Understanding Smart Sensors - Nomads.usp

The Nature of Semiconductor Sensor Output 63accomplish that with additional circuitry. However, the industry’s quest is asensor or sensor family with an inherent digital output. On-chip or external signalconditioning may achieve “inherent” capability, but it must cost considerablyless and be more accurate than units using existing techniques.3.4.1 Incremental Optical EncodersStructures that contain a light source and a photodetector provide a digitalmeans of measuring displacement or velocity when an alternating opaque andtranslucent grid is passed between them. As shown in Figure 3.7, a macrosizedexample of uniformly spaced apertures on a wheel allows logic circuitry tocount the number of pulses in a given time frame to determine shaft velocity orangular displacement [16]. Linear measurements of displacement and velocityalso are possible.Basic presence sensing can be accomplished through the use of a singleoptical channel (or emitter-detector pair). Speed and incremental position sensingmust use two channels. The most commonly used approach is quadraturesensing, in which the relative positions of the output signals from two opticalchannels are compared. That comparison provides the direction information,and either of the individual channels gives the transition signal to derive eithercount or speed information. A typical application can use the direction outputas the up/down input and either channel A or channel B as the count input fora common up/down counter. The counter in an incremental system will incrementor decrement as required to maintain a relative position or count output.Quadrature direction sensing requires that the two optical channels createelectrical transition signals that are out of phase with each other by 90 degreesnominally. When the code wheel is spinning, the two electrical output signals,A and B, will be 90 degrees out of phase, because the wheel windows are90 degrees out of phase with the two sensor channels. The interpretation ofClockCh AphotodetectoroutputInputconditioningφAStepdeterminationTimerSpeedCh BphotodetectoroutputInputconditioningφBDirectiondeterminationUP/ DNUp/downcounterCountFigure 3.7 Incremental optical encoder.