Did you know that your smartphone can contain up to 18 different types of sensors? In this article we review them and explain what they can be used for!
Table of content
The different categories of sensors
We usually distinguish 5 categories of sensors, depending on the type of usage:
Motion sensors can analyze the movement of the phone: is it horizontal or vertical, does it move from right to left, does it vibrate or is it in free fall. They are used in games or interactions with the smartphone, but also to detect emergency situations such as a fall or an earthquake. These sensors include the accelerometer and the gyroscope.
Environmental sensors measure data about the outside world such as temperature, atmospheric pressure or humidity. Not all smartphones are equipped with these sensors. Some are present but are not used to inform the user but to detect events that are dangerous for the smartphone, such as overheating. You can also connect the phone to external sensors such as fine particle detectors.
Position sensors inform the user about his position in space. GPS, magnetometer or LiDAR allow you to know in which direction your smartphone is facing, or what its exact position on the map or its altitude is. Thanks to these sensors it is now impossible to get lost!
Light and sound sensors detect and record information about light rays or sound waves. They can be simple like photoelectric cells or a microphone, or more complex like the grids of cells like the photographic sensors which make it possible to record an image or a video in black and white or color.
Finally we must not forget the action sensors which are used to detect a direct action of the user on the smartphone. For example detecting if a finger approaches the device, or if the user uses his finger on the screen.
What is MEMS technology?
How to put all these sensors in a mobile phone without it being the size of a suitcase?
This is where MEMS technology, short for Micro Electro Mechanical System, comes in. MEMS are small integrated circuits comprising integrated mechanical and electronic parts. This technology was developed in the 1970s and its advantages include low power consumption, small size, affordable cost and high accuracy.
What does a MEMS accelerometer look like? The attached photo shows an example of a MEMS accelerometer in an iPhone 4 (https://www.memsjournal.com/2010/12/motion-sensing-in-the-iphone-4-mems-accelerometer.html).
One can observe on this photo the springs which are used for the detection of the acceleration, as well as the mass which surrounds the object. One can also discern the capacitors oriented along two axes. These two groups of capacitors are arranged at right angles to measure the acceleration in two directions.
Without MEMS technology, today we wouldn't have all these sensors in smartphones and all these possibilities!
The accelerometer
Acceleration is the change in speed over small time intervals. When we see velocity we immediately think about the GPS, but unfortunately GPS is not precise enough and would consume too much electricity.
Other methods had to be found. The one that is currently used in smartphone is to measure the acceleration of a small mass located inside the mobile phone and connected to the frame of the smartphone by a spring is subjected.
When the smartphone is set in motion, this small mass will have a movement which will depend on the stiffness of the spring to which it is connected. The force that the spring creates is F = k*x where x is the displacement of the weight. And we also have F = m*a, with m the weight and a the acceleration. Using a capacitor we can measure the displacement x, and we deduct the acceleration. To discover more on the equation, follow this link.
One of the peculiarities of the measurement of acceleration using this method is that it takes into account the acceleration of terrestrial gravity. Indeed, if we put the smartphone vertically, we see that the spring will have a tension equal to the weight of the mass. The acceleration that is measured by smartphones is the so-called absolute acceleration or also acceleration with g.
There is another type of "acceleration, called linear acceleration or acceleration without g. This is the acceleration due solely to the movement of the user. To calculate it, you must use another sensor, the magnetometer, which will allow deduce the acceleration due to gravity from the absolute acceleration vector.
Smartphone accelerometers are very precise, at around 0.01 m/s2, and give measurements at a frequency of 150 hertz, or 150 measurements per second, an asset for making quality measurements!
The gyroscope
A gyroscope is an instrument that measures the rotational speed of an object. They are essential for the navigation of aircraft or satellites and allows them to detect whether they are pointing up, down or sideways.
Usually, a gyroscope consists of a wheel or disk that rotates around another disk or axis. This device keeps its orientation because of the gyroscopic effect, and it is easy to measure the new orientation with respect to this reference.
In mobiles, the MEMS component works in a similar way to the accelerometer but with components that measure rotation rather than displacement.
The orientation sensor
If we combine the data provided by the accelerometer and the gyroscope, we are able to know the position of the smartphone at all times. Indeed, from the initial position of the smartphone at rest, and by applying to this position all the linear and rotary movements to which the mobile phone is subjected, we are able to determine what its new position is at any time.
In smartphones this function is ensured by a specialized circuit called the orientation sensor, which takes data from the gyroscope and the accelerometer, plus possibly that of the magnetometer, to calculate the orientation in space of the smartphone. To learn more about the orientation sensor, you can follow this link.
The pedometer
The pedometer is often referred to as a sensor, but in fact the pedometer is not a sensor per se. This is a small electronic circuit that detects large and regular variations in acceleration that indicate that the user is walking.
When the movement is repeated several times, the device starts counting, which explains why there is always a delay when using a pedometer.
The thermometer
Present in all mobile phones, the thermometer allows you to measure the ambient temperature inside the smartphone. The problem with this measurement is that as the device heats up, the thermometer no longer simply measures the ambient temperature, but also the heating of the smartphone, and therefore the measurement is false. For this reason, very few smartphones allow access to the external temperature, and this sensor is used mainly to detect overheating of the device.
