Wireless sensors connected in a local network offer revolutionary exploration capabilities, but the current solutions do not work in extreme environments of low temperatures (200K) and low to moderate radiation levels (
Supercapacitors are nanotechnologybased electrochemical capacitors that can be cycled millions of times, compared to tens to hundreds of times for batteries. This allows supercapacitors to be used in conjunction with batteries by acting as a charge conditioner, storing energy for load-balancing purposes and then using any excess energy to charge the batteries at a suitable time. Supercapacitors are insensitive to radiation past l Mrad. JPL has demonstrated supercapacitor electrolytes that function to 189K.
This technology uses the ultra-lowpower flash-based FPGA as the communication protocol (physical and medium layers) generator that is powered by a hybrid combination of lithium-based batteries and supercapacitors. A SiGe based RF front end can be used to provide transmitter/receiver capability for the 2.45-GHz and 858-MHz ISM frequency bands at low temperatures. The low power is critical because it defines how long the system can operate. The cold temperatures will reduce the performance of the batteries. Instead of lasting a year at room temperature, a lithium battery may only last a few weeks at cold temperatures. Even at cold temperatures, the battery output will be reduced. This means only a very-lowpower circuit can be considered for use. The supercapacitors provide additional direct power capability for short-burst communication, and they provide the ability to extend the life of the battery by maintaining the voltage longer than the battery could alone. The FPGA can implement load-balancing and control logic to enable the battery to either operate completely alone, in combination with the supercapacitors, or have the supercapacitors alone power the FPGA in standby mode and/or be the startup power source if the FPGA is completely powered off.
This powering-off capability exists due to the use of the non-volatile flash memory cell-based FPGAs. This allows the supercapacitor to act also in countdown circuit mode, where the discharge time constant of the circuit is used as the time to keep the FPGA power off in order to save power. Once the supercapacitorbased circuit reaches a given level, this voltage is sensed and the wake-up sequence to the FPGA begins.
The supercapacitors also provide the ability to store and harvest recharge energy for the battery. RF energy can be beamed into the system and then fed back into the battery/supercapacitor network. Alternatively, mechanical energy from a MEMs device can be used to re-charge the supercapacitor. The capacitors can be quickly charged up and then act as a power reservoir for the battery. The completely described system above is currently in development.
This work was done by Douglas J. Sheldon of Caltech for NASA’s Jet Propulsion Laboratory.
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