Classifications of Sensors

Table of Contents

Various 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 below 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.

*Animation Showing the Resistive Sensors Change*

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

Additional Common types of Sensors

Sensor	Description [5]	Image of Sensor [5]
Temperature	For analog temperature sensors, any change in temperature causes a change in a physical property such as voltage or resistance.
Proximity	Proximity sensors detect a nearby object without direct contact. An electromagnetic field is emitted and any change in the field returns a signal back to the sensor.
Infrared (IR)	An infrared sensor detects and measures IR in its surroundings. The two main types are transmissive and reflective. In the transmissive sensor, there is a IR detector and transmitter facing each other. It is able to sense an object between these two components. In the reflective IR sensor, the two components are adjacent to each other.
Ultrasonic	An ultrasonic sensor is an electronic device that measures the distance of a target object by emitting ultrasonic sound waves, and converts the reflected sound into an electrical signal. Ultrasonic waves travel faster than the speed of audible sound (i.e. the sound that humans can hear). Ultrasonic sensors have two main components: the transmitter (which emits the sound using piezoelectric crystals) and the receiver (which encounters the sound after it has travelled to and from the target)
Light	A light sensor indicates the intensity of light. It converts light energy into an electrical signal which it outputs. A popular type of light sensor is a light dependent resistor (LDR). The resistance is inversely proportional to the ambient light.
Smoke and gas	These sensors detect and identify different types of gases and are often apart of a larger embedded system. A metal oxide based gas sensor is the most common type of gas sensor. Its electrical resistance increases as it comes into contact with a specific gas.
Alcohol	These sensors can detect alcohol concentration. It is a metal oxide sensor which also referred to as a chemiresistor. When the sensing material is exposed to alcohol there is a change in resistance. This sensor is placed in a voltage divider network.
Touch	In a touch sensor, when there is some touch or pressure on the surface and electrical circuit opens up which allows current to flow. The two main types are capacitive and resistive touch. In capacitive touch sensors capacitance decreases when a touch is sensed and allows the touchpoint to be determined. In voltaic touch sensors, when the surface is pressed the inner and outer conductive layer is pressed against each other causing voltage changes. These changes are compared to the starting voltage were allows the touch point to be calculated.
Color	A color sensor is similar to a light sensor. It emits light via a transmitter and detects the reflected light. It can detect red, green and blue light intensity which allows it to determine the color of an object.
Humidity	Humidity sensors work by detecting any change which cause electrical currents or temperature in the air to be modified. They fall into the category of thermal, capacitive and resistive.
Tilt	Tilt sensors detect orientation and inclination. They detect any change in angle from a zero state. There is a metallic ball which moves two pins which determines the on and off position until it reaches the specific angle.

References

[1] Y. Yu and S. Bedi, "Sensors", University of Waterloo, 2020.

[2] "Learn.Digilentinc | Resistance and Ohm's Law", Digilent Inc. [Online]. Available: https://learn.digilentinc.com/Documents/331. [Accessed: 16- Dec- 2020].

[3] "Capacitive Sensor Introduction", Cs.cmu.edu. [Online]. Available: http://www.cs.cmu.edu/~ram/capsense/intro.html. [Accessed: 16- Dec- 2020].

[4] E. Csanyi, "Purpose and Working Principle of Inductive Sensors", EEP - Electrical Engineering Portal, 2014. [Online]. Available: https://electrical-engineering-portal.com/purpose-and-working-principle-of-inductive-sensors. [Accessed: 16- Dec- 2020].

[5] "What is a Sensor," ElectronicsHub, 2021. [Online]. Available https://www.electronicshub.org/different-types-sensors/. [Accessed: 6-Aug-2021]

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