Design of power sensor limit subsystem for wireless sensor nodes
If the systems around us are able to detect and respond to their own environmental changes, this will undoubtedly revolutionize our lives. A wireless sensor network is one in which distributed sensor implementations (nodes) in the system communicate with each other wirelessly to respond to physical stimuli. This article provides an overview of some of the latest developments in nodes to help you understand the system-level design methodology.
Figure 1 shows an example network and the subsystems of each node. Based on ease of deployment and lower installation costs, each node is required to be able to communicate wirelessly. In order to reduce communication overhead and reduce response time, we want nodes to be able to process sensor data locally and control the actuator. The daily maintenance of a large number of nodes (for example, battery replacement, etc.) can be extremely costly. Ideally, it would last for years to rely solely on storage/acquisition of energy sensors.
The sensor node of Figure 1 independently determines its environmental changes by collecting energy from the energy source, and can communicate using multiple protocols.
The choice of sensors, radios, and microcontrollers (MCUs) depends on the specific application properties. This article focuses on sensor networks in an office environment, and its applications include energy management, security, or resource planning.
Energy and storage
Light energy is usually the most abundant form of environmental energy in indoor environments. Some modern solar cells (made of amorphous silicon) produce about 5 uW/cm2 under the illumination of a 200 lux fluorescent light source.
Microthermal generators use a certain temperature gradient to generate electrical energy. But to produce a power density of 15 uW/cm3, the thermal collector requires a thermal gradient of about 10oC. Many application environments, especially indoor environments, do not have large temperature fluctuations. Therefore, the applicability of the heat collector is limited by these environments.
Some of today's vibration energy harvesters require accelerations of about 1.75–2.00 g (in general, indoor environments are not so large) to produce 60 microwatts of power.
The capacity of the energy storage board is very limited, and the opportunity to collect environmental energy is limited, so the sensor needs to use energy very economically. For example, a solar cell with a battery capacity of 100 mAh gets 70 uW, which provides half the power for 10 years of node life. The node must have its subsystems working and the average power consumption must not exceed 39 uW.
MCUs, radios, sensors and actuators have very different power/performance characteristics. To meet system power budgets, sensor nodes are required to manage their subsystems in an optimal manner. Figure 1 shows some of the subsystems used to implement a node.
Some modern low-power MCUs operate at approximately 1MHz clock frequency with a peak power consumption of approximately 345 uW. Assuming the sensor data processing requirements are generally medium, the MCU's duty cycle can be extremely small (eg, less than 1%) to reduce average power consumption.
Sensor nodes typically transmit information such as physical phenomena and associated control messages at relatively low rates.
Only as a general guideline for system design. As transceiver designs evolve, their power consumption is getting lower and lower. When choosing a transceiver architecture, it is important to consider all aspects of the design. Wireless local area network (LAN) transceivers consume less energy/bit than Zigbee® transceivers, but are optimized for higher data rates with higher peak power consumption.
Some examples of sensors associated with indoor applications include thermometers, temperature sensors, microphones, and passive infrared sensors. Some current temperature and humidity sensors and microphones have a peak power consumption of approximately 70–80 uW. Some passive infrared sensors capable of detecting human activity typically have a peak power consumption of 100–500 uW. Temperature and humidity sensors monitor slowly changing phenomena and operate at low duty cycles, while other sensors used to detect motion are turned off to reduce detection performance. In many applications, sensors require more energy than data processing or wireless communication. Therefore, meeting system power budgets requires innovative methods to manage sensors.
Despite tremendous advances in computing, communications, and sensing, the lack of adequate power and energy is still a serious challenge for wireless sensor networks. Some technological advances in energy harvesting and storage are continually alleviating power supply bottlenecks, but some of the needs of end applications are constantly pushing up their requirements. If you want to close this persistent power-demand gap, you need a system-level design approach that best compromises performance to achieve energy savings while ensuring minimal service quality. Future wireless sensor nodes will adapt to the changing application needs and energy supply over time.