Car Sensors: Classification and Application

Catalog

I Development History

In the 1960s, there were only oil pressure sensors, oil quantity sensors, and water temperature sensors in cars, which were connected to instruments or indicator lights.

After entering the 1970s, in order to control emissions, catalytic converters, electronic ignition, and fuel injection devices are required to maintain a certain air-fuel ratio. Therefore, some sensors were added to help control the car's power system. And in the 1980s, anti-lock braking systems and airbags improved vehicle safety.

Today, there are car sensors used for:

(1) Measuring the temperature and pressure of various fluids (such as intake air temperature, airway pressure, cooling water temperature, and fuel injection pressure, etc.),

(2) Determining the wheel speed and position of various parts (such as vehicle speed, throttle percentage, camshafts, crankshafts, transmission angles and speeds, positions of exhaust gas recirculation(EGR) valve, etc.).

(3) Measuring engine load, knocking, fire cutoff, and oxygen content in the exhaust gas.

(4) Determining seat position.

(5) Measuring wheel speed, road surface height difference, and tire pressure in anti-lock braking systems and suspension control devices.

Airbags for protecting front passengers require more impact sensors and acceleration sensors. As researchers use anti-collision sensors (ranging radar or other ranging sensors) to determine and control the lateral acceleration of the car, the instantaneous speed of each wheel, and the required torque, the braking system becomes a part of the vehicle stability control system.

Airbags

The old-style oil pressure sensor and water temperature sensor are independent of each other. Due to the clear maximum or minimum limit, some of the sensors on cars are equivalent to switches. As sensors develop in the direction of electronics and digitization, their output values will be more widely utilized.

II Classification and Application

Types of Sensors in Cars

Sensors for engine control systems are the core of the entire car sensors, which includes the auto temperature sensor, pressure sensor, position and speed sensor, flow sensor, gas sensor, and knock sensor. These car sensors could provide precise control of engine working conditions for the electronic control unit (ECU) to improve the dynamic property, reduce fuel consumption and exhaust emissions and perform fault detection.

ECU

1. Temperature Sensors

The car temperature sensor is mainly used to detect the engine temperature, intake gas temperature, cooling water temperature, fuel temperature, and catalytic temperature. There are three main types of vehicle temperature sensors, wire-wound resistor type, thermistor type, and thermocouple resistor type. The three types of sensors have their own characteristics, and their applications are slightly different.

(1) Wirewound resistor temperature sensors have high accuracy, but poor response characteristics.

(2) Thermistor temperature sensors have high sensitivity, good response characteristics, but have poor linearity and adapt to low temperatures;

(3) Thermocouple resistor temperature sensors have high accuracy and a wide temperature range for measurement, but they need to be used together with amplifier and cold-end treatment.

Car Coolant Temperature Sensor

2. Pressure Sensors

The automotive pressure sensor is mainly used to detect the negative pressure of the cylinder, atmospheric pressure, the step-up ratio of the turbine engine, the internal pressure of the cylinder, and the oil pressure. The suction negative pressure sensor is mainly used for the detection of suction air pressure, negative pressure, and oil pressure. There are many types of pressure sensors used in automobiles: capacitive, piezoresistive, linear variable differential transformer(LVDT), and surface elastic wave type (SAW).

(1) The capacitive pressure sensor is mainly used to detect negative pressure, hydraulic pressure, air pressure, measuring from 20-100kPa, with high input energy, good dynamic response characteristics, and good environmental adaptability.

(2) The piezoresistive pressure sensor is greatly affected by temperature, so a separate temperature compensation circuit is often needed, but it’s suitable for mass production.

(3) LVDT pressure sensor has a large output, easy for digital output, but has poor anti-interference ability.

(4) SAW pressure sensor has small size, lightweight, low power consumption and high reliability, high sensitivity, and high resolution, which is used to detect the pressure of the suction valve of the car and can work stably at high temperatures.

Pressure Sensor used in Automotives

3. Flow Sensors

The flow sensor is mainly used for the measurement of engine airflow and fuel flow. The measurement of airflow is used by the engine control system to determine the burning conditions, control the air-fuel ratio, start-up, ignition, etc. There are four types of airflow sensors: rotary vane type (blade type), Karman vortex type, hot wire type, and hot-film type.

(1) Rotary vane (vane) airflow sensor has a simple structure and low measurement accuracy. The measured flow needs temperature compensation.

(2) Karman vortex airflow sensor has no moving parts, which is sensitive and requires high accuracy and temperature compensation.

(3) Hot-wire airflow sensor has high measurement accuracy, no temperature compensation is required, but it is easily affected by gas pulsation and easy to break.

(4) Hot-film airflow sensor has the same measuring principle as the hot-wire airflow sensor, but the volume is small and is suitable for mass production at a low cost.

