Output Drift Research Articles

Development and Evaluation of A Novel and Cost-Effective Approach for Low-Cost NO₂ Sensor Drift Correction.

Emerging low-cost gas sensor technologies have received increasing attention in recent years for air quality measurements due to their small size and convenient deployment. However, in the diverse applications these sensors face many technological challenges, including sensor drift over long-term deployment that cannot be easily addressed using mathematical correction algorithms or machine learning methods. This study aims to develop a novel approach to auto-correct the drift of commonly used electrochemical nitrogen dioxide (NO2) sensor with comprehensive evaluation of its application. The impact of environmental factors on the NO2 electrochemical sensor in low-ppb concentration level measurement was evaluated in laboratory and the temperature and relative humidity correction algorithm was evaluated. An automated zeroing protocol was developed and assessed using a chemical absorbent to remove NO2 as a means to perform zero correction in varying ambient conditions. The sensor system was operated in three different environments in which data were compared to a reference NO2 analyzer. The results showed that the zero-calibration protocol effectively corrected the observed drift of the sensor output. This technique offers the ability to enhance the performance of low-cost sensor based systems and these findings suggest extension of the approach to improve data quality from sensors measuring other gaseous pollutants in urban air.

Development of a Flow Injection Based High Frequency Dual Channel Quartz Crystal Microbalance.

When the quartz crystal microbalance (QCM) is used in liquid for adsorption or desorption monitoring based bio- or chemical sensing applications, the frequency shift is not only determined by the surface mass change, but also by the change of liquid characteristics, such as density and viscosity, which are greatly affected by the liquid environmental temperature. A monolithic dual-channel QCM is designed and fabricated by arranging two QCM resonators on one single chip for cancelling the fluctuation induced by environmental factors. In actual applications, one QCM works as a specific sensor by modifying with functional membranes and the other acts as a reference, only measuring the liquid property. The dual-channel QCM is designed with an inverted-mesa structure, aiming to realize a high frequency miniaturized chip and suppress the frequency interference between the neighbored QCM resonators. The key problem of dual-channel QCMs is the interference between two channels, which is influenced by the distance of adjacent resonators. The diameter of the reference electrode has been designed into several values in order to find the optimal parameter. Experimental results demonstrated that the two QCMs could vibrate individually and the output frequency stability and drift can be greatly improved with the aid of the reference QCM.

Integral based sliding mode stabilizing a camera platform using Kalman filter attitude estimation

This paper proposes an Integral Sliding Mode Control (ISMC) strategy to stabilize a camera platform. The main goal of this research is to achieve precise inertial stabilization of the platform while attenuating wide range of perturbations including external rates and accelerations. So firstly, a mathematical model for the platform is developed. Then considering the available sensors, the unmeasurable terms are extracted as system uncertainties. The upper bounds of these uncertainties are calculated theoretically and based on them, an appropriate ISMC is designed to stabilize the camera platform and regulate its absolute attitude in local horizon. MEMS gyroscopes are utilized to feedback the angular rate and absolute attitude in inertial coordinate system. To overcome attitude deviations caused by integrating the MEMS gyro's output drift, the attitude is estimated through fusion of gyroscopes with accelerometers using Kalman filter. The closed loop system stability is proved mathematically and finally, performance of the proposed attitude estimation method and the ISMC are validated experimentally.

Twenty-four hour intraocular pressure monitoring with the SENSIMED Triggerfish contact lens: effect of body posture during sleep

PurposeTo determine the difference in relative intraocular pressure (IOP) measured by the SENSIMED Triggerfish (TF) contact lens in flat compared with 30° head-up sleeping positions in patients with progressive primary...

Compensation to the output drift for cooled infrared imaging systems at various ambient temperatures

A method is proposed to compensate the output drift for cooled infrared imaging systems at various ambient temperatures. By calibrating the cryogenic infrared detector which absorbs the radiant flux of blackbody directly, the internal factors can be obtained. Then, by combining the calibration result of infrared imaging system at an arbitrary ambient temperature, the output drift can be calculated and compensated at various integration time and ambient temperatures. Experimental results indicate that the proposed method can eliminate the effect of ambient temperature fluctuation on the system output efficiently.

