Sensors are fundamental measurement devices for acquiring data and performing actions on such measurements. An inappropriate choice of these devices can negatively affect our project.
What we are going to see today are three types of sensors: Resistive, Capacitive, Inductive.
Resistive sensors are sensors that convert the quantity to be measured with a variation in the resistance of the component. They are also very common due to the numerous physical quantities that can alter the electrical resistance of the material.
- Potentiometers: Particular type of resistance that is able to modify the geometry through a sliding contact. It is used both as a command and as a position sensor
- Strain gauges: sensor for measuring surface deformation. The sensor, consisting of a metal wire applied to a plastic support, once glued to the surface to be measured, follows its deformations by varying its resistance based on the expansion of the wire.
- Resistance thermometers: Sensors based on the fact that the resistance to current grows linearly with increasing temperature, generally the range of these sensors are really wide but suffer from self-heating.
- Thermistors: These exploit the property of semiconductors of increasing resistance as the temperature varies. Unlike the resistance thermometers, they have very limited renge and linearity zones.
- Photoresistors: This type of resistance is able to vary its resistive component as the surrounding light intensity varies.
Capacitive sensors are sensors capable of varying their capacity as the size we want to measure varies. There are 2 ways to vary the capacity of these sensors:
- The first is to vary the distances of the 2 capacitor plates
- The second, on the other hand, consists in modifying the dielectric constant (ie “the tendency of the material to contrast the intensity of the electric field present inside it” ¹) by modifying the material between the 2 plates of the capacitor.
These sensors are used for:
- To measure the level of liquids: in this case, with the variation of the liquid level, the value of the dielectric constant also varies;
- To measure mechanical quantities: such as displacement. In fact they are often used in MEMS sensors (Micro Electro-Mechanical Systems, known as: “the sensors that are in the smartphone”);
- Proximity sensors: in this sensor there is only one face of the capacitor, and a conductive or partially conductive object (such as skin) passes near the face of the capacitor and transforms into the second face forming a detectable capacity.
These sensors will barely be mentioned, as I wanted to focus more on resistive and capacitive sensors. These sensors convert the measured quantities into a change in inductance, usually the change in this inductance is caused by a change in the geometry of the sensor or by a change in the magnetic field.
Each type of sensor has its positives and some negatives, this is the reason why the choice of the sensor is not something to be taken for granted. To best choose a sensor, you have to take a look at the Datasheet of the company that produces them by looking at the various input and output parameters, operating range and linearity of the device.