing the oscillating voltage, i.e. one kind of sinusoidal waveform signal (∼ AC voltage).
When the trigger wheel with the teeth passes in enough close distance (G) to the pole pin of the sensor, the magnetic field surrounding the coil is changed. As the result of the magnetic field changes, in the coil a voltage is induced, which is proportional to the strength and rate of change of the magnetic field. One complete oscillation is produced for each tooth that passes beside to the sensor pole pin. The basic integral components and the shape of the generated signal is shown in the figure 2.
Figure 2. Inductive sensor:
1. Sensor housing
2. Output signal wires
3. Coaxial coated protection
4. Permanent magnet
5. Inductive coil
6. Pole pin
7. Trigger wheel
G. Air gap
Depending upon the manufacturer application and type of the sensor, the electrical resistance of the coil is typically in the range between 500 ohms and 1.500 ohms. In some extreme cases, the lowest value can be about 200 ohms, as well as in some cases, the highest value can be up to 2.500 ohms.
The voltage signal produced by the sensor depends on the speed of the trigger wheel and the number of turns in the coil, so an output voltage could be expected between 1 V and 2 V during the engine cranking for example, but in cases at higher rpm, can expected more. The output voltage signal produced by the sensor is weak, i.e. low energy level, so could easily be degraded by other external stronger signals, such as the ignition system for example. For that reason, to eliminate the external influences, the signal wires from the sensor to the control unit are usually shielded with a coaxial coated wires type of protection.
Hall Effect Sensor – Operating Principles and Specification
Unlike inductive sensors, the output signal from a Hall effect sensor is not effected by the rate of change of the magnetic field. The produced output voltage typically is in the range of milli volts (mV) and is additionally amplified by integrated electronics, fitted inside of the sensor housing. On the figure 3 is shown typical build of a Hall Effect sensor. The final output voltage signal usually is in digital waveform pulses (square form). Depending upon the internal electronics of the sensor, the output signal of the sensor can be either positive or negative with peak voltage usually up to 5 V depending upon the type of the integrated electronics and requirements of the used system. The amplitude of the output signal remains constant, only the frequency increases proportionally with rpm. Unlike inductive sensors which generate a voltage signal by itself, the Hall Effect sensors must be additionally supplied by external voltage needed for integrated electronics. The usual supplying voltage (+Vcc) is mainly 5 V but in some cases can be 12 V.
Figure 3. Hall Effect sensor:
1. Sensor housing
2. Output wires (+Vcc, −Vcc and signal)
3. Integrated electronics
4. Permanent magnet
5. Hall Effect device
6. Trigger wheel
G. Air gap
