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Understanding Automotive Electronics 5E Episode 8

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Tham khảo tài liệu 'understanding automotive electronics 5e episode 8', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | SENSORS AND ACTUATORS 6 The voltage induced in the sensing coil varies with the rate of change of the magnetic flux. When the tab is centered between the poles of the magnet the voltage is zero because the flux is not changing. reaches a maximum when the tab is exactly between the pole pieces and then decreases as the tab passes out of the pole piece region. In most control systems the position of maximum magnetic flux has a fixed relationship to TDC for one of the cylinders. The change in magnetic flux induces a voltage V in the sensing coil that is proportional to the rate ofchange of the magnetic flux. Since the magnetic flux must be changing to induce a voltage in the sensing coil its output voltage is zero whenever the engine is not running regardless of the position of the crankshaft. This is a serious disadvantage for this type of sensor because the engine timing cannot be set statically. As shown in Figure 6.8 the coil voltage Vo begins to increase from zero as a tab begins to pass between the pole pieces reaches a maximum then falls to zero when the tab is exactly between the pole pieces see Figure 6.8a . Note that although the value of magnetic flux is maximum at this point the rate of change of magnetic flux is zero therefore the induced voltage in the sensing coil is zero. Then it increases with the opposite polarity reaches a maximum and falls to zero as the tab passes out of the gap between the pole pieces. The coil voltage waveform shown in Figure 6.8b occurs each time one of the Figure 6.8 Output Voltage Waveform from the Magnetic Reluctance Crankshaft Position Sensor Coil UNDERSTANDING AUTOMOTIVE ELECTRONICS 199 6 SENSORS AND ACTUATORS Engine speed can be calculated in a number of ways. Digital circuits use counters and crankshaft sensors to calculate actual engine speed. cylinders reaches TDC on its power stroke. It should be noted that if the disk is mounted on the crankshaft then the number of tabs for this crankshaft position sensor always will