The main technical indicators of the airflow sensor are:

• working range: 0.11-103 m³/ min

• working temperature: -40 ℃-120 ℃

• accuracy ≤  1%

(5) The fuel flow sensor is used to detect the fuel flow, which mainly divided into the water-wheel type and a circulating ball type. Its technical indicators are:

• dynamic range: 0-60kg/h

• working temperature: -40℃-120℃

• Accuracy: 1%

• response time: <10ms

Mass Airflow Sensor

4. Position and Speed Sensors

Position and speed sensors are mainly used to detect crank angle, engine speed, throttle percentage, vehicle speed, etc. The car position sensor and speed sensor used in automobiles are mainly generator type, magneto-resistive type, Hall effect type, reed switch type, optical type, semiconductor magnetic transistor type, etc... The measurement range is 0 -360, the accuracy is less than 0.5, and the measured bending angle is 0.1.

There are many types of vehicle speed sensors, like those with the rotation of sensitive wheel rotation, sensitive power transmission rotation, and sensitive differential speed driven shaft. When the vehicle speed is higher than 100km/h, the general measurement method will have a large error, and a non-contact photoelectric speed sensor is required. The technical indicators are:

• speed range: 0.5-250km/h

• repeat accuracy: 0.1%

• distance measurement error: > 0.3%

Hall Effect Sensors

5. Gas Sensors

The gas sensor is mainly used to detect gas and exhaust emissions in the vehicle body. The most important one is the car oxygen sensor, practical ones include zirconia sensors (operating temperature -40℃-900℃, accuracy 1%), zirconia concentration battery gas sensors (operating temperature 300℃~800℃), solid Electrolyte zirconia gas sensors (operating temperature 0℃~400℃, accuracy 0.5%), and titanium dioxide car O₂ sensor. Compared with the zirconia sensor, the titanium dioxide oxygen sensor in the car has a simple structure, lightweight, low cost, and strong resistance to lead pollution.

Gas Sensor

6. Knock Sensors

The car knock sensor, or the car vibration sensor,  is used to detect the oscillation of the engine by adjusting the ignition advance angle to control and avoid engine knock.

We can detect the knocking by testing the cylinder pressure, engine body vibration, and combustion noise.

Knock sensors have the magnetostrictive type and piezoelectric type.

The operating temperature of the magnetostrictive detonation sensor is -40℃-125℃, and the frequency range is 5-10kHz.

The piezoelectric knock sensor has a sensitivity up to 200mV/g at the center frequency of 5.417khz and has good linearity at the amplitude of 0.1g-10g.

Knock Sensor

7. Radar Sensors

The 24GHz radar sensor for cars is used in the anti-collision installation system, which emits radar waves to judge the size, distance, and moving speed of objects in front to prevent collision with the use of the display and the braking system. The transmitting frequency of the sensor is about 24.125GHz, and the adjustable frequency range is about 50KHz.

Radar Sensor

8. Vehicle Body Control Sensors

The sensor for vehicle body control is mainly used to improve the safety, reliability, and comfort of automobiles. Because its working conditions are not as bad as those of the engine sensors and chassis, the general industrial sensors can be applied after a little improvement. This kind of automobile sensors can be mainly divided into 6 groups according to their applications:

(1) In automatic air conditioning systems, there are temperature sensors, humidity sensors, air volume sensors, and sunshine sensors.

(2) Acceleration sensors used in airbag systems

(3) Car speed sensor used in the door lock control

(4) Car light sensor used in automatic brightness control

(5) Car ultrasonic sensor or laser sensor used in reversing control

(6) Car distance sensor used to maintain a safe distance

(7) Image sensors used to eliminate driver's blind zone

(8) In the navigation system, the compass sensor, gyroscope, vehicle speed sensor, and steering wheel angle sensor are used to determine the driving direction of the car.

Other car sensors used in the vehicle body: anti-collision acceleration sensor, ultrasonic short-range sensor, and car infrared sensor, millimeter-wave radar, and ambient gas electrochemical sensor. The new type of sensors for cars includes car reverse sensor, side road deflection alarms, and infrared thermal imaging night vision sensor.

9. Car Sensors for Chassis Application

(1) In transmission control systems: vehicle speed sensors, accelerator pedal position sensors, acceleration sensors, car throttle position sensors, engine speed sensors, water temperature sensors, oil temperature sensors, etc.

(2) In suspension control systems: speed sensors in cars, car throttle sensor, acceleration sensor, body height sensor, steering wheel angle sensor, etc.

(3) In a power steering system: mainly include vehicle speed sensor, engine speed sensor, torque sensor, oil pressure sensor, and so on.

Digital Chassis Acceleration Sensors

III Detection of Car Sensors(Taking throttle position sensor as the discussing object )

The driver operates the throttle of the car with the accelerator pedal to change the air inflow of the engine, so as to control the operation of the engine. Different throttle percentage indicates different operating conditions of the engine.

