Tag Archives: Wireless Sensors

Sensors for Structural Health Monitoring

Public bridges and roads require their structural health to be monitored, and engineers use sensors for continuous measurement. To power these embedded sensors, they exploit several sources of ambient energy. This can include vibrational energy obtained from vehicular traffic, which can generate adequate power for sensor nodes that engineers have built into the infrastructure. Off-the-shelf devices make it easier for engineers to design structural monitoring devices. Many manufacturers now provide such sensors.

Drivers are rather well-acquainted with potholes on the bridges and roads on which they frequently travel. However, apart from the surface damage, there are more insidious structural damages that may be less obvious. One of them is stress corrosion cracks in structural components that may lead to a bridge collapse.

Therefore, engineers are rightly concerned about existing infrastructure developing similar defects. The rise in vehicular traffic over bridges and roads, often going beyond the original design specifications, together with rapid aging from the stress, can lead to their continual wear and tear and deterioration. Engineers use Structural Health Monitoring or SHM based on continuous monitoring of infrastructure. This is critical for identifying structures at risk.

Monitoring the system through wireless means is more practical, as this avoids the expenses of using wired system monitoring. Wireless monitoring also leads to the simpler placement of sensors within the existing infrastructure. Powering the wireless sensors with energy harvesting techniques further enables avoiding the cost and maintenance concerns related to using batteries and their periodic replacement.

Engineers use various ambient sources for powering the nodes of SHM wireless sensors. This includes vibrational, thermal, and solar sources. Ultimately, the optimum choice depends less on the technical requirements but rather on the logistics, cost, and maintenance requirements related to the target structure. For instance, noise barriers may be necessary for roads in urban areas with heavy traffic. These noise barriers may double as solar panels for energy harvesting.

Some situations may offer alternative sources of energy for powering sensors. These could be thermoelectric generators or TECs, which generate power based on the temperature differential across them. Such differentials often exist between the subgrade layers and the pavement surface of a road. Although using TECs in new constructions may be quite effective, retrofitting in existing roads may involve prohibitive costs.

Engineers often use a heavier tip mass to augment the mechanical loading of a piezoelectric device. Such loading helps to reduce the natural frequency of the device, bringing it closer to the predominant frequencies from the ambient vibrational energy source, enabling maximization of power generation.

In some cases, the ambient vibrational energy source may have frequencies well below the tunable range of the piezoelectric devices available. Engineers then turn to alternative low-frequency vibrational energy transducers like electromagnetic generators. The low-frequency vibrations cause a spring-mounted magnetic core to move through a coil, thereby converting the energy of vibrations to a current following Faraday’s law of induction.

Ambient-powered wireless sensors also require power conditioning and management. Power management circuits monitor the energy harvested, regulate the voltage applied to the load, and use the excess energy to charge external energy storage devices like a rechargeable battery or a supercapacitor.

Waspmote Plug & Sense! : Solar-Powered Wireless Sensor Platforms

Today, we use sensors for a myriad of activities such as intrusion detection, fall detection, patient surveillance, art and goods preservation, offspring care, animal tracking, selective irrigation, and many more. Where the sensor network has to operate outdoors, what can be a better way of powering them other than through solar means?

Using an external or internal solar panel, one can safely recharge batteries for the system. For external solar panels, the panel is usually mounted on a holder tilted at a suitable angle ensuring the maximum performance of the outdoor installation. When space is a major challenge, such as indoors, the solar panel can be embedded on the front of the enclosure. Typical rechargeable batteries used for powering loads are rated 6600mAh, and this ensures the sensors do not stop working even when the sun is not providing adequate light.

Such platforms of wireless sensor networks provide solutions for Smart Cities. Waspmote Plug & Sense! from Libelium is a system of encapsulated wireless sensor devices that allow system integrators to implement modular wireless sensor networks in a scalable manner. The Libelium system reduces the installation from days to just hours.

Each node of a Waspmote Plug & Sense! comes with six connectors. You can connect sensor probes to these connectors directly and the system is ready to install and easy to deploy. Using connectors ensures that the services remain scalable and sustainable. The possibility of powering the platform through solar power allows energy harvesting and years of autonomy.

Once the sensors have been installed, the nodes on the Waspmote Plug & Sense! can be programmed wirelessly. This is possible because of the special feature, OTAP or Over The Air Programming, incorporated into the platform. Thanks to OTAP, users can replace or add sensors without having to uninstall any of the nodes. This helps to keep the maintenance levels within reasonable limits. For example, to extend the service, you can easily add a noise sensor to a network consisting of CO2 probes, simply by attaching it.

The applications are endless for the Waspmote Plug & Sense! platforms. Apart from Smart Cities, the models are preconfigured for creating other widely applicable services out of the box, such as radiation control, ambient control, smart security, air quality, smart agriculture, smart parking and so many more.

You can use these sensor platforms anywhere in the world, as they use the generally available radio frequencies 2.4GHz and 868/900MHz, besides complying with certification standards such as CE, FCC, and IC. Usually, these sensor platforms send information to a sensor gateway that in turn, uploads the data to a cloud service. Therefore, the data is accessible from anywhere in the world and users can integrate it easily into third-party applications.

Use of solar-powered wireless sensor networks makes it so easy for adding a new sensor that municipalities find they do not have to reinstall the network for Smart Cities. The solution reduces the complexity of the installation and its maintenance, while providing it with a high degree of scalability. Available with IP65 enclosures for outdoor deployment and no software license fees, these platforms offer remarkable opportunities.

