Wireless Technology is evolving from communications to between people and computers to communications between machines. There is a third wave of wireless that is following the almost ubiquitous integration of cell phones and wireless Internet (Wi-Fi) into our lives.
This third wireless wave consists of wireless sense and control networks that can connect and control all kinds of equipment in our homes and businesses – from freezers to light switches, from consumer electronics (TV, DVD-player) and remote controls to sensors, for detection or protection, and to central door locking and window locking in our homes (as we are used to in our cars).
Unfortunately, using today’s wireless technologies, most of those wireless sensors and controls require the use of a significant quantity of batteries creating environmental concerns (think toxic chemicals and heavy metals) as well as a serious maintenance problem (continuously exchanging batteries). Therefore ultra low power wireless networks that require very little power are of great interest.
This includes systems that can run off of a single cell battery for the life of a device as well as wireless networks and sensors that can be powered by energy harvesting (sometimes called energy scavenging). Creating ultra low power wireless networks and systems that can run off the energy that is available in the environment instead of batteries is a very exciting emerging technology.
Last year, the ZigBee organization partnered with several of the largest consumer electronics companies in the world (Panasonic, Philips, Sony and Samsung) to form what is known as ZigBee RF4CE (Radio Frequency for Consumer Electronics). This industry partnership signals the development of an entire new generation of remote control devices – for TVs, for home and office automation, for many other types of remote control products that communicate via low power RF instead of the decades old IR (infrared). By using these new communication technologies, we soon shall be seeing a wide range of remote devices that are not only interoperable among brands and models, but require so little power that their batteries will never have be changed or recharged. It is even possible to design and build remotes that will not require any batteries at all and will get their power from energy harvesting.
Challenges of wireless sensor networks
The biggest technical challenge for developing these 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. There are several essential issues key to developing low power wireless sensor applications, but it all starts with the development of an ultra low power transceiver radio chips.
By using a communication controller centric chip design instead of a microcontroller centric design, along with synchronized wake-ups, it is possible to reduce overall power consumption by 65% or more.
Most transceiver solutions require that the MCU be switched on the whole time during the transmission of a package. By using GreenPeak Technology’s GP500 communication controller, the MCU is only required to process the data to be transmitted or received.
Most low power radio networks rely on a processor centric approach that requires a microcontroller to handle all the intelligence for the transceiver. This requires the microcontroller to be awake the entire time that in turn requires additional power. By using a more energy efficient communication controller approach, the transceiver can transmit and receive the data independently from the microprocessor and the microprocessor is only awakened and used when it is needed to further process the data.
By using a hardware based scheduler and synchronizer within the chip itself, the radio only wakes up as needed to see if there is any data that needs to be sent. If not, it returns to sleep. If there is data to be sent, the controller then wakes up the microcontroller. The chip then communicates the information and then goes back to sleep until the next time it is scheduled to wake. 9999 times out of 10,000 – there is no message to be sent and the controller does not need to energize the microprocessor. Every time that data is sent, the chips also transmit a synchronization message to ensure that they all wake up together on the next duty cycle.
By letting the communications controller decide when to wake up and check for messages, it is possible to greatly reduce overall energy consumption. Because of the scheduler and synchronizer inside the communication controller, the system only wakes up for a brief moment to check to see if there are any messages and goes back to sleep. By letting the microprocessor sleep until it is needed, it is possible to save over 65% of energy usage as compared to a the typical always on traditional transceiver
If you multiply this individual node power saving by a wireless network of over 100 nodes, it is obvious that the entire network will be able to operate using vastly less power than a traditional microprocessor based network.
Peak current savings
There are three typical wireless sensor node states for a commonly used wireless sensor platform. Each has its own level of current consumption. In state one, the microprocessor and transceiver are in sleep mode (10µA). In state two, the microprocessor is switched on while the transceiver is asleep (10 mA). In state three, both the transceiver and the microprocessor are awake (27 mA).
When closely examining the power consumption behavior of electronic circuits, it becomes apparent that what initially looks like a flat current curve actually bears more resemblance to a mountain range with peaks and valleys. When certain functional blocks become active, they draw peak current. When two functional blocks switch on simultaneously, the peak amplitude doubles.
The secret to reducing the peak power lies in carefully managing the turn-on and turn-off time for key functions so that double peaks can be avoided.
