Sensors used in automobiles

A short introduction to the common types of sensors used in modern automobiles and their functions. Please note this is only an overview, and does not include ultramodern sensors used in high end cars.

1. SENSORS used in Automobiles BY: Siju K Abraham smbs vit University chennai

2. What is a sensor ? Device which detects or measures a physical property Records or indicates the value proportional to the physical property Responds to the physical property

3. What physical properties? Temperature Pressure / Force Velocity / Flow Acceleration Position / Angle / Displacement Current / Voltage Magnetic flux pH value / Chemical properties Light / Optical Properties Proximity / Presence Ionization / Radiation Acoustics / Vibration

4. Temperature sensors Thermocouple Two wires of different material joined at bot ends, both at different temperatures For high temperature applications like exhaust gas temperature and catalytic convertors Thermistors PTC / NTC Semiconductors used in ECT Resistance Temperature Detectors Positive Resistance Coefficient materials : Platinum Integrated Silicon Temperature sensors Ambient temperature monitoring, air conditioning Pyrometric Sensors Passenger / occupant detection, air conditioning, Disc and tire temperature Thermal Imaging Cameras Obstacle detection

5. Engine Coolant T emperature sensor (ECT) To measure engine temperature The readings from this sensor are then fed back to the Engine control unit (ECU), which uses this data to adjust the fuel injection and ignition timing . To control the cooling fan To provide data for temperature gauge on dashboard Cold engine requires different injection and ignition timings compared to hot conditions, in order to operate smoothly

6. Types of sensors Thermistors Negative temperature coefficient (NTC) sensors: - Internal resistance will decrease as the temperature rises (and vice versa) Positive temperature coefficient (PTC) sensor :- Resistance will increase with rising temperature. Most automotive coolant temperature sensors are NTC sensors.

7. Outside Air Temperature Sensor (OAT) RTDs or Integrated Silicon Temperature sensors (LM34 /LM35 etc ) Usually mounted inside or near the front bumper or in the side mirror Automatic Climate controller uses this temperature to help control the interior temperature of the vehicle Provides the vehicle occupants with the outside temperature readings

8. Exhaust Gas Temperature sensor EGT Thermocouple based sensor High temperature operation Directly measures the temperature of exhaust gas

9. Pressure sensors 1. Gravitational transducer a) Manometer b) Dead weight tester 2. Elastic transducer a) Bourdon tube pressure gauge b) Elastic diaphragm pressure gauge c) Bellows type pressure gauge 3. Electrical transducer a) Variable resistance type b) Strain gauge type c) Variable capacitance type d) LVDT type 4. Thermal transducer 5. Piezo - electric transducer

10. Manifold absolute pressure (MAP) sensor Used to sense the load on the engine Measures the amount of vacuum in the intake manifold Amount of intake manifold vacuum is directly related to engine load The ECU uses this information to calculate the correct  amount of fuel to inject, when to ignite (ignition timing retard or advance) and other functions.

11. Working Piezo resistive type A silicon chip is mounted inside a reference chamber . One side of the chip is the reference pressure (Calibrated pressure , usually vacuum) O n the other side is the pressure to be measured . The silicon chip changes  its resistance with the change in pressure. This change in resistance alters the voltage signal to the ECU ( 1V – 4.5V)

12. Tire pressure monitoring system (TPMS) employs capacitive or piezoresistive sensors to measure the pressure of tire directly. Some systems use vibration and sound sensors combined with the wheel speed sensor to monitor the tire pressure

13. Mass Air Flow Sensor (MAF) Used to find the mass flow rate of air entering the engine The MAF data is used by the ECU to deliver the correct fuel mass to the engine for providing the exact air fuel ratio required. The air density varies with the ambient temperature, altitude and the use of forced induction. Two types of MAF – Vane type and Hot wire The Vane type sensor measures the air flow into the engine with a spring-loaded air vane (flap/door) attached to a variable resistor (potentiometer). The vane moves in proportion to the airflow . A voltage is applied to the potentiometer and a proportional voltage appears on the output terminal of the potentiometer in proportion to the angle the vane rotates

14. Hot Air Mass Meter

15. To provide precise pilot control of the air-fuel ratio, it is essential for the supplied air mass to be exactly determined in the respective operating status. The wire's electrical resistance increases as the wire’s temperature increases, which varies the electrical current flowing through the circuit, according to Ohm's law. When air flows past the wire, the wire cools, decreasing its resistance, which in turn allows more current to flow through the circuit, since the supply voltage is a constant. As more current flows, the wire’s temperature increases until the resistance reaches equilibrium again. The current increase or decrease is proportional to the mass of air flowing past the wire

16. Hall Effect Sensor Varies its output voltage in response to a magnetic field W hen a current carrying conductor is placed in a magnetic field, the electrons will be deflected from a straight path Consequently, one plane of the conductor will become negatively charged and the opposite side will become positively charged The voltage between these planes is called Hall voltage This type of sensors are non contact sensors and are used in speed measurement of rotating parts, position sensing, etc

17. Torque sensor 1 Torsion rod (twisting area internal) 2 Input shaft (from the steering wheel) 3 Housing for the coil spring making the electrical connections 4 Sensor module with Magneto resistive sensor chip and signal amplification 5 Steering pinion/ balancer shaft 6 Magnetic multipole wheel

18. Passive Wheel Speed Sensor

19. Active wheel speed sensor

20. Working

21. Exhaust Gas Sensor – O 2 sensor Monitor the concentration of residual oxygen within the exhaust gases Reduce engine emissions Best economy and performance The ideal ratio of air to fuel to achieve complete combustion is 14.7:1 The term Lambda refers to the ratio of air to fuel; this chemically correct air/fuel ratio is known as a stoichiometric ratio or Lambda ( λ ) =1.0. A fuel rich mixture would have a lower value e.g. 0.8 and a fuel lean mixture would have a higher value e.g. 1.2. Lambda sensor works on closed-loop control system

22. Working

23. “Knocking” occurs when the air-fuel mixture self-ignites prematurely. Sustained knocking combustion causes damage primarily to the cylinder head gasket and cylinder head . Damage to pistons, rings, and exhaust valves can result if sustained heavy knock occurs The knock sensor is mounted on the crankcase and measures the structure-borne noise using a piezoelectric measuring element. Knocking is discernible by its higher sound frequencies. Knock Sensor

24. Working A piezo -electric element is translating the vibrations into an electric signal proportional to the acceleration Due to the vibration, a counter weight inside the sensor is applying pressure on the piezo element, this pressure creates an electric charge in the piezo element T his is the output signal of the sensor. For a knock application on a gasoline engine, the ECU adjusts the spark timing and on a diesel application the ECU controls the pilot injection quantity.

25. Throttle Position Sensor Senses the position of the butterfly valve in the throttle body Contact type – Potentiometer type Non contact type – Hall effect or inductive or magnetoresistive   A magnet/inductive loop is mounted on the throttle spindle    The change in the magnetic field is sensed by the sensor and the voltage generated is given as the input to the ECU

26. Crank Position sensor Senses the position of the crankshaft This is very important for adjusting the ignition and injection timing Non contact type sensor Hall effect sensor Crank angle encoder

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