Ever wondered why your laptop turns into sleep when you close it?
It is because of magnetic sensors! There is a magnet on the screen-side, and a sensor on the keyboard-side. When they get close, the sensor feels it and turns a switch.
Picture showing how a sensor on the keyboard-side of the device can be used to detect a magnet on the screen-side, and use the output to switch the power on/off. Â Source: Toshiba
Following last weekÂŽs post on the use of magnets in motors, this post will be about the magnetic sensors we find in cars.
Lets start at the beginning. There are numerous types of magnetic sensors, and they can be divided into three classes, depending on how they are used in relation to the ever-present magnetic field of the Earth.
There are the sensors for High sensitivity (that are highly sensitive to the magnetic field of the Earth) called MAD, which can detect ferromagnetic objects from long distances. These sensors are used to detect mines, ships and airplanes etc., and are based on the fact that the magnetic fields of ferromagnetic materials (its dipole moment) will distort the field lines of the ambient magnetic field, or create anomalies, why MAD is short for Magnetic Anomaly Detection. See picture below.
Picture showing how MAD (Magnetic Anomaly Detection) works. The dipole moment from ferromagnetic materials will distort the ambient magnet field lines.Â Source: Lenz, 2006.
Then there are the sensors of Medium sensitivity that are working with (sensing) the Earths magnetic field, and they are known as compasses.
Finally, we have the sensors that are not very sensitive, and it is in this segment we find the two types of magnetic sensors that dominate in cars.
2 main types: The Hall sensor, and the AMR sensor.
The Hall sensor contains a conducting material through which a current is applied, and the voltage across the material is measured. When a magnetic field acts on the system, the voltage will change (the Hall Effect), and indicate the strength of the magnetic field.
AMR is short for Anisotropic Magneto Resistivity, which is the basis for this type of sensor. The main constituent is a Permalloy (80 % Nickel, 20 % Iron) thin film deposited onto a silicon wafer. The films properties are such that the resistivity can change 2-3 % in the presence of a magnetic field. As with the Hall sensor, the voltage (= resistance x current) is measured to indicate the strength of the magnetic field.Â
Both these types of sensors are of simple design, cheap to manufacture. There are differences; for example that Hall sensor detects fields that are perpendicular to the current, while AMR senses parallel fields. The AMR has a wider detection area, and are faster, but Hall sensors are still cheaper, and can be smaller (down to a couple of millimetres) why they are more popular. With that said, they are still used for similar applications.
Now, lets focus on the cars again and how these sensors are used in them. We start with the wheel speed sensor.
Connected to the wheels of your car, there is a wheel speed sensor that measures the rotation speed of the pulse wheel (a wheel that is connected to the actual driving wheel, so it has the same speed). A sensor – often a Hall sensor with an incorporated magnet â is positioned so that the magnetic field from the magnet âcoversâ the cogged pulse wheel. Since the cogged pulse wheel is ferromagnetic, it will change the magnetic field depending on the cog is up or down, and thus also change the output voltage of the sensor. A microprocessor then keeps track of how many ups and downs there has been per time unit, and calculates the speed from that. See picture below.
Picture showing the graph for the output voltage from a Hall sensor next to the Pulse wheel.Â Source: Hella
Wheel speed sensors improve comfort and safety
Apart from telling you the speed of your car, the information from the speed sensors can be used to make driving more comfortable, for example with the use of cruise control. You set a speed you want to keep, and when the speed drops below, a signal is sent to the fuel pump to pump harder.
It is also used for safety applications. Heard of ABS? It is short for Anti-lock Braking System, and is a control system that modulates the break pressure in response to the detected wheel speed deceleration. So if you do an emergency break, the wheels wonât lock, instead it makes sure that the car decelerates in a controlled manner, so that the steer ability is not lost and the car wonât slide. (Read more here.)
Ever wondered how the fuel level is measured?
A Hall-type magnetic sensor is installed in the ceiling of the tank, and a floating magnet is put into the tank. The higher the level is, the closer the magnet is to the sensor and the higher the output will be. See picture below.
Picture showing how fuel level can be measured. By using a Hall sensor at the top, the distance to a floating magnet, and thus the level, can be measured.Â Source: Nagarjuna College
Volvos aims to revolutionise road travel with magnetic sensors
The magnetic sensors can be used for a lot of things. In fact, Volvo thinks that they could be used to guide tomorrowâs autonomous cars.
The idea is to put magnets into the roads, and then use magnetic sensors on the underside of the car to sense where itâs going and what is in the surroundings. This would improve both comfort and safety.
I bet you wonder which kind of sensor they use?
With car speeds of 90 km/h, the sensors need a sampling rate of 400 readings per second.Â And since the sampling rate of the Hall sensor is far from this, they have to use AMR sensors, which are much faster.
Volvo actually built a test road (in HĂ€llered, Sweden) and tried the technology out, and if they can reduce the costs ($24,405 per kilometer highway), the future where we drive autonomous cars might not be that far away after all.