In-field Calibration Research Articles

Inertial-Magnetic Sensors for Assessing Spatial Cognition in Infants

This letter describes a novel approach to the assessment of spatial cognition in children. In particular, we present a wireless instrumented toy embedding magneto-inertial sensors for orientation tracking, specifically developed to assess the ability to insert objects into holes. To be used in naturalistic environments (e.g., day cares), we also describe an in-field calibration procedure based on a sequence of manual rotations, not relying on accurate motions or sophisticated equipment. The final accuracy of the proposed system, after the mentioned calibration procedure, is derived by direct comparison with a gold-standard motion tracking device. In particular, both systems are subjected to a sequence of ten single-axis rotations (approximately 90°, back and forth), about three different axes. The rms of the angular error between the two measurements (gold-standard versus proposed systems) was evaluated for each trial. In particular, the average rms error is under 2°. This study indicates that a technological approach to ecological assessment of spatial cognition in infants is indeed feasible. As a consequence, prevention through screening of large number of infants is at reach.

Field calibration of dosemeters used for routine measurements at flight altitudes

Intercomparisons of dosemeters used in radiation protection are performed routinely in reference radiation fields under well-defined conditions. In the case of the radiation protection of aircrew members, such reference fields either do not exist or they can be partially simulated by accelerator-based fields. Another method is to perform simultaneous measurements on board an aircraft under constant flight conditions, i.e. at defined latitude, longitude and altitude. The intercomparison presented in this work is the second one of its kind. As reference instruments, two types of tissue-equivalent proportional counters (TEPC) were used in comparison with different silicon detector systems. The excellent agreement of the FDOScalc code, which is based on measurements made by PTB during different phases of solar cycle 23 with the TEPC measurements and the results of the preceding project "Coordinated Access to Aircraft for Transnational Environmental Research" (CAATER) even allows the in-field calibration of the Si dosemeters.

The dual-beam mini-DOAS technique—measurements of volcanic gas emission, plume height and plume speed with a single instrument

The largest error in determining volcanic gas fluxes using ground based optical remote sensing instruments is typically the determination of the plume speed, and in the case of fixed scanning instruments also the plume height. We here present a newly developed technique capable of measuring plume height, plume speed and gas flux using one single instrument by simultaneously collecting scattered sunlight in two directions. The angle between the two measurement directions is fixed, removing the need for time consuming in-field calibrations. The plume height and gas flux is measured by traversing the plume and the plume speed is measured by performing a stationary measurement underneath the plume. The instrument was tested in a field campaign in May 2005 at Mt. Etna, Italy, where the measured results are compared to wind fields derived from a meso-scale meteorological model (MM5). The test and comparison show that the instrument is functioning and capable of estimating wind speed at the plume height.

Quantification of Hourly Speciated Organic Compounds in Atmospheric Aerosols, Measured by an In-Situ Thermal Desorption Aerosol Gas Chromatograph (TAG)

The Thermal desorption Aerosol Gas chromatograph (TAG) is a recently developed instrument for the in-situ, hourly measurement of speciated organic compounds in atmospheric aerosols. This paper presents a method for the in-field calibration of this instrument, with the objective of providing quantitative concentrations for a large suite of low polarity organic compounds. A new collection and thermal desorption cell was developed that incorporates an injection port for in-situ calibrations with liquid standard mixtures. Two classes of injection standards, instrument tracking and auxiliary, provide the means to calibrate the instrument in the field for a wide range of compounds. A routinely injected tracking standard suite of compounds generates a time-dependent correction of detector drift through the course of a measurement study that accounts for the bulk of the change in response of the TAG instrument. Injection response data for the tracking standard is also used to measure instrument precision and limits of quantitation. Auxiliary standards extend the range of compounds calibrated through use of relative response factors. The accuracy of this in-situ calibration approach is assessed through comparisons of TAG analyzed reference filter punches to published NIST assay values. A subset of compound classes, alkanes and PAHs, are used to illustrate the method and provide a means of reducing an 11-day period of data collected in Riverside, CA during the fall of 2005.

Methods for in-field user calibration of an inertial measurement unit without external equipment

This paper presents methods to calibrate and compensate for non-zero biases, non-unit scale factors, axis misalignments and cross-axis sensitivities of both the tri-axial accelerometer and gyroscopic setups in a micro-electro-mechanical systems (MEMS) based inertial measurement unit (IMU). These methods depend on the Earth's gravity as a stable physical calibration standard. Specifically, the calibration of gyroscopes is significantly improved by comparing the outputs of the accelerometer and the IMU orientation integration algorithm, after arbitrary motions. The derived property and proposed cost function allow the gyroscopes to be calibrated without external equipment, such as a turntable, or requiring precise maneuvers. Both factors allow the IMU to be easily calibrated by the user in the field so that it can function as an accurate orientation sensor. A custom-made prototype IMU is used to demonstrate the effectiveness of the proposed methods, with data that are carefully obtained using prescribed motions, as well as those less rigorously collected from the IMU when it is mounted on the head of a user or held in hands with brief random movements. With calibration, the observed average static angular error is less than a quarter of a degree and the dynamic angular error is reduced by a factor of 2 to 5.