Optimized Thermal Compensation Method Using Clustering and Drifted Response Stability for Total Power Radiometer Calibration

Radiometer calibration using a compensation method based on temperature drift under a variety of environmental conditions can be used for airborne applications because of limitations on weight and power. Accordingly, the observation of the variation of the system states via analysis of the obtained response with temperature data is necessary to compensate the fluctuation of physical temperature for accurate calibration of the radiometer. However, the accuracy of the previous method (which uses one or more temperature probe) can rarely obtain an accurate estimated result; its accuracy is insufficient for high-precision measurement applications. Additionally, previous methods may cause unpredictable estimation errors when using regression methods with fewer measurable sensors. Therefore, more advanced compensation methods, which can compensate for the drift in radiometer output using temperature sensors (based on the analysis of the distinctive features of the temperature), are required to estimate the brightness temperature. In this paper, an optimized thermal compensation method using a selection of the optimal point for gain and offset probing to control the coefficients is analyzed using both the correlation-matrix-based hierarchical clustering and a comparison with the response stability for the training period. Then, the clustered temperature sensors of the radiometer system are used in the predictor variable to achieve optimum compensation of radiometer response variation. Next, the regression model of the multiple linear clustering method is compared the estimation accuracy for the selected training period and temperature sensors. Improved results relative to the reference method are achieved using the proposed model in experiments.

An Accurate BJT-Based CMOS Temperature Sensor With Duty-Cycle-Modulated Output

This paper describes the design of a precision bipolar junction transistor based temperature sensor implemented in standard 0.7-μm CMOS technology. It employs substrate p-n-ps as sensing elements, which makes it insensitive to the effects of mechanical (packaging) stress and facilitates the use of low-cost packaging technologies. The sensor outputs a duty-cycle-modulated signal, which can easily be interfaced to the digital world and, after low-pass filtering, to the analog world. In order to eliminate the errors caused by the component mismatch, chopping and dynamic element matching (DEM) techniques have been applied. The required component shuffling was done concurrently rather than sequentially, resulting in a fast DEM scheme that saves energy without degrading accuracy. After a single-temperature trim, the sensor's inaccuracy is ±0.1 °C (-20 to 60 °C) and ±0.3 °C (-45 to 130 °C), respectively. Measurements of sensors in different packages show that the package-induced shift is less than 0.1 °C. Measurements of eight sensors over 367 days show that their output drift is less than 6 mK. While dissipating only 200 μW, the sensor achieves a resolution of 3 mK (rms) in a 1.8-ms measurement time, and a state-of-the-art resolution figure of merit of 3.2 pJK <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . This combination of high accuracy, high resolution, high speed, and low-energy consumption makes this sensor suited for commercial and industrial applications.

Pulse reference-based compensation technique for intensity-modulated optical fiber sensors

We propose a compensation technique based on pulse reference for intensity-modulated optical fiber sensors that can compensate the power fluctuation of the light source, the change of optical components transmission loss, and the coupler splitting ratio. The theoretical principle of this compensation technique is analyzed and a temperature sensor based on fiber coating-covered optical microfiber is carried out to demonstrate the compensation effect. The system noise is measured to be 0.0005 dB with the temperature sensitivity reaching ?0.063 dB/°C, and the output drift is 0.006 dB in 2 h at room temperature. The output shows a slight variation (0.0061 dB) when the light source and the common light path suffer a 3 dB attenuation fluctuation.

Hydrostatic pressure device for evaluation of contact pressure sensor characteristics

In order to gain more insight in the characteristics of Tekscan contact pressure mapping sensors, a hydrostatic pressure cell is designed. The main research topics are load history dependency, inter-pixel output variation and output drift behaviour of the sensors. This leads to a state of the art preconditioning and post processing method which yields a higher accuracy of the measurement data.

Effects of dielectric charging on the output voltage of a capacitive accelerometer

Output voltage drifting observed in one typical capacitive microelectromechanical system (MEMS) accelerometer is discussed in this paper. Dielectric charging effect is located as one of the major determinants of this phenomenon through a combination of experimental and theoretical studies. A theoretical model for the electromechanical effects of the dielectric surface charges within the electrode gap is established to analyze the dielectric charge effect on the output voltage. Observations of output voltage drift against time are fitted to this model in order to estimate the possible dielectric layer thickness. Meanwhile, Auger electron spectroscopy is carried out to analyze the electrode surface material composition and confirms a mixture layer of dielectric SiO2 and Si with a thickness about 5 nm, which is very close to the model estimation. In addition, observation of time-varing output drift in the variable bias voltage experiment indicates the movement of dielectric charge can be controlled by the applied electric field.

