Understanding Smart Sensors - Nomads.usp

266 Understanding Smart Sensors11.5.1 BatteriesPortable applications have generated considerable system design activity toobtain extended performance and improved life from the batteries that powerthese systems. By including semiconductor components in the battery, a smartbattery is created. A smart battery is a battery or battery pack with specializedhardware that provides information regarding the state of charge and calculatespredicted run time [15]. Information regarding the type of battery chemistryand the battery-pack voltage, capacity, and physical packaging is conveyed by aprotocol to power-related devices in the system. The system can provide controlto a smart battery or batteries, a smart battery charger, and various regulatorsand switches. Other battery-charging efforts are directed at eliminating thememory effect in nickel-cadmium (NiCd) batteries, reducing the chargingcycle time, and avoiding overcharging, which reduces battery life. Portable dataacquisition systems and remote sensing devices will benefit when smart batteriesare incorporated into their design.Reduced battery size through thin-film manufacturing technology willalso affect sensors. A thin-film solid-state lithium-titanium-sulphur (Li/TiS 2 )microbattery has been reported that has properties suitable for long-liferechargeable applications [16]. The microbatteries range in size from 8 to 12mm in thickness and have a capacity between 35 and 100 mA-hr/cm 2 . Theopen-circuit voltage is approximately 2.5V. Batteries have been cycled over10,000 cycles at 100 mA/cm 2 . Microbatteries have routinely withstood 1,000cycles between 1.4V and 2.8V at current densities as high as 300 mA/cm 2 .Any reasonable smooth surface is a potential substrate for the thin-filmbattery. A chromium, TiS 2 , and solid electrolyte layer are sputtered onto thesubstrate. LiI, Li, and a protective coating are vapor deposited over the sputteredlayers. The construction technique will allow the microbattery to beincorporated with many semiconductor devices, including microsensors, duringtheir manufacture.Photopatterned carbon that is subsequently heat-treated at various temperatureshas been explored for its potential in MEMS [17]. The material producedby this process permits a variety of new MEMS applications throughnew shapes, resistivities, and mechanical properties. Carbon surfaces can beused to form electrochemical electrodes with deposition of a wide variety oforganic molecules. Polymeric batteries have been developed using the photopatternedcarbon and surface micromachining.11.5.2 Field Emission DisplaysEfficient low-power displays are an essential part of many systems, includingportable digital assistants, virtual reality–driven robots, and automotive GPSs.