RTC Magazine

Today, we are entering this third wireless wave. Also known as “The Internet of Things,” the third wave utilizes wireless sense and control technology to bridge the gap between the physical world of humans and the virtual world of electronics. Sense and control networks do not enhance human communication. Instead, they allow sensors to interact with actuators, creating a more dynamic world and avoiding error-prone, monotonous and costly human intervention. However, the strength of wireless sensor networks can only be fully achieved when the wiring for both the data communication and the power supply is eliminated.

The standard wireless sensor network solutions that are available today only solve a wiring problem in sensor applications: making networks easy to install. However, ultra-low-power wireless network solutions can also address the maintenance problem inherent to networks with a high number of nodes and a limited battery life. For example, a network of 4,000 nodes and a battery life of 10 years, means that on average 1 battery per day needs to be changed.

Because the true cost of wireless sensor networks has shifted into the area of the maintenance cost, wireless sensor networks that gather industrial or machine data have yet to become a cost-effective solution. The majority of the sensors used are still battery powered; these batteries require regular changing and or recharging. In addition, reintegrating the downed nodes after battery maintenance, further adds to this onerous labor expense. To avoid these high costs, the industrial sector and its applications require self-powered nodes that require very little power to operate for long periods of time. This is generating the rapidly emerging opportunity for ultra-lower-power wireless and energy harvesting.

As you can imagine, there are many different types of applications that will be able to benefit from ultra-low-power wireless sensor networks. These include monitoring of temperature, vibrations, humidity, position, tank levels, etc. in industrial plants and manufacturing. They can also be linked to the control and actuation of HVAC systems, storage, robot movements, temperature control, etc. But there are many others that are not so obvious. For example, agricultural applications now benefit from the use of wireless sensor networks when temperature sensors or soil moisture sensors are used for remote monitoring of test fields, vineyards or green houses and to control irrigation and fertilization.

For many real-world applications, the third wave of wireless–ultra-low-power wireless sensor networks–will provide many advantages including the cost elimination of hard-wiring, the enhanced flexibility in constricted or dangerous areas, ease of installation, increased safety and reduced maintenance costs of sensor deployments.

The challenge of designing wireless sensor applications is not limited to deploying reliable wireless communication. Power management is an even bigger challenge. This should not be a surprise since the real benefit in wireless communication is primarily to avoid the wiring cost, so the data cables as well as the power cable need to be eliminated.

The biggest technical challenge for developing ultra-low-power sensor networks is managing the energy consumption without reducing range or functionality, like speed and standards compliance. The resulting elimination of battery replacement will then simplify maintenance and provide a higher level of ease of use and safety.

Ultra-Low-Power Consumption

It is obvious that current consumption–milli-amps–and duty cycling are important in wireless sensor networks. However, minimizing current consumption is only part of the solution. Five other essential issues are key to developing low-power wireless sensor applications, the first of which is low-power Wireless Mesh Routing.

One of the most dramatic differences between wireless sensor communication technology and other well-known wireless technologies is the ability of sensor nodes to forward messages from other nodes further down a communication chain. This is called mesh routing or multi-hop networking. Mesh networking is an effective and reliable solution for spanning large infrastructures beyond the range of what a single wireless link can do.

In a low-power mesh network, all the nodes, including the mesh routing nodes, operate in low power. Figure 1 depicts how low-power routing works when Node A wants to send a message to Node C, through Node B. All nodes in the pictures are low-power nodes and are in sleep mode most of the time.

The breakthrough lies in synchronizing the sleep/wake-up cycles of the nodes to each other. This means a node wakes up when it can expect a message from a neighbor node. This works through very precise synchronization of the transmitting and receiving nodes. As a result, the routing nodes will also be in a nearly powerless sleeping state most of the time, achieving ultra-low-power operation. The more accurately the wake-up schedule can match the communication expectations, the less power is consumed by unnecessarily long wake-up periods.