Understanding Smart Sensors - Nomads.usp

64 Understanding Smart Sensorswhether signal A leading B is clockwise or counterclockwise is a matter ofchoice. Figure 3.8 shows the ideal quadrature outputs. The two waveforms arein four equal quadrants, exactly 90 degrees out of phase with each other.The detector is on when light is present and turns off when the web blocks thebeam.3.4.2 Digital TechniquesA study of available digital techniques has been reported [15]. Sensing techniquesbased on a resonant structure or a periodical geometric structure arepotentially the most direct approach to digital sensing. However, other techniquescan be used as well.Micromachined silicon can act as a resonant structure if it is designedwith a membrane or tuning fork. Electric activation is achieved by using a piezoelectricfilm such as ZnO on the micromachined structure. Most resonantstructures are implemented in pairs. One element is not interfaced to the inputand acts as a reference. Comparison of the output frequency of the sensing elementto the reference element reduces the influence of unwanted parameters.The technique is commonly used in chemical sensors using surface acousticalwave (SAW) delay line oscillators.Position3Position4Position1Position2Position3Position4Output AOutput BFigure 3.8 Quadrature detection.Clockwise rotation