Many modern building automation systems today rely on wireless connectivity, which not only simplifies the installation process, but also facilitates rapid adjustment and expansion. However, while eliminating the need for wiring, designers need to be powered by batteries, but they have to change batteries regularly, affecting overall system cost. . The difficulty with these systems is to create a high-performance power management solution that, in addition to extending battery life, ensures greater stability in life-critical applications.
Overview
According to the 2013 “Market and Market†projections and other data sources, the global building automation market will increase to nearly $50 billion in 2018 as people seek to make their work and living spaces safer, more comfortable and more efficient.
Part of the reason for this market expansion is the availability of wireless sensors, so it can be easily installed, expanded and adjusted. The majority of installation costs are due to the artificially erected lines and copper prices. The wireless automation infrastructure can significantly reduce the cost, but it also adds one to the system owner. Long-term cost, because there is no line, the system must rely on battery operation.
Battery replacement costs vary, and depending on the location of the sensor (such as a 6-meter-high ceiling or hard-to-reach location), a lot of labor may be required. In an ideal world, the battery never has to be replaced within the sensor's useful life, but it is limited by the aging of the battery's chemical reaction or the old sensor's power consumption. This idea is impractical, so in most wireless building automation systems, the battery life is 5 years. The basic requirements are 10 to 15 years.
Reduce energy consumption
Designers can extend the battery replacement period through a variety of fields. First, understand the RF environment that requires sensor operation. Usually, it is combined with other sensors to form a network. In the mesh structure, some sensors are terminals or repeaters.
Since RF transmission is the largest source of energy for the sensor, this configuration can improve energy efficiency. By reducing transmission energy consumption and repeating messages through the router, a very low power network can be formed. This topology is usually used in IEEE 802.15.4. A wireless network based, such as ZigBee or 6LoWPAN.
In a mesh structure such as ZigBee, how to synchronize is a problem. Router nodes must be on standby. If a node sends a message or a message is sent to a node, it will be stored (first stored) and then forwarded to other available nodes.
In ultra-low power devices, the sleep mode will greatly reduce the power consumption, and most of the blocks in the device, including the RF receiver, will be suspended. At this time, the router node must be powered (for example, using AC power) to monitor the sleep node when it is running. The message, so the power consumption is quite asymmetrical, the sensor node consumes very low energy and sleeps for a long time, while the router node (and possibly the sensor) continues to supply power and maintains the receiver operation.
If you want to omit a line-powered router in an ultra-low power network, the timing must be very precise, but this may increase the cost and complexity of each node. The entire network must wake up, communicate, and sleep again in a very short time. The longer the distance between each cycle, the higher the timing requirement. If a node in the network does not catch up with the synchronization, it will continue to operate until the next waking cycle and then resynchronize. This situation must be avoided as much as possible in battery power. Occurs within the network.
In the most practical wireless networks, the router is continuously powered by the line or Ethernet network, and the node must be as efficient as possible under battery power. Usually the microcontroller will turn off the radio and all unnecessary devices, and the sensor will turn off all Analog front end electronics.
The microcontroller then enters the low-power mode, relying on the timer to wake up periodically, start the system, transmit various messages, and then enter sleep mode.
The duty cycle will determine the amount of electricity used. The following formula estimates the battery life based on the amount of electricity used for sleep and wake up:
The operating time (T, unit hours) depends on the duty cycle. The available energy (EA, watt-hours) divided by the amount of electricity used (P, watts), the formula can be further expanded.
Capacity (C, unit amp-hour), VS and VE are the discharge starting point and end point voltage respectively, PW and PS are the power consumption (in watts) of the awake period and the sleep period, respectively, and D is the working period of the waking stage (zero to one). In applications with a service life of more than five years, the down-counting factor α is used to adjust the battery capacity loss; the power converter performance (eff) affects performance, so it should also be taken into consideration, usually with a performance range of 80% ( Eff = 0.8) to 95% (eff = 0.95).
Part of this design will be the source of choice for power supplies that must not be significantly attenuated after 10 to 20 years of continuous power and maintain high performance power converters (such as switching regulators) under low power conditions. The former can be used with lithium sulphite battery (Li/SOCl2). Since its introduction in the 1970s, this product has a very long battery life (10 to 25 years) and is also used in remote meters and other battery-powered wireless devices. In the system, the voltage is typically 3.6 V and the operating temperature range is large (–55 ° C to 125 ° C).
If a single lithium battery is used, the sensor node design may increase the output from 3.0 V to 3.6 V to 5.0 V, or use a buck-boost converter such as the TPS63001 to maintain the output at 3.3 V and provide the highest in buck-boost conditions. 800 mA. This feature is very important because the RF transmitter may require a large amount of instantaneous current. Moreover, during the sleep cycle, the converter is mostly unloaded and must be able to automatically enter the pulse frequency modulation or other pulse omitting technique to switch the power supply.
Even if the microcontroller enters the low-power mode, it will continue to operate during the sleep cycle and still cause energy loss. But at least the timer must be turned off with the main core to save energy. Even with this configuration, however, a small amount of mA may still be present, even though a top-of-the-line low-power microcontroller such as the MSP430 requires approximately 0.3 μA in standby mode.
The new solution features a special timing device for long sleep periods. The Texas Instruments TPL5000 features a programmable splitter that provides up to 64 seconds of wake-up pulses. The power consumption is only 30 nA during operation, and can be up to 30 nA during extremely long sleep periods. Battery-powered wireless sensors extend the life of two years. (Please see the picture below)
in conclusion
With the increasing number of battery-powered wireless networks, installers and owners want to extend battery life, even within the network life (25 years or more), no need to replace the battery, lithium sulfite battery, pulse frequency modulation converter The new nano-energy long-cycle timer can maintain extremely low power consumption during the sleep cycle, allowing the wireless building automation system to surpass the wired system.
Richard Zarr is a Texas Instruments technician specializing in high-speed signal and data path technology. He has more than 30 years of engineering practice experience and has published numerous papers and articles around the world. He is an IEEE member and a Bachelor of Science from the University of South Florida. LED lighting and cryptography patents.
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