Understanding Smart Sensors - Nomads.usp

Standards for Smart Sensing 2911 0 0 0 0 0 0m ⋅kg ⋅s ⋅A ⋅K ⋅mol ⋅ cd(12.1)The standard provides two derived units, radians (rad) and steradians (sr)to the seven base SI units and addresses units that are given in meters per meter,logarithm of quantities, and logarithms of dimensionless numbers, as well ashardness and digital data through an enumeration field (enum). Exponents areencoded using an unsigned byte integer. The exponent and its sign are multipliedby 2, and 128 is added to achieve an encoded byte integer with a resolutionof 1 2. Using this technique, exponents between –64 to +63 can beexpressed.As an example of how measurement units are handled in IEEE 1451.2,consider a pressure measurement that is expressed in pascals, that is, kilogramper meter per seconds squared. Table 12.6 shows the field units for this example.The enumeration 0 means that the units are the product of the base units.Examples of how distance, area, resistance, noise spectral density, massfraction, strain, power quality, counts, and switch positions are shown in onereference [11].12.5 IEEE P1451.3IEEE P1451.3 defines a digital interface for connecting multiple, physicallyseparated sensors [12]. This is one of two mixed-mode interfaces that allowsdigital information to be stored with the transducer and transmitted over analogdata wires. The multidrop transducer bus standard is a minibus implementationsmall enough and cheap enough to integrate into a transducer. Theamount of overhead for the proposed 1451.3 is considerably less than existingfieldbuses, which use up to 32 bits for node addressing [1].Table 12.6IEEE 1451.2 Pressure Measurement in Pascals [10]Pressure (pascals = m −1⋅kg ⋅s − 2)enum rad sr m kg s A K mol cdExponent 0 0 0 −1 1 −2 0 0 0 0Decimal 128 128 126 130 124 128 128 128 128