1. Detection of the Throttle Switch

(1) Structure and Circuit

The throttle switch is also called the switch output type throttle position sensor. It has two pairs of contacts, idle contact (IDL) and full load contact (PSW). A cam coaxial with the throttle valve controls the opening and closing of the two switch contacts.

TPS Structure

When the throttle valve is in full close, the idle contact is closed, and the ECU determines that the engine is in idling mode according to the closing signal of the idle switch, so as to control the fuel injection quantity according to the requirements of the idling mode.

When the throttle valve is opened, the idle contact is open, the ECU performs fuel injection control during the transition from idle to small load based on this signal. The full load contact is always open from the full close to the small and medium opening of the throttle.

When the throttle is opened at a certain angle, the full-load contact starts to close, sending a signal to the ECU that the engine is in full-load operation, and the ECU performs full-load enrichment control based on this signal.

(2) Check and Adjustment

• Check the conductivity between the terminals in the car

Step 1: Set the ignition switch to the "OFF" position, remove the connector of the throttle position sensor, and insert a thickness gauge of appropriate thickness between the throttle stop screw and the limit rod;

Step 2: Connect a multimeter to the connector to test the conduction mode of the idle contacts and full load contacts.

When the throttle is fully closed, the idle contact should be turned on; when the throttle is fully opened or close to fully opened, the full load contact should be turned on; at other opening angles, neither of the contacts should not be turned on. The specific situation is shown in Table 1. Otherwise, the throttle position sensor should be adjusted or replaced.

Table 1.

2. Detection of Throttle Valve with Linear Variable Resistance Output

(1) Structure and Circuit

The throttle position sensor of the linear variable resistance type is a kind of linear potentiometer whose sliding contact is driven by the throttle shaft.

Under different throttle percentage, the resistance of the potentiometer is also different, so that the throttle percentage is converted into a voltage signal and sent to the ECU. Through the throttle position sensor, the ECU can obtain continuously changing voltage signals representing all opening angles of the throttle valve, and the rate of change of the throttle percentage, so as to more accurately determine the operating conditions of the engine. Generally, this type of throttle position sensor is also provided with an idling contact IDL to determine the idling operating conditions of the engine.

(2) Inspection and adjustment

• Conductivity Detection of Idle Contact

• Set the ignition switch to the "OFF" position, remove the wire connector of the throttle position sensor, and use a multimeter to measure the conductivity of the idling contact on the connector. When the throttle is fully closed, the resistance between the terminal IDL and E2 should be 0. When the throttle is open, the resistance between the two terminals should be ∞. Otherwise, the throttle position sensor should be replaced.

• Measure the resistance

Set the ignition switch to the OFF position, unplug the wire connector of the throttle position sensor, and measure the resistance of the linear potentiometer with the &Omega; gear of the multimeter. The resistance should increase linearly with the increase of the throttle percentage.

IV Development Trend of Car Sensors

The development trend of car sensor technology is miniaturization, multi-function, integration, and intelligentization.

Nowadays, the development of design technology, material technology, especially Mems (microelectronic mechanical system) technology brought micro-sensors to a new level. By using microelectronic mechanical processing technology, we can pack the micron-level sensitive components, signal processors, and data processing device on one chip, which has a small size, low price, and high reliability, and can significantly improve the system test accuracy. Besides, Mems technology can be used to make miniature sensors that detect mechanical, magnetic, thermal, chemical quantities and biomass. Because Mems miniature sensors can greatly reduce the cost of automotive electronic systems and improving their performance, they have begun to gradually replace sensors based on traditional electromechanical technology and will become an important part of the world's automotive electronics.

Mems auto sensors have low cost, good reliability, small size. They can be integrated into new systems, and work for millions of hours. The earliest Mems devices are absolute pressure sensors and airbag acceleration sensors. At present, the MEMS/MST products under development and small batch production are wheel speed rotation sensor, tire pressure sensor, refrigeration pressure sensor, engine oil pressure sensor, car brake sensor, and deviation rate sensor, etc. In the next 5-7 years, Mems devices will be widely used in automotive systems.

With the development of microelectronic technology and the rapid increase in the application of electronic control systems in automobiles, the market demand for car sensors will maintain rapid growth. The miniaturized, multifunctional, integrated, and intelligent sensors based on Mems technology will gradually replace traditional sensors and become the mainstream of the automotive sensor.

• Multifunctional means that one sensor can detect two or more characters or chemical parameters, thereby reducing the number of vehicle sensors and improving car sensor system reliability.

• Integration refers to the use of IC manufacturing technology and fine processing technology to make IC-type sensors.

• Intelligentization refers to the combination of sensors and large-scale integrated circuits with a CPU, which has an intelligent function to reduce the complexity of the ECU, reduce its volume and costs.

Mems automotive Sensors

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