Dealing with Measurement UncertaintyJuly 1, 2006 By: Patrick L. Walter, PhD Sensors
When environmental conditions can affect transducer measurements, data validation helps ensure that each transducer is responding only to the physical property that it is designed to measure. Placebo transducers are one of the data validation tools at your disposal.
A transducer is only as good as the signal it initiates through a measurement system. Important design decisions affecting human lives and costly structures (e.g., automobiles, aircraft, and buildings) frequently depend on the analysis of these signals. To make intelligent decisions, it is critical to perform measurement uncertainty analysis on the signal. When using transducers where their operating conditions vary with time and/or location, the analysis can be performed only after several requirements have been fulfilled. First and foremost, the transducers, as well as the measurement systems they operate with, must have been designed using good practices. These include adequate low- and high-frequency response and data-sampling rates, appropriate anti-aliasing filter selection, and proper grounding and shielding. In addition to these requirements, performing data validation is necessary to establish that each transducer responds only to the environmental stimulus that it is intended to measure.
Placebos and Data Validation
For piezoelectric transducers, "placebo transducers" (IEST-RP-DTE011.1) enable data validation. The referenced IEST standard defines a placebo transducer as "identical to a 'live' unit in every parameter except for mechanical sensitivities." The placebo transducer should respond only to extraneous environmental factors. Ideally, its output would be zero. Any signal output other than zero indicates that the signals from the "live" transducers can be corrupted.
Some examples of test environments where validation should be performed include flight tests, highly hazardous testing (e.g., blast and munitions), and electrodynamic shaker testing. In the last example, in addition to acceleration, the shaker produces (as a minimum) temporally and spatially varying magnetic fields.
Transducers respond to their environment in every way they can. For example, accelerometer specifications include their response to thermal, acoustic, strain, and radiation stimuli, to name just a few. While accelerometers must have their response to acoustic pressure specified, pressure transducers must have their response to acceleration specified. Thus, one transducer's desirable response becomes another's undesirable response.
Transducer manufacturers try to minimize the effect of these undesirable responses in the design process, but they can never completely eliminate them. Undesirable responses can cause a change in transducer sensitivity or result in additive, spurious signals at the transducer's output attributable to thermoelectric, electromagnetic, triboelectric, and other self-generating noise phenomena. Because the test or instrumentation engineer has the best understanding of the test environment, he or she becomes responsible for validating the data. The transducer manufacturer can assist by supplying "placebo" transducers to support the validation process.
Figure 1. Quartz boule
Consider how placebo transducers are manufactured. Figure 1 (page 28) shows a boule of quartz from which piezoelectric elements are cut to be used in the manufacture of force, pressure, and acceleration transducers. The boule's piezoelectric properties vary according to the direction of the cut, as illustrated by Equation 1. Note that the third equation in the set shows that there is one direction (Z axis) that produces no piezoelectric output. Cuts along this axis provide the quartz for placebo transducers.
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