Types of Sensors

Resistive Sensors

These sensors can convert the stress applied onto it into a resistance value that is measured. This helps to determine the amount of load that is being put on the object or the direction at which the bending is occurring in an object. A way to measure the resistance is by applying Ohm’s law.V stands for voltage, I stands for current and R stands for resistance. The resistance can be found by measuring the voltage drop across the unknown resistor/sensor and the current can be found by using the initial voltage and a known resistance value. In the animation bellow it shows a strain gauge, a resistive sensor, and how when force is applied, the current and wire area decrease while the resistance and wire length increase.

Voltaic Sensors

These sensors create a voltage output based on a change of electricity within a system. An example of this is a thermocouple.

Thermocouple

A thermocouple is a thermoelectrical devices that is used for measuring temperature. They are usually made from two dissimilar metals that are joined to form two junctions as shown in the photo.

The set up of the thermocouple

One of the junctions is placed in or on the changing environment, the hot junction, and one junction is placed in or on an environment that is constant, the cold junction. The change in the hot junction creates a voltage reading which will flow through a loop. An equation is created to convert the voltage reading into a temperature reading.

Capacitive Sensors

These sensors can detect nearby objects based on the affect they have on the electrical magnetic field that is created by a capacitive sensor. The objects they detect are conductive materials such as glass. These sensors can even measure a presence of an object based on a medium that transmits electric force without using conduction but instead by using an insulator for example(e.g. wood, plastic).

How a capacitive sensor works

A common example of this sensor is a touch screen sensor. The sensor can measure the current across two dissimilar materials that are separated by air gaps. When a person touches the screen, the air gapes change and as a result the capacitive value changes. This change in capacitive value can be used to map out where the person has touched the screen.

Touch screen sensor

Inductive Sensors 

These sensors use current induced by magnetic fields to detect nearby metal object. The sensor uses a coil, an inductor, to produce a magnetic field. If there is a metal near the magnetic field, a current will flow through the metal. As a result, a new magnetic field is created which opposes the original magnetic field. This changes the inductance of a coil in the inductive sensor and that can be measured to determine when the object was brought near the sensor. 

A typical inductive sensor set up

References