Understanding Smart Sensors - Nomads.usp

154 Understanding Smart SensorsPositioncommandPositioncontroller−SpeedcommandSpeedcontrollerDecoupling−− −CurrentcontrollerI iConverterCounterSpeed feedbackCommutating signalsPosition feedbackRotary encoder (actual position)Tachogenerator (actual speed)Hall elements (commutation signals)(a)Rotary encoder (actual position, actual speed, commutation signal)Encoder feedbackAC signal−PositioncommandPositioncontrollerSpeedcomputationSpeed feedback−SpeedcontrollerSpeedcommandDecoupling(b)−−−SubdivisionCurrentcommandCurrentcontrollerI iConverterFigure 7.3 (a) Analog versus (b) digital speed control. (After: [6].)7.2 State MachinesThe state machine is one of the most commonly implemented functions in programmablelogic [7]. The state machine is actually a sequencing algorithm. Itallows an interrupt service routine to execute an algorithm that extends overmany interrupts. State machines can be open loop or closed loop [8]. Statemachines are developed from a state diagram and a state table. The state diagramis determined at the beginning of the design process from a description ofthe problem. A natural language describes the intended circuit function. A keyarea of interest is the synthesis of the state machine, a process that will allowrapid and easy development of the circuit from a functional description of thestate machine. If only a small subset of inputs is sensed while the circuit is inany given state, then an algorithmic state machine is used in place of a statediagram.A finite-state machine is a mathematical representation of a digitalsequential circuit in which abstract symbols, not binary codes, represent thestates. Finite-state machines can be specified in state transition tables or circuits