The barometer
The barometer is used to measure the atmospheric pressure and is an important measure for predicting the weather, for measuring the altitude or the depth of a dive. All recent iPhones have a barometric chip, and some Android phones too.
Barometric pressure sensors use an aneroid cell that expands or contracts as atmospheric pressure changes. A small MEMS system detects the variations of the diaphragm of the cell to deduce the pressure. The more the diaphragm deforms, the higher the pressure.
The humidity sensor
The hygrometer or humidity detector measures the amount of water vapor present in the ambient air. Humidity sensors are used in several industries to protect equipment and ensure safe and comfortable environments.
Typically, humidity sensors contain a humidity sensing element and a thermistor, which is used to measure temperature. Indeed the temperature often influences the measurements which need to be corrected.
There are three main types of humidity sensors, but for smartphones or microcontrollers we mainly use the capacitive method: we measure the electrical capacitance of a small band of metal oxide located between two electrodes, which changes with the level moisture in the air.
The magnetometer
The magnetometer makes it possible to calculate the magnetic field to which our mobile phones is subjected. In the absence of any other magnetic field (such as a magnet or a fero-magnetic object), the magnetometer gives the coordinates of the earth's magnetic field, which makes it possible to know the north for example.
The advantage of a MEMS magnetometer is that it consumes very little power, and therefore can replace the accelerometer and the gyroscope to calculate the position of a smartphone in space. With a disadvantage: if a magnetized or iron-magnetic object is close to the sensor, its measurement will be affected and the reference frame will be wrong.
Smartphone magnetometers generally use the Hall effect or the magneto-resistive effect. In the Hall effect, a magnetic field deflects the flow of an electric current through a plate. The electrons would be deflected to one side of the plate and the positive poles to the other side of the plate. If we measure the potential difference between the two sides of the plate we obtain a measurement of the magnetic field.
Other sensors use the magnetoresistive effect. These sensors use materials sensitive to the magnetic field, generally composed of Iron (Fe) and Nickel (Ne). When these materials are exposed to a magnetic field, their resistance changes.
The GPS
The GPS or Global Positioning System is probably the tool that has most revolutionized the development of mobile applications. Countless new applications have been made possible by the Americans opening up the GPS system to the general public in 1995 and creating GPS chips small enough to fit in a smartphone.
GPS uses the difference in time of reception of signals from different satellites and calculates the position on earth by triangulation. To have a precise position, it is therefore necessary to be able to receive information from several satellites (at least 4), which excludes applications inside buildings or in the basement.
GPS chips make it possible to deduce a lot of information from signal analysis: position (latitude and longitude), speed, altitude. they also give a universal clock as well as an estimate of the precision of the measurements. With the right number of satellites we can have an accuracy of less than 1 meter, with a frequency of 1 hertz.
The LiDAR
LiDAR (Light Detection and Ranging) is a scanner used to measure the distance between an object and oneself. It is used in smartphones to recognize faces when connecting, to measure distances for photo taking and adjust depth of field, or to calculate the distance to objects and their shape.
It emits a beam of pulsed laser light and calculates the time it takes for this light to bounce off the object and be picked up again by the sensor. Unlike radar, which uses radio waves, LiDAR can collect information at 360°, thus generating a set of points to recreate in 3D with great precision the space surrounding the sensors.
This system has long been used in industry, advanced security, aviation to map the earth's surface and even archeology to uncover buried ruins. It is mainly used in recent Apple devices and in some high-end Androids.
The photoelectric cell
The photoelectric cell of smartphones is a small component usually present on the front of the device next to the camera and which measures ambient light. This measurement has several functions such as adjusting the brightness of the screen, detecting the night, or adjusting the sensitivity of the camera during photos and videos.
Some ambient light sensors can also detect luminosity for different wavelengths, blue, red and green, as well as UV and infrared.
The detectors are photo-diodes or photo-transitors which use the photo-electric effect. When light strikes the depletion layer with sufficient energy, it ionizes the atoms in the crystal structure and generates electron-hole pairs. The existing electric field, due to polarization, causes electrons to move towards the cathode and holes towards the anode, giving rise to a photocurrent. The higher the light intensity, the greater the photocurrent.
The photographic sensor
If we gather thousands of photoelectric cells in a matrix we obtain a CMOS (Complementary Metal Oxide Semiconductor) sensor, and if we place a lens in front of this sensor, we then obtain a camera. Capturing photography and movies is one of the main uses of smartphones today, to the point that they have replaced cameras for non-professionals. These digital images can then be shared using mobile technology.
If the digital photographic sensors were invented in 1969, it is only gradually that they became essential means of taking photos, when the resolution reached 5 mega-pixels. In smartphones, it is especially with the arrival of the iPhone and a quality sensor that the smartphone has become a photographic tool and of course, a video creation tool.
In its original version, the CMOS is a black and white detector. To detect the color, a Red, Green, Blue filter is added to it, which filters the different wavelengths. This filter, called the Bayer filter, makes it possible to simultaneously record three simultaneous images, each in a different spectrum, and which will be brought together to display the image.
