Principles of sensors. Part 2
Last time we talked about the basic principles of sensors widely used in daily life and industry. However, the development of technology and overall miniaturization of electronics-packed devices have already significantly affected our lives. With the help of wrist gadgets, most people are accustomed to track their activity and sleep quality. Many people monitor their heart rate during training. An unstable health situation is forcing more and more people to monitor saturation, record electrocardiogram or measure blood pressure. All this and even more are already within the capabilities of a smartwatch, although not so long time ago numerous standalone devices were needed for getting such diverse medical information. So, below, we'll go over how to cram that much technology into a small wristwatch, and how the device's sensors handle everything.
Complex sensors and systems
Systems based on photodetectors. Systems based on photosensitive elements are very diverse. We have already mentioned optocouplers used determine the number of revolutions of a moving element. It is also possible to check whether there is a foreign object between the light source and the photosensor, for example, for alerting. Optical and laser displacement sensors further develop the ideas on which optocouplers are based on. In everyday life, we use such sensors in computer mice. Unlike optocouplers, such sensors have two optical circuits: one redirects light from the source (LED or laser) to the object or surface, the second optical circuit redirects the reflected light to the detector. Such detector is typically a matrix of 2x2 (or more) photosensitive elements. By processing the data stream from the matrix one can determine not only the direction of the surface movement relative to the sensor but also the speed. Similar sensors are used in industrial equipment for non-contact position control, as well as in printers to control the movement of the paper.
Pulse and oxygen in the blood measurement. The ability of the blood to absorb and reflect light is essential for medical and household devices that control heart rate and oxygen saturation. It is even can be checked visually that venous blood, which has little oxygen, and arterial blood, which has more oxygen, have different colors, although still two shades of red. We do not notice this, but depending on the filling of the capillaries with blood, the color of our skin also changes slightly, i.e., it changes its reflection and absorption properties. Blood absorbs the most in the green spectrum region, so to determine the pulse, it is sufficient to measure the consistent change of green laser absorption by the skin. Of course, measurements should be made over a certain time frame, and the frequency of the sampling should be at least twice the maximum possible heart rate. The sensor must be above the same area of skin at all times, too. That's why if your fitness band or smartwatch is loose during sports activities, its readings may differ significantly from reality. A similar principle is used to determine saturation, but in this case, the absorption or reflection must be measured for two different laser wavelengths. Oxygen-enriched and unenriched blood cells absorb light in different ranges in different ways. Devices measured absorption (this is how finger clips work) and reflection (this is how fitness trackers and smartwatches work) in the visible (red or green) and infrared ranges. The difference in absorption or reflection in different ranges determines the ratio between the blood saturated with oxygen erythrocytes and the one that is not.
It should be noted that a significant advantage of optical saturation sensors is their non-invasiveness, i.e., no need to pierce the skin to determine blood parameters, and the sensors themselves are relatively cheap and easy to manufacture. In fact, the sensors in laser mice, and fitness trackers are similar. The difference is mostly in the data processing.
Laser rangefinders and LIDARs. Laser rangefinders also use laser emitters and photodetectors. The laser pulse is sent in the direction of the object to which the distance is to be measured, and the reflected light is recorded by the sensor. Knowing how long it takes for the pulse to travel to the object and back and considering the speed of light, it is possible to determine the distance. However, the speed of light is very high, so the time to be measured is extremely short. Therefore, most rangefinders simulate a variable signal and determine the phase change between the transmitted and reflected signals, and based on the phase change determine the time. LIDAR, or laser radar, is built on the base of rangefinders. They measure the distance sequentially in different directions using optical schemas with mirrors. It allows building an exact map of the surrounding. The first LIDARs were bulky and expensive devices. A known example is a 3D-LIDAR on an experimental Google self-driving car. Modern LIDAR is smaller in size and significantly cheaper. LIDARs are now used in electric cars, quadcopters and various robotic devices such as vacuum cleaners.
Image sensor. A matrix of light-sensitive elements is required to capture the image. The increase in the number of matrix elements has led to the use of the large integrated circuit as an image sensor. There are now two main technologies: CCD – a charge-coupled device (named after the technology of information reading) and CMOS – a complementary metal-oxide-semiconductor (named after the technology of production). CCD matrices have better sensitivity, but CMOS matrices are more energy-efficient and cheaper to manufacture. CMOS sensors are in use in most small portable devices and video surveillance cameras.
There are also special sensors, such as arrays sensitive to infrared radiation in thermal imagers.
Depth cameras and ToF sensors. Depth chambers consist of an infrared light emitter spaced a certain distance apart from two or more CCD cameras sensitive to light frequency range of its illumination source. Depth images (cloud of points, in fact) are based on the displacement of the resulting images from different cameras. In fact, they work like human binocular vision, and the infrared backlight allows such cameras to "see" at night. A slightly different principle is exploited in ToF (time-of-flight) light cameras. It consists of a laser and a highly sensitive CCD. This is how this technology works: a short laser pulse illuminates the scene, and the ultra-sensitive CCD camera opens its fast shutter for only a few hundred picoseconds. A 3D scene is calculated from a sequence of 2D images recorded with different delays between the laser pulse and the opening of the shutter.
