Sensors

What are sensors?

In the broadest sense, a sensor is a measuring instrument.
Environmental influences such as temperature, moisture, light and gas, etc. are measured and converted into electrical signals.
This can then be recorded and interpreted by an analysis system (control unit).
The control units subsequently send signals to actuators in order to carry out any necessary adjustments or actions.

For example,
The temperature inside the vehicle is measured by a temperature sensor.
Control electronics interpret the measured temperature as too low and turn on the heating.

Sensoren – im Kraftfahrzeug

When used in motor vehicles, three main categories can generally be distinguished:

• Sensors in the drive train/motor
  The drive train or motor unit are a group of components in the motor vehicle which generate torque and deliver power to the driving wheels. In this area, sensors ensure that this process runs smoothly. Examples are exhaust sensors, lambda(oxygen) sensors and pressure sensors in the engine, etc.

• Safety sensors
  These sensors ensure the safety of the vehicle and its occupants. For example, an ABS speed sensor prevents the wheels from blocking when braking. Shock sensors make sure that the airbag is released in an accident, etc.

• Sensors for enhanced comfort
  Comfort sensors are responsible for ensuring the driver’s comfort. An example of this is a rain sensor.

This detects moisture on the windscreen so that a control device can switch on and control the windscreen wipers electronically.

What type of sensors exist in vehicles and how are they used?

This glossary elaborates on the sensors most regularly used in cars and their operating principles. To include all sensors that exist here would go beyond the scope available.

Optical sensors

Optical or optoelectrical sensors detect objects within a certain range by means of changes in light, and subsequently trigger various actions.
A few typical examples of optical sensors are listed below.

LiDAR (Light Detection and Ranging)

A LiDAR/laser sensor scans, measures and analyzes a vehicle by means of a rotating laser beam. The data recorded is used to generate a three-dimensional image of the vehicle’s surrounding environment. (Also see “LiDAR” in the glossary)

Rain sensors

Light diodes emit infrared light which is reflected onto the outer surface of the windscreen. The light is reflected back onto receptor diodes (photo diodes). If the surface is dry, the infrared light is reflected back onto the receptor diode at virtually full intensity. This is what is known as “total reflection”. However, when it rains, the beam of light is refracted by the water droplets and therefore only partially reflected. The difference between the signals provides the control electronics with information about the intensity of the rain so that they can then adjust the wiping frequency accordingly.

Cameras

The use of camera systems in vehicles is extremely varied:

• Front camera: Mono (1 lens) and stereo (2 lenses)
• Reversing camera
• 360° and blind spot detection (wing mirror)
• Night driving and infrared camera
• Interior camera

Camera systems can be used in combination with other sensors to considerably improve safety. The disadvantage of camera systems is that they cannot measure distances. Cameras record two-dimensional images of their surroundings.

Air quality sensors

Air quality sensors for the interior and external areas are becoming increasingly significant for automobile manufacturers. As development is clearly moving towards electromobility, the energy consumption factor is a major concern.
 A considerable part of the battery’s capacity is used to heat or cool the passenger area. However, if it can be ensured that the air in the passenger compartment can circulate as long as possible, and that fresh air is only supplied if the air quality deteriorates, this considerably reduces the energy requirements.
The measurement itself is carried out by an optical sensor which detects and analyzes particles of dirt.

Inductive/magnetic sensors

Inductive or magnetic sensors are mainly used for determining the vehicle’s position and orientation. These components are used to measure anything from a vehicle’s axle speed (ABS) and engine speed to its three-dimensional orientation (rollover sensor).
Hall sensors are often used here.

Capacitive sensors

The use of capacitive sensors is very varied. Acceleration sensors are a typical example in cars. Vibrations and accelerated movements in the car can be measured in this way. For instance, this is where the airbag crash sensor is used. A further example of a capacitive sensor is better known as a “kick sensor”. This ensures that the car boot opens or closes with a “kick” movement of the foot below the rear bumper.

Ultrasound sensors

Ultrasound or sonar (Sound Navigation And Ranging) is used to measure distances to objects in a short distance range (15 cm to 5.5 m max.).
It is mainly used as a parking aid. Pulsed ultrasound signals are emitted and reflected back onto an object, and the time that this takes (transit time) is measured. The distance to the object can now be calculated via this transit time.

Radar sensors

 A pulsed signal is sent out in this case too, and the transit time until the reflected signal is received is measured. Distance measurements over 160 m are possible here. Radar sensors have been used in premium vehicles for a long time now but are increasingly finding their way into the medium class segment too. A typical use of radar sensors can be found in adaptive cruise control. The distance to the vehicle in front is calculated using the radar sensor. In this way, the system can automatically maintain a constant safety distance. (Also see “radar” in the glossary)

Pressure sensors

Pressure sensors generally operate according to the differential pressure principle. Two chambers are hermetically separated from each other by a thin membrane. If there is a difference in pressure, the membrane deforms. This deformation is converted by measuring bridges into a proportional electric signal.

Typical uses for pressure sensors are:

• measuring pressure in gas recirculation systems
• detecting leaks in fuel systems
• cabin pressure
• detecting occupants
• etc.

Temperature sensors

Automotive temperature sensors are typically implemented with NTC thermistors which alter their resistance characteristics with temperature. This means that the higher the temperature, the better the current is conducted, i.e. resistance decreases with temperature.

Typical uses are:

• Engine temperature monitoring
• exhaust temperature monitoring
• air-conditioning system
• battery temperature monitoring
•  etc.

Moisture sensors

From the combined data provided by a moisture and a temperature sensor (e.g. in the base of the interior rearview mirror), the air conditioning control unit calculates the dew point temperature of the air, i.e. the temperature at which the air humidity would condense and cause the windscreen to mist up. This avoids condensation and ensures that the driver’s field of vision remains clear. In this application, a temperature and a moisture sensor are often combined in a single chip.

Current sensors

Current sensing is becoming increasingly important as e-mobility becomes more popular. It is implemented via shunts (low resistance measuring resistors) or magnetic field sensors. The measurement must be carried out bidirectionally as it must metrologically capture and cover both the motor’s and the generator’s operation (recuperation) in electric drives, as well as external charging.
The consumption and therefore the remaining range of an e-vehicle can be measured with the aid of current sensors. Current sensing also prevents overloading when charging the battery, thus ensuring a long battery life.

Conclusion

Without sensors, it would not be possible to operate a modern vehicle.
Every vehicle contains hundreds of these measuring devices. Due to the switch to electric drives, the type of sensors used in vehicles is going to change.
While those for the classic engine (e.g. exhaust gas recirculation) are steadily losing significance, sensors such as those for measuring current or air quality are gaining ground. There will also be a significant increase in all sensors required to implement ADAS and AD, such as LiDAR, radar and cameras.