传感器网络讲座 L5-sensing.ppt

June 2008 WEI - L05 sense Wireless Embedded InterNetworkingFoundations of Ubiquitous Sensor Networks Triggers and Sensing David E. Culler University of California, Berkeley An Analog World Everything in the physical world is an analog signal Sound, light, temperature, gravitational force Need to convert into electrical signals Transducers: converts one type of energy to another Electro-mechanical, Photonic, Electrical, … Examples Microphone/speaker Thermocouples Accelerometers And digitize Then manipulate An Analog World Transducers Allow us to convert physical phenomena to a voltage potential in a well-defined way. Simplest Analog Device Often think of it as an actuator, rather than a sensor But that’s because of the circuit we put it in It is binary (two states) but why is it not digital? To Sample a switch, make it digital Many sensor are switches Two “states” but not digital Open = no current Closed = no voltage drop Cap charges to Vacc when open Cap discharges to GND when closed EPIC I/F Board - Digital Inputs Getting Input into the MCU MCU – a system on a chip Programmed IO Interrupts Getting a hold of the event Making Sense of Physical Information Digital representation of physical phenomenon Transducer = Signal Conditioning = ADC = Conversion to physical units Calibration and correction Here: 0 / 1, True / False Associating meaning to the reading Open / Closed Empty / Full In Position / Not Depends on the specific device taking the reading The Context of the device Analog to Digital What we want How we have to get there Ratiometric sensor Va = Vacc* Rsens / (Rcomp+ Rsens) use Vref = Vacc D = M * Rsens / (Rcomp+ Rsens) Sampling Basics How do we represent an analog signal? As a time series of discrete values ? On the MCU: read the ADC data register periodically Sampling Basics What do the sample values represent? Some fraction within the range of values ? What range to use? Sampling Basics Resolution Number of discrete values that represent a