Introduction
We don't always think about it, but any modern device makes use of dozens or even hundreds of sensors. Of course, there are dedicated devices, like flood or temperature sensors, or home weather stations and so on. But our smartphones, smartwatches, or fitness trackers are just "stuffed" with sensors too. Using the engineering menu of the smartphone or some software, you can even see what parameters are constantly measured by integrated sensors. However, not all sensors’ data can be seen (in fact, there are many more sensors). Obviously, fitness trackers that measure the parameters of human activity utilize sensors by design. But sensors are also integrated, for example, in printers, refrigerators, and washing machines and numerous sensors are on the motherboard of a PC or a laptop. And in an ordinary car there are more built-in sensors than one can imagine, not to mention electric cars with self-driving systems.
1. So, What is a Sensor?
Of course, we should start with the questions of what a sensor is, why it is needed and how to use it.
To begin with, let’s consider the human body. We have sense organs we use constantly and through which we communicate with each other. Computing systems, which evolved into PCs and smartphones, also need some means to exchange information. Early input devices were quite inconvenient and eventually disappeared, but, for example, the mouse manipulator is still in use. And, surprisingly enough, it still has sensors that work on the very same principles as in early prototypes.
The development of robotics simply required sensors that would replicate the humanlike senses (see our article on the history of robots). The automatic control systems were developed alongside computers and robots. For example, when one needs to maintain a stable temperature, it is inconvenient to operate the system manually, especially if stability is required over a longer timeframe. Such systems consist of a sensor (in this case a temperature sensor) and a control device to change the heating or cooling of the object, depending on the sensor readings and desired temperature value. Automatic control systems, in general, have been in production for a long time, and used, for example, the conversion of thermal energy into mechanical energy to open and close a supply valve, e.g., on a fuel pipe. These mechanical devices were sensors of some sort.
Modern sensors also use the energy conversion principle, but more often not into a mechanical, but into an electromagnetic signal that can be measured. The sensor's proliferation in the modern world, especially in recent decades, has become so great that it has become the basis of a multibillion-dollar business for the production of elements. Today it is almost impossible to create any home appliance without sensors.
The assessment of open statistics says that the growth of the market of sensors in monetary terms is not very large and is somewhere around 8-10% per year. Forecasts for the next 5-7 years say that nothing will change significantly. However, the absolute number of to-be-produced sensors is much larger, and the slow growth of the monetary equivalent is associated with the use of new production technologies to make sensors cheaper. But even so, forecasts for 2025-2028 indicate a market of 300-400 billion USD.
Of course, major changes in society may change the statistics somewhat, for example, the COVID-19 pandemic has caused a significant demand for oxygen saturation sensors, but this had little effect on the overall market situation. However, advances in robotics and IoT fields can change the market due to the significant need for sensors of varying complexity.
2. How to Convert Physical Parameters Into Electrical Signals?
Modern microcomputers work with the so-called digital signal. However, all physical quantities are continuous under normal conditions, so to work with them, one must first convert this value into an electrical signal, and then into digital values. To obtain numerical values from an electrical signal, special devices are employed – those are called analog-digital converters (ADC). Modern ADCs can be made as individual chips or integrated into the system-on-chip. The last option is useful for the Internet of Things because it saves space, requires fewer external components, and reduces the energy consumption of the market-ready device.
Thus, sensors consist of two main elements: a sensing element that interacts with a certain force or medium, and a transducer that converts the result of the interaction into an electrical signal, or even into a digital information signal that is ”understandable” by computer or microcontroller. Most of the sensitive elements have three basic operation principles:
- Voltage measurement. Sensitive elements are often based on different physical phenomena that create a potential difference in the contacts. An excellent example of such a sensitive element is a conductive circuit in the field of a permanent magnet. In the circuit moving through a magnetic field occurs an electromotive force, i.e., a voltage that can be measured. ADC itself can be used to measure the resulting signal, however, in most cases, additional electronic circuits must be needed.
- Resistance measurement. Numerous sensitive elements behave as resistors, their resistance changes with the change of a certain characteristic. This can be a change in resistance under the action of a force, a chemical, or a change in the characteristics of the environment. Data from such a sensitive element can be obtained using a voltage divider from two resistors connected in series. One should be a resistor with a known resistance and the other a sensitive element. A known voltage is applied to them, and the ADC measures the voltage drop across the known resistor.
- Capacity measurement. Using a capacitive sensing element, it is possible to measure changes in the distance: one of the capacitor plates must be fixed, and the other can be movable to change capacity. One can also measure changes in the capacity of certain substances between the plates of the capacitor under the action of forces or chemicals. However, the capacitance cannot be measured directly, so in addition to the capacitor, a power supply and a large resistor must be included. The supply will charge the capacitor, and a voltage drop on the resistor will be the output signal. This circuit has a high resistance, so it requires an amplifier.
Of course, there are other types of sensing elements based, for example, on the determination of the resonant frequency.
Therefore, to measure the same characteristic, one can use sensors built on different physical principles. Of course, depending on the specifics of the sensing part, a certain connection circuit, and conversion into a digital signal, or even further data processing may be required. The range and accuracy of measurements of a certain characteristic also often depend on the sensitive element. But, of course, the price of the sensor is often defined by its complexity. Let’s consider in more detail what characteristics and how we can measure with sensors.
