The Internet of Things (IoT) brings the value of information technology to the physical world. By combining sensors and actuators with computer networks, we can detect what is happening in the physical world, make these readings available to software algorithms, calculate results based on the readings and, finally, direct actuators to use these calculations to modify the physical world. Generally, these networks are closed-loop, which means that the physical parameter that an actuator controls is immediately read back into the system by a sensor, closing a continuous loop in real time and allowing tight monitoring and control of physical processes.
IoT sensors and actuators
The IoT adds actuators to computer networks and the control of the physical world they provide adds layers of complication to IoT applications. In the past, system problems such as software errors, computer security breaches or component failures could cause IT managers significant concern because their data could be corrupted, lost or stolen. When actuators are added to the network – especially if these control powerful or dangerous systems such as locomotives, reactors, substations, vehicles or medical devices – if the systems are hacked or suffer failures, serious property damage, personal injury or even death can occur. We therefore need to take special care with connected devices (the IoT things) such as system integrity and security, if actuators are included.
Most sensors and actuators include transducer types. Transducers are devices that convert one form of energy into another. For example, the breaks in your car convert mechanical energy into thermal energy. For IoT systems, almost all sensors use some physical parameters and transform them into electrical signals. Likewise, almost all actuators in IoT systems pick up electrical signals and convert them into some kind of physical output. The physical parameters used in IoT systems run through the book of physics. They can include electrical (voltage, current, power, resistance, capacitance, inductance, frequency, phase, etc.), mechanical (position, velocity, acceleration, weight, compass direction, gravity, force, tension, pressure, flow, torque, magnetic field, etc.), acoustic (sound, vibration, seismic movement, etc.).), imaging (light intensity, cameras, monitors, infrared (IR), light detection and ranging (LiDAR, etc.), chemical (hydrogen potential (pH ) concentrations, composition, purity, etc.), medical (heart rate, respiration, blood pressure, temperature, electroencephalogram (EEG), etc.) and much more. There are literally thousands of types of sensors for all of these physical parameters and for most of the parameters that can be detected and there is an analogous actuator to modify that parameter in the physical world.
Sensors usually include a raw sensing element or transducer and a signal processing chain to make the raw readings available to networked computers. Often, the raw sensor element – something like a thermistor, accelerometer, microphone or light sensor – produces a modest/simple analog signal. This signal must pass through a signal processing chain that amplifies, filters and converts the raw signal into a format that our control computers and their software can input. This usually involves analog to digital conversion and some kind of computer interface such as I²C or USB. Sophisticated sensors use digital signal processing techniques to filter, condition, average and format sensor readings in the digital domain.
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Actuators have inverse functions. The digital output of the control computers and their software is delivered via an interface to the drivers that pick up the signals and convert them into whatever inputs the transducer in the actuator requires. This usually consists of a digital-to-analog converter, an output filter and some kind of amplifier. Increasingly, digital techniques such as digital signal processors (DSPs) and class D amplifiers are being employed in actuators to make them more precise, responsive and energy-efficient.
Example of using actuators and sensors for simple IoT functions, such as making a cup of tea
Often, the number and diversity of sensors and actuators in networks is unexpectedly high. Stop for a minute and consider your smartphone and all the types of sensors and actuators it includes. To further illustrate this point, let’s consider a robot system for making cups of tea. In Star Trek the Next Generation, Captain Pickard would approach his food replicator and say “Tea, Earl Grey, Hot” and in a few seconds he would be served. Although the transducers needed to materialize a tasty drink in seconds from pure energy may not exist, at least not yet, let’s consider a practical system that can perform a similar function and examine all the sensors and actuators that a fully possible version might contain:
The first step in this process is to accept the user’s request. The request can be made via a keyboard (simple button sensors), a capacitive keyboard (capacitance sensors), a microphone (sound sensors) or a gesture-seeking camera (image sensors). Requests can also arrive via a computer network using connection types that can include fiber (optical sensors), wireless (RF sensors) or wired (electrical sensors). A local processor accepts this input and coordinates the sensors and actuators in the machine as a real-time control system. If it’s a vending machine, additional sensors and actuators accept the currency or credit cards and check that they are genuine (Figure 1).
Figure 1: Image of a beverage machine (Source: Mouser)
Sensors continuously monitor the temperature and pressure of the infusion water in the reservoir. A resistance heater serves as the actuator that controls the water temperature. If there is a cold water reservoir for cold brewed beverages, it has its own temperature and pressure sensors, and perhaps a Peltier module as an actuator to provide thermoelectric cooling. There are feedback loops from the sensors, through the control processor and the actuators, to ensure that the water is at the desired temperature for optimal fermentation.
Next, we have to select a tea bag. Our machine may have a magazine of tea bags of different types that represent the varieties it can make. Sensors confirm the presence of the bags – perhaps optically or mechanically. Robotic actuators use motors to index the magazine to the correct position and lower the selected tea bag into the brewing station. Another robot actuator can move the cup to the brewing station and the sensors confirm that it is positioned correctly.
Now, the valve actuators open to let the water into the fermentation station. Ultrasonic level sensors measure the amount of cup and cut the valves at the right moment. Thermal sensors monitor the temperature of the tea as it is brewed. Motors that lower the tea bag can gently dip it up and down for easier dispensing. An optical sensor can measure the color of the tea to remove the tea bag when the desired strength is reached. An actuator, such as a solenoid, can release the used tea bag into the waste container.
Additional actuators can add the selected condiments, for example a motor-controlled dispenser for measuring sugar or cream, or a valve for adding honey. Optical, weight or ultrasonic sensors in the condiment boxes check that the supply is adequate and the dispensers are measuring correctly. A little robotic spoon – requiring several motor-driven actuators to position, move and clean it – gives a final stir and the actuators open the delivery door and present the cup to the user.
So using IoT techniques to automate something as simple as making a cup of tea could take several dozen sensors and actuators, their interface circuits, a fairly sophisticated processor and a lot of software. I don’t know if it’s worth making such a machine today, but as the capabilities of sensors and actuators continue to increase and their costs continue to fall, it’s likely that such systems will start to appear.
Key points:
- Sensors and actuators are the elements that define Internet of Things networks.
- Many electronic components are needed to amplify, condition and interface the signals from the raw transducers to the control processors.
- It takes a surprisingly large number of different types of sensors and actuators to perform simple IoT functions, even to make a cup of tea.
Article originally written by Charles Byers for Mouser Electronics: Access the original text at the link. Translated by Equipe Embarcados.
