How Electronic Sensors Work on Cars

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Variable Reluctance Sensors

  • Variable reluctance sensors work on the principle of magnetic inductance and are used in a variety of applications. Wheel speed sensors, vehicle speed sensors, camshaft position sensors and crankshaft position sensors are all examples of the variable reluctance sensor.

    The sensor consists of a coil of wire wrapped around a permanent magnet, and a toothed tone ring turning with a moving part of the vehicle. The permanent magnet covers the coil in a magnetic field, and as long as the field is not broken, nothing happens. As a tooth of the tone ring passes through the magnetic field, the field collapses and this collapsing magnetic field induces positive voltage into the coil. As the valley between the teeth of the tone ring passes the magnet, the field builds, only to be collapsed by the next tooth on the tone ring. This collapsing and expanding of the magnetic field causes an alternating current to pass through the coil of the sensor and into the computer, where it is processed into a digital signal and used for speed and position calculations.

Variable Resistance Sensors

  • Other sensors function by varying their resistance and causing a voltage to drop across them. There are two types of variable resistance sensors: reostats, like the throttle position sensor, and vacuum sensors, like the manifold absolute pressure sensor.

    These all vary resistance, but each does it a little bit differently.

    The reostat uses a coil of wire and a wiper arm that changes position on the coil as it moves through its sweep. As the position is changed, resistance is changed across the coil. A constant 5-volt signal is applied to the coil, and as the resistance rises and falls with the sweep of the wiper, more or less voltage is dropped across the coil, and more or less voltage finds its way back to the computer. This is how a common TPS sensor works

    A vacuum sensor works in much the same way, except that instead of a coil and wiper, it uses a flexible crystal that varies the resistance. A constant voltage is applied, and as the sensor is exposed to engine vacuum, the resistance drops, allowing more voltage to return to the computer. Because this sensor relies on engine vacuum to operate, it makes a good engine load sensor and is used as a manifold absolute pressure sensor.

Magneto Resistive Sensors

  • The newest design of speed and position sensor is the magneto-resistive sensor. This sensor operates the same as the variable reluctance sensor, except that the magnetic field is provided by an electromagnet instead of a permanent magnet. There is also a built-in integrated circuit called a magneto resistive bridge that gives the computer a digital signal instead of an analog signal.

    Again, as with the variable reluctance sensor, when the tooth of the tone ring breaks the magnetic field, a voltage is induced. But this time, the signal is processed in the magneto resistive bridge and a digital square wave is sent to the computer. Calculations for speed or position are then made by the computer.

Oxygen Sensors

  • Oxygen sensors generate their own voltage through a chemical reaction between oxygen and the catalyst contained in the sensor.

    As the vehicle's engine runs rich, less oxygen is present in the exhaust, and less voltage is fed back to the computer. When the vehicle goes lean, more exhaust is present, and more voltage is fed back to the computer. The computer is always trying to keep the engine running at a perfect air/fuel ratio, so when it sees more voltage it recognizes a lean condition and adds fuel. If it sees lower voltage it recognizes a rich condition and subtracts fuel.

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