Inductive and Hall Effect RPM Sensors

← Back to Auto Repair Articles

Inductive and Hall Effect RPM Sensors Explained

Figure 1. RPM Sensors

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

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

 

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

 

Diagnostics and Testing Procedures

Inductive Sensor

• Unplug the sensor and check that the electrical resistance of the inductive coil is roughly between 500 ohms and 1.500 ohms. If the reading value is drastically different, including zero or infinite, replace the sensor. NOTE: In some extreme cases, the lowest resistance can be about 200 ohms, as well as in some cases, the highest resistance can be up to 2.500 ohms.

Figure 4. Inductive sensor coil resistance testing

• Check the size of the air gap (G) between the sensor and the trigger wheel, G ≈ 0.8 – 1.5 mm (0.03 – 0.06 inch).

• Check the cleanliness of the sensor pin (sometimes may have cumulated metal turnings).

• Check the continuity and condition of the wires, connectors, terminals and the condition of the shielding.

• Unplug the sensor and check that there is an output AC voltage when cranking the engine (for engine rpm sensors) or when a wheel is rotated (for ABS wheel sensors). The output voltage signal could be expected between 1 V and 2 V (∼AC voltage) during the engine cranking for example, but in cases at higher rpm, can expected more. Also, this operation can be performed and when the connector of the sensor is plugged in.

Hall Effect Sensor

Figure 5. Hall sensor power supply testing

• Check the power supply to the sensor. The usual supplying voltage is 5 V  (in some cases can be 12 V).

• Check the size of the air gap (G) between the sensor and the trigger wheel, G ≈ 0.8 – 1.5 mm (0.03 – 0.06 inch).

• Check the continuity and condition of the wires, connectors and terminals.

• Check the cleanliness of the sensor pin (sometimes may have cumulated metal turnings).

• Check that there is an output signal when cranking the engine (for engine rpm sensors) or when a wheel is rotated (for ABS wheel sensors). NOTE: Unlike inductive sensors, at Hall sensors the connector must be plugged in, because is needed power supply for integrated electronic components, which are inside of the sensor.

Figure 6. Testing with LED lamp
For testing can be used: test LED lamp, electrical multimeter or oscilloscope. When is used test LED lamp, during the engine cranking, the LED should fast blinking according to the engine rpm, but in cases at higher rpm, the blinking is difficult to be follow. Then is better to use multimeter or oscilloscope for check the frequency as well as voltage of the signal.

Important advice: When testing the signal of a sensor, never use a test lamp with tungsten filament, may cause an extra current overload and produce damage of the sensor. It is recommended always to use some of the more sensitive tools, like test lamp with LED light or electrical multimeter for example.

If you have a trouble to find out where is sensor location, or to detect which type is, or how to test it, any time you can  contact us for additional assistance or advice.

Designed and Published by Kiril Mucevski

About 

More than 15 years experience in Automotive Engineering:

Diagnostics, Maintenance & Repair of Motor Vehicles
Technical Trainings of Road Patrolmen for Road Assistance
Tuning of Racing Vehicles, Engine Modification, Maintenance & Testing
Laboratory Researches in the field of Gasoline IC Engines:
Propulsion Fuels, Engine Oils & Additives,
Engine Adjustments & Power Performance
Technical Support & Sales of Tires & Alloy Wheels
Interest in New Automotive Technologies, Vehicle Systems & Equipment

Tags: , , , , , , , , , , , , , , , , , ,

Leave a Reply

Your email address will not be published. Required fields are marked *

Connect with Us