Sensors
Sensors are devices that can detect the physical properties of the environment around them, convert the physical properties into electrical signals (analog or digital), and transmit the information to a processor either directly or through a gateway. Sensors can be electrical, electronic, mechanical, biochemical, or a combination.
Physical properties that sensors can detect and capture include almost everything humans can sense or experience and many others that humans cannot. Sensors can detect temperature, humidity, pressure, light, motion, touch, sound, acceleration, vibrations, moisture, and many more properties. The only exclusions at this time probably are taste and smell. Although, sensors can detect the presence of a gas using chemical reactions, which humans may detect with their sense of smell.
Sensors can also do something with these properties that humans cannot. They can measure and quantify the properties they detect. Humans can sense if it is hot or cold, or relatively hotter or colder, but they cannot determine the precise temperature. Humans can feel that a car is going fast but cannot measure the speed precisely. And this is true about everything that humans can sense. There is no quantified value since it is always relative and even subjective; for example, at the same temperature, one person may feel it is cold while the other may not.
The sensing process could be electrical, mechanical, chemical, based on light, based on properties of a material, or other such mechanisms.
Sensors may generate an analog output, which is a continuous electrical signal that varies in magnitude based on the changes in the measuring property, or a digital output with values at points when the property is measured. Digital sensors may also generate only binary values (zero or one) to indicate the absence or presence of a certain property.
The choice of analog or digital depends on the application. The table below lists some common sensors and how their analog or binary measurements can be useful:
| Sensor | Analog/Digital | Purpose |
|---|---|---|
| Temperature | Analog | A temperature sensor will send an analog signal that varies in magnitude as the temperature goes up or down. |
| Motion | Digital | A motion sensor will send a binary reading either in true or false value, which indicates some motion within a configured distance. It does not vary depending on how close the object is or how fast it is moving. |
| Sound | Analog | A sound sensor can measure sound signals and send an Analog signal whose intensity varies based on its intensity level. |
| Speed | Analog | Speed and acceleration sensors will send an Analog signal with values measured continuously over time. |
| Light | Digital/Analog | Light sensors will send a binary signal to indicate the presence or absence of light. They may also send an Analog signal that varies based on the intensity of light. |
| Smoke/Gas | Digital/Analog | Smoke and gas sensors will send a digital signal with a true value to indicate the presence of smoke or gas. They may also send an Analog signal that varies based on the concentration of the smoke or gas. |
| Vibrations | Analog | Vibration sensors will send an Analog signal that will indicate the level of vibrations over time. |
| Accelerometers | Analog | These sensors detect changes in position, velocity, and orientation and send a continuous Analog signal as any of those parameters change. |
Quality of a Sensor
The primary function of a sensor is to detect ambient properties, convert that to data and transmit the data to a processor. They are not designed to have much (if any at all) processing power since they are considerably small and low-powered. And since they could be located in harsh environmental conditions, they need to be sturdy.
The quality of a sensor is determined by a few parameters, the key ones are listed below:
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Accuracy:
This parameter suggests how accurately the sensor converts the physical property to the value transmitted. The sensor must consistently convert and transmit the same value for a given state for any property. A human would only be able to identify a broken or inaccurate sensor if the values given by the sensor are way off the mark. Small errors would go undetected as the human experience of a physical property is subjective.
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Range:
This parameter indicates the minimum and maximum values of the physical property the sensor can convert without error. The range required of a sensor depends on the application it is being used for.
For example, in a temperature sensor at home, the range can be between a few degrees Celsius below zero up to about 40 degrees Celsius above zero since temperatures are unlikely to be outside that range. Whereas a temperature sensor in a factory may need a range that goes up to a few hundred degrees.
An air pressure sensor for a car tyre will need a much smaller range of measurement than the air pressure sensor on an airplane which will need to read the pressure from the ground level to the maximum altitude the airplane can fly to. If an insufficient or incorrect range of a sensor is used in the IoT system, it will provide inaccurate readings outside of its range.
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Resolution:
This parameter indicates the smallest change the sensor can detect in the property that it is measuring. Again, the requirement of a high or low resolution depends on the application.
For example, a temperature sensor used at home to switch on or off an air-conditioner does not need a very high resolution, up to a degree is enough. But in the case of a laboratory for medical or scientific experiments sensors may need to provide readings to a fraction of a degree.