Synchronized Wake Up and Sleeping enables reduction of power consumption for low power mesh networks
One of the most dramatic differences between wireless sensor communications technology and other well known wireless technologies is the ability of sensor nodes to forward messages from other nodes located further down the communications chain. This technique, known as mesh routing or multi-hop networking, provides an effective and reliable means of spanning large infrastructures, beyond the range of what a single wireless link can do.
For a node to forward a message received from another node, it needs to be in an awake and receiving mode when the original wireless message arrives. Unfortunately, the reception mode requires so much power that it can drain batteries in a matter of a few days. As this power lifespan is too short for most real-life applications, the most straightforward solution, as specified by most industry standards, is to limit the multi-hop capability to the nodes that are permanently connected to the main power. In such a framework, low-power devices, which are assumed to be in a power-down mode most of the time, are not capable of retransmitting messages from other devices. These low-power devices, known as end-devices, are located at the end or beginning of the communications chain.
This framework, which combines mains-powered mesh routing devices and low-power end-devices, works for some applications. Take, for example, an office lighting application utilizing interconnected wireless lamps and light switches. The lamps, which are connected to the main power source, house the mesh routing communication nodes. The switches, which are not mains powered, are a natural place for the end-devices.
Many other applications do not fit well in such a framework. In applications like gas detection, fire detection, access control, precision farming, battlefield monitoring, perimeter surveillance, warehouse temperature monitoring, etc., mains power is not readily available or even present. Running a power cable in these applications would be cost prohibitive, offsetting the benefit of wireless communication.
To address this class of applications requires low-power multi-hop networking, or low-power routing, in which all of the nodes, including the mesh routing nodes, operate in low-power mode.
By using a “synchronized wake-up” scheme, it is possible to coordinate receiving activity in a way that eliminates the need for the mesh routing nodes to continually operate in receive mode, thereby significantly reducing power consumption. The picture below 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, sleeping most of the time.
By synchronizing the sleep/wake-up cycles of the nodes to each other, nodes wake up when they expect a message from a neighboring node. This enables the routing nodes to operate in a nearly powerless sleeping state most of the time, thereby achieving ultra-low-power operation. Clearly, more wake-ups will occur than strictly required to carry the data, as neighboring nodes will not always have data to transmit. However, the additional power required for periodic wake-ups and synchronization is more than offset by the power saved by eliminating the need for continuous receive mode operation.
Since its inception, wireless sensor technology has been linked with low-power electronics. Most low-power wireless sensor networks have been designed for low power, meaning that they consume little power when switched on. That is not enough. By using communication centric transceiver chips, wireless mesh networks, and synchronized wake up and sleep cycles, developers can now create systems that don’t even need batteries and instead, can utilize energy harvesting to power the sensor network from environmental power sources.
The wireless sensor network standard – IEEE 802.15.4
For wireless sensor transceivers the dominant and probably only real standard is the IEEE 802.15.4 specification. However, there have been efforts to use Bluetooth and Wi-Fi for low power sensor applications. In most of the cases reported, Bluetooth and W-Fi were used in a non-standard way, in fact weaving the principles of IEEE 802.15.4 in their native implementation. It is nowadays widely accepted that the IEEE 802.15.4 offers the best basis for wireless sensor network applications.
Besides the IEEE 802.15.4 standard, a number of technology suppliers have chosen to build proprietary transceivers. The main motivation seems to be a reduction of the complexity and thus a potential lower cost point. However, it remains to be seen if a proprietary solution will ever reach sufficient volumes to actually reach that theoretically lower cost point. Additionally, reducing the complexity automatically goes hand in hand with sacrificing performance and thus limiting the applicability.
Proprietary technologies are vulnerable, for two reasons: (1) the owner of the technology controls the specification and thus also the price, and (2) the customer depends on the technology owner for upgrades and uninterrupted sourcing.
Even within the boundaries of standards, technology providers can discover and leverage differentiation opportunities.
As an example GreenPeak has developed transceiver and network stack technology that is compliant to the IEEE 802.15.4/2.4 GHz standard but includes additional functionalities that enable its use for ultra low power applications. An ultra-low-power application is defined as an application that is able to live off a coin cell battery or off energy harvested from the environment through a solar cell, a vibration energy harvester or any other environment energy converter.