3. Main Types of Sensors
The most convenient for use in electronics are sensors that behave like resistors. They are easy to manufacture, and information from such sensors can be obtained using ADCs built into the controllers simply by determining the voltage drop across such a resistor. Consider the most common designs of simple sensors used to measure basic parameters.
3.1 Temperature sensors
Probably the most common in electronics is the thermal sensor. Perhaps thermoelectric sensor or thermocouple should be the first mentioned in this category. The principle of its operation is quite simple: if a connected pair of different conductors are placed in a temperature gradient when one end of the connection is hotter than the other, then at the other end there will be an electric potential difference, and it can be measured. This so-called thermoelectric effect was discovered by German physicist Thomas Seebeck. With the help of several thermocouples, one can directly measure the temperature difference between environments. Depending on the materials the thermocouple is made of, it can be used in different temperature ranges up to -200 +1500 C (-300 +3000 F). Such sensors are often used in industry and control the temperatures of certain mechanical elements, heaters, refrigerators etc. The problems of thermocouples are often high cost, low accuracy in some temperature ranges and nonlinearity of characteristics. It makes measurement more difficult.
3.2 Thermoresistive sensor
The first thermistors consisted of a piece of metal conductor, the electrical resistance of which increases with increasing temperature. However, such a sensor was too cumbersome, and the nonlinearity of its characteristics created additional problems. So, the vast majority of modern thermistors are semiconductor devices. Using multilayer semiconductor structures, it was possible to change the resistance of the thermistor linearly with respect to temperature and reduce their size. Such sensors are not only used as standalone devices but also are integrated into almost any semiconductor electronic equipment because they are easily placed on a substrate with a microchip of a smartphone or PC. The only problem with such sensors is the limitation of the measured range because they cannot withstand temperatures above 160 C (320 F) and at low temperatures (less than -55 C (-70 F)) they do not work. But the cheapness and versatility of modern thermistors make them ideal for mass use to monitor the state of not only devices but also buildings (possibly with the integration of numerous sensors at the construction stage) and environmental ecology.
Of course, in addition to the above, there exist temperature sensors based on other principles, such as piezoelectric: they determine the resonant frequency of the crystal, which depends on the temperature. Also, there are sensors that use nuclear paramagnetic resonance. However, such sensors are complex to manufacture and expensive. Because of this, they are used for specific tasks, often in single copies.
3.3 Pressure sensors
With the pressure changes, the distance between the condenser plates and, consequently, the capacity will change too. This sensor can measure excess pressure in the heating or water supply system. The other type, strain gauge pressure sensor also uses a membrane, but its deformation is determined by the change in resistance of a conductor or semiconductor of a certain shape, which is tightly glued to the membrane. Yet another pressure sensor is again based on the piezoelectric effect. This sensor measures absolute pressure and can be very compact, allowing it to be integrated into smartphones, watches, and other microelectronics. In addition to atmospheric and pipe pressure measurement systems, such sensors can be found in electronic sphygmomanometers and digital scales. In the latter case, the measurement of weight is no different from the measurement of excess pressure on the surface of the scales.
3.4 Accelerometers
Acceleration sensors or g-sensors consist of a spring-loaded weight. Under the action of external acceleration, the weight deforms the springs and shifts in the opposite direction. These shifts are determined by piezoelectric or capacitive sensing elements. Accelerometers are integrated into smartphones to determine screen orientation, and, of course, in fitness trackers. With the help of constant determination of acceleration and special processing of the obtained data, the statistic on activity and movements is calculated. Accelerometers can also be in use to monitor the vibration of devices, buildings etc.
3.5 Humidity sensors
The main purpose of humidity sensors, of course, is meteorology and humidity control in production, ventilation systems, or the premises where the pools are located. Also, humidity sensors are sometimes integrated into household appliances critical to moisture (dryers, microwave ovens, steamers). The main types of humidity sensors in most cases are based on capacitive and resistive elements. They have an active element consisting of a specific hydrophilic polymer the capacity or resistance of which is measured. The thermal humidity sensor is structurally different. It has two chambers: closed with steam (wet) and open, humidity in which is measured. Humidity is determined by the temperature difference between wet and dry chambers. These sensors can operate at higher temperatures than resistive or capacitive ones but respond to changes in the atmosphere, such as the appearance of another gas. This often distorts the measurement.
3.6 Photosensors
Unlike previous types, photosensors work only on the photoelectric effect. Photons of light hit a sensitive element and knock out electrons. This creates a current that can be measured. This effect also defines the operation of solar panels. Photosensors are used both independently, for example, in smartphones for automatic adjustment of screen brightness, and as a part of more sophisticated systems. Some of them will be covered in next publications. One of the applications in electronics is the so-called optocoupler. It consists of an emitter (LED) and a photosensor. The optocoupler is often used to count the number of rotations. To do this, a wheel with holes is scrolled between the light source and the sensor. Rotations are calculated by the number of pulses received on the photosensor. This is how the wheel of the mouse manipulator works, for example.
Conclusion
This time we have considered the basic operation principles of certain sensors with relatively simple structure. As we can see, most of them are based on measuring the capacity and resistance of a sensitive element or based on a certain physical effect (piezoelectric effect, Seebeck effect, photoelectric effect etc.). However, the need to measure new characteristics with increasing accuracy is growing day by day. In the next part, we will talk about more complex sensors. Most of them do not determine the characteristics directly but calculate them from a specific data set. We will try to see what sensors will appear in the near future and how they can influence our daily lives.