Simultaneous Temperature Compensation and Synchronous Error Elimination for Axial Displacement Sensors Using an Auxiliary Probe

The output stability of the displacement sensors is an important issue for safe operations in magnetic bearing supported high-speed rotor system. This paper proposes an effective postprocessing solution to the problems of temperature drift and synchronous measuring error for the axial displacement sensor in a 10-kW magnetically suspended motor. The proposed configuration of the axial displacement sensor consists of three probes: one pair is used to eliminate the synchronous measuring error, and the third probe is employed for the consideration of temperature drift. First, the generating mechanism of the synchronous measuring error caused by the improper assemble is presented, and an operational amplifier is proposed to obtain an accurate position of the axial center by adjusting the weights of sensing signals from one pair of probes. Then the scheme of temperature compensation using an auxiliary probe is presented. In order to obtain accurately the characteristics of temperature drift at the operating point, the detailed procedure is given to determine the temperature–drift ratio. Finally, the terms related to temperature drift and synchronous error in the final output of the displacement sensor are both eliminated. Experimental results on a magnetically suspended motor test rig show the effectiveness of the proposed solution.

A double differential torsional accelerometer with improved temperature robustness

This paper reports the design, realization and characterization of a novel double differential torsional MEMS accelerometer with improved temperature robustness. Based on the structure of conventional torsional accelerometers, normally composed of two unbalanced proof masses and an identical torsional beam, this work explores a torsional accelerometer with two structural optimizations aiming to improve the temperature robustness: a double differential configuration and an on-chip integrated reference capacitor configuration. The double differential configuration realizes the temperature self-calibration by making the two pairs of unbalanced masses have the same sensitivity to temperature but opposite sensitivity to acceleration. And the on-chip integrated reference capacitor realizes the temperature self-calibration by transferring the temperature dependent reference capacitor from ASIC or PCB into the MEMS structure. Totally ten accelerometer prototypes are fabricated with the pre-buried mask wet-etching method and tested. Comparison tests show that, for most of the accelerometers, by taking the double differential configuration and the on-chip integrated reference capacitor configuration, the temperature sensitivity of the scale factor, the bias stability and the output drift of in the full temperature range are greatly improved. The testing results experimentally demonstrate the effectivity of the optimization for improving temperature robustness.

Design, simulation, and fabrication of silicon-on-insulator MEMS vibratory decoupled gyroscope

This paper reports the design and fabrication of a 2-degree-of-freedom (DOF) decoupled vibratory gyroscope based on a silicon-on-insulator (SOI) MEMS process. The 2-DOF capacitive comb structure is deliberately designed to have a decoupled drive and sense mode oscillation to prevent the unstable operation due to mechanical coupling, resulting in a low zero rate out-put drift. It is well known that the closer are the drive and sense resonances, the higher is the angular rate resolution of the gyroscope. Generally, this is achieved by using symmetric suspensions, but it results in a reduced bandwidth. The proposed design has been configured to achieve a bandwidth of about 150 Hz, while ensuring the decoupled operation of the drive and sense modes. An analytical method has been employed to study the steady state response of the 2-DOF structure. FEM analysis has been carried out in CoventorWare® MEMS Design software and the simulation results show that the drive resonance occurs at 21.48 kHz and sense resonance at 21.63 kHz, which are in close agreement with the theoretical results. The structure is designed with a 15 µm thick device layer. Fabrication of the design is proposed using a two mask process based on Deep reactive-ion etching (DRIE) and sacrificial wet release etching on a SOI wafer. DRIE etching with an aspect ratio of 1:5 has been successfully carried out as desired and the results have been presented.

Enhanced analogue front-end for the measurement of the high state of wide-band voltage pulses with 87 dB common-mode rejection ratio and ±0.65 ppm 1-day offset stability.

An improved analogue front-end for measuring the high state of trapezoidal voltage pulses with transition duration of 3 μs is presented. A new measurement system, composed by a front-end and the state-of-the-art acquisition board NI PXI-5922, has been realized with improved Common Mode Rejection Ratio (CMRR) of more than 87 dB at DC and 3-sigma stability of ±0.65 ppm over 1 day. After highlighting the main design enhancements with respect to state-of-the-art solutions, the CMRR measurement is reported. The output drift due to temperature and humidity is assessed to be negligible. Finally, the worst-case repeatability is measured both with shorted-to-ground inputs and with an applied common-mode voltage of 10 V, which represents the nominal working condition.