Using Smart Sensor Strings for Continuous Monitoring of Temperature Stratification in Large Water Bodies

A "smart" thermistor string for continuous long-term temperature profiling in large water bodies is described allowing highly matched yet low-cost spatial and temporal temperature measurements. The sensor uses the three-wire SDI-12 communications standard to enable a low-powered radio or data logger on supporting buoys to command measurements and retrieve high-resolution temperature data in digital form. Each "smart" temperature sensor integrates a thermistor element, measurement circuitry, power control, calibration coefficient storage, temperature computation, and data communications. Multiple addressable sensors at discrete vertical depths are deployed along a three-wire cable that provides power and allows data transfer at regular intervals. Circuit, manufacturing, and automated calibration techniques allow temperature measurements with a resolution of plusmn0.003degC, and with intersensor matching of plusmn0.006degC. The low cost of each sensor is achieved by using poor tolerance thermistor and circuit components in conjunction with a 15-bit charge-balance analog-to-digital converter. Sensor inaccuracies and temperature coefficients are corrected by a two-point calibration procedure made possible by a standard-curve generator within the sensor, based upon the method of finite differences. This two-point calibration process allows in-field sensor string calibration in stratified water bodies and provides a means to correct for long-term calibration drift without having to return the string to a laboratory

In‐field calibration of a dielectric soil moisture meter designed for use in an access tube

Abstract. The in‐field calibration of a dielectric probe to measure soil water content is described. The probe uses an access tube analogous to that of the neutron probe. The dielectric constant was measured at soil depths of 10, 20, 30, 40, 60 and 100 cm. Cores of soil were then taken from the face of pits dug 30 cm from the access tube and their soil water contents determined by oven drying. The dielectric constant values measured by the probe were calibrated against water contents from these cores. We found that sensor depth needed to be included to achieve a good calibration model that explained 72% of the variance. It is argued that depth needs to be included because of artefacts introduced during the installation of the access tube.

In-field calibration of a dielectric soil moisture meter designed for use in an access tube

In-field calibration of a dielectric soil moisture meter designed for use in an access tube

Field evaluation of a calibration method for air-assisted sprayers involving the use of a vertical patternator

Vertical spray patternators are used in Europe for the inspection and calibration of orchard and vineyard sprayers. However, little information is available about their reliability, particularly when used for crop-specific calibration, involving the adjustment of nozzle position and orientation according to canopy size and geometry. A method, based on the use of a lamellate patternator, was evaluated for adjusting the nozzle orientation of a conventional, axial-fan sprayer. The treatments included: (a) keeping the nozzle orientation as determined by the manufacturer (no calibration); (b) adjusting the nozzle orientation in the field, following visual observation (in-field calibration); and (c) adjusting the nozzles in order to obtain, on the lamellate patternator, a vertical spray pattern reflecting the canopy outline of the vineyard (patternator calibration). The above treatments were applied in a hedgerow vineyard, at beginning of flowering and at fruit set, and spray deposits on the canopy determined by colorimetry, using a water-soluble dye (Tartrazine) as a tracer. No calibration gave the most uniform deposit distribution in the first experiment, whereas in-field calibration was best in the second experiment. In both experiments, Patternator calibration gave the largest differences between deposits at different heights on the canopy, related to low collecting efficiency of the patternator, particularly at the middle and top canopy locations.

Field evaluation of a calibration method for air-assisted sprayers involving the use of a vertical patternator

Vertical spray patternators are used in Europe for the inspection and calibration of orchard and vineyard sprayers. However, little information is available about their reliability, particularly when used for crop-specific calibration, involving the adjustment of nozzle position and orientation according to canopy size and geometry. A method, based on the use of a lamellate patternator, was evaluated for adjusting the nozzle orientation of a conventional, axial-fan sprayer. The treatments included: (a) keeping the nozzle orientation as determined by the manufacturer (no calibration); (b) adjusting the nozzle orientation in the field, following visual observation (in-field calibration); and (c) adjusting the nozzles in order to obtain, on the lamellate patternator, a vertical spray pattern reflecting the canopy outline of the vineyard (patternator calibration). The above treatments were applied in a hedgerow vineyard, at beginning of flowering and at fruit set, and spray deposits on the canopy determined by colorimetry, using a water-soluble dye (Tartrazine) as a tracer. No calibration gave the most uniform deposit distribution in the first experiment, whereas in-field calibration was best in the second experiment. In both experiments, Patternator calibration gave the largest differences between deposits at different heights on the canopy, related to low collecting efficiency of the patternator, particularly at the middle and top canopy locations.

WEED DETECTION USING COLOR MACHINE VISION

Many weed species have reddish stems, but stems of wheat and soybean are green. These color features were used in this study to establish a simple weed-detection method using a color machine-vision system. This method is more practical than texture- or shape-based methods because of its low sensitivity to canopy overlap, leaf orientation, camera focusing, and wind effect. Four types of relative color indices formed by RGB gray levels were designed. The most effective combinations of these color indices were selected using a statistical method. These combinations were used as the input variables for a statistical classifier based on discriminant analysis (DA) and two artificial neural-network (NN) classifiers. These classifiers were trained and tested using three weed species (Johnsongrass, redroot pigweed, and yellow foxtail) with soybean and three weed species (wild buckwheat, cheat, and field bindweed) with wheat. Preprocessing and postprocessing algorithms were developed to shorten the processing time and to reduce noise. The results showed that the statistical DA classifier was more accurate than the NN classifiers in classification accuracy. The least-square means of the classification rates using the DA classifiers for soybean and wheat were 54.9% and 62.2%, respectively. The misclassification rates for most weed species were below 3%. Because the reddish colors on the stems of some weed species vary as the plants grow, an in-field calibration procedure will be needed to make the classifiers more adaptive to different circumstances.

A Field Experiment on the Calibration of Radars with Raindrop Disdrometers

Radar reflectivity factors determined from disdrometer measurements of drop spectra are compared with simultaneous WSR-57 radar measurements in two Oklahoma thunderstorms. The possibility of using a disdrometer for an in-field calibration check of a radar is examined and found to have limited usefulness for convective precipitation sampled at long ranges.