51 Electromagnetic sensors

Electromagnetic sensors are often used to sense the speed and/or angular position of a rotating object. Two common uses are: (1) crankshaft position for ignition and fuel injection control; and (2) road wheel rotational speed relative to vehicle frame for anti-lock braking (ABS) and traction control (TCS). The interactions between electricity and magnetism are used in various ways to produce the desired sensing effect. However, there are two types of sensor that are widely used in vehicle systems: variable reluctance and Hall type sensors.

5.1.1 the variable reluctance type sensor

This type of sensor is used in many vehicle applications, such as ignition systems, engine speed sensors for fuelling, and wheel speed sensors for anti-lock braking etc. Air has a greater reluctance (resistance to magnetism) than iron and this fact is made use of in many sensors. The basic principle of operation of a variable reluctance type sensor (Fig. 5.1) may be understood from the following description.

The principal elements of the sensor are:

• a metallic path (the pole piece) for carrying the magnetic flux;

• a coil, wound around the metallic path, in which a voltage is induced.

The reluctor disc has a number of tabs on it and these tabs are made to move through the air gap in the magnetic circuit. The movement of the reluctor tabs, through the air gap is achieved by rotation of the reluctor shaft. The voltage induced in the sensor coil is related to the rate of change of magnetic flux in the magnetic circuit. The faster the rate of change of magnetic flux the larger will be the voltage that is generated in the sensor coil. When the metal tab on the reluctor rotor is outside the air gap, the sensor voltage is zero. As the tab moves into the air gap the flow of magnetism (flux) increases rapidly. This causes the sensor voltage to increase, quite quickly, to a maximum positive value. Figure 5.2 shows the approximate behaviour of the voltage output as the reluctor is rotated.

Figure 5.2(a) shows the reluctor tab moving into the air gap. As the metal tab moves further into the gap the voltage begins to fall and, when the metal tab is exactly aligned with the pole piece, the sensor voltage falls back to zero.

(Although the magnetic flux is strongest at this point, it is not changing and this means that the voltage is zero.) Figure 5.2(b) shows that there is zero voltage when the reluctor tab is in alignment with the pole piece. As the metal tab continues to rotate out of the air gap and away from the pole piece, the rate of change of the magnetic flux is rapid, but opposite in direction to when the tab was moving into the air gap. This results in the negative half of the voltage waveform as shown in Fig. 5.2(c). When the tab has moved out of the air gap the sensor voltage returns to zero. While the rotor shaft continues to turn another tab will enter the air gap and the above process will be repeated. If the sensor coil is connected to an oscilloscope the pattern observed will be similar to that shown in Fig. 5.2(d).

Crankshaft position sensor

Figure 5.3 shows a crankshaft sensor. Here the reluctor disc is attached to the engine flywheel. The permanent magnet, the pole piece and the sensor coil are attached to the cylinder block. As each metal tab on the reluctor disc passes the sensor pole piece a voltage is induced in the sensor winding.

The size of this voltage, induced in the sensor winding, depends on engine speed; the faster the engine speed the higher the sensor voltage. Each time a reluctor passes the pole piece an alternating current waveform is produced and at high engine speed the voltage produced by the sensor can be of the order of 100 V and some sensor circuits are designed to restrict the maximum voltage. In order to provide a top dead center (TDC) reference, there is a missing tab on the reluctor disc which means that the TDC position is marked by the absence of a voltage and this 'gap' is used to indicate to the ECM that the TDC position has been reached. The voltage waveform to be expected from this type of sensor is

Fig. 5.3 Variable reluctance crank speed and position sensor
Fig. 5.4 A crank sensor voltage pattern

shown in Fig. 5.4. The missing wave at the TDC position is evident at the left-hand side of the pattern.

ABS wheel sensor

The principle of operation of many ABS wheel sensors is the same as for the crank sensor. However, the purpose for which it is used is somewhat different. To obtain the most effective braking, and to allow the driver to retain control of the vehicle, the wheels should not lock up under braking. The ABS sensor is used to assess slip between the tyre and the surface on which the tyre is working. The purpose of the ABS sensors is to detect when wheel lock-up is about to occur. This condition is indicated when the rotational speed of the reluctor ring (sensor rotor) is slow in relation to the sensor pick-up, which is fixed to the brake back plate, or equivalent. The layout of the sensor and its voltage waveform is shown in Fig. 5.5.

Wheel sensor output Fig. 5.5 ABS wheel speed sensor layout and voltage waveform

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