Suppression of Thermal Effect on Closed Loop Fiber Optic Gyroscope

Fiber Optic Gyroscope (FOG), an inertial sensor measures the angular rotation of an object based on the principle of Sagnac effect. FOG is subjected to various environmental disturbances such as temperature, vibration etc., when it is operated in real time. Temperature variation is the major factor that affects the gyro performance. In this paper, a novel compensation approach called temperature soaking method has been presented which makes the gyro insensitive to temperature. This method has been carried out for a 3-axis (Gx, Gy and Gz) FOG and its outputs (bias values) were taken at different temperature points. In order to reduce the bias drift of FOG output, this method uses linear curve fitting function to generate the bias error coefficients. The generated bias error coefficients are subtracted from the existing bias data, which in turn reduces the bias error. The bias values obtained for 3-axis gyro at temperature points from -20°C to +60°C before compensation is then compared with the bias values obtained for after compensation at same temperature points. Experimental results demonstrate that, after compensation the bias drift greatly reduced from 15 to 0°/hr approximately. This method effectively improves the stability and performance of a gyro and makes it to withstand to different environmental conditions which in turn increases the range of applications from military, navigation to space.

Multivariate detrending of fMRI signal drifts for real-time multiclass pattern classification

Signal drift in functional magnetic resonance imaging (fMRI) is an unavoidable artifact that limits classification performance in multi-voxel pattern analysis of fMRI. As conventional methods to reduce signal drift, global demeaning or proportional scaling disregards regional variations of drift, whereas voxel-wise univariate detrending is too sensitive to noisy fluctuations. To overcome these drawbacks, we propose a multivariate real-time detrending method for multiclass classification that involves spatial demeaning at each scan and the recursive detrending of drifts in the classifier outputs driven by a multiclass linear support vector machine. Experiments using binary and multiclass data showed that the linear trend estimation of the classifier output drift for each class (a weighted sum of drifts in the class-specific voxels) was more robust against voxel-wise artifacts that lead to inconsistent spatial patterns and the effect of online processing than voxel-wise detrending. The classification performance of the proposed method was significantly better, especially for multiclass data, than that of voxel-wise linear detrending, global demeaning, and classifier output detrending without demeaning. We concluded that the multivariate approach using classifier output detrending of fMRI signals with spatial demeaning preserves spatial patterns, is less sensitive than conventional methods to sample size, and increases classification performance, which is a useful feature for real-time fMRI classification.

Visual Analysis of the Daily QA Results of Photon and Electron Beams of a Trilogy Linac over a Five-year Period.

Data visualization technique was applied to analyze the daily QA results of photon and electron beams. Special attention was paid to any trend the beams might display. A Varian Trilogy Linac equipped with dual photon energies and five electron energies was commissioned in early 2010. Daily Linac QA tests including the output constancy, beam flatness and symmetry (radial and transverse directions) were performed with an ionization chamber array device (QA BeamChecker Plus, Standard Imaging). The data of five years were collected and analyzed. For each energy, the measured data were exported and processed for visual trending using an in-house Matlab program. These daily data were cross-correlated with the monthly QA and annual QA results, as well as the preventive maintenance records. Majority of the output were within 1% of variation, with a consistent positive/upward drift for all seven energies (~+0.25% per month). The baseline of daily device is reset annually right after the TG-51 calibration. This results in a sudden drop of the output. On the other hand, the large amount of data using the same baseline exhibits a sinusoidal behavior (cycle = 12 months; amplitude = 0.8%, 0.5% for photons, electrons, respectively) on symmetry and flatness when normalization of baselines is accounted for. The well known phenomenon of new Linac output drift was clearly displayed. This output drift was a result of the air leakage of the over-pressurized sealed monitor chambers for the specific vendor. Data visualization is a new trend in the era of big data in radiation oncology research. It allows the data to be displayed visually and therefore more intuitive. Based on the visual display from the past, the physicist might predict the trend of the Linac and take actions proactively. It also makes comparisons, alerts failures, and potentially identifies causalities.

