Understanding Smart Sensors - Nomads.usp

112 Understanding Smart Sensorsthat generate RFI to the sensor element. An example of power management isquiet-time sampling, a circuit technique that performs analog switching whendigital switching is not present [6]. Cleaner samples of data are taken by haltinghigh-current, high-frequency activity while the sensor input is being measured.Normal digital processing functions continue once those values are in memory.5.5.6 Indirect (Computed, Not Sensed) Versus Direct SensingInputs to the MCU can be manipulated to provide additional data for a system.For example, the MCU can use an input pressure signal to provide maximumpressure, minimum pressure, an integrated (averaged) pressure, and timedifferentiatedpressure data to a system. An accelerometer signal processed by asignal processor can be integrated once to provide velocity and twice to providedisplacement information. Also, a single sensor input to an MCU can replaceseveral switches sensing the same parameter and provide programmable switchpoints for outputs.The ability to compute rather than sense is among the solutions thatMCUs bring to control applications. For example, in three-phase motor controlsystems, a Hall-effect sensor is used to sense the location of the magneticfield for each phase or the rotor speed in induction motors. An MCU uses thesignal to switch output drivers for pulse-width modulated control. The sensorsin this system have been a target for cost reduction for many years. Recent solutionshave eliminated the sensors by using the MCU to compute the rotorspeed and slip angular frequency from other available information, includingthe primary resistance [15].5.6 Software, Tools, and SupportCreating a new approach or an alternative to existing control technologiesrequires much more than the architecture. The software used to program thecontrol portion and the tools that allow the system to be developed are equallyimportant. Portable code (software compatibility) is essential if future endproducts require migration to a higher performance MCU. Also, keeping thesensor’s design and process simple and separate from the MCU allows implementationwith other available processors.One main advantage of using existing MCUs or DSPs is that the developmenttools already exist and allow the designer to quickly and easily developboth system hardware and software. For example, data entry, program debugging,and programming of an MCU’s OTP, EPROM, and EEPROM canbe accomplished by utilizing an evaluation module (EVM) (Figure 5.10)