The data stored in the sensor’s EEPROM or a VTEDS file is compressed to save space. To retrieve and interpret the information, the DAQmx driver applies a template to the binary data. There are separate templates for each type of sensor from charge amplifiers to thermistors. In addition to these sixteen standard templates, there are three calibration templates: a calibration table, calibration curve (polynomial), and frequency response table. See Table 1 below.
Table 1. IEEE 1451.4 Standard Templates
template describes a lookup table with the electrical-to-physical transfer function of a transducer. The table requires multiple sets of values for the CalPoint_DomainValue and CalPoint_RangeValue parameters. See Table 2 below.
Table 2. Calibration Table Template (ID = 40) Summary
CalTable_Domain is used to select the domain parameter for the calibration table as either electrical (0) or physical (1). For example, an electrical domain parameter would be voltage, whereas a physical domain parameter would Celsius. CalPoint_DomainValue is the domain parameter for the calibrated value, expressed as a percentage of the full range of the transducer. If a thermocouple has a full range of -300 to 300°C, then CalPoint_DomainValue would be 50% if the thermocouple were to be calibrated at 0°C. CalPoint_RangeValue is the deviation of the range measurement from the expected value, expressed as a percentage of the full range of the transducer. A reading that is off by 5°C would correspond to a CalPoint_RangeValue of .8333% for the same thermocouple. Figure 1 below illustrates the use of these properties to specify the transfer function of the transducer.
Figure 1. Calibration Table Transfer Function
The calibration table template provides a simple means of recording a few data points for calibration purposes. This is particularly useful when calibrating around a very narrow range of values.
Sometimes, however, it is necessary to precisely characterize a transducer over its entire range. This may prove difficult, especially in the case of a transducer with a highly nonlinear transfer function, such as a thermocouple. Accordingly, TEDS provides an alternative means of calibration. The calibration curve template specifies a multi-segment polynomial curve describing the electrical-to-physical transfer function of a transducer. See Table 3 below.
Table 3. Calibration Curve Template Template (ID = 41) Summary
The overall function is described in a piecewise manner, each segment defined by an array of coefficients and powers. For example, a curve may be described by [(3,1),(2,5),(1,-2),(0,1)], where the numbers in parentheses are interpreted as (CalCurve_Power,CalCurve_Coef). This would translate to the polynomial function f(x) = x3 + 5x2 – 2x + 1. See Figure 2 below.
Figure 2. Calibration Curve Transfer Function
|the number of coefficients for segment k, read from the TEDS as CalCurve_Poly
|the ith polynomial coefficient for segment k, read from the TEDS as CalCurve_Coef
|the power to which domain variable x is raised for coefficient Ck,i in segment k
Each segment of the function is delineated by the CalCurve_PieceStart parameter, which specifies the starting value of each piece. Each segment is bounded by sequential values of CalCurve_PieceStart, closed below and open above. For example, the segment k interval in the figure above includes CalCurve_PieceStart
but excludes CalCurve_PieceStart
, where PieceStart
designates the k
value read for the CalCurve_PieceStart property. This segment is one of potentially several polynomials that may describe the transfer function of a transducer. Many RTDs, for example, exhibit linear behavior for part of their measurement range, and polynomial for the rest.
To make effective use of the calibration curve template, several calibration points must be taken and then fitted to a polynomial curve. This may take significantly more time than collecting a few data points for a calibration table. However, the tradeoff is greater accuracy and a wider range of calibrated values.
Some measurements may require information beyond the time domain. The frequency response transfer function of a transducer is particularly useful for microphones and accelerometers. The frequency response of any vibration transducer, in fact, is very important when comparing readings. Sensors with a broader frequency response will see more vibration if it is present than a narrower bandwidth transducer.
The frequency response table template specifies the frequency response transfer function of a transducer with a lookup table of frequency-amplitude pairs. Similar to the CalTable and CalCurve properties, TF_Table_Freq and TF_Table_Ampl define data points in the amplitude-versus-frequency transfer function. See Table 4 and Figure 3 below.
Table 4. Frequency Response Table Template (ID = 42) Summary
Figure 3. Frequency Response Transfer Function
Any combination of these calibration templates may be used; however, each is tailored to a different type of measurement. Calibration tables offer an easy means of thoroughly characterizing a narrow range of transducer readings. Calibration curves completely specify the range of transducer, though they require more effort due to the calculation of a polynomial from discrete measurements. Frequency response tables may be combined with either of the first two templates to provide more information about the dynamic response of a transducer.