Relationship Between Temperature Drift and Thermal Expansion of Sensing Structure in Sandwich Micro-Accelerometer

To cope with the temperature drift of capacitive sandwich type micro-accelerometer, based on mechanics and thermal principle, mechanics equilibrium of sensing structure after thermal expansion is obtained. Then, in light of the accelerometer’s closed loop operation mode, the electrical equilibrium is obtained. On the basis of two equilibriums, the output drift induced by thermal expansion of sensing structure is attained, which implies that the drift is less than 0.002 gn per 10oC. Meanwhile, an experiment is conducted which indicates that the drift due to the temperature is from 0.1 gn to 0.2 gn per 10oC. The comparison implies that the thermal expansion of sensing structure is not a dominant reason for temperature drift of the accelerometer. Keywords: Sandwich micro-accelerometer, sensing structure, temperature drift, thermal expansion.

SU-E-T-227: Least Restrictive Assignment of Dose-Distance Differences (LRAD) in IMRT On a Conventional Linac

Purpose: To develop a method to compare dose distributions based on the least restrictive of dose and distance differences, and to evaluate the method for IMRT QA on a conventional linac. Methods: Lrad is defined as the least restrictive of the dose difference at each point in the distributions being compared (%D), with dose normalized by the same factor, and the distance to agreement (DTA in millimeters), limited to directions parallel and perpendicular to MLC motion. If %D<%Do, Lrad=%D. Otherwise, if L=%D/DTA>Lo, Lrad=DTA, else Lrad=%D. Lrad fails at any point when Lrad>DTAo or Lrad>%Do. We compared LRAD (Lo=1%/mm, DTAo=2mm, %Do=2%) with Gamma index tool (2%/2mm pass/fail criteria) for IMRT QA with 2D detector arrays. Dose distribution measured for 6 MV and 18 MV x-rays for various field sizes and shapes, including 6 IMRT plans, were compared to measurements made with output, beam flatness, beam symmetry, MLC leaf positions, and detector array translation and rotation adjusted at up to twice published acceptable tolerance. Results: LRAD was more effective than Gamma in detecting a 2% drift in output (dose per monitor unit), 1–2 mm drifts in MLC leaf position for 1–8 leaves in the field, and beam flatness and symmetry drifts that resulted in a 3% change in the off-axis ratio. LRAD and Gamma were equally effective for larger output changes and for detector translations and rotations. For all ∼400 measurements performed, LRAD detected 78% of the error and Gamma detected 52% of the errors. By using a color scale to display dose differences and grey scale for spatial errors, the reason for failure was self evident, whether it be beam or mechanical changes. Conclusion: LRAD proved to be advantageous in its ability to detect and distinguish between realistic changes in beam and mechanical settings in comparison to the Gamma Index.

SU-C-18C-07: Quantitative Measurement of Arterial Arrival Time in Pial Collaterals: Effect of Physical Parameters On Performance

Introduction: During Middle Cerebral Artery Occlusion (MCAO), the presence pial collateral capillaries can affect a stroke's severity and subsequent recovery Arterial arrival time (AAT) is the difference in contrast arrival time between a reference artery and a measured vessel, and can serve as an indirect angiographic measure of pial collateral recruitment during acute ischemic stroke. No automated method for AAT assessment is currently available on digital subtraction angiography. We report on an automated method for AAT maps derived from DSA and potential pitfalls that confound the accuracy of these maps. Methods: Our dataset consisted of digital angiographic images of five canines obtained on a GE OEC 9800 Plus Cardiac system using, 490 × 492 pixel images and iohexol, Images had been obtained after an induced MCAO. Our automatic method corrects for drift in x-ray output and measured noise in a region with no contrast. For each pixel, contrast arrival time was calculated using thresholding. AAT is the difference in contrast arrival time between a reference artery and a measured vessel, and yields information on the extent of pial collateralization in a measured vessel. This method was also validated against a manual determination of AAT. Deviations between manual and automatic methods were assessed across various physical parameters. Results: Factors that degrade the automatic determination of the AAT included motion, vessel orientation, the partial volume effect, pixel location from the vessel's center, and contrast agent dispersion. For example, contrast agent dispersion caused errors in the AAT from 33 to 891 milliseconds, across three dogs, for arrival times between a reference artery of the middle cerebral artery and a pial collateral vessel. Conclusions: For parameters studied, temporal distance can be validated down to milliseconds, thus allowing for the objective assessment of AAT and the potential appraisal of a stroke's severity.