Wireless sensors sans batteries

The Internet of Things has led to several simple sensors being used for applications requiring reporting of their readings wirelessly to a gateway or hub. However, most sensors require to be powered from batteries, creating logistical and cost barriers to several use cases. Now, many wireless sensor modules appearing on the market do not require batteries, as they are ultra-low power types.

Several key building blocks are necessary to make up a wireless sensor module meant for IoT use. The first among these is the sensor itself, its signal feeding a micro-controller that processes and packages it for transmission. The final part consists of a radio transceiver to send the information to its destination. Even with the most careful logic design, these building blocks work at a minimum of 1.8V, using up several tens of microamperes at modes requiring the lowest power.

However, in the last decade, extensive research has resulted in development of sub-threshold circuits involving logic, memory and RF. Transistor switching, in conventional logic design, takes place between saturation and an on-off state, dominated by leakage currents. Switching mostly occurs at a gate-to-source voltage or VGS of about 0.5V, which is the threshold voltage or VT for the transistor. In conventional logic, VGS < VT, is the condition for the transistor to remain in the off state. Sub-threshold circuits use this off-state region for the two operational states of a transistor. With the transistor's gate voltage operating below the threshold, the supply voltage can go lower than the conventional 1.8V. An active logic circuit consumes power relative to the square of the supply voltage. Therefore, operating at lower supply voltages can mean considerable power savings. The drawback in this manner of operation is that switching speeds slow down – but that does not hamper many applications. Another requirement of sub-threshold circuits is that a careful control is to be exercised on device physics, including circuit structures. These are necessary to mitigate the effects of temperature variation and noise. However, researchers have provided answers for these problems as well and the solutions have proven themselves practically. Functioning circuits are available for analog, microprocessors and memory devices. Sub-threshold designs are now starting to appear in the market as full SOCs. Universities of Michigan, Virginia and Washington have culminated their research efforts as a two-year old startup, PsiKick. They are preparing a sub-threshold circuitry based wireless sensor module that will operate without batteries. Aside from the RF transceiver, a micro-controller and a sensor front-end, the module will include blocks for energy harvesting. This makes it a self-powered sensor platform that can be used in a wide array of applications. Another design, a second-generation version, is on the cards. This is based on standard CMOS technology and a demonstrable product is due any time soon. The sub-threshold module requires astonishingly small power to operate. Compared to sensor platforms currently available, these modules will consume 100 to 1000 times less power. When fully operating, the micro-controller consumes only 400nW while the RF transmitter generated 10µW, which is effective within a 10m range. The module operates within a supply voltage range of 0.25 to 1.2V. That makes the module eminently suitable to the output capabilities of most energy harvesting methods.

Why Are Industrial Sensors Going Wireless?

Industries are increasingly opting for low-power wireless photoelectric sensors with extended range of signals that carry for miles. Such improvements have been made possible with the proliferation of low-power micro-controllers that have boosted the range of the sensors and enhanced their battery life.

In general, wireless sensors conserve and extend battery life by switching themselves off when they are not taking measurements. This allows the sensor to spend most of its time not consuming any power. With this simple technique itself, the battery life of the sensor is boosted by a factor of 100 or more in comparison to that of a continuously powered sensor. However, as the sensor does not sense when it is off, the response time suffers.

To understand how much the battery life can be extended, consider a dry contact wireless sensor that typically dissipates about 100 to 200 µW of power. Such a sensor operates on two AA batteries, which last for five years with the dry contact wireless sensor sampling at 10 times or more every second. In comparison, a powered sensor system can remain on continuously and can respond more quickly. It is also possible to run them at higher power levels to produce a longer wireless range.

To provide reliable and interference-free communication, FHSS or Frequency-Hopping Spread Spectrum techniques are used in industrial wireless sensors. Basically, FHSS switches a carrier rapidly among several possible frequencies, using a pseudorandom sequence. When bound or paired devices communicate with each other, data and control packets are interchanged using these frequency channels randomly, but in a pattern known only to the communicating pair.

Typically, the bandwidth necessary for frequency hopping is much larger than that required for transmitting the same information on just one carrier frequency. However, the transmission takes place only on a small portion of the bandwidth at any given time. Since the effective bandwidth of any interfering signal is the same as that for a narrow carrier, frequency hopping greatly diminishes interference from narrowband sources. Usually, a site survey is conducted before installation of wireless sensors to determine if there is RF interference and whether this is strong enough to be a problem.

Modern wireless sensor systems have a radio master device or gateway that polls all its sensor nodes at specific intervals to ascertain radio communications are still operating. If there is no response from one of the sensors, the system reacts deterministically; the system enters a state to maintain control in a fail-safe way.

The radio master connects to multiple sensors allowing many dozens of wireless sensor nodes to work within a single radio network. Using a TDMA or Time-Division Multiple-Access technique, ensures that all the sensors in the network have adequate time to transmit their data and receive their individual instructions. This effectively eliminates the possibility of multiple sensors trying to communicate simultaneously.

One of the major advantages of using wireless sensors and indicator lights is the elimination of complex cable installation. Rearrangement can easily be done if the plant layout changes. The modern wireless sensor with its own battery, radio and sensor in a single housing, allows higher productivity with real-life status of the production line.