Understanding Smart Sensors - Nomads.usp

Standards for Smart Sensing 289performs the correction, the other processors must be programmed to get dataonly from the host and not directly from the transducer or the NCAP. If theconversion is performed in multiple places, the copy of the calibration TEDSmust be changed each time a transducer is changed.Figure 12.12(b) shows the conversion performed in the NCAP. The calibrationTEDS is copied from the STIM into the NCAP, where the conversionto engineering units is performed. That simplifies the management of thesource of data and eliminates multiple copies of the data. The NCAP processorwill cost more to provide this capability.Figure 12.12(c) shows the STIM running the correction process. Thistechnique is useful in systems with a large array of transducers or with groups ofvery high speed transducers. For wide acceptance, ICs must be developed thatprovide this capability.12.4.5 Sourcing Power to STIMsVoltage on the power line at the NCAP is specified at 5V ±0.20V dc withrespect to the common. Power for the STIM interface control circuitry must beprovided only through the primary communications interface. Power normallyis supplied to the STIM as shown in Figure 12.13(a) [6]. An alternative sourceof power for the STIM for applications where the sensor or actuator circuitryexceeds the current or voltage levels that the NCAP can provide is shown inFigure 12.13(b).12.4.6 Representing Physical Units in the TEDSIEEE 1451.2 has a simple and easily stored method of identifying physicalunits for use with smart transducers, allowing them to provide output in termsany user can understand [11]. This capability enhances sensor plug-and-playwithout writing special software. The approach uses a standard set of units todetermine the calibration constants. For manufacturers, that means they canalways perform final tests and calibrations the same way. For users, a standardtechnique exists for a variety of system functions by understanding the methodologyused in IEEE 1451.2. The standard represents the International Systemof Units (SI) units, as listed in Table 12.5.For an example of how to express a measurement of distance, consider(12.1) [11], which shows how a measurement in meters could be written. Toprovide this to a computer, only the exponential values are used, that is, 1, 0, 0,0, 0, 0, 0, 0.