The heart rate sensor
The heart rate sensor is a quasi-medical device that monitors the heart rate of patients, but also their oxygen level in the blood.
This sensor is based on the principle of photoplethysmography, demonstrated in 1937 by Hertzman and Spealman (5). These two scientists found that they could measure, using a photoelectric cell, variations in the transmittance of light through the finger and that these variations made it possible to accurately estimate the heart rate.
With each heartbeat, blood is pumped through the blood vessels during the systole phase. This action leads to a rapid increase in blood volume in the capillary vessels, which causes a slight increase in tissue thickness and redness, due to the presence of hemoglobin in the blood. During the diastolic phase, the blood flow in the tissues decreases, which makes them less opaque. By examining the variations in opacity or color of sufficiently transparent tissues, such as those of a finger or an earlobe, it is possible to determine the phases of systole and diastole, and to calculate the heart rate.
Note that the sensors emit a green light. Hemoglobin in its oxygenated form absorbs green radiation. During the systole phases, the green radiation emitted by the light source will be more widely absorbed than during the diastole phase, during which the blood is less oxygenated. The use of green light makes it possible to magnify the photoplethysmography effect.
The microphone
We often tend to forget the microphone among smartphone sensors. Probably because it is an old technology to which we are accustomed, compared to those of MEMS.
Invented in 1876 by Graham Bell, the microphone is not new, and yet it is an essential element of smartphone sensors, both for oral exchange, dictation, but also for sound analysis that we use with FizziQ. .
Cell phones use tiny capacitors called electret mics. These mics require very little power to operate and also fit perfectly into the circuitry of a typical cell phone.
An electret microphone is a type of condenser microphone with a permanently polarized capsule. Therefore, it does not require an external power supply. This microphone is coupled to a preamp and then to an analog - digital converter which converts the sound into digital signals.
The microphones are able to capture the pressure variations of the sound wave at a very high frequency, approximately 44,000 hertz, or 44,000 data per second, with a resolution of 16 bits, i.e. with an accuracy of 0.003 %!
From the sounds of the microphone we can calculate the frequency of a sound signal, its intensity, its frequency spectrum or even build the curve of the sound wave as with an oscilloscope.
The touch screen
Another sensor that is not often thought of and which has been a phenomenal advancement for digital tools is the touch screen. On the old Blackberrys there was a mechanical keyboard, which I must say allowed you to type quite quickly. The technology of capacitive screens has made it possible to create very pleasant keyboards directly on the screen and to gain the place of the numeric keypad to make larger trans.
There are two technologies for touch screens:
resist screens that use two conductive layers that come into contact when pressed. The advantage is to operate with the bare finger, but also with a glove, for example during an intervention in the medical field. Resistive touch screens consume little electrical current and are low in price but have only average durability and the outer surface is vulnerable to scratches
capacitive screens that detect the touch of a finger. This modifies the voltage of the electric field which is applied to a conductive layer above the glass plate. The advantage of capacitive touch screens is their long service life as well as fast touch reaction time, high resolution (accuracy), high optical clarity and high resistance to dust, water and to scratches. On the other hand, handling is only possible with the bare finger or with a stylus, and the cost of these screens is higher.
Today almost all smartphones are equipped with capacitive touch screens, but some medical equipment continues to use resistive technology, which can be used with gloves.
The proximity sensor
The proximity sensor detects when an object is approaching the smartphone. For example, it causes the brightness of the screen to be reduced when on the phone.
There are many technologies for these sensors but in smartphones we generally use a combination of an infrared light emitter and an infrared receiver. If there is no obstacle, the infrared light will not be reflected and the sensor will not receive anything, if there is an object, the infrared light will be reflected and will be detected by the receiver. The detection distance is low, less than 5 cm in general.
They are available in almost all smartphones at the top of the screen. Infrared light passes through this sensor. When a physical object comes into contact with this light, it senses it and reacts to it. For example, when you talk on your phone and put your phone to your ear, the infrared light detects a physical object, i.e. your ear. Detecting this, the screen light will turn off automatically. This both saves battery life and prevents accidental screen contact.
The fingerprint reader
Many phones come with a fingerprint sensor to help you log in quickly.
There are three main types of digital reader: optical (scanning with light), capacitive or CMOS (scanning with electronic capacitors) and ultrasonic (scanning with sound waves). Although the best technology seems to be ultrasound, all biometric sensors can be fooled in one way or another but they are generally safer and much more convenient than using a PIN or a diagram on the screen.
The latest technologies use facial recognition coupled with a Lidar, which makes it safer than facial recognition technology by camera.
Buttons
Finally we must mention the buttons! These are sensors in their own right, certainly very simple, because they just give yes/no information, but when they are broken, it's a smartphone that can be thrown in the trash. Since the power and volume keys are the most frequently used, the button click test sees each button tapped 100,000 times, and the fingerprint key tapped 1 million times to ensure that the keys are comfortable to the touch, functional and intact.
When we think about it, isn't it the start button which is the most important sensor of your smartphone!