512 Hall Effect Sensors

Figure 5.6 shows the principle of a Hall type sensor. The Hall element is a small section of semiconductor material such as silicon. When connected as shown in

Circuit 1

Circuit 1

Fig. 5.6 The principle of a Hall type sensor

Fig. 5.6(a), the battery will cause current to flow through the semiconductor Hall element and battery circuit, but there will be no current in the circuit which is at right angles to the battery circuit, as shown by a zero reading on the voltmeter.

When a magnetic field is imposed on the Hall element, as shown in Fig. 5.6(b), a current will flow in circuit 2. When the magnetic effect is prevented from reaching the Hall element, as in Fig. 5.6(c), the current will cease to flow in circuit 2. The result is that the current in circuit 2 can be switched on and off by shielding the Hall element from the magnetic field. When the metal plate that is inserted between the magnet and the Hall element is mounted on a rotating shaft, the Hall current can be switched on and off at any desired frequency. The Hall type sensor produces an output power that is virtually constant at all speeds. Hall effect sensors are used wherever other electromagnetic sensors are used, e.g. engine speed and crank position, ABS wheel sensors, camshaft (cylinder) identification (for ignition and fuelling) etc.

The voltage from a Hall element is quite small and it is common practice for Hall type sensors to incorporate an amplifying and pulse-shaping circuit. The result is that the sensor produces a digital signal, i.e. it is a rectangular waveform as shown in Fig. 5.7.

1 The upper horizontal lines should reach reference voltage.

2 Voltage transitions should be straight and vertical.

3 Peak-Peak voltages should equal reference voltage.

4 The lower horizontal lines should almost reach ground.

1 The upper horizontal lines should reach reference voltage.

2 Voltage transitions should be straight and vertical.

3 Peak-Peak voltages should equal reference voltage.

4 The lower horizontal lines should almost reach ground.

The duty cycle of the signal remains fixed, determined by the spacing between shutter blades.

Frequency of the signal increases as the speed of the engine increases.

Fig. 5.7 A Hall sensor output signal

5.2 Optical sensors

When light is directed onto semiconductor materials, energy is transferred to the semiconductor and this produces changes in the electrical behaviour of the semiconductor. This effect is used in optoelectronic devices, either as a photodiode, or as a phototransistor.

Figure 5.8 shows a vehicle speed sensor. The photocoupler consists of an infrared beam that is directed onto a photodiode. The infrared beam is interrupted

by the light-shielding rotor (chopper) that is driven by the speedometer drive. In this way the light-sensing element is switched on and off at a frequency that is related to speed.

Optical sensors may be used in any application where electromagnetic sensors are used. They are found in vehicle speed sensing, ignition systems, steering systems etc. These sensors require a power source, and the voltage pattern that is typical of the signal from this type of sensor is shown in Fig. 5.9.

1 The upper horizontal lines should reach reference voltage.

2 Voltage transitions should be straight and vertical.

3 Peak-Peak voltages should equal reference voltage.

4 The lower horizontal lines should almost reach ground.

1 The upper horizontal lines should reach reference voltage.

2 Voltage transitions should be straight and vertical.

3 Peak-Peak voltages should equal reference voltage.

4 The lower horizontal lines should almost reach ground.

Voltage drop to ground should not exceed 400 mV.

If the voltage drop is greater than 400 mV, look for a bad ground to the sensor or ECU.

Signal frequency increases as the speed of the vehicle increases. Fig. 5.9 Signal from an optoelectronic (light sensitive) sensor

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