Pressure sensors can be found all around us, from the most commonly used devices, for example, scales, to technically complicated devices with high precision, such as vacuum gauges. There are many pressure sensors for various applications, but in general, their principle is the same, the conversion of mechanical energy (pressure) to another, mainly the electrical energy (voltage).
What are those sensors measuring, when it is said that sensors measure mechanical stress? The mechanical stress of the object represents the internal resisting forces to the applied mechanical pressure. In the RVmagnetics article Pressure Sensors Types and future trends we already discussed different types of pressure sensors, let’s take a better look at their functionality.
MEMS represent pressure sensors that are mostly used in devices, where pressure (atmospheric, in vacuum systems, differential, …) is converted to the electrical signal, which is proportional to the applied load.
MEMS consist of a few important parts: silicon wafers, electrical membrane, and circuit. Silicon wafers represent the mechanical holders of the membrane. If pressure is applied to the membrane, it is bent, which causes the electrical elements of the membrane to change their properties and create an electrical signal. This kind of electrical impulse is registered by the circuit of the MEMS pressure sensor and consequently evaluated. Electrical signal differs based on the MEMS type of sensing, for example – capacitive MEMS, and piezoresistive MEMS.
MEMS can work as capacitors, where stored electric energy is indirectly proportional to the distance of the electrically charged surfaces, one surface is in the membrane, and another is opposite but parallel to the membrane. When the pressure is applied the distance between those surfaces will shorten, which means that capacitance will increase, and the electrical circuit will detect and evaluate those changes. Its main advantage is sensitivity, as the membrane is elastic and can detect even the smallest changes. However, membrane elasticity brings also one of the biggest disadvantages, which is low withstanding of the higher pressures, as the system can be easily damaged. Also, the temperature drift of the capacitive MEMS pressure sensor can drastically change the measurement, as the components are from materials, that change properties with the change of the external temperature. Not only the materials of the system, where the sensor is mounted are temperature-dependent, but the sensor itself is non-invariant to external conditions, such as temperature. This can lead users and manufacturers to the reduction of the components for the pressure sensors themselves.
The reduction of the size of sensor components brings us to the piezoresistive MEMS pressure sensors, which use piezoresistors for measuring the applied pressure. The number of piezoresistors in the membrane differs based on the measuring precision. Similarly, as capacitive MEMS, piezoresistive MEMS also produce the voltage output, which is proportional to the applied mechanical stress . The most precise piezoresistive MEMS, which are used in technical practice use the Wheatstone bridge conformation, which means four piezoresistive elements are sensing the pressure on the four places of the membrane, which can also evaluate the distribution of the applied force along the diaphragm. The piezoresistors themselves represent strain gauges, which means that for the construction of the MEMS pressure sensors, other types of pressure sensors are needed.
Piezoelectric sensors are built from materials that can show the piezoelectric effect, which represents the ability to generate the electrical response from the mechanical stress applied to the sensor.
Piezoelectric sensors are usually made from ceramic or crystal materials, such as quartz, lithium niobate, or lead zirconate titanate. After applying mechanical stress (pressure, vibrations , …) the piezoelectric part of the sensor converts this stimulus into the electrical impulse output. The advantage of this conversion is, that it is happening inside of the material, and does not require other powering. Depending on the materials used in the piezoelectric sensor it has wide ranges of applicability, in practice, as some of them can precisely detect changes even at the ranges of MPa. The output information represents the voltage inducted inside of the material, which can be interpreted through the calibration of the real pressure applied on the sensor.
Strain gauges represent useful devices, as they are sensors that can function by themselves, but they are also being used in other pressure sensors, such as piezoresistive MEMS. There are many types of strain gauges, but in the end, every one consists of the measuring grid or resistive foil.
A strain gauge is glued on the surface of the material, by special glues that are made only for strain gauges. The applied mechanical stress on the material is transformed into the strain of the material. Strain is transfered also on the mounted strain gauge and depending on the direction of the strain, the resistivity of the measuring grid changes. It applies that with the strain gauges it is not possible only to detect the mechanical stress applied on the material, but also the direction of the force creating the straining. Different types of strain gauges are available depending on the application, which means these sensors are not very universal, as there are strain gauges for low pressures, high pressures, wide-range thermal stability, with almost no thermal stability, and so on. It means the user has to know his system well to apply a certain strain gauge.
From all the sensors mentioned above, MicroWire is not converting mechanical energy to electrical energy, but it is changing its magnetic properties, which are being sensed, by the outer circuit.
MicroWire glued on the material, or incorporated inside of the material, changes its magnetic properties proportionally to the applied mechanical stress on the material, and the change in the magnetic properties is sensed by the outer electronic circuit. It is important to understand, that the produced MicroWire is already a sensor by itself, the electronics is present just to extract and evaluate the data from the MicroWire sensor . As it is a micro-sensor, the measurements of the mechanical pressure applied are local, which leads to many applications, as you can monitor local strains from the outside, or inside of the material. For longer-scale measurements, there is a necessity to place other MicroWires, but only one outer electrical circuit for signal evaluation is needed. A pressure sensor in the form of MicroWire is thermally stable, which is given by the composition of the MicroWire, as mechanically dependent sensors are thermally independent, and temperature sensors are mechanically invariant. The glass-coating of the sensor makes it suitable for harsh environments, so it can measure the pressure from the inside of the material such as concrete, and signal extraction can be performed from the outside. Even if the MicroWire represents the magnetic sensor, due to the unique sensing method, the surrounding magnetic fields do not affect the signal from the sensor.