IoT Endpoints: What You Need to Know

Sensors and actuators—the endpoints that give “Things” in the Internet of Things their smarts—are the basic components of IoT infrastructure. A wide range of sensors and actuators are available and they vary in their features, capabilities, and application. It takes a judicious combination of these fundamental components to get IoT off the ground.

How Sensors and Actuators Work

Sensors imitate the capability of human sensory organs to detect changes in the surrounding environment. However, with sensors, the sensing capability is several times stronger and their reach far exceeds those of humans.

Sensors detect physical stimuli (heat or force, for example) in their environment and convert them into measurable signals (such as temperature or pressure). Together with actuators—the devices that convert electrical inputs into action—sensors enable complex machine-to-machine and human-to-machine interactions.

Commonly Used Sensors

Temperature Sensors

The principal uses of temperature sensors include monitoring and controlling temperature as well as preventing equipment failure through the detection of overheating. Temperature sensors differ according to various parameters such as temperature range, accuracy, calibration, response time, among several others. Examples: Thermocouple, Resistance Temperature Detector, Thermistor.

Parameter Thermocouple Resistance
Temperature Detector(RTD)
Thermistor
Operating Temperature Range -200 °C to 1750°C -240 °C to 650 °C -40 °C to 250 °C
Accuracy Low High Medium
Sensitivity Low Medium Medium to Fast
Response Time Medium to Fast Medium Medium to Fast
Cost Low High Low

Pressure Sensors

Pressure sensors are used to measure the pressure of liquids or gases as well as variables such as flow, speed, and water level. Since pressure reading is required for a wide range of conditions and materials, pressure sensors vary in their design. Pressure measurements can be absolute, gauge, or differential and are taken relative to vacuum, ambient atmospheric pressure, and specific reference pressure respectively.

Flowmeters

Flowmeters help measure how much fluid is flowing through a point at any given time. While there are several flowmeters, the appropriate one is chosen based on parameters such as the type of fluid, its pressure, temperature, viscosity, size of the pipe, and so on. The determining characteristics of the sensor include accuracy, costs, and ease of installation and maintenance.

Accelerometers

Accelerometers help measure the forces of acceleration, which can be either static or dynamic. Static sensors help measure the angle at which a device is tilted in relation to the earth. Dynamic sensors help analyze the movement or vibration of a device. Selection is based on frequency response, number of axes to be measured, variable capacitance, interface used (analog, digital, or pulse-width modulated connection), etc.

Beacons

Beacons are small, short-range devices that can signal their location to Bluetooth devices in their vicinity. The signal strength conveys the location of the beacon in relation to the receiving device. Beacons leverage Bluetooth Low Energy (BLE) protocol for their short-range communication and therefore consume very little power. Beacon-enabled location-based applications are widely used in retail marketing.

Infrared Sensors

Infrared sensors can sense changes in their environment by emitting or detecting and measuring the infrared radiation emitted by an external source. There are two types of infrared sensors based on their function – thermal infrared sensors and quantum infrared sensors. Thermal infrared sensors have lower detection capabilities and response times compared to quantum infrared sensors.

Proximity Sensors

The presence of objects can be detected by proximity sensors. These sensors predict presence by sensing and analyzing electromagnetic fields, light, or sound (and not through physical contact). Proximity sensors are of many types: inductive, capacitive, photoelectric, ultrasonic, magnetic, and so on. Their response time is high compared to limit switches, which require physical contact.

Vision Sensors

Vision sensors or vision systems can scan scenes to detect objects that are in their field of view and inspect variations from preset tolerance parameters. They can detect dimensions and compare contours of objects. Vision sensors comprise a camera that captures images and an engine to process the images.

Choosing the Right Sensors

Deciding which sensors to use involves careful consideration of several aspects.

  • Application Requirements

    Requirements vary application to application. It can be remote sensing, condition monitoring, location tracking, color detection, displacement measurement, or many others. Another consideration is the kind of object that needs to be sensed.

  • Operating Environment

    What is the ambient temperature? Will vibration impact sensor performance? What are the other sensors deployed in the environment? These are some of the many questions that arise in relation to the operating environment at the time of sensor selection.

  • Performance Requirements

    What is the permissible time delay? What is the expected accuracy? What should be the sensing range? Differences in performance parameters can affect the results and the usefulness of the sensor network.

  • Cost Benefits

    Cost is a major consideration in sensor selection. The performance benefits need to be weighed against the cost of the devices, their installation, and maintenance.

Sensor Characteristics

Accuracy

Accuracy refers to the correctness of a sensor’s output compared to an absolute standard.

Resolution

The smallest change that a sensor can reliably indicate is referred to as its resolution.

Precision

Precision is the capacity of a sensor to give the same output when the same parameter is repetitively measured in the same conditions.

Response

The time taken by a sensor to approach its true output when subjected to a step input is referred to as its response time.

Sensitivity

Sensitivity refers to the change in sensor output per unit change in the measured parameter.

Operating Range

The operating range of a sensor refers to the maximum and minimum values that it can measure.

Using Multiple Sensors

In industrial deployments, a combination of sensors is used to make a high-level inference about a machine or operating conditions. Multiple and heterogeneous sensor inputs are combined to obtain more reliable and accurate information. Having a mix of sensors has many advantages. It reduces the risk of information loss due to the failure of any one sensor. It improves the spatial and temporal coverage, reduces errors and wrong interpretations, and provides an output with a higher resolution.

Power Considerations

Sensors are power-operated. Their sensing, transmission, and data processing capabilities are as only as reliable as the source that powers them. Choosing the most efficient power source is therefore of utmost importance.

Some of the technical challenges with respect to powering sensor systems arise from the fact that sensors are often located in remote and inaccessible locations. In some cases, they are also on the move. This makes wired networks out of the question.

In industry settings, sensors are operated with the help of one or a combination of the following power systems:

Batteries

The most common types include alkaline, lithium, and nickel metal hydride batteries.

Alkaline Lithium Ion Nickel Metal Hydride
Pros
  • Available in different sizes
  • Low self-discharge rate
  • Low cost
  • Lightweight and compact
  • Low self-discharge rate
  • High energy density
  • Can be recharged more than 500 times
  • Suited for high-drain devices
  • High energy density
Cons
  • Not rechargeable
  • Heavy
  • Not suited for high-drain devices
  • High cost
  • Not available in standard cell sizes
  • Discharge-lifespan trade-off
  • High self-discharge rate
  • Limited life – needs to be recharged every 1-2 months
  • Poor performance in high temperatures

Capacitors

Capacitors are devices that store potential energy electrostatically by holding apart pairs of opposite charges (unlike in batteries, where electrochemical processes are involved). The basic design consists of two parallel plates, one positively charged and the other negatively charged, separated by an insulator block. The amount of energy stored depends on two factors: the size of the plates and the type of insulator used.

Parameter Batteries Capacitors
Charging time Low High
Energy Density High Low
Lifecycle Limited Long
Cost Low High

Energy Harvesting Devices

Large-scale deployment of wireless sensors entails the use of a high number of batteries. With their limited lifespan, the cost of maintenance runs high. For low-power devices, energy harvesting technologies offer a cost-effective alternative. Energy harvesting devices convert ambient energy such as heat, light, or vibration to electrical energy. The physical and environmental constraints play a role in determining whether they are a more viable option compared to batteries.

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