Both these types of sensors receive data to build a three-dimensional map of some environment or a specific part of it. In everyday life, these sensors are used to build a 3D image of a face and subsequent authentication of users.
Touch sensors and fingerprint scanners. Touch sensors have been used in recent decades to enter data or interact with persons. In the beginning, these were plain separate touch areas or buttons, and now it is impossible to imagine a smartphone without a touch screen. Capacitive touch sensors have practically supplanted all other technologies. These sensors consist of a lattice of transparent electrodes, the capacitance between which is measured. The human finger adds extra capacity to the space between the contacts. Positioning in two directions is determined either by two sets of electrodes (X and Y) or by the specific shape of the electrodes.
Interestingly, the same technology is used in capacity fingerprint sensors. The difference is that the touchscreens determine only whether the finger touches it and where, and the fingerprint sensors scan the microscopic pattern of finger capillaries. In the latter case, incredibly high measurement accuracy is required. The main problem here is the noise from the environment and the device itself. Noise can be orders of magnitude greater than the useful signal.
Sensors in the near future
In the last years, technology has significantly advanced in the miniaturization of sophisticated devices for use in portable appliances and everyday life.
An example of a sensor that has come a long way but has not yet appeared in the household is a compact micro-spectrometer. Some time ago, an optical spectrum measuring device occupied almost an entire room. Over time, the spectrometers decreased in weight and size and could fit on the table. Advances in technology have made it possible to measure spectra almost instantly, and the size of the spectrometer has been reduced to a small black box. But the latest versions are already less than an inch in size and can be integrated into a device slightly larger than a smartphone. Amazingly enough, the principle of device operation has not changed. The spectrometer still has an aperture, a set of lenses, a diffraction grating for the decomposition of light into a spectrum, and a light-sensitive element (a matrix similar to the image sensor is now used for faster measurements). The sensor based on a micro-spectrometer is perfect to configure video cameras or studio lighting. But more interesting is the application for monitoring of air contamination by micro and nanoparticles. The presence of them is difficult or almost impossible to determine by other sensors.
Another widespread application of micro-spectrometers is for the determination of Raman scattering spectra. This technology allows to determine the component composition of materials and lays a potential groundwork for the non-invasive measurement of blood glucose. The advent of such a sensor integrated into an everyday device like a smartwatch will forever change the early diagnosis of diabetes. But so far, active surface sensors are used to determine blood glucose. These consist of disposable microchips with a specific active structure to determine blood parameters. The development of this technology is moving forward, and in a few years, we will probably see similar devices for fast reading of DNA – pocket sequencers. The sensor element of such a device has a porous nanostructure for studying a DNA molecule, which is still difficult to produce cheaply enough. However, even state-of-the-art models already have very high accuracy and this will allow the use of sequencers in next-level protection systems.
Sensors are evolving, using more and more complex technologies, becoming smaller, more accurate and efficient.
How sensors are used in IoT
Sensors are a fundamental part of the modern Internet of Things concept, so the development of one without the other is now almost impossible to imagine. IoT devices are now used to monitor all aspects of human life. The powerful drivers of the Internet of Things in general and the development and modification of sensors are medicine, ecology and manufacturing.
Sensors and sensor systems for medical use have always been and still are the most complex and technological. Patient monitoring within hospitals has long been the standard. But modern approaches in addition to standard measurements of the human body like temperature, blood pressure, heart rate, cardiogram and saturation also include non-standard approaches: for example, the collection of large data arrays on the pressure in different places of the medical bed. Data processing from such an IoT system allows identifying problems during sleep, incorrect body position and in some cases predicting the deterioration or improvement of the patient's condition. Some of the technologies recently were only available in medical facilities are already becoming commonplace due to the already mentioned smartwatches and fitness trackers, however, over time there will be more and more such examples. Cardiogram measurements gadgets will allow for so-called Holter monitoring for early diagnosis of cardiovascular disease.
In the near future, the Internet of Things could have a significant impact on transport infrastructure. Systems for determining the speed of vehicles and photo-fixing are now widespread on the road. But soon a network of cheap sensors to monitor various parameters of the road condition will be installed on highways. Self-driving cars will be integrated into the same system. The constant interaction of the car with the road will significantly increase safety and increase the capacity of highways.
The impact of sensors and IoT on our lives is already great, and it will only grow over time. Internet of Things technologies improve people's quality of life, control life processes and anticipate problems that may arise. In the near future, the world around us will adapt to the demands and needs of people, for example health conditions will be monitored for immediate assistance in case of problems. On the other hand, all this will be achieved by obtaining data sets from sensors around and on people's bodies. Perhaps the IoT systems around us will know more about us than we do. Is it good or bad? Who knows. But this is our future. The science fiction